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Vol. 8 . No.1 . 2001s

An Applications Reference for Siemens MAGNETOM UsersM A G N E T O M

FlashFlashFlash

:In this issue:BLow-Field MRI in Pediatric

Radiology Possibilitiesand Limitationsof the Method . . . . . . . . . . . . . .1

BDiffusion and Perfusion

Weighted MRI in AcuteNeurological Studies . . . .12

BAdvancements inMRI Scanner technologylead to improved functionalimaging . . . . . . . . . . . . . . . . . .18

BApplication News Software Release B33F forMAGNETOM Open /Open viva . . . . . . . . . . . . . . . .22

BProduct News CP Body Array Flex Coil

and CP Body Extender . . .28

:MRI in Pediatric Radiology Possibilities and Limitations ofLow Field Magnetic ResonanceImaging:M. Wagner, P. Kreissler1, R. Kuth1, T. Rupprecht

1 Siemens Medical Engineering,MR Research and Development,Erlangen, Germany

Technical Characteristics Monitoring and Sedation of Patients Possible Interventions Disadvantages in Comparison to High-Field Magnetic Resonance Systems Fields of Application Typical Diagnoses Prospects and Perspectives

Department of Pediatric Radiology

Hospital and Outpatient Clinic forChildren and Adolescents

(Professor Dr. W. Rascher, Director)

Friedrich-Alexander University,Erlangen-Nuremberg

Abstract:MRI has been used in pediatric imagingfor several years. It provides excellentanatomic detail and tissue characterizationcombined with the advantage that it isnot associated with the application of

ionizing radiation to the radiation sensi-tive infant organism. Low field MRIprovides some additional advantageslike a lower rate of sedations, easiermonitoring of sedated patients and theoption of interventional examination andtherapy. The disadvantages, however,are the slightly prolonged examinationtimes and the lower signal-to-noise ratiocompared to high-field MR scanners.In the future, new techniques usingmodern gradient echo sequences willprovide fast imaging methods witha very high signal-to-noise ratio which

continuation next page*Prior to 3 weeks out of the body,

infant imaging is not determined to be safe.


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For the last two-and-a-half years, anopen low-field magnetic resonancesystem from Siemens (MAGNETOMOpen) has been installed in the Hospitaland Outpatient Clinic for Children and

Adolescents at Erlangen-NurembergUniversity. This paper is a presentationof our experiences using this methodas well as a collection of interestingpatients.

Opportunit ies andLimitations of Open MagneticResonance in theRoutine ExaminationsWe have examined approximately1500 children in the last two and a halfyears using magnetic resonanceimaging (MRI). Approximately 80% ofthese examinations involved indicationsin the central nervous system (CNS);15% involved thoracic diseases, diseasesof the abdomen, and diseases of themusculo-skeletal system; finally, 5% ofthese examinations involved congenitalheart defects.

In the following, we present a selectionof typical diagnoses for each individualgroup of examination indications, anddiscuss the opportunities and limitations

of the method.

Examinations of CNSMagnetic resonance imaging is astandard diagnostic technique for theCNS (1,2). In pediatric medicine, it isused in the case of congenital diseases(deformities in the brain or spinal cord[figs. 1-3], metabolic defects [4]); fortumor location; for follow-up of tumors[5], hydrocephalus, and chronic inflam-matory diseases [6]; as an imagingtechnique in cases of convulsions [7] or

equivocal neurological symptoms;and in the diagnosis of infarctions orhemorrhage [8]. Figures 1-9 presenta range of typical diagnostic situations.

All customary examination proceduresfor the CNS area are also available inlow-field magnetic resonance tomogra-phy. In some cases, the inferior signal-to-noise ratio due to physical constraintsresults in a slight increase in examinationtime over that required for a high-fieldexamination. As figs. 1,4 and 9 demon-strate, inversion recovery (IR) sequencesare outstandingly well suited to improveand clear imaging of gyration. At the pre-sent time, image quality in angiography

sequences [figs. 1, 8, 9] is still inferior tothat obtainable in a high-field system.In case of doubt, the MR tomographyresults should be verified using colorDoppler sonography and quantified by

pulsed Doppler sonography [figure 1].

Currently, the principal technical limitationon low-field MRI in the CNS area is inimaging very thin slices. Slice thicknessesof 1-2 mm, which we and many otherinstitutions use for planning surgical inter-ventions for epilepsy, can be success-fully achieved using 3D sequences.Images can be reconstructed with a slicethickness of 2 mm when a Fast Low-Angle Shot (FLASH) 3D examination isconducted. The datasets are obtainedin a way suitable for image fusion using

single-photon emission computedtomography (SPECT) and magneticencephalography (MEG) examinations[figure 10].

As a result of the inferior signal-to-noiseratio, currently neither functional MRsequences nor spectroscopy can berealized on low-field magnetic resonancesystems. At the moment, this is theonly real limitation on this examinationmethod in the field of pediatric CNSdiagnosis.

Cardiac andThoracic Examinations

MRI has been a standard technique forsome years now in the diagnosticimaging of the thorax and anomalies ofthe great vessels (3-5). But it is alsopossible using this technology to producegood quality images of congenitalcardiac defects and anomalies in thethoracic venous course, as well asanomalies in the area of the trachea (6-9).

Finally, there are textural anomalies of

the myocardium (arrhythmogenicright ventricle) that can be diagnosedalmost exclusively with MRI (10).Here,T1-weighted spin echo sequencesin all 3 orientations (axial, coronal,sagittal) are chiefly used. In addition,double oblique sequences are usedfor imaging of the course of the aorta.

In recent years, we have examined5 children whose severe ventriculararrhythmias led clinically and electro-cardiographically to the suspicion of anarrhythmogenic right ventricle. Thissuspicion was confirmed in three of thechildren by means of a cardiac low-field magnetic resonance examination.

2

From the EditorChange of address?Would you like to be addedto our mailing list?In the US, please contact theApplications Helpline(phone 1-800-888-SIEM (7436) in theUS) and give us your name and businessaddress (no home addresses, please).MAGNETOM Flash is distributedoutside the US by local Siemens offices.Please contact me and I will makesure that you are included on your localsupport offices distribution list.

We hope you will find the informationin this issue of MAGNETOM Flashhelpful.

Dagmar Thomsik-Schroepfer, Editor

could partially replace conventional x-rayimaging. In the presented article wereport our experiences in low-field MRimaging of pediatric patients. Thepossibilities as well as the limitations ofthis imaging modality are pointed out.

Keywords: MRI, low-field MRI,children, sedation and monitoring.

IntroductionMagnetic resonance imaging is animaging modality free of ionizing radiation.For this reason, especially in pediatricpatients*, it is frequently applied. Thedevelopment of low-field magneticresonance imaging systems in recent

years has opened up new avenuesfor the use of diagnostic MR imaging inpediatric radiology. On the one hand,magnetic resonance imaging using fieldstrengths of around 0.2 Tesla (T) issubject to physical limitations comparedto traditional high-field magnetic reso-nance technology; on the other hand,though, the different constructionof these systems (open design) resultsin improved opportunities for patientmonitoring and new possibilities forinterventional procedures.

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Fig. 1a Fig. 1b

Fig. 2a Fig. 2b

Fig. 3a Fig. 3b

Figure 1a: Two-month-old boy withcongenital malformation of the right cerebralhemisphere which is lisencephalic andsmaller than the left hemisphere which

appears normal.

Figure 1b: MR angiography of the samepatient revealed a missing lenticulostriateartery which was confirmed by Dopplersonography. Additionally, the right anteriorcerebral artery was missing in this patient.If the malformation was the result of an earlyintrauterine infarction or caused by a primarymalformation of the arteries is not known.The quality of MR angiography is poorercompared to high-field imaging and is evenmore difficult to interpret in this patient,due to small size of the anatomic structures.

Figure 2a: T1-weighted image of a two-year-old girl with postoperative spina bifida and

Chiari II malformation. The typical features ofthe malformation are seen in the medialsagittal plane. Low posterior fossa contentswith herniated tonsils and elongated fourthventricle, partial agenesis of the corpuscallosum as well as polymicrogyria, a largemassa intermedia and the beaking of the tec-tum are characteristics of this malformation.

Figure 2b: Midsagittal T1-weighted spin echoof the spine in the same patient. The largelumbar defect is easily depicted. The spinalcord is fixed to the operation site indicatinga tethered cord which in this patient hadbecome symptomatic in terms of a decreaseof the motor function of the left leg.Additionally, two arachnoid cysts are noted at

the lower end of the spinal cord.Figure 3a: Eleven-month-old patient withfrontal encephalocele. The T1-weightedcoronal spin echo revealed a small cele withinthe nasal cavity. The intracranial connectionis clearly depicted. Note the wide arachnoidspace which is considered normal in theyoung child.

Figure 3b: Sagittal T1-weighted spin echosequence of the same patient. The ence-phalocele is seen very softly within thenasopharynx, occluding most of the rightupper airway.


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Figure 4a: 14 year-old girl with mucopolysac-

charidosis III (Sanfilippo) and severe physicaland mental retardation. Axial T2-weightedspin echo shows dilated ventricles and severeatrophy of the brain.

Figure 4b: Sagittal IR sequence of the samepatient. The atrophy is easily depicted.Most of the white matter is destroyed in thispatient secondary to the underlying disease.However, exact differentiation of the whiteand gray matter is no longer possible in thispatient.

Fig. 4a Fig. 4b

Fig. 5a Fig. 5b

Figure 5a: Sagittal T1-weighted spin echoof a seven year-old patient presenting withupper extremity paresis. The image showsan intraspinal tumor over 3 cervical segments.Partial destruction of the seventh cervicalvertebra is present.

Figure 5b: Postoperative T1 weighted spinecho of the same patient. No residual tumoris seen within the spinal canal. A fine syrinxis observed as well as a persistent spinaledema causing a mild thickening of the spinalcord. Note the scarring and the partialresection of the spinous processes at theoperation site.

Definite histology is pending.

Figure 6: Sagittal T2-weighted image of a14 year-old girl suffering from cerebral involve-ment of systemic lupus erythematodus. Theareas of cerebral vasculitis are easily depicted

as regions of high signal intensity.

Fig. 6


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Figure 7a: 16 year-old boy with Sturge-Weber

disease. Coronal T1-weighted image aftercontrast enhancement. Left cerebral hemia-trophy with and associated expansion ofthe subdural space. Cortical and meningealcontrast enhancement due to multiplevascular malformations is observed withinthe left hemisphere.

Figure 7b: Same patient, axial T2-weightedimage. The hemiatrophy and expansion ofthe arachnoid space is observed in this planeas well. Note the expansion of the left frontalsinus consistent with the cerebral hemia-trophy. In addition to that, multiple enlargedvessels are seen within the left choroidplexus which were identified as veins in anadditionally performed venous MR-angio-

graphy.

Figure 8a: Cerebral MR-angiography.Occlusion of the right middle cerebral artery(MCA) in a four-year-old girl with an infarctionof the right MCA.

Figure 8b: T2-weighted spin echo sequenceof the same patient. The large periventriculardefect is readily apparent as well as theconcomitant atrophy of the surrounding gyri.

Figure 9a: Four month-old girl with largevein of Galen aneurysm following inter-ventional angiography with coil and fibrin glueembolization of the aneurysm (ProfessorW. Huk, Neuroradiology, University ClinicErlangen/Nuremberg). Coronal IR sequence.The aneurysm is easily depicted.

Figure 9b: MR-angiography, sagittal view ofthe same patient. The supplying vesselsare clearly imaged while the aneurysm itselfshowed a minimal flow, indicating residualperfusion after the embolization. A completethrombosis of the aneurysm ensued.

Fig. 7a Fig. 7b

Fig. 8a Fig. 8b

Fig. 9b

Fig. 10

Fig. 9a

Figure 10: Image fusion from a FLASH 3Dexamination using interictual single-photonemission computed tomography (SPECT) ofa 6-year-old patient with therapy-resistantcerebral convulsive disorder associated withRasmussens encephalitis. The image showsa frontally reduced perfusion, with basalaccentuation. The image was reconstructedin a slice thickness of 2 mm. The datasetswere obtained in a way suitable for imagefusion using single-photon emission com-puted tomography (SPECT) and magneticencephalography (MEG) examination.(Image fusion: Dr. Platsch, Nuclear Medicine

Clinic, University of Erlangen/Nuremberg).


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Figure 11: Cardiac MR images (modifiedlongitudinal axis with cardiac triggering) ofa 12 year-old boy suffering ventricular

tachycardia. The pronounced thinning of theright ventricular wall and the increasedsignal intensity within the right ventricularmyocardium are typical evidence for fattydegeneration consistent with an arrhythmo-genic right ventricle.

Figure 12: 16 year-old boy with coarctationof the aorta status post repair in his infancy.Routine follow-up MRI. Modified longaxis T1-weighted spin echo with cardiactriggering. A large aneurysm of the aorta atthe former operation site was diagnosed.In addition, an aneurysm of the left carotidartery was clearly depicted on the images.The patient underwent surgical repair of theaneurysms.

Figure 13a: Cardiac MRI of a 14 year-oldpatient status post repair of a coarctation ofthe aorta. The MRI revealed a hypoplastic

aortic arch with prestenotic enlargement ofthe ascending aorta.

Figure 13b: Flow curve for the same patient.The result in the post-stenotic segment is amaximum systolic flow velocity of 291 cm/sec., which (in accordance with the modifiedBernoulli equation) corresponds to acalculated pressure gradient of 34 mm Hg.A slight diastolic gradient is suggested.

Figure 14: Comparison of maximum systolicpressure gradients (pmax) for 9 patientsdetermined by Doppler sonography and by

magnetic resonance tomography.The difference between the two methodsappears to be acceptable at high pressuregradients (r = 0.93, p< 0.001). The pressuregradient was calculated from the maximumflow velocities using the modified Bernoulliequation (pmax = 4v2).


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Fig. 15bFig. 15a

Fig. 16bFig. 16a

Fig. 17bFig. 17a

Figure 15a: Cardiac MRI in a 13 year-old girl

with right cervical aortic arch. The ascendingaorta is elongated and orientated to theright. The left carotid artery is easily depictedin this slice.

Figure 15b: Anatomical sketch of the rightcervical aortic arch. The aortic arch is drawnup and deflected to the right. The rightexternal and internal carotid arteries arise asindependent vessels from the aortic arch;the right subclavian artery arises from an aorticdiverticulum. The descending aorta intersectsthe central line and runs along the left side inthe thoracic region. The left subclavian arteryarises aberrantly from a diverticulum.(Modified from Finch MA, in: Amplatz, Moller,Radiology of Congenital Heart Disease;

1992, p. 1031).

Figure 17a: 7 year-old girl presentingwith swelling and pain of the left thigh. X-rayrevealed a cystic process with periostalreaction. Coronal T1-weighted image withoutcontrast enhancement. The isointenseprocess is clearly seen within the proximalfemur. Cortical thickening and periostalreaction are observed as well as an involve-ment of the surrounding soft tissues.

Figure 17b: Coronal fat suppressed sequenceof the same patient. An increased signal

intensity is noted at the site of the process aswell as a bone edema involving most ofthe femur diaphysis and the surrounding softtissue. Histology of the process revealedchronic osteomyelitis. Malignancy was themost important differential diagnosis andhas to be excluded by histology in such cases.

Figure 16a: 41/2year-old girl presenting with alarge abdominal tumor. Coronal T1-weightedimage without contrast enhancement.A large left-sided abdominal mass with hete-rogeneous medium-signal-intensity is seen,while the left kidney could not be located.

Figure 16b: Same slice after contrast mediuminjection. A heterogeneous contrast enhance-ment was seen while the right kidney showedno pathological contrast enhancement.Histology confirmed a unilateral Wilms tumor.


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Fig. 18b Fig. 18c Fig. 18a

Figure 18a: Coronal T1-weighted spinecho after Gadolinium administration in an11 month-old boy with a presacral massof unknown origin. The mass is seen at thelower boarder of the image. Additionally,marked dilatation of both renal pelvices isnoted.

Figure 18b: More anterior slice of the samepatient. The bladder is dilated and shifted tothe right. The bladder neck is clearly seen.The tumor compresses the urethra leading tomassive urine retention in this patient. Asno contrast medium is collected in the bladderduring the investigation an inhibited urinaryexcretion may be assumed secondary to the

morphological findings.Figure 18c: Sagittal T1-weighted image aftercontrast enhancement of the same patient.The presacral mass is clearly seen. Aftercontrast enhancement it showed a heteroge-neous signal intensity. In the sagittal planethe intraspinal spread of the tumor is easilydepicted. Histology is pending. Note theonly mild motion artifacts (respiration, bowelmovement) which are notably less pro-nounced than they would be in high-fieldMR imaging.

Figure 19: Typical examination of a consciouslittle girl. There is direct visual and physicalcontact with the child during the examination,which has a positive effect on the cooperationof the young patient.


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Figure 20a: Fast thick-slice MRI of the lungin a 16 year-old boy with cystic fibrosis.Multiple patchy areas of increased signal arenoted consistent with diffuse pneumonia.Interstitial disease is seen in its typical streakyappearance. Additionally, hilar attenuation isnoted. Acquisition time for 3 slices (slicethickness 50 mm each) covering the wholethorax 3.6 sec!

Figure 20b: Follow-up of the same patientafter 1 week of treatment. The pneumonicinfiltrates have markedly resolved indicatingthe usefulness of thick-slice MRI to diagnoseand later follow up acute changes in patientswith chronic lung disease.

Figure 21: Fast thick-slice true FISP imagingof the paranasal sinuses in an eight year oldgirl presenting with fever. The left maxillarysinus is completely mucous-filled.The ethmoidal cells were involved while thesphenoid sinus was found to be free ofinfection. Despite antibiotic therapy the girldeveloped an orbital phlegmon whichresolved within three days of IV antibiotictreatment. Acquisition time per slice 1.7 sec!

Fig. 20bFig. 20a

Fig. 21

Fig. 11 shows as an example the cardiac

low-field MR results for a twelve-year-old boy (modified longitudinal axis). Inaddition to electro-physiological criteria,the cardiac low-field MR diagnosisis a mainstay in diagnosing this clinicalpicture.

Coarctation of the aorta is the most easilyimaged congenital cardiac anomaly. Inour clinic, patients are regularly subjectedto a follow-up MR examination aftersurgical repair of a coarctation in order todetect as early as possible the develop-ment of any restenosis or an aorticaneurysm. The method also enables

good pre-operative aortic imaging ofthe morphological isthmus. In additionto the morphological image, it is alsopossible using MRI with the newly deve-loped phase-sensitive flow measure-ment sequences to evaluate the hemo-dynamic relevance of the stenosis.

Figure 12 presents an aneurysmformation of the aortic arch followingsurgical repair of a coarctation of theaorta in a 16 year-old boy; figures 13a-bshow the morphological image of ahypoplasic aortic arch and the flowcurve. Figure 14 is a comparison of thepressure gradients of 9 children deter-mined by Doppler sonography and byMRI, respectively.

As a result of the good quality image ofboth the vascular system and the tracheaprovided by MRI, anomalies of the greatvessels can often be better evaluatedby that technology than by angiographyor bronchography.

Figure 15a shows the results for a patientwhose right cervical aortic arch wasdiagnosed using sonography. Low-fieldMRI here enables easy imaging of therelationship between the bronchial andvascular structures.

Figure 15b visualizes the findings in ananatomical diagram.

Low-field MRI is also a valuable methodin the field of thoracic and cardiac diag-

nosis; here, though, its usefulness isrestricted by its somewhat inferiorsignal-to-noise ratio with very thin slices.Here at the Hospital and OutpatientClinic for Childrenand Adolescents at theUniversity of Erlangen, we have suc-ceeded for the first time in establishingfunctional low-field MR sequences thatalso enable quantitative flow measure-ment in the region of the great vessels.

In the future, it will also be necessary torealize additional functional sequences(ejection fraction, cardiac output) for low-field diagnostics. This seems feasible

in terms of the physics involved, althoughhere too one must expect that exami-nation times will be lengthier than they arefor comparable high-field examinations.

Abdominal ExaminationsThe most salient feature of abdominaldiagnosis in pediatric imaging is the limi-ted degree of patient cooperation com-pared to adults. MRI that are customaryin use with adult patients (breath-holdexaminations) are of only limited practicalvalue in pediatrics (11). This difficulty

must be overcome by an optimizedexamination process that includes thepatients regular and even breathing(breath triggering, special sequencesthat perform an averaging of individuallines rather than of the entire image),and, if necessary, pharmaceuticalsedation.

Figure 16 shows the examination of aninfant with a central Wilms tumor ofthe left kidney; figures 18a-c depict theMR results for an 11 month-old boywith a low abdominal tumor. Special line-averaged sequences for better suppres-

sion of motion artifacts due to breathingwere not applied in this patient becauseof the even breathing during sedation.

Overall, MR diagnosis is a valuablecomponent of pediatric abdominalimaging. Here, and especially in the caseof diagnosis of intra-abdominal tumors,examination should be conductedfollowing the administration of contrastmedium. MRI is often superior to CTexamination here, especially in connec-tion with the extent of paravertebraltumors in the vertebral canal, as shownin the examples in figures 5 and 18 oftumors with intraspinal dissemination ororigin in the vertebral canal.


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Sedation ofUncooperative ChildrenIn a retrospective and prospective studyconducted in 1997-1998, we compareda cohort of children requiring sedationduring use of our current system withdata for 111 patients examined byus using a closed high-field MR system.Strikingly, we found that the sedationand anesthesia rate for our patients haddecreased significantly, from 47% to27% (p < 0.001), following introductionof the open magnetic resonance tomo-graph. This is principally the result of aconsiderably lower sedation rate for theage group up to 10 years (12).

Table 1 presents an overview of the

sedation rate in a closed magneticresonance system versus the sedationrate in an open system, broken downinto 2 age groups. In the age group over10 years, the decision for pharmaceuti-cal sedation is determined by the patientsphysical condition (for example, inten-sive care patients) or mental situation(statomotoric and mental retardation)rather than the design of the system andthe examination process (13,14).

Examinations in high-field systems at ourclinic were conducted exclusively usinggeneral anesthesia. In our experience,

this can almost always be avoided in anopen MRI, where sedation combining0.2 mg/kg KG etomidate and 0.2 mg/kgbody weight midazolam is sufficient fornearly all patients to ensure an uninter-rupted examination. Although therehave been no incidents during sedationin the last two and a half years, the pre-sence of an investigatorwith experiencein pediatric intensive medicine is neces-sary.

10

Special Technical MonitoringFeaturesWe use a monitoring system from Bruker(Maglife) for technical monitoring ofpatients in our department; this systemenables continuous measurement oftranscutaneous pCO2 and pO2, capillaryoxygen saturation, and terminal expira-tory CO2, as well as oscillatory andinvasive arterial blood pressure measure-ment. The routine monitoring of sedatedpatients chiefly involves the measure-ment of capillary oxygen saturation andof the oscillatory blood pressure.

In our experience, the use of this moni-toring system in its standard commer-cially available form results in considerable

RF interference, with a significant dele-terious effect on the quality of MR tomo-graphy images. This is the result of theincreased sensitivity of open MRI systemsto RF interference, which can reachthe receiving coil more easily than it isthe case for closed tubes with bettershielding.

We have solved this problem bydeveloping additional RF shielding forthe monitoring system. This shieldingis already available from the companiesinvolved.

Overall, the ease of patient monitoringis significantly increased by the opensystem design: direct access to thepatient is always available, and evenmalfunctions occurring during theexamination process (for example, sensordislocations) can easily be correctedwithout delay or interruption of the inves-tigation.

For examinations in the abdomen,the use of special sequences forartifact suppression and an examinationtechnique that accommodates thespecial needs of pediatric patients are

recommended.

MusculoskeletalExaminations

MR imaging of the musculoskeletalsystem is less frequently performed inpediatric patients. X-ray is still theimaging method of choice for boneevaluation. Indications for MRI arebetter differentiation of bone defectsdiagnosed on radiographs (tumors,

infections, benign cysts), secondaryimaging when clinical findings cannot beexplained by the radiographic finding,especially when the condition is causedby a soft tissue process, rheumaticdisease and follow-up of musculoskeletaltumors. One of the important indi-cations is the diagnosis and follow-up ofosteomyelitis in pediatric patients asradiographs are often negative while MRImight reveal a skeletal edema consis-tent with the disease before any otherimaging method becomes positive. Inaddition to the routine sequences withand without contrast enhancement,special fat suppressed sequences shouldtherefore always be performed whenevaluating the pediatric musculoskeletalsystem [figure 17].

Design-dependent FeaturesAffecting Patient Monitoringand Sedation

The open design of the MAGNETOMOpen system provides good visual andphysical contact during the entireexamination and thus facilitates patientmonitoring; this is particularly advan-tageous when pharmaceutical sedativesare used. In our experience, the presenceof the mother during the examinationand the possibility of direct physicalcontact with her have a very positiveeffect on the cooperation of non-sedatedchildren. Fig. 19 shows the process ofa typical examination.

Open Low- Closed High- Significance

Field System Field System Level (Sedation) (General Anesthesia)

Age 120 Monthsexamined/sedated 232/68 (29%) 84/51 (61%) p < 0.0001

Age > 120 Monthsexamined/sedated 42/6 (14%) 25/1 (4%) n.s.

Totalexamined/sedated 274/74 (27%) 111/51 (47%) p < 0.001

Table 1:Overview of sedation rates in a closedmagnetic resonance system in comparisonto an open magnetic resonance system,

broken down into 2 age groups.


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InterventionsNot only does the open system designallow for better contact with the patientin respect to monitoring, but it also

enables intervention for diagnostic andtherapeutic purposes. The open systemdesign makes it easier to reach thetarget regions. Fast sequences can beused to track the needle in real timeduring a biopsy. These interventions canbe performed with short-term pharma-ceutical sedation.

In our center, for example, we performeda biopsie in a 7-year-old boy with statuspost treated oncological disease andcurrent suspicion of a metastasis in avertebral body. The conventional MRIof the spine had shown a destroyed tho-racic vertebra. As an increased sensi-tivity to ionizing radiation was included inthe differential diagnosis of this patientthe puncture had to be undertaken underradiation free conditions. The biopsywas performed in a dorso-lateral positionusing MR monitoring. Necrotic tissuewas confirmed histologically. The necro-sis of the vertebral body was assumedto be a radiation side-effect resultingfrom the patients increased sensitivityto radiation as further proven in thefibroblast cultures. Another patient whounderwent an interventional biopsy

presented with a cystic intra-abdominallymphangioma. During the procedurea sclerosing agent (OK 432) was injectedinto the lymphangioma.

Overall, our experience shows thatinterventional procedures are easilyperformed in the open MR system.Methodological difficulties do arise,however, in the positioning of MR sliceorientation for interventions; the useof markings on the body surface or needleprobings at likely points of access canmake this complicated. Further develop-

ment of biopsy holders for automaticneedle positioning would significantlyincrease the importance of MR-sup-ported interventions by simplifying thenecessary methods.

Prospects and PerspectivesWhile low-field MRI provides obviousadvantages in the areas of sedation andpatient access, this methods principallimitation lies in its inferior signal-to-noise ratio in comparison with high-fieldsystems. If modern gradient echosequences, for example, are used withconsiderably increased slice thicknes-ses to counteract this, the method will

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FlashFlasFlashallow for MR images that are in theore-tical physical terms qualitatively betterthan those obtained with high-fieldsystems: some of the artifacts thatoccur with MRI increase with increasing

field strength. With the use of verythick slices in the area of body diameter,projection volume images can beobtained that can be interpreted similarto conventional X-ray images in thearea of the lung or the paranasal sinuses.

As an example, figure 20a shows thevolume projection examination usingthick slices in the area of the lung ofa patient with cystic fibrosis. The followup image [figure 20b] shows that acutechanges are clearly noted and radiationfree follow-up is possible. At a veryshort examination time (1.2-3.6 seconds),this examination method might beused in the future as an alternative toconventional X-ray examination of thethorax in pediatrics. This is currentlybeing investigated in a prospective studyconducted at our clinic (15). Due to theextremely fast imaging real-time MRIduring respiration can also be performedeasily. Another possible perspective isthe fast imaging of the paranasal sinusesas presented in figure 21.

Conclusion

MRI is an important element in pediatricdiagnostic imaging. Without usingionizing radiation, it provides outstandingimaging of anatomical relationshipsand tissue limits. In the low-field range,the open system design provides signi-ficant advantages such as a decreasein sedation rates, easier monitoring ofsedated patients, and interventionaldiagnostics and therapy. At the sametime, examination times are slightlylengthened and the signal-to-noise ratiois slightly inferior to those for high-fieldMR systems.

The future prospects for low-field MRIare new methods relying on moderngradient echo systems, which willenable fast imaging with a high signal-to-noise ratio. In some areas, thesemethods could come to supplant con-ventional X-ray exposures.

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6. Malmgren N, Hochbergs P, Holmqvist C,Sandstrm S, Laurin S, Bjrkhem G. Complexcongenital heart malformation evaluated withMR imaging at 0.3 T. Pediatr Radiol

1996;26:470-477.7. Mc Kenna W et al. Diagnosis ofarrhythmogenic right ventricular dysplasia/cardiomyopathia. British Heart Journal1994;71:215-218.

8. Rupprecht T, Knig M, Nitz WR, HofbeckM, Singer H, Bwing B. Mglichkeitender Niederfeld NMR-Diagnostik bei kardio-pulmonalen Erkrankungen im Kindesalter.Monatsschr Kinderheilkd 1997;145 (S1):S 155 (Abstract)

9. Rupprecht T, Bwing B, Rascher W, KuthR, Deimling M. Steady State Free PrecessionProjektions NMR - Eine Alternative zukonventionellen Rntgenthorax Unter-suchungen? Ergebnisse einer pdiatrischenPilotstudie. Radiologe 1999;8:732-733.(Abstract)

10. Siegel MJ. Chest applications ofmagnetic resonance imaging in children.Top Magn Reson Imaging 1990;3:1-23.

11. Sieverding L, Jung W I, Fleiter T H, et al.Progress of MRI and variation of thediagnostic procedure in congenital andacquired heart disease. Klin Pdiatr1992;204:340-347.

12. Rupprecht T, Kuth R, Bwing B,Gerling S, Wagner M, Rascher W. Sedationand Monitoring of Pediatric Patientsundergoing Open Low Field MRI.Acta Paediatr 2000;89:1077-1081.

13. Carr JJ, Hatabu H, Gefter WB: Thorax;in Edelmann RR, Hesselink JR and Zlatkin

MB. (eds): Clinical Magnetic ResonanceImaging. Philadelphia, WB Sunders Company,1996, pp 1615-1682.

14. Cohen MD: Gastrointestinal System; inCohen MD and Edwards MK. (eds): MagneticResonance Imaging of Children. Philadelphia,BC Decker, Inc. 1990, pp 611-678.

15. Volle E, Park W, Kaufmann HJ.MRI examination and monitoring of pediatricpatients under sedation. Pediatr Radiol1996;26:280-281.

T. Rupprecht, MDUniversity Pediatric HospitalLoschgestrasse 15D-91054 ErlangenGermany


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:Exemplary Studies on Diffusion and

Perfusion Weighted Magnetic ResonanceImaging in Acute Neurological Disease:A. Gass, J. G. Hirsch, S. Behrens, K. Szabo, J. Gaa, M. G. Hennerici

NMR Research Depts. of Neurology/Radiology, Universitaetsklinikum Mannheim,University of Heidelberg, Mannheim, Germany

IntroductionMagnetic resonance imaging (MRI) is

increasingly used for the diagnosis andmonitoring of neurological disorders,while established diagnostic methodsshow limitations in their ability to demon-strate the underlying pathophysiologyand pathology. MRI frequentlyoffers thebackground information needed fora rational basis for therapeutic decisionmaking.

High resolution MRI using conventionalcontrasts (proton density-, T2-, T1-weigh-ted,T1-weighted after Gadolinium) candemonstrate with high sensitivity the

site and extension of focal abnormalityeven in structures as small as the opticnerve or brain stem nuclei [1, 2]. Echoplanar acquisition techniques havefacilitated the use of perfusion weighted(PW) and diffusion-weighted (DW)magnetic resonance imaging (MRI) ina clinical setting [3, 4]. In addition tostructural MRI they have the potential tovisualize the earliest hemodynamic andtissue changes induced by ischemia andnon-ischemic CNS pathologies. This isexemplified in serial MRI case studies of4 patients with acute severe neurologicaldeficits due to cerebral ischemia, focalepilepsy, acute relapse in multiplesclerosis (MS) and in suspected toxic/nutritional demyelination. All patientswere investigated early after symptomonset and in follow-up studies withstructural and echo planar diffusion andperfusion weighted MRI.

Materials and Methods

MRI data acquisition

MRI was performed on a 1.5 Teslaclinical scanner with EPI hardware(VISION, Siemens Medical EngineeringGroup, Erlangen).

1. Transverse, coronal, and sagittallocalizing sequences followed bytransverse oblique contiguous images

(slice thickness 5 mm) alignedwith the inferior borders of the corpuscallosum (sequences 2.-5.).

2. Proton density (PD)- and T2-weightedTSE (TR 2620 ms/ TE 14 ms/85 ms,FOV 180 x 240 mm2,matrix size 192 x 256)

3. T1-weighted SE (TR 530 ms/TE 12 ms,FOV 180 x 240 mm2,matrix size 192 x 256)

4. DW SE-EPI (TR 4000 ms/TE 110 ms,b = 0/160/360/640/1000 s/mm2,

FOV 240 mm2

, matrix size 128 x 128,sequential application of 3 separatediffusion sensitising gradients inperpendicular directions).

5. Perfusion-weighted FID-EPI sequencefollowing the first pass of a contrastbolus through the brain (TR 2000 ms/TE 65 ms/Flip 90, 13 slices, 40 acqui-sitions, 1:20 min, FOV 240 mm2,matrix size 128 x 128). Contrast wasinjected manually through largegauge venous canula at the antecubitalvein following a standardised protocol.

Data Processing and Analysis

DW MRI: ADC-maps were calculated ona pixel-by-pixel basis by a linear least-squares fit after averaging the direction-dependent DW images [3]. Manuallydefined regions-of-interest (ROI) werepositioned in the regions of mostpronounced signal change on DW images(b = 1000 s/mm2) and correspondingnormal appearing tissue. These regionswere superimposed on the calculatedADC maps.

PW MRI: From the data set a global time-intensity curve (a plot of the average signalas a function of time) was calculated for

quality assurance of the bolus-passage(fig. 1). Two parameter maps, thatevaluate qualitatively the main charac-

teristics of the contrast bolus passage(i.e. the timing of arrival in the capillarybed and the amplitude of signal change)were calculated: A time-to-peak (TTP)map (i.e. the intensity of each pixel isrelated to the relative position of thepeak of the bolus-passage curve), whichshows the time of arrival of the contrastbolus in the parenchyma/veins and asignal change map (PBP percentage ofbaseline at peak), that considers theamplitude of signal loss induced by thecontrast bolus passage through thecapillary bed.

Case Report 1A 62 year old man presented 2 hoursafter acute onset of severe right sidedweakness and speech disturbance.On clinical examination high grade rightcentral hemiparesis and dysphasiawere noted. Cerebral ischemia in the leftmiddle cerebral artery (MCA) territorywas suspected and emergency MRI was

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Figure 1

Plot of average signal as a function of time(global time-intensity curve), that is calculatedfor quality assurance of the bolus-passage.


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performed demonstrating a right MCAinfarct in evolution. Intravenous rtPAtherapy was initiated and on follow-upMRI reopening of the previously occludedMCA main stem and improvement ofhypoperfusion in the MCA territory wasobserved. Doppler and duplex ultra-sound studies revealed subtotal steno-sis of the left internal carotid artery

(ICA) and on day 3 carotid endarterectomywas successfully performed. Follow-upMRI demonstrated restoration of ICAflow signal and normalisation of MCAterritory perfusion and the patienthad a favorable outcome with only slightresidual finger weakness.

Case Report 2

A 58-year-old patient was admittedto the emergency department 2 hoursafter sudden onset of left sidedhemiparesis. The referring physicianalso reported a nystagmus to the leftand phases of altered consciousness.

Previous history: Five months earlierangiographically confirmed cortical veinthrombosis had occurred and one monthlater symptomatic epilepsy was diag-nosed after a first complex focal seizureand electroencephalographic confirmationof right centro-parietal rhythmic spike-slow-wave and sharp-slow wave activity.

On clinical presentation in the emer-gency room he had a hemianopic visualfield defect to the left and a left sensory-motor paresis. Cranial CT examinationdetected no definite change comparedto the previous scan 4 months earlier.MRI was performed 3.5 hours aftersymptom onset. At the end of the MRIexamination clonic epileptic motoractivity of the left leg occurred, thatpersisted when the patient was trans-ferred from the scanner to his bed.Clonic activity stopped immediatedlyafter a dose of 7.5 mg diazepam. A diag-nosis of Todds paresis after suspectedprolonged complex-focal ictal activity(probably for several hours) was made.

MRI demonstrated a close spatialcorrelation of signs of regional hyperper-fusion and a 22% reduction of the ADCin corresponding right parieto-occipitalcortical tissue. Single-photon emissioncomputed tomography (SPECT)confirmed focal hyperperfusion and EEGlocalized periodic lateralised epilepti-form discharges in corresponding areas.

Along with medical seizure control,perfusion and diffusion abnormalitieswere reversible.

Case Report 3

A 27-year-old woman with knownrelapsing-remitting MS was referred1 day after onset of left sided weakness.On clinical examination there wassevere pyramidal weakness of the rightarm and leg. An acute relapse wassuspected and conventional and DWMRI was performed. A new subcorticallesion involving pyramidal tract fiberson the right could be distinguished from

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Figure 2aSuccessful i.v. thombolytictherapy and subsequentcarotid endarterectomy inacute stroke. MRI 2 hours aftersymptom onset (first column),2 hours after i.v. thombolytictherapy (second column) andon day 3 after carotidendarterectomy is shown

(third column).

Initial T2- and T1-weightedimages shows minimalabnormality, while diffusionweighted images and the ADCmap reveal an early ischemiclesion in the left middlecerebral artery (MCA) territory.The area of hemodynamiccompromise is considerablylarger encompassing

the whole MCA territory.

Figure 2bOn MRA no flow signal isdetected in the left internalcarotid artery (ICA) and MCA(second column, left). Afteri.v. thombolytic therapy flowsignal is detected in the leftMCA and the perfusion is nor-malized in the anterior partof the MCA territory (second

column, middle and right).T1-weighted shows transienthemorrhagic transformation

of the ischemic lesion (thirdcolumn). Subtotal ICA stenosiswas diagnosed by Duplexultrasound and carotid endar-terectomy was performed.Postoperatively flow signal canbe detected in the left ICA andMCA while perfusion showscomplete normalization (firstcolumn). The area of infarction

remained relatively limitedcompared to the initial risk oflarge MCA infarction.


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14

Figure 3a-fIctal/postictal perfusion (left, center)and diffusion MRI (right) in the acute phase(top row) and follow-up (bottom row).

Signs of hyperperfusion are detected on PWMRI (TTP map left, PBP map center).

TTP and PBP maps show lower signal

intensity indicating marked hyperperfusionin the right parieto-occipital region(encircled), accentuated in cortical regions.On the corresponding strongly DW image(b = 1000 s/mm2) hyperintensity can beseen in large parts of the right hemisphere(occipital and parietal lobes, parieto-occipital

junction).

Hyperintensity is accentuated along thecortical band and in the paramedian occipitalcortex.

The ADC was reduced by 22% ( 1%) com-pared to normal appearing ROIs in the contra-lateral hemisphere. Diffusion abnormalitieshave normalized on day 13 (bottom row).

Follow-up perfusion weighted MRI on day 3shows resolution of perfusion changeswith a normal, symmetric bolus passage onTTP and PBP maps.

3a 3b 3c

3d 3e 3f


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chronic subcortical T2 hyperintenselesions. This lesion showed initially areduced ADC, minimal T2 hyperintensityand only very faint contrast enhancementon late post contrast images (doubledose Gadolinium-DTPA). High dose i.v.corticosteroid treatment was started for3 days. On day 5 contrast enhancementand T2 hyperintensity of the lesion weremuch stronger, while the ADC waspseudo normalized. The follow-up MRIat 5 weeks after the patient had madea nearly complete functional recoveryshowed a characteristic chronic constel-

lation of a slight ADC elevation, T2 hyper-intensity and only slight T1 hypointensityof the residual lesion.

Case Report 4

A previously healthy 29-year-old patientwas referred from another institutionwith unexplained subacute onset ofneurological symptoms. She had givenbirth to her first baby without anycomplications 6 weeks earlier. On theinitial clinical examination she was fullyoriented with psycho-motor slowingand slight dysarthria. Her motor functionwas impaired with abnormal choreatiform

movements and her gait was markedlyataxic. On a cranial CT examination nodefinite abnormality was noted. EEGshowed some generalized slowing butno focal abnormality. CSF examinationwas normal without any indication of CNSinflammation. Laboratory examinationswere unremarkable.

Conventional and DW MRI was per-formed. Symmetrical white matterlesions with marked ADC reduction andT2 hyperintensity were observed in asymmetrical distribution in the corpuscallosum and in subcortical white matter

in the centrum semiovale. There was nocontrast enhancement and no evidenceof any chronic lesions. A diagnosis ofsuspected toxic/nutritional demyelinationwas made. Over the following 8 weeksshe made a continuous gradual improve-ment with a favorable outcome withsome residual dysarthria and slight gaitabnormalities.

Discussion

In these case studies of patients pre-senting with acute neurological deficits,lesions with a reduced ADC are describedin 4 different disorders: cerebral ischemia,

after a prolonged complex-partial seizure,in the early stage of the developmentof an inflammatory-demyelinating lesionand in suspected toxic/nutritionaldemyelination. In all cases the locationand the extent of the acute abnormalitywere identified, which correlated wellwith the clinical features at presentation.PW MRI clearly separated the typicalpattern of hypoperfusion in cerebralischemia from the state of hyperperfusionas a sequel of increased glucoseutilisation due to prolonged epilepticactivity. In acute stroke, MRI identified

not only the vasular pathology, thearea of hypoperfusion und a reducedapparent diffusion coefficient (ADC)in the infarct core, but visualized success-ful recanalisation therapy (i.v. thomboly-tic therapy and carotid end-arterectomy)in follow-up studies.

Reduction of the ADC delineated anacute phase of disease which wasfollowed by elevated or normalized ADCvalues in chronic stages. This patternis well recognized in stroke but is muchmore rarely seen in postictal MRI andin acute MS lesions [5-7]. In patients 1-3chronic lesions (with T2 hyperintensityand elevated ADCs) were easily differen-

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Figure 4Serial MRI of an MS patient with an acuterelapse and a new MS lesion.

T2-, diffusion weighted, ADC, and contrastenhanced T1 -weighted MRI at presentation(top row), at day 5 (middle row), and at4 weeks (bottom row):

At presentation the lesion shows slightT2 hyperintensity, prominent ADC reduction,and minimal contrast enhancement on latepost contrast images after appropriatewindowing. At day 5, T2 hyperintensity hasincreased while the ADC shows pseudonormalization, and contrast enhancementshows up early and more prominently thanbefore. At 5 weeks the lesion shows residualT2 hyperintensity, slight ADC elevation,contrast enhancement has stopped.


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tiated from the acute pathology indicatedby the ADC reduction in symptomaticlesions. Chronic lesions in stroke and MSusually show signs (T

2hyperintensity,

T1 hypointensity, ADC elevation) of persis-tent tissue destruction (due to necrosis,demyelination, axonal loss) [8]. In patients3 and 4 (after epileptic activity, aftersuspected toxic/nutritional demyelination)there were no unequivocable signs oftissue destruction on follow-up ADC mapsor T2-weighted images. This indicatesthat low ADCs in acute human whitematter lesions occuring in the context ofdifferent pathophysiological mecha-nisms do not share the same prognosis.

The location of all lesions with an ADCreduction correlated with severe neuro-logical deficits. The relationship of

functional neurological status and ADCreduction has rarely been addressed inanimal experiments. ADC reductionsof about 25% as in the patient withprolonged ictal activity may be sufficientto compromise neuronal/axonal electricalcapabilities [9]. Correlations of clinicalstatus and lesion volumes have beenrepeatedly reported in acute stroke,when a more pronounced ADC decreaseis usually found (40-50%) [4, 10].Further studies into the relationship ofthe ADC and functional status areneeded to substantiate how closely ADCreductionscorrelate with neuronal/axonal dysfunction.

There is no definite explanation whydifferent pathophysiologies may lead toa reduced ADC. Experimental work in

ischemia and epilepsy suggests, thatcompromised energy metabolism anda rise of lactate induce ADC reductions[9, 11]. Prolonged ictal activity isknown to increase glucose utilization,the increase of which is not adequatelymatched by the enhanced blood flow(12, 13). As a result, blood flow-meta-bolism uncoupling leads to a reductionof high-energy adenosine phosphatesand tissue hypoxia, thereby stimulatinganaerobic glycolysis (14). In experimen-tal inflammatory white matter lesionsit has been suspected that toxic inflam-matory changes (cytokines, oxidativeproducts, proteolytic enzymes) in activelesions may cause mitochondrialdysfunction and lead to metabolic com-promise inducing a reduction of theADC [15,16].

16

Left: Extensive white matterlesions with ADC reductions(top row) are noted in the genuand splenium of the corpuscallosum and in the subcortical

white matter in a symmetricaldistribution. On T2-weightedimages correspondinghyperintense lesions are seenexcept for the lesion in thegenu of the corpus callosumthat appears isointense tonormal white matter.

Figure 5T2- and diffusion weightedfollow-up MRI in suspectedtoxic demyelination.

Left box at presentation.Right box follow-up at4 weeks:

Right: At 4 weeks the ADChas normalised and no obviousresidual T2abnormality ortissue atrophy has developed.


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PW and DW MRI can provide comple-mentary information to high resolutionstructural MRI providing new insightsinto the pathophysiology of acuteneurological diseases. This offers new

approaches for research as wellas valueable diagnostic information inclinical practice.

Literature[1] Gass, A., Filippi, M.,Grossman, R. I.:The contribution of MRI in the differen-tial diagnosis of posterior fossa damage.J Neurol Sci 2000; 172 Suppl 1:43-S49.

[2] Gass, A., Moseley, I. F.:

The contribution of magnetic resonanceimaging in the differential diagnosisof optic nerve damage.J Neurol Sci 2000;172 Suppl 1:17-S22.

[3] Le Bihan, D., Breton, E.,Lallemand, D. et al.:MR Imaging of Intravoxel IncoherentMotions: Application to Diffusionand Perfusion in Neurologic Disorders.Radiology 1986;161:401-407.

[4] Warach, S., Dashe,J. F., Edelman, R. R.:Clinical outcome in ischemic stroke

predicted by early diffusion-weightedand perfusion magnetic resonanceimaging: a preliminary analysis.J Cereb Blood Flow Metab 1996;16(1):53-59.

[5] Warach, S., Gaa, J.,Siewert, B., et al.:Acute Human Stroke Studied by WholeBrain Echo Planar Diffusion-weightedMRI.Ann Neurol 1995; 37: 231-241.

[6] Wieshmann,U. C., Symms, M. R., Shorvon, S. D.:

Diffusion changes in status epilepticus.Lancet 1997;350(9076):493-494.

[7] Tievsky, A. L., Ptak, T., Wu, O.,Farkas, J., Gonzalez, R. G.,Rosen, B. R., Sorensen, A. G.:Evaluation of MS lesions with fulltensor diffusion weighted imagingand anisotropy mapping.ISMRM 1997:666-666.

[8] Horsefield, M., Larsson,H., Gass, A.:Diffusion weighted Magnetic resonanceimaging in multiple sclerosis.J Neurol Neurosurg Psych suppl 1997;

64(suppl):80-S84.

[9] Zhong, J., Petroff, O. A. C.,Pleban, L. A., Prichard, J. W., Gore, J. C.:Reversible, reproducible reduction ofbrain water apparent diffusioncoefficient by cortical electroshocks.Magn Reson Med 1997;37(1):1-6.

[10] Lovblad, K.O., Baird, A.E.,Schlaug, G., et al.:Ischemic lesion volumes in acute strokeby diffusion-weighted magneticresonance imaging correlate with clinicaloutcome.

Ann Neurol 1997;42(2):164-170.

[11] Gyngell, M., Back, T.,Hoehn-Berlage, M., Kohno, K.,Hossmann, K.-A.:Transient cell depolarizationafter permanent middle cerebral arteryocclusion: An observation by diffusion-weighted MRI and localized 1H-MRS.Magn Reson Med 1994;31:337-341.

[12] Hoshi, Y., Tamura, M.:Cerebral oxygenation state in chemically-induced seizures in the rat study bynear infrared spectrophotometry.

Adv Exp Med Biol 1992;316:137-42.

[13] Blennow, G., Nilsson, B.,Siesjo, B. K.:Influence of reduced oxygen availabilityon cerebral metabolic changes duringbicuculline-induced seizures in rats.J Cereb Blood Flow Metab 1985;5(3):439-45.

[14] Blennow, G., Folbergrova, J.,Nilsson, B., Siesjo, B. K.:Cerebral metabolic and circulatorychanges in the rat during sustainedseizures induced by DL-homocysteine.

Brain Res 1979;179(1):129-46.

[15] Heales, S. J., Bolanos, J. P.,Stewart, V. C., Brookes, P. S., Land, J.M., Clark, J. B.:Nitric oxide, mitochondria and neuro-logical disease.Biochim Biophys Acta 1999 Feb 9;1410(2):215-28.

[16] Blamire, A. M., Anthony, D. C.,Rajagopalan, B., Sibson, N. R.,Perry, V. H., Styles, P.:Cytokine Induced Inflammation CausesAlterations in Brain Water Diffusionand Cerebral Perfusion. Proc. Intl. Sot.Mag. Reson. Med. 8 (2000) 485.

Authors address

PD Dr. A. GassNMR Research Neurology/RadiologyUniversittsklinikum MannheimUniversitt Heidelberg

Theodor-Kutzer-UferD-68137 Mannheim, GermanyTel: +49-6 21-3 83-28 85Fax: +49-6 21-3 83-38 07e-mail: [email protected]

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:Advancements in MRI Scanner Technology

Lead to Improved Functional Imaging:A. G. SorensenMGH-NMR Center, Massachusetts General Hospital, Charlestown, USA

Introduction

MRI has generally excelled at depictinganatomic changes in the brain. Over thepast decade, however, MRI has increas-

ingly been used to image physiologicchanges in the brain, including changesassociated with brain function anddysfunction. Responding to the demandfor physiologic imaging, equipmentcapabilities are beginning to be expanded;this in turn is allowing improvementsin physiologic brain imaging to begin toreach clinical practice. This article willreview a few of the basic technicaladvances that suggest near term andlong term clinical applications. Clinicalimage examples shown in this articlehave been acquired on a Siemens1.5T MAGNETOM Sonata anda 3T MAGNETOM Allegra system.

The Need for More SNR

Perhaps as much or more than any otherMRI technique, physiologic imaging islimited by the signal to noise ratio (SNR).

Broadly defined, physiologic imaging,also termed functional imaging includesdiffusion-weighted imaging (DWI);perfusion-weighted imaging (PWI),including techniques with and withoutexogenous contrast agents; brainactivation imaging using blood oxygenlevel-dependent (BOLD) and bloodflow techniques; and various otherexperimental techniques. Physiologicalimaging also includes spectroscopicimaging; although issues surroundingspectroscopic techniques warrant aseparate discussion than will be under-taken here, to first approximation manyof the same technical requirementsimposed by the already mentionedadvanced techniques are also neededby spectroscopic imaging. Conversely,spectroscopic imaging stands tobenefit from the hardware and softwareadvances that have been recentlymade just as other types of physiologicalimaging do.

MR Signal Increases withField Strength

Investigators have used numerousapproaches to try to overcome theseSNR limitations. Many of these techniquessuch as acquiring multiple signal aver-ages, use of surface coils, and sequenceoptimization often still leave physiologicimaging limited by SNR. As a result,additional approaches are being utilized.One approach is to move to higher fieldstrengths. Work at a number of institu-tions has confirmed that higher fieldstrength does indeed provide improve-ments in SNR and thereby in imagingperformance for physiological imaging.Fig. 1 shows an example of this forBOLD imaging in a cognitive paradigm.

Because BOLD imaging detects onlya small change in signal compared tobackground fluctuations, the improve-ment in SNR translates effectively intoa reduction in background fluctuationsand therefore improved statisticalconfidence regarding what changesrepresent signal and what changesrepresent noise. This improvement inraw SNR by moving to higher field willbenefit most if not all forms of functionalimaging. Many investigators believethat such improved SNR will be essentialif functional imaging is to meet its fullclinical potential.

OvercomingHigh Field Artifacts

Some investigators have felt thatmoving to higher field strength such as3T might come at too great a cost. Inparticular, since the susceptibility sen-sitivity also increases roughly linearlywith field strength, there has been someconcern that in areas near the base ofthe skull or in the posterior fossa therewould be artifacts that would precludegaining useful information from thebrain in these areas. However, this doesnot appear to be necessarily the case.We have found that by moving to thinnerslices (something that one wishes to do

typically with increased SNR anyway),the intravoxel dephasing and incoherenceis decreased (as a result of the smallervoxel size) and in fact susceptibilityeffects can decrease. An example ofthis is shown in fig. 2, where single shotecho-planar imaging is obtained through-out the brain including the cerebellumand brain stem and down in to thesuperior cervical spine without imagedegradation.

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Figure 13T versus 1.5T activation.Comparison of activation in a group ofsubjects performing a word-generationtask to visual stimul i at 3.0T (top row)and at 1.5T (bottom row).

Note the more extensive and stronger(as evidenced by the higher intensity color)activation in all regions. Note that additionalnoise is not produced, but rather additionalsignal consonant with the type of paradigmused.

Courtesy of Russell Poldrack, PhD.

3.0T

1.5T

z = 0 mm z = 18 mm z = 24 mm

Figure 2Coverage at 3.0T of brain, cerebellum/brainstem, and superior cervical spine.

Note the lack of warping, signal dropout,or other overt signs of susceptibilityeffects.

Technical parameters:Gradient Echo EPI, 3.0T, TR 3000 ms,TE 50 ms, 3 mm slice thickness,no gap, single shot interleaved acquisition,48 slices.

Courtesy of Larry Wald, PhD.


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20

Figure 3Very high b value diffusionweighted imaging. All images

are trace-weighted from fulltensor acquisitions at 1.5Twith a 40 mT/m system.Images through the brain inthis normal volunteer demon-strate routine appearanceat b = 1,000 s/mm2

(top row, 3 signal averages).

Note that at b = 5,000 s/mm2,there is now markedgray-white contrast presentwhereas at 1,000 s/mm2thereis little contrast (and whatcontrast is present at b = 1,000is based mainly on T2differen-ces). However, at b = 5,000

s/mm2

(second row, 12 signalaverages) there are clearlynew phenomena occurringincluding apparently differentrates of water diffusion in grayversus white matter, as wellas a drop in signal to noise ratiodue to the very high b valueused.

This trend continues atb = 10,000 s/mm2(third row,20 signal averages), wherethere are still increased gray-

white differences comparedto b = 1,000 s/mm2, until atb = 20,000 s/mm2the gray

matter is essentially invisible.This is probably due to non-monoexponential decay in thebrain, but this is a matter ofongoing research.

Note that even forb = 20,000 s/mm2on a 40 mT/msystem with all three axes ableto operate simultaneouslythe TE is 136 ms (as comparedto 67 ms for b = 1,000 s/mm2).This makes the visualizationof white matter possible atb = 20,000 s/mm2 with20 signal averages; if less SNRwere available (if, for example,

the TE was substantiallylonger), many more signalaverages would be neededsince SNR goes as thesquare root of the numberof signal averages.

Note that the system iscapable of 40 mT/m in all threegradient directions simulta-neously, for a net performanceof (square root of 3 times40 mT/m) approximately69 mT/m.

Figure 4Diffusion tensor tractography*of the in vivo human brain.

This tractography study isfrom a 68 year old female withmotor weakness but noevidence on any imaging study(including this tractographystudy) of any physical defect.

The multicolor area is an

enlarge-ment of a coronalsection through the brain.

The eigenvalues and eigen-vectors were then computed,weighted by the fractionalanisotropy in the image, andoverlaid on to an anatomicscan.

Note the clear directionality ofthe fiber bundles including thecorpus callosum, cortico-spinaltracts, and subcortical U fibers.

Data were obtained on a 1.5 Tsystem with 40 mT/m gradientsand 18 signal averages ofa single tensor acquisitionsequence.

Courtesy of Mette Wiegell,

David Tuch, and Van WedeenMD.


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Balanced SystemArchitectureOther approaches to improving SNRcan be as important and as helpful as theincrease in field strength. Indeed, theentire MRI system must be engineeredto optimize SNR at each step: themagnetic field, the gradient subsystem,the RF chain, and the computer systemsto handle both control of the systemand the acquisition and processing ofthe data that are obtained. For example,it is well known that surface coils canimprove SNR in a local area just under-neath or near the surface coil. Nume-rous groups have demonstrated thatone approach to improving SNR over alarger area than just a surface coil is to

create arrays of surface coils. However,for functional imaging, an array-awaresystem is not enough. The RF receiversand digitizing hardware must be ableto handle the high throughput of data oneach of the multiple channels, ratherthan just on a single channel if phasedarray functional imaging is to actually beusable. This means four, eight, or morefast receivers, something that needsto be deliberately engineered in to theMR system. Similar issues arise with thecomputational chain: there is no pointin having fast RF and gradient hardware

to sample the magnetization rapidly ifthe computing subsystem cannot storeor process the incoming data.

Gradient SystemPerformanceA key subsystem that has been a focusof improvement recently is the gradientsubsystem. While slew rates on theorder of 100 T/m/s were once consideredto be high, such systems are now under-powered for the rapid read-out pulsesequences in use today. Similarly, whilepeak gradient strength of 20 mT/m wasonly recently considered high performance,many users find their needs for peakgradient strength to keep increasingwell beyond the 40 mT/m routinelyoffered today. This need for high peakgradient performance is particularlytrue in diffusion-weighted MR imaging.Fig. 3 demonstrates images from anormal volunteer taken at multiple highb values, including up to b = 20,000s/mm2. (Normal DWI is obtained atb = 1,000 s/mm2). Such very high b valueimages are highly dependent on ade-quate SNR, since the large diffusiongradients cause extensive signal loss.

By increasing peak gradient strength,one can shorten the duration of thegradient pulses and markedly reducethe overall echo time of a given pulsesequence. Because diffusion-weighted

images typically have long TE values(due to the long diffusion gradient pulselength), decreasing the TE from 100 ms(typical with gradient capabilities ofapproximately 20 mT/m) to TE of 65 ms(typical with gradient capabilities of40 mT/m) one can expect a doubling ofSNR. Particularly because very highb values are highly dependent on peakgradient strength and amplifier dutycycle, engineering the system from theground up to handle such advancedperformance requirements is essential.

Towards New Frontiersin NeuroscienceOnce these systems are in place, how-ever, the type and quality of functionalimaging that can be done is dramaticallyincreased. To close with just a singleexample, Fig. 4 demonstrates diffusiontensor tractography* performed using40 mT/m gradients at 1.5 T in the brain.The type of tractography and the reso-lution available are highly dependent onthe shortened TE, the faster gradientand RF performance, the amplifier

capabilites, and the computer controland data processing subsystems.With the advent of this type of functionalimaging, diagnoses of more subtleabnormalities and insight into morecomplex disease processes will beavailable in ways that were heretoforeunavailable.

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Authors address

A. Gregory Sorensen, MDMGH-NMR CenterMassachusetts General HospitalBuilding 149, 13th Street

Charlestown, MA 02129USA

* This information about diffusion tensortractography is preliminary. The product is

under development and not commerciallyavailable in the U.S., and its future availabilitycannot be ensured.


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The literal meaning of Viva is Livea long life. The word Viva is used toexpress Goodwill and Approval. TheNumaris 3B33F software is preciselyachieving this objective. Numaris 3B33F

for the MAGNETOM Open/Open Viva/Open Viva #P is further enhancing thecapabilities of the system. Most of thesequences (.vhc) exploit the 15 mT/mgradient strength. MAGNETOM Opencan do the routine Bread and Butterexaminations with excellent Imagequality.

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APPLICATION NE:Software upgrade Numaris 3B33Ffor MAGNETOM Open/Open Viva/Open Viva #P:

OverviewThe 3B33F software has the following new sequences:

s Sequence with low TR and

low TE (8 ms) for better T1 contrast

for increased anatomical coverage as more slices per TR are possible

for use in C-Spine and orthopedic imaging as well

s Sequences forMR Cholangiography

for non invasive study of the biliopancreatic ductal system

for imaging the extraductal structures as well

with thick slab and thin slice (HASTE ) techniques

s LOTA Sequences

for reducing respiratory and motion artifacts

for abdomen, C-Spine imaging

s Single Shot U-FLARE sequencefor Diffusion Imaging

for diffusion imaging in a few seconds (9 sec)

with a b value of 600 sec/mm2

for advanced neuro applications

s In Phase/Opposed Phase Sequence for Body Imaging

for liver diseases

for pancreatic imaging

for adrenal abnormalities

s Sequences forMusculoskeletal Imaging

for better resolution knee, shoulder and other joints

small FOV (130 mm) DESS, FLASH and FISP sequences

3D water excited sequences for pre and post contrast T1 weightedimages with fat suppression

s 2D True FISP Sequences forLung Imaging*

for very fast T1/T2 contrast of the lung with or without breathhold

* Cardio Option


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FlashFlasFlashSSpin Echo sequencewith low TR and low TE (8 ms)The sequence se_8b130.vhc providesa short TE of 8 ms for better T

1

contrast.The short TE reduces the flow artifactsfrom the carotids. There are 19 slicesavailable within a TR of 395 ms. Thismaintains the T1 contrast and has a goodanatomical coverage. The results arebetter than the sequence with TE of15 ms, which was previously available.This sequence is also useful for T1C-Spine imaging as well as in jointimaging.

Sequences forMR Cholangiography MRCPThe sequences for MRCP are the singleshot TSE slab and HASTE thin slicetechnique. Similar to the sequenceson the high field systems thesesequences allow non-invasive breath-hold studies of the biliopancreaticsystem in few seconds. MRI has theadded advantage of imaging the extra-ductal structures. Transverse T1 andT2 weighted imaging using LOTA pro-vides additional information.

Courtesy Bhavin Jankharia M.D.,JIC Mumbai, India

Case of Cholangiocarcinoma

Thick Slab MRCP aquired in2.89 seconds

Spin Echo, 5:31 min, 5 mm, FOV 230 mm,168 x 256 matrix

Spin Echo, 4:57 min, 5 mm, FOV 230 mm,144 x 256 matrix

Thin Slice HASTE aquired in15 seconds

T1 LOTA tra, 4:31 min


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LOTA SequencesLOng Term Averaging is a techniquethat reduces respiratory and motionartifacts. This technique is best used in

body imaging. LOTA sequences can beused for good C-Spine imaging as well.T1 and T2 LOTA sequences are availablewith the Numaris 3B33F software.

Single Shot U-FLARE sequencefor Diffusion ImagingA few years ago diffusion imaging onthe low field was a difficult hypothesis initself. With the Single shot U-FLAREsequence Siemens has achieved diffusionweighted imaging on the MAGNETOMOpen, using all 3 diffusion sensitizinggradients simultaneously. U-FLARE takesonly 9 seconds!!!! The followingclinical examples demonstrate the S/Nand excellent image quality.

The U-FLARE sequence is a single shotTurbo Spin Echo variant. In its simplestform it has 10 single shots averagedrequiring 0.5 seconds/image with a matrixof 128 x128. The scan has two setsof images, one with a b value of0 sec/mm2 called T2 reference imageand a second set of images with ab value of 600 sec/mm2.


Standard averaging LOTA averaging

T2weighted reference imageb 0 sec/mm2

TE = 102 ms

Diffusion weighted imageb 600 sec/mm2

TE = 102 ms


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Example of a 60-year-oldfemale with right MCA infarctCourtesy Bhavin Jankharia M.D.,JIC Mumbai, India

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FlashFlasFlash

Example of a 40-year-old malewith venous infarctCourtesy Bhavin Jankharia M.D.,JIC Mumbai, India

Spin Echo T1, 4:57 min Turbo SE T 2, 4:59 min

T2weighted reference image,b 0 sec/mm2

Diffusion weighted image,b 600 sec/mm2

Dark Fluid T2, 5:16 min

T1 sagittal MR Venogram

Multiple collateralvenous channelsdue to thrombosisof sagittal sinus

Diffusion weighted image,b 600 sec/mm2

Note:The hyperintensesignal on DWI isrepresented byhypointense signalon the T2 referenceimage an area ofdecreased diffusion.

T2weighted reference image,b 0 sec/mm2


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In Phase Opposed PhaseSequence for Body ImagingAt a particular TE (TE = 5.4 ms), whenfat and water are in phase both signalsadd within a voxel, while at anotherTE (TE = 18.1 ms), fat and water protonsare out of phase and their signals canceleach other in the same voxel. In thedouble echo sequence, 2 sets of imagesare acquired (in the same sequence)with two different TEs. In the imageswith opposed phase, hypointenseregions contain fatty components. Thisis particularly useful in adrenal imaging.Secondly, the viscera get a sharper edge,differentiating the structuresbetter. Inthe example below besides the viscerahaving a sharper edge, the signal from

the bone marrow changes significantly.The Out of phase image has a better S/Nas its bandwidth is narrower than the inphase image with very short echo time.Low bandwidth sequences improve S/N a definite advantage on Low field!

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In phase image (TE = 5.4 ms) Out of phase image (TE = 18.1 ms)

DESS 3D, 2 mm, 130 mm FOV Water excitation, 3D, 3 mm, 220 FOV

Sequences for

Musculoskeletal ImagingDESS, FLASH and FISP 3D sequencespermit small FOV (130 mm) imagingfor higher resolution. This will furtherenhance joint imaging on MAGNETOMOpen. The two dimensional tablemovement enables isocentre positioningof the joint, like the shoulder yieldingoptimum image quality. The 3D waterexcited sequence is useful for pre- andpost-contrast T1 weighted joint imagingwith fat suppression. Accurate fre-quency for water excitation has to bedetermined before the scan for good

results.

Left: Courtesy Bhavin Jankharia M.D.,JIC Mumbai, IndiaRight: Courtesy MRI ofEaston, USA


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2D True FISPfor LUNG Imaging*Siemens is currently the only vendoroffering True FISP, especially on lowfield. The 2D True FISP sequence isa very fast (subsecond to a few seconds)sequence. It is well suited for imaginglung parenchyma without contrast. Initialstudies have shown that it is a promisingalternative to conventional chest x-rayin pulmonary disease particularly in thepediatric age group where repeatedx-ray should be avoided. A low magneticfield is an advantage in using this pro-cedure.

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FlashFlasFlash

X-ray 2D True FISP in 3 sec, 256 matrix


Case of Pneumonia logarithmic scale

Courtesy: Dr.Th.Rupprecht et.al.,Kinderklinik Uni-Erlangen, Germany

Pulmonary nodule

Pulmonary Vasculature

* Cardio Option


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CP Body Array Flex and CP Body ArrayExtender are suitable for all high resolution

body examinations such as thorax,abdomen, pelvis, etc. Each coil contains2 CP elements for optimal SNR. Theflexible design creates high patient com-fort and ease of use. The coils are usedin combination with the CP Spine ArrayCoil. They can easily be used togetherproviding large field-of-view coverage toacquire for example thorax-liver or wholebody MR angiography studies. Com-bining the coil element from within theuser interface allows large anatomicalcoverage or targeted high resolution with-out the need of repositioning the patient

or the coil by using remote table control.

CP Body Flex Array/Peripheral CP Angio Array forperipheral MRACP Body Array Flex can be combinedwith Peripheral CP Angio Array Coilto perform high-resolution peripheralangiography including the lowerabdominal aorta and iliac arteries.

Clinical result

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PRODUCT NEWS:CP Body Array Flex Coil andCP Body Array Extender forMAGNETOM Harmony/Symphony/Sonata:

Fig. 1

Technical data for both coils:

4 coil design with 4 integrated preamplifiers

Integrated preamplifiers increases the signal-to-noise ratio and eliminates degradation ofthe signal during transfer

CP design offers up to 40% higher signal-to-noise than linear design

Array technology allows the coverage oflarge fields-of-view for high quality abdominalimaging

IPA coil concept allows intelligentcombination of coil elements from differentcoils providing easy access to all anatomical

areas

No-tune coil(no patient-dependent tuning is necessary)

Size: 316 mm x 460 mm x 36 mm

Weight: only 1.1 kg !

Fig. 2

Coils positioned on patient for large FoVimaging. Remote Table motion allows the

region of examination (pelvis ) to be placedin the center of the magnet. Integratedpanoramic positioning with automatic coilelement switch chooses the coil at iso-center automatically from upper elements tolower elements


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FlashFlasFlash

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Fig. 5FLASH 1, 45 sec postGd or recently VIBE1,45 sec post Gd(Volume Interpolated Breathhold Examinationallowing the examination of the parenchymaand the vasculature with one study)

Fig. 6Fat suppressed FLASH 90 sec postGdor VIBE 90 sec postGd(Liver, mid abdomen axial )

Fig. 3liver coronal

Fig. 4FLASH in-out phase (Liver axial)

Fig. 7FLASH 5-min postGd (transverse )

Fig. 9

Fat suppressed FLASH or recently VIBE(axial, sagittal)

Fig. 10

T2high resolution TSE(axial, sagittal)

Fig. 8Localizer (pelvis )


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:SiemensMedical.com

Joining the Siemens MR Community MR Clinics and MAGNETOM World:Nejat Bengi, MD, Siemens Medical Solutions, Erlangen, Germany

The results of the advances in MR tech-nology are numerous new applications

that directly affect todays clinical prac-tice. However, our rapidly changingworld makes it difficultat times to followtheir clinical impact. For this reasonwe present examplified procedures andtheir effect on a sites clinical routineand on a patients diagnostic workup.Examples are collected from variousparts of the world.

To bring new applicaitons into the clini-cal world, Siemens Medical Solutionscreated the MR Community in theInternet. It is a place where SiemensMAGNETOM users provide clinical

case studies in various clinical areas,future views, imaging and applicationtips to our MR community.

How to get there?The MR Clinics and MAGNETOM Worldare linked to the home page of MagneticResonance in both the international aswell as the German version. In the nearfuture, the US page will include theMR Clinics and MAGNETOM World aswell.

You can directly get to the page via:www.siemensmedical.com/mr.

The first time you visit the page you willhave to select the international, Germanor US version.

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MAGNETOMWorld

MR

Clinics

Now you have the possibility toview the MAGNETOM World or visit

the MR Clinics.Select: MAGNETOM World:

Take a look at the case studies, clinicalmethods, imaging, tips, or communitysites that support selected clinical areas.

At the moment, we provide thefollowing content for MR Clinics andMAGNETOM World

1. Royal Brompton Hospital, UnitedKingdom: The image gallery includestwenty-seven different cases showingthe importance of cardiac MR.A case study shows that segmentedcine TrueFISP increases the diagnosticreliability of MR cine imaging.

2. St. Johannes Hospital, Dortmund,Germany: Cardiac case study shows theimportance of Siemens-patentedviability technique in diagnosing infarcts.Application tips for optimal use ofCardiac MR Imaging.


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FlashFlasFlash

3. Massachusetts General Hospital,Boston, USA: Future views, casereports, application tips, and clinicalmethods describing new techniques in

stroke imaging.4. University Essen, Germany:Twenty-five angiography studies plustips are presented, showing how toget the most out of MR angiographytechniques.

5. Dr. Jankharias Imaging Center,Mumbai, India: Case reports showingTrueFISP lung imaging for tubercolosis.Case reports, clinical studies, applicationtips.

Choosing MR Clinics provides threeclinical areas at the moment: Cardiology,Neurology and Angiography/Vascular.Selecting one of the areas will provide

more detailed information.We kindly invite you to join the SiemensMR Community at MAGNETOM Worldand MR Clinics.


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FlashFlashFlash

Published by

Siemens AG, Medical Solutions

P.O.Box 3260D-91052 Erlangen

CORRESPONDENCE AND

INTERNATIONAL DISTRIBUTION

Dagmar Thomsik-Schroepfer,PhD, EditorMAGNETOM FLASHSiemens AG, Medical Solutions,Med MRM3Allee im Rthelheimpark 3

D-91052 ErlangenPhone: 49 - 91 31 - 84 - 58 73Fax: 49 - 91 31 - 84 - 21 86

e-mail: [email protected]

US DISTRIBUTION

Siemens Uptime Service CenterMR Applications110 MacAlyson Court

Cary, NC 27511Phone: 800 - 888 - 7436

TECHNICAL EDITOR

Helmuth Schultze-Haakh, PhDSiemens Medical Systems, Inc9742 Ravenscroft RoadSanta Ana, CA 92705Voicemail: 800 - 753 - 63 36, ext 21 17

e-mail: [email protected]

Internet:www.siemensmedical.comwww.siemensmedicalacademy.com

All articles represent the techniquesand opinions of the authors and may notrepresent specific recommendationsor endorsements from Siemens MedicalSolutions. Contact the authors directlyfor further information about theirtechniques and opinion A

91100-M2220-E813-4-7600

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Documents

[1] Magnetom Flash_Jan 2001