Martin N. Yongbi, PhDFrancesco Fera, MDYihong Yang, PhDJoseph A. Frank, MDJeff H. Duyn, PhD

Index terms:Brain, diffusion, 10.121413, 10,12144Brain, MR, 10.121413, 10,12144Brain, perfusion, 10.121413Magnetic resonance (MR), functional

imaging, 10.121413, 10,12144Magnetic resonance (MR), perfusion

study, 10.121413

Published online before print10.1148/radiol.2222001697

Radiology 2002; 222:569–575

Abbreviations:ASL � arterial spin labelingCBF � cerebral blood flowPW � perfusion weightedROI � region of interestSNR � signal-to-noise ratio

1 From the Laboratory of Functionaland Molecular Imaging, Clinical Cen-ter (M.N.Y., Y.Y., J.H.D.), and Labora-tory of Diagnostic Radiological Research(F.F.), National Institutes of Health, Be-thesda, Md; Department of Psychiatry,Functional Neuro-Imaging Laboratory,Cornell University Medical College, NewYork, NY (J.A.F.); and Institute of Exper-imental Medicine and Biotechnology,National Research Council, Cosenza, It-aly (F.F.). Received October 23, 2000;revision requested December 6; final re-vision received May 30, 2001; acceptedJune 20, 2001. Y.Y. supported by theWhittaker Foundation. Address corre-spondence to J.H.D., NMR Center,Building 10, Room BID109, National In-stitutes of Health, 9000 Rockville Pike,Bethesda, Md 20892.© RSNA, 2001

Author contributions:Guarantor of integrity of entire study,M.N.Y.; study concepts, all authors;study design, M.N.Y., J.A.F., J.H.D.; lit-erature research, M.N.Y.; clinical stud-ies, M.N.Y., F.F., J.A.F.; data acquisition,M.N.Y., F.F.; data analysis/interpreta-tion, M.N.Y., Y.Y., F.F., J.A.F., J.H.D.; sta-tistical analysis, M.N.Y., F.F.; manuscriptpreparation, M.N.Y., F.F., J.H.D.; manu-script definition of intellectual content,M.N.Y., Y.Y., J.A.F., J.H.D.; manuscriptediting, revision/review, and final ver-sion approval, all authors.

Pulsed Arterial SpinLabeling: Comparison ofMultisection Baseline andFunctional MR ImagingPerfusion Signal at 1.5 and3.0 T: Initial Results in SixSubjects1

Flow-alternating inversion-recoverymagnetic resonance imaging wasperformed at 3.0 T to measure cere-bral perfusion during rest and motoractivation in six healthy adult volun-teers. Results were compared withthose at 1.5 T. The mean signal-to-noise ratio for both gray matter andwhite matter perfusion measuredwith and without vascular suppres-sion at 3.0 T was significantly (P �.01) higher (n � 6) than that at 1.5 T.Brain perfusion activation maps col-lected during a motor task showed asubstantially larger number of acti-vated pixels (�80%) at 3.0 T, withactivation in the supplementary mo-tor area in the 3.0-T data that wasnot present on 1.5-T perfusion maps.

Perfusion is a fundamental physiologicprocess that is highly sensitive to normaltissue function as well as a broad range ofpathologic conditions. The potential ofmagnetic resonance (MR) imaging in thenoninvasive measurement of tissue per-fusion has been shown in studies of sev-eral pathologic conditions, such as stroke(1), dementia (2,3), and epilepsy (4).Techniques for perfusion measurementwith MR imaging include monitoring ofa bolus of gadolinium-based contrastagent (5,6) and arterial spin labeling(ASL) (7–13). Although techniques basedon monitoring of an intravenously ad-ministered bolus of contrast agent with

dynamic serial imaging are known toprovide better signal-to-noise ratio (SNR),the techniques have a number of draw-backs, including possible adverse patientreaction to the contrast agent and cost.ASL techniques, on the other hand, areentirely noninvasive, but they providelower SNR. With ASL, arterial water pro-tons that flow into an organ are magnet-ically labeled and used as an endogenoustracer. The exchange between the labeledarterial blood water and the unlabeledwater in tissue leads to a change in tissuemagnetization that can be detected atMR imaging.

There are two ASL approaches: contin-uous and pulsed labeling. In continuouslabeling (7,10), tagging is achieved bymeans of continuous inversion of inflow-ing arterial water in blood by using a long(�3-second) adiabatic radio-frequencypulse. Pulsed labeling (8,9,11–13), on theother hand, involves use of a relativelyshort (�10-msec) hyperbolic secant in-version (14) or a frequency-offset–cor-rected inversion (15) radio frequency formore efficient inversion of the arterialwater (16). Although these techniqueshave become popular for experimentswith functional MR imaging (17,18),they are still not used routinely in clini-cal examinations, primarily because oftheir characteristic low SNR. This prob-lem is further compounded by additionalloss in perfusion signal through T1 as thelabeled arterial water protons migratefrom the tagging zone to the imagingsection. Since the T1 decay rate decreasesat higher magnetic field strengths (�3.0T), the associated perfusion loss is sub-

569

stantially lower owing to the longer T1 ofblood (17).

Despite the apparent advantages ofhigh-field-strength systems, the bulk ofASL studies performed to date have beenperformed at 1.5 T, with a limited num-ber (reported in abstract form [18]) per-formed at high field strength. The lattermay be due in part to the limited numberof whole-body high-field-strength MRimaging systems. Given the expected im-provement in perfusion SNR at high fieldstrengths and the progressive increase inthe number of 3.0-T MR imagers, there isa need to evaluate the performance ofASL perfusion techniques at higher fieldstrengths, particularly in light of the re-cent approval of 3.0-T magnets for clini-cal use by the U.S. Food and Drug Admin-istration.

The goals of this study were to (a) eval-uate the feasibility and performance ofpulsed ASL techniques for perfusion mea-surements at 3.0 T and (b) measure theperfusion SNR for gray matter and whitematter with and without vascular sup-pression at 3.0 T and compare with thesame data at 1.5 T by means of t testanalysis. Additionally, the perfusion acti-vation area after a simple motor task wasmeasured at both field strengths, and theactivation area was assessed and com-pared by using standard methods basedon functional MR imaging. For thisstudy, we used software and hardwarefrom the same manufacturer.

Materials and Methods

Summary of Perfusion(flow-alternating inversion-recovery)Method

For perfusion measurements in healthycontrol cerebrum, a modified flow-alter-nating inversion-recovery sequence (16)was used that incorporates an inversionpulse of the frequency-offset–corrected in-version design (15) for optimal inversion.Although the mode of operation of theflow-alternating inversion-recovery tech-nique has been described elsewhere (8,9),we provide a brief summary as backgroundinformation.

Perfusion-weighted (PW) MR imagesare acquired with the flow-alternatinginversion-recovery technique by perform-ing two experiments: In the first experi-ment, a selective inversion radio-frequencypulse (section-selective frequency-offset–corrected inversion in this study) is appliedto (ideally) selectively invert only the spinsin the imaging volume. As shown in Figure1, this inversion radio frequency is fol-

lowed by a delay period (inversion time)during which uninverted fresh blood flowsinto the imaging section, and some ex-change with the tissue protons occurs. Thisexchange increases the bulk apparent re-laxation rate in the voxel, which results inan image with signal intensity that is flowweighted. In the second experiment (inter-leaved with the first), image acquisition isperformed with identical timings but withthe inversion radio frequency renderednonselective by switching the frequency-offset–corrected inversion gradient off. Inthis manner, all incoming blood spins intothe section are inverted so that any subse-quent exchange with tissue spins does notalter the relaxation rate; hence, a non–flow-weighted image is acquired. The PWimages are obtained by subtracting thenon–flow-weighted image from the flow-weighted image.

Clinical Studies

Clinical studies were performed with1.5- and 3.0-T MR imaging systems (Lx;GE Medical Systems, Milwaukee, Wis)that included identical hardware andsoftware platforms. A quadrature headcoil was used for radio-frequency trans-mission and reception. The studies wereperformed as part of a protocol approvedby the Intramural Review Board at theNational Institutes of Health. Informed

consent was obtained from all volunteersbefore they participated in the study.

Multisection brain perfusion images(10 transverse 5-mm-thick sections) wereacquired in six healthy volunteers (threemen and three women; age range, 23–30years; mean age, 26 years). In each sub-ject, 1.5- or 3.0-T MR imaging was per-formed on a different day. The age rangewas limited to between 23 and 30 years tominimize age dependency on the perfu-sion signal. Since the total imaging vol-ume was 50 mm thick (ie, 10 sections � 5mm), an inversion width of the same sizeshould (ideally) be used in the selectiveinversion radio-frequency pulse to mini-mize the transit time of blood to the im-aging volume. Unfortunately, use of aninversion width equal to the width of theimaging slab causes interaction betweenthe pulses, which results in contamina-tion from static tissue. On the basis offindings in previous calibration experi-ments (16), we used a selective inversionwidth of 80 mm. Bipolar crusher gra-dients with an amplitude of 15 mT/m(2-msec duration and separation) wereapplied (Fig 1) to selectively eliminatecontributions from large vessels (10,13).In some experiments, the crusher-gradi-ent pulses were switched off to allow theacquisition of PW images with vascularsignal intensity.

Figure 1. Schematic depicts the flow-alternating inversion-recovery perfusion sequence: a fre-quency-offset–corrected inversion pulse (FOCI) is followed by a delay (TI) to allow transit of bloodspins to the imaging plane. Signal readout is accomplished with a fast spiral scheme. Gradients(Gx, Gy, Gz) within the inversion time are used to spoil unwanted transverse magnetizationcreated by the inversion pulse. The frequency-offset–corrected inversion gradient waveform wasswitched on for selective inversion and off for nonselective (control) flow-alternating inversion-recovery acquisitions. The bipolar gradient lobes (diagonal lines) within the echo time (TE)denote the gradients are applied to minimize signal contributions from the vascular bed. RF �radio frequency, �1 � recovery delay.

570 � Radiology � February 2002 Yongbi et al

For absolute regional cerebral bloodflow (CBF) quantification, PW imageswere acquired with a series of 11 inver-sion time values that ranged from 0.5 to3.3 seconds. These data permitted the es-timation of transit times, on the basis ofsubsequent signal versus inversion timecurves for PW images, which were thenused to quantify CBF, as previously de-scribed (13). Other imaging parametersincluded recovery delay, 2.5 seconds;echo time, 17 msec; field of view, 240mm; 40 flow-weighted and non–flow-weighted raw images to calculate thePW images. To estimate the perfusionSNR, 120 flow-weighted and non–flow-weighted raw images were used to cal-culate the PW images. Inversion time forthe latter measurement was set to 1.8 sec-onds with crusher gradients and 1.3 sec-onds without crusher gradients, becausethese values are known to provide opti-mal perfusion sensitivity under the twogiven crusher-gradient conditions (13,17).The spiral readout involved use of trapezoi-dal gradients to achieve minimal T2*weighting and to minimize the effects ofblood oxygenation level dependence. Theduration of this pulse was 22 msec, whichis substantially shorter than that imple-mented in most current echo-planar se-quences.

Given the widespread use of ASL tech-niques for functional MR imaging inves-

tigations, we also conducted experimentsto compare the perfusion-related activa-tion signals at both field strengths by us-ing a simple motor task. Forty perfusionimages were collected in 4 minutes (ie, 6seconds for each flow-alternating inver-sion-recovery image) during which sub-jects (n � 4) alternated between rest (offstate) and finger tapping (on state). Therewere four off and four on states, whicheach lasted 30 seconds. The finger-tap-ping frequency was paced at 1 Hz by us-ing a computer-controlled digit that wasregularly projected on a visible screen toindicate when the subject should fingertap with the thumb of the subject’s dom-inant hand (right in all cases). The otherparameters (recovery delay, 1,575 msec;echo time, 7 msec; inversion time, 1,200msec; field of view, 24 cm; matrix, 64 �64; five sections acquired) combined toyield a PW image every 6 seconds. Ineach examination, the preceding experi-ment was repeated five times in the samesubject to test for reproducibility.

Data Analyses

Both baseline perfusion and functionalMR imaging data were analyzed offlinewith a workstation (SunSparc; Sun Micro-systems, Palo Alto, Calif) by using func-tional MR imaging software (MEDX; Sen-sor Systems, Sterling, Va) and programs

written in-house (Interactive Data Lan-guage [IDL]; Research Systems, Boulder,Colo). To obtain perfusion data, the non-selective raw images were subtractedfrom the corresponding selective flow-weighted images. The resultant PW im-ages were averaged to provide a PW im-age with a relatively high SNR for eachinversion time. For SNR estimates, re-gions of interest (ROIs) (12–15 pixels)were manually prescribed in maps of seg-mented gray matter and white matter byusing the functional MR imaging displaysoftware. Segmentation of the gray mat-ter and white matter was based on pre-calculated T1 maps by using nonselectiveinversion-recovery images for each sub-ject. The T1-based ROIs for segmentationof gray matter and white matter werethen transferred to the same spatial loca-tions on the flow-alternating inversion-recovery PW images, and their mean sig-nal intensity was calculated.

Noise was computed as the SD in a12–15-pixel ROI that was free of signalintensity and placed outside the PW im-ages in the upper left corner of the image.Images for which the measured noise wasmore than 5% of the noise estimated ona reference (signal intensity–free) imageacquired with parameters identical tothose used to acquire the PW images, butwith the radio-frequency pulses switchedoff, were excluded. In this study, the

Figure 2. Top: Multisection PW images acquired in human brain without vascular signal suppression at 1.5 and 3.0 T are displayed with the samewindow and level settings. Localized areas of high signal intensity (artifacts) represent intravascular signal contributions (eg, from the sinus andother peripheral vessels). Bottom: Corresponding PW images in the same subject were acquired with vascular signal suppression. The bulk of thevascular artifacts have been eliminated by using the crusher gradients.

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number of excluded PW images rangedbetween zero and three. The mean per-fusion SNR with and without vascularsuppression was measured for both graymatter and white matter at both fieldstrengths. A t test analysis was then per-formed to assess for significant differ-ences between the mean gray matter per-fusion SNR and the mean white matterperfusion SNR across the two magneticfields. These analyses were conductedseparately within each technique (ie,with and without vascular suppression).

To further compare the perfusion per-formance at both field strengths and toestimate absolute CBF, the normalizedperfusion-related signal change was inte-grated in gray matter and white matterROIs. The ROI approach provided ade-quate SNR to allow multiparameter fit ofthe wash-in and washout curves fromwhich transit times were estimated byusing previously described methods (13).All ROI prescriptions in this study wereperformed by one neuroradiologist (F.F.).

Automated image registration per-formed with the software sought to min-imize the sampling coefficient of varia-tion of ratios of voxel values between thefirst raw image and each successive rawimage. Perfusion activation maps wereobtained with a pixel-wise cross correla-tion of perfusion signal-to-time courseswith a boxcar reference function that re-flected the activation paradigm. For thisstudy, only pixels with a significant acti-vation (P � .05) were included.

Results

Figure 2 shows representative PW im-ages acquired at both field strengthswithout (top) or with (bottom) the appli-cation of vascular signal suppressioncrusher-gradient pulses. Images obtainedwith both field strengths manifestedclean subtraction and excellent gray mat-ter and white matter contrast, which are

characteristic of spin-tagging perfusionimages (10–13). However, improved per-formance at 3.0 T in terms of better SNRwas observed, and a quantitative analysiswas performed. Typical signal intensitycomponents from large vessels (vascularartifacts), which were observed as brightspots on the perfusion images obtainedwith crusher gradients, were effectively

Summary of Transit Times and Absolute CBF Measured in Gray Matterand White Matter at 1.5 and 3.0 T

Finding

Subject No.

1 2 3 4 5 6

Transit time (msec)1.5 T

Gray matter* 0.66 0.70 0.72 0.70 0.72 0.60White matter† 0.88 0.79 0.70 0.83 0.81 0.68

3.0 TGray matter‡ 0.54 0.62 0.53 0.77 0.65 0.78White matter§ 0.70 0.82 0.72 0.85 0.70 0.78

CBF (mL/100 g/min)Gray matter

1.5 T� 71.2 60.0 62.2 62.0 57.0 72.63.0 T# 73.0 66.3 59.2 70.8 58.2 63.0

White matter1.5 T** 22.7 20.9 26.6 31.3 22.9 20.83.0 T†† 27.6 22.0 19.7 25.2 26.8 36.6

* Mean � SD, 0.68 � 0.05.† 0.78 � 0.08.‡ 0.65 � 0.11.§ 0.76 � 0.06.� 64.1 � 6.3.# 65.1 � 6.1.** 24.2 � 4.1.†† 26.3 � 5.0.

Figure 3. Bar graphs depict perfusion SNR at 1.5 and 3.0 T. Left: Plot for data acquired without vascular signal suppression. Right: Plot for dataacquired with vascular signal suppression. The higher perfusion SNR on the left is primarily due to contributions from the unsuppressed flowingspins within the vascular bed. Higher perfusion SNR at 3.0 T is attributed mainly to the increase in T1 of arterial blood, which in turn minimizesthe amount of decay of the tag in transit to the imaging volume. Error bars indicate the SD.

572 � Radiology � February 2002 Yongbi et al

eliminated at both field strengths by us-ing a crusher-gradient intensity of ap-proximately 7 sec/mm2.

The mean gray matter perfusion SNR(23.7 � 9.3 [SD]) acquired with vascularsuppression at 3.0 T was found to be sig-nificantly higher (P � .01) than the cor-responding gray matter perfusion SNR(8.3 � 2.2) obtained at 1.5 T. For thewhite matter images acquired with vas-cular suppression, the mean perfusionSNR (6.8 � 2.3) was also significantlyhigher (P � .01) than the correspondingmean value (2.7 � 1.5) at 1.5 T. Theseresults are depicted in Figure 3. The meanperfusion SNR obtained without vascularsuppression also manifested a similartrend in both the mean gray matter(30.0 � 6.1) and white matter (8.4 � 2.2)

at 3.0 T, which indicates significantlyhigher (P � .01) values than the corre-sponding mean gray matter (17.1 � 5.2)and mean white matter (2.7 � 1.3) valuesat 1.5 T. These results are also shown inFigure 3.

The average perfusion signals in graymatter or white matter were measuredand normalized to the equilibrium mag-netization. For the central section, thechange (qualitative and quantitative) insignal with vascular signal suppressionmanifested the typical wash-in and wash-out characteristic curve (13). At both fieldstrengths, the perfusion signal progres-sively increased as inversion time in-creased and reached a broad plateau cen-tered at an inversion time of around 1.8seconds. The perfusion signal then de-

clined as a function of inversion timeowing to the competing mechanisms ofthe inflow of fresh spins and T1 effectswith increasing inversion time (17).However, the perfusion signal was con-sistently higher at 3.0 T throughout theentire inversion time range.

For comparison with results in othergroups, the perfusion signal was normal-ized by dividing it by the equilibriummagnetization (ie, the normalized perfu-sion-related signal change), since this ra-tio is independent of the total number ofimages. Gray matter normalized perfu-sion-related signal change was a mean0.708% � 0.109 at 3.0 T and 0.598% �0.079 at 1.5 T. The corresponding whitematter normalized perfusion-related signalchange at 3.0 T was 0.184% � 0.063 ofequilibrium magnetization and 0.120% �0.044 at 1.5 T.

Mean CBF values for the gray matterand white matter ROIs at 3.0 T in thehealthy volunteers were 65.1 mL/100g/min � 6.1 and 26.3 mL/100 g/min �5.9, respectively, with a mean gray mat-ter to white matter CBF ratio of 2.5. Sim-ilar results were obtained at 1.5 T. Theyare summarized in the Table because theyhave already been reported (10,13).

The activation maps obtained from onesubject at both field strengths (overlaidon high-spatial-resolution T1-weightedimages) are shown in Figure 4a. Despitesimilarities in the location of motor acti-vation, the 3.0-T images showed a sub-stantially greater extent of activation.The mean number of activated voxels fordata from four subjects were 128 vox-els � 34 at 3.0 T (z score, 6.79 � 0.45)compared with 70 voxels � 12 at 1.5 T(z score, 5.52 voxels � 0.32).

The increased number of activatedvoxels at 3.0 T can be primarily ascribedto the better perfusion SNR at 3.0 T and,to a lesser extent, to reduced transit timelosses owing to the longer T1 of blood.More important, perfusion-related acti-vation was consistently observed in theipsilateral primary and contralateral sup-plementary motor area on 3.0-T images.On the contrary, no activation of theseregions appeared on the 1.5-T images forthe same statistical threshold used in theanalysis. An overlay plot of the perfu-sion-related time courses obtained atboth field strengths in identical ROIs inthe primary motor cortex is shown inFigure 4b. The signal plots represent themean of five measurements in the samesubject, and they show a perfusion in-crease during activation of around 70%in both cases. The similarity in activa-tion-induced blood flow change is ex-

Figure 4. (a) Perfusion activation maps at 1.5 and 3.0 T obtained in a motor activation studyduring finger tapping overlie T1-weighted images. Activated pixels in the supplementary andprimary motor regions are depicted in red. (b) Graph depicts mean perfusion signal change(n � 4) from the primary motor area in the images in a at both field strengths. Activation insupplementary and contralateral motor areas was observed at only 3.0 T mainly because of higherperfusion SNR.

Volume 222 � Number 2 Pulsed Arterial Spin Labeling: MR Perfusion Signal at 1.5 and 3.0 T � 573

pected since this increase is independentof field strength.

Discussion

To date, the bulk of spin-labeling stud-ies designed to measure tissue perfusionhave been performed at 1.5 T (8–13).Higher magnetic field strengths providehigher SNR. Even though T2 losses andsusceptibility effects increase with fieldstrength, findings in this study clearlyshow that high-quality pulsed arterialspin-tagging perfusion images in humanbrain are readily obtained at 3.0 T withexcellent suppression of static back-ground tissue signal. Absolute CBF andmeasured values for change in signal di-vided by the signal were in excellentagreement with previously published ex-perimental data (for 1.5 T) (10,13) andtheoretically predicted values for bothfield strengths (17).

Compared with findings on perfusionimages acquired at 1.5 T, the improve-ment in perfusion signal intensity af-forded by the increased field strength isreadily apparent from our results. Themean increase in perfusion SNR in graymatter at 3.0 T was substantially greaterthan that observed for white matter. Thisdiscrepancy may be due to the differ-ences in transit time between gray matterand white matter, which was shown in arecent study (19–21) to be approximately0.5 second longer in white matter than ingray matter. Given the exponential lossof perfusion signal with transit time(10,13), the perfusion signal increase inwhite matter that results from the in-creased field strength is likely to be se-verely offset by transit time losses. In ourimplementation, we attempted to mini-mize this problem by using the highlyselective frequency-offset–corrected in-version pulse (15,16) in place of the con-ventional hyperbolic secant pulse (14).

SNR values reported for both fieldstrengths depend primarily on the mea-surement parameters used and thehardware resources that are available.For example, the perfusion SNR will bedifferent for different imaging sectionthickness, as will other measurementparameters, such as echo time, repeti-tion time, or both. Furthermore, the useof multiple receive (phased-array) coilswill likely enhance the perfusion signal.Because T2* losses are lower at 1.5 T, itmay be possible to use a longer spiralreadout, which would result in the useof a smaller receiver bandwidth andconsequently reduce receiver fold-back

noise into the MR images. While thisapproach may be suitable for single-sec-tion ASL studies, it is not appropriatefor multisection investigations becauseit can cause big intersection inversiontime differences and quantitative prob-lems. Additionally, the shorter readoutduration is desired especially at higherfield strengths (3.0 T in this study) tominimize the effects of both T2* andblood oxygenation level dependenceon the PW images.

In most functional MR imaging inves-tigations, the simultaneous measure-ment of blood oxygenation level depen-dence and perfusion-based activation canprovide complementary information.Therefore, a more complete investiga-tion would incorporate measurements ofblood oxygenation level dependence ob-tained with functional MR imaging.However, the measurements of blood ox-ygenation level dependence were pre-cluded primarily because they have beenperformed previously (22,23).

In performing the perfusion compari-son, we attempted to control for possiblevariations between the two imagers thatmay bias the results. For example, wecontrolled for experimental parameters,manufacturer hardware, and processingsoftware. Owing to the possibility of spa-tial mismatch in section positioning be-tween the two field strengths, normaliza-tion of the data to a standardized spacemay have been necessary. The latter re-quirement was minimized by using stan-dard in-house equipment at both fieldstrengths to secure the volunteer’s headin the same spatial position within thestandard head coil. Further consistencyin spatial localization was ensured by us-ing high-spatial-resolution T1-weightedimages as a guide. Additionally, the lattertask was performed in all subjects by thesame observer (F.F.).

Findings in our study demonstrate thathigh-quality ASL perfusion images can beacquired at 3.0 T with significant im-provement in SNR over measurementsperformed at 1.5 T. ASL approaches toMR imaging of CBF offer the potential forcompletely noninvasive quantitative im-aging of an important physiologic anddiagnostic quantity. For clinical perfu-sion studies, however, approaches thatinvolve use of dynamic contrast agentsstill remain the main choice primarilybecause of better SNR performance. Inad-equate SNR continues to be the maindrawback in the clinical application ofASL-based techniques. For example, de-spite the increased sensitivity observed inthe current 3.0-T study, absolute quanti-

fication of perfusion was possible onlywith measurements performed over rela-tively large ROIs from data acquired dur-ing 45 minutes. However, the situation iscompletely different for functional MRimaging applications for which the im-proved perfusion sensitivity at 3.0 Tclearly results in more precise mapping ofbrain function. Future studies are neededthat focus on further improvements inASL perfusion sensitivity, particularly inconjunction with the use of multiple re-ceiver coils.

References1. Detre JA, Alsop DC, Vives LR, Maccotta L,

Teener JW, Raps EC. Noninvasive MRIevaluation of cerebral blood flow in cere-brovascular disease. Neurology 1998; 50:633–641.

2. Frackowiak RSJ, Pozzilli C, Legg NJ, et al.Regional cerebral oxygen supply and uti-lization in dementia. Brain 1981; 104:753–778.

3. Waldemar G. Functional brain imagingwith SPECT in normal aging and demen-tia. Cerebrovasc Brain Metab Rev 1995;7:89–130.

4. Spencer SS. The relative contributions ofMRI, SPECT, and PET imaging in epilepsy.Epilepsia 1994; 35(suppl):S72–S89.

5. Belliveau JW, Rosen BR, Kantor HL, et al.Functional cerebral imaging by suscepti-bility contrast. Magn Reson Med 1990;14:538–546.

6. Ostergaard L, Sorenson AG, Kwong KK,Weisskoff RM, Glyldensted C, Rosen BR.High resolution measurement of cerebralblood flow using intravascular tracer bo-lus passages. II. Experimental comparisonand preliminary results. Magn Reson Med1996; 36:726–736.

7. Williams DS, Detre JA, Leigh JS, KoretskyAP. Magnetic resonance imaging of per-fusion using spin inversion of arterial wa-ter. Proc Natl Acad Sci U S A 1992; 89:212–216.

8. Kwong KK, Chesler DA, Weisskoff RM,et al. MR perfusion studies with T1-weighted echo planar imaging. Magn Re-son Med 1995; 34:878–887.

9. Kim SG. Quantification of relative cere-bral blood flow change by flow sensitivealternating inversion recovery (FAIR)technique: application to functional map-ping. Magn Reson Med 1995; 34:293–301.

10. Ye FQ, Pekar JJ, Jezzard P, Duyn JH, FrankJA, McLaughlin AC. Perfusion imaging ofthe human brain at 1.5 T using a single-shot EPI spin tagging approach. Magn Re-son Med 1996; 36:219–224.

11. Tanabe JL, Yongbi M, Branch CA, Hrabe J,Johnson G, Helpern JA. MR perfusion im-aging in human brain using the UNFAIRtechnique. J Magn Reson Imaging 1999;9:761–767.

12. Wong EC, Buxton RB, Frank LR. Quanti-tative imaging of perfusion using a singlesubtraction (QUIPSS and QUIPPS II).Magn Reson Med 1998; 39:702–708.

13. Yang Y, Frank JA, Hou L, Ye FQ, Mc-Laughlin AC, Duyn JH. Multislice imag-ing of quantitative cerebral perfusionwith pulsed arterial spin labeling. MagnReson Med 1998; 39:825–832.

574 � Radiology � February 2002 Yongbi et al

14. Silver MS, Joseph RI, Hoult DI. Selectivespin inversion in nuclear magnetic reso-nance and coherence optics through anexact solution of the Bloch-Riccati equa-tions. Phys Rev 1985; A31:2753–2755.

15. Ordidge RJ, Wylezinska M, Hugg JW, But-terworth E, Franconi F. Frequency offsetcorrected inversion (FOCI) pulses for usein localized spectroscopy. Magn ResonMed 1996; 36:562–566.

16. Yongbi MN, Yang Y, Frank JA, Duyn JH.Multislice perfusion imaging in humanbrain using the C-FOCI inversion pulse:comparison with hyperbolic secant.Magn Reson Med 1999; 42:1098–1105.

17. Kim SG, Tsekos NV, Ashe J. Multi-sliceperfusion based functional MRI using theFAIR technique: comparison of CBF andBOLD effects. NMR Biomed 1997; 10:191–196.

18. Kastrup A, Li TQ, Glover GH, Kruger G,

Mosley ME. Gender differences in cere-bral blood flow and oxygenation re-sponse during focal physiologic neuralactivity. J Cereb Blood Flow Metab 1999;19:1066–1071.

19. Calamante F, Williams SR, van BruggenN, Kwong KK, Turner R. A model forquantification of perfusion in pulsed la-beling techniques. NMR Biomed 1996;8:79–83.

20. Duvvuri U, Roberts DA, Bolinger L,Schnall MD. Quantitative perfusion im-aging at 4.0 T (abstr). In: Proceedings ofthe Sixth Meeting of the InternationalSociety for Magnetic Resonance in Medi-cine. Berkeley, Calif: International Soci-ety for Magnetic Resonance in Medicine,1998; 1201.

21. Ye FQ, Mattay VS, Frank JA, WeinbergerDR, McLaughlin AC. Comparison of whiteand gray matter aterial transit times in

spin-tagging experiments (abstr). In: Pro-ceedings of the Seventh Meeting of the In-ternational Society for Magnetic Resonancein Medicine. Berkeley, Calif: InternationalSociety for Magnetic Resonance in Medi-cine, 1999; 1847.

22. Bandettini PA, Wong EC, Jesmanowicz A,et al. MRI of human brain activation at0.5 T, 1.5 T, and 3.0 T: comparison of �R2and functional contrast to noise ratio (ab-str). In: Proceedings of the First Meetingof the Society of Magnetic Resonance.Berkeley, Calif: Society of Magnetic Reso-nance, 1994; 434.

23. Yang Y, Wen H, Mattay VS, Balaban RS,Frank JA, Duyn JH. Comparison of 3DBOLD functional MRI with spiral acquisi-tion at 1.5 T and 4.0 T. Neuroimage 1999;446–451.

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