Principles of magnetic resonance imagingimul.umlub.edu.pl/en/system/files/Principles of... · 15.03.2020 Principles of magnetic resonance imaging - UpToDate

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Principles of magnetic resonance imagingAuthors: Daniel Chernoff, MD, PhD, Paul Stark, MDSection Editor: Nestor L Muller, MD, PhDDeputy Editor: Lisa Kunins, MD

All topics are updated as new evidence becomes available and our peer review process is complete.

Literature review current through: Feb 2020. | This topic last updated: Jan 17, 2020.

INTRODUCTION

Magnetic resonance imaging (MRI) is an important tool in the diagnosis and evaluation ofdiseases [1]. In the early 1970s, Paul Lauterbur and Raymond Damadian applied nuclearmagnetic resonance (NMR) technology to the imaging of living organisms, generating imagesreferred to as zeugmatographs [2-5]. Subsequent refinements in image acquisition andprocessing, developed by Sir Peter Mansfield and others, allowed improved visualization ofanatomic detail and broader clinical application of MRI [1,6-8]. Lauterbur and Mansfield wereawarded the 2003 Nobel Prize in Medicine and Physiology for their contributions to medicalimaging.

This topic will review the principles of MRI. Clinical applications of MRI are discussed in individualtopic reviews.

MAGNETIC RESONANCE PHYSICS

The phenomenon of nuclear magnetic resonance (NMR) derives from spin angular momentum ofatomic nuclei in quantum mechanics, which has no direct equivalent in classical physics.Nevertheless, a classical mechanical description of NMR captures nearly all of the microscopicand macroscopic properties predicted from quantum mechanics and is much easier to grasp.

Atoms are characterized by mass, electrical charge, and a magnetic property called spin. Atomicnuclei that contain an odd number of protons or neutrons possess a magnetic moment, whichdescribes the strength and direction of a microscopic magnetic field surrounding the nucleus. Inthe presence of a strong, constant external magnetic field, such as that produced inside animaging magnet, a small excess fraction of polarized nuclei, on average, align themselves with themagnetic field, producing a macroscopic, measurable magnetic moment (figure 1) [9-11].

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In addition, the interaction between the magnetic moment of the nucleus and the external fieldcauses each spinning nucleus to precess (ie, change the orientation of the rotation axis of thespinning nucleus). Each nucleus precesses at a characteristic (resonant) frequency that isproportional to the strength of the external field. The resonant frequency can be calculated withthe Larmor equation:

Resonant frequency F = B x Larmor constant

where B is the magnetic field strength and the Larmor constant (also called gyromagnetic ratio orgamma) is the specific precession frequency of the nucleus at a specified magnetic field strength.For example, a hydrogen nucleus (a proton) precesses at a frequency of 42.57 MHz per Tesla (1Tesla is approximately 25,000 times the earth's magnetic field strength). At 1.5 Tesla, the Larmorfrequency of hydrogen is 63 MHz; at 3 Tesla, 126 MHz; and at 7 Tesla, 298 MHz [12]. Because ofthe high abundance of hydrogen ( H) nuclei in tissue compared with other atomic nuclei, theproton signal from hydrogen is used in virtually all clinical MRI. It is estimated that one volumeelement (voxel) contains 10 water molecules.

The fraction of protons aligned with the magnetic field can be perturbed by application ofradiofrequency (RF) energy at the Larmor (resonant) frequency. In essence, a transiently appliedRF pulse at the Larmor frequency induces a change in the vector direction of the net magneticmoment. The density of protons as well as the rate at which the magnetization returns toequilibrium (a phenomenon referred to as “relaxation”) can then be measured by the RF signal orecho emitted by the protons (referred to as "spins") as they relax. Two forms of relaxation can bemeasured:

Phase coherence refers to a collection of neighboring spins whose magnetization vectors point inthe same direction. Loss of phase coherence occurs when these vectors get out of alignment dueto precession of neighboring spins at slightly different velocities, reflecting small, randomvariations in the local magnetic field. Both longitudinal and transverse relaxation occur at singleexponential rates, with time constants T1 for longitudinal and T2 for transverse relaxation. T1 isthe time constant of the exponential describing the rate of realignment with the longitudinal axis ofthe main magnetic field. T2 is the time constant of the exponential describing the decay of thetransverse magnetization. These time constants, T1 and T2, in turn depend on the local chemicalmicroenvironment, which varies between tissues. Therefore, three independent properties oftissue (proton density, T1, and T2) can be determined by magnetic resonance imaging (MRI).Various methods are used to emphasize one property over another in a given MRI paradigm

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Relaxation produced by realignment of the net magnetization vector with the external field(spin-lattice or longitudinal relaxation) (figure 2A-B).

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Relaxation produced by loss of phase coherence of spins in a plane perpendicular to theexternal field (spin-spin or transverse relaxation) (figure 3A-B).

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(“pulse sequence”), allowing tremendous flexibility in determining contrast between differenttissues based on the particular pulse sequence utilized.

MRI TECHNOLOGY AND PULSE SEQUENCES

The appearance and signal intensity of tissues in a magnetic resonance image (MRI) dependheavily upon the type of imaging technique used.

Magnet and coil design — MRI requires a large, constant, spatially homogeneous magnetic field.Most clinical MRI systems use a superconducting magnet to generate the required magnetic field.The superconducting magnet is typically composed of windings of special metal alloys (niobium-titanium fibers in an aluminum or copper matrix) cooled by liquid helium, in which electrical currentflows without loss of energy. The circular flow of electrical current in the windings induces aconstant magnetic field oriented perpendicular to the windings. Additional complex windings (shimcoils) are incorporated to allow application of small correction fields, improving field homogeneity.Alternative magnet technologies in clinical use include permanent and resistive magnets, whichare of lower field strength than superconducting magnets.

The majority of clinical MRI systems are based on cylindrical bore magnets, in which the mainmagnetic field is oriented parallel to the bore and the patient placed inside the bore.

Imaging speed and resolution is defined by magnet strength, which is between 1 and 7 Tesla inmost clinical scanners [12]. MRI signal is linearly proportional to the applied static B magneticfield strength, while system noise is relatively independent of field strength [13]. Open or largebore scanners are available for large and claustrophobic patients and for image-guidedinterventional procedures. However, magnet strength for these systems are ≤1.5 Tesla, whichlimits image quality, although a lower field strength has the potential to decrease magneticsusceptibility artifacts when imaging the lungs [13,14]. While a higher field strength is generallydesirable, an increased field strength has several problems including increased cost andcomplexity of scanners, greater siting and safety issues, increased nonlinearity of radiofrequency(RF) fields, increased severity of various imaging artifacts, higher energy deposition in tissue(heating), and other issues that impact clinical practice [15].

The major alternative geometry for MRI systems is the dipole magnet, in which two separatemagnets of equal strength are held apart by strong beams, creating a gap in which the fieldstrength is uniform. This geometry is advantageous for large or claustrophobic patients, for someinterventional procedures, and for imaging off-center anatomy (eg, shoulder, elbow, or wrist), sincethere is much more room to position the region of interest at the center of the magnet where theuniformity is highest. The tradeoff is typically cost and upper limit of field strength, since thisgeometry is generally more expensive for a given field strength. Available dipole systems havemaximum field strength of 1.2 Tesla in a vertical field configuration. Some of the advantages of

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dipole magnets, in particular for larger or claustrophobic patients, have been addressed bymanufacture of cylindrical bore magnets with larger and/or shorter bores.

Four or more additional windings (or coils) are incorporated in an imaging magnet. Three gradientcoils are used to create small linear perturbations of the magnetic field in any spatial directions.Because the proton precession frequency is linearly related to magnetic field strength, thesemagnetic gradients can encode the spatial location of protons within the body by the frequency ofprecession. An RF coil is used to provide the bursts of RF energy required to perturb themagnetization of spins within the body. Most MRI systems have a "whole body" RF coilpermanently installed next to the gradient coils. This provides homogeneous RF excitation of alarge volume of tissue within the body. Although the same RF coil can be used to receive the echosignal(s) from the body, in most cases smaller coils, tailored to different body parts, are connectedvia electrical cable to the RF system and placed in close proximity to the body part being imaged.

Although a full discussion of the physics of coil design is beyond the scope of this chapter, thegeneral principles guiding coil design are:

The field of view prescribed in image acquisition is important. If the field of view is too small andleads to undersampling of data, aliasing and so-called wrap-around artifacts may ensue [16]. Toolarge a field of view decreases the spatial resolution achievable for a given length of imaging time.

The remainder of the MRI system consists of a patient transport system (the table); RF andmagnetic shielding; MR-compatible electrocardiographic and respiratory monitoring systems; RFand gradient coil power amplifiers; and a computer to set up the pulse sequences, performcollection of the echo data, and transform the raw data into images (figure 4).

Pulse sequences — Unlike computed tomography (CT), where tissue contrast depends almostentirely upon electron density, contrast in MRI is a complex function of proton density, T1relaxation, T2 relaxation, and local chemical environment. The relative contributions of theseparameters to the image vary with the manner in which the tissue is excited. The temporal patternand shape of RF and gradient coil waveforms used to obtain an image is referred to as a pulsesequence [17]. For thoracic imaging, the most commonly used pulse sequence is called "spin

Closer proximity to the body part being imaged increases signal.●

Unwanted RF noise is proportional to the volume of sensitivity of a coil.●

It is possible to create one large coil out of many small coils with the low RF noise of a smallcoil, provided that each coil element has its own independent RF channel. Modern MRIscanners typically use multielement coils coupled to multichannel RF systems (multi-arraycoils), improving the signal-to-noise ratio over single channel designs. Some coils are alsoable to provide the RF excitation pulses, which can be helpful in limiting the overall powerdeposition into (heating of) the body.

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echo" or "black blood imaging." For specific applications, such as imaging blood flow or rapidimaging (eg, cardiac imaging), other types of pulse sequences are employed, as described below.

Spin echo — A spin echo (SE) pulse sequence typically consists of a 90 degree RF pulse toexcite the tissue followed by a 180 degree refocusing pulse. The refocusing pulse reverses anydephasing effects that have occurred due to heterogeneities in the external magnetic field, formingan RF "echo" when the spins come back into phase. The elapsed time from the center of the 90degree pulse to the peak of the echo is called the echo time (TE). The elapsed time betweensuccessive 90 degree pulses is called the repetition time (TR). A spin echo sequence with a shortTR (eg, 300 to 1000 msec) and a short TE (less than 20 msec) emphasizes the T1 differencesbetween tissues (T1-weighting). A spin echo sequence with a long TR (3000 to 6000 msec) and along TE (greater than 80 msec) emphasizes T2 differences (T2-weighting). A long TR sequencewith a short TE minimizes the effects of T1 and T2 relaxation and thus reflects proton density,which in turn primarily reflects water content. In order to increase the image contrast, inversionrecovery pulses can be added.

Gradient echo — A gradient echo (GE) pulse sequence, also called “bright blood imaging,”typically consists of small-angle (20 to 60 degrees) RF pulses applied in rapid succession (TR lessthan 100 msec). GE imaging uses a reversal of the magnetic field gradients rather than a 180degree RF pulse to refocus spins. Image contrast in GE imaging is a complex function of TR, TE,and RF flip angle, but the imaging parameters can be adjusted to produce T1, T2, and protondensity weighting similar to spin echo sequences. GE sequences generally can suffer fromsusceptibility artifacts related to magnetic field inhomogeneity, and disturbances in resonantfrequency with subsequent loss of signal, when compared with spin echo sequences, but they aremore efficient, ie, more information is acquired per unit time. Therefore, GE sequences are oftenused for very fast imaging or 3D volumetric imaging, in which data from an entire volume areacquired with nearly isotropic voxels, rather than the typical 2D sequences, in which the slicethickness is much greater than the in-slice pixel dimensions. Examples of GE sequences includespoiled GE imaging and balanced steady state free precession. GE imaging in the thorax ispreferentially used in fast imaging, angiographic imaging, and cardiac imaging.

With fast, spoiled-gradient imaging or single-shot imaging, images are obtained much morerapidly than with an equivalent spin echo sequence. As a result, individual images can beacquired during a single breath hold, which improves image quality due to elimination ofrespiratory motion artifacts. The transverse magnetization returns to zero prior to each RFpulse. In addition, the temporal pattern of enhancement of tissues following intravenouscontrast material injection can be assessed by repetitive rapid imaging at one or morelocations, a procedure useful in the imaging of tumors.

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Magnetic resonance angiography (MRA) uses ultrafast acquisition of T1-weighted three-dimensional GE data sets, typically obtained during a breath hold [18]. In this setting,magnetization of stationary tissue within the imaged slice becomes partially "saturated,"

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Balanced steady-state free precession is preferentially used for cine MRI of the heart andcoronary arteries. In contrast with fast gradient imaging, the transverse magnetization reaches asteady state. Its signal is dependent on the relaxation time ratio between T2 and T1, and itprovides excellent contrast between the blood and the endocardium while being independent ofcontrast from blood flow and its artifacts. (See "Clinical utility of cardiovascular magneticresonance imaging", section on 'Techniques'.)

Other sequences — In an effort to achieve the most rapid imaging possible, various pulsesequences have been developed, all of which have in common the collection of a large amount ofdata in the form of multiple echoes following each RF excitation.

Fast spin echo is a common modification of the spin echo sequence in which multiple 180 degreerefocusing pulses follow a single 90 degree excitation. Fast spin echo can reduce the timerequired to produce a set of images by a factor equal to the number of refocusing pulses (the echotrain length) applied. Fast spin echo is often used in the thorax to allow spin echo imaging withbreath holding. In the most extreme form of rapid imaging, called echo planar imaging, all of thedata needed to reconstruct a single two-dimensional slice are acquired after a single RF pulse, ina time as short as 30 to 40 msec [24]. Echo-planar technique can be used for perfusion imaging oftissues after injection of gadolinium-based contrast agents (eg, in myocardial perfusion studies).(See "Clinical utility of cardiovascular magnetic resonance imaging", section on 'Pharmacologicstress CMR'.)

Chemical shift imaging is a powerful tool for investigating the cellular composition of tissue. Ittakes advantage of the slight differences in resonant frequency between water protons and fatprotons by using echo times when the fat and water signals are in phase or out of phase. It allowsfor determination of the relative amount of fat and water within an individual voxel. Loss of signalintensity between in-phase and opposed-phase MRI implies intravoxel fat or lipid. an example ofthe utility of chemical shift imaging is in establishing the presence of intracellular lipid in adrenalmasses, which is characteristic of a benign adenoma [25].

MAGNETIC RESONANCE CONTRAST AGENTS

Paramagnetic contrast agents, usually administered intravenously, have a role in magneticresonance imaging (MRI) similar to the use of iodinated contrast agents in computed tomography

leading to decreased signal. Blood flowing into the imaged slice, however, will not haveexperienced repetitive excitations and therefore will generate higher signal intensity. Thepulse sequence design for contrast-enhanced MRA is based on a three-dimensional Fouriertransform GE sequence, using rapid RF pulsing [18-20]. The excellent contrast between thebright blood and darker stationary tissue makes GE a useful angiographic sequence forimaging of arterial and venous structures [21-23].

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(CT). However, most MR contrast agents in clinical use are chelates of gadolinium. Afterintravenous injection, these agents initially partition into the vascular space before diffusingextravascularly. Gadolinium, a paramagnetic element with seven unpaired electrons, influencesthe surrounding water molecules, resulting in an increased relaxivity with shortening of both T1and T2; it is primarily the T1-shortening effect that is clinically relevant at typical tissueconcentrations. On T1-weighted images, relatively greater delivery of contrast material to a regionin certain conditions (eg, due to increased vascularity), or trapping of gadolinium in myocardialinfarcts, scars, inflammation, or infiltration, produces an increase in T1-weighted signal intensityover baseline.

MR contrast agent safety is discussed below. (See 'Gadolinium contrast agent' below.)

MOTION COMPENSATION TECHNIQUES

The relatively long time required to collect all the data necessary to reconstruct an image rendersmagnetic resonance imaging (MRI) susceptible to motion artifacts, so-called ghosting artifacts.The types of motion encountered include cardiac, respiratory, flowing blood, swallowing,peristalsis, and gross patient movements. Motion artifacts tend to be most noticeable along theaxis in which position is encoded by the phase of the received echo signal. Judicious choice of thephase-encoding axis is helpful in minimizing motion artifact in an area of clinical interest. Mostclinical MRI systems now have rapid imaging options, which typically can acquire all data in abreath hold, albeit with somewhat reduced spatial resolution and/or signal-to-noise ratio. This canobviate the need for motion compensation and has become common for MRI of the chest,abdomen, and pelvis.

More sophisticated techniques are also available to compensate for predictable, repetitive forms ofmotion such as cardiac motion, respiratory motion, and blood flow.

Cardiac gating — Artifacts created by cardiac motion can be minimized if pulses aresynchronized with the cardiac cycle. This requires an electrocardiographic- or pulse-monitoringsystem. To achieve T1-weighting, repetition time (TR) is usually set to one R-R interval. For T2-weighting (long TR), TR is chosen to be a multiple of an R-R interval. Because of the dominanteffects of cardiac motion in the chest, virtually all thoracic imaging is performed with eitherprospective electrocardiogram (ECG) triggering or retrospective ECG cardiac gating [26].

Respiratory compensation — Normally, the phase-encoding gradient is incrementedsequentially on successive radiofrequency (RF) excitations until sufficient data are collected toperform Fourier reconstruction. Respiratory compensation consists of monitoring the respiratorycycle with a flexible belt placed around the patient and reordering the phase-encoding gradientssuch that adjacent phase encodes occur during similar degrees of inspiration. In thereconstruction process, the data are reordered to make it appear that only one very long




respiratory cycle was sampled. This greatly reduces respiratory motion artifact without changingimaging time. This type of motion compensation is therefore used routinely in thoracic imaging.

Periodic respiratory motion can be synchronized by adding a navigator pulse to the pulsesequence. This approach entails the application of a small, one-dimensional spatial-encodinggradient in a plane perpendicular to the diaphragm. It ensures that only imaging data acquired inexpiration with the diaphragm in its most cranial position are utilized for the reconstruction of theimage.

Flow compensation (gradient moment nulling) — Motion occurring between the application ofRF excitation and reception of the echo causes changes in the phase of the received signal fromthe moving tissue, thereby causing misregistration of position along the phase-encoding axis. Thisartifact can be minimized by altering the gradient waveforms in the slice-selective and frequency-encoding axes to produce no net phase-shift for constant velocity motion, a technique called flowcompensation (referring to blood flow) or gradient moment nulling (GMN).

GMN produces an increase in signal within blood vessels and is therefore used routinely inangiographic (gradient echo [GE]) sequences. The GMN gradient waveforms increase theminimum achievable echo time (TE), which reduces signal-to-noise ratio in short TE (T1- andproton density-weighted images). It is generally less effective than respiratory compensation inreducing breathing motion artifact, and its use in spin echo imaging is therefore limited.

SPATIAL AND CHEMICAL PRESATURATION

Spatial presaturation refers to spatially selective prepulses applied immediately before the imagingpulse sequence to reduce signal arising from undesired spatial locations. Its main use is inreducing artifact from flowing blood on spin-echo images, but the concept is also used forsuppression of respiratory motion artifact. Chemical presaturation uses a spectrally selectiveprepulse to reduce or eliminate signal from fat or water, taking advantage of the fact that protonsbound to fat molecules resonate at a slightly different frequency than water protons, aphenomenon known as chemical shift.

Suppression of signal from flowing blood — In conventional multislice spin echo imaging, theregion outside the imaged volume does not absorb radiofrequency (RF) energy, and spins fromthis region are therefore fully relaxed (maximum magnetic moment). If these spins flow into theimaged volume during data acquisition, ie, arterial or venous inflow, they will generate relativelyhigh signal intensity and will also produce phase-shift artifacts. (See 'Flow compensation (gradientmoment nulling)' above.)

This undesired blood flow artifact can be suppressed by presaturating the nonimaged volume witha 90 degree RF excitation and randomizing the phase of these spins with a large magnetic field



gradient, at a small cost in increased imaging time. Spatial presaturation is routinely used in spinecho thoracic imaging sequences to reduce blood flow artifact.

Suppression of respiratory motion artifact — Suppression of respiratory motion artifact can beachieved by presaturating the anterior chest wall to reduce its emitted signal. This type ofsaturation within the imaged slice is used mainly in conjunction with surface phased-array coils.These receiver coils greatly increase the signal-to-noise ratio obtained per unit of imaging time;however, they suffer from nonuniform spatial sensitivity. Thus, signal from protons close to the coil,ie, the chest wall, is received with greater efficiency than signal from protons deep within thethorax. Spatial presaturation can deemphasize the signal from the chest wall and therefore reducerespiratory artifact arising from the chest wall as well. The main limitation of spatial presaturationfor this application is that only a linear band of tissue can be suppressed with a single pulse. Tosuppress signal from the entire chest cage, multiple presaturation pulses must be applied, at agreater cost in imaging time.

Navigator gating of the diaphragmatic movements can effectively freeze respiratory motion whilethe patient is freely breathing and is a newer technique for suppression of respiratory motionartifacts.

Single-shot sequences acquire each section during a single breath hold, with data acquired duringeach heartbeat or every other heartbeat in cardiac imaging.

Suppression of signal from fat — As noted above, chemical shift fat suppression is commonlyused to reduce the high signal intensity of fat-containing tissues. This comes at a modest cost inimaging time. Chemical fat suppression (CHEM SAT) is effective but relies on a high degree ofmagnetic field homogeneity within the imaged volume: Since the difference in resonant frequencyof water and fat protons is only 3.5 parts per million (at 1.5 Tesla), any variation in the magneticfield of this order or larger will cause uneven fat suppression. The problem is exacerbated onlower field strength imaging systems since the fat-selective prepulses have a finite bandwidth thatis more likely to overlap the water resonance frequency due to the very narrow difference inresonant frequencies.

A common alternative method of fat suppression is known as short tau inversion recovery (STIR),which exploits the fact that the relaxation time of fat protons is much shorter than water protons.By exciting all tissue within a volume to invert the magnetization (180 degree inversion orpresaturation pulse), followed by a delay chosen when fat protons have no net magnetic moment,followed by a 90 degree pulse and then readout pulses, signal from fat can be suppressed withoutrequiring high field homogeneity. The main disadvantage of this method over CHEM SAT is that itis slower.

Use of fat suppression within the thorax enables tissue characterization when a fat-containinglesion is suspected (either method effective), and facilitates differentiation of fat from contrast


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enhancement after intravenous contrast administration, where enhancing tissue andnonsuppressed fat may approach equal signal intensity (only CHEM SAT effective).

PRACTICAL ASPECTS

During the magnetic resonance imaging (MRI) examination, the patient typically lies supine on asliding table, which is advanced into the bore of the magnet for imaging. Since noise levels can behigh during imaging due to vibration of the gradient coils, earplugs or specially designed in-earaudio systems are commonly used. An imaging sequence may be as short as one second or aslong as 10 minutes, and the entire exam typically lasts 20 to 60 minutes as multiple imagingsequences are performed in multiple planes. More limited and rapid MRI examinations aresometimes used in the setting of trauma or with a patient unable to tolerate a more completeexamination for whatever reason.

PRECAUTIONS

Conventional clinical magnetic resonance imaging (MRI) is rarely associated with any adverseeffects. Most contraindications are relative precautions, which can be divided into five groups:implanted devices and foreign bodies, unstable patients, pregnancy, gadolinium contrast agents,and other.

Implanted devices and foreign bodies — Prior to scanning, a thorough safety screen of eachpatient for any implanted, embedded, or attached devices or foreign objects is mandatory. Theseconsiderations and associated screening procedures are discussed elsewhere. (See "Patientevaluation for metallic or electrical implants, devices, or foreign bodies before magnetic resonanceimaging".)

Unstable patient — Because of difficult access to a patient within the magnet and because mostresuscitation equipment cannot be brought into the scanning room, an unstable patient probablyshould only undergo MRI for an urgent clinical indication without another acceptable imagingalternative.

Pregnancy — The risks posed to the developing fetus by MRI are unknown. These risks, if any,are probably low and use of MRI may help avoid ionizing radiation associated with other imagingmodalities. Gadolinium-based contrast agents cross the placenta and are not recommended foruse in pregnant patients unless the potential benefit justifies the potential risk to the fetus. MRI isindicated for use in pregnant women if the potential benefit outweighs the potential risk. (See"Diagnostic imaging in pregnant and nursing women".)

Gadolinium contrast agent — The major route of MRI contrast excretion is renal. However,unlike iodinated contrast media, MRI contrast agents rarely cause anaphylactoid reactions. (See

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"Diagnosis and treatment of an acute reaction to a radiologic contrast agent", section on'Gadolinium-based contrast'.)

Patient safety considerations in giving gadolinium-based contrast agents for MRI are discussedelsewhere. (See "Patient evaluation before gadolinium contrast administration for magneticresonance imaging" and "Diagnosis and treatment of an acute reaction to a radiologic contrastagent", section on 'Patients with past severe allergic-like reactions'.)

Other — Several other problems may occur:

SUMMARY

Some patients may experience claustrophobia when asked to lie within an MRI scanner.Although estimates vary and the prevalence is likely a function of culture, in the largest studyincluding over 55,000 patients, the rate of MRI-induced claustrophobia necessitatingintervention was 2.1 percent [27].

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For some patients, claustrophobia may be severe enough to require sedation. Variousremedies, including MR-compatible music and video systems to “distract” the claustrophobicpatient, and acoustic insulation to reduce the loud noise from the energizing/de-energizing ofradiofrequency (RF) and gradient coils, are commercially available. In addition, a largenumber of so-called "open" MRI systems have been installed to cater to patients whoexperience claustrophobia or are too large for a cylindrical bore magnet. Furthermore, smallcylindrical bore MRI systems are available for imaging of the upper or lower extremities(excepting the shoulders and hips), which allow the rest of the body to remain outside of themagnet.

An agitated patient or one who cannot hold still within the magnet is a poor candidate for MRIunless sufficiently sedated; motion artifacts resulting from gross patient movement aretypically severe enough to interfere with accurate diagnostic imaging.

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Some obese or large patients may not be able to fit into the bore of a typical cylindricalmagnet. Such patients can usually be accommodated by an open MRI system or by some ofthe wide-bore MRI scanners.

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Magnetic resonance imaging (MRI) is an imaging technology that uses nonionizingradiofrequency (RF) radiation inside a strong magnetic field to detect the location and localchemical environment of protons. The imaging process can be summarized as induction ofprotons by a magnetic field, followed by excitation with RF pulses, subsequent relaxation, andeventual readout with RF receiver coils. (See 'Magnetic resonance physics' above and 'MRItechnology and pulse sequences' above and "Patient evaluation before gadolinium contrastadministration for magnetic resonance imaging".)

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REFERENCES

1. Andrew ER. Nuclear magnetic resonance and the brain. Brain Topogr 1992; 5:129.

2. Lauterbur PC. Progress in n.m.r. zeugmatography imaging. Philos Trans R Soc Lond B BiolSci 1980; 289:483.

3. Damadian R. Field focusing n.m.r. (FONAR) and the formation of chemical images in man.Philos Trans R Soc Lond B Biol Sci 1980; 289:489.

4. Lai CM, Lauterbur PC. True three-dimensional image reconstruction by nuclear magneticresonance zeugmatography. Phys Med Biol 1981; 26:851.

5. Seynaeve PC, Broos JI. [The history of tomography]. J Belge Radiol 1995; 78:284.

6. Andrew ER. The Wellcome Foundation lecture, 1981. Nuclear magnetic resonance imagingin medicine: physical principles. Proc R Soc Lond B Biol Sci 1985; 225:399.

7. Garroway AN. Solid state NMR, MRI and Sir Peter Mansfield: (1) from broad lines to narrowand back again; and (2) a highly tenuous link to landmine detection. MAGMA 1999; 9:103.

Unlike computed tomography (CT), which detects only differences in electron density, thetissue contrast that can be achieved with MRI is extremely flexible and orders of magnitudegreater. (See 'Magnetic resonance physics' above and "Patient evaluation before gadoliniumcontrast administration for magnetic resonance imaging".)

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Limitations of the use of MRI include hazards posed by certain indwelling metallic devices (forwhich safety references are available) and claustrophobia. So-called open MRI scanners andshort-bore conventional scanners can accommodate most claustrophobic patients, some ofwhom may still require sedation to complete the imaging study. (See 'Precautions' above.)

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As with CT, intravenous injection of MRI contrast agents is often used to improve contrastbetween pathologic and normal tissues or to perform angiography. (See 'Magnetic resonancecontrast agents' above.)

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Conventional clinical MRI is rarely associated with any adverse effects. Most contraindicationsare relative precautions, which can be divided into five groups: implanted devices and foreignbodies, unstable patients, pregnancy, gadolinium contrast agents, and other. (See'Precautions' above.)

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8. Mustarelli P, Rudnicki M, Savini A, et al. Synthesis of magnetic gradients for NMRtomography. Magn Reson Imaging 1990; 8:101.

9. Morelli JN, Runge VM, Ai F, et al. An image-based approach to understanding the physics ofMR artifacts. Radiographics 2011; 31:849.

10. Pooley RA. AAPM/RSNA physics tutorial for residents: fundamental physics of MR imaging.Radiographics 2005; 25:1087.

11. Elster AD: Questions and Answers in Magnetic Resonance Imaging www.mri-q.com (Accessed on July 15, 2015).

12. Hoff MN, McKinney A 4th, Shellock FG, et al. Safety Considerations of 7-T MRI in ClinicalPractice. Radiology 2019; 292:509.

13. Grist TM. The Next Chapter in MRI: Back to the Future? Radiology 2019; 293:394.

14. Hong C, Lee DH, Han BS. Characteristics of geometric distortion correction with increasingfield-of-view in open-configuration MRI. Magn Reson Imaging 2014; 32:786.

15. Ladd ME, Bachert P, Meyerspeer M, et al. Pros and cons of ultra-high-field MRI/MRS forhuman application. Prog Nucl Magn Reson Spectrosc 2018; 109:1.

16. Yanasak NE, Kelly MJ. MR imaging artifacts and parallel imaging techniques with calibrationscanning: a new twist on old problems. Radiographics 2014; 34:532.

17. Bitar R, Leung G, Perng R, et al. MR pulse sequences: what every radiologist wants to knowbut is afraid to ask. Radiographics 2006; 26:513.

18. Vogt FM, Goyen M, Debatin JF. MR angiography of the chest. Radiol Clin North Am 2003;41:29.

19. Finn JP, Baskaran V, Carr JC, et al. Thorax: low-dose contrast-enhanced three-dimensionalMR angiography with subsecond temporal resolution--initial results. Radiology 2002;224:896.

20. Riederer SJ, Bernstein MA, Breen JF, et al. Three-dimensional contrast-enhanced MRangiography with real-time fluoroscopic triggering: design specifications and technicalreliability in 330 patient studies. Radiology 2000; 215:584.

21. Holmqvist C, Larsson E-M, Ståhlberg F, Laurin S. Contrast-enhanced thoracic 3D-MRangiography in infants and children. Acta Radiol 2001; 42:50.

22. Okuda S, Kikinis R, Geva T, et al. 3D-shaded surface rendering of gadolinium-enhanced MRangiography in congenital heart disease. Pediatr Radiol 2000; 30:540.

















23. White CS. MR imaging of thoracic veins. Magn Reson Imaging Clin N Am 2000; 8:17.

24. Cohen MS, Weisskoff RM. Ultra-fast imaging. Magn Reson Imaging 1991; 9:1.

25. Shetty AS, Sipe AL, Zulfiqar M, et al. In-Phase and Opposed-Phase Imaging: Applications ofChemical Shift and Magnetic Susceptibility in the Chest and Abdomen. Radiographics 2019;39:115.

26. Finn JP, Nael K, Deshpande V, et al. Cardiac MR imaging: state of the technology. Radiology2006; 241:338.

27. Dewey M, Schink T, Dewey CF. Claustrophobia during magnetic resonance imaging: cohortstudy in over 55,000 patients. J Magn Reson Imaging 2007; 26:1322.

Topic 5309 Version 22.0








GRAPHICS

Creation of macroscopic magnetic moment in MRI

Excess of magnetic moments aligned parallel to the main magnetic field (B )creates a macroscopic magnetic moment (M ). The random phases of theindividual magnetic vectors cause cancellation of the transverse component ofmagnetization.

MRI: magnetic resonance imaging.

Graphic 66019 Version 2.0

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Longitudinal relaxation

Evolution of magnetization after a 180° radiofrequency excitation. Magnetizationis flipped 180° at time zero (t 0). Magnetization relaxes toward equilibrium asenergy is lost. Magnetization recovery is 63% complete at time t=T1 and 100%complete at time t>>T1.




Rate of recovery during longitudinal relaxation in MRI

Recovery of the longitudinal component of magnetization to its equilibrium value isexponential with a time constant T . T depends upon field strength, temperature,and tissue composition.



1 1



Transverse relaxation in MRI

Evolution of transverse magnetization following a 90° radiofrequency excitation.There is maximal alignment of magnetization vectors at time t=0 immediatelyafter excitation. Vectors have lost phase coherence at time t=T . At timet>>T , the magnetization vectors are randomly distributed and no macroscopicsignal can be measured.



2

2



Rate of recovery during transverse relaxation in MRI

Following 90° radiofrequency relaxation, transverse magnetization is initiallymaximal and then decays exponentially due to dipolar interactions among spins(spin-spin relaxation) with a time constant T2.





Schematic diagram of imaging magnet

A large hollow cylinder contains windings of superconducting wire bathed in cryogen(coolant). Additional shim windings provide small magnetic field corrections. 3 sets ofnonsuperconducting gradient coils provide spatial localization of spins. An RFtransmit/receive coil provides the RF excitation for each MR sequence and collects thesignals used to create an image. Control electronics are located outside the roomcontaining the magnet. The part of the patient being imaged is moved to the center ofthe magnet by the transport table.

RF: radiofrequency; MR: magnetic resonance.* Liquid helium/nitrogen.




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Daniel Chernoff, MD, PhD Nothing to disclose Paul Stark, MD Nothing to disclose Nestor L Muller, MD,PhD Nothing to disclose Lisa Kunins, MD Nothing to disclose

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