Download - From the RSNA Refresher Courses - radiology.queensu.ca · From the RSNA Refresher Courses 1009 Abbreviation: ECG = electrocardiography Index terms: Aorta, dissection, 56.74, 94.74
From the RSNARefresher Courses
Abbreviation: ECG = electrocardiography
Index terms: Aorta, dissection, 56.74, 94.74 • Heart, diseases, 50.14 • Heart, MR, 50.12141 • Heart, ventricles,52.12141 • Pericardium, 55.12141
RadioGraphics 1999; 19:1009–1025
1From the Department of Radiology, Beth Israel Medical Center, First Ave and 16th St, New York, NY 10003. Presentedas a refresher course at the 1998 RSNA scientific assembly. Received July 28, 1998; revision requested August 26 and re-ceived September 25; accepted September 25. Address reprint requests to the author.
Cardiac MR Imaging: A Guide for theBeginner1
Lawrence M. Boxt, MD
The complex motion of the heart during contraction is a serious chal-lenge to the diagnostic radiologist and to the capabilities of the mag-netic resonance (MR) imaging unit itself, but electrocardiographic (ECG)gating “stops” motion and thus allows acquisition of diagnostic-qualityimages. Attention to placement of chest-wall ECG electrodes and thecourse of ECG leads from the patient will maximize the quality of thegating signal and result in better-quality images. Before commencing theMR imaging examination, the clinical questions for the examinationmust be identified so that a clinical protocol can be applied to acquirerelevant morphologic and physiologic data. In addition to the standardorthogonal views (axial, coronal, and sagittal), oblique and complex sec-tions parallel and orthogonal to intrinsic cardiac axes may be necessaryto portray the relevant anatomy to best advantage. Construction ofthese views requires an understanding of basic normal and pathologiccardiac anatomy. If care is taken in the planning and execution of theMR imaging examination, the radiologist will be able to exploit this ex-citing technology to its full noninvasive potential.
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■■■■■ INTRODUCTIONElectrocardiographically (ECG) gated cardiacmagnetic resonance (MR) imaging is useful forevaluating both congenital and acquired heartdisease (1,2). Despite the usefulness of this tech-nique, it remains underused in part due to thedifficulty of managing heart disease; patients maybe clinically unstable and often receive a con-stant infusion of cardioactive drugs. Further-more, MR imaging suites are often recent addi-tions to medical centers and have been sited insecluded areas away from the clinical core ofthe facility, a location that makes movement ofpatients to and from the imager difficult. De-pendence on ECG gating for cardiac evaluationprolongs cardiac examination. Variation in intra-cardiac anatomy may be confusing and thus fur-ther prolong examination time by requiring ad-ditional views. We have found that many patientswith heart disease, especially those with ischemicor cyanotic heart disease, are claustrophobicand may require additional anxiolytic or seda-tive medication before or during the examination.
In this article, ECG-gated cardiac MR imagingis reviewed. Specific topics discussed are acqui-sition techniques and pitfalls, planning the ex-amination, performing the examination, indica-tions for cardiac MR imaging, and completingthe examination.
■■■■■ ACQUISITION TECHNIQUES ANDPITFALLSCardiac MR imaging makes use of ECG gating tosuppress motion artifacts caused by complexcardiac motion and great artery pulsation (3–5).In conventional MR imaging, the preset repeti-tion time determines the timing of phase-encod-ing gradient stepping. Each phase-encoding stepfollows the previous step until an image of nphase-encoding steps (usually 256–512) is ob-tained. The length of time needed to obtain animage with one signal acquired is the numberof phase-encoding steps times the repetitiontime. This scheme works well if the object be-ing imaged does not move. One can imaginethe image of a rapidly beating heart obtainedwith a continuous 15-second exposure time.
To suppress the periodic motion of cardiaccontraction, ECG gating is used to provide a pe-riodic timing function to image acquisition. Inother words, rather than a series of phase-en-coding steps being performed, each indepen-dent of a particular phase of cardiac contrac-tion, the ECG R wave is used to signal the ad-vancement of phase encoding. Each of thephase-encoding steps of an image is thus col-
lected from each of a series of heartbeats witheach step obtained at the same phase (time de-lay after a reference point in the ECG cycle) ofcardiac contraction. Phase-encoding step 1 isobtained from beat 1, phase-encoding step 2 isobtained from beat 2, and so on until data fromall 256 or 512 phase-encoding steps are col-lected. The ECG gate thus tells the imager whento advance phase encoding. Therefore, an im-age made of 256 “strips” of the heart with eachstrip obtained at the same phase of the cardiaccycle will be a coherent image of the beatingheart at that particular phase of the cardiac cycleand will be free of motion-related artifacts.
If the image is obtained at or very close tothe R wave, then image acquisition correlateswith electromechanical end diastole and anend-diastolic image is obtained. If the imagerwas instructed to wait two-thirds of the timebetween two R waves to step phase encoding,then a late systolic image would be obtained.The R wave is conventionally chosen as the gat-ing signal because it has the greatest voltageand is therefore more easily identified from theECG signal by the imager (Fig 1). If the voltageof the QRS complex is diminished (as in patientswith pericardial effusion) or if the P- or T-wavevoltage is increased (as in patients with atrialenlargement or hyperkalemia), then the imagermay be “confused” and trigger off of a non-Rwave; the result will be impaired image quality.That is, if the device senses an R wave followedby a P or T wave followed by another R waveand so on, the imager may presume a series oflong followed by short followed by long R-R in-tervals and produce incoherent images (Fig 2).
Spin-echo acquisitions produce series of im-ages with each series obtained at a differentanatomic level within the chest and each imageobtained at a different phase of the cardiac cycle.With this technique, several sections can be ob-tained in the same time that used to be neces-sary to obtain a single section. This techniquehas made cardiac MR imaging clinically feasible.However, since each image in such an acquisi-tion is obtained at a different phase of the car-diac cycle, functional comparison of anatomicsections may be confusing if not impossible.
Short flip angle gradient reversal (gradient-echo) cine imaging (6,7) provides images withincreased temporal resolution, which allowsevaluation of ventricular function, valvular re-gurgitation or stenosis, and shunts. With thispulse sequence, the same anatomic section isexcited with radio-frequency pulses by using ashort repetition time. The temporal resolutionof the examination depends on the repetitiontime and heart rate. In other words, a faster
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heart rate results in a shorter R-R interval (thetime between R waves) and, for any given num-ber of discrete phases of the cardiac cycle, lesstime between phases. After a fixed number ofsignals are acquired, the imaging system waits forthe next R wave to advance to the next phase-encoding step.
Occasionally, an adequate ECG tracing maynot be obtained. This problem may occur in pa-tients with severe ventricular ectopia (eg, after
acute myocardial infarction) or arrhythmogenicright ventricular dysplasia or in patients withvery low voltage ECGs (eg, patients with peri-cardial effusion). In these individuals, periph-eral pulse gating may be used.
■■■■■ PLANNING THE EXAMINATIONCommunication between the referring physicianand the radiologist performing the examinationis essential for efficient and accurate cardiac MRimaging. The purpose of the examination mustbe clear: to identify a specific problem or toconfirm or exclude a specific diagnosis. For ex-ample, ruling out aortic dissection is a straight-forward clinical problem. The MR imaging ex-amination will naturally include evaluation ofthe pericardial space, the aortic valve and leftventricle, and the origins of the arch vessels.On the other hand, a referral for evaluation of“heart failure” is vague and makes performanceof a complete examination difficult to achieve.
●●●●● Selecting a Pulse SequenceFor both spin-echo and gradient reversal pulsesequences, the spatial resolution of an MR im-age is dependent inversely on the size of thefield of view and directly on the number ofphase-encoding steps chosen. If morphologicanalysis will provide the specific answer to aclinical question, then spin-echo acquisition,which provides the highest contrast resolution,should be used. The downside of spin-echo ac-quisition is the temporal resolution of the ex-amination. The necessity to rephase spins bymeans of a second radio-frequency pulse (the180° pulse) limits the number of images that canbe obtained from a patient with a given heartrate. Thus, functional abnormalities may not beexamined directly. Analysis of resultant mor-phologic changes (eg, diagnosing aortic regurgi-tation by demonstration of a dilated and hyper-trophied left ventricle and dilated aorta) mustsuffice.
Cine imaging (gradient reversal images viewedin the loop cine mode) can be performed muchmore rapidly than performing series of 90° and180° radio-frequency pulse pairs and thereforeallows acquisition with greater temporal resolu-tion, which is ideal for functional analysis (eg,diagnosis and quantitation of aortic regurgita-tion by direct demonstration of the signal voidof the regurgitant jet). On the other hand, cineacquisitions have lower contrast resolution dueto the short flip angle and short repetition time
Figures 1, 2. (1) ECG gating. (a) From an engineer-ing point of view, the R waves should be preciselyspaced and unambiguously greater in voltage thanany other ECG signal. (b) In the clinical setting, dif-ferentiation of R waves from background noise maybe less precise. (c) R waves are identified by com-paring the voltage of a signal with a known (“thresh-old”) voltage. Thus, an R-wave triggering signal is asignal greater than the threshold. (2) Pseudo–R-R in-tervals. If the T-wave voltage exceeds the thresholdvoltage, then the T waves are interpreted by the im-ager as R waves. The assumed R-R intervals (arrows)are unequal; the result is temporally irregular phase-encoding steps and subsequently incoherent images.
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used (Fig 3). Furthermore, they are more sensi-tive to artifacts caused by sternal sutures, valvu-lar prostheses, or surgical clips. Although thereis no improvement in contrast resolution, k-spacesegmented gradient reversal acquisitions are veryrapid and may be used to overcome the acquisi-tion time limitations of conventional gradientreversal sequences.
●●●●● Contraindications to Cardiac MR Im-agingCardiac MR imaging is absolutely contraindicatedin patients with an active pacemaker (8) or otherimplanted electrical stimulator, free particulateiron in the optic globe, or intracerebral aneu-rysm clips. Mediastinal sutures and clips do notcontraindicate MR imaging. With the exceptionof pre–6000 series Starr-Edwards valves, thereare no cardiac prostheses that pose a knownrisk to the patient or are adversely affected byexamination at the magnetic field strengthspresently used (<2 T). Untoward effects of MRimaging on cardiac support hardware, such asimplantable ventricular assistance devices, arenot well established. Thus, it is prudent to ex-clude patients with these devices from MR im-
aging. Valvular prostheses and arterial stents mayproduce artifacts that diminish diagnostic sensi-tivity (Fig 4), but examination in their presenceis not precluded. Pregnancy is a relative contrain-dication. MR imaging may be performed if thebenefit to the mother is deemed greater than theunknown risk to the fetus.
Figure 3. Comparison of spin-echo acquisition with gradient reversal acquisition. (a) Midsystolic spin-echo MRimage (short-axis section obtained 138 msec after the R wave) shows a difference in signal intensity betweenthe left (L) and right (R) ventricular myocardium and the epicardial fat (arrows). (b) Gradient reversal MR im-age obtained in the same location at the same phase of the cardiac cycle shows that the difference in signal in-tensity between the cavitary blood and the ventricular myocardium and between the myocardium and fat ismarkedly decreased.
Figure 4. Image defect caused by an aortic valvularprosthesis in a 38-year-old man treated for congenitalaortic regurgitation. Horizontal long-axis gradient re-versal MR image shows a huge signal void in the cen-ter of the heart (V). The signal void causes the sizeof the prosthesis to be exaggerated but otherwisedoes not prevent interpretation of the image.
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●●●●● Choosing Imaging ParametersRegardless of the pulse sequence chosen, thenumber of signals acquired (NSA) will affect thespatial resolution of the image and the examina-tion time. The greater the NSA, the greater thesignal-to-noise ratio (SNR) of a voxel. The SNRof a voxel increases by the square root of theNSA used (eg, doubling the NSA from two tofour increases the SNR by nearly 41%). On theother hand, increasing the NSA prolongs theexamination time. Furthermore, prolonging theexamination only increases patient restlessness,which often results in motion artifacts. Wehave found that using four signals acquired inour conventional spin-echo acquisition proto-cols provides superior imaging without unnec-essarily prolonging examination time. Commer-cially available wraparound chest coils may beused to improve SNR. By placing the receivercloser to the body part being interrogated, sig-
nal is increased. We have found that such devicesallow acquisition of spin-echo images with twosignals acquired at no cost to image quality (Fig5). Cine acquisitions performed with two signalsacquired provide adequate spatial resolutionand SNR in reasonable acquisition times.
Increasing section thickness increases thesignal recovered but also results in loss of spa-tial resolution due to the greater amount of tis-sue interrogated. Generally, 8–10-mm-thick sec-tions in adult patients and 5–8-mm-thick sectionsin children provide sharp images without sig-nificant loss of spatial resolution (increased spa-tial resolution is limited by the increasing noisefound with thinner sections). Thinner sectionsmay be acquired by imaging small children withina head coil or by using a torso coil with an adult.
Figure 5. Use of a torso coil to increase signal. (a) Horizontal short-axis spin-echo MR image (two signals ac-quired) of a 68-year-old man with an infiltrating sarcoma (S) of the posterior wall of the left ventricle (LV) showsa clear difference in signal intensity between the epicardial fat and the free-wall myocardium of the right ven-tricle (arrowheads). The left main coronary artery is seen in cross section (arrow). Note the increased signal in-tensity of the anterior chest wall compared with that of the posterior chest wall. (b) Spin-echo MR image (samesection as in a, four signals acquired) of a 50-year-old man evaluated for tricuspid regurgitation shows interven-tricular septal trabeculae in the right ventricle (arrows). However, the diagnostic quality of both images is nearlyequal.
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The section-selection gradient is perpendicu-lar to the phase- and frequency-encoding direc-tions. Motion in the phase-encoding directioncauses a banding artifact along the direction ofthe applied gradient. Suitable selection of thephase-encoding direction can minimize this arti-fact. During spin-echo acquisition, the phase-en-coding gradient is switched on for a relativelyshort time in comparison with the frequency-encoding gradient. Motion occurring duringphase encoding acquires only a small phase shift.Similar motion occurring during application ofthe frequency-encoding gradient acquires agreater phase shift, which creates greater arti-fact. Therefore, motion artifact can be minimizedif the phase-encoding direction is horizontalduring sagittal acquisition and vertical duringcoronal acquisition (9). In axial acquisition, mostmotion (blood flow) is through the imagingplane; there is little difference in image qualitybetween horizontal or vertical phase encoding.In addition, the phase-encoding direction shouldbe chosen to minimize the amount of tissue out-side the field of view to avoid wraparound arti-facts.
■■■■■ PERFORMING THE EXAMINATIONThe MR imaging examination is limited to someextent by patient anxiety caused by confinementwithin a narrow magnet bore, coil noise, andstudy duration (10,11). In addition, adult patientswith congenital or acquired heart disease are of-ten mildly hypoxic and tend to be truly claus-trophobic; thus, such patients often require se-dation for MR imaging. If these patients are accom-panied by a friend or relative who can transportthem from the imaging facility, then they maybe studied as outpatients. An occasional patientmay not respond to mild, outpatient sedation.In this situation, as well as in patients with se-vere cyanosis or hemodynamic instability, MRimaging should be performed as an inpatientprocedure with appropriate anesthesia backupas needed.
●●●●● Preparing for ImagingMost patients who need mild sedation may begiven 2–10 mg of diazepam by mouth; if the pa-tient is unusually anxious, 2–20 mg of diazepammay be given intravenously and slowly (1 minutefor every 5 mg administered). Oral diazepam maytake a while to achieve a desired anxiolytic ef-
fect, whereas intravenous administration has animmediate effect. The effect of light anesthesiain children is augmented if the child is deprivedof sleep the night before the examination. Chil-dren may be sedated with orally or rectally ad-ministered chloral hydrate (up to 100 mg/kg)(12). Newborns may have their last feeding with-held and then be given a bottle of formula im-mediately before the examination. The newbornwill fill its stomach and sleep without medication.
The ECG tracing obtained for gating cannotbe used for diagnostic purposes. Peripheral pulseoximetry should be used in all sedated patients,any cyanotic patient, and any patient who hasreceived intravenous contrast material. Hypoxicpatients are especially sensitive to changes inambient oxygen levels. Therefore, although it isunlikely that cryogens used in superconductingmagnet systems may vent into the examinationroom, the constant oxygen concentration of theexamination room should be monitored.
An ideal team for performing cardiac MR im-aging would include a radiologist experiencedin managing cardiovascular disease, a nurse ex-perienced in patient sedation and monitoring,and a technologist experienced in examiningcritically ill patients. MR facilities are often recentadditions to an existing facility or in remodeledspace, so resuscitation teams should be madeaware of their location. Needless to say, com-plete “crash carts” should be readily available inall MR facilities.
Figure 6. Placement of ECG leads. Leads should beplaced within a 10-cm radius over the precordium.Wires should pass over the shoulder parallel to thelong axis (B field) of the magnet.
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The ECG gating signal may be obtained byplacing monitoring electrodes on the anterioror posterior chest wall (Fig 6); we prefer to ap-ply leads to the anterior chest for patient com-fort and ease of use. The ECGs of patients lyingwithin an MR imager are degraded by superim-posed electrical potentials resulting from the flowof blood in the magnetic field (13), the magni-tudes of which are proportional to the magneticfield strength and are maximal when flow isperpendicular to the magnetic field lines. In asupine individual, the left ventricle and ascend-ing aorta conduct blood nearly orthogonally tothe magnetic field, a phenomenon that increasesthe amplitude of the T wave (14) and possiblyconfuses R-wave detection by the imager. As-signment of specific leads to each position is ar-bitrary; the combination of electrode placementand lead attachment that produces the tallest Rwaves with the least noisy background is thebest lead selection. Superimposed blood flow–derived potentials may be minimized by appli-cation of electrodes relatively close together onthe chest wall and by braiding the leads to avoidforming loops (15). The leads should not passacross the patient’s chest and should be con-ducted out of the magnet bore parallel to thelong axis of the magnet. We occasionally shavethe patient’s chest to ensure optimal contactwith the ECG electrodes to provide a clear trac-
ing for ECG gating. However, as uncosmetic assuch shaving may be, it is important to providea clear ECG tracing for gating.
●●●●● Constructing Imaging SectionsThe primary axes of the heart and left ventricleare not aligned parallel to the axes of the body.Evaluation in views orthogonal to body axes(axial, coronal, or sagittal) may distort the thick-ness of ventricular myocardium or the shape ofthe cardiac chambers. Specifically, in the supineposition, the long axis of the body is parallel tothe B field vector of the magnet but the long andshort axes of the left ventricle are not. Thus, al-though image data obtained from axial, sagittal,or coronal acquisitions provide useful clinicalinformation, these data may be degraded becausethe intracardiac anatomy is distorted by obliqueacquisition. The size and shape of the ventricularcavity may be exaggerated and thus give a falseimpression of the functional state of the heart.
All cardiac MR imaging examinations in ourdepartment begin with a series of ungated gradientreversal “scout” images with one signal acquiredin the coronal plane through the middle third ofthe chest (Fig 7). This rapid, low-resolution se-ries provides cephalic and caudal landmarks for
Figure 7. Scout images. (a) Coronal gradient reversal MR image (one signal acquired) shows the aortic root(Ao), left ventricle (LV), main pulmonary artery (PA), and right atrium (RA). (b) Coronal gradient reversal MRimage (one signal acquired) obtained 3 cm posterior to a shows the distal aortic arch (AA), left atrium (LA), pos-terior left ventricle (LV), and right pulmonary artery (RP).
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the prescription of the next acquisition, a seriesof axial images. Most of our cardiac MR imagingexaminations are spin-echo examinations supple-mented by gradient reversal sequences performedto address specific functional questions.
The advantage of acquiring axial cardiac im-ages lies in the ease with which they are set upand the conventional manner in which anatomicdata are presented. Axial chest MR images ap-pear very similar to computed tomographic (CT)images obtained at the same anatomic level.Thus, individuals experienced in interpretationof chest CT images will immediately recognizedetails of intracardiac and mediastinal anatomyfrom axial MR images; the left-to-right and ante-rior-to-posterior relations among structures areapparent.
Axial images are necessary in the evaluation ofany congenital heart lesion (Fig 8). The range ofimage acquisition with respect to the patient’slong axis depends on the problem to be solvedwith MR imaging. In general, investigation ofthe aortic arch extends from cephalad to theaortic arch (to include the origins of the great
arteries) to just caudad to the aortic valve andleft ventricular outflow tract. Examination ofthe heart and pericardium generally extendsfrom just cephalic to the main pulmonary artery(the pericardial reflection) to just below the dia-phragm. The latter landmark is chosen to dem-onstrate the intrahepatic inferior vena cava andthe hepatic veins as well as the inferiormostportion of the pericardial space.
Examination of the left ventricle may be opti-mally performed with evaluation in the horizon-tal short-axis and four-chamber views, planes or-thogonal to its intrinsic long and short axes. Thefootball-shaped left ventricle has two orthogo-nal short axes, which form a plane orthogonalto the long axis of the ventricle. Construction ofimaging planes orthogonal to intrinsic ventricu-lar axes is based on identification of landmarksand starts with those identified in the initial scoutseries (Fig 7). Each successive acquisition pro-vides the landmarks for the next acquisition andthus provides a logical and reproducible meansof constructing rather complex, compound,angulated anatomic sections (Fig 9).
Figure 8. Series of axial spin-echo MR images with each image obtained 1 cm caudad to the previous one.(a) The ascending aorta (Aa), main pulmonary artery (MP), and superior vena cava (S) are outlined by mediasti-nal fat. The transverse portion of the right pulmonary artery (RP) is behind the aorta and superior vena cava andanterior to the right bronchus (RB). The left upper lobe pulmonary vein (arrow) is anterior to the left bronchus(LB). The descending aorta (Ad) lies to the left of the spine. (b) The left upper lobe pulmonary vein (open ar-row) is seen entering the left atrium. Posterior to the ascending aorta (Aa), medial to the superior vena cava (S),and anterior to the right pulmonary artery (RP) is the signal void of the superior pericardial recess (solid arrow).(c) The signal void of the anterior aspect of the pericardial space (straight solid arrow) is seen between the as-cending aorta (Aa) and proximal main pulmonary artery (MP). The right upper lobe pulmonary vein (open ar-row) enters the left atrium (LA) posterior to the superior vena cava (S). The left lower lobe pulmonary vein (ar-rowheads) passes anterior to the descending aorta (Ad) before entering the left atrium. The left atrial appendage(curved arrow) forms the left border of the heart. (d) The left lower lobe pulmonary vein (Ll) is seen enteringthe left atrium (LA). The three sinuses of the pulmonary artery may be identified (1, 2, and 3). (e) Just caudad tothe pulmonary valve, the right ventricular outflow tract (RVO) is seen. The posterior left aortic sinus (PLS) issituated between the left atrium (LA) and right ventricular outflow tract. The trabeculated right atrial append-age (arrows) lies anterior to the root of the ascending aorta. (f) The entrance of the superior vena cava (S) intothe right atrium and the ostium of the right atrial appendage (RAA) are seen. The commissures of the aortic valvedefine the anterior, posterior right, and posterior left aortic sinuses (AS, PRS, and PLS, respectively). The rightlower lobe pulmonary vein (open arrow) enters the left atrium (LA). Note the fatty infiltration of the sinus veno-sus portion of the interatrial septum (solid arrows). (g) The signal void of the descending portion of the rightcoronary artery (arrow) is seen within the fat of the anterior atrioventricular ring. The posterior right aortic si-nus (PRS) is interposed between the right atrium (RA) and left atrium (LA). The anterior mitral leaflet (arrow-head) is continuous with the aortic annulus. (h) The tricuspid valve (small white arrows) separates the rightatrium (RA) from the right ventricle (RV). The muscular interventricular septum (black arrow) is shared by theright ventricle and left ventricle (LV). The atrioventricular septum (large white arrow) separates the left ven-tricle from the right atrium. The mitral valve separates the left atrium (LA) from the left ventricle. (i) The infe-rior vena cava (IVC) is seen entering the right atrium (RA). The coronary sinus (C) enters the right atrium me-dial to the increased-signal-intensity fat of the posterior atrioventricular ring (arrow).
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a. b. c.
d. e. f.
g. h. i.
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From the coronal scout image through themiddle of the heart, a section is obtained throughthe aortic valve and left ventricular apex. Fromthis scout image, axial spin-echo images are ob-tained from the level of the pulmonary arterythrough the diaphragm. The image in this seriesthat contains the left atrium, mitral valve, andleft ventricular apex is chosen as the scout im-age for the next acquisition. A line drawn be-tween the middle of the mitral valve and theapex of the left ventricle defines the plane con-taining the true long axis of the left ventricle.This plane is nearly parallel to the plane of theinterventricular septum. Images obtained paral-lel to this plane are right anterior oblique sagit-tal images. From this series of right anterior ob-lique sagittal images, the image that best dem-
onstrates the mitral valve and cardiac apex isagain chosen. In this image, a line drawn be-tween the middle of the mitral valve and theleft ventricular apex defines the long axis of theleft ventricle. Images obtained orthogonal tothis line are in the cardiac short-axis view.
When viewed in the short-axis section, theleft ventricular myocardium appears as a seriesof intermediate-signal-intensity “doughnuts” sur-rounding the signal void (or increased signal in-tensity in gradient reversal acquisition) of theventricular cavity. In diastole, the left ventricularmyocardium in this section should be no morethan 1 cm thick in normal adults and should behomogeneous in signal intensity. The left ven-tricular cavity appears as a circle in this view.
Figure 9. Constructing the left ventricularaxes. (a) Axial spin-echo MR image shows aline (1) drawn between the middle of the mi-tral valve and the left ventricular apex. This linedefines the plane containing the true long axisof the left ventricle. (b) Right anterior obliquesagittal MR image containing the mitral valveand left ventricular apex shows a line (2) drawnbetween the middle of the mitral valve and theleft ventricular apex. This line defines the longaxis of the left ventricle. Images obtained nor-mal to this line are along the cardiac short axis.(c) Cardiac short-axis MR image shows the leftventricle (LV) as a round structure. The normalright ventricle seems to be an appendage of theleft ventricle.
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The papillary muscles of the left ventricle ap-pear as round loci of increased signal intensity(or signal voids in gradient reversal acquisition)in the lower left quadrant of the left ventricularcavity. Left ventricular cavity volume and myo-cardial mass may be accurately calculated withplanimetry of the endocardial and epicardial ven-tricular borders viewed in this section (Fig 10).
The four-chamber view displays the heart in aseries of parallel planes orthogonal to the short-axis sections (Fig 11). In this view, not only canthe ventricular myocardium and ventricular cham-bers be evaluated but the aortic and mitral valvesand their function may be scrutinized as well.The fibrous continuity between the two valveanuli is seen, and the alternation of mitral andaortic valvular opening and closing is demon-strated in cine mode. Regional changes in leftventricular myocardial thickness and cavity sizeare well displayed in this section.
Evaluation of the right ventricle with orthogo-nal axial, sagittal, and coronal sections simpli-fies three-dimensional reconstruction of the un-usual shape of the right ventricular cavity and
Figure 10. Short-axis section. (a) End-diastolic gradient reversal MR image shows a sharp borderbetween the high-signal-intensity moving blood of the left ventricular cavity and the intermediate-sig-nal-intensity myocardium. However, it is difficult to differentiate the right ventricular cavity fromfree-wall myocardium. (b) End-systolic gradient reversal MR image shows the papillary muscles of theposterior wall of the left ventricle more clearly (arrows). Left ventricular cavity size has decreased,and myocardial thickness has increased.
Figure 11. Four-chamber section. Spin-echo MRimage shows the aortic sinuses (1, 2, and 3) in thecenter of the heart. Within the fat of the anterioratrioventricular ring lies the right coronary artery(solid arrow) and tricuspid valve. The trabeculatedright ventricular (RV) free-wall myocardium is dem-onstrated. The anteroposterior relationships be-tween the left atrial appendage (LAA) and left upperlobe pulmonary vein (open arrow) as well as the leftbronchus (LB) and descending left pulmonary artery(LP) are well demonstrated.
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provides landmarks for longitudinal and intra-patient comparison of regional morphology (Fig12). Quantitation of right ventricular myocar-dial thickening and ventricular function is reli-ably performed with oblique short-axis sections.Furthermore, since the left and right ventriclesshare the interventricular septum, quantitative(or even qualitative) analysis of short-axis im-ages allows comparison of right with left ven-tricular function and evaluation of the role ofthe interventricular septum in cardiac function.
■■■■■ INDICATIONS FOR CARDIAC MR IM-AGING
●●●●● Aortic DissectionThe most common diagnosis excluded with car-diac MR imaging is aortic dissection. The highcontrast resolution, ability to image in arbitrarysections, and wide field of view for evaluatingassociated lesions make MR imaging a highlysensitive and extremely specific means of evalu-ating the thoracic aorta (16–18) (Fig 13). MRimaging is highly sensitive not only to the pres-ence or absence of aortic dissection but fordemonstration of the extent of the dissection,including entry and exit points, and extension
Figure 12. The right ventricle in orthogonal sec-tions. (a) Axial spin-echo MR image at the level of thedome of the right diaphragm (D) shows the cavity ofthe right ventricle (RV) appearing to drape aroundthe left ventricle (LV). The moderator band (arrow)is found on the right ventricular side of the interven-tricular septum and connects the free wall with theseptum. (b) Coronal spin-echo MR image shows thecavity of the right ventricle extending from the fat ofthe anterior atrioventricular ring, which contains thesignal void of the right coronary artery (arrow), tothe fat of the anterior interventricular groove. Theright atrial appendage (RA) is seen in cross section.(c) Sagittal spin-echo MR image of a patient with pul-monary hypertension and right ventricular hypertro-phy clearly shows the hypertrophied right ventricularmyocardial trabeculae (T) and supraventricular crest(C). Note the straightening of the interventricularseptum.
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of the dissection into one of the great arteriesof the aortic arch as well as associated aortic in-sufficiency. This complete examination may beperformed in a rapid, clinically relevant mannerin clinically stable patients, as well as in manyclinically unstable patients, by experienced ex-aminers in an appropriately prepared setting.
Although hemodynamic stability is a precon-dition for MR imaging performed to excludeaortic dissection, one must assume the possibil-ity of imminent cardiovascular collapse and actquickly to establish or exclude the diagnosis.From the coronal scout image, one prescribesdual spin-echo axial sections 10 mm thick with1-mm intersection gaps. The repetition time isset according to the length of the patient’s R-Rinterval. To maximize signal intensity, we usu-ally prescribe a 20-msec echo time. When weacquire dual-echo images, we use echo times of20 and 40 msec. The examination extends from
above the branches of the arch vessels to justbelow the aortic valve. If this territory cannotbe covered in one acquisition, an additional ac-quisition in this plane is performed. An obliquesagittal spin-echo acquisition through the aorticarch (section thickness = 10 mm, intersectiongap = 1 mm) displays the ascending aorta, aorticarch, and descending aorta in a plane orthogo-nal to the axial section and helps determine theextension of a dissection and possibly demon-strates entry and exit points. This acquisition isfollowed by an axial gradient reversal acquisi-tion extending from the aortic valve and evenlyspaced through the ascending aorta to demon-strate a thin, nonfatty intimal flap within a di-lated aorta. The examination is completed with
Figure 13. Type I aortic dissection. (a) Axial spin-echo MR image through the transverse right pulmonary ar-tery (RP) shows a dilated ascending aorta (Aa), which contains the high-signal-intensity intimal flap of the dis-section (arrows). The descending aorta (Ad) is seen as a homogeneous signal void without a flap. (b) Off-coro-nal gradient reversal MR image through the aortic valve in another patient shows the dilated ascending aorta(Aa), left ventricle (LV), main pulmonary artery (PA), and right atrium (RA). A calcification of the aortic valve(large arrow) appears as a signal void in the aortic anulus. A jet of accelerating flow leaves the stenotic aorticvalve and is deflected off the flap of an ascending aortic dissection into the true lumen of the mid–ascendingaorta (small arrows). The false lumen (F) is separated from the true lumen.
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a gradient reversal vertical long-axis acquisitionthrough the aortic valve and left ventricular cav-ity to demonstrate the presence or absence ofaortic regurgitation and the status of the leftventricular cavity and myocardial function.Spin-echo acquisitions are performed with twoto four signals acquired (depending on the useof a chest coil); gradient reversal acquisitionsare performed with two signals acquired.
●●●●● Evaluation of the PericardiumECG-gated cardiac MR imaging is an extremelyimportant means of differentiating restrictivecardiomyopathy from constrictive pericarditis.In the latter, the pericardium may be thickenedto at least 4 mm and thus increase the distancebetween the inner chamber of the heart andthe outer cardiac borders (Fig 14). The abilityto image in any arbitrary section allows MR im-aging to demonstrate the entire pericardialspace regardless of body habitus, thoracic de-formity, or pulmonary hyperaeration. Further-more, repeated images obtained over time maybe compared for objective evaluation of the ef-fects of medical or surgical therapy.
Complete examination of the pericardium(19–21) begins with a dual spin-echo acquisi-tion extending from above the pulmonary ar-
Figure 14. Pericardial disease. (a) Short-axis gradient reversal MR image obtained at end diastole ina 52-year-old man with congestive heart failure shows the right ventricular (RV) and left ventricular(LV) myocardium surrounded by a high-signal-intensity pericardial effusion (arrowheads). (b) Coronalspin-echo MR image of a 59-year-old woman with uremic pericarditis shows the attachment of thepericardium over the main pulmonary artery (PA) and ascending aorta (Aa). Note the thickened peri-cardium (arrowheads) as well as the increased-signal-intensity fibrinous exudate within the pericar-dial space (arrows).
Figure 15. Ectopic fat in a 65-year-old man withshortness of breath. Axial spin-echo MR imageshows a high-signal-intensity mass (M) immediatelyposterior to the left atrium (LA). The mass, whichhad been diagnosed as a mural left atrial tumor atechocardiography, is revealed to be mesenteric fatassociated with a hiatal hernia. In addition, note thethickened left ventricular myocardium of hyper-trophic obstructive cardiomyopathy (arrow) and es-pecially the anteriorly displaced anterior mitral leaf-let (arrowheads), which results in left ventricularoutflow obstruction. Secondary signs of mitral regur-gitation include left atrial enlargement and bilateralpleural effusions.
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other hand, has no such axial or sagittal symme-try. Modeling of the right ventricle is difficult,and the variance between models and the ac-tual appearance of the ventricle increases withthe severity of right ventricular disease. Further-more, MR imaging allows direct demonstrationof fatty infiltration and failure of the right ven-tricular free-wall myocardium to thicken duringsystole, which are the basis for the radiologicdiagnosis of arrhythmogenic right ventriculardysplasia (Fig 16).
Functional evaluation of the heart can beperformed by assessing the morphology of thecardiac chambers and the myocardial wallthickness on serial images, but the value of cineacquisition cannot be overstated. Cine examina-tion provides images of the cardiac chambersthrough the cardiac cycle and thus allows di-rect demonstration of temporal changes in car-diac morphology. Furthermore, cine acquisitiondemonstrates the cardiac chambers at end dias-tole and end systole and thus provides the sub-strate for calculation of ventricular myocardialmass (22,23) and chamber volume (24,25). Pro-tocols for acquisition of quantitative informa-tion are predicated on short-axis gradient rever-sal acquisition. The first image in the series ob-tained at a particular section is the end-diastolicphase (be certain of the phase of the cardiaccycle if you are using peripheral pulse gating).
Figure 16. Sustained ventricular tachycardia resulting from arrhythmogenic right ventricular dysplasia. (a)Axial spin-echo MR image of a 34-year-old man shows a thin veneer of epicardial fat that extends from the ante-rior atrioventricular ring (short large arrow) and then stops. The right ventricular free-wall myocardium (smallarrows) from this point to the moderator band (long large arrow) is less than 2 mm thick. (b) Axial gradient re-versal MR image of a 40-year-old woman with palpitations shows that a portion of the right ventricular (RV)free-wall myocardium fails to thicken (arrow). As a result, the right ventricular cavity appears to extend to theanterior chest wall.
tery (to include the pericardial reflection) tothe upper abdomen (to include the hepaticveins and suprahepatic inferior vena cava). Ac-quisition of coronal or off-sagittal (right anterioroblique) sections should be added to demon-strate the infracardiac pericardial space and thelateral border of the right atrium. In patientswith pericardial constriction, the dual spin-echoacquisition (vide supra) will provide sharp dem-onstration of the pericardium as well as thepericardial space and myocardial wall. Further-more, analysis of signal intensities within thepericardial space will help differentiate massesfrom moving pericardial fluid.
Tumors of the heart and pericardium as wellas paracardiac masses may be characterized ac-cording to their appearance and evaluated forpossible medical or surgical therapy by usingthe multiplanar approach available with cardiacMR imaging. Demonstration of an intact pericar-dium or the presence of specific cardiac in-volvement provides useful information for accu-rate staging and for estimating clinical prognosis.Furthermore, differentiation of true paracardiacmalignancy from ectopic or malplaced normalthoracic or abdominal contents (Fig 15) obvi-ates expensive and potentially risky evaluationof such masses.
●●●●● Evaluation of Ventricular FunctionTraditional evaluation of the left ventricle isbased on modeling of its symmetric shape tothat of a football. The right ventricle, on the
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The end-systolic phase is that image at a par-ticular section with the minimum cavity area.Ventricular volume is computed as the sum ofthe cavitary volume of each section (cavity area× section thickness) over the entire chamber.Myocardial mass is calculated by summation ofmyocardial section mass ([epicardial area − cavityarea] × section thickness) over the entire ven-tricle (Table).
●●●●● Congenital Heart DiseaseExamination of patients with congenital heartdisease begins with axial images obtained froma coronal scout image. Acquisition should ex-tend from the level of the aortic arch to belowthe diaphragm to include the inferior vena cavaand hepatic veins. This larger imaging range fa-cilitates demonstration of lesions associatedwith the specific clinical question as well as un-suspected additional lesions. Shunt lesions maybe small and may not be directly demonstratedwith spin-echo acquisition. However, if a shuntis present, one can expect to find sequelae ofthe shunt including chamber and arterial dilata-tion. Furthermore, covering the same anatomicterritory with a gradient reversal sequence mayhelp by directly demonstrating the jet of a shunt.Images in children should be thinner than inadults (5–8 mm). Since children have higherheart rates, their R-R intervals and thus calcu-lated repetition times are shorter; each acquisi-
tion therefore has fewer sections, but the higherheart rate reduces the duration of each acquisi-tion. Careful examination with axial sections willreveal the defect itself or associated findingspointing toward one diagnosis or another (Fig17). Orthogonal views provide additional find-ings that increase confidence in the diagnosis.
■■■■■ COMPLETING THE EXAMINATIONThe MR imaging examination of a patient’sheart is completed when the specific questionsasked by the referring physician are satisfacto-
Figure 17. Intracardiac shunts. (a) Axial spin-echo MR image of a 26-year-old man with a sinus venosus atrialseptal defect shows that the superior portion of the interatrial septum is incomplete (arrow). There is dilatationof the right ventricular outflow tract (RVO) and a normal-size left atrium (LA). (b) Axial spin-echo MR image ofa 4-month-old boy with a large ventricular septal defect and a primum atrial septal defect shows a discontinuityin the interventricular septum (arrows) and a break in the posterior medial interatrial septum (arrowhead).Note the increased myocardial thickness and cavity size of the right ventricle (RV) and the dilatation of the leftventricle (LV).
Right and Left Ventricular Volume andMyocardial Mass
Parameter by Ventricle Value*
Right ventricleEnd-diastolic volume (mL/m2) 67.9 ± 13.4End-systolic volume (mL/m2) 27.9 ± 7.5Stroke volume (mL/m2) 40.1 ± 9.7Ejection fraction 0.59 ± 0.09Mass (g/m2) 23.3 ± 1.4
Left ventricleEnd-diastolic volume (mL/m2) 68.9 ± 13.1End-systolic volume (mL/m2) 27.1 ± 7.8Stroke volume (mL/m2) 41.8 ± 10.9Ejection fraction 0.60 ± 0.11Mass† (g/m2) 91.6 ± 3.2
Source.—References 22 and 24.*Mean ± standard deviation.†The mass of the interventricular septum is as-signed to the left ventricle.
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rily answered. Doing so requires the examiningphysician not only to be aware of the clinical in-dication for examination but also to be knowl-edgeable about associated issues related to par-ticular clinical questions. For example, exami-nation to exclude an intracavitary ventriculartumor is pretty straightforward; once the ven-tricular cavity has been completely displayed,the diagnosis is made or excluded and the ex-amination can be terminated. On the other hand,evaluation of a patient with a dilated ascendingaorta involves not only estimation of maximumaortic caliber (and the site of that dilatation) butevaluation of the aortic anulus, aortic valvularfunction, and the volume and mass of the leftventricle as well. Thus, increasing familiarity ofexamining physicians with the diseases beinginvestigated will enhance their ability to usecardiac MR imaging to its fullest capacity.
■■■■■ REFERENCES1. Boxt LM. MR imaging of congenital heart dis-
ease. MR Clin North Am 1996; 4:327–359.2. Boxt LM. MR imaging of acquired heart disease.
MR Clin North Am 1996; 4:253–268.3. Lanzer P, Botvinick EH, Schiller NB, et al. Car-
diac imaging using gated magnetic resonance.Radiology 1984; 150:121–127.
4. Lanzer P, Barta C, Botvinick EH, et al. ECG-syn-chronized cardiac MR imaging: method andevaluation. Radiology 1985; 155:681–686.
5. Lieberman JM, Alfidi RJ, Nelson AD, et al. Gatedmagnetic resonance imaging of the normal anddiseased heart. Radiology 1984; 152:465–470.
6. Pflugfelder PW, Sechtem U, White RD, HigginsCB. Quantification of regional myocardial func-tion by rapid cine MR imaging. AJR 1988; 150:523–529.
7. Sechtem U, Pflugfelder PW, White RD, et al.Cine MR imaging: potential for the evaluationof cardiovascular function. AJR 1987; 148:239–246.
8. Hayes DL, Holmes DR Jr, Gray JE. Effect of 1.5Tesla nuclear magnetic resonance imaging scan-ner on implanted permanent pacemakers. J AmColl Cardiol 1987; 10:782–786.
9. Thickman D, Rubinstein R, Askenase A, Cabellero-Saez A. Effect of phase-encoding direction uponmagnetic resonance image quality of the heart.Magn Reson Med 1988; 6:390–396.
10. Quirk ME, Letendre AJ, Ciottone RA, Lingley JF.Anxiety in patients undergoing MR imaging.Radiology 1989; 170:463–466.
11. Quirk ME, Letendre AJ, Ciottone RA, Lingley JF.Evaluation of three psychologic interventionsto reduce anxiety during MR examination. Ra-diology 1989; 173:759–762.
12. Committee on Drugs, American Academy of Pe-diatrics. Guidelines for monitoring and manage-ment of pediatric patients during and after se-dation for diagnostic and therapeutic proce-dures. Pediatrics 1992; 89:1110–1115.
13. Gaffey CT, Tenforde TS. Alterations in the ratelectrocardiogram induced by stationary mag-netic fields. Bioelectromagnetics 1981; 2:357–370.
14. Dimick RN, Hedlund LW, Herfkens RJ, et al.Optimizing electrocardiograph electrode place-ment for cardiac-gated magnetic resonance im-aging. Invest Radiol 1987; 22:17–22.
15. Wendt RE III, Rokey R, Vick GW III, JohnstonDL. Electrocardiographic gating and monitoringin NMR imaging. Magn Reson Imaging 1988; 6:89–95.
16. Nienaber CA, Spielmann RP, von Kodolitsch Y,et al. Diagnosis of thoracic aortic dissection: mag-netic resonance imaging versus transesophagealechocardiography. Circulation 1992; 85:434–447.
17. Nienaber CA, von Kodolitsch Y, Brockhoff CJ, etal. Comparison of conventional and transesoph-ageal echocardiography with magnetic resonanceimaging for anatomical mapping of thoracicaortic dissection: a dual noninvasive imagingstudy with anatomical and/or angiographicvalidation. Int J Card Imaging 1994; 10:1–14.
18. Nienaber CA, von Kodolitsch Y, Nicholas V, etal. The diagnosis of thoracic aortic dissectionby noninvasive imaging procedures. N Engl JMed 1993; 328:1–9.
19. Amparo EG, Higgins CB, Farmer D, et al. GatedMRI of cardiac and paracardiac masses: initialexperience. AJR 1984; 143:1151–1156.
20. Masui T, Finck S, Higgins CB. Constrictive peri-carditis and restrictive cardiomyopathy: evalua-tion with MR imaging. Radiology 1992; 182:369–373.
21. Mulvagh SL, Rokey R, Vick GW III, JohnstonDL. Usefulness of nuclear magnetic resonanceimaging for evaluation of pericardial effusions,and comparison with two-dimensional echo-cardiography. Am J Cardiol 1989; 64:1002–1009.
22. Katz J, Whang J, Boxt LM, Barst RJ. Estimationof right ventricular mass in normal subjectsand in patients with primary pulmonary hyper-tension by nuclear magnetic resonance imag-ing. J Am Coll Cardiol 1993; 21:1475–1481.
23. Keller AM, Peshock RM, Malloy CR, et al. Invivo measurement of myocardial mass usingnuclear magnetic resonance imaging. J Am CollCardiol 1986; 8:113–117.
24. Boxt LM, Katz J, Kolb T, et al. Direct quantitationof right and left ventricular volumes with nuclearmagnetic resonance imaging in patients withprimary pulmonary hypertension. J Am CollCardiol 1992; 19:1508–1515.
25. Sechtem U, Pflugfelder PW, Gould RG, et al.Measurement of right and left ventricular vol-umes in healthy individuals with cine MR imag-ing. Radiology 1987; 163:697–702.