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European H eart Journal (1998) 19, 1939

Task Force Report

The clinical role of magnetic resonance in cardiovascular

disease

Task Force of the European Society of Cardiology, in Collaborationwith the Association of European Paediatric Cardiologists

Introduction

M agnetic resonance is helpful in the diagnosis andassessment of cardiovascular disease. However, thereare no published guidelines on the clinical role of thetechnique, although with the growing interest of clini-cians in cardiovascular magnetic resonance their need isnow pressing.

The board of the European Society of Cardi-ology has created a Task Force to establish the currentclinical role of magnetic resonance imaging and spec-troscopy in the diagnosis and assessment of diseases ofthe heart and great vessels. Because magnetic resonanceis a multidisciplinary technique, the task force wascomposed of cardiologists, paediatric cardiologists andradiologists and theseguidelines arejointly supported by

theEuropean Society of Cardiology and theA ssociationof European Paediatric Cardiology. Task Force recom-mendations arebased on evidencefrom published litera-ture and from the clinical experience of its members.Whenever there is little published evidence for a recom-mendation this is indicated, but usually no recommen-dation is made in this circumstance. To strengthen theclinical value of the guidelines, the role of magneticresonance is described alongside that of other imagingtechniques, with emphasis on the relative role withrespect to echocardiography. The rapid development ofmagnetic resonance means that its indications are ex-panding. Thus, these guidelines will require updatingwith time.

The value of imaging techniques in individualcircumstances is often defined as appropriate, accept-able, rarely justified, and not indicated. Consequently,

the classification of the ACC/AHA Task Force onAssessment of Diagnostic and Therapeutic Cardiovascu-lar Procedures[1] has been adapted:

Class I provides clinically relevant informationand is usually appropriate; may be usedas a first line imaging technique

Class II provides clinically relevant informationand is frequently useful, but similar in-formation may be provided by otherimaging techniques

Class III may provide clinically relevant infor-mation but is infrequently used becauseinformation from other imaging tech-niques is usually adequate

Class IV does not provide clinically useful infor-

mationInv potentially useful, but still under investi-gation

Technical aspects

Although an understanding of the physics of magneticresonance is important in appreciating the capabilitiesof thetechnique, a full description is beyond thescopeofthis report. A brief description of the technical aspectsof magnetic resonance is included to aid understandingof the terms used.

Nuclear magnetization

The hydrogen nucleus is abundant in water and it istherefore themost clinically useful of thenuclei exhibit-ing thephenomenon of magnetic resonance. Theproton,which forms the hydrogen nucleus, behaves as a smallmagnet. If placed in a magnetic field, the nucleus willalign with the field and it will precess in the same waythat a spinning top precesses in a gravitational field. Thefrequency of precession depends on the strength of the

KeyWords:M agnetic resonance imaging, magnetic reso-nance spectroscopy, clinical cardiology.

Task Force M embers are listed in the Appendix.

M anuscript submitted 22 J uly 1997, and accepted 15 August 1997.

Correspondence: Udo Sechtem, M D, F ESC, Abteilungfuer K ardi-ologie und Pulmologie, Robert-Bosch-K rankenhaus, Auerbach-strasse 110, 70376 Stuttgart, G ermany.

0195-668X/98/010019+21 $18.00/0 hj970787 1998 The European Society of Cardiology


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magnetic field and the type of nucleus. For hydrogen,this precessional or resonance frequency is approxi-mately 21 M Hz for a field of 05 Tesla and 63 M Hz fora field of 15 Tesla. Thenuclei align parallel or antipar-allel to the field, and only a small net magnetizationvector arises from tissue containing many hydrogennuclei, since almost equal numbers align in oppositedirections at body temperature. The small excess ofnuclei in the parallel state leads to a net magnetizationvector in the direction of the applied field, but atequilibrium there is no magnetization in the planeperpendicular to this field because the precession ofindividual protons is uncoordinated.

The magnetic resonance signal

If the nuclei are exposed to radiowaves at the resonantfrequency, the net magnetization vector is rotated at anangle to theapplied field depending upon theamount ofenergy applied, and the angle is termed the flip angle.After such a displacement, the net magnetization vectorrelaxes back to its former position tracing out a spiral.Until it reaches this equilibrium position, a componentof magnetization exists perpendicular to the appliedmagnetic field. This component can inducea radio signalat the frequency of precession in an aerial and such asignal is called the free induction decay. The net mag-netization vector as it returns to equilibrium can bedescribed by two components. T he component parallelto the main field returns to equilibrium by interactingwith surrounding molecules. The component perpen-dicular to the field returns to zero by interaction of thenuclei with each other and also because of local field

inhomogeneities, and this is more rapid. Both processesare exponential with time constants T1 (longitudinalrelaxation) and T2 (transverse relaxation). Dependingupon the sequence of radiofrequency pulses used todisturb the net magnetization vector and the time atwhich the magnetic resonance signal is recorded, themagnitudeof signal and hence thecontrast in theimagecan be made to reflect proton density, T1, T2 ormixtures of each. If the contrast mainly reflects T2, thenthese images are T2 weighted, etc.

I mage formation

M agnetic resonance imaging is theprocess of localizingtheradio signal from relaxing nuclei and displaying thesignal as an image. L ocalization is accomplished byadditional magnetic fields applied as gradients super-imposed upon the externally applied main field. Thegradients vary the field strength along their directionwhich means that the nuclei are excited or irradiate atfrequencies depending upon their position.

Images arereconstructed from a number of linesof data in frequency or k spaceasopposed to imagespace. In conventional cardiac magnetic resonance

imaging, each line is acquired from the same part ofsuccessive cardiac cycles and so ECG triggering of theacquisition is essential. The number of lines determinesthe resolution of the image but also directly influencesthe duration of the acquisition. Typically, 256 cardiaccycles are used in conventional imaging.

Pulse sequences

A pulse sequence is a combination of radiofrequencypulses and magnetic field gradients. Thepulse sequencesmost commonly used for theheart arethespin echo andgradient echo sequences. I n the spin echo sequence, themagnetization vector is tilted by 90 and after a brieftimeby another 180. Thissecond tilt leadsto coherenceof the spins and hence to an echo of the free inductiondecay signal at a time after the 90 pulse called theecho time (TE). In the images formed from a spinecho sequence, moving blood gives no signal andappearsblack becauseit does not experienceboth radio-

frequency pulses. Other tissues give signal which willdepend on proton density, T1 or T2 according to thetiming of the sequence. Echo times ranging from 2040 ms are used most commonly. The longer echo timeshave greater contrast between moving blood and othertissues but lower signal and more motion artefact thantheshorter echo times. Spin echo sequences areroutinelyused for multi-sliceanatomical imaging.

Thegradient echo sequence consists of an initialradiofrequency pulse tilting the magnetization by avariable angle usually between 10 and 30. This isfollowed by switching magnetic field gradients, whichdephase and rephase precession of the protons to forman echo. A n advantage of this sequence is images of thesame plane can be acquired at multiple points of thecardiac cycle, producing a cineacquisition. Because thissequence can be very short, moving blood gives a highsignal and appears white unless it is turbulent, whensignal may belost. Conventional gradient echo imagingacquires one line of the image each cardiac cycle, butif a number of lines is acquired in rapid successionoverall imaging time is reduced at the expense of adecrease in temporal resolution. For instance, imagingtimecan bereduced to 1530 s and hencewithin a singlebreath-hold.

Gradient echo sequences haverelatively low softtissue contrast and they are mainly used for studies of

ventricular or valve function. However, they can alsobe adapted to provide high signal from moving bloodand low signal from static tissue, allowing two dimen-sional or three dimensional magnetic resonance angi-ography. Turbulent flow leads to loss of signal in thegradient echo sequenceand thetechniquecan beused tolocate assess abnormal jets through valves and otherstructures.

M odifications of any sequence allow a grid oflines of magnetic saturation to be applied to an image,and this has been termed myocardial tagging. Thisprovides a tool unique to magnetic resonance and it

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allows threedimensional tracking of myocardial motionand sophisticated analysis of myocardial strains.

With the development of stronger magnetic gra-dient systems, several methods of rapid imaging cannow be implemented. Echo planar imaging involvesacquisition of all image lines (in k-space) after a singleexcitation. Variants of thetechniqueinvolvecoverageofk-spacein a spiral rather than rectilinear fashion (spiralecho planar), and segmental coverage of k-space withseveral (perhapseight) excitations to formanimage. Theecho planar technique is applicable to both spin echoand gradient echo sequences and so there is considerableflexibility in its application. It is however technicallydemanding and it is not yet used routinely in cardiacimaging.

Velocity mapping is a technique that encodesvelocity in a chosen direction in the phase of themagnetic resonance signal. A quantitative image ofphase can beformed and the pixel values are velocity inmm. s1. Thetechnique is accurate and flexible with awidedynamic rangeallowingencodingof velocitiesfrom

several mm. s1

to at least 10 m. s1

. Integration ofthese velocities over a vessel area and over the cardiaccycleallows flow to bemeasured, and pressuregradientsformed by discrete jets can be estimated using themodified Bernoulli equation. Themethod can beappliedto any sequence but most commonly a cine gradientecho sequence is used. Appropriate selection of echotimeis important sincesignal is lost in turbulent jetswithlong echo times. L eft sided jets in aortic stenosis andcoarctation may require echo times down to 3 ms foraccurate visualization of thejet.

Contrast agents

Specific contrast agents are available for magnetic reso-nance and these are mainly gadolinium complexes withsimilar physiological propertiesto theiodinated contrastmedia used in general radiology. Although natural con-trast is inherent to themagnetic resonancetechnique, theadditional flexibility of extrinsic contrast canbevaluablein certain circumstances. These agents act by shortening

T1 or T2 or both. T he main uses in cardiac imaging areto increase contrast between blood and soft tissue forcine functional imaging or angiography, and to assesssignal within masses such as tumours and cysts. Bolusinjections of contrast can also be combined with rapidgradient imaging to assess the passage of the contrastthrough the myocardium and hence myocardial per-fusion. This techniqueis still under investigation.

The scanner

A magnetic resonance scanner consists of five majorcomponents. The magnet, which is usually supercon-ducting, produces thestatic magnetic field, which has tobe homogeneous and stable with time, and yet large

enough to contain a human body. Resistive gradientcoils within the bore of the magnet produce the gradi-entssuperimposed upon themain field, and thecurrentswithin these coils are driven by the gradient amplifiers.

The performance of the gradient system determinesthe speed of acquisition. A radiofrequency aerial, oftenreferred to as a transmitter coil, is coupled to a radio-frequency amplifier to irradiate the patient with thepulses needed to disturb themagnetization. Thesameordifferent aerials can be used to receive the radio signal,and surface coils designed to be applied closely to thepart of the body to be imaged arefrequently used.

Safety

M agnetic resonance imaging is safeand no long-term illeffects have been demonstrated. Claustrophobia occursin approximately 2% of patients. Very rapidly changinggradients may inducemuscle twitching or effects on theretina, but clinical systems operate below the thresholdfor such effects. M etallic implants such as hip pros-theses, mechanical heart valves and sternal suturespresent no hazard since the materials used are notferromagnetic. Patients with electrical stimulators such apacemakers or cardioverter-defibrillators should not bestudied because of therisk of causing arrhythmias frompotentials induced in theendocardial wire and therapidrhythms that some pulsegenerators develop in a rapidlychanging magnetic environment. With appropriatesafe-guards however patients with pacemakers have beenscanned on occasion.

Congenital heart disease

General aspects

Evaluation of congenital heart diseaseis oneof themainstrengths of magnetic resonance (Table 1), but its valuedepends upon the age and clinical condition of thepatient. In general, magnetic resonanceis more valuablein the older patient with complex anatomy. Sedation isrequired in small children and physiological monitoringmaybedifficult in theconfines of themagnet in criticallyill infants. Thus, magnetic resonance imaging is usuallyperformed after transthoracic echocardiography in

neonates and infants. In contrast, body habitus andinterposition of lungs becomean increasing problem forechocardiography in adolescentsor adults, or at any ageafter surgery. Although some of these problems can beovercome by using the transoesophageal approach,expertise in magnetic resonance imaging is desirable incentres specializingin thecareof patientswith grown-upcongenital heart disease[2,3].

Cardiac catheterization will not be replaced inthe near future by magnetic resonance imaging, but theneed for diagnostic catheterization and the length andrisks of the examination can beminimized by its use[2,4].

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As catheter-based interventions are used increasingly inpaediatric cardiology, diagnosis should be as completeas possible before catheterization which then shouldbecome a onestage procedure with completion of diag-nosis (if necessary) and intervention being performed atthe same time.

M agnetic resonance imaging is generally lessoperator dependent than echocardiography, but a thor-

ough understanding of the anatomical and functionalprinciples of congenital disease is required for a reliablestudy. All parts of the cardiovascular system can beimaged, a feature which makes magnetic resonanceimaging especially useful in complex cases. For acomplete magnetic resonance examination, both spinecho and gradient echo series should be performed.Preferably, spin echo images in the transverse and one

Table 1 I ndications for magnetic imaging in patients with known or suspected

congenit al heart disease

Indication Class

GeneralEvaluation of anatomy and/or function if questions remain after

echocardiography or X-ray angiocardiographyI

Prior to cardiac catheterization in complex malformations IF ollow-up studies when echocardiography does not provide completeinformation and cardiac catheterization is not desirable because information

about pressures and vascular resistance is not the primary concern

I

SpecificViscero-atrial situs

Isolated anomalies IIAnomalies associated with complex congenital disease I

Atria and venous returnAtrial septal defect (secundum and primum) I IAnomalous pulmonary venous return, especially in complex anomalies and

cor triatriatumI

Anomalous systemic venous return Ibaffle repair or correction of anomalous pulmonary venous return I

Atrioventricular valvesAnatomical assessment of mitral and tricuspid valves IIIValve function I IIEbsteins anomaly I IIAtrioventricular septal defect I II

The ventriclesIsolated ventricular septal defect I II

Ventricular septal defect with other complex anomalies IVentricular aneurysms and diverticula IIEvaluation of right and left ventricular function I

The semilunar valvesI solated pulmonary valve stenosis and dysplasia I IISupravalvular pulmonary stenosis IIPulmonary regurgitation I

Isolated aortic valve stenosis I IISubaortic stenosis I IISupravalvular aortic stenosis I

The arteriesM alposition of the great arteries IIPostoperative follow-up of shunts IAortic (sinus Valsalvae) aneurysm IAortic coarctation IVascular rings IPatient ductus arteriosus I IIAorto-pulmonary window ICoronary artery anomalies in infants InvAnomalous origin of coronary arteries in adults IPulmonary atresia ICentral pulmonary stenosis I

Peripheral pulmonary stenosis Inv




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additional orthogonal plane(sagittal or coronal depend-ing on the case) should be available. For answeringspecific clinical questions, additional oblique sectionsmay berequired. Functional information is provided bygradient echo sequences, which haveto betailored to theclinical question. When clinically indicated, the exami-nation should use velocity mapping to assess flow.

Viscero-atrial situs

Theviscero-atrial situs (situs solitus, situs inversus, situsambiguus) and malposition of the heart (dextrocardia,levocardia) areeasily identified by conventional diagnos-tic tools (ECG, X-ray, echocardiography, abdominalultrasound). However, in the presence of additionallesions such asatrio-ventricular discordance, ventriculo-arterial discordance, anomalous pulmonary or systemicvenous connections, difficulties may arise in defining thetopographic relationship of themajor cardiac segments.M agnetic resonance provides images of the heart whichare easily related to surrounding structures[4] and thusit provides reliable and highly accurate diagnoses[5].In patients with complex anomalies, especially in olderones, magnetic resonance imaging is usually comp-lementary to echocardiography indicating that it shouldbeused routinely to maximize non-invasiveinformationbefore catheterization.

The veins and atria

M agnetic resonance imaging is potentially valuable inthe detection and quantification of atrial septal defects.In older patients, the best technique for imaging theinter-atrial septum is transoesophageal echocardiogra-phy and magnetic resonance imaging will provide ad-ditional information in only a few patients. In infants,however, magnetic resonance imaging may be usedinstead of transoesophagel echocardiography followinga transthoracic study. Pulmonary and systemic flow canbe measured more accurately than by any other tech-niqueand so magnetic resonancehas a clinical rolewhensuch measurements are important[6].

One limitation of echocardiography is the diffi-culty of evaluating anomalous pulmonary venous re-turn, but magnetic resonance imaging appears to bethe

best non-invasive techniquefor this[79]

. I n particular incomplex cases, magnetic resonance imaging is superiorto conventional imaging techniques sincecompletedem-onstration of thepulmonary veins may not be achievedby echocardiography or by X -ray angiography (sensi-tivity of magnetic resonance imaging 95%, X -rayangiography 69%, echocardiography 38%[8]). M agneticresonance imaging may also be indicated to identifypartial anomalous venous return in patients with atrialseptal defects.

When gradient echo sequences are used, mag-netic resonanceimaging successfully reveals thepresence

and severity of systemic or pulmonary venous obstruc-tion, which may occur pre- or post-operatively and maybedifficult to diagnose by echocardiography. Therefore,a major indication for magnetic resonance imaging isthepostoperativeevaluation of patientsafter intra-atrialbaffle repair in transposition of thegreat arteries or cor-rection of anomalous pulmonary venous return[8,10,11].M oreover, systemic venous anomalies (bi-lateralsuperior caval vein, interrupted inferior caval vein) arecorrectly identified by magnetic resonance imaging[5,12].

In cor triatriatum, magnetic resonance imagingmay be used if echocardiography is unclear, sincethe membrane within the atrium is well demonstratedas well as anomalous pulmonary venous return ifpresent[8,13].

The atrioventricular valves

In general, real-timeechocardiography, especially by thetransoesophageal route, is better suited to show theanatomy and motion of the atrioventricular valves[14].M agnetic resonance imaging may provide additionalinformation in selected patients[15], especially in infantswith an atrioventricular canal defect, where the attach-ment of the superior and inferior bridging leaflets to thecrest of theinterventricular septummay bedefined moreaccurately than by transthoracic echocardiography[16].

Anatomy of the tricuspid valve in Ebsteinsanomaly is best seen by echocardiography, in adults bythe transoesophageal route. However, in cases of doubtthethree dimensional arrangement of themalformationand theresulting tricuspid incompetenceis well depictedby magnetic resonance imaging[17,18].

T he ventr icles

M agnetic resonance imaging is sensitive and specificfor thedetection of ventricular septal defects(sensitivity84%100%, specificity 95%)[5,12,14,16,1921]. Difficultiesmay arise with small peri-membranous or musculardefects if spin echo images are used alone, but cinegradient echo imaging improves sensitivity by virtue ofits ability to detect and localizeabnormal jets[22]. As withan isolated atrial septal defect, magnetic resonance im-aging usually does not add much information in isolated

ventricular septal defect when the diagnosis is alreadyestablished by echocardiography. M agnetic resonancevelocity mapping, however, allows the shunt size to bedetermined accurately[6], and this may become in-creasingly important with the move to rely solely oninformation obtained non-invasively before referral forsurgery.

M agnetic resonance imaging has an importantrole in demonstrating ventricular anatomy in complexanomalies such as in tetralogy of Fallot, double out-let ventricle, pulmonary atresia, tricuspid atresia, anduniventricular hearts[5,7,21,23,24], but discrepancies with




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X-ray angiography arise in only a few cases (38%)[5,15].Thus, magnetic resonance imaging is often complemen-tary to echocardiography and both techniques should beused before catheterization. Follow-up studies by mag-netic resonance imaging and echocardiography mayreduce theneed for invasive investigation in these cases.

Because of its limited field of view, echocardiog-raphy has limitations in the evaluation of congenitalventricular aneurysms, particularly if it is necessary todefine the site, size and relationships of congenitaldiverticulae[25,26]. Thus, magnetic resonance imaging isindicated whenever such anomalies are suspected.

Right ventricular function is an important deter-minant of outcome in may forms of congenital heartdisease, and magnetic resonance imaging has the abilityto measure both right and left ventricular volume andmass accurately and reproducibly[2731]. This may beparticularly helpful in patients after the M ustard pro-cedure. The consequences of pulmonary regurgitationon right and left ventricular function can be evaluatedby combining measurements of ventricular volume and

diastolic function[32,33]

. Such functional measurementsarelikely to haveimportant prognostic and therapeuticimplications for the post-operative management ofpatients with congenital heart disease.

The semilunar valves

Congenital stenoses of the semilunar valves are easilyidentified by echocardiography, and maximal and meangradients can be determined by Doppler ultrasound.Although cinegradient echo imaging with velocity map-ping can also detect and quantify stenoses[34,35], Doppler

echocardiography is usually adequate. M agnetic reso-nance imaging may be particularly useful in localizingstenoses below, at, or above the valve if echocardiogra-phy fails to givecomplete information[36].

Patients with previous pulmonary valve surgeryoften haveimportant pulmonary regurgitation, and thismay be difficult to assess by echocardiography. In suchcases, magnetic resonance imaging can aid the decisionon replacement of the valve[37].

The arteries

M agnetic resonance imaging is an excellent method ofdemonstrating the size, position and connections of thecentral great arteries[23,38,39]. Similarly, abnormalities ofthethoracic and abdominal aorta can bedemonstrated,including aneurysm, dilation, stenosis, duplication, andvascular rings[36,4042].

Two-dimensional-echocardiographyandDopplerultrasound are usually adequate for the diagnosis andassessment of coarctation, but difficulties may be en-countered in older children or adults when magneticresonance imaging has a clear role, particularly fordiffusenarrowings of thearch. Velocity mappingmaybe

helpful for assessing the severity of coarctation eitherfrom the pressure gradient[43] or from measurements ofcollateral flow[44]. F or follow-up after surgical repair orangioplasty magnetic resonance imaging is probably thetechnique of choice[4547].

Visualization of a patent arterial duct is normallyachieved in infants by echocardiography, but magneticresonance imaging has a role in older patients where itis often better than echocardiography[48]. In patientswith a suspected aorto-pulmonary window, magneticresonance imaging may be helpful to establish the dif-ferential diagnosis or to demonstrate the additionaldefect[7].

M agnetic resonance imaging can demonstratecongenital anomalies or inflammatory changes of thecoronary arteries as in Bland-White-Garland syn-drome[49] or K awasaki disease[50]. Recent reports indi-cate that congenital anomaliesof theproximal coronaryarteries can be shown reliably in adults[51,52], and thetechnique may even have advantages over X -ray angi-ography, since the relationship of the arteries to the

aorta and pulmonary artery is animportant determinantof risk in these patients, and this is not always clearfollowing invasive angiography.

Visualization of the pulmonary artery and itsmain branches is essential in the surgical managementof patients with diminished pulmonary blood flow.Echocardiography is of limited value because of reflec-tion of the sound beam by the chest wall and lungs.Similarly, X -ray angiocardiography has its problems;since it can be dangerous in cyanotic children, smallpulmonary arteries may be difficult to catheterize, andnon-confluent vessels may not be seen even with pul-monary venous wedge contrast injection. Therefore thisgroup of patients is particularly suited to magneticresonanceimaging. Studiesof pulmonary atresia, tricus-pid atresia, tetralogy of Fallot and unilateral pulmonaryartery anomalieshaveshown very few discrepancieswithX-ray angiocardiography in defining theanatomy of thecentral and hilar pulmonary arteries as well as in deter-mining the sources of pulmonary blood flow[24,5357]. Inaddition, magnetic resonance imaging can reveal un-detected pulmonary and collateral vessels. A furthermajor impact of magnetic resonance imaging is inserial follow-up examinations, where it can show theshape and size of the pulmonary arteries, the patencyof systemic-to-pulmonary shunts, and extra-cardiacconduits[5760], as well as measure pulmonary flow[61,62].

Extra-cardiac ventriculo-pulmonary shunts often degen-erate in the long term, but this is not easy to assess byechocardiography because of the retro-sternal positionof thegraft. M agnetic resonanceimaging provides accu-rate anatomical and functional information and is thetechnique of choice for following these patients[63].

Acquired disease of the great vessels

In addition to congenital heart disease, theclinical valueof magnetic resonance imaging is well documented in




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patients with disease of the great vessels, more specifi-cally with acquired disease of the aorta (Table 2). T helarge field of view and the flexibility of imaging planesprovides a clear description of anatomy and of relation-ships to neighbouring structures. Rapid flow leads tohigh contrast between blood and the vessel wall, andvelocity mapping can beused to assess flow both quali-tatively and quantitatively.

Thoracic aortic aneurysm

Spin echo magnetic resonanceimaging depicts theextentand diameter of fusiform or saccular aneurysms of

whole thoracic aorta, regardless of aetiology[6466]. Inconditions such as mycotic pseudo-aneurysm and peri-valvular pseudo-aneurysm complicating bacterial endo-carditis, magnetic resonance imaging is more valuablethan transthoracic echocardiography[67,68]. Gradientecho imaging can be used to detect and characterizemural thrombus[6971], and velocity mapping provides ameasure of associated aortic regurgitation[72,73]. Thinslice imaging (5 mm) or segmented k-space breath-holdangiography can be used to identify theinvolvement ofaortic branches. Three dimensional angiography pro-vides attractive and easily interpretable images, but itsadditional value over simpler techniques is not yet well

defined

[74]

.

M arfans syndrome

Thecardiovascular complications of M arfanssyndromeinclude aortic dilation and dissection, and this is themain cause of death. X-ray angiography[75], echocardi-ography[76], computed X- ray tomography[77], and mag-netic resonance imaging[7880] have all been used toimage the aorta in M arfan patients, with excellentagreement between echocardiography and magnetic

resonance imaging for measuring the size of the aorticroot[78]. Aortic distensibility can be measured in bothchildren[81] and adults[82], although the clinical impor-tance of such measurements is still to be defined. Thetechnique is valuable for follow-up since aneurysmexpansion and dissection are clearly defined and areoften detected in asymptomatic patients[83,84]. Thus thelack of ionising radiation and the ability to visualizethewhole thoracic aorta make magnetic resonance a firstline imaging technique in these patients.

A ort ic dissection

There is substantial evidence that magnetic resonanceimaging has the highest accuracy for the detection ofaortic dissection of all imaging techniques[8587]. It notonly identifies the intimal flap and the siteof tear, but itcan also demonstrate associated abnormalities such asthrombus, aortic regurgitation, and pericardial effu-sion[87]. Although transoesophageal echocardiography isalso sensitive, it has several pitfalls that reduce specifi-city[87]. Computed X -ray tomography is also accurate,but it cannot evaluate functional aspects such as bloodflow, entry jets into the false lumen, and aortic regurgi-tation. Spiral computed X-ray tomography shortensimaging time without loss of accuracy[49], but there are

not yet sufficient comparisons with magnetic resonanceimaging to assess their relative clinical roles.

In populationswith anintermediatelikelihood ofdissection (disease prevalence 10%), the positive predic-tive value of magnetic resonance imaging, computedX-ray tomography and transoesophageal echocardiog-raphy is higher (>90%) than that of X-ray angiography(65%)[88]. Invasive angiography should therefore nolonger be used as a primary investigation, although itremains valuable for assessment of thecoronary arteriesif this is required. Practical aspects such as local exper-tise and availability of techniques will often determine

Table 2 I ndicati ons for magneti c resonance imaging in acquired disease of t he great

arteries

Indication Class

Diagnosis of thoracic aortic aneurysm IDiagnosis and follow-up in M arfan disease IAortic dissection

Diagnosis of acute aortic dissection IIDiagnosis of chronic aortic dissection I

Diagnosis of aortic intramural haemorrhage IDiagnosisof penetrating atheromatous ulcers of theaorta IFollow-up of acquired aortic disease IPulmonary artery anatomy IPulmonary emboli

Diagnosis of central pulmonary emboli IIIDiagnosis of peripheral pulmonary emboli I nv

Assessment of pulmonary flow and pulmonary hypertension IIIAssessment of thoracic veins I




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the best order of investigations. The problems of imag-ing haemodynamically unstable patients by magneticresonance favour transoesophageal echocardiographyin this situation, with magnetic resonance imaging orcomputed X-ray tomography reserved for equivocalcases or for a morecompleteassessment oncethepatientis stable. In subacute situations or for chronic follow-up, magnetic resonance imaging is the technique ofchoice[89].

Although aortic intramural haemorrhage wasonceconsidered a separateentity, it is nowthought to bea precursor of dissection. M agnetic resonance imagingcan detect such haemorrhage[80,90], and it has been usedto show that the clinical profile and outcome of thecondition is similar to classic dissection, and hencerequires a similar therapeutic approach[91]The recogni-tion of intramural haemorrhagerelieson visualizing wallthickening (>7mm) with a smooth surface and possiblycontaining areas of high signal. The high signal is theresult of methaemoglobin which develops after severaldaysand persists for several months, although it may be

absent in the acute phase[80]

. Both computed X-raytomography and transoesophageal echocardiographyarealternatives to magnetic resonance imaging[90,91] butX-ray angiography is not useful. Because intramuralhaemorrhage of the thoracic aorta may be associatedwith true dissection of the abdominal aorta, the lattershould also be evaluated.

Penetrating ulcers and atheroscleroticdisease

Penetrating ulcers are almost always associated withextensiveatherosclerosis of theaorta, primarily affectingthe intima and only secondarily the media. The diseasetherefore differs from dissection or intramural hae-matoma which are diseases of the media, althoughsymptoms of all three may besimilar. Penetrating ulcersusually arise in the middle and distal third of thedescending thoracic aorta[92]. Treatment may besurgicalor conservative and close follow-up is required in orderto detect complications. M agnetic resonance imaging isaccuratein making thediagnosis[93], and typical featuresinclude a thickened and irregular wall, an ulcer-shapedirregularity at a typical site, and haematoma in thewall of the aorta with high signal intensity caused by

methaemoglobin. Flexible imaging planes and theabilityto image without contrast material are advantages overcomputed X-ray tomography, although X -ray tomogra-phy is able to show displacement of intimal calcificationwhich can be a helpful diagnostic sign[94]. The roleof transoesophageal echocardiography is currentlyunknown.

Atherosclerotic plaques thicker than 4 mm in theaortic arch predict cerebral infarction and other vascularevents[95]. Although such plaques can be detected bymagnetic resonance imaging[96], its role in estimatingrisk is unknown.

Postoperat ive fol low-up

All of the above conditions require follow-up whethertreatment is conservative or surgical. The accuracy ofmagnetic resonanceimaging, its versatility, non-invasivenature, and lack of necessity for ionizing radiation orcontrast media, make it the method of choice for thispurpose[85,97,98].

Pulmonary artery anatomy

M ost pulmonary artery pathology occurs in the settingof congenital anomalies, which are discussed above.Dilation, thrombo-embolic involvement, and rare com-plications such as dissection are well demonstrated bymagnetic resonance imaging[99,100].

Pulmonary embolism

Early studies suggested a possible role for magneticresonance imaging in detecting central pulmonaryembolism[101103], but it cannot currently be recom-mended for routine use and ventilation-perfusion scin-tigraphy supplemented by X-ray angiography remainsthe diagnostic standard. Recent successful studies usingspiral X-ray tomography indicatethat thistechniqueis auseful diagnostic alternative. The value of this cross-sectional technique, however, suggest that newer mag-netic resonance techniques may find a role once moreexperience has been acquired. These techniques includemagnetic resonance angiography using fast two dimen-sional time-of-flight gradient echo sequences combinedwith maximum intensity projections, and this appearsto be sensitive (92100%), but only moderate specific(62%)[104]. Better results have been obtained usingphased-array coils and three dimensional angiogra-phy[105,106], or contrast-enhanced angiography[107].

Tagging can also be helpful to differentiate thrombo-embolism from flow related signal[108].

Pulmonary hypertension

Flow can bemeasured accurately by magnetic resonancevelocity mapping in the main and left and right pul-monary arteries[109]. The cyclical pattern of flow is

altered by pulmonary hypertension which leads to alower peak velocity and greater retrograde flow at theend of systole[110]. Although the significance of theseobservationsis not fully understood, it mayberelated topulmonary vascular resistance and hence serve as amethod of measuring resistance.

T horacic veins

Thesuperior and inferior caval veins and their relation-ship to the right atrium can be visualized by magnetic




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resonanceimaging, and this has been useful for assessingthrombus and invasion or compression by tumour[111].In addition, flow measurements using velocity mappinghave shown altered patterns in pulmonary hyperten-sion, pericardial constriction, and tricuspid regurgi-tation[112,113]. Pulmonary venous flow has been used to

assess diastolic left ventricular filling and theseverity ofmitral regurgitation[114].

Valvular heart disease

Normal heart valves arethin, rapidly moving structuresand they arepoorly seen in spin echo images. Abnormalvalves aremore easily seen because they arethicker andthey may be less mobile. Occasionally it is possible toobtain diagnostic information from spin echo imagesbut cine gradient echo imaging is more informative.Valve calcification may lead to loss of signal but theabsence of this sign cannot rule it out.

Regurgitation

The measurement of regurgitation is an important butdifficult aspect of theassessment of valvular disease. Theneed for intervention is determined partly by theseverityof symptoms, but other measurements such as theseverity of regurgitation and ventricular function areimportant. Echocardiography and radionuclide ven-triculography arecommonly used to assess left ventricu-lar function, but quantification of the regurgitation isoften felt to require invasivestudies with left ventricular

cine X-ray angiography. M agnetic resonance imaginghas the potential to help in several ways (Table 3).

Thesignal intensity of flowing blood during cinegradient echo imaging depends upon the nature of theflow. In general, flowing blood generates uniform highsignal because of continuous replacement of magneti-cally saturated blood by fresh blood. Turbulence leadsto loss of signal and so the turbulent jet of mitralregurgitation can beseen in the left atrium[115]. T hesizeof the signal void can be used as a semi-quantitativemeasureof regurgitation[116]but thesignal void does notnecessarily correspond to the area of turbulence, and

that it will vary with imaging parameters such as echotime. This is similar to colour flow Doppler wheretechnical factors such as gain adjustment and filtersetting areimportant[117]. A more fundamental problemcommon to both is that the sizeof the regurgitant jet isinfluenced by many factorsin addition to the severity of

regurgitation, such as the shape and size of the regur-gitant orifice and thesize of thereceiving chamber.

There is a correlation between the angiographicgrading of mitral regurgitation and the length and areaof the regurgitant jet imaged by Doppler echocadiogra-phy. A similar correlation existsbetween colour Dopplerand cine magnetic resonance although magnetic reso-nance shows a smaller area of flow disturbance[118]. Thismay be the result of different imaging planes or thefactthat the techniques use different physical principles. Apotential advantage of magnetic resonance is that it isable to samplea completethreedimensional volumeandso may provide a more accurate assessment of thevolume occupied by theregurgitant jet.

If only a single valve is regurgitant, comparisonof left and right ventricular stroke volumes allows theregurgitant fraction to becalculated asdescribed above.If singlevalves on both sides of theheart areregurgitant,the method can be extended by comparing ventricularstroke volumes with great vessel flow measured bymagnetic resonance velocity mapping. The regurgitantfraction then compares well with the regurgitant gradeassessed by Doppler echocardiography[37]. The methodstill fails if both valves on one side of the heart areregurgitant, but flow studies in the proximal aorta (orpulmonary artery) can be used to measure aortic (orpulmonary) regurgitation alone from the amount of

retrograde diastolic flow in the artery[72]

, and it is thenpossible to assess even the most complex cases.

Stenosis

As with regurgitant jets proximal to a valve, a turbulentdistal jet can be used to detect potential stenosis.Although thesizeof theregion of signal loss is related tothe pressure gradient across a lesion, it has not provedsimple to usethis asa measureof stenosis because otherfactors can lead to turbulence and signal loss without

Table 3 I ndications for magneti c r esonance imaging in patients wit h valvular heart

disease

Indication Class

Valve morphology IVAssociated cardiac chamber morphology II IAssociated intracardiac thrombus II I

Associated ventricular or atrial function II IDetection and quantification of regurgitation I IDetection and quantification of stenosis II ID etection of infectiveendocarditis and vegetations I VDetection of paravalvular abscess IIAssessment of prosthetic valves II I




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stenosis. For instance, it is not uncommon to see turbu-lence distal to a rheumatic valve that is not stenosedbecause of the disturbed flow across it. The degree towhich this signal loss is seen depends to a large extent

upon the echo timeof the gradient echo sequence used.Short echo times are less susceptible to signal loss.Despite these problems, the technique has been usedsuccessfully in the follow-up of patients after balloonmitral valvuloplasty[119]. The modified Bernoulli equa-tion makes a number of assumptions which when validallows the pressure gradient across a stenosis to becalculated from thepeak velocity. Themethod is widelyused in Doppler ultrasound and it can be applied in thesame way to velocity measurements made by magneticresonance. A sequence with a short echo time is import-ant in order to avoid loss of signal from areas ofturbulence. The technique is accurate both in vitro andin vivo[120] and a close agreement between magnetic

resonance and Doppler measurements has been demon-strated in patients with mitral and aortic stenosis andwith stenoses of conduits and great vessels[35,63].

An advantage of magnetic resonance overDoppler measurements of peak velocity is that themagnetic resonance technique allows velocities in anydirection to be measured without the limitation ofacoustic windows and the requirement to align a beamalong thedirection of a jet. Provided that an appropriatevelocity sensitivity is used during theacquisition aliasingdoes not occur, although if the anticipated velocity isunderestimated this can be a problem. The resolutionallows unambiguous distinction between the jets of

mitral stenosis and aortic regurgitation. A disadvantageof magnetic resonance is that it is not yet real time andso careful alignment of theimaging plane is required inorder to obtain an accurate measurement.

Prosthetic valves

Patients with prosthetic valves are commonly encoun-tered but prostheses arenot visualized because thealloyfrom which they aremade contains no mobile hydrogenatoms and so does not givea magnetic resonancesignal.

There is distortion of the applied magnetic field by thedifference of susceptibility between prosthesis and bio-logical tissueand by eddy currents induced in thevalve,and this leadsto loss of signal fromtissues for a variable

distancearound theprosthesis. This distanceis small forspin echo images and neighbouring structures are seennormally, but the defect in the image is much larger ingradient echo images making it difficult to assess turbu-lent jets in the region of the valve. M etal valves arenot ferromagnetic, however, and at currently availablemagnetic field strengths there is no effect of the fieldupon the working of the valve[121,122].

Cardiomyopathies and transplantation

Cardiomyopathy describes a number of conditions witheither primary or secondary myocardial involvement.M agnetic resonanceimaging hasthepotential to identifyand differentiate the cardiomyopathies and it is alsouseful in secondary myocardial hypertrophy (Table 4).

H ypert rophic cardiomyopathy

Two dimensional and Doppler echocardiography arecommon methods of diagnosing and assessing functionin hypertrophic cardiomyopathy[123,124], but they do notprovidecompleteinformation in all cases, particularly incases with localized apical hypertrophy[125]. I n contrast,spin echo magnetic resonance imaging provides reliable

images of the whole myocardium and gives a three-dimensional assessment of regional hypertrophy. Cinegradient echo imaging and velocity mapping alsoprovide functional measurements[126128], including ameasure of outflow tract gradient, if present, but mag-netic resonanceimaging has not been shown to bebetterthan Doppler echocardiography for this parameter.M agnetic tagging demonstrates regional myocardialmotion and strain in a unique fashion[129]. Thus, mag-netic resonance imaging is a second line technique inhypertrophic cardiomyopathy if echocardiography doesnot provide complete information.

Table 4 I ndications for magneti c r esonance imaging in patients wit h pericardial

disease, cardiac tumours, cardiomyopathies, and cardiac transplants

Indication Class

Pericardial effusion I IIConstrictive pericarditis IID etection and characterization of cardiac tumours I

Hypertrophic cardiomyopathy IIDilated cardiomyopathy I IIA rrhythmogenic right ventricular dysplasia I nvRestrictive cardiomyopathy IIPost-cardiac transplantation

Acute rejection IVChronic rejection I IIOther complications IV




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Secondary hypertrophy

M easurements of myocardial mass have been validatedin animals[130132] and in humans[133,134] and magneticresonance imaging is now the standard against whichother techniques are judged. Increased muscle volume(and hence mass) can be observed in athletes andpatients with left ventricular hypertrophy, and theregression of hypertrophy following treatment ofhypertension can be monitored[135].

Dilated cardiomyopathy

The anatomical and functional abnormalities of dilatedcardiomyopathy areclearly demonstrated and measuredby magnetic resonance imaging. The appearance is,however, non-specific and it is not possibleto distinguishdilated cardiomyopathy from other causes of left ven-tricular dysfunction such as ischaemic heart disease.

Advantages over echocardiography includebetter depic-tion of the right ventricle[136] and low variability offunctional measurements[137], but echocardiographynormally provides adequate clinical information, andtherole of magnetic resonance is mainly in quantifyingthe effects of therapy[138]

Arrhythmogenic right ventricular dysplasia

M agnetic resonance imaging is probably thebest imag-ing technique for demonstrating structural and func-tional abnormalities of the right ventricle. Global and

regional abnormalities can be seen in arrhythmogenicright ventricular dysplasia and also in patients withright ventricular tachycardia who do not have ab-normalities on other imaging techniques[139,140]. Theseabnormalities includeregional thinning, failureof systo-lic thickening, ventricular dilation, impaired globalsystolic and diastolic function, and areas of high signalintensity within the myocardium which indicate fattyinfiltration[141,142].

Restr icti ve cardi omyopathy

M agnetic resonance imaging can show anatomical andfunctional abnormalities associated with restrictive car-diomyopathy[143], although echocardiography remainsthe initial diagnostic tool in patients with unexplainedsymptoms of right and/or left ventricular failure. Themain contribution of the technique is the straightfor-ward and accurate differentiation of restrictive cardio-myopathy from constrictive pericarditis by virtue ofvisualization of the abnormal pericardium in the lattercondition in spin echo images[144]. Cine gradient echoimaging to assess the functional consequences ofboth diseases may be helpful in patients with limited

ultrasonic windows. Computed X -ray tomography (con-ventional, spiral or electron beam) may have similaraccuracy in making this distinction.

Cardiac transplantation

Imaging may play a rolein solving two common clinicalproblems in patients after heart transplantation. First,the early detection of acute episodes of rejection andsecond, theidentification of accelerated coronary arterydisease commonly considered as chronic rejection.

Acute rejection is characterized on spin echoimages as an increase in myocardial mass[145] and areasof high myocardial signal[146], which indicatemyocardialoedema and/or infiltration by mononuclear cells.Changes in relaxation times and regional functionalabnormalities may also be observed, but none of theseabnormalities is sensitivefor detecting early acuterejec-tion. Similarly, magnetic resonance imaging is not help-ful in chronic rejection, although it may reveal othercomplications of transplantation such as pericardialdisease or intracavitary masses[145].

Cardiac tumours

Echocardiography, especially using the transoesopha-geal approach, is well suited to detecting and localizingtumours and it may provide complete information onorigin, extent, and resectability. M agnetic resonanceimaging is, however, valuable as a secondary techniqueto confirm or exclude the diagnosis when echocardiog-

raphy gives incomplete information

[147]

(Table 4). Im-portant information provided includes the relationshipand involvement of surrounding structures such as ad-

jacent vessels and mediastinal structures[148]. Pericardialinvolvement is shown more clearly by magnetic reso-nanceimaging than by echocardiography[149], and it maybe particularly helpful in planning surgery[147].

Some features of the images allow limited tissuecharacterization but a definitive distinction betweenmalignant or benign masses can only rarely be made.High signal intensity on T1 images may represent fattytumours such as lipoma or liposarcoma, recent haemor-rhage, cystic lesions with high protein content, ormelanoma. L ow signal intensity may represent a cyst

with low protein content, signal loss in a vascularmalformation, calcium, or air[150]. On T2 images, cystshavehighsignal intensity irrespectiveof protein content.Solid tumoursincrease in signal with following injectionof contrast, but cysts are unchanged.

Pericardial disease

Both magnetic resonance imaging and computed X-raytomography arewell suited to imaging thepericardium,




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but magnetic resonance is also able to quantify associ-ated functional abnormalities[112] (Table 4). Both tech-niques have a much larger field of view thanechocardiography, but echocardiography remains theusual initial technique because of its simplicity andavailability.

Peri cardial effusion

Gradient echo images normally show pericardial fluidwith high signal because of motion related enhancementcaused by cardiac pulsation. T1 spin echo images showpericardial fluid with variablesignal depending upon thecontent and the motion of the fluid[151]. Transudates

normally have relatively low signal intensity, whereasexudates and subacute and chronic haemorrhagic effu-sions have high signal. Imaging is particularly helpfulin patients with loculated or complex configurationsof effusions which may be difficult to localize by echo-cardiography. As in other conditions, magnetic reso-nance imaging can be helpful whenever there is doubtfollowing echocardiography.

Constrictive pericarditis

Thecharacteristic appearance of constrictive pericardial

disease is pericardial thickening, an elongated andnarrow right ventricle, a sigmoid shaped septum withabnormal motion, enlargement of the right atrium andinferior caval vein, and stagnant blood in the atria.Pericardial thickening is the hallmark of constriction,although rare cases without thickening have been de-scribed[144]. All of these changes are readily seen usingspin echo imaging[144,152] and the positive predictiveaccuracy of the technique in one study was 100%[144].Calcification is moreeasily identified bycomputed X-raytomography, although large deposits of calcium can beseen by magnetic resonance as areas of low signal.

Calcification is less commonly a diagnostic feature inrecent years[153] and so this is not a major disadvantage.M agnetic resonance imaging and computed X-ray tom-ography arebetter than echocardiography in measuringpericardial thickness, but magnetic resonance has theadvantage of permitting assessment of haemodynamicimpairment. Velocity mapping may behelpful in assess-ing haemodynamics since there is attenuation of thediastolic peak of flow in the caval veins with any causeof impaired right ventricular filling[154]. Because com-puted X-ray tomography does not require ECG trigger-ing and because imaging time is shorter, it may bepreferred in sick patients or in those with arrhythmias.

Congenital pericardial anomalies

Pericardial cysts can be distinguished from other tu-mours by their characteristic low signal in T1 weightedimages and high signal in T2 images. However, differen-tiation from a necrotic or cystic mediastinal tumour canbe difficult.

Absence of the pericardium is associated with aleftward shift of thelongaxis of theleft ventricle. Partialabsencemaylead to a localized protrusion, although thedefect itself may be difficult to see[155].

Coronary artery disease

M agnetic resonance imaging can contribute to the as-sessment of patients with coronary artery disease in anumber of ways, although coronary and myocardialperfusion imaging havenot reached thestagewheretheycan beuseful in routinepractice. Themost useful area isin the assessment of myocardial thickness and thicken-ing, particularly for assessing myocardial viability andhibernation (Table 5).

Table 5 I ndicationsf or magneti c resonance imaging in patients with coronary artery

disease

Indication Class

Assessment of myocardial function II IDetection of coronary artery disease

Analysisof regional left ventricular function during stress III

Assessment of myocardial perfusion InvCoronary angiography I nvBypass graft angiography II IAssessment of coronary flow I nv

Detection and quantification of acute myocardial infarcts IVSequelae of myocardial infarction

M yocardial viability IIVentricular septal defect II IM itral regurgitation II IIntraventricular thrombus II




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V entri cular function

M agnetic resonance imaging appears to be the mostaccurate technique for assessing left ventricularmass[130,131,156,157]. Although spin echo images depict theendocardial boundary more clearly, cine gradient echoimaging is faster and equally accurate in practice[158].

Because the technique makes few assumptions aboutventricular geometry it is particulary useful in irregularventricles such as after infarction[134]. L eft and rightventricular volumes[159161], left ventricular ejection frac-tion[162,163], and regional left[164167] and right ventricularfunction[168,169] can all be assessed. Reproducibility isexcellent[133,137,138,170,171]. Thus, magnetic resonance im-aging should be considered whenever accurate measure-ments are required, such as in monitoring therapy. Inclinical practice, regional and global function is oftenestimated visually and without quantification from thecomputer screen.

M yocardial tagging is a technique unique tomagnetic resonanceimaging[172,173] and it can beused to

measure regional strain, motion and thickening withsufficient spatial resolution to distinguish endocardialand epicardial function[174176]. It has proved a valuableresearch tool but it is not commonly used in routinepractice.

D etecti on of coronary art ery disease

Stress imaging of left ventricular functionSeveral approaches to detecting flow limiting coronarystenoses have been described. Cine gradient echo imag-

ing can beused to assess regional ventricular function atrest and during a number of interventions. Dynamicexerciseis difficult within theconfines of a magnet and itleads to unacceptable motion artefact. As with echocar-diography, dobutamine appears to bemore appropriatethan dipyridamole for pharmacological stress because itmore consistently provokes ischaemia rather than per-fusion inhomogeneity[177,178]. Thediagnostic accuracy ofthis approach is between 75% and 90% dependingon thenumber of diseased vessels[178182], and this can beimproved by quantification especially in patients withsingle vessel disease[183]. To date, there areno compari-sons of stress magnetic resonance imaging and stressechocardiography but the techniques are similar inprinciple, if different in thequality of myocardial imagesobtained.

M yocardial perfusion imagingM agnetic resonance is able to assess myocardial per-fusion at rest and during pharmacological stress in aresearch setting[184]. Rapid imaging with acquisitiononce each cardiac cycle allows the myocardial transitof a bolus of contrast to be observed and parametersrelated to myocardial perfusion to be derived[185187].

The typical pattern in a territory served by a diseased

artery is delayed and attenuated appearance of con-trast[185,188]. Comparisons with myocardial perfusionscintigraphy are, however, limited and it is not clear howrobust thetechnique will prove in clinical practice. Oneimportant limitation that is being addressed is thelimited coverage of theventricle that can beobtained ina single study, but multi-slice perfusion studies maybe possible.

Coronary angiographyProbably the most important approach to detectingcoronary disease is magnetic resonance coronary arteri-ography, since a successful non-invasive techniquewould transform assessment of the patient with knownor suspected coronary disease. M agnetic resonanceangiography is already used routinely in many centresfor the carotid and intracerebral arteries, for aortogra-phy and for the ileo-femoral system. Imaging of thecoronary arteries is, however, much more demandingbecause of their size, complex three dimensionalanatomy, and rapid motion.

Rapid gradient echo imaging within a singlebreath-hold was the first technique to provide prom-ise[189], and sensitivities for proximal stenoses rangefrom 63%[190] to 90%[191], with best results for the leftmain coronary artery[192]. Spatial resolution, signal lossfrom turbulence and other artefact, and the inability toevaluate the distal coronary tree currently limit clinicalapplication to the assessment of anomalous coronaryarteries and other special circumstances, but severaldevelopments may overcome these limitations. Theseinclude more powerful gradients for rapid imaging,better surface coils, and real time tracking of diaphrag-matic motion in order to avoid the need for breath

holding[193195]

. Similar principles can be used to imagecoronary bypass grafts. U sing both spin echo[196,197] andgradient echo imaging[198,199] accuracies of around 90%for predicting graft patency have been reported. How-ever, anastomotic stenoses or progression of nativevessel disease are not easy to demonstrate reliably.M easurement of graft flowis possibleand thisobviouslyprovides more important information than patencyalone[200202].

Further development of all of these techniquesand comparative evaluation alongside computed X-raytomography of the coronary arteries can beexpected inthe coming years.

Cor onar y flowA further approach to detecting and characterisingcoronary stenosis is the measurement of coronary vel-ocity andflow[203206]. Usingrapid gradient echo velocitymapping coronary flow velocities can be measuredat rest and after maximal coronary dilation with adeno-sine, and good agreement has been demonstratedwith invasive measurements in animals[207] and inhumans[208]. I n theory, it should be possible to measurecoronary flow reserve and preliminary results areencouraging.




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Acute myocardial ischaemia and infarction

Areas of acute ischaemia and infarction develop highsignal intensity on T2 weighted spin echo images[209212],and regional myocardial thinning is the most specificfeature of acute necrosis[213215]. Paramagnetic contrastagents may help in the detection and sizing ofinfarction[216218]. However, the difficulties of imagingacutely ill patients means that these techniques have norole in assessment of acute infarction.

M agnetic resonance imaging may be helpful indetecting the short and long-term sequelae of acuteinfarction. For instance, regional thinning is indicativeof infarction and in the long term severe thinningindicates transmural scar[219]. Assessment of myocardialthickness and thickening are therefore finding a rolein the assessment of myocardial viability and hiberna-tion. There is a correlation between these parametersand uptake of 18F-fluorodeoxyglucose assessed bypositron emission tomography[220,221]. In addition, inakinetic regions, improvement in response to low dose

dobutamine infusion also indicates viability and likelyhibernation[221].

Complications of acute infarction such asaneurysm[25], ventricular septal defect[22], and mitralregurgitation are readily demonstrated. Becauseechocardiography can give misleading results whenlooking for ventricular thrombus after infarction[222],magnetic resonance imaging may be warranted wheninvestigating embolic complications[150,223]. It alsohas advantages over other techniques in identifyingpseudoaneurysms of the left ventricle[224].

Magnetic resonance spectroscopy

The signal source for magnetic resonance imaging isexclusively the 1H nucleus, more specifically the abun-dant 1H nuclei in water (H2O) and fat (CH2 and CH3groups). M agnetic resonance spectroscopy also involvesother nuclei with a net nuclear spin such as 32P. Animportant problemfor spectroscopyis its low sensitivity,sinceall nuclei other than 1H havea substantially lowermagnetic resonance sensitivity and are present in con-centrations orders of magnitudelower than 1H in waterand fat.

31P spectroscopy allows high energy phosphate

metabolismto beinvestigated and it therefore providesanon-invasive estimate of theenergetic stateof theheart.Six resonances can beidentified in themyocardium: thethree31P atoms of AT P (, , ) phosphocreatine(PCr),inorganic phosphate (Pi) and monophosphate esters(MPE). Each of these resonates at a different frequencyby virtue of their chemical environments and so eachappears with a different chemical shift, expressed inpartsper million relativeto themain magnetic field. Thearea under each resonanceis proportional to theamountof signal and hence to the amount of the nucleus.Absolute concentrations can be evaluated by acquiring

spectra with a known external standard. In addition,intracellular pH can bemeasured from thechemical shiftdifference between PCr and Pi.

Clinical magnetic resonancespectroscopy faces anumber of problems, such as long examination time,cardiac and respiratory motion, localization of signal,low sensitivity and hence a large voxel size (currentlyapproximately 30 cc), and the difficulties of measuringabsolute concentrations of metabolities. Higher fieldsstrengths[225], improvements in hardware and softwareand the nuclear Overhauser effect may allow some ofthese obstacles to be overcome. I f so, we can anticipatetrue three-dimensional metabolic imaging with voxelsizes of less than 5 cc.

Almost without exception, human cardiac spec-troscopy has been confined to the31P nucleus. Althoughit has contributed to our understanding of cardiacpatho-physiology it remains a research technique with-out a validated clinical role. At present, cardiac31Pspectroscopy has been applied in three main areas[226].

Coronary artery disease

In patients with left anterior descending stenosis and areversible thallium perfusion defect, the anterior myo-cardial PCr/ATP ratio is normal at rest[227], but itdecreases during isometric exerciseand returns towardsnormal after exercise[228]. In areas with fixed thalliumdefects PCr/A TP is reduced at rest and it does notdecrease with exercise[229]. M easurement of absoluteconcentrations of PCr and ATP has recently been re-ported[230]. A TP at rest is reduced in patients with fixedthallium defects but unchanged in those with reversible

defects. This suggests that these measurements may beuseful for theassessment of myocardial metabolism andviability after myocardial infarction.

Heart failure

ThePCr/ATP ratio is reduced in patientswith advancedheart failure of ischaemic[231] and non-ischaemic ori-gin[227], but it may be unchanged in the earlystages[227,232]. The reduction correlates with the clinicalseverity of heart failure, and it improves with medicaltreatment[227]. Thus, 31P-magnetic resonance may

provide an objective index of the severity of heart fail-ure and it may allow treatment and progression ofdiseaseto bemonitored. Theratio mayalso berelated toprognosis[233].

Valve disease

In patients with aortic and mitral valve disease thePCr/ATP ratio is unchanged in mild stenosis andincompetence, but it is reduced with the onset of




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heart failure[233]. Further studies are required toassess whether this might be helpful in timing valvereplacement.

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