- Email: [email protected]
Clinical cardiovascular magnetic resonance imaging
--
Clinical Cardiovascular Magnetic Imaging
-
Resonance
Ronald M. Peshock, MD Magnetic resonance imaging (MRI) la a powerful tool prodding high-resolution images of the heart and great vessels without the use of ionizing radiation or contrast agents. MRI systems currently in use at many hospitals can be used effectively in the routine, clinical evaluation of many forms of cardiovascular disease, including great vessel disease, ischemic cardiac disease and congenital cardiac disease. Moreover, quantltative and tine MRI techniques are now available, which provide highly accurate measures of chamber size, wall motion and wall thickening. Recent developments in the areas of myocardial tagging, high-speed imaging and MR assessments of flow and perfusion suggest potential for an increasing role of MRI in the clinical evaluation of the cardiovascular system. (Am1 Cardiol 1990;66:4lF-44F)
From the Departments of Radiology and Internal Medicine (Cardiology), University of Texas, SouthwesternMedical Center at Dallas, Dallas, Texas. This study was supported in part by Ischemic Specialized Centers of Research grant HL-17669, National Heart, Lung, and Blood Institute, Bethesda,Maryland, and the Moss Heart Fund, and a grant from Toshiba America Magnetic ResonanceImaging, Inc. Address for reprints: Ronald M. Peshock, MD, Mary Nell and Ralph B. RogersMagnetic ResonanceCenter, 5801 ForestPark Road, Dallas, Texas 75235-9085.
agnetic resonanceimaging (MRI) is a powerful and flexible tool for the evaluation of the cardiovascular system.It can provide high-resolution images without the use of ionizing radiation or contrast agents and is intrinsically sensitive to flow. These features make MRI of potential use’in a variety of clinical conditions, including great vesseldisease,ischemic cardiac diseaseand congenital cardiac disease.’The purposeof this review is to: (1) provide a brief introduction to the basicprinciples of MRI; (2) describethe practical clinical applications; and (3) discuss several recent significant developments.
M
BASIC PRINCIPLES
The basic principles of MRI have been extensively revjewed.2,3Briefly, magnetic resonancehas 3 requirements. First, the presencein the body of atoms whose nuclei have a magnetic moment-meaning that the nucleus of each atom will behave like a tiny bar magnet when exposedto an external magnetic field. In the caseof MRI, these are nuclei of hydrogen atoms in water and fat. Second, an external magnetic field to align these nuclei is required. This is achieved by using a large magnet surrounding the patient. Third, a radio transmitter/ receiver to produce a secondexternal magnetic field that changes the orientation of the nuclear magnets and follows their return to the original orientation is necessary. This is doneusing a coil of wire placed closeto the patient. All MRI and MR spectroscopyinvolves the changes in orientation of these tiny nuclear magnets by external magnetic fields. These steps are illustrated in Figure 1. The tiny bar magnets (the nuclei of hydrogen atoms in the body in the case of MRI) are normally randomly oriented. When placed in an external magnetic field, the nuclear magnets tend to line up with the external field. If these nuclei are then exposedto a secondmagnetic field (provided by the radio transmitter) perpendicular to the original magnetic field, the nuclei can be tipped from their original orientation. If this secondmagnetic field is then turned off, the nuclei gradually return to the original orientation. As they return to the original orientation, they induce a current that can be detected. The energy releasedduring the return of the nuclei to the original orientation is used to form the image in MRI. By varying the timing, orientation and strength of magnetic fields to which the nuclei are exposed, it is possibleto obtain a wide range of image contrasts.This is illustrated in Figure 2 for 2 techniques commonly usedin clinical cardiovascular MRI: spin-echo imaging and tine MRI. Note that each of theseimagesis obtained without the use of contrast agent. The differences in contrast between the blood and other structures is obtained by
THE AMERICAN JOURNAL OF CARDIOLOGY
OCTOBER 26,199O
41F
A SYMPOSIUY:
CARDIOVASCULAR
IMAGING
IN THE 1999s
a Random nuclear momenfs.
a I/~ifofm
l
magnefic field aligns mugnefic momeflis.
Applied RF bufstperfurbs magnetic moments, RFifl
m Realignment emits chorocferistic frequency sensed by RF co/%
L FIGURE 1. Basic principles of magnetic resonance imaging. See text. N = north; RF = radiofrequency; S = south. (Reproduced with permission from Am I Cardio/m2y
simply altering the magnetic field pulsesthat are applied. At present, spin-echo MRI is used primarily to obtain high-resolution structural images, whereas tine MRI is usually used to obtain functional images. In general, the spin-echomethod is usedto obtain multiple slicesthrough the heart and great vessels,whereasthe tine MRI method is used to obtain 1 or a few slices at multiple points in the cardiac cycle to examine aspectsof motion and flow. Currently, the presence of a cardiac pacemaker, a ferromagnetic cerebral aneurysm clip, cochlear implant, or a metal fragment in the eye are consideredcontraindications for MRI. Patients with sternal wires, clips usedin coronary artery bypass grafting, and prosthetic cardiac valves can generally be safely imageda CURRENT APPLICATIONS There are 3 general areas of current clinical application for cardiovascular MRI: (1) great vesseldisease;(2) particular aspects of ischemic cardiac disease;and (3) congenital cardiac disease.Again, theseapplications have been the subject of a number of recent, extensive reviews.5,6Our purpose is only to provide a brief, practical review. The use of cardiovascular MRI is best established in the evaluation of diseaseinvolving the great vessels.In particular, aortic dissection, aortic aneurysm, congenital abnormalities of the great vesselsand vena cava obstruction have been extensively studied. An example of an image obtained in a patient with aortic dissection is shown in Figure 2 using both spin-echo and tine MRI techniques. The intimal flap is evident, using both imaging techniques. Typically, standard multislice, spin-echo imaging is usedto evaluate the extent of dissection with a 42F
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 66
FIGURE 2. Spin-echo versus tine magnetic resonance imaging images. Transaxfal images obtained through transverse portion of the aortic arch in a patient with aortic dissection using spin-echo (fop) and tine magnetic resonance imaging @oftom) techniques. The intimal flap is demonstrated using both techniques.
FIGURE 3. Spin-echo tasia with a markedly
image in a patient with annuloaortic dilated aortic root.
ec-
large number of slices,while tine MRI is performed at selectivelevels to allow evaluation of flow in the true and false channels. In the hands of experienced users, the sensitivity of MRI in the diagnosisof dissectionis comparable to that of x-ray computed tomography.7 MRI has also been shown to be useful in the evaluation of aortic aneurysm. Figure 3 showsa spin-echoim-
larly useful in the evaluation of malrotations of the heart and assessmentafter surgery. Although ultrasound is the primary modality in the evaluation of congenital cardiac disease,MRI can be a useful tool in a number of situations. Theseinclude the assessmentof patients with complex congenital cardiac disease,pulmonary atresia, and palliative shunts and conduits. It is also useful in older children, adults, and patients after surgery in whom acoustic windows may be limited for ultrasound. RECENT
FIGURE 4. Myocardial tagging using spatial modulation of magnetization (SPAMM). The tagging lines (dark) were applied at end-diaetole. With cardiac motion, the tagged points move relative to the tagged lines in stationary tissue allowing the determination of absolute cardiac wall motion.
age obtained in a patient with anuloaortic ectasia and clearly demonstratesthe dilated ascendingaorta. In Marfan’s syndrome,MRI can be usedto follow aortic sizeand to determine whether aortic dissection has occurred.8 MRI, with its lack of ionizing radiation, may be particularly useful in the evaluation of pregnant women with Marfan’s syndrome in whom dissection is a consideration. The technique typically usesgated,multislice, spinecho imaging to cover the region of the aneurysm with tine MRI to examine aspectsof flow. Coarctation of the aorta has also been extensively studied using MR1.9 Typically, transaxial spin-echo imagesare obtained to determine the minimum diameter of the aorta, with parasagittal spin-echoimaging performed along the long axis of the descendingaorta to establishthe extent of the narrowing. In this way, MRI can be usedto establishthe initial diagnosis,determine the adequacyof surgical repair and evaluatepotential complications such as pseudoaneurysm. MRI offers significant potential in the evaluation of ischemiccardiac disease.‘O MRI can obtain a complete 3dimensional dataset without the limitations of acoustic window or need for ionizing radiation. Hence, in cases where other imaging modalities are technically limited, MRI can be usedto examine the heart for regions of wall thinning and intracardiac thrombus. In addition, using multislice, multiphase and tine MRI methods, one can readily obtain assessmentsof chamber size, myocardial mass,wall motion and wall thickening.” MRI can also be usedto detect regionsof acute infarction, due to its inherent sensitivity to changes in tissue water content that occur in the setting of infarction. The application of MRI in the evaluation of congenital cardiac diseasehas beenextensive.l 2It can be particu-
DEVELOPMENTS
Recently, there have been a number of developments that may lead to the increased application of MRI in clinical cardiology. The first is in the area of myocardial “tagging.” I3 Standard evaluation of wall motion and wall thickening by ventriculography or echocardiography requires the identification of a given myocardial segment at multiple points in the cardiac cycle. Proving that the samesegmentis being evaluated at both end-systoleand end-diastole requires the attachment of some physical marker to the myocardial tissue. In the caseof tine angiography, this has required the attachment of beads or clips to the myocardium. I4 However, there has been no noninvasive means of applying such markers. In MRI, it is possible to apply additional pulsesof energy to label or tag given portions of the myocardium. The absolute motion of thesetags can then be usedto measurewall motion and wall thickening unequivocally. Additional variants of this method (Fig. 4) can be used to label many points in the heart simultaneously for rapid assessmentof wall motion.15These methods allow unambiguous evaluation of cardiac segmental function and should become the “gold standard” for assessingwall motion and wall thickening.lb The second major development relates to the availability of high-speed MRI. Conventional MRI of the
FlGURE 5. Echo-planar magnetic resonance imaging. Image of the heart obtained in approximately 30 ms using echo-planar magnetic resonance imaging (obtained in collaboration ith Drs. Richard Rzedzian and Ian Pykett at Advanced NM stems, Inc.).
THE AMERICAN JOURNAL OF CARDIOLOGY OCTOBER 26,1990
A SYMPOSIUI:
CARDIOVASCULAR
IMAGING
IN THE 19BOS
heart requires gating becauseof the time neededto collect REFERENCES 1. Council on Scientific Affairs. Report of the Magnetic ResonanceImaging all of the information to construct a complete image. Panel: magnetic resonance imaging of the cardiovascular system. JAMA Standard spin-echo and tine MRI sequencesacquire a 1988;259:253-259. series of images in a period of approximately 4 to 6 2. PeshockRM, Moore DM. Magnetic resonanceimaging:technical aspects.In: Brundage B, ed. Comparative Cardiac Imaging. Rockville, MD: AspenPublicaminutes. Recently, a number of techniques have emerged tions, 1990;41-46. that allow imaging of the heart using MRI in a fraction of 3. Young SW. Magnetic ResonanceImaging: BasicPrinciples.New York: Raven a second.The most rapid method currently available is Press, 1988. Soulen RL, Budinger TF, Higgins CB. Magnetic resonanceimaging of proscalled echo-planar MRI and can acquire an entire image 4. thetic heart valves. Radiology 1985;154:705-707. in approximately 30 ms.l7 An example of an echo-planar 5. PeshockRM. NMR imaging of the great vessels.In: Marcus M, Schelbert H, DJ, Wolf G, eds.Cardiac imaging: principles and practice.Philadelphia: image is shown in Figure 5. There is someloss of resolu- Skorton WB Saunders, 1990, in press. tion compared to standard images;however, the effect is 6. Peshwk RM. Heart and great vessels.In: Stark DD, Bradley WG, eds. small considering that the imaging time has been short- Magnetic ResonanceImaging. St. Louis: CV Mosby, 1988;887-920. Kersting-Sommerhoff BA, Higgins CB, White RD, Sommerhoff CP, Lipton enedby a factor of approximately 10,000.Unfortunately, 7. MJ. Aortic dissection: sensitivity and specificity of MR imaging. Radiology this very-high-speed imaging requires a specializedMRI 1988;166:651-655. 8. Schaefer S, Peshock RM, Malloy CR, Katz J, Parkey RW, Willerson JT. system not currently widely available. magnetic resonanceimaging in Marfan’s syndrome.J Am Coil Curdiol Another form of rapid imaging is termed turbo- Nuclear 1987;9:70-74. FLASH and acquires an image in approximately 300 9. Boxer RA, LaCorte MA, SinghS, CooperR, FishmanMC, GoldmanM, Stein Nuclear magnetic resonanceimaging in evaluation and follow-up of children ms.1s,19 This form of high-speed imaging can be imple- HL. treated for coarctation of the aorta. J Am Co1 Cardiol 1986;7:1095-1098. mented on many conventional MRI systems and could 10. Wisenberg C, Finniew KJ, Jablonsky G, Kostuck WJ, Marshal T. Nuclear magneticresonanceand radionuclide angiographicassessment of acutemyocardibecome more widely available in the near future. infarction in a randomized trial of intravenous streptokinase.Am J Car&al The final area of recent development involves assess- al1988;62:1011-1016. ments of blood flow and perfusion. BecauseMRI is in- 11. PeshockRM, Rekey R, Malloy CM, McNamee P, Buja LM, Parkey RW, trinsically sensitive to motion, it is possible to accentuate Willerson JT. Assessmentof myocardial systolic wall thickeningby nuclear magresonanceimaging. J Am Coil Cardiol 1989;14:653-659. the effects of blood motion and obtain angiographic-like netic 12. Fletcher BD, Jacobstein MD. Magnetic ResonanceImaging of Congenital images. MR angiography is undergoing rapid develop- Heart Disease.St. Louis: CV Mosby, 1988. 13. Zerhouni EA, Parish DM, Rogers WJ, Yang A, Shapiro EP. Human heart: ment for the noninvasive evaluation of cerebral and pe- tagging with MR imaging-a method for noninvasiveassessmentof myocardial ripheral vascular disease.2oParamagnetic contrast agents motion. Radiology 1988;169:59-63. (discussedin more detail by Wolfzl in this supplement), 14. Mitchell JH, Wildenthal K, Mullins CB. Geometrical studies of the left using biplane cinefluorography. Federation Proceedings1969;28:1334which distribute into tissue on the basisof blood flow, can ventricle 1343. be used to alter local tissue relaxation times and, hence, 15. Axe1 L, Dougherty L. MR imaging of motion with spatial modulation of provide a means to assessregional flow. With the devel- magnetization. Radiology 1989;171:841-845. 16. Beyar R, Shapiro EP, Graves WL, Rogers WJ, Guier WH, Carey GA, opment of faster imaging techniques, it is possible to Soulen RL, Zerhouni EA, Weisfeldt ML, WeissJL. Quantification andvalidation obtain at least qualitative estimates of tissue blood of left ventricular wall thickening by a three-dimensionalvolumeelementmagnetic resonanceimaging approach. Circulation 1990;8 1:297-307. flow.“-24 17. Rzedzian RR, Pykett IL. Instant imagesof the human heart using a new, In conclusion, numerous studies now support the use whole body MR imaging system.Am J Radio1 1987;149:245-250. of MRI in cardiovascular imaging. In particular, MRI is 16. HaaseA. SnapshotFLASH MRI. Applications to Tl, T2 and chemical-shift Magn Reson Med 1990;13:77-89. an effective tool in the evaluation of patients with great imaging. 19. Frahm J, Merboldt D, Bruhn H, Gygell ML, Haenickle W, Chien D. 0.3~ vessel disease,ischemic cardiac disease and congenital secondFLASH MRI of the human heart. Magn Reson Med 1990;13:150-157. cardiac disease.Importantly, MRI is now widely avail- 20. Edelman RR, Wentz KU, Mattle H, Zhao B, Liu C, Kim D, Laub G. Projection angiography and venography:initial clinical resultswith MR. Radioloable, with over 1,500units operating in the United States gy 1989;172:351-355. alone. Recent developmentsin the areas of myocardial 21. Wolf G. The role of MR contrast agentsin cardiac imaging. Am J Cardiol 1990;66:59F-62F. tagging, high-speed imaging and assessmentsof perfu- 22. Atkinson DJ, Burstein D, EdelmanRR. First-passcardiac perfusion:evaluasion suggestan increasing role for MRI in the evaluation tion with ultrafast MR imaging. Radiology 1990;174:757-762. 23. Belliveau JW, Rosen BR, Kantor HL, Rzedzian RR, Kennedy DN, of cardiovascular diseasein the future. The author would like to thank Dorothy Gutekunst, Ginny Vaughn, Jerry Cheek, Cindy Miller, Maria Morgan, Kevin Baker and Christi Ward for their technical expertise in performing the illustrated imaging studiesand in the preparation of the manuscript. Acknowledgments:
44F
THE AMERICAN JOURNAL OF CARDIOLOGY
VOLUME 66
McKinstry RC, Vevea JM, Cohen MS, Pykett IL, Brady TJ. Functional cerebral imaging by susceptibility-contrast NMR. Magn Reson Med 1990;14:538-546. 24. Shaefer S, Lange R, Kulkarni PV, Katz J, Willerson JT, Parkey RW, PeshockRM. In&o nuclear magneticresonanceimaging of myocardial perfusion using the paramagnetic contrast agent manganesegluconate. J Am Coil Curdiol 1989;14:472-480. 25. Goldman MR, Pohost GM, Ingwall JS, FossellET. Nuclear magneticresonanceimaging: potential cardiac applications.Am J Cardiol1980;46: 1278-1283.
Clinical Cardiovascular Magnetic Imaging
-
Resonance
Ronald M. Peshock, MD Magnetic resonance imaging (MRI) la a powerful tool prodding high-resolution images of the heart and great vessels without the use of ionizing radiation or contrast agents. MRI systems currently in use at many hospitals can be used effectively in the routine, clinical evaluation of many forms of cardiovascular disease, including great vessel disease, ischemic cardiac disease and congenital cardiac disease. Moreover, quantltative and tine MRI techniques are now available, which provide highly accurate measures of chamber size, wall motion and wall thickening. Recent developments in the areas of myocardial tagging, high-speed imaging and MR assessments of flow and perfusion suggest potential for an increasing role of MRI in the clinical evaluation of the cardiovascular system. (Am1 Cardiol 1990;66:4lF-44F)
From the Departments of Radiology and Internal Medicine (Cardiology), University of Texas, SouthwesternMedical Center at Dallas, Dallas, Texas. This study was supported in part by Ischemic Specialized Centers of Research grant HL-17669, National Heart, Lung, and Blood Institute, Bethesda,Maryland, and the Moss Heart Fund, and a grant from Toshiba America Magnetic ResonanceImaging, Inc. Address for reprints: Ronald M. Peshock, MD, Mary Nell and Ralph B. RogersMagnetic ResonanceCenter, 5801 ForestPark Road, Dallas, Texas 75235-9085.
agnetic resonanceimaging (MRI) is a powerful and flexible tool for the evaluation of the cardiovascular system.It can provide high-resolution images without the use of ionizing radiation or contrast agents and is intrinsically sensitive to flow. These features make MRI of potential use’in a variety of clinical conditions, including great vesseldisease,ischemic cardiac diseaseand congenital cardiac disease.’The purposeof this review is to: (1) provide a brief introduction to the basicprinciples of MRI; (2) describethe practical clinical applications; and (3) discuss several recent significant developments.
M
BASIC PRINCIPLES
The basic principles of MRI have been extensively revjewed.2,3Briefly, magnetic resonancehas 3 requirements. First, the presencein the body of atoms whose nuclei have a magnetic moment-meaning that the nucleus of each atom will behave like a tiny bar magnet when exposedto an external magnetic field. In the caseof MRI, these are nuclei of hydrogen atoms in water and fat. Second, an external magnetic field to align these nuclei is required. This is achieved by using a large magnet surrounding the patient. Third, a radio transmitter/ receiver to produce a secondexternal magnetic field that changes the orientation of the nuclear magnets and follows their return to the original orientation is necessary. This is doneusing a coil of wire placed closeto the patient. All MRI and MR spectroscopyinvolves the changes in orientation of these tiny nuclear magnets by external magnetic fields. These steps are illustrated in Figure 1. The tiny bar magnets (the nuclei of hydrogen atoms in the body in the case of MRI) are normally randomly oriented. When placed in an external magnetic field, the nuclear magnets tend to line up with the external field. If these nuclei are then exposedto a secondmagnetic field (provided by the radio transmitter) perpendicular to the original magnetic field, the nuclei can be tipped from their original orientation. If this secondmagnetic field is then turned off, the nuclei gradually return to the original orientation. As they return to the original orientation, they induce a current that can be detected. The energy releasedduring the return of the nuclei to the original orientation is used to form the image in MRI. By varying the timing, orientation and strength of magnetic fields to which the nuclei are exposed, it is possibleto obtain a wide range of image contrasts.This is illustrated in Figure 2 for 2 techniques commonly usedin clinical cardiovascular MRI: spin-echo imaging and tine MRI. Note that each of theseimagesis obtained without the use of contrast agent. The differences in contrast between the blood and other structures is obtained by
THE AMERICAN JOURNAL OF CARDIOLOGY
OCTOBER 26,199O
41F
A SYMPOSIUY:
CARDIOVASCULAR
IMAGING
IN THE 1999s
a Random nuclear momenfs.
a I/~ifofm
l
magnefic field aligns mugnefic momeflis.
Applied RF bufstperfurbs magnetic moments, RFifl
m Realignment emits chorocferistic frequency sensed by RF co/%
L FIGURE 1. Basic principles of magnetic resonance imaging. See text. N = north; RF = radiofrequency; S = south. (Reproduced with permission from Am I Cardio/m2y
simply altering the magnetic field pulsesthat are applied. At present, spin-echo MRI is used primarily to obtain high-resolution structural images, whereas tine MRI is usually used to obtain functional images. In general, the spin-echomethod is usedto obtain multiple slicesthrough the heart and great vessels,whereasthe tine MRI method is used to obtain 1 or a few slices at multiple points in the cardiac cycle to examine aspectsof motion and flow. Currently, the presence of a cardiac pacemaker, a ferromagnetic cerebral aneurysm clip, cochlear implant, or a metal fragment in the eye are consideredcontraindications for MRI. Patients with sternal wires, clips usedin coronary artery bypass grafting, and prosthetic cardiac valves can generally be safely imageda CURRENT APPLICATIONS There are 3 general areas of current clinical application for cardiovascular MRI: (1) great vesseldisease;(2) particular aspects of ischemic cardiac disease;and (3) congenital cardiac disease.Again, theseapplications have been the subject of a number of recent, extensive reviews.5,6Our purpose is only to provide a brief, practical review. The use of cardiovascular MRI is best established in the evaluation of diseaseinvolving the great vessels.In particular, aortic dissection, aortic aneurysm, congenital abnormalities of the great vesselsand vena cava obstruction have been extensively studied. An example of an image obtained in a patient with aortic dissection is shown in Figure 2 using both spin-echo and tine MRI techniques. The intimal flap is evident, using both imaging techniques. Typically, standard multislice, spin-echo imaging is usedto evaluate the extent of dissection with a 42F
THE AMERICAN JOURNAL OF CARDIOLOGY VOLUME 66
FIGURE 2. Spin-echo versus tine magnetic resonance imaging images. Transaxfal images obtained through transverse portion of the aortic arch in a patient with aortic dissection using spin-echo (fop) and tine magnetic resonance imaging @oftom) techniques. The intimal flap is demonstrated using both techniques.
FIGURE 3. Spin-echo tasia with a markedly
image in a patient with annuloaortic dilated aortic root.
ec-
large number of slices,while tine MRI is performed at selectivelevels to allow evaluation of flow in the true and false channels. In the hands of experienced users, the sensitivity of MRI in the diagnosisof dissectionis comparable to that of x-ray computed tomography.7 MRI has also been shown to be useful in the evaluation of aortic aneurysm. Figure 3 showsa spin-echoim-
larly useful in the evaluation of malrotations of the heart and assessmentafter surgery. Although ultrasound is the primary modality in the evaluation of congenital cardiac disease,MRI can be a useful tool in a number of situations. Theseinclude the assessmentof patients with complex congenital cardiac disease,pulmonary atresia, and palliative shunts and conduits. It is also useful in older children, adults, and patients after surgery in whom acoustic windows may be limited for ultrasound. RECENT
FIGURE 4. Myocardial tagging using spatial modulation of magnetization (SPAMM). The tagging lines (dark) were applied at end-diaetole. With cardiac motion, the tagged points move relative to the tagged lines in stationary tissue allowing the determination of absolute cardiac wall motion.
age obtained in a patient with anuloaortic ectasia and clearly demonstratesthe dilated ascendingaorta. In Marfan’s syndrome,MRI can be usedto follow aortic sizeand to determine whether aortic dissection has occurred.8 MRI, with its lack of ionizing radiation, may be particularly useful in the evaluation of pregnant women with Marfan’s syndrome in whom dissection is a consideration. The technique typically usesgated,multislice, spinecho imaging to cover the region of the aneurysm with tine MRI to examine aspectsof flow. Coarctation of the aorta has also been extensively studied using MR1.9 Typically, transaxial spin-echo imagesare obtained to determine the minimum diameter of the aorta, with parasagittal spin-echoimaging performed along the long axis of the descendingaorta to establishthe extent of the narrowing. In this way, MRI can be usedto establishthe initial diagnosis,determine the adequacyof surgical repair and evaluatepotential complications such as pseudoaneurysm. MRI offers significant potential in the evaluation of ischemiccardiac disease.‘O MRI can obtain a complete 3dimensional dataset without the limitations of acoustic window or need for ionizing radiation. Hence, in cases where other imaging modalities are technically limited, MRI can be usedto examine the heart for regions of wall thinning and intracardiac thrombus. In addition, using multislice, multiphase and tine MRI methods, one can readily obtain assessmentsof chamber size, myocardial mass,wall motion and wall thickening.” MRI can also be usedto detect regionsof acute infarction, due to its inherent sensitivity to changes in tissue water content that occur in the setting of infarction. The application of MRI in the evaluation of congenital cardiac diseasehas beenextensive.l 2It can be particu-
DEVELOPMENTS
Recently, there have been a number of developments that may lead to the increased application of MRI in clinical cardiology. The first is in the area of myocardial “tagging.” I3 Standard evaluation of wall motion and wall thickening by ventriculography or echocardiography requires the identification of a given myocardial segment at multiple points in the cardiac cycle. Proving that the samesegmentis being evaluated at both end-systoleand end-diastole requires the attachment of some physical marker to the myocardial tissue. In the caseof tine angiography, this has required the attachment of beads or clips to the myocardium. I4 However, there has been no noninvasive means of applying such markers. In MRI, it is possible to apply additional pulsesof energy to label or tag given portions of the myocardium. The absolute motion of thesetags can then be usedto measurewall motion and wall thickening unequivocally. Additional variants of this method (Fig. 4) can be used to label many points in the heart simultaneously for rapid assessmentof wall motion.15These methods allow unambiguous evaluation of cardiac segmental function and should become the “gold standard” for assessingwall motion and wall thickening.lb The second major development relates to the availability of high-speed MRI. Conventional MRI of the
FlGURE 5. Echo-planar magnetic resonance imaging. Image of the heart obtained in approximately 30 ms using echo-planar magnetic resonance imaging (obtained in collaboration ith Drs. Richard Rzedzian and Ian Pykett at Advanced NM stems, Inc.).
THE AMERICAN JOURNAL OF CARDIOLOGY OCTOBER 26,1990
A SYMPOSIUI:
CARDIOVASCULAR
IMAGING
IN THE 19BOS
heart requires gating becauseof the time neededto collect REFERENCES 1. Council on Scientific Affairs. Report of the Magnetic ResonanceImaging all of the information to construct a complete image. Panel: magnetic resonance imaging of the cardiovascular system. JAMA Standard spin-echo and tine MRI sequencesacquire a 1988;259:253-259. series of images in a period of approximately 4 to 6 2. PeshockRM, Moore DM. Magnetic resonanceimaging:technical aspects.In: Brundage B, ed. Comparative Cardiac Imaging. Rockville, MD: AspenPublicaminutes. Recently, a number of techniques have emerged tions, 1990;41-46. that allow imaging of the heart using MRI in a fraction of 3. Young SW. Magnetic ResonanceImaging: BasicPrinciples.New York: Raven a second.The most rapid method currently available is Press, 1988. Soulen RL, Budinger TF, Higgins CB. Magnetic resonanceimaging of proscalled echo-planar MRI and can acquire an entire image 4. thetic heart valves. Radiology 1985;154:705-707. in approximately 30 ms.l7 An example of an echo-planar 5. PeshockRM. NMR imaging of the great vessels.In: Marcus M, Schelbert H, DJ, Wolf G, eds.Cardiac imaging: principles and practice.Philadelphia: image is shown in Figure 5. There is someloss of resolu- Skorton WB Saunders, 1990, in press. tion compared to standard images;however, the effect is 6. Peshwk RM. Heart and great vessels.In: Stark DD, Bradley WG, eds. small considering that the imaging time has been short- Magnetic ResonanceImaging. St. Louis: CV Mosby, 1988;887-920. Kersting-Sommerhoff BA, Higgins CB, White RD, Sommerhoff CP, Lipton enedby a factor of approximately 10,000.Unfortunately, 7. MJ. Aortic dissection: sensitivity and specificity of MR imaging. Radiology this very-high-speed imaging requires a specializedMRI 1988;166:651-655. 8. Schaefer S, Peshock RM, Malloy CR, Katz J, Parkey RW, Willerson JT. system not currently widely available. magnetic resonanceimaging in Marfan’s syndrome.J Am Coil Curdiol Another form of rapid imaging is termed turbo- Nuclear 1987;9:70-74. FLASH and acquires an image in approximately 300 9. Boxer RA, LaCorte MA, SinghS, CooperR, FishmanMC, GoldmanM, Stein Nuclear magnetic resonanceimaging in evaluation and follow-up of children ms.1s,19 This form of high-speed imaging can be imple- HL. treated for coarctation of the aorta. J Am Co1 Cardiol 1986;7:1095-1098. mented on many conventional MRI systems and could 10. Wisenberg C, Finniew KJ, Jablonsky G, Kostuck WJ, Marshal T. Nuclear magneticresonanceand radionuclide angiographicassessment of acutemyocardibecome more widely available in the near future. infarction in a randomized trial of intravenous streptokinase.Am J Car&al The final area of recent development involves assess- al1988;62:1011-1016. ments of blood flow and perfusion. BecauseMRI is in- 11. PeshockRM, Rekey R, Malloy CM, McNamee P, Buja LM, Parkey RW, trinsically sensitive to motion, it is possible to accentuate Willerson JT. Assessmentof myocardial systolic wall thickeningby nuclear magresonanceimaging. J Am Coil Cardiol 1989;14:653-659. the effects of blood motion and obtain angiographic-like netic 12. Fletcher BD, Jacobstein MD. Magnetic ResonanceImaging of Congenital images. MR angiography is undergoing rapid develop- Heart Disease.St. Louis: CV Mosby, 1988. 13. Zerhouni EA, Parish DM, Rogers WJ, Yang A, Shapiro EP. Human heart: ment for the noninvasive evaluation of cerebral and pe- tagging with MR imaging-a method for noninvasiveassessmentof myocardial ripheral vascular disease.2oParamagnetic contrast agents motion. Radiology 1988;169:59-63. (discussedin more detail by Wolfzl in this supplement), 14. Mitchell JH, Wildenthal K, Mullins CB. Geometrical studies of the left using biplane cinefluorography. Federation Proceedings1969;28:1334which distribute into tissue on the basisof blood flow, can ventricle 1343. be used to alter local tissue relaxation times and, hence, 15. Axe1 L, Dougherty L. MR imaging of motion with spatial modulation of provide a means to assessregional flow. With the devel- magnetization. Radiology 1989;171:841-845. 16. Beyar R, Shapiro EP, Graves WL, Rogers WJ, Guier WH, Carey GA, opment of faster imaging techniques, it is possible to Soulen RL, Zerhouni EA, Weisfeldt ML, WeissJL. Quantification andvalidation obtain at least qualitative estimates of tissue blood of left ventricular wall thickening by a three-dimensionalvolumeelementmagnetic resonanceimaging approach. Circulation 1990;8 1:297-307. flow.“-24 17. Rzedzian RR, Pykett IL. Instant imagesof the human heart using a new, In conclusion, numerous studies now support the use whole body MR imaging system.Am J Radio1 1987;149:245-250. of MRI in cardiovascular imaging. In particular, MRI is 16. HaaseA. SnapshotFLASH MRI. Applications to Tl, T2 and chemical-shift Magn Reson Med 1990;13:77-89. an effective tool in the evaluation of patients with great imaging. 19. Frahm J, Merboldt D, Bruhn H, Gygell ML, Haenickle W, Chien D. 0.3~ vessel disease,ischemic cardiac disease and congenital secondFLASH MRI of the human heart. Magn Reson Med 1990;13:150-157. cardiac disease.Importantly, MRI is now widely avail- 20. Edelman RR, Wentz KU, Mattle H, Zhao B, Liu C, Kim D, Laub G. Projection angiography and venography:initial clinical resultswith MR. Radioloable, with over 1,500units operating in the United States gy 1989;172:351-355. alone. Recent developmentsin the areas of myocardial 21. Wolf G. The role of MR contrast agentsin cardiac imaging. Am J Cardiol 1990;66:59F-62F. tagging, high-speed imaging and assessmentsof perfu- 22. Atkinson DJ, Burstein D, EdelmanRR. First-passcardiac perfusion:evaluasion suggestan increasing role for MRI in the evaluation tion with ultrafast MR imaging. Radiology 1990;174:757-762. 23. Belliveau JW, Rosen BR, Kantor HL, Rzedzian RR, Kennedy DN, of cardiovascular diseasein the future. The author would like to thank Dorothy Gutekunst, Ginny Vaughn, Jerry Cheek, Cindy Miller, Maria Morgan, Kevin Baker and Christi Ward for their technical expertise in performing the illustrated imaging studiesand in the preparation of the manuscript. Acknowledgments:
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McKinstry RC, Vevea JM, Cohen MS, Pykett IL, Brady TJ. Functional cerebral imaging by susceptibility-contrast NMR. Magn Reson Med 1990;14:538-546. 24. Shaefer S, Lange R, Kulkarni PV, Katz J, Willerson JT, Parkey RW, PeshockRM. In&o nuclear magneticresonanceimaging of myocardial perfusion using the paramagnetic contrast agent manganesegluconate. J Am Coil Curdiol 1989;14:472-480. 25. Goldman MR, Pohost GM, Ingwall JS, FossellET. Nuclear magneticresonanceimaging: potential cardiac applications.Am J Cardiol1980;46: 1278-1283.