InternationalJournal of CardiacImaging8: 217-227, 1992. 9 1992KluwerAcademic Publishers, Printedin the Netherlands.
Magnetic resonance imaging vs. ultrafast computed tomography for cardiac diagnosis Robert M. MacMillan Departments of Medicine and Radiology, Hahnemann University School of Medicine, 230 N. Broad Street, Philadelphia, PA 19102, USA Accepted 29 April 1992
Key words: heart, magnetic resonance imaging, ultrafast computed tomography Abstract
Ultrafast computed tomography (CT) and magnetic resonance imaging (MRI) generate high resolution tomographic cardiac images. Ultrafast CT requires intravenous injection of x-ray contrast combined with an image acquisition time of 50 msec. MRI requires no contrast injection, but has relatively long acquisition times due to gating. Both technologies can be used to evaluate cardiac chamber and great vessel dimensions, intracardiac and extracardiac masses, ventricular hypertrophy, left ventricular mass, congenital heart disease, regional and global left ventricular function, right ventricular function and pericardium. MRI is highly useful for detection and semi-quantitation of valvular regurgitation while ultrafast CT is not. Aortic and mitral valve stenosis can be detected by both, but MRI is the preferred study. Though both techniques can be used to assess coronary artery bypass graft status, ultrafast CT is the preferred method. It is concluded that ultrafast CT and MRI have broad applications for cardiac diagnosis.
Magnetic resonance imaging (MRI) and ultrafast computed tomography (CT) are complex tomographic imaging techniques which became available in the mid 1980s for clinical evaluation of cardiac anatomy and function [1, 2]. Both technologies offer high resolution images of the heart and can produce cine images throughout the cardiac cycle. MRI and ultrafast CT are relatively expensive technologies to acquire and maintain. Few centers have the resources to obtain both devices. Relatively few physicians have had direct experience with the cardiac applications of MR1 or ultrafast CT. The purpose of this communication is to contrast and compare MRI and ultrafast CT as they are currently employed for evaluation of cardiac structure and function.
The ultrafast CT scanner The scanner I operates with an electron beam (650 mA, 130 kv) electromagnetically focused over a target track length of 330 cm onto four parallel 210-degree tungsten rings (90cm radius). Target rings can be sequentially or individually activated at a rate of 17 times per second (scan speed, 50 ms; interscan delay, 8 ms). The resultant x-ray beam is collimated onto two parallel stationary crystal photodiodes, each with 432 channels. Individual slice thickness is 8 ram. There is a 4 mm inter-ring gap between paired slices. For calculation of ventricular volumes, slice thickness is taken as 10 mm by 11matron C-100, San Francisco, CA
218 convention. The in-plane resolution for a 10 mm slice is 1.8 mm and 0.7 mm for a slice thickness of 3 ram. The attenuation coefficient is represented by the standard Hounsfield system ( - 1 0 0 0 to 1000) with continuous window widths. The CT number for each pixel can be determined and represented by a shade of gray proportional to the attenuation coefficient.
Advantages of ultrafast CT The rapid image acquisition times eliminate blurring artifact created by cardiac motion . Up to eight slices through the heart can be obtained over as few as seven cardiac cycles using cine mode . Tomographic images have 0.7 mm-2 mm in-plane resolution . Image reconstruction time is one image per 6seconds . X-ray contrast may be injected through a median antecubital vein, thus avoiding the need for a central venous catheter in the superior vena cava or right atrium . Rapid image acquisition times allow tracking of x-ray contrast boluses through the heart as change in density per time . Density resolution permits separation of about 2000 gray levels . Scanning may be easily accomplished as an outpatient procedure.
Disadvantages of ultrafast CT The subject must be able to lie supine on a table. To minimize motion artifact, breath-holding may be required for up to 30-45 seconds . The technique is suboptimal for hemodynamically unstable patients. Planar slice orientation is limited to pivoting the table left-to-right and some feet down tilt due to the size of the reconstruction circle . X-ray contrast is required since the CT number for blood and myocardium are nearly identical. X-ray contrast must be injected through a large vein, usually only the median antecubital vein, in order to accommodate rapid injection rates. Metal clips and wires can create bright artifacts often obscuring surrounding structures. The method is not portable. A large lead-shielded room is required to house the scanner. Radiation exposure is relatively small (0.35
rad/slice) and is highly collimated, but must be considered potentially harmful [8, 9].
The MRI scanner The MRI scanner used at our institution is a 1.5 tesla superconductiong magnet. 2The spin-echo images are obtained using ECG gating. For cardiac cine, a matrix of 128 x 256 at two excitations yields an acquisition time of 256 cardiac cycles with ECG gating. Paired slice acquisition using a concatenated technique to minimize interslice interference is used. The data acquisition per sample is 25 ms. Images are reconstructed at 16 frames distributed through the cardiac cycle.
Advantages of MRI Rapid image acquisition times coupled with gating eliminates blurring artifact from cardiac motion . Multilevel slices through the heart can be obtained . Sequential images of one cardiac slice through the cardiac cycle permits viewing as a cine . Slice thickness may be 3, 5 or 10mm. Image reconstruction time is completed at the end of scanning sequence permitting immediate review of images. Tomographic images have an in-plane resolution of 2 mm . Compared to 2D echocardiography, resolution is uniformly good over a large field without problems of bone or air transmission . No breath-holding is required, but patients must remain still during scans. No X-ray contrast or intravenous catheters are necessary. Any planar orientation can be obtained. There is no recognized biohazard at the field strengths employed . It is an outpatient procedure.
Disadvantages of MRI The need for gating results in relatively long image acquisition times compared to ultrafast CT. Cine acquisition requires a regular cardiac rhythm, thus 2 GE Signa System, Milwaukee, WI
219 excluding patients with atrial fibrillation or frequent extrasystoles. Subjects with claustrophobia cannot tolerate the scanner enclosure [16, 17]. Critically ill patients, especially on ventilators, are from a safety standpoint, suboptimal for scanning . Ferromagnetic implants are not an absolute contraindication for MRI; however, permanent pacemakers, brain clips, metal filings in the eye cannot be scanned ]19]. Ferromagnetic metal implants such as prosthetic valves and clips can produce dropout artefact. The MRI scanner requires a large room especially shielded from external magnetic/electrical fields and is therefore not portable.
Scanning technique Ultrafast CT Patients are studied fasting. An 18 or 20 gauge venous cannula is placed in the median antecubital vein. The patient is placed head first supine into the scanner. The table is positioned within the scanning circle for axial, long axis or short axis planar orientation. An eight-level localization scan without contrast is obtained to ensure proper positioning of the patient. A flow study is performed by injecting 35 cc of radiographic contrast by powered injector at 5-7 cc per sec while timing the bolus for peak appearance in the heart. An eight-slice cine mode scan is obtained during peak passage of contrast through the right and left ventricles by injecting 42 cc of contrast at 3 cc per sec.
MRI Patients must be in sinus rhythm (for cine), free of internal or external ferromagnetic implants and not subject to claustrophobia to be scanned. Permanent pacemakers, defibrillators and other electronic devices cannot be scanned . The nonfasting patient is placed supine in the scanner. A localization scan is obtained followed by spin-echo or cine images acquisition in any plane desired. No x-ray contrast is required.
Clinical applications Anatomy The high resolution images of ultrafast CT and MRI are sufficient to permit quantitative measurements of relatively large cardiac structures such as individual cardiac chambers, aorta, pulmonary artery, vena cavae and the walls of the left ventricle [4, 14, 20]. Ultrafast CT and MRI can detect right and left ventricular hypertrophy by wall thickness measurement and quantitatively measure left ventricular mass [21-26]. Ultrafast CT has also been demonstrated to be useful for measurement of right ventricular mass . MRI and uttrafast CT are useful for detection and diagnosis of intracardiac and extracardiac masses such as atrial myxomas [28-30], lipomas (MRI) , thrombi (MRI)  and malignant metastatic masses [33, 34], (Fig. 1). Asymmetric septal hypertrophy as part of the spectrum of hypertrophic cardiomyopathy can be recognized using the long axis central left ventricular slice with ultrafast CT or MRI , (Fig. 2).
Congenital heart disease As a result of high quality spatial resolution of ultrafast CT and MRI, the abnormal anatomic structure of congenital heart disease is readily demonstrated. Atrial and ventricular septal defects can be qualitatively assessed by both technologies , (Fig. 3). Ultrafast CT can quantitate intracardiac shunt ratios by analysis of flow-density curves which have been compared to cardiac catheterization . MRI has been shown to be useful in diagnosis of congenital anomalies of the great vessels , prenatal diagnosis of congenital aortic stenosis , congenital tricuspid valve stenosis , double outlet right ventricle , tetralogy of Fallot  and Ebstein's anomaly . MRI has also been applied to postoperative evaluation of patients with D-transposition of the great arteries . MRI was able to evaluate patency of the vena cavae, right ventricular function, tricuspid and mitral regurgitation and obstruction of the right and left ventricular outflow tracts. With the exception
Fig. 1. (A) Ultrafast CT long axis planar slice through the heart demonstrating a large tumor mass (T) arising from the interventricular septum (S) biopsy proven to be a lipoma. LA = left atrium; LV = left ventricle; RV = right ventricle. (B) Spinecho MR image in axial plane demonstrating a large mass (arrows) on the posterior wall of the atria representing benign lipomatous hypertrophy. of quantitation of intracardiac shunts, M R I is preferred to ultrafast CT for evaluation of congenital heart disease since it avoids the problems of ionizing radiation and contrast injection.
Fig. 2. (A) Ultrafast CT axial slice through the ventricles in diastole demonstrating asymmetricseptal hypertrophy (ASH). R = right ventricle; L = left ventricle. (B) Cine MR long axis slice through the heart in diastole demonstrating asymmetric septal hypertrophy (S). A = left atrium; L = left ventricle.
Cardiac function Ultrafast CT and M R I can measure right and left ventricular volumes and ejection fraction using a modified Simpson's rule whereby the sum of the individual slice volumes in end-diastole and endsystole yields the total chamber volume [45, 46]. Comparison of derived volumes with cardiac catheterization is good. A corollary to measuring chamber volumes is derivation of stroke volumes and cardiac output. Cardiac output can also be mea-
sured using ultrafast CT by analysis of contrast density flow curves using a g a m m a variate fit technique . When this method is employed, nonionic contrast must be used since ionic contrast will cause rapid changes in the cardiac output secondary to peripheral vasodilation. Left ventricular segmental wall motion can be assessed tomographically in cine m o d e for ultrafast CT and M R I . Both techniques can detect akinesis, dyskinesis, hypokinesis and normal segments (Fig.
Fig. 3. (A) Ultrafast CT axial slice demonstrating an atrial septal defect (arrows), secundum variety at surgery. L = left atrium; R = right atrium. (B) Axial spin-echo MR image demonstrating a large atrial septal defect (arrow). A secundum variety was noted at surgery. L --- left atrium; R = right atrium.
4), [48, 49]. Neither technique is reliable for grading degrees of hypokinesis; i.e., mild, moderate or severe. For thorough evaluation of left ventricular segmental wall motion, both long and short axis images must be acquired. For ultrafast CT, this may require up to three separate contrast injections. For MRI, no contrast is injected, but equivalent acquisition times to ultrafast CT will be necessary because of gating.
Ultrafast CT has been shown to be useful for diagnosis of aortic and mitral valve stenosis. It is possible to detect valve calcification, chordae tendineae and papillary muscle thickening and restriction of leaflet opening excursion. In selected cases, aortic valve area can be directly measured and
Fig. 4. (A) short axis ultrafast CT images in diastole (lower) a systole (upper) from a patient with previous anterior transmural myocardial infarction. Note the anterior wall fails to contract in systole (arrow). (B) Short axis cine MR images in diastole (lower) and systole (upper) from a patient with previous anteroseptal myocardial infarction. Note the anterior septum and anterior free wall (arrows) fail to contract.
Fig. 5. Long axis cine MR image through a central left ventricular (LV) slice in diastole demonstrating a dilated aortic root (A) and aortic regurgitation seen as a dark cloud (arrows) in the white LV blood pool.
bicuspid vs. tricuspid valves distinguished . U1trafast CT is unable to detect valve regurgitation directly. When isolated mitral or aortic valve regurgitation is known to exist, regurgitant fraction can be calculated by taking the difference between left and right ventricular stroke volumes . MRI is useful for diagnosis of aortic, mitral, pulmonic and tricuspid valve regurgitation , (Fig. 5). Using gradient echo (cine) imaging regurgitant blood creates a signal loss in the recipient chamber which can be semiquantitated based upon the percent volume of signal loss vs. size of the chamber . Mitral valve prolapse can be seen as protrusion of one or both leaflets into the left atrium. Valve stenosis is qualitatively recognized by restricted leaflet mobility using cine MRI. With aortic and pulmonary stenosis, turbulence distal to the valve in systole produces signal loss. The linear distance of the signal loss in the aorta from the valve has been shown to be directly proportional to the severity of the aortic valve gradient . A narrow jet of signal loss in the LV outflow tract is often detected in aortic stenosis as a result of prevalve acceleration of flow. MRI also provides nonspecific evidence of aortic stenosis by demonstra-
Fig. 6. (A) Axial ultrafast CT image through the aortic root from a patient with Bjork-Shiley prosthetic aortic v a n e (VS) with endocarditis. Note the paravalvular abscesses. (MA). LA = left atrium; LV-left ventricle; RV = right ventricle; RA = right atrium. (B) Long axis cine MR image from a patient with a St. Jude aortic v a n e prosthesis (J) and perivalvular aortic regurgitation seen as signal loss (arrows) in the left ventricular cavity.
tion of dilated ascending aorta and increased LV mass. Spin-echo imaging can be used to detect anatomic abnormalities associated with mitral stenosis such as left atrial enlargement, thickened chordae tendineae and mitral leaflets and focal areas of signal loss in the valve and annulus indicative of calcification . Using cine MR, a central jet of signal loss corresponding to turbulent blood flow can be seen in early diastole entering the LV. A significant correlation exists between the size of the signal loss and the mitral valve gradient .
Prosthetic valves Mechanical valves can be detected by ultrafast CT (Fig. 6A). Because of their metallic components, they are densely white (CT number = + 1000) and little detail can be distinguished making identification difficult, though high profile (Starr-Edwards) valves may be differentiated from low-profile valves. Shellburst artifact from metal components may also obscure adjacent structures. Using MR, all mechanical valves can be safely imaged at high-field strengths except the StarrEdwards mitral pre-6000 series which should not be scanned at greater than 0.35 Tesla if valve dehiscence is suspected [3, 21]. Mechanical valves are seen as an area of discrete signal loss with little or no detail being discernible. Unlike ultrafast CT, MR images of prosthetic valves cause minimal distortion or artifact formation of surrounding structures. Valvular dysfunction due to disc or ball restriction cannot be identified, but valve or paravalvular regurgitation can be seen as a jet of signal loss in the recipient chamber (Fig. 6B).
Coronary artery disease Ultrafast CT has been demonstrated to have applications for evaluation of coronary artery disease (CAD). Noncontrast CT quantification of epicardial coronary artery calcification has a high negative predictive value for clinical coronary artery disease superior to fluoroscopy, exercise testing and exercise thallium scanning . Ultrafast CT can be performed in conjunction with exercise . Roig et al.  showed that exercise ultrafast CT assessing left ventricular global and regional function is a useful technique for evaluation of CAD. A multicenter study of coronary artery bypass graft patency using ultrafast CT had a predictive accuracy of 92% , (Fig. 7A). MRI is useful in coronary artery disease for evaluation of regional left ventricular function, quantitation of myocardial infarct size and detection of coronary bypass grafts. Higgins et al.  suggested that cine MR may be more accurate than angiography for identifying regional left ventricular dys-
Fig. 7. (A) Axial ultrafast CT image through the ascending aorta (A). Note the vein graft (black arrow) and the left internal mammary artery graft (white arrow). (B) Axial spin-echo MR image through the ascending aorta (A). Note the origins of two saphenous vein grafts (small arrows) and a longitudinally sliced vein graft to the left oronary artery (large arrow).
function since it can measure wall thickening as well as inward wall motion. Johns et al.  determined infarct location and size in 20 patients with good correlation using angiography. MRI has been employed for detection of coronary artery bypass graft patency with predictive accuracy similar to that obtained using ultrafast CT (Fig. 7B), [62, 63]. MRI differs from ultrafast CT in that grafts are recognized solely by anatomic location while ultrafast CT can detect and semiquantitate flow through grafts making ultrafast CT the procedure of choice for graft evaluation.
Pericardium The pericardium can be evaluated using ultrafast CT or MRI. Both methods can detect abnormal thickening of pericardium, infiltrative masses and
Fig. 8. (A) Axial ultrafast CT image through the heart from a patient with an aortic valve prosthesis (AVR). Note the anterior pericardium (P), small pericardial effusion (PE) and pleural effusion (PLE). SVC = superior vena cava; AO = aorta; LV = left ventricle. (B) Axial spin-echo MR image from a patient with pericardial effusion (arrows) secondary to metastatic carcinoma. effusions (Fig. 8). Ultrafast CT is the choice for detection of calcification. Pericardial constriction diagnosed by recognition of thickened pericardium, a dilated inferior vena cava, a small or normal descending thoracic aorta and deformity of the right ventricle has been described by both technologies [64, 65]. Because of greater soft tissue resolution, M R I is the preferred technique for evaluation of pericardium.
Fig. 9. (A) Axial ultrafast CT image through ascending and descending aorta from a patient with type I aortic dissection. Note the ascending aortic dissection flap (AD). DD = descending aorta false lumen. SVC = superior vena cava. (B) Axial spin-echo MR image through the ascendingaorta from a patient with type II aortic dissection. Note the dissection flap (arrows) between true and false lumens.
vena cava, superior vena cava and pulmonary artery. Multiplanar, multithickness slicing is available by both which permits detection of coarctation of the aorta, aortic aneurysm , aortopulmonary window and aortic root abscesses . Both technologies are highly useful for diagnosis of aortic dissection and can be used to classify by type and extent for assisting in clinical decisions (Fig. 9), [68, 69].
Summary The great vessels Ultrafast CT and M R I produce high quality images with excellent anatomic detail of the aorta, inferior
Ultrafast CT and M R I provide a broad base of cardiac diagnostic information. High resolution images by both can be used to assess cardiac structure, the great vessels and pericardium. Cine imag-
225 es f r o m b o t h p r o v i d e data o n global a n d r e g i o n a l v e n t r i c u l a r f u n c t i o n . M R I can detect a n d semiq u a n t i t a t e v a l v u l a r r e g u r g i t a t i o n while ultrafast C T c a n n o t . M R I is p r e f e r r e d for d e t e c t i o n of valve stenosis. T h o u g h ultrafast C T a n d M R I are e q u a l l y useful for e v a l u a t i o n of c o n g e n i t a l h e a r t disease, M R I is p r e f e r r e d m o d a l i t y as it avoids e x p o s u r e to x-rays a n d i o d i n a t e d c o n t r a s t in the p e d i a t r i c age
group. Since t i m e - d e n s i t y curves c a n b e g e n e r a t e d b y c o n t r a s t passing t h r o u g h a p a t e n t vein graft using ultrafast CT, thus n o t relying p u r e l y o n anat o m i c l o c a t i o n as with M R I , the f o r m e r is the preferred t e c h n i q u e for e v a l u a t i n g graft p a t e n c y . N e w
a p p l i c a t i o n s of b o t h t e c h n o l o g i e s are u n d e r develo p m e n t a n d p r o m i s e g r e a t e r roles in cardiac diagnosis.
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