(Radiographics. 2002;22:649-650.)
© RSNA, 2002
Invited Commentary
Lawrence M. Boxt, MD
Department of Radiology, Beth Israel Medical Center, New York, New York
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Commentary
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Progress in the diagnosis and management of heart disease has been closely linked with progress in cardiac imaging. Increased sensitivity and specificity of catheter-based left ventriculography and coronary arteriography for the diagnosis of regional wall motion abnormalities and segmental coronary stenosis have driven advances in cardiac surgery for valvular and ischemic heart disease, and these imaging tools have formed the basis for recent advances in percutaneous interventional techniques of cardiac revascularization. Furthermore, these catheter-based imaging techniques have also provided the basis for the tremendous growth in early diagnosis and successful palliation and cure of congenital cardiac malformations.
Application of these techniques to the demonstration and evaluation of right ventricular disease, however, has lagged far behind their use for left ventricular disease for a number of reasons. First, there was no good imaging method for reliably evaluating the right ventricle. The success in visualizing the left ventricle rests significantly on the axis symmetry of its particular left ventricular shape, on the uniform thickness of all wall segments, and on its relatively symmetric mode of chamber contraction. That is, the left ventricle was rather easy to visualize, its function was easily modeled, and its pathologic changes could be analyzed objectively. Furthermore, since the vast majority of cardiac disease involved the left ventricle, there was more than ample clinical case material to drive investigations focused on improving chamber visualization.
The shape of the right ventricle is unusual (1), with no intrinsic symmetries that lend themselves to an imaging strategy. That is, with no true right ventricular "axes," one cannot readily construct "orthogonal imaging planes" for chamber visualization. Furthermore, the difference in free wall and septal myocardial thickness makes analysis of changes at rest and during cardiac contraction difficult to assess.
Most right ventricular disease is caused by left ventricular dysfunction and pulmonary disease. Consequently, right ventricular changes in many patients were thought to be not only secondary to left ventricular changes but also less important as well, thus limiting efforts to develop imaging technologies for accurate diagnosis of right ventricular disease. In particular, the strong association between obstructive pulmonary disease and chronic pulmonary hypertension with right ventricular dysfunction and failure precluded an imaging work-up in many patients. Conventional catheter-based right ventriculography is contraindicated in patients with pulmonary hypertension, and the hyperaerated lungs of patients with obstructive lung disease, as well as the anterior location of the right ventricle, preclude reliable echocardiographic evaluation. Thus, primary and secondary right ventricular disease have not received great attention in the cardiac imaging world. And as such, our ability to diagnose and evaluate right ventricular disease has been the less for it.
ARVD is a cardiomyopathy involving predominantly the right ventricle that results in characteristic electrocardiographic changes, structural changes in the right ventricular free wall, and ventricular tachyarrhythmia, which may cause sudden death. In the preceding article, Kayser and colleagues point out the significant role that MR imaging plays in the identification of morphologic and functional criteria for the diagnosis of this disease. They discuss the value of spin-echo acquisition for providing image data for assessing the morphology and histologic state of the right ventricular myocardium and the complementary value of cine gradient reversal acquisition for regional functional evaluation.
For reasons not yet understood, in ARVD the myocyte content of the right ventricular free wall is markedly reduced, and the wall itself is replaced with a fatty or fibrofatty infiltrate. Thus, the wall is abnormally thin; is histologically abnormal; and, as a result of these two processes, functions abnormally, exhibiting focal areas of akinesia or focal or diffuse aneurysm formation. As Kayser and colleagues mention, these nonspecific changes are regional in distribution and are not diagnostic of the disease. Rather, their presence provides objective criteria for making the diagnosis of this disease. Thus, MR imaging becomes an important means of evaluating both the morphology and function of the right ventricle. The observations that must be elicited in studying a patient are difficult to obtain. The normal right ventricular free wall is only about 3 mm or so in thickness. Visual determination of abnormally thin regions or of regions of abnormal (increased) signal intensity may be limited by the spatial and contrast resolution of the MR imaging unit. However, differentiation between regions of normal systolic wall thickening and segments that fail to thicken or bulge as dyskinetic areas may provide a reliable indication of abnormal ventricular myocardium. Thus, the value of the MR imaging examination may lie not only in its wide field of view and ability to image in arbitrary section, but also in its ability to display abnormal myocardium and to provide a functional means of testing the significance of the visually abnormal segments.
One may generalize this strategy to all right and left ventricular disease, as well as to congenital heart disease. MR imaging evaluation of the heart provides not only a means of evaluating the morphology of the heart and its internal structure, but also a means of evaluating the functional sequelae of such morphologic changes. The added benefit of using MR imaging to evaluate patients with ischemic and other forms of left ventricular heart disease must be compared with the results from conventional diagnostic tools, including echocardiography; nuclear cardiac imaging; and, especially in the case of coronary imaging, conventional coronary arteriography. However, for the investigation and clinical diagnosis of right ventricular heart disease in general and in ARVD in particular, as Kayser et al point out, MR imaging techniques may provide the most sensitive, specific, and meaningful diagnostic information that we obtain in these patients and the techniques should be on the front lines in the diagnostic work-up of these patients.
Gaining experience with the use of MR imaging for cardiac diagnosis will familiarize us with the normal and pathologic appearance of the right ventricle. This will lead to new and important insights into the normal and abnormal function of the right ventricle, and will do no less than improve our ability to diagnose and manage heart disease.
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References
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Boxt LM. Radiology of the right ventricle. Radiol Clin North Am 1998; 37:379-400.
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