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Invited Commentary |
Department of Radiology, University of California, San Francisco
Valvular heart disease is an important cause of morbidity and mortality. Major causes of aortic stenosis include bicuspid valve, valve degeneration, and rheumatic heart disease. Mitral stenosis is most commonly caused by rheumatic heart disease, and tricuspid stenosis is usually the result of rheumatic heart disease or endocarditis. Pulmonary valve stenosis is usually caused by congenital fusion of the valve leaflets (1). Causes of aortic valve regurgitation include rheumatic heart disease, annuloaortic ectasia, hypertension, and myocardial infarction. Mitral regurgitation may be caused by ischemic heart disease, myocardial infarction, or endocarditis. Tricuspid and pulmonary regurgitation may be related to pulmonary arterial hypertension, endocarditis, myocardial infarction, or surgery for congenital heart disease (1).
The need for surgical intervention in valvular heart disease depends on the symptoms, the severity of regurgitation or stenosis, and the degree of ventricular impairment. The imaging evaluation of valvular heart disease relies primarily on echocardiography with Doppler and color flow mapping, which provides a noninvasive means of appraising the regurgitation or stenosis and ventricular function. However, echocardiography allows only semiquantitative assessment of valvular disease because its effectiveness depends on measurement of the size of the regurgitant or poststenotic flow jet (2). Moreover, echocardiography is subject to technical limitations such as a limited acoustic window or complex flow patterns. When echocardiographic findings are inconsistent with clinical findings, further imaging evaluation is usually required.
Cardiac scintigraphy can be used to quantify regurgitation by measuring the difference between right and left ventricular stroke volumes. However, this method can be of limited value, especially if more than one valve is regurgitant. Cardiac angiography is invasive, and the grading of regurgitation and stenosis is subjective.
In contrast, MR imaging is a noninvasive, accurate, and reproducible method of evaluating function and anatomy in valvular heart disease (3). MR imaging yields quantitative data regarding the severity of valvular regurgitation or stenosis. Unlike echocardiography, it does not depend on an acoustic window and provides a wide field of view. In patients with complex flow patterns caused by cardiac valve disease, MR imaging can depict the entire abnormal flow jet in any plane or direction. Furthermore, in patients with pulmonary valve disease, MR imaging is more accurate than echocardiography in the depiction of right ventricular enlargement and hypertrophy and in volume quantification (4,5).
There are three techniques for evaluating valve regurgitation: measurement of the area of the signal void caused by valve regurgitation or stenosis (6), calculation of regurgitant fraction by measuring right and left ventricular stroke volumes (7), and direct quantification of regurgitation with VEC MR imaging (8,9). Measurement of the signal void is semiquantitative and is subject to error depending on technical variables such as TE and display settings. Measurement of stroke volumes is not valid in patients with more than one regurgitant valve. However, in patients with a single diseased valve, stroke volume measurement and velocity-encoded cine MR imaging can both be performed and the results compared as an internal check on the accuracy of the measurements.
The two primary methods of assessing valvular stenosis are evaluation of the flow void (10) and quantification of the pressure gradient and valve area with VEC MR imaging (11,12). Measurement of the flow jet is semiquantitative and is subject to the limitations mentioned earlier. In contrast, VEC MR imaging provides accurate quantitative information regarding the severity of valvular stenosis.
One drawback of MR imaging is poor image quality in patients with arrhythmia. In addition, MR imaging does not depict cardiac valves as readily as does echocardiography. Direct visualization of the valve may be important in patients with suspected endocarditis or bicuspid valve. Contraindications to MR imaging include claustrophobia and the presence of an implanted pacemaker device.
In the preceding article , Didier et al present a comprehensive, easy-to-understand review of the use of MR imaging in the evaluation of valvular heart disease. The authors describe pertinent applications of cardiac MR imaging techniques thoroughly and accurately with interesting examples and illustrations.
Despite the strengths of MR imaging in the assessment of valvular heart disease, the actual clinical use of this technique remains limited. MR imaging of the heart has not yet achieved its potential: It is performed at relatively few centers, and the majority of patients are referred for anatomic imaging rather than functional evaluation. To secure a more visible role for cardiac MR imaging in the clinical setting, the following obstacles must be overcome:
1. Cardiac MR imaging is relatively expensive and time-consuming compared with echocardiography. With the advent of stronger gradients and faster imaging sequences, imaging time should decrease substantially. As scan times become shorter, costs should also decline.
2. Flow and volume calculations are typically performed manually and are therefore time-consuming. However, the latest software programs permit semiautomated analysis of the MR imaging data. For example, edge-detection software allows fast measurement of ventricular volumes. Nevertheless, the operator must exercise caution to ensure the accuracy of the software in defining the regions of interest.
3. Many radiologists and most cardiologists are unfamiliar with the indications and techniques for MR imaging of the heart. However, as education improves, cardiac MR imaging will become an indispensable imaging modality in the new century.
References
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