Cardiac magnetic resonance imaging
Many noninvasive methods exist for cardiac imaging, such as echocardiography and magnetic resonance imaging (MRI), but only cardiac MR (CMR) can provide high-resolution volumetric imaging of the heart in correct three-dimensional spatial and temporal resolution without ionizing radiation or nephrotoxic contrast. This enables CMR to furnish a detailed interpretation of cardiac function and anatomy. Furthermore, by using MRI contrast agents, CMR can provide valuable information about regional myocardial perfusion and myocardial viability. This viability technique, known as delayed enhancement (DE) imaging, is able to distinguish reversible myocardial dysfunction from acute or chronic myocardial infarction.1-6 Hence, by combining these features, CMR can provide valuable information of unparalleled quality that cannot be provided by any other single imaging method.
CMR images are created using a variety of signal acquisition strategies or “sequences.” Volumetric imaging of cardiac structure and function, including regional myocardial function, is provided by cine imaging using an electrocardiogram-gated steady-state free precession (SSFP) sequence. Compared with traditional two-dimensional imaging, CMR is able to provide a superior, more reliable interpretation of ventricular anatomy and function. The accuracy and reproducibility of measurements, as well as the spatial and temporal resolution exhibited by CMR, have led to its currently being considered the gold standard for quantification of ventricular volume, function, and mass.7-11
Evaluation of left ventricular function and mass provides essential data for continued management of the cardiac patient. Clinical implications range from medication choices to the placement of automated internal cardiac defibrillators. These measurements, therefore, require accurate, reliable, and reproducible values. Although echocardiography is accepted as the initial test of choice for this evaluation, CMR is now considered the preferred method. Furthermore, left ventricular mass is known to be an important marker of subsequent cardiovascular morbidity and mortality.7-15 Compared with left ventricular mass evaluation by CMR, two-dimensional echocardiography is relatively unreliable, partly because of interobserver variation.7 These attributes, provided by a single study, along with limited patient and operator interference, make CMR a desirable alternative to other imaging methods.
The precision and clarity of the images generated by CMR have also led to its firm establishment as a method for evaluation of various cardiac disorders, such as congenital heart disease and cardiac masses, as well as aortic and pericardial diseases.14 Congenital heart disease has been particularly revolutionized by the use of CMR. CMR imaging relies on two separate techniques. The morphologic evaluation is completed by spin-echo and MR angiographic techniques, whereas the functional and flow information is provided by the previously described fast cine-SSFP and velocity-encoded sequences.16-20 Partly because of this technology, CMR appears to be widely accepted as the imaging method of choice for evaluating anomalies involving the aorta and central pulmonary arteries and for postoperative follow-up of patients with congenital heart disease. The combination of such specific functional and morphologic assessment generates an unrivaled description of complicated anatomy that is essential for successful surgical correction.16-20
Another well-established area of CMR imaging excellence involves the investigation of cardiac masses. Until recently, the evaluation of a cardiac mass relied predominantly on echocardiography with subsequent biopsy. In addition to the aforementioned accuracy of spatial and temporal detail, using contrast techniques as well as noncontrast signal intensity and anatomical-topographical features allows CMR to distinguish the presence of perfusion, infarction, inflammation, and fibrosis within masses. Through this combination of detail, CMR can be used to discriminate thrombi, myxomas, and malignant tumors.21-27 It can therefore provide an accurate noninvasive method of evaluation and identification of a large variety of cardiac masses with less risk to the patient.
The combination of volumetric and flow velocity imaging has broadened the utility of CMR to encompass valvular imaging and quantitation of the severity of valvular stenosis or regurgitation. By using CMR, valvulopathy can be assessed with such exactitude that some authors suggest that there is less need for invasive evaluation.14 Although echocardiography remains the currently accepted initial mode of evaluation for valvular disease, CMR has much to offer selected patients. For example, patients who are being assessed for possible surgical intervention require exact valvular measurements, as well as accurate estimation of overall cardiac function. Although this can be provided by echocardiography, the windows can often be limited by concomitant valvulopathies, which can lead to overestimation or underestimation of the severity of the valve disease, and inaccurate ejection fractions and body habitus interference, which can lead to poor imaging.28-30
CMR, however, can provide measurements of peak velocity and flow that are at least as accurate as echocardiography. Additionally, the severity of val-vular disease can be measured through several different methods using CMR, which improves the accuracy of the findings. One of the techniques includes the use of cine gradient echo MR imaging, which allows measurement of the area of the signal void corresponding to the abnormal flow jet. Alternatively, this method can be used to obtain ventricular volumetric measurements, which are then used to calculate the regurgitant fraction. A third method involves the use of velocity-encoded cine MR imaging, which actually quantifies regurgitant blood flow.
There are also several methods available to evaluate stenoses, including flow jet evaluation and calculation of transvalvular pressure gradients and valve area, again attainable by a single imaging study.28,30-32 These imaging options also allow measurements to be adjusted for existing valvulopathies. This means that errors associated with estimation are reduced with CMR while extremely accurate and reproducible values are provided. With respect to limitations resulting from body habitus, CMR provides unequalled imaging, in part, because of negligible interference from surrounding tissue. These values are often critical factors in determining when surgical intervention is indicated.
Another recent advancement in CMR is the ability to distinguish areas of infarction from ischemia. Infarct imaging is performed using an inversion recovery turboflash sequence. To distinguish viable myocardium, the contrast agent gadolinium chelate (Gd) is required with subsequent DE imaging. The Gd migrates into the cardiac cells, and the rate of washout from the cells can determine functionality. Cells with expanded extracellular space, such as those seen in areas of infarction, have a slower washout of Gd compared with areas of reversible ischemia. Thus, these areas of DE can be distinguished as nonviable myocardium, presumably secondary to acute infarction and necrosis or old scarring.1,4,33,34 In fact, as the transmural extent of DE increases, the prognosis for functional recovery of that myocardial segment worsens.1,4,33 Because of the precise spatial detail and established accuracy of DE imaging, many investigators have confirmed the utility of CMR DE imaging as the gold standard for detection of infarcts and quantitation of infarct size and shape.
The diagnosis of arrhythmogenic right ventricular dysplasia/cardiomyopathy (so-called ARVD/C), is another useful application of CMR. This electrophysiologic abnormality is characterized by fibro-fatty replacement of the right ventricle, leading to arrhythmias, right ventricular failure, and sudden death.35,36 Until recently, analysis of these fibro-fatty changes required a myocardial biopsy for diagnosis. Noninvasive detection by CMR has an excellent correlation with histopathology and can predict inducible ventricular tachycardia. Because of this specificity, cardiac MR imaging is developing into the method of choice for the evaluation of patients who are suspected of having ARVD/C.35,36
Conclusion
Although limitations of CMR exist, such as certain physical limitations, availability, and patient tolerance, the many applications for the use of CMR are still being determined. With improving cost effectiveness and more advanced techniques, it is likely that CMR imaging will become a routine procedure for the evaluation of multiple areas of heart disease.
