| Anthon R Fuisz, MD Gerald M Pohost, MD |
Mar 29, 2000 |
Magnetic resonance imaging (MRI) is the newest and most complex of the cardiovascular imaging modalities. Although MRI has been used for more than a decade to image the brain and other stationary organs within the body, only recently has it become widely available as a cardiovascular diagnostic approach. This has been possible because of more sophisticated cardiac gating and the commercial development of echo-planar imaging which provides cross sectional images of the heart within 50 msec per tomographic cut, leading to the term "real-time MRI."
There are many other techniques embodied in an MRI system, two of which are the mainstays of clinical cardiovascular MRI.
• Spin echo imaging ¡ª Spin echo imaging depicts the tissue structures of the heart as bright and the blood pool as dark (black blood approach). The spin echo method is predominantly used for assessing myocardium, including myocardial mass and zones of acute infarction, and for identifying the fatty infiltration seen in arrhythmogenic right ventricular dysplasia (ARVD) [1]. (See "Arrhythmogenic right ventricular dysplasia").
• Gradient echo or cin¨¦ MRI ¡ª Cin¨¦ MRI generates images of the heart in which the blood pool appears as bright and cardiac tissue structure appears dark (bright blood approach). Cin¨¦ MRI is used to evaluate left and right ventricular function and size, and valvular and shunt lesions, and to detect intracardiac masses.
Other MRI techniques are useful for quantifying the severity of valvular lesions, shunt size, and the severity of arterial vascular stenosis (phase velocity mapping) [2], and for precise assessment of myocardial dynamics (radiofrequency or RF tagging) [3]. Looking into the future, MRI methods are being developed to assess regional myocardial perfusion and to evaluate myocardial viability.
CLINICAL APPLICATIONS ¡ª MRI uses high intensity magnetic fields and radiofrequency to generate 3D/tomographic images with high resolution and excellent contrast. The blood's motion through the magnetic field can generate signal, obviating the need for contrast agents. However, contrast agents are presently under investigation that may be useful in the evolving application of perfusion imaging. The strengths and weaknesses of these approaches are summarized in Table 1 (show table 1).
Aortic disease ¡ª The diagnosis and assessment of diseases of the aorta is an area in which MRI is preeminent. Spin-echo and cin¨¦ MRI can precisely define the dimensions and extent of complex aortic aneurysms, false aneurysms, dissection flaps, periaortic abscesses (eg, with endocarditis), aortic arch abnormalities, and coarctation. It can also be used to monitor patients with thoracic and abdominal aortic aneurysms and, in patients with homozygous familial hypercholesterolemia, spin MRI can detect atherosclerotic plaque and supravalvular aortic stenosis [4]. High resolution MRI is highly accurate for measurement of arterial wall area and has the potential for monitoring atherosclerotic plaque size [5].
MRI has a major role in the diagnosis, characterization, and determination of the extent of aortic dissection (show radiograph 1).
It can provide information about the differential flow velocity in the true and false lumens, and can identify involvement of branch arteries anywhere from coronaries to the femoral arteries. Because of the inherent three dimensional nature of the technique, the images of the aorta and its pathology can be reconstructed, usually in a matter of minutes, into 3D display of the aorta and affected areas, thereby facilitating planning for surgical repair. (See "Clinical manifestations and diagnosis of aortic dissection").
One limitation to the use of MRI is that patients who are hemodynamically unstable or actively rupturing are not good candidates because of the time of acquisition, the difficulty with access to the patient while in the magnet bore, and distortion of the electrocardiogram by the magnetic field, thereby making monitoring more difficult. The newest MRI systems, optimized for cardiac and vascular imaging, can acquire a three dimensional representation of the entire thoracic aorta in a 90 second study.
Ischemic heart disease ¡ª Magnetic resonance imaging can be useful in the assessment patients with possible or definite ischemic heart disease. Since MRI provides a dimensionally accurate, three-dimensional perspective of the heart, it has been considered by many to be the gold standard for measurements of left and right ventricular ejection fraction, volumes, and myocardial mass. It can also detect regional wall motion abnormalities. In addition, magnetic resonance spectroscopy can be used to detect high energy phosphate abnormalities in ischemic tissue [6], and ischemic territories can be identified by using intravenous gadolinium for a first-pass perfusion study; in one study a myocardial perfusion reserve after dipyridamole of
1.5 had a sensitivity, specificity and diagnostic accuracy of 90, 83, and 87
percent, respectively, for the detection of a coronary artery stenosis [7].Cardiac function has also been evaluated before and during the infusion of dobutamine, allowing one to visualize segments that become ischemic (ie, demonstrate reduced motion) during stress. While this latter technique should be considered experimental, it does provide a high resolution alternative for the diagnosis of coronary disease and for quantification of "myocardium at risk"; it may be particularly useful for patients in whom adequate echocardiograms cannot be obtained [8]. One study compared dobutamine stress echocardiography with dobutamine stress MRI in 208 patients with suspected coronary disease who underwent angiography. Compared to dobutamine echocardiography, dobutamine MRI increased the sensitivity from 74 to 86 percent and specificity from 70 to 86 percent; the results were similar in men and women (show figure 1) [9].
(See "Stress echocardiography in the diagnosis of coronary heart disease").
Dobutamine MRI may also be a predictor of left ventricular functional recovery after coronary revascularization [10]. (See "Evaluation of hibernating myocardium", see "Dobutamine echocardiography in the evaluation of hibernating myocardium", and see "Assessment of myocardial viability by nuclear imaging in coronary heart disease-I").
Coronary angiography ¡ª The production of diagnostic quality angiograms of the coronary arteries with MRI (MR angiography [MRA]) is another area of rapid development and will likely become clinically important in the near future [11]. However, this technique is limited by cardiac and respiratory motion, the need for submillimeter spatial resolution, and suppression of signals from adjacent epicardial fat and muscle (improved contrast-to-noise ratio). One study found that T2-weighted, three-dimensional coronary MRA with a shorter acquisition window (60 ms) improved contrast-to-noise ratio and allowed for adequate assessment of the coronary arteries during free breathing [12]. (See "Principles of magnetic resonance imaging").
Despite technical advances, the clinical role of coronary MRA for the identification of native vessel disease remains to be defined by multicenter trials; the use of coronary MRA for the identification or characterization of anomalous coronary arteries appears to have a clinical role [13,14].
MRI can also be used to evaluate bypass graft patency [15,16,17] and to measure flow velocity in bypass grafts and native left anterior descending arteries using phase velocity mapping [18,19]. Preliminary animal data suggest that MRI may have a role in documenting arterial remodeling in native atherosclerosis and after percutaneous interventions [20].
After myocardial infarction ¡ª Assessment of ejection fraction is an important prognostic indicator after acute myocardial infarction. (See "Risk stratification for arrhythmic and nonarrhythmic death after acute myocardial infarction"). As with radionuclide studies, MRI with its three dimensional perspective permits the calculation of left or right ventricular volumes without geometric assumptions which may lead to inaccuracies (especially in focally deformed ventricles) [21]. In addition, MRI can visualize the heart with high resolution and can detect and quantify the many potential complications of myocardial infarction. These include:
• Mitral regurgitation and ventricular septal defect (see "Mechanical complications of acute myocardial infarction")
• Thrombus (see "Echocardiography in detection of intracardiac sources of embolism")
• Left ventricular aneurysm and pseudoaneurysm (see "Left ventricular aneurysm and pseudoaneurysm following acute myocardial infarction")
• Pericardial effusion (see "Pericardial disease associated with myocardial infarction")
It is also possible to characterize myocardial tissue as recently infarcted by visualizing a myocardial territory with increased intensity on spin-echo images [22,23].
Contrast-enhanced MRI might establish the presence of microvascular obstruction, defined as hypoenhancement seen one to two minutes after contrast injection, which corresponds to greater myocardial damage. The extent of microvascular obstruction and infarct size increase significantly over the first 48 hours after a myocardial infarction [24]. The presence of microvascular obstruction after a myocardial infarction appears to have prognostic importance and to be associated with fibrous scar formation and left ventricular remodeling.
One study of 44 patients reported that patients with MRI-determined microvascular obstruction had more frequent cardiovascular events during a 16 month follow-up (45 versus 9 percent for those without this abnormality) [25]. The clinical utility of this information remains to be defined in larger series.
Myocardial viability ¡ª MRI myocardial tagging is a noninvasive method that quantifies local myocardial segment shortening throughout the left ventricular myocardium at sites across the left ventricular wall thickness [3]. When combined with low dose dobutamine, this method can quantify the amount of myocardial viability after an acute myocardial infarction and may provide prognostic information. In one study of 20 patients with a first reperfused myocardial infarction, a normal increase in shortening within the midwall and subepicardium, but not the subendocardium, during a low-dose dobutamine infusion predicted greater functional recovery eight weeks after the infarction [26]. A second report compared dobutamine MRI with thallium and tetrofosmin SPECT in 30 patients [27]. The sensitivity and specificity of dobutamine MRI was 50 and 81 percent, respectively; SPECT imaging with thallium or tetrofosmin had higher sensitivity (76 and 66 percent, respectively), but lower specificity (44 and 49 percent, respectively).
Myocardial viability can also be established with contrast MRI using gadolinium diethylenetriamine pentaacetic acid, a paramagnetic contrast agent. One animal study found that, independent of wall motion or infarct age, the size and shape of regions exhibiting delayed hyperenhancement are identical to regions of myocardial necrosis and irreversible injury; regions that fail to hyperenhance are viable [28]. In one series of 24 patients with stable coronary artery disease and left ventricular dysfunction, delayed (three to 15 minute) hyperenhancement of contrast-enhanced MRI images were associated with nonviability as established with rest-distribution thallium imaging and dobutamine echocardiography; the absence of enhancement correlated with radionuclide and echocardiographic viability, regardless of the status of resting contractile function (show figure 5) [29].
This is in contrast to what has been seen in the post infarction patient, as discussed above. (See "Evaluation of hibernating myocardium" , see "Dobutamine echocardiography in the evaluation of hibernating myocardium", and see "Assessment of myocardial viability by nuclear imaging in coronary heart disease-I").
Acute myocarditis ¡ª Preliminary data from 44 patients suggest that MRI may also be helpful in "timing" of acute myocarditis [30]. Using Gd-DTPA contrast, MRI demonstrated focal enhancement initially, which then became diffuse within the first two weeks after disease onset. The extent of relative myocardial enhancement correlated with clinical status and left ventricular function. (See "Clinical manifestations and diagnosis of myocarditis").
Cardiomyopathy ¡ª The strength of MRI to assess ventricular volumes, ejection fraction, and myocardial mass and wall thickness provides a means for evaluation of patients with heart failure and for the diagnostic evaluation of cardiomyopathy.
• Dilated cardiomyopathy is readily diagnosed. In addition, high resolution evaluation of regional myocardial function provides a means to differentiate dilated cardiomyopathy from the cardiomyopathy of coronary artery disease.
• The asymmetric thickening of the interventricular septum in hypertrophic cardiomyopathy can be detected and MRI may be of value in the assessment of variant types of hypertrophic cardiomyopathy [31,32]. It is also possible to visualize the turbulence in the left ventricular outflow tract created by the dynamic obstruction in obstructive hypertrophic cardiomyopathy. (See "Evaluation of obstructive hypertrophic cardiomyopathy").
• The cardiomyopathy associated with hemochromatosis produces a diagnostic pattern in which the myocardium and liver have very low signal intensity due to the deposition of iron. (See "Pathophysiology and diagnosis of iron overload syndromes").
• Magnetic resonance imaging is also an excellent approach to differentiate between restrictive and constrictive disease [33]. (See "Hemodynamics in constrictive and effusive constrictive pericarditis versus restrictive cardiomyopathy"). As an example, the infiltrated myocardium in restrictive cardiomyopathy (eg, due to amyloidosis) is easily assessed and the associated atrial enlargement defined [34]; there is a homogeneous increased thickness of ventricular and atrial walls, including the interatrial septum and right atrial free wall. Additionally there is a significant decrease in signal intensity ratio of myocardial and skeletal muscle (show figure 7).
These abnormalities are not seen in patients with an idiopathic restrictive cardiomyopathy (show figure 8) [35].
(See "Amyloid cardiomyopathy"). (See "Amyloid cardiomyopathy").
Pericardial disease ¡ª Gated MRI provides direct visualization of the normal pericardium, which is composed of fibrous tissue and has a low MRI signal intensity [36]. The normal pericardium measures
3
mm in thickness. Although pericardial fluid also has a low signal intensity,
it can be distinguished from pericardium with cin¨¦ MRI images on which it has
a bright signal in contrast to the dark line from the pericardium.MRI can be used for the detection of pericardial disease [36,37]. It can diagnose congenital absence of the pericardium and pericardial thickening (constrictive pericarditis). In addition, the presence of pericardial effusion can be established and the distinction made between a hemorrhagic and nonhemorrhagic effusion.
Valvular heart disease ¡ª The turbulence created by valvular stenosis and regurgitation is easily identified on gradient echo (cin¨¦) magnetic resonance images. As noted above, the blood pool appears bright with this imaging technique. When turbulent blood enters the blood pool, it causes loss of signal, an effect not generally seen with normal valves. This effect allows qualitative assessment of valvular lesions [38,39]. However, changes in the acquisition parameters can markedly alter this effect, so that qualitative assessment of valvular lesions must be done with caution and by those familiar with parameters involved. In addition, measurement of the ejection fraction and ventricular volumes can help to determine the timing of valvular surgery.
Regurgitant valvular lesions produce a zone of proximal isovelocity surface area (PISA) on the side of the valve opposite from the direction of regurgitant flow with both MRI and echocardiography [40]. The amount of PISA is directly related to the amount of regurgitation. Phase velocity mapping is a newer method that allows quantification of the amount of regurgitation or the severity of stenosis by depicting velocity, similar to Doppler echocardiography [41].
The valvular regurgitation caused by endocarditis is well seen with gradient echo images, but the vegetations are only occasionally visualized with conventional imaging sequences. New faster pulse sequences may make it possible to detect vegetations [42].
Peripheral vascular disease ¡ª The ability to generate images of blood vessels without giving contrast media coupled with the capability of producing high resolution three dimensional images gives MRI a particular advantage in assessing peripheral vascular disease. Carotid arteries can be imaged along their entire length, identifying stenoses and resultant turbulent flow. The iliac and femoral arteries can also be readily imaged [43]. These images permit precise planning for peripheral interventions, without exposure to contrast medium or the post-procedure recovery associated with arterial puncture [11]. (See "Noninvasive diagnosis of peripheral vascular disease").
Other uses ¡ª MRI has many other uses that are clinically important. Patients with complex congenital heart disease can be studied accurately and noninvasively with spin-echo and gradient echo MRI [44,45]. In addition, tissue characterization can give important diagnostic information in patients with primary and metastatic cardiac tumors [46,47].