MRI & CAD
Magnetic Resonance Imaging & the Evaluation of CAD
Magnetic resonance imaging (MRI) is another imaging technique that obtains images of the body in a non-invasive fashion. Unlike CT and nuclear techniques, however, MRI does not require ionizing radiation to produce pictures of the human body. Rather, MR works by applying a radiofrequency pulse to human tissue, which imparts energy to the tissue, and then observing the rate at which that energy leaves the tissue. The various tissues in the human body have different compositions which cause them to release the imparted energy in unique ways, which allows a computer to create a map of how this energy dissipates and thereby reconstruct images of the human body. The application of the radiofrequency pulse is completely painless and does not create tissue damage.
How is MRI used to evaluate heart disease and CAD? There are a number of techniques that can be used to assess the heart with MRI. For the evaluation of suspected CAD, perfusion MRI may be performed. This technique is similar to stress echocardiography and nuclear perfusion techniques in theory. Images are obtained at rest and then stress is applied to the heart, usually by the use of certain drugs (usually adenosine or dipyridamole), to increase the heart rate. Intravenous contrast (based on a heavy metal called gadolinium, unrelated to the iodinated contrast used for CT) is injected to create a map of blood flow to all areas of the heart. As discussed above, areas of coronary artery narrowing in the range of 60-70% or greater will not allow sufficient contrast opacified blood flow to reach heart muscle, whereas other coronary arteries allow the injected intravenous contrast material to pass, creating a difference in enhancement between affected and unaffected areas. As the heart beats, when blood flow is restricted by areas of significant CAD, these regions of heart muscle do not enhance, or "light up" as much as areas with normal blood flow, highlighting the distribution of the diseased vessel. This technique is often referred to as "rest-stress first-pass cardiac perfusion MRI" because this phenomenon is observed very quickly after the intravenous contrast is injected, just as the contrast makes its first appearance in the left side of the heart.
Recent studies suggest that rest-stress first-pass cardiac perfusion MRI has a high diagnostic accuracy for the detection of significant CAD. Rest-stress first-pass cardiac perfusion MRI has several advantages over nuclear perfusion techniques, including lack of use of ionizing radiation, fewer artifacts, and improved spatial resolution (ability to see small, but potentially important, details). The improved spatial resolution is important because MRI may allow detection of diffuse CAD in all vessels (balanced ischemia, as discussed above), and may also allow detection of earlier, milder areas of CAD which tend to only affect the inner line of the heart, so-called subendocardial ischemia.
Much like coronary CTA, MRI may directly visualize the coronary artery lumen and wall through a procedure called coronary artery magnetic resonance angiography (MRA), or coronary MRA. Because MRI and MRA techniques do not use ionizing radiation, they enjoy a major advantage over CT, nuclear perfusion techniques, and catheter coronary angiography, all of which employ ionizing radiation to create medical images. Nevertheless, coronary MRA is a rather demanding technique to perform because the heart and coronary vessels are moving and because of motion related to the diaphragm. Additionally, examining the small coronary vessels requires very high spatial resolution. For this reason, most investigators favor the use of CT if non-invasive anatomic imaging is required. However, in selected circumstances, particularly suspected anomalous coronary arteries in younger patients, coronary MRA may be quite useful (Figure 7). Other abnormalities of the coronary arteries, particularly in young patients, may be well evaluated with MRA (Figure 8).
MRI techniques are also useful for assessing patients who have already had MIs, to determine if revascularization techniques (heart surgery to restore blood flow called coronary artery bypass grafting surgery, or coronary artery stent placement) would be useful treatments for the patient's CAD. After a patient has an MI, some heart muscle is permanently damaged, but some heart muscle is only transiently damaged or is chronically not receiving enough blood to function properly. Both situations may produce reduced heart function, but the latter situation is reversible, whereas the former is not. If the patient's MI produced mostly irreversible heart muscle damage, then coronary stent placement or heart surgery to restore blood flow to areas of the heart affected by CAD will have little use. Permanently damaged heart muscle on contrast-enhanced MR appears as brightly enhancing regions (Figure 9). On the other hand, if the patient has a significant amount of living (or viable) but dysfunctional heart tissue, restoring normal blood flow to these regions of the heart will provide improved heart function. Differentiating dead heart muscle from viable but dysfunctional heart muscle is called myocardial viability imaging and may be performed with MRI (called delayed enhancement MRI) or PET.
