Specific Applications of Magnetic Resonance Imaging

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ACUTE MYOCARDIAL INFARCTION Infarction can be visualized following intravenous administration of a contrast agent: gadolinium-DTPA. Within the infarct, there is a central "core" of tissue within which there is microvascular obstruction, despite patency of the epicardial vessel ( Fig 57-14). Surrounding this is nonviable tissue, with yet a still larger area of myocardium that remains "stunned." Signal intensity of myocardial territory corresponding to microvascular occlusion is reduced (hypoenhanced) compared with normal myocardium immediately after contrast administration. In reperfused infarcts, myocardial image intensity is increased (.hyperenhanced).—l3 The ability to discern these changes with fidelity may play a role in the detection of myocardial infarcts and the determination of their size. Large human infarcts, associated with prolonged obstruction of the infarct-related artery, frequently have central areas of hypoenhancement during the first several minutes after myocardial infarction. Smaller, reperfused infarcts tend to demonstrate more uniform signal hyperenhancement.13

FIG. 57-14. Magnetic resonance imaging of myocardial infarction. Short-axis images of the heart show a focal area of decreased signal in the circumflex artery territory of the left ventricle. Images were acquired in a breath hold, after injection of a gadolinium contrast agent.

First-pass MRI myocardial perfusion methods may be applied to the assessment of coronary flow reserve. Coronary flow reserve is defined as the ratio of blood flow at maximal vasodilatation, divided by flow at rest. Maximal flow is measured under conditions of vasodilatation, as achieved with dipyridamole or adenosine. Typically, coronary flow reserve ranges from 3 to 5 in normal individuals. In the presence of fixed coronary stenoses, coronary flow reserve is markedly reduced. 14 Coronary flow reserve may be measured with an intracoronary Doppler flow ultrasound probe, but first-pass contrast MR has also been found to be useful for flow reserve assessment. In both animal and patient studies, a linear relationship between coronary flow reserve and first-pass MR imaging has been found This indicates that patients with microvascular dysfunction, either from epicardial vascular disease or diseases altering the microvasculature, such as hypertension or diabetes, may be assessed by using these methods.15

CHRONIC MYOCARDIAL INFARCTION This results in thinning of the left ventricular wall (remodeling) that is distinct from the normal myocardial wall thickness. These changes are better seen during systole, because of increased thickness of normal myocardium relative to the region of infarction. This finding is specific for chronic myocardial infarction on MRI.1 I7

Left ventricular aneurysms may occur following myocardial infarction and are depicted as areas of focally protruding myocardium ( Fig 57-15), with transmural thinning and associated with mural thrombus. Although distinguishing between a true aneurysm and false aneurysm (ruptured myocardium contained by pericardium) is usually straightforward by echocardiography, MR is useful in cases in which an adequate acoustic window is not obtained. Pseudoaneurysms are characterized by a relatively small neck communicating with the left ventricular cavity, whereas true aneurysms have a relatively wide neck. 18

FIG. 57-15. Magnetic resonance imaging of left ventricular aneurysm. A focal outpouching of the left ventricle is present ( arrow) during both systole and diastole. The patient had a prior myocardial infarction, with resulting thinning of the ventricular wall and aneurysm formation. (Reprinted from Bluemke and Boxerman, 19 with permission.)

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