Contrast-Enhanced Cardiac MRI

Contrast-enhanced cardiac MRI requires the administration of IV GBCA. As discussed previously, clinicians considering a study that requires the administration of GBCA need to carefully weight the potential risks and benefits in certain patient groups, including those who are pregnant, those who have exhibited a previous sensitivity to contrast agents, and those with advanced kidney disease.3 Contrast-enhanced MRI is particularly useful for obtaining high-resolution 3-dimensional (3D) MR angiography studies to evaluate the great vessels, cardiac perfusion imaging studies, and cardiac viability sequences. Technologists should note that the FDA has approved IV GBCA for contrast studies at doses of 0.1 to .0.2 mmol/kg (20–40 mL).

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Perfusion cardiac MRI is a particular application of contrast-enhanced techniques that can overcome limitations of traditional imaging studies. Although nuclear medicine scans with thallium or sestamibi imaging agents are the most routine techniques to evaluate patients for suspected myocardial ischemia, there are important limitations to these scans. The clinical value of nuclear medicine scans can be constrained by limited spatial resolution, difficulties in imaging certain areas of the cardiac anatomy, and a lack of scan penetration in obese patients. Meanwhile, perfusion studies with cardiac MRI are not limited by these factors and can be used in patients who present with challenges to effective imaging.

First pass or near-real time perfusion imaging, with the rapid administration of MRI contrast agent, can be used to effectively evaluate the adequacy of blood delivery to the myocardial tissue based on patterns of tissue enhancement. This technique requires a 6- to 10-mL bolus injection of MRI contrast, at a rate of 3 to 5 mL per second. Studies demonstrating a low perfusion of myocardial tissue are consistent with a myocardial infarct resulting in ischemic or necrotic tissue.

Meanwhile, delayed imaging or viability techniques are also useful for the detection of necrotic myocardial tissue. This technique is particularly useful to differentiate between ischemic tissue versus non-viable myocardium. Delayed imaging is obtained between 15 and 30 minutes after the injection of IV GBCA (to incorporate a washout phase), using black blood spin-echo/turbo spin-echo or double inversion-recovery techniques. Time for inversion (TI) is used to suppress the appearance of normal myocardium. To obtain high TI tissue contrast, effective TI times used are between 175 to 250 msec during the washout phase. The normal myocardium signal is suppressed or nulled by choosing the correct TI time. Necrotic or infarcted myocardium will appear white on these delayed images, whereas normal myocardium will appear dark.7

Contrast-enhanced cardiac MRI perfusion studies are particularly important in clinical practice, because having accurate viability information allows the cardiologist to determine whether a patient who has suffered from an MI is likely to benefit from a therapeutic revascularization procedure.