Toward necrotic cell fraction measurement by contrast-enhanced MRI of reperfused ischemically injured myocardium

Delayed contrast enhancement on MR images of myocardium: past, present, future

Differential enhancement of myocardial infarction was first recognized on computed tomographic (CT) images obtained with iodinated contrast material in the late 1970s. Gadolinium enhancement of myocardial infarction was initially reported for T1-weighted magnetic resonance (MR) imaging in 1984. The introduction of an inversion-recovery gradient-echo MR sequence for accentuation of the contrast between normal and necrotic myocardium was the impetus for widespread clinical use for demonstrating the extent of myocardial infarction. This sequence has been called delayed-enhancement MR and MR viability imaging. The physiologic basis for differential enhancement of myocardial necrosis is the greater distribution volume of injured myocardium compared with that of normal myocardium. It is now recognized that delayed enhancement occurs in both acute and chronic (scar) infarctions and in an array of other myocardial processes that cause myocardial necrosis, infiltration, or fibrosis. These include myocarditis, hypertrophic cardiomyopathy, amyloidosis, sarcoidosis, and other myocardial conditions. In several of these diseases, the presence and extent of delayed enhancement has prognostic implications. Future applications of delayed enhancement with development of MR imaging and CT techniques will be discussed.

The influence of myocardial blood flow and volume of distribution on late Gd-DTPA kinetics in ischemic heart failure

PURPOSE

To determine the mechanism of enhancement of contrast-enhanced MRI (ceMRI) in chronic ischemic myocardium. While ceMRI can identify scar tissue in chronic ischemic myocardium, the mechanism of enhancement is not completely understood.

MATERIALS AND METHODS

A total of 11 patients with ischemic heart failure (ejection fraction [EF] 28 +/- 9%) were imaged with ceMRI and positron emission tomography (PET) to measure myocardial blood flow (MBF). Longitudinal relaxation rate (T1) of blood, normal tissue, and scar tissue defined by ceMRI was determined before and two to 50 minutes after contrast (Look Locker technique), and the partition coefficient (lambda) and volume of distribution (VD) were calculated.

RESULTS

In scar and viable tissue, T1 was significantly different over the whole period after contrast, but not before contrast. However, T1 of scar and blood were similar five to 15 minutes post contrast, making the detection of subendocardial defects difficult. lambda reached an initial steady state in viable tissue, but was delayed (20 minutes) in scar tissue. VD in scar was double that of viable tissue (0.54 +/- 0.01 vs. 0.29 +/- 0.02, respectively) indicating an increased interstitial space. Contrast wash-in kinetics correlated moderately with MBF (r = -0.36), but well with the combination of MBF and VD (r = 0.59).

CONCLUSION

Late myocardial contrast kinetics depend on both MBF and VD; however the increased VD seems to be the main mechanism for the late enhancement effect.

MR contrast media for myocardial viability, microvascular integrity and perfusion

Cardiovascular imaging requires an appreciation of rapidly evolving MR imaging sequences as well as careful utilization of intravascular, extracellular and intracellular MR contrast media. At the present time, clinical studies are restricted to the use of extracellular MR contrast media. MR imaging has the potential to noninvasively measure multiple parameters of the cardiovascular system in a single imaging session. Recent advances in fast and ultrafast MR imaging have considerably enhanced the capability of this technique, beyond the assessment of left ventricular wall motion and morphology into visualization of the coronary arteries and measurement of blood flow. During the course of the last several years, multiple strategies for imaging viable myocardium have been developed and validated using MR contrast media. Contrast enhanced dynamic MR imaging provides information regarding microvascular integrity and perfusion. Because these information can be provided noninvasively by MR imaging, repeated measurements can be performed in longitudinal studies to monitor the progression or regression of myocardial injury. Similar studies are needed to examine the effects of newly developed cardioprotective therapeutics. Development of suitable intravascular MR contrast medium may be essential for visualization of the coronary arteries and interventional therapies. MR imaging may emerge as one-stop-shop for evaluating the heart and coronary system. This capability will make MR imaging cost-effective in the first decade of this millennium.

Reperfused rat myocardium subjected to various durations of ischemia: estimation of the distribution volume of contrast material with echo-planar MR imaging

PURPOSE

To estimate and compare the fractional distribution volume (fDV) of gadodiamide injection and technetium 99m-diethylenetriaminepentaacetic acid (DTPA) in the reperfused myocardium of rat hearts subjected to various durations of ischemia.

MATERIALS AND METHODS

Magnetic resonance (MR) imaging and autoradiography were performed in rats subjected to 20, 30, 40, or 60 minutes of regional ischemia followed by 1 hour of reperfusion. The fDVs of gadodiamide injection and (99m)Tc-DTPA were measured and compared by using inversion-recovery echo-planar imaging and autoradiographic phosphor imaging, respectively.

RESULTS

The mean fDV of both tracers (gadodiamide and (99m)Tc-DTPA) in normal myocardium was 18% +/- 1, whereas that in the entire area at risk increased significantly (P <.05) with 20, 30, 40, and 60 minutes of ischemia to 32% +/- 1, 57% +/- 4, 66% +/- 2, and 68% +/- 2, respectively. The fDV was significantly (P <.05) greater in the core of infarction-78% +/- 4, 89% +/- 5, and 88% +/- 5 with 30, 40, and 60 minutes of ischemia, respectively-than in the normal myocardium or in the area at risk.

CONCLUSION

The fDV of MR contrast material in the periinfarcted rim was significantly (P <. 05) greater than that in the normal myocardium, but significantly less than that in the core of infarcted myocardium.

Reperfused myocardial infarction as seen with use of necrosis-specific versus standard extracellular MR contrast media in rats

PURPOSE

To measure the difference in size of reperfused myocardial infarction with necrosis-specific (bis-gadolinium-mesoporphyrin [hereafter, mesoporphyrin]) and standard extracellular (gadopentetate dimeglumine) magnetic resonance (MR) contrast media.

MATERIALS AND METHODS

Echo-planar (for T1 measurement) and spin-echo (for infarction size) MR imaging were conducted in 32 rats subjected to reperfused reversible (n = 16) and irreversible (n = 16) myocardial injuries. All animals received gadopentetate dimeglumine 1 hour after reperfusion and underwent imaging. Sixteen rats received mesoporphyrin at 2 hours, the other 16 rats received gadopentetate dimeglumine at 24 hours, and all animals underwent imaging at 24 hours.

RESULTS

Mesoporphyrin produced prolonged (22 hours) reduction in T1 in irreversibly, but not in reversibly, injured myocardium. The size of the mesoporphyrin-enhanced region (37% +/- 4 [SEM] of left ventricular surface area) closely correlated with the true infarction size as measured by means of histomorphometry (36% +/- 3, r = 0.90). The size of the gadolinium-enhanced region overestimated (48% +/- 2 and 43% +/- 1 at 1 and 24 hours of reperfusion, respectively) the size of true infarction (36% +/- 3, P < .05, r = 0.02), but it was close to the size of the area at risk (r = 0.93).

CONCLUSION

The sizes of hyperenhanced regions displayed by using mesoporphyrin and gadopentetate dimeglumine differed from each other. The difference in size of the hyperenhanced region demarcated by mesoporphyrin and gadopentetate dimeglumine may provide an estimation of potentially salvageable myocardium.