Normalized apparent diffusion coefficients are more accurate in differentiating between viable and necrotic tumor than are T2 relaxation times or relative signal intensity increases on contrast-enhanced images. Signal intensity overlap between viable and necrotic tumor on gadolinium-enhanced images may be caused by the small molecular size of the agent, which permeates the interstitial space freely, thereby also enhancing necrosis. Diffusion-weighted MR imaging depicts differences in diffusion and, ultimately, in membrane integrity between viable and necrotic tumor and may be used to monitor tumor viability during treatment.
Rapid echo-planar (EP) magnetic resonance (MR) imaging was used to monitor the first pass of a bolus of gadodiamide injection in the hearts of normal rats and rats subjected to left coronary artery occlusion. Inversion-recovery EP imaging combined with a low dose (0.05 mmol/kg) of the contrast agent caused signal enhancement of normal myocardium from 19% +/- 4 to 63% +/- 5 (mean +/- 1 standard error of the mean) of fully relaxed intensity at the peak of the bolus but only slight increase in signal intensity of the ischemic zone. Thus, ischemic myocardium was demarcated as a hypointense zone (cold spot) during passage of the bolus. A higher dose (0.20 mmol/kg) of the same agent caused signal loss of normal myocardium from 100% to 39% +/- 7 of control at the peak of the bolus on gradient-recalled echo EP images, and ischemic myocardium was visualized as a hyperintense zone (hot spot). With either method of monitoring bolus transit, myocardial signal intensity recovered slowly following the peak bolus effect, consistent with substantial extraction of the agent during the first pass through the heart. Use of gadodiamide injection can allow discrimination between ischemic and nonischemic myocardium on both T1- and susceptibility-weighted EP images during bolus transit.
The purpose of this study was (1) to monitor the dynamic effects of T1-enhancing and magnetic susceptibility contrast material on normal canine myocardium using inversion recovery (IR)- and driven equilibrium (DE)-prepared fast gradient-recalled echo (GRE) sequences and (2) to determine the relative value of T1-enhancing and magnetic susceptibility contrast material in detecting regions of ischemia in the same animal. Normal dogs (n = 5) and dogs with acute occlusion of the left anterior descending (LAD) coronary artery (n = 11) were studied using a 1.5-T MR imager. ECG-gated fast IR-prepared GRE images were acquired using TI/TR/TE of 700/7.0/2.9 msec and a flip angle of 7 degrees. Fast DE-prepared GRE images were obtained using a flip angle of 12 degrees and a DE delay/TR/TE of 60/10.2/4.2 msec. Sequential images were acquired to monitor transit of 0.05 mmol/kg gadodiamide injection and 0.2 and 0.4 mmol/kg sprodiamide injection. On slice-nonselective IR fast GRE images, gadodiamide caused significant enhancement of the normal myocardium and the left ventricular (LV) chamber blood. In dogs with LAD occlusion, the ischemic region was defined as an area of low signal intensity (SI). On DE-prepared GRE sequences, administration of sprodiamide resulted in a substantial decrease in signal from normal myocardium and LV chamber blood in normal dogs. In animals subjected to LAD occlusion, this contrast medium produced a transient decrease in SI from normal myocardium (P < .05) and no significant change in SI from ischemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)
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