T2 quantification for improved detection of myocardial edema

Abstract

Background: T2-Weighted (T2W) magnetic resonance imaging (MRI) pulse sequences have been used to detect edema in patients with acute myocardial infarction and differentiate acute from chronic infarction. T2W sequences have suffered from several problems including (i) signal intensity variability caused by phased array coils, (ii) high signal from slow moving ventricular chamber blood that can mimic and mask elevated T2 in sub-endocardial myocardium, (iii) motion artifacts, and (iv) the subjective nature of T2W image interpretation. In this work we demonstrate the advantages of a quantitative T2 mapping technique to accurately and reliably detect regions of edematous myocardial tissue without the limitations of qualitative T2W imaging.

Methods: Methods of T2 mapping were evaluated on phantoms; the best of these protocols was then optimized for in vivo imaging. The optimized protocol was used to study the spatial, view-dependent, and inter-subject variability and motion sensitivity in healthy subjects. Using the insights gained from this, the utility of T2 mapping was demonstrated in a porcine model of acute myocardial infarction (AMI) and in three patients with AMI.

Results: T2-prepared SSFP demonstrated greater accuracy in estimating the T2 of phantoms than multi-echo turbo spin echo. The T2 of human myocardium was found to be 52.18 +/- 3.4 ms (range: 48.96 ms to 55.67 ms), with variability between subjects unrelated to heart rate. Unlike T2W images, T2 maps did not show any signal variation due to the variable sensitivity of phased array coils and were insensitive to cardiac motion. In the three pigs and three patients with AMI, the T2 of the infarcted region was significantly higher than that of remote myocardium.

Conclusion: Quantitative T2 mapping addresses the well-known problems associated with T2W imaging of the heart and offers the potential for increased accuracy in the detection of myocardial edema.

Figure 1

T2 mapping scheme. Three images were acquired with different T2 preparation times with a gap of 2 RR intervals to allow for sufficient T1 recovery. Seven heartbeats were required for image acquisition performed during breath-hold. Trigger delay (TD) was adjusted for each of the three images to ensure that the readout was always in the same phase of cardiac cycle. The three acquired images were processed to fit the T2 decay curve at each pixel to yield a T2 map.

Figure 2

Centric versus Linear k-space ordering in SSFP. The raw images and T2 maps acquired with centric (top row) and linear (bottom row) ordering of k-space. Note the artifact in the raw images with centric ordering which is not present with linear ordering. The T2 maps generated using centric ordering showed greater variability than those acquired using linear k-space ordering.

Figure 3

Spatial variability in 16 LV myocardial segments. The 95% CI of the adjusted T2 value (segment T2 - mean T2 in subject) across all subjects is plotted for each segment. The adjusted T2 values among LV segments reached statistical significance (p-value < 0.001) due to the higher apparent T2 measured in the apical slice.

Figure 4

Effect of surface phased array coils on T2 maps and T2W images; images acquired using anterior coils only. (A) The graphs show normalized T2 values from T2 Maps (top) and normalized intensities from T2W images (bottom), averaged for all the 14 subjects. Myocardial Segments: 1 = Anterior, 2 = Anteroseptal, 3 = Inferoseptal, 4 = Inferior, 5 = Inferolateral, 6 = Anterolateral. (B) Spatial variability in T2W images (squares) and T2 Maps (dots) acquired using only anterior coil elements expressed as coefficient of variation. The mean spatial variation was 27.7% for T2W STIR and 4.25% for T2 Maps.

Figure 5

Effect of cardiac motion on T2W images and T2 Maps. Images were acquired 400 ms after R-wave (left column) and in mid-diastole (right column). T2W imaging (top row) showed sensitivity to cardiac through-plane motion with a near complete myocardial signal dropout. T2 map (bottom row) showed consistent results in both cardiac phases and demonstrated insensitivity to cardiac motion. Note that these images are from a more mobile basal slice and the images in left column were deliberately acquired in a highly dynamic cardiac phase to demonstrate insensitivity of T2 Maps to motion.

Figure 6

Animal study results. T2 maps from three pigs are shown in the top row and LGE images from two of the three pigs in the bottom row. Note that the LGE image for pig #2 was acquired in a slightly more basal slice position than the T2 map, but nevertheless demonstrates the expected myocardial infarction in the LAD territory. Pig # 3 died before the LGE image could be acquired, but 90 minutes of LAD occlusion virtually assures myocardial infarction.

Figure 7

SAX and VLA images from patient # 1. Coronary angiography confirmed >90% occlusion of RCA. STIR images (1st column) did not show hyperintensity in inferior regions. In addition, the VLA images show high signal from static endocardial blood. Similarly, the infarcted region could not be detected in T2p-SSFP image with T2p = 55 ms (column 2). T2 maps (column 3) demonstrate increased T2 in the inferior regions (see Table 5) and the endocardial borders are clearly demarcated. LGE images (column 4) show the location and extent of the infarct. S = Signal Intensity.