With the MCODE method, both T1
- and T2
-weighted images are acquired within the same breath-hold, and may be displayed either side-by-side in separate windows (as in the figures) or in the same window, alternating between images. The latter display method (i.e., flickering between T1
- and T2
-weighted images) was found to be an effective means of detecting the MI because the images are already registered and the endocardial border becomes readily apparent. Another approach () that takes advantage of the inherent image registration is to trace the endo- and epicardial borders on the T2
-weighted image and then copy these contours onto the T1
-weighted image (TrueFISP example). These contours may also be used for segmentation in conjunction with objective computer-based infarct sizing (21
). The T2
-weighted image may be amenable to automatic segmentation, which would provide greater automation.
Figure 9 Approaches for visualizing MI. a: Illustration of copying endo- and epicardial contours from a T2-weighted image to a T1-weighted image. b: Illustration of merging T1-and T2-weighted images to create a single image with enhanced blood-to-MI contrast using (more ...)
It may be possible to merge the images into a single image with enhanced contrast, but there are also pitfalls with this approach. A simple approach is to divide the T1-weighted image by the T2-weighted image. This has the effect of increasing the signal intensity of the MI proportional to the blood (). However, when the MI signal intensity is below the blood pool intensity, as in Fig. or , the division may actually increase the MI level such that it is closer to that of the blood, thereby reducing the net contrast. Furthermore, any misregistration will lead to artifacts with this approach. More sophisticated schemes, such as PCA, might be appropriate because the three tissue species appear well clustered in the scatter plot. PCA was tried on a couple of cases, but the overall image quality was reduced (e.g., the blood pool was somewhat mottled). Since the merging of both images into a single image entails a loss of information, we feel that it is preferable to display both images. This is an open topic for further investigation.
The multicontrast method was implemented with single-shot or segmented (results not shown) TrueFISP, as well as segmented TurboFLASH sequences. The large number of variables, such as flip angles, preparation times, matrix size, segmentation, and others, leads to a large parameter space that may be further optimized. A comparison of the sequences is beyond the scope of this paper. Several other issues (discussed below) are continuing to be explored.
As shown in the simulations, the T2
-weighted image does not have pure T2
contrast, as desired, but includes some T1
contrast due to longitudinal relaxation following the T2
-preparation. This may contribute to a loss of contrast between the blood and MI, although this contrast is quite large in the case of chronic MI. The T1
contrast may degrade the ability to measure or assess elevated T2
due to edema, although the T2
-weighted images exhibited very little T1
contrast for the chronic cases examined. While the simulations (Appendix
) show that a large T2
contrast is achieved even at the elevated T2
values associated with acute MI, these cases remain to be studied experimentally. Another potential variable is the value of T2
in the blood after contrast administration, which may be reduced at very high dosage and may differ between the left and right ventricles due to oxygenation. The residual T1
contrast may be reduced by using a smaller number of views per segment. This results in a longer acquisition, which may be offset by the use of parallel imaging and/or a segmented TrueFISP sequence.
The use of cine images in conjunction with delayed-enhancement images has been cited as a means of addressing the issue of subendocardial MI cases with low contrast between the MI and blood pool (4
). Cine images offer excellent resolution and contrast between blood and myocardium, and this technique is often very helpful, although it is sometimes time-consuming to interpret the results due to differences in spatial and temporal resolution. It is also noted that cine and delayed-enhancement images are acquired during separate breath-holds and may have different slice positions, even though the slice prescription is the same, due to differences between breath-holds, as shown in the example of . Even small differences may complicate the interpretation or make it impossible to resolve, since the subendocardial MI may be very narrow or small. The accuracy of the subendocardial border will affect the assessment of transmurality, an important prognostic indicator. This is not to imply that cine imaging is not useful or that one should not use all available data to achieve the best interpretation, but rather to stress potential pitfalls. The proposed MCODE method may be used with a small additional cost in imaging duration to provide an easy first means of enhanced detection, and to determine whether a more complicated analysis is warranted.
For single-shot imaging, there is also an issue of image misregistration even though all images are acquired during the same breath-hold. This arises in cases of imperfect breath-holding, during which there is slight diaphragmatic motion. Unlike the case of , in which there is significant slice motion, there may be a small shift (typically less than a pixel) that may occur between three heartbeats due to a smooth respiratory drift. This may be corrected by rigid body registration, or a simple reacquisition. Since both T1- and T2-weighted images are presented immediately in the same image series and may be displayed alternately as a movie, the user is quickly alerted to any slight misregistration and can take corrective action. This was not deemed to be a problem in the cases examined.
The multicontrast approach using T1
- and T2
-weighted images has the potential to be extended to the acquisition of full parametric maps. For instance, the single-shot TrueFISP sequence might easily acquire additional image frames with variable parameters. Parametric maps for T1
have been estimated by measuring the signal intensity with TrueFISP readout following an IR preparation at several values of TI and readout flip angle (22
In conclusion, MCODE imaging offers a significant improvement in the ability to detect subendocardial MI by providing a T2-weighted image with high contrast between blood and MI. MCODE should improve both the detection and accuracy of sizing of subendocardial MI. MCODE has the potential to enable imaging of edema, which would allow the differentiation of acute vs. chronic MI.