Quick Fire Session
SCMR 22nd Annual Scientific Sessions
Background: Diffusion tensor cardiovascular magnetic resonance (DT-CMR) can provide insight into the microstructure of healthy and diseased myocardium1-3. Commonly-used single-shot techniques provide a suboptimal spatial resolution to image patient cohorts with a thinned myocardium. Recently an interleaved spiral stimulated echo (STEAM) sequence was demonstrated with an increased spatial resolution4,5. However, this technique uses long readouts, during which T2* decay results in image blurring. In this work a T2* correction is proposed to increase the sharpness of the high-resolution DT-CMR data.
Methods: 10 healthy volunteers were imaged with both a single-shot and an interleaved variable density spiral stimulated echo acquisition mode (STEAM) DT-CMR sequence4 at 2.8x2.8x8mm3 and 1.8x1.8x8mm3 and with a STEAM EPI sequence at 2.8x2.8x8mm3. Mid-ventricular short-axis images were acquired in peak-systole on breath-hold on a 3T Siemens Skyra. 6 diffusion directions were obtained at b=600 and 150 s/mm2 with 8/2 averages respectively. Motion-induced phase was corrected for the interleaved spirals and frequency segmented reconstruction off-resonance correction4,5 based on field-maps acquired in each breath-hold was performed. At the beginning of each breath-hold, 4 stimulated echoes with two readouts each were obtained and used for the T2* correction (4th STEAM) and field-map calculation (2nd and 3rd STEAM), see fig. 1. Myocardial T2* was calculated based on the magnitude of the first 1000 points of two spirals acquired during the same stimulated echo separated by 9.2ms. Imaging spiral data was corrected based on exponential decay with the estimated T2*. To measure image sharpness, line profiles were drawn from the centroid of the left ventricle to the epicardium on b=600s/mm2 image. The sigmoid function6 f(x,a0,a1,a2,s)=a1/(1+10s(a0-x))+a2 was fitted to the profiles where x is increasing along the profile, a1+a2 are the upper and a2 the lower plateau and a0 is the location. Median endocardial sharpness (s) of the septal profiles was calculated subject-wise.
Results: The median and interquartile range of calculated T2* over all subjects was T2*=34±7ms. Fig. 2 shows typical diffusion weighted images for before and after T2* correction while the sharpness and some DT-CMR parameters are compared subject-wise in fig. 3. Sharpness significantly increases with T2* correction. Helix angle gradient and fractional anisotropy increase in magnitude after correction, which may be a consequence of the improved spatial resolution.
Conclusion: The proposed T2* correction increases significantly the sharpness of spiral DT-CMR data and could also be beneficial in other spiral CMR applications. The increased sharpness will be most beneficial when imaging patient groups with a thinned myocardium, focal disease and in the right ventricle.