Directions vs. averages: an in-vivo comparison for cardiac DTI
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: P25 (2015)
The ability to interrogate cardiac microstructure has led to much recent interest in in-vivo cardiac diffusion tensor imaging (cDTI). However, when compared to studies performed in neuro-imaging, very little work has been done to determine the optimal diffusion encoding schemes. Previous work has suggested that accuracy is improved by increasing the number of diffusion encoding directions(Ndirs)1,2, but comparisons in the heart have been limited to fixed animal specimens3. Here we compare parameters derived from cDTI using data acquired in vivo with an increasing Ndirs.
10 healthy subjects were imaged on a Siemens Skyra using the STEAM-EPI cDTI sequence4 in a short-axis slice of the mid left-ventricle with the optimal protocol recently described5 (b-values: 150 and 750 smm-2, 2.8x2.8x8mm3 resolution). This was repeated with Ndirs=6, 10, 12 and 20 (standard Siemens product directions) and 12 averages (Navs) were acquired in each direction. The diffusion tensor and parameter maps including mean diffusivity (MD), helical angle (HA) and fractional anisotropy (FA) were calculated as previously described5, using all averages and all directions together to provide a reference data set. The processing was then repeated for each set of diffusion encoding directions with varying numbers of averages chosen to match the total images used Ntot = Navs x Ndirs, as closely as possible to 24, 36 and 60 and also using Navs=12.
Figure 1 shows example parameter maps (HA, MD and FA) calculated using all directions (Ndir=48) and all averages (Nav=12) together compared to each diffusion encoding scheme processed with Ntot=60. There was no consistently visible difference between the encoding schemes. Figure 2 shows mean MD and FA values for each Ndirs plotted with the Ntot and the average variation (standard deviation) in these parameter maps over the left ventricle. For a given Ntot, Ndirs=10 appears to have the minimum variation of FA across the left ventricle and most commonly has FA closest the reference value and Ndirs=12 appears to be superior when considering MD. However, a comparison of MD and FA values when Ntot=60, showed no statistically significant difference between Ndirs (1-way repeated measures ANOVA; MD: p=0.59; FA: p=0.82).
While simulations in previous work have found increasing Ndirs to result in more accurate results, our results suggest that any resultant changes in MD or FA measured in in-vivo myocardium are small.
This work was performed at The National Institute for Health Research Funded Cardiovascular Biomedical Research Unit at The Royal Brompton Hospital and Imperial College London.
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Scott, A.D., Ferreira, P., Nielles-Vallespin, S. et al. Directions vs. averages: an in-vivo comparison for cardiac DTI. J Cardiovasc Magn Reson 17 (Suppl 1), P25 (2015). https://doi.org/10.1186/1532-429X-17-S1-P25