Increased maximum gradient amplitude improves robustness of spin-echo cardiac diffusion-weighted MRI

Background Cardiac motion presents a major challenge in diffusion weighted MRI (DWI), often leading to large signal dropouts that necessitate repeated measurements (Pai, V.M., MRM 2011). While cardiac DWI is generally ECG gated to apply diffusion weighting during peak-systole or enddiastole, these intervals can be short and difficult to pinpoint, resulting in poor sequence reproducibility. Recent improvements in gradient hardware provide larger maximum gradients than current systems (Gmax=80mT/m), which can substantially reduce the temporal footprint of diffusion preparation and make cardiac DWI more robust to bulk motion.

Increased maximum gradient amplitude improves robustness of spin-echo cardiac diffusionweighted MRI Eric Aliotta 1,2* , Stanislas Rapacchi 1 , Peng Hu 1,2 , Daniel B Ennis 1,2 From 18th Annual SCMR Scientific Sessions Nice, France. 4-7 February 2015 Background Cardiac motion presents a major challenge in diffusion weighted MRI (DWI), often leading to large signal dropouts that necessitate repeated measurements (Pai, V.M., MRM 2011). While cardiac DWI is generally ECG gated to apply diffusion weighting during peak-systole or enddiastole, these intervals can be short and difficult to pinpoint, resulting in poor sequence reproducibility.
Recent improvements in gradient hardware provide larger maximum gradients than current systems (G max =80mT/m), which can substantially reduce the temporal footprint of diffusion preparation and make cardiac DWI more robust to bulk motion.

Methods
A left ventricular (LV) motion model simulated motion of the healthy heart with 30-70ms quiescent intervals (t Q ). Monopolar encoded SE-DWI (b=500 s/mm 2 , 3 directions) was simulated using: G max =40 and 80mT/m with diffusion gradients centered at mid-quiescence and with a range temporal offsets (ΔT=±20ms). Complex Gaussian noise was added such that SNR=50 for b=0 images. Bulk motion induced error was measured by the bias in apparent diffusion coefficient (ADC) recovery from the programmed value (ADC=1x10 -3 mm 2 /s). Sequences that recovered ADC with bias<10% for ΔT= ±10ms were deemed robust to motion.

Conclusions
Simulations show that G80 recovered ADC more accurately than G40 for all t Q and ΔT and was robust to motion for t Q ≥30ms. This is likely due to the shorter diffusion preparation (G40 t prep =39ms, G80 t prep =28ms) and indicates that G80 will perform more consistently for short t Q (fast heart rates, systolic imaging) or changes in heart rhythm.
With fixed b-value=300mm 2 /s in vivo, G80 had consistently better image quality than G40. In agreement with simulation, this indicates that G80 improves the robustness of cardiac DWI for the same b-value. With fixed TE=36ms, G40-b100 was acceptable more frequently than G80-b300, but with insufficient diffusion weighting. Increased G max can thus improve diffusion sensitivity with less loss of robustness.

Funding
This research was supported by Siemens Medical Solutions and the Department of Radiological Sciences at UCLA.