Diffusion-weighted imaging of the brain: comparison of stimulated- and spin-echo echo-planar sequences

We used a rat model of focal cerebral ischaemia to compare stimulated-echo (STE) and spin-echo (SE) echo planar (EPI) diffusion-weighted sequences as regards image quality and accuracy of calculation of apparent diffusion coe fficients (ADC). Focal cerebral ischaemia was induced by endovascular occlu sion of the middle cerebral artery in five rats. MRI was performed on a 2.3 5 tesla imager. For diffusion-weighted imaging (DWI) we used STE-EPI and SE -EPI with different diffusion times (a) of 15, 30, 45, 60, 75 and 90 ms usi ng values of b of 200, 300, 400, 500, 600 and 700 s/mm(2). We assessed imag e quality, the signal-to-noise-ratio (SNR) and the accuracy of the ADC calc ulated from both sequences. Infarcts were delineated in all cases, independ ent of sequence type and Delta. The image quality and SNR of the SE-EPI ima ges were significantly better, with Delta higher SNR than STE-EPI images fo r short and intermediate values of Delta. However, when Delta reached 75 ms STE-EPI became superior to SE-EPI. ADC calculated from STE-EPI images were smaller than those from SE-EPI images for short and intermediate diffusion times, possibly because of the lower SNR of the former. We suggest that SE -EPI sequences be used for DWI of the brain, particularly on experimental s ystems and whole-body imagers with enhanced gradient hardware, where it is possible to run highly diffusion-weighted sequences (b > 500 s/mm(2)) with Delta less than 50 ms. However, when using very long values of Delta becaus e of hardware restrictions or for measurement of restricted diffusion, STE sequences give better results.