Correcting for imaging gradients–related bias of T(2) relaxation times at high‐resolution MRI

PURPOSE: High‐resolution animal imaging is an integral part of preclinical drug development and the investigation of diseases' pathophysiology. Quantitative mapping of T(2) relaxation times (qT(2)) is a valuable tool for both preclinical and research applications, providing high sensitivity to subtle tissue pathologies. High‐resolution T(2) mapping, however, suffers from severe underestimation of T(2) values due to molecular diffusion. This affects both single‐echo and multi‐echo spin echo (SSE and MESE), on top of the well‐known contamination of MESE signals by stimulated echoes, and especially on high‐field and preclinical scanners in which high imaging gradients are used in comparison to clinical scanners. METHODS: Diffusion bias due to imaging gradients was analyzed by quantifying the effective b‐value for each coherence pathway in SSE and MESE protocols, and incorporating this information in a joint T(2)‐diffusion reconstruction algorithm. Validation was done on phantoms and in vivo mouse brain using a 9.4T and a 7T MRI scanner. RESULTS: Underestimation of T(2) values due to strong imaging gradients can reach up to 70%, depending on scan parameters and on the sample's diffusion coefficient. The algorithm presented here produced T(2) values that agreed with reference spectroscopic measurements, were reproducible across scan settings, and reduced the average bias of T(2) values from −33.5 ± 20.5% to −0.1 ± 3.6%. CONCLUSIONS: A new joint T(2)‐diffusion reconstruction algorithm is able to negate imaging gradient–related underestimation of T(2) values, leading to reliable mapping of T(2) values at high resolutions.