Phase‐based T2 mapping with gradient echo imaging.

Purpose: Transverse relaxation time (T2) mapping with MRI has a plethora of clinical and research applications. Current T2 mapping techniques are based primarily on spin‐echo (SE) relaxometry strategies that rely on the signal magnitude, and often suffer from lengthy acquisition times. In this work, we propose a phase‐based T2 mapping technique where T2 information is encoded into the signal phase of rapid gradient echo (GRE) acquisitions. Theory: Bloch equation simulations demonstrate that the phase of GRE acquisitions obtained with a very small inter‐repetition RF phase increment has a strong monotonic dependence on T2, resulting from coherent transverse magnetization. This T2‐dependent phase behavior forms the basis of the proposed T2 mapping technique. To isolate T2‐dependent phase from background phase, at least 2 data sets with different RF phase increments are acquired. The proposed method can also be combined with chemical shift encoded MRI to separate water and fat signals. Methods: The feasibility of the proposed technique was validated in a phantom experiment. In vivo feasibility was demonstrated in the brain, knee, abdomen, and pelvis. Comparisons were made with SE‐based T2 mapping, spectroscopy, and T2 values from the literature. Results: The proposed method produced accurate T2 maps compared with SE‐based T2 mapping in the phantom. Good qualitative agreement was observed in vivo between the proposed method and the reference. T2 measured in various anatomies agreed well with values reported in the literature. Conclusion: A phase‐based T2 mapping technique was developed and its feasibility demonstrated in phantoms and in vivo. [ABSTRACT FROM AUTHOR]