Phase‐based T ₂ mapping with gradient echo imaging

Abstract

Purpose Transverse relaxation time (T 2 ) mapping with MRI has a plethora of clinical and research applications. Current T 2 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 T 2 mapping technique where T 2 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 T 2 , resulting from coherent transverse magnetization. This T 2 ‐dependent phase behavior forms the basis of the proposed T 2 mapping technique. To isolate T 2 ‐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 T 2 mapping, spectroscopy, and T 2 values from the literature. Results The proposed method produced accurate T 2 maps compared with SE‐based T 2 mapping in the phantom. Good qualitative agreement was observed in vivo between the proposed method and the reference. T 2 measured in various anatomies agreed well with values reported in the literature. Conclusion A phase‐based T 2 mapping technique was developed and its feasibility demonstrated in phantoms and in vivo.