Modeling of T₂* decay in vertebral bone marrow fat quantification

Bone marrow fat fraction mapping using chemical shift encoding-based water-fat separation is becoming a useful tool in investigating the association between bone marrow adiposity and bone health and in assessing cancer treatment-induced bone marrow damage. Vertebral bone marrow is characterized by short T₂* relaxation times, which are in general different for the water and fat components and can confound fat quantification. The purpose of the present study is to compare different approaches to T₂* correction in chemical shift encoding-based water-fat imaging of vertebral bone marrow using single-voxel MRS as reference. Eight-echo gradient-echo imaging and single-voxel MRS measurements were made on the spine (L3-L5) of 25 healthy volunteers. Different approaches were evaluated for correction of T₂* effects: (a) single-T₂* correction, (b) dual-T₂* correction, (c) T₂' correction using the a priori-known T₂ from the MRS at each vertebral body and (d) T₂' correction using the a priori-known T₂ equal to previously measured average values. Dual-T₂* correction resulted in noisier imaging fat fraction maps than single-T₂* correction or T₂' correction using a priori-known T₂. Linear regression analysis between imaging and MRS fat fraction showed a slope significantly different from 1 when using single-T₂* correction (R²=0.96) or dual-T₂* correction (R²=0.87). T₂' correction using the a priori-known T₂ resulted in a slope not significantly different from 1, an intercept significantly different from 0 (between 2.4% and 3%) and R²=0.96. Therefore, a T₂' correction using a priori-known T₂ can remove the fat fraction bias induced by the difference in T₂* between water and fat components without degrading noise performance in fat fraction mapping of vertebral bone marrow.