3D MR Fingerprinting for Quantitative Bladder Wall T₁, T₂, and M₀ Mapping in Healthy Subjects at 1.5 T and 3 T

Current diagnostic tests for lower urinary tract symptoms (LUTS) do not assess tissue-level alterations in the bladder wall. This study assesses the feasibility of 3D magnetic resonance fingerprinting (MRF) for simultaneous T₁, T₂, and proton density (M₀) mapping of the bladder wall in healthy subjects at 1.5 T and 3 T. A 3D MRF acquisition was combined with a deep image prior reconstruction to achieve whole-bladder T₁, T₂, and M₀ mapping in 11.8 min at an acquired voxel size of 1 × 1 × 3 mm³ (interpolated to 0.5 × 0.5 × 1.5 mm³). Twenty-nine healthy subjects (14 female, 18-78 years) were enrolled, with 16 scanned at both 1.5 T and 3 T. Conventional maps were acquired using MOLLI and T₂-prepared GRE sequences. Maps were analyzed by measuring mean global and regional T₁ and T₂ values, with spatial heterogeneity assessed using the coefficient of variation (CV). Bladder wall thickness (BWT) was measured from synthetic weighted images derived from MRF maps, compared to reference values from conventional T₂-weighted MRI. Mean global MRF bladder wall measurements were 984 $\pm$ 53 ms (T₁), 51.4 $\pm$ 5.6 ms (T₂), and 4.5 $\pm$ 1.6 mm (BWT) at 1.5 T and 1468 $\pm$ 118 ms (T₁), 43.5 $\pm$ 4.0 ms (T₂), and 4.3 $\pm$ 1.6 mm (BWT) at 3 T. MRF T₁ values were significantly lower than MOLLI at 1.5 T (bias -151 ms) but not significantly different at 3 T. MRF T₂ values were significantly lower (by 7-8 ms) than T₂-prepared GRE at both field strengths. MRF exhibited lower CV than MOLLI T₁ mapping at 1.5 T. BWT from synthetic images exhibited excellent agreement (bias $\le 0$ .1 mm) with conventional T₂-weighted images. Regionally, T₁ and T₂ tended to be higher in the dome and posterior wall. In an exploratory analysis, a positive association between age and T₂ was observed. In conclusion, 3D MRF of the bladder wall is feasible in healthy subjects, providing a baseline for translational studies to characterize pathological remodeling in LUTS.