Impact of MRI slice thickness and fiducial marker orientation on geometric accuracy in multi-echo GRE imaging for prostate radiotherapy planning.

BACKGROUND: Precise target definition in prostate radiotherapy requires accurate computed tomography (CT)-magnetic resonance imaging (MRI) image registration using gold fiducial markers (GFMs). While multi-echo gradient recalled echo (ME GRE) sequences facilitate GFM identification, the quantitative impact of acquisition parameters on physical measurement accuracy remains insufficiently understood. PURPOSE: This study evaluates how MRI slice thickness and marker orientation angle jointly affect GFM visualization and geometric accuracy to characterize the resulting localization uncertainties. METHODS: A kiwifruit-based phantom (T1: 1603.3 ms; T2: 72.3 ms) that mimics the relaxation times of the human prostate was imaged with 3.0T MRI. Imaging was performed using a 3D ME GRE sequence with three slice thicknesses (1.0, 2.0, and 3.0 mm) and 12 orientation angles ranging from horizontal to 90°. Five multidisciplinary observers independently identified marker coordinates while blinded to the acquisition parameters. Subjective confidence scores were also recorded. RESULTS: Two-way analysis of variance (ANOVA) revealed significant main effects for slice thickness (F(2, 144) = 32.03, p < 0.001) and orientation angle (F(11, 144) = 4.06, p < 0.001), with significant interaction between them (F (22, 144) = 2.72, p < 0.001). Mean measurement errors with 95% confidence intervals (CI) were 0.96 mm (95% CI: 0.70-1.22) for 1.0 mm, 1.49 mm (95% CI: 1.23-1.75) for 2.0 mm, and 2.00 mm (95% CI: 1.74-2.26) for 3.0 mm slices. The peak error reached 3.10 mm under the combination of a 3.0 mm slice and 15° orientation. Subjective confidence scores showed a significant negative correlation with physical measurement errors (r = -0.296, p < 0.001). CONCLUSIONS: MRI slice thickness and marker orientation significantly impact GFM visualization accuracy. The results suggest that a 1.0 mm slice thickness offers a technical advantage for maintaining sub-millimeter localization accuracy across various marker orientations and highly desirable for high-precision radiotherapy planning.