A nonmagnetic micropositioner for displacement measurement validation in the MRI environment

Objective The purpose of this study was to implement a nonmagnetic micropositioner in the MRI environment to validate displacement estimates of magnetic resonance acoustic radiation force imaging (MR-ARFI). Methods The micropositioner consisted of a stage driven by a piezoelectric stepper motor in closed-loop operation with an optical quadrature encoder. A 100-gram agar gel phantom was prepared, and three MR-ARFI pulse sequences were used to generate displacement maps. MR-ARFI measured displacements were compared to ground truth data from the optical encoder. Results The micropositioner demonstrated consistent performance with positioning times of 1.6 ± 0.4 1.6 ± 0.4 ms for extension and 2.2 ± 0.2 2.2 ± 0.2 ms for return to baseline position. The micropositioner decreased the signal-to-noise ratio of magnitude images due to increased electronic noise. Linear regression analysis showed that displacement measurements were highly linear with R² ≥ 0.98 R 2 ≥ 0.98 but exhibited scaling biases that may have been due to the experimental setup. Conclusion The proposed instrument can potentially improve the accuracy and precision of MR-ARFI-based applications, including focused ultrasound dosimetry and mechanical biomarker imaging.