A nonmagnetic micropositioner for displacement measurement validation in the MRI environment

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).

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.

The micropositioner demonstrated consistent performance with positioning times of $1.6\pm 0.4$ ms for extension and $2.2\pm 0.2$ 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 $R^2\ge 0.98$ but exhibited scaling biases that may have been due to the experimental setup.

The proposed instrument can potentially improve the accuracy and precision of MR-ARFI-based applications, including focused ultrasound dosimetry and mechanical biomarker imaging.