MR-ARFI to evaluate intensity variability and enhance targeting of FUS: A brain study in rats.

Purpose: Inconsistencies in focused ultrasound (FUS) transducer positioning and skull-induced aberrations can reduce the targeting accuracy and cause inconsistencies in the intensity delivered during FUS neuromodulation procedures. This study aimed to evaluate the use of MR-acoustic radiation force imaging (MR-ARFI) in improving the targeting accuracy and assessing the variation in the pressure delivered during FUS procedures.

Methods: An MR-guided FUS system was used to bilaterally target the nucleus accumbens region of Sprague-Dawley rats. Displacement maps were acquired with MR-ARFI to refine the targeting accuracy by adjusting the focal point position electronically and mechanically as necessary. In addition, the ARFI measurements were used to establish the relationship between the displacement and input power, and to assess intra- and inter-subject variability in FUS pressure.

Results: ARFI displacement was a strong predictor of FUS power (r = 0.935, p < 0.001), with displacements showing quadratic dependence on FUS pressure. Intra-site and intra-subject coefficients of variation in ARFI displacement measurements were 9.0% and 18.6%, whereas the variation across animals was 43.4%. Initial MR-guided targeting required secondary adjustments in 21% of cases.

Conclusion: Despite initial MRI guidance, substantial targeting errors were observed, highlighting the need for advanced imaging techniques like MR-ARFI in neuromodulation procedures. Moreover, ARFI displacements in the target region across subjects varied over three-fold, indicating very high variation in the delivered acoustic pressure. By improving the precision of FUS targeting and estimating deviations in the FUS pressure, MR-ARFI can improve therapeutic outcomes and reduce risks in transcranial FUS neuromodulation.