Magnitude preparation-based MR-acoustic radiation force imaging

To visualize ultrasonic foci in tissue without phase subtraction, to reduce sensitivity to motion and respiration. A secondary purpose is to provide inherently fused anatomy and focus depiction across various image contrasts.

A magnitude-contrast MR-acoustic radiation force imaging (Mag-ARFI) magnetization-preparation sequence was implemented that converts displacement-induced phase shifts into modulated longitudinal magnetization that can be read out by a subsequent imaging sequence. Phantom experiments compared focus visualization between conventional phase contrast-ARFI and Mag-ARFI, and demonstrated the ability to quantify displacement using multiple Mag-ARFI measurements. In vivo phase contrast-ARFI maps and $T_1$ - and $T_2$ -weighted Mag-ARFI images were acquired using a loop coil that was pulsed with DC to mimic an ultrasound focus-induced phase shift. Breathing motion artifacts in head images were compared between the phase-contrast MR-ARFI and Mag-ARFI methods.

Phantom experiments demonstrated strong agreement (R = 0.88) between Mag-ARFI and phase contrast-ARFI displacement values. Mag-ARFI and phase-contrast ARFI peak displacements were 3.02 $\mu$ m and 3.91 $\mu$ m, respectively. Mag-ARFI FWHM was [9.45, 10.35] mm, compared to phase contrast-ARFI FWHM of [7.65, 8.56] mm. Head Mag-ARFI images showed the focus fused with background anatomy, correlating well with phase contrast-ARFI (R = 0.87 for T₁-weighted, R = 0.79 for T₂-weighted). Mag-ARFI had reduced breathing motion artifacts, with 3.5 $\times$ lower coefficient of variation than phase contrast-ARFI.

Mag-ARFI allows ultrasound focus localization, with reduced breathing motion artifacts, on $T_1$ - and $T_2$ -weighted magnitude images containing background anatomical details.