Alternating-contrast single-shot spiral MR-ARFI with model-based displacement map reconstruction

To improve single-shot spiral MR-Acoustic Radiation Force Imaging (MR-ARFI)'s robustness to dynamic phase errors and evaluate it in non-human primates (NHPs) with a low-f-number transducer.

A single-shot spiral MR-ARFI pulse sequence with 2 mm in-plane resolution and alternating displacement phase contrast was implemented to visualize the focus generated by a 128-element ultrasound transducer in the NHP brain. A model-based displacement map calculation was implemented to remove dynamic phase errors. MR-ARFI scans were acquired at pressure levels above and below FDA mechanical index (MI) limits, and reconstructed displacement maps were compared to maps generated by a 3D EPI MR-ARFI scan and a spiral MR-ARFI scan with blocked ultrasound triggering.

The proposed sequence and processing detected focal tissue displacements of 160 nm at a transcranial mechanical index of 0.96, which the 3D EPI could not detect, and with 9.7 $\times$ -improved precision. The model-based reconstruction suppressed background phase errors and maximized precision. Alternating contrast yielded displacement maps with 4.9 $\times$ -improved precision compared to blocked contrast.

Single-shot spiral MR-ARFI can provide robust focus visualization in MR-guided ultrasound in the brain at MI levels well below the FDA limit.