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

## TL;DR

This paper introduces a new MR-ARFI technique that improves imaging precision and robustness in the brain using a novel spiral sequence and model-based processing.

## Contribution

The novel alternating-contrast single-shot spiral MR-ARFI sequence with model-based reconstruction improves precision and reduces phase errors in brain imaging.

## Key findings

- The proposed sequence detected 160 nm displacements at a low mechanical index, which 3D EPI could not detect.
- Model-based reconstruction improved precision by 9.7× and suppressed background phase errors.
- Alternating contrast improved precision by 4.9× compared to blocked contrast.

## Abstract

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×‐improved precision. The model‐based reconstruction suppressed background phase errors and maximized precision. Alternating contrast yielded displacement maps with 4.9×‐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.