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

Area of Science:

• Medical Imaging
• Biophysics
• Neuroimaging

Background:

• Acoustic Radiation Force Imaging (ARFI) combined with Magnetic Resonance (MR) offers unique insights into tissue mechanics.
• Dynamic phase errors in MR-ARFI can limit the accuracy and robustness of displacement measurements.
• Improving MR-ARFI's precision is crucial for visualizing subtle mechanical effects in the brain.

Purpose of the Study:

• To enhance the robustness of single-shot spiral MR-Acoustic Radiation Force Imaging (MR-ARFI) against dynamic phase errors.
• To evaluate the improved MR-ARFI technique in non-human primates (NHPs) using a low-f-number transducer.
• To achieve reliable visualization of focused ultrasound in the NHP brain.

Main Methods:

• Implementation of a single-shot spiral MR-ARFI pulse sequence with alternating displacement phase contrast.
• Utilization of a model-based displacement map calculation to mitigate dynamic phase errors.
• Acquisition of MR-ARFI scans at varying mechanical index (MI) levels and comparison with existing methods (3D EPI, blocked triggering).

Main Results:

• The developed MR-ARFI sequence successfully detected focal tissue displacements as small as 160 nm at a transcranial MI of 0.96.
• The technique demonstrated a 9.7-fold improvement in precision compared to 3D EPI MR-ARFI.
• Model-based reconstruction significantly reduced background phase errors, and alternating contrast improved precision by 4.9-fold.

Conclusions:

• Single-shot spiral MR-ARFI, with the implemented enhancements, provides robust visualization of ultrasound focus in the brain.
• The method operates effectively at mechanical index (MI) levels well below the FDA safety limit.
• This advancement holds promise for improved MR-guided focused ultrasound applications in neuroimaging.