# Enabling in vivo comparisons of different four-dimensional magnetic resonance imaging sequences for radiotherapy guidance using visual biofeedback

## TL;DR

This study introduces a method using visual biofeedback to improve breathing consistency during 4D-MRI scans, enabling more accurate comparisons of MRI sequences for radiotherapy.

## Contribution

A novel approach combining visual biofeedback with 4D-MRI sequences to enable in vivo comparisons and reduce breathing variability.

## Key findings

- Visual biofeedback reduced breathing variability by 37% in amplitude and 64% in period.
- Motion amplitude agreement improved from 3.5 mm to 1.8 mm with biofeedback guidance.
- 4D-MRI-derived amplitudes were consistently smaller than breathing waveform amplitudes across sequences.

## Abstract

Managing respiratory motion is essential for effective radiotherapy in the abdominothoracic regions. Respiratory-correlated four-dimensional magnetic resonance imaging (4D-MRI) can provide accurate motion estimation to help define treatment volumes for adaptive radiotherapy. However, validating and comparing 4D-MRI sequences in vivo is challenging due to the presence of breathing variability. This study combines visual biofeedback (VBF) with 4D-MRI sequences to facilitate in vivo comparisons.

Fourteen healthy volunteers and one patient were scanned on a 1.5 T Unity MR-linear accelerator (Elekta AB, Stockholm, Sweden) at two institutions. A radial stack-of-stars (SoS), a simultaneous multi-slice (SMS), and a Cartesian acquisition with spiral ordering (CASPR) 4D-MRI sequence were acquired. These acquisitions were performed without and with VBF based on an interleaved one-dimensional respiratory navigator (1D-RNAV) acquisition. Breathing variability across sequences was quantified using 1D-RNAV-derived breathing waveforms. Reconstructed 4D-MRI data were used to extract the motion amplitude, which was compared intra-volunteer across sequences and to the amplitudes of the breathing waveforms.

Breathing variability across sequences decreased by 37% (amplitude, p= 0.039) and 64% (period, p< 0.003), and the median intra-volunteer 4D-MRI-derived motion amplitude agreement improved from 3.5 mm to 1.8 mm (p= 0.064) across sequences due to VBF guidance. Four-dimensional MRI-derived amplitudes were smaller than breathing waveform amplitudes, with median differences of -31% (SoS), -17% (SMS), and -9% (CASPR). The average breathing waveform amplitude was 8% larger than instructed.

This methodology enables in vivo comparisons of 4D-MRI sequences for adaptive radiotherapy, with guidance improving anatomical consistency and ensuring more reliable comparisons.

•Three different 4D-MRI sequences acquired in 14 healthy volunteers and 1 patient.•Visual biofeedback improved breathing regularity across 4D-MRI sequences.•Inter-sequence 4D-MRI-derived amplitude agreements improved from 3.5 mm to 1.8 mm.

Three different 4D-MRI sequences acquired in 14 healthy volunteers and 1 patient.

Visual biofeedback improved breathing regularity across 4D-MRI sequences.

Inter-sequence 4D-MRI-derived amplitude agreements improved from 3.5 mm to 1.8 mm.