Reduced-distortion diffusion weighted imaging for head and neck radiotherapy.

Area of Science:

• Medical Imaging
• Radiotherapy Physics
• Quantitative MRI

Background:

• Quantitative Diffusion Weighted Imaging (DWI) is crucial for head and neck (HN) cancer radiotherapy guidance.
• Clinical translation is limited by distortions in single-shot EPI (ssEPI) and low SNR/long scan times in Turbo-Spin-Echo (ssTSE).
• Multi-shot (ms) EPI and TSE sequences are evaluated to overcome these limitations.

Purpose of the Study:

• To assess the performance of multi-shot EPI (msEPI) and multi-shot TSE (msTSE) DWI sequences.
• To compare geometric distortions, apparent diffusion coefficient (ADC) values, and signal-to-noise ratio efficiency (SNR_eff) against standard ssEPI and ssTSE.
• To determine the optimal sequence for HN cancer radiotherapy applications.

Main Methods:

• Acquired ssEPI, ssTSE, 2-shot EPI (2sEPI), 3-shot EPI (3sEPI), and msTSE DWI.
• Scans performed on a phantom, 10 volunteers, and 5 HN cancer patients using a 3T MRI.
• Quantified geometric distortions (DVF), ADC values, and SNR_eff at matched spatial resolution and b-values (0, 200, 800 s/mm²).

Main Results:

• All ms techniques significantly reduced distortions compared to ssEPI (mean DVF 3.1 ± 1.3 mm).
• msTSE showed the lowest distortion (1.5 ± 0.6 mm), followed by ssTSE (1.8 ± 0.9 mm), 3sEPI (2.2 ± 1.0 mm), and 2sEPI (2.6 ± 1.0 mm).
• SNR_eff decreased with distortion reduction; msTSE/ssTSE had the lowest SNR_eff (5.7 ± 1.9, 6.0 ± 1.6). In vivo ADC values were ~4% lower with msEPI and ~12% lower with ssTSE/msTSE compared to ssEPI.

Conclusions:

• msEPI and TSE acquisitions improve geometric distortion at the expense of SNR_eff and scan time.
• msTSE offers the least distortion, but 3sEPI presents a favorable balance.
• 3sEPI provides improved geometric fidelity, superior efficiency, and accurate in vivo ADC quantification for HN cancer radiotherapy.