Quantitative Assessment of Restriction Spectrum Imaging Using a Diffusion MRI Phantom
Abstract
Purpose
Restriction Spectrum Imaging (RSI) is a multi-compartment diffusion MRI model developed to enhance sensitivity to restricted diffusion associated with tumoral tissue, addressing known limitations of the apparent diffusion coefficient (ADC). Unlike ADC, which reflects an aggregate diffusion signal, RSI leverages an extended b-value range to extract restricted diffusion from hindered and free water components. Although RSI has demonstrated improved lesion conspicuity in vivo across multiple organs, including the brain, prostate, and breast, its stability and reproducibility under controlled experimental conditions remain largely uncharacterized. The purpose of this work was to evaluate the repeatability and quantitative behavior of RSI using a diffusion phantom with known diffusivities.
Methods
Diffusion-weighted images (DWI) of a NIST-traceable MRI diffusion phantom (CaliberMRI, USA) were acquired over five consecutive days, with three repeated runs per day, using two head coils (20/64ch). A multishell diffusion scheme with b-values ranging from 0 to 8000 s/mm2 was implemented on a 3T system (Siemens Healthineers, Germany). RSI Cellularity (CELL) and Free Water fraction maps were computed and compared against reference diffusivities associated with known polyvinylpyrrolidone (PVP) concentrations. Intra- and inter-day repeatability were quantified using the repeatability coefficient (RC).
Results
RSI CELL demonstrated a strong linear relationship with increasing %PVP (slope=0.065, R2=0.973), consistent with increasing restriction of diffusion, while Free Water fraction map showed a corresponding inverse relationship with %PVP diffusivities. As expected, CELL maps decay monotonically with increasing ADC reference diffusivities. RSI maps illustrated high intra- and inter-RC across different %PVP (RC=0.03), where higher variability with the 20ch head coil (RC=0.175) was attributed to the noise floor.
Conclusion
RSI demonstrated high intra- and inter-day repeatability across %PVP, with RSI CELL showing robust stability under controlled phantom conditions. Variability observed at low %PVP was primarily driven by noise-floor effects, supporting the reliability of RSI for quantitative and longitudinal diffusion imaging.