All procedures were performed using a 0.4-T permanent MRI system (APERTO Inspire; FUJIFILM, Tokyo, Japan).

The phantoms were prepared with MnCl₂ (Wako Pure Chemical Industries, Osaka, Japan) at seven different concentrations (0, 0.02, 0.05, 0.10, 0.20, 0.40, and 0.80 mM). The phantoms were placed in acrylic resin containers (Kyoto Kagaku, Kyoto, Japan) and surrounded by fluid paraffin. Fig 1a–c show an overview of the phantoms. The phantoms were left in the MRI room (with a temperature of 24°C) for 24 h.

Image acquisition

First, the T₂ values of the phantom were measured. Then, RADAR DWI and EPI DWI were performed. Additionally, varying the b value from 0 to 700 in 100-s/mm² steps, RADAR DWI and EPI DWI imaging was performed and the signal intensity (SI) was measured.

The parameters for T₂ value, EPI and RADAR DWI measurment are tablulated in Table 1. T₂ maps were obtained by exponentially fitting the acquired SIs. The ADC maps were derived by calculating pixel-by-pixel from b0 to b700 images according to the Stejskal–Tanner relationship. The region-of-interest (ROI) was analyzed using the generated T₂ and ADC maps with ImageJ. Circular ROIs were placed in each phantom, and the T₂ and ADC values were measured. Further, the coefficient of variation (CV) of ADC values obtained by RADAR and EPI DWI was calculated. In addition, the relative ADC change (rADC)) of EPI and RADAR DWI was estimated as rADC =ADC_RADAR - ADC_EPI / ADC_RADAR × 100%.

Data were presented as means ± standard deviation or medians with interquartile range based on their distribution. The normality of data was assessed using the quantile–quantile plots.

One-way analysis of variance followed by the Dunnett’s test was used to compare the ADC values obtained using RADAR and EPI DWI. The Kruskal–Wallis test followed by the Dunn’s test was used to compare the rADC. Multiple comparison tests of the measured data were performed using the ADC and rADC obtained in the phantom with a MnCl₂ 0 mM as reference values. We used linear regression analysis to assess the logarithms of SI and b value of RADAR and EPI DWI.

All Statistical analysis were performed using GraphPad Prism 10.3.1 (San Diego, CA, the USA). A P value of < 0.01 indicated statistically significant difference.