This study showed that RADAR DWI can calculate ADC values more accurately than EPI DWI in the presence of MnCl₂.

Our phantoms were created by adjusting the MnCl₂ concentration, and the T₂ values decreased as the MnCl₂ concentration increased. MnCl₂, a paramagnetic material, is used as a contrast medium in MRI, and it shortens T₂ values by disrupting the local magnetic field due to its magnetic susceptibility [14,15]. Moreover, our phantoms reflected the T₂ value shortening effect of MnCl₂, with a 96.8% reduction in T₂ values in the phantom with an MnCl₂ concentration of 0.8 mM.

Theoretically, even if the MnCl₂ concentration increases, the ADC value should not change, as the viscosity of the liquid does not change or a physical barrier is not generated. However, the ADC values obtained using EPI DWI changed significantly as the MnCl₂ concentration increased. Additionally, CVs of RADAR DWI were 2.27%, and EPI DWI were 27.37%. Especially, the linear correlation between b value and the logarithm of the SI was observed in all phantoms for RADAR DWI, whereas for EPI DWI the linear correlation was disrupted in phantoms with MnCl₂ concentrations of 0.40 and 0.80 mM. Therefore, RADAR DWI is robust to changes in magnetic susceptibility caused by the presence of MnCl₂ and is effective in calculating ADC values than EPI DWI. RADAR DWI applies multiple 180° pulses based on the blade width and k-space filling factor [10]. This may reduce the magnetic susceptibility change caused by MnCl₂, thereby allowing more accurate ADC calculations.

The ADC values obtained with FSE-based DWI slightly differ from those obtained with EPI DWI [10, 12, 16, 17]. The ADC values obtained with RADAR DWI were higher than those obtained with EPI DWI [18]. They reported that the rADC was approximately 10 to 20% for phantoms with ADC values of 0.5 to 1.0 (× 10^-3 mm²sec^-1), which is similar to the rADC for phantoms with MnCl₂ concentrations of 0 to 0.20 mM in the present study. Therefore, the difference in ADC values between RADAR DWI and EPI DWI (rADC) at MnCl₂ concentrations of 0 to 0.20 mM is considered to be an effect of differences in imaging sequences. However, the rADC of phantoms with MnCl₂ concentrations of 0.40 and 0.80 mM were significantly higher than those of phantoms with MnCl₂ concentration of 0 mM.  Based on these results, the ADC values obtained with DWI EPI were low if the influence of magnetic susceptibility increased. Meanwhile, the ADC values obtained with RADAR DWI almost did not change.

The current study had a few limitations. First, the effect of magnetic susceptibility caused by MnCl₂ in a low-field MRI system is lower than that in 1.5- or 3.0-T MRI systems, which are commonly used in clinical settings [19, 20]. Second, the ADC values obtained using FSE-based DWI are affected by blade thickness [16]. Hence, this study was conducted under fixed blade thickness parameters. Theoretically, increasing the blade thickness reduces the phase dispersion effect caused by the paramagnetic material [21, 22]. Therefore, it is assumed that the measurement of ADC values can be more accurate. 

In conclusion, in the presence of MnCl₂, RADAR DWI is more effective in obtaining accurate ADC values than EPI DWI.