Low b-values in apparent diffusion coefficient calculations overestimate diffusion in rectal cancer

Background and purpose: The apparent diffusion coefficient (ADC), derived from diffusion-weighted MRI (DWI), is commonly calculated using a monoexponential model. However, there is no consensus on optimal b-value selection for ADC quantification in rectal cancer. This prospective observational study evaluated how varying b-value combinations influence ADC values. Patient/material and methods: DWI with seven b-values (b = 0, 25, 50, 100, 500, 1,000, and 1,300 s/mm2) was acquired from 23 rectal cancer patients in the OxyTarget study (NCT01816607) using a 1.5T Philips Achieva scanner. Two radiologists independently delineated whole-tumour volumes of interest. ADC values were calculated using 18 different b-value combinations and compared with a biexponential reference. Results: Tumour ADCs varied significantly across b-value combinations. Excluding low b-values (b ≤ 100 s/mm²) led to reduced ADCs. Although b = 0 s/mm² is commonly included in ADC calculations, this study demonstrates that its inclusion leads to substantial overestimation. The use of two or three b-values from b = 500, 1,000, and 1,300 s/mm² yielded the smallest deviations from the biexponential reference. Interpretation: In rectal cancer, tumour ADC calculated using the monoexponential model is strongly influenced by the choice of b-values. By eliminating the contribution from perfusion (b ≤ 100 s/mm2) the uncertainty in the calculations is significantly reduced. Our findings support the use of b-values exceeding 100 s/mm², ideally in combination with a high b-value of at least 1,000 s/mm², when assessing diffusion using the monoexponential model. Consistent b-value combinations across studies are recommended for reliable quantitative comparisons of ADC values.