High b-value Diffusion-weighted Imaging in Normal and Malignant Peripheral Zone Tissue of the Prostate: Effect of Signal-to-Noise Ratio

Purpose: To determine whether the apparent diffusion coefficient (ADC) obtained using a high b-value (2,000 s/mm2) is superior to that using a standard b-value (1,000 s/mm2) for discriminating malignant from normal peripheral tissue in the prostate. Methods: Twenty-six patients with biopsy-proven prostate cancer underwent 1.5T magnetic resonance (MR) imaging including single-shot, echo-planar diffusion-weighted imaging (DWI) with repetition time/echo time, 3500/88 ms; 4-mm slice thickness; 1-mm interslice gap; 144×128 matrix; field of view, 250×250 mm; number of excitations, 10; and b-values, 0, 1,000, and 2,000 s/mm2. For each patient, ADC values were obtained for malignant and normal tissue using b=1,000 and 2,000 in a monoexponential model. Signal-to-noise (SNR) and contrast-to-noise (CNR) ratios in DWI were also evaluated. Results: At b=1,000, the mean ADC (×10-3 mm2/s) for malignant tissue was 0.82±0.27 (range 0.43-1.29) and for normal tissue, 1.69±0.23 (1.31-2.18). At b=2000, the mean ADC for malignant tissue was 0.61±0.19 (0.30-0.94) and for normal tissue, 1.01±0.14 (0.73-1.35). Significant ADC overlap between the malignant and normal tissue was recognized at b=2000. As b-value increased, the mean SNR within malignant tissue decreased by 21.6%, and mean CNR decreased 17.3%. Conclusions: Under the same imaging conditions, measuring ADC using a high b-value (2,000 s/mm2) in a monoexponential model has little diagnostic advantage over using the standard b-value (1,000 s/mm2) in discriminating malignant from normal prostate tissue.