Relevance of Prostatic Fluid on the Apparent Diffusion Coefficient: An Inversion Recovery Diffusion-Weighted Imaging Investigation

Objectives: Diffusion-weighted imaging (DWI) is pivotal for prostate magnetic resonance imaging. This is rooted in the generally reduced apparent diffusion coefficient (ADC) observed in prostate cancer in comparison to healthy prostate tissue. This difference originates from microstructural tissue composition changes, including a potentially decreased fluid-containing lumen volume. This study explored the nature of the observed ADC contrast in prostate tissue through inversion recovery-prepared DWI examinations that generated varying levels of fluid suppression.

Materials and methods: This institutional review board-approved, single-center, prospective study was conducted from 2023 to 2024; all participants underwent magnetic resonance imaging including DWI with b-values of 50 and 800 s/mm2 at 16 inversion times (TI; 60-4000 milliseconds). The measured ADC was interpreted with a 2-compartment model (compartments: tissue and fluid). Descriptive statistics were computed for all analyzed parameters.

Results: Twelve healthy male volunteers (45 ± 17 years) and 1 patient with prostate adenocarcinoma (66 years) were evaluated. The ADC map appearance depended heavily on the TI, and we observed a feature-rich ADC(TI) curve. The ADC in the transition zone (TZ) of healthy volunteers increased between TI = 60 milliseconds and approximately 1100 milliseconds, then dropped drastically before increasing again, stabilizing at a very high TI. This effect was greatly reduced in the patient's prostate cancer lesion. The 2-compartment model described this behavior well. After the inversion, tissue magnetization recovers faster, decreasing its signal contribution in absolute terms and resulting in an increase in the ADC. At the tipping point, the total magnetization is zero at b = 0, when the positive tissue magnetization and still-inverted fluid magnetization cancel out. A small diffusion encoding leads to a positive signal, thus generating an infinite ADC. After the tipping point, the fluid magnetization remains negative and thereby reduces the ADC.

Conclusions: Prostate fluid appears to contribute significantly to prostate ADCs. Its contribution could be adjusted by choosing an appropriate inversion recovery preparation, potentially enhancing contrast for prostate cancer lesions.