In years prior, the basis for biopsy rested entirely within the hands of the urologist, mainly through PSA evaluation and digital rectal exam. As prostate MRI and the use of PSA density have gained traction, the weight of the physical exam has diminished, evidenced by its lack of inclusion in current AUA guidelines for the early detection of prostate cancer. Prostate MRI has revolutionized how we diagnose prostate cancer, particularly since the advent of magnetic resonance/ultrasound-guided fusion prostate biopsy. Unfortunately, it has also made urologists heavily dependent on our radiology colleagues. Though there is nothing inherently wrong with this dependency in the setting of a radiologist who is well versed in reading prostate MRIs, many urologists are not fortunate enough to work with such experts. This limitation highlights the need to formalize training residents and attendings on how to interpret prostate MRIs. Training urologists can be accomplished in various ways, whether hands-on small group sessions, grand round lectures, or formalized courses.¹ This skill has far-reaching impacts, including biopsy decision-making, focal therapy candidacy, and surgical treatment planning. Mastery of prostate MRI interpretation will not only make us better diagnosticians but also better surgeons. A recent study demonstrated an approximately 50% decrease in the incidence of positive margins during prostatectomy by providing real-time 3D visualization of tumor and prostate gland relationships.²

The section below aims to provide a brief overview of prostate MRI, including what to look for in each sequence (see Figure).

Image Quality

Consistently obtaining high-quality images for the detection, localization, staging, and follow-up of clinically significant prostate cancer (csPCa) can be challenging due to variations in MRI scanners, software levels, and the experience of prostate MRI technologists.

It is crucial that prostate MRI scans adhere to the Prostate Imaging Reporting & Data System (PI-RADS) v2.1 guidelines and are conducted by trained MRI technologists using a standardized protocol comprising T2-weighted (T2WI), diffusion-weighted (DWI), and dynamic contrast-enhanced imaging (DCE-MRI). Modern scanners offer improved consistency and high-quality images. Although the PI-RADS v2.1 standard does not provide specific recommendations for patient preparation, proper preparation is essential to achieve optimal image quality. Patient preparation involves ensuring patient comfort, providing clear instructions and communication before and during the scanning procedure, and considering using antispasmodics and rectal air removal (micro-enema) where applicable. While artifacts can still affect multiparametric MRI images, using modern machines, techniques, faster image protocols, and appropriate patient preparation can help minimize their impact.

T2WI

T2WI provides anatomical and morphological information about the prostate and surrounding structures. These images are typically acquired in 3 perpendicular planes, allowing visualization of the zonal anatomy and the relationship between the prostate and its surroundings. T2WI helps differentiate the high-signal (bright) peripheral zone (PZ), the heterogeneous mixed-signal transition zone (TZ), and the low-signal (dark) central zone (CZ). The high signal in the PZ is often caused by cystic degeneration with high fluid content and is usually surrounded by a thin hypointense rim, representing a pseudocapsule.

The TZ exhibits mixed signal intensity due to various stages of benign prostatic hyperplasia (BPH) nodules. The CZ contains more dense fibrous tissue and thus appears as a low-signal area.

Lesions can be anatomically localized, and their shape, form, and size can be assessed on T2WI. Zonal distinction of the prostate is important for PI-RADS assessment. In the TZ, T2WI is used to detect csPCa, while in the PZ, DWI is more dominant. The high-signal PZ may show areas of lower signal intensity due to the presence of prostate cancer (PCa; see Figure), although PCa can also present as iso-signal areas or nonfocal mildly hypointense abnormalities. Low-grade PCa or nonmalignant conditions like scar tissue, hemorrhage, atrophy, postradiation changes, and prostatitis (including granulomatous prostatitis) often have low signal (dark) intensity on T2WI, making it challenging to differentiate them from csPCa based solely on T2WI signal. However, differentiation between csPCa, low-grade PCa, and benign pathology can be achieved by considering the anatomical and morphological characteristics. csPCa is likelier to exhibit a focal, round, or irregular structure. At the same time, postprostatitis fibrosis is characterized by a wedge-shaped appearance, and prostatitis by a more diffuse appearance.

Features indicative of csPCa in the TZ on T2WI include areas with ill-defined margins, focal homogeneous intermediate-low signal (referred to as the “erased charcoal drawing sign”), a noncircumscribed, lenticular, or fusiform shape, and invasion of surrounding structures (“disruption of organized chaos”). To determine whether an abnormal region is suspicious for csPCa, T2WI should be used in conjunction with the other 2 functional imaging techniques.

DWI

DWI is considered the most important functional technique, corresponding to histopathological findings. DWI provides information on the diffusion of intracellular water, which is reduced in dense, hypercellular tissue, leading to restricted diffusion. This is visible as a low signal (dark) on the DWI-derived apparent diffusion coefficient (ADC) map. Conversely, low cell density appears as a high signal on the ADC map. Another DWI-derived image is the high b-value (>1,400 s/mm²) image, where hypercellularity appears as a high signal (bright) and low cell density appears dark. The normal PZ, which contains fluid-filled glandular structures with high water molecule velocity, has a high signal on the ADC map. csPCa replaces normal glandular tissue and exhibits hypercellularity, resulting in a low signal on the ADC map (restricted diffusion). There is an inverse relationship between ADC values and Gleason score, meaning that decreasing ADC values correlate with increasing Gleason score. However, in the TZ, BPH can also exhibit restricted diffusion. Therefore, DWI is more accurate for csPCa detection in the PZ than in the TZ. Focal lesions are more likely to be csPCa than diffuse lesions, such as those caused by prostatitis. It is important to note that DWI is susceptible to artifacts, which can be caused by factors such as bowel peristalsis, total hip prosthesis, or rectal gas.

DCE and T1-Weighted Imaging

DCE-MRI involves T1-weighted images that show tissue enhancement (vascularization) after the injection of a contrast agent. Due to tumor angiogenesis and higher vessel permeability, low-grade PCa and csPCa, cellular-BPH, and inflammation show earlier, and more pronounced, enhancement than other prostate tissue. Therefore, accurately differentiating benign structures such as highly vascularized prostatitis in the PZ or highly perfused cellular BPH in the TZ from csPCa is challenging. DCE-MRI is valuable for detecting local recurrences (eg, postradiotherapy or after radical prostatectomy) and can help identify prostatitis in untreated patients and provide additional information in equivocal cases.

Interpretation

To enhance standardization, lesions should be scored using the PI-RADS assessment system. TZ lesions showing the “erased charcoal drawing” or disruption of “organized chaos,” as well as PZ lesions appearing black on the ADC map and white on the high b-value DWI, should be evaluated for a high likelihood of csPCa using the PI-RADS system. When multiparametric MRI is of good quality and evaluated according to PI-RADS v2.1 recommendations, it provides valuable information that can be used alongside other clinical data to reliably exclude csPCa and avoid unnecessary biopsies, guiding the targeting of MRI-directed biopsy cores. The Figure demonstrates an excellent MRI quality with clear delineation of the PZ vs TZ. Within the TZ one can identify nodules with a distinct border as well as the right mid–anterior peripheral zone lesion. The prostatic capsule is visualized with a dark line around the PZ posteriorly and TZ anteriorly.

In conclusion, these are transferable skills with respect to prostate MRI interpretation, which will impact your magnetic resonance/US fusion–guided prostate biopsies, focal therapy planning, surgical planning, and most importantly your patients’ prostate cancer outcomes.