Residency · Residency · Diagnostic Radiology
Prostate MRI and PI-RADS
Introduction
Multiparametric MRI (mpMRI) of the prostate has become the cornerstone imaging tool for detecting, localizing, and staging clinically significant prostate cancer (csPCa). The Prostate Imaging Reporting and Data System (PI-RADS) provides a standardized framework for interpretation and reporting, currently in its version 2.1 (2019).
Indications for Prostate MRI
Prostate MRI is indicated for elevated or rising PSA with clinical suspicion for prostate cancer, risk stratification prior to initial biopsy (pre-biopsy MRI is now standard of care), active surveillance monitoring of known low-grade prostate cancer, staging of biopsy-proven prostate cancer (extraprostatic extension, seminal vesicle invasion), and evaluation of suspected recurrence after treatment.
Prostate Anatomy on MRI
Zonal Anatomy
The peripheral zone (PZ) is located posteriorly and laterally and is the site of approximately 70% of prostate cancers. The transition zone (TZ) surrounds the urethra, enlarges with BPH, and accounts for approximately 25% of cancers. The central zone surrounds the ejaculatory ducts and is rarely the site of primary cancer. The anterior fibromuscular stroma is non-glandular tissue where tumors are uncommon.
Normal Signal Characteristics
The PZ demonstrates high T2 signal (uniform bright) and low signal on the ADC map. The TZ shows heterogeneous T2 signal due to BPH nodules, with an organized nodular pattern described as the "erased charcoal" sign in BPH.
mpMRI Sequences
T2-Weighted Imaging (T2W)
T2W is the primary sequence for morphologic assessment and TZ lesion evaluation. Cancer in the PZ appears as focal low T2 signal replacing the normal bright PZ. Cancer in the TZ appears as a non-circumscribed, lenticular, homogeneous low-signal lesion.
Diffusion-Weighted Imaging (DWI) and ADC Map
DWI is the dominant sequence for PZ lesion assessment in PI-RADS. Cancer shows restricted diffusion with high signal on high b-value DWI (b >= 1400) and low ADC. ADC values inversely correlate with Gleason grade, and a quantitative ADC threshold for csPCa is generally less than 750-800 x 10^-6 mm2/s.
Dynamic Contrast-Enhanced (DCE) Imaging
Cancer demonstrates early, focal enhancement (type 2 or 3 curves). DCE plays a limited but supportive role in PI-RADS v2.1, used only to upgrade PI-RADS 3 to 4 in the PZ. Positive DCE is defined as focal enhancement that is earlier than or contemporaneous with adjacent normal tissue.
PI-RADS v2.1 Scoring
Assessment Categories
| PI-RADS Score | Likelihood | csPCa Probability | Management |
|---|---|---|---|
| 1 | Very low | Highly unlikely | No biopsy |
| 2 | Low | Unlikely | No biopsy |
| 3 | Intermediate | Equivocal | Consider biopsy based on clinical factors |
| 4 | High | Likely | MRI-targeted biopsy recommended |
| 5 | Very high | Highly likely | MRI-targeted biopsy recommended |
PI-RADS 1 indicates very low likelihood (csPCa highly unlikely). PI-RADS 2 indicates low likelihood (csPCa unlikely). PI-RADS 3 indicates intermediate likelihood (equivocal for csPCa). PI-RADS 4 indicates high likelihood (csPCa likely). PI-RADS 5 indicates very high likelihood (csPCa highly likely).
Dominant Sequence by Zone
In the peripheral zone, DWI/ADC is the dominant sequence, and DCE can upgrade PI-RADS 3 to 4. In the transition zone, T2W is the dominant sequence, and DWI can upgrade PI-RADS 3 to 4.
Size Thresholds
The distinction between PI-RADS 4 and 5 in the PZ is based on a lesion >= 1.5 cm on DWI/ADC or evidence of extraprostatic extension, which upgrades to 5. In the TZ, a lesion >= 1.5 cm on T2W or evidence of invasion upgrades to 5.
Staging
T2 disease is organ-confined. T3a indicates extraprostatic extension (EPE), identified by an irregular capsular bulge, capsular disruption, or neurovascular bundle infiltration. T3b indicates seminal vesicle invasion, manifesting as focal T2 low signal, restricted diffusion, or enhancement within the seminal vesicle. T4 indicates invasion of adjacent structures (bladder, rectum, pelvic sidewall).
Pitfalls and Mimics
Prostatitis can cause diffuse or focal T2 low signal and restricted diffusion and should be correlated with clinical history. Post-biopsy hemorrhage produces high T1 signal; a minimum wait of 6 weeks after biopsy is required before MRI. BPH nodules are organized, circumscribed, and often have a pseudocapsule in the TZ. Anterior fibromuscular stroma lesions are difficult to assess due to inherently low signal on all sequences.
Key Clinical Pearls
Pre-biopsy MRI followed by MRI-targeted biopsy detects more clinically significant cancer and fewer indolent cancers compared to systematic biopsy alone. A negative MRI (PI-RADS 1-2) has a negative predictive value of 85-95% for csPCa (Gleason >= 3+4). Always report lesion size, PI-RADS score, index lesion location using the PI-RADS sector map, and staging features. Biparametric MRI (T2W + DWI without DCE) is an emerging alternative that reduces scan time with comparable accuracy.
References
- Turkbey B, et al. Prostate Imaging Reporting and Data System Version 2.1. Eur Urol. 2019;76(3):340-351.
- Kasivisvanathan V, et al. MRI-targeted or standard biopsy for prostate-cancer diagnosis (PRECISION trial). N Engl J Med. 2018;378(19):1767-1777.
- Weinreb JC, et al. PI-RADS Prostate Imaging Reporting and Data System: 2015, Version 2. Eur Urol. 2016;69(1):16-40.
- Ahmed HU, et al. Diagnostic accuracy of mpMRI and TRUS biopsy in prostate cancer (PROMIS). Lancet. 2017;389(10071):815-822.