Genitourinary · Prostate MRI · 1.0
PI-RADS v2.1: Prostate MRI assessment categories
Zone-specific scoring system for multiparametric prostate MRI that assigns an overall assessment category 1-5 reflecting the likelihood of clinically significant prostate cancer.
Purpose
PI-RADS v2.1 standardises acquisition, interpretation and reporting of multiparametric prostate MRI in treatment-naive patients with suspected prostate cancer, and assigns each lesion an overall assessment category reflecting the likelihood of clinically significant prostate cancer.
| Overall category | Likelihood of clinically significant cancer | Peripheral zone (DWI is the dominant sequence) | Transition zone (T2W is the dominant sequence) |
|---|---|---|---|
| PI-RADS 1 | Very low | DWI 1: no abnormality on ADC and high b-value images | T2W 1: normal-appearing transition zone, or a round completely encapsulated nodule |
| PI-RADS 2 | Low | DWI 2: indistinct hypointensity on ADC | T2W 2: mostly encapsulated nodule, homogeneous circumscribed nodule without encapsulation, or a homogeneous mildly hypointense area between nodules |
| PI-RADS 3 | Intermediate (equivocal) | DWI 3: focal mild or moderate hypointensity on ADC and isointense or mildly hyperintense on high b-value images; upgraded to 4 if DCE is positive | T2W 3: heterogeneous signal intensity with obscured margins; upgraded to 4 if DWI score is 5 |
| PI-RADS 4 | High | DWI 4: focal markedly hypointense on ADC and markedly hyperintense on high b-value images, less than 1.5 cm in greatest dimension | T2W 4: lenticular or non-circumscribed, homogeneous, moderately hypointense, less than 1.5 cm in greatest dimension |
| PI-RADS 5 | Very high | DWI 5: features of DWI 4 but 1.5 cm or larger, or with definite extraprostatic extension or invasive behaviour | T2W 5: features of T2W 4 but 1.5 cm or larger, or with definite extraprostatic extension or invasive behaviour |
How to use it
- Score by zone: in the peripheral zone the DWI/ADC score determines the category, and in the transition zone the T2-weighted score determines the category. Assign each lesion to the zone in which the bulk of the lesion lies.
- Dynamic contrast enhancement is used only as a tie-breaker in the peripheral zone: a positive DCE (focal enhancement earlier than or at the same time as surrounding normal tissue, corresponding to a suspicious T2W or DWI finding) upgrades a peripheral-zone DWI 3 to overall category 4. A DCE-positive transition-zone lesion is not upgraded on that basis.
- In the transition zone, a T2W score of 3 is upgraded to overall category 4 when the DWI score is 5.
- Measure the lesion in its greatest dimension on the axial image of the sequence that determined the score (DWI/ADC for peripheral zone, T2W for transition zone); if the largest dimension is on the sagittal or coronal plane, report that measurement and state the plane. The 1.5 cm threshold separates category 4 from category 5.
- Version 2.1 refined the transition-zone T2 criteria for scores 1-3, restricted DCE to a binary positive/negative role, and tightened technical requirements including high b-value acquisition of at least 1400 s/mm2 (acquired or calculated), DWI slice thickness of 3 mm or less and DCE temporal resolution of 15 seconds or less.
Common mistake
Treating PI-RADS as a histological or staging system, or scoring the peripheral zone off the T2-weighted images. PI-RADS categorises the likelihood of clinically significant cancer on a per-lesion basis; the peripheral zone is scored on DWI/ADC and the transition zone on T2W, and DCE never independently creates a category 4 or 5 lesion.
Exam pearl
Peripheral zone equals DWI dominant, transition zone equals T2 dominant; DCE only upgrades a peripheral-zone 3 to a 4, and 1.5 cm (or definite extraprostatic extension) is what turns a 4 into a 5.
Viva questions
- What does PI-RADS v2.1 actually assess?
- It assesses the likelihood that a lesion on multiparametric prostate MRI represents clinically significant prostate cancer, on a per-lesion five-point scale, in treatment-naive patients. It is not a histological grade and not a staging system.
- Which sequence is dominant in each zone?
- Diffusion-weighted imaging with the ADC map is dominant in the peripheral zone, and T2-weighted imaging is dominant in the transition zone.
- What role does dynamic contrast enhancement play in v2.1?
- It is binary. A positive DCE study upgrades a peripheral-zone DWI score of 3 to an overall category 4. It does not upgrade transition-zone lesions and never independently produces a category 5.
- How is a transition-zone T2 score of 3 handled?
- A transition-zone lesion scored T2W 3 is upgraded to overall category 4 when the DWI score is 5; otherwise it remains category 3.
- What separates category 4 from category 5?
- Size of 1.5 cm or more in greatest dimension, or definite extraprostatic extension or invasive behaviour, moves an otherwise category 4 lesion to category 5.
- How should a suspicious lesion be measured?
- Measure the greatest dimension on the axial image of the sequence that determined the score, that is the ADC map for peripheral-zone lesions and T2-weighted images for transition-zone lesions. If the largest dimension is on a sagittal or coronal image, report that value and state the plane.
- Name the key technical requirements introduced or tightened in v2.1.
- High b-value imaging of at least 1400 s/mm2, acquired or calculated from lower b-values, DWI slice thickness of 3 mm or less with no gap, and dynamic contrast-enhanced temporal resolution of 15 seconds or less.
- What is the commonest interpretive pitfall with PI-RADS?
- Scoring the wrong dominant sequence for the zone, or using positive DCE to justify a category 4 or 5 transition-zone lesion. A further pitfall is quoting a PI-RADS category as if it were a Gleason grade or a stage.