Multiparametric prostate MRI (mpMRI) has become clinically significant for prostate cancer assessment. It is especially recommended when there are clear risk factors increase the probabilities of prostate disease such as family history, prior negative biopsy, active surveillance, and risk calculator. With PI-RADS version 2.1, classification of prostate lesions has become more standardized among radiologists. PIRADS v2.1 divides the prostate anatomically in 41 regions, with two additional sectors in the right and left posterior medial Peripheral Zone (PZ) when compared to v2 (Fig. 2).
It also defines clinically significant cancer as Gleason score >7, and/or volume >0.5 cc, and/or extra prostatic extension. PI-RADS uses anatomical location and lesion size when scoring.
Clinical Considerations for Prostate MRI
- Timing of MRI after prostate TRUS biopsy
- at least 6 weeks or more to avoid misleading assessment and grading due to the risk of hemorrhage (most common in PZ and seminal vesicles).
- Patient preparation
- Light diet preceding day
- Evacuate rectum just prior to exam since they may cause artifactual distortion in DWI, for which an enema may be considered
Normal Prostate
The appearance of a normal prostate in mpMRI is homogeneous low signal in T1WI and low signal in central zone (CZ) (TZ and CZ appear similar in SI and loosely termed central gland) and higher signal in peripheral zone (PZ).
Prostate muliparametric MRI (mpMRI)12
Prostate mpMRI (T2W + DWI + DCE combination, pre biopsy) has a high sensitivity of 86%, high specificity of 94%, and high NPV of 95%, making it more specific than “traditional” prostate MRI.
It has two parts (Fig. 5):
- T1WI/T2WI to evaluate anatomy and morphology
- DWI/ADC and DCE to evaluate functional and physiologic assessment
It can be performed either in a 3.0T or a 1.5T MRI, the main difference being the use of an endorectal coil (ERC) when performing a 1.5T MRI. The pros of the ERC are that it increases SNR and spatial resolution at any magnetic field strength, which is valuable for DWI and DCE, especially in larger or obese patients. The cons of the ERC are that it increases cost, introduces artifacts, is uncomfortable for patients, deforms the prostate gland, and the air needed for the coil balloon could produce magnetic field inhomogeneity.
Protocol12
Standard sequences for prostate mpMRI are the following:
- Axial T1W pelvis (large FOV)
- LAD, bone marrow, overview of pelvic pathology
- High resolution Ax,Sag and Cor T2W (small FOV)
- T2WI should be performed in the axial plane to the patient or oblique axial perpendicular to the prostatic long axis. An additional plane, either sagittal or coronal, should also be obtained
- Prostate lesions; more important in TZ
- DWI
- low b-value (0-100 sec/mm2)
- intermediate b–value (800-1000sec/mm2)
- high b-value (>1,400 sec/mm2); mandatory
- Prostate lesions (cellularity); more important in PZ
-DCE
- temporal resolution <15 seconds
- 3D T1W GRE images preferred
- evaluate angiogenesis, very sensitive, less specific
- Not performed if GFR < 30
- Spectroscopy (not needed)
Radiology Report12:
The radiology report of a prostate MRI should include the following:
- Prostate size and volume
- Post biopsy hemorrhage
- Lesion (each one described with: size, side, level,location,zone, T2W, DWI, DCE)
- Signal in TZ
- Signal in PZ
- Capsule/EPE
- Seminal vesicles
- Lymph nodes
- Osseous structures
- Other findings (rectum, diverticulosis, hernia, spine)
Advantages and Pitfalls (Fig. 3)
Use of prostate MRI has its advantages. MRI has superior tissue characterization and visualization of the prostate gland. It is a non-invasive procedure that has no ionizing radiation. Multiparametric MRI (mpMRI) is more accurate when compared to other diagnostic approaches by reducing detection of low-risk prostate cancer, reducing number of men requiring biopsy, and improving rate of detection of intermediate/high-risk prostate cancer. It is also better for staging, as it also allows for a more accurate assessment of tumor size and invasion and metastatic disease evaluation.
Pitfalls of prostate MRI are that it may be technically challenging, time consuming, costly, and inaccessible. There is also the risk of interobserver variability and depends on urologist acceptance. It is also important to be aware that prostatitis may cause PZ signal abnormalities, resulting in equivocal results.
PI-RADS V2.1 Objectives (as stated in the ACR PI-RADS 2019 2.1 Version)12
The Objectives of PI-RADS v2.1 are the same as v2 and are as follows:
- Establish minimum acceptable technical parameters for prostate mpMRI.
- Simplify and standardize the terminology and content of radiology reports.
- Facilitate the use of MRI data for targeted biopsy.
- Develop assessment categories that summarize levels of suspicion or risk and can be used to select patients for biopsies and management (e.g., observation strategy vs. immediate intervention).
- Enable data collection and outcome monitoring.
- Educate radiologists on prostate MRI reporting and reduce variability in imaging interpretations.
- Enhance interdisciplinary communications with referring clinicians.
PI-RADS v2.1 Update4
*Changes are shown in images and tables with red asterisk.
Image Acquisition Changes
- Plane of T2W images: T2WI should be performed in the axial plane to the patient or oblique axial perpendicular to the prostatic long axis. An additional plane, either sagittal or coronal, should also be obtained.
- DWI b-value uses : ADC map calculation should use a
- low b-value (0-100 secmm2)
- intermediate b–value (800-1000sec/mm2)
- high b-value (>1,400 sec/mm2)
- Dynamic Contrast Enhancement (DCE) temporal resolution
- temporal resolution <15 seconds
- 3D T1W GRE images preferred
Image Interpretation Changes
- Sector map has two additional sectors: Right and left posterior medial PZ (PZpm) at the base for a total of 41 sectors (Fig. 2)
- Prostate Volume Calculation
- Criteria for DWI scores of 2 and 3 (Table 1)
- A DWI score of 4 or 5 in the TZ elevates the overall PI-RADS category from a 2 to a 3 for a lesion with a T2W score of 2
- New score 2: linear/wedge shaped hypointense on ADC and/or linear/wedge shaped hyperintense on high b-value DWI
- New score 3: focal (discrete and different from the background) hypointense on ADC and/or focal hyperintense on high b-value DWI; may be markedly hypointense on ADC or markedly hyperintense on high b-value DWI, but not both
- TZ overall assessment categories (Table 2)
- Changes in T2WI scores of 1 and 2 in the TZ
- Score 1: Normal appearing TZ or a round, completely encapsulated nodule
- Score 2: A mostly encapsulated nodule or a homogeneous circumscribed nodule without encapsulation or a homogeneous mildly hypointense area between nodules
- Only nodules or lesions/regions between nodules that differ from the background TZ will be scored
- Typical BPH nodules are NOT scored
- Negative enhancement criteria on DCE (Table 4)
- New negative score: no early or contemporaneous enhancement; or diffuse multifocal enhancement not corresponding to a focal finding on T2W and/or DWI or focal enhancement corresponding to a lesion demonstrating features of BPH on T2WI (including features of extruded BPH in the PZ)
- Evaluation of Central Zone (CZ) and Anterior Fibromuscular Stroma (AFMS) lesions
- One should try to identify where the lesion is coming from in order to identify which dominant sequence will be used for PI-RADS assessment (T2W for TZ and DWI for PZ)
Biparametric MRI (bpMRI)
- No specific recommendation for bpMRI (without DCE)
- mpMRI recommended when:
- Prior negative bpMRI
- Suboptimal DWI image quality
- Clinical risk factors that indicate significant disease
PI-RADS Assessment Category Examples