The Prostate Imaging – Reporting and Data System Version 2 (PI-RADS™ v2) is the product of an international collaboration of the American College of Radiology (ACR), European Society of Uroradiology (ESUR), and AdMetech Foundation. It is designed to promote global standardization and diminish variation in the acquisition, interpretation, and reporting of prostate multiparametric magnetic resonance imaging (mpMRI) examination, and it is based on the best available evidence and expert consensus opinion. It establishes minimum acceptable technical parameters for prostate mpMRI, simplifies and standardizes terminology and content of reports, and provides assessment categories that summarize levels of suspicion or risk of clinically significant prostate cancer that can be used to assist selection of patients for biopsies and management. It is intended to be used in routine clinical practice and also to facilitate data collection and outcome monitoring for research.
Keywords: Prostate mpMRI, Prostate MRI, Magnetic resonance imaging, Prostate, Prostate cancer.
Magnetic Resonance Imaging (MRI) has been used for noninvasive assessment of the prostate gland and surrounding structures since the 1980s. Initially, prostate MRI was based solely on morphologic assessment using T1-weighted (T1W) and T2-weighted (T2W) pulse sequences, and its role was primarily for locoregional staging in patients with biopsy proven cancer. However, it provided limited capability to distinguish benign pathological tissue and clinically insignificant prostate cancer from significant cancer.
Advances in technology (both in software and hardware) have led to the development of multiparametric MRI (mpMRI), which combines anatomic T2W with functional and physiologic assessment, including diffusion-weighted imaging (DWI) and its derivative apparent-diffusion coefficient (ADC) maps, dynamic contrast-enhanced (DCE) MRI, and sometimes other techniques such as in-vivo MR proton spectroscopy. These technologic advances, combined with a growing interpreter experience with mpMRI, have substantially improved diagnostic capabilities for addressing the central challenges in prostate cancer care: 1) Improving detection of clinically significant cancer, which is critical for reducing mortality; and 2) Increasing confidence in benign diseases and dormant malignancies, which are not likely to cause problems in a man's lifetime, in order to reduce unnecessary biopsies and treatment.
Consequently, clinical applications of prostate MRI have expanded to include, not only locoregional staging, but also tumor detection, localization (registration against an anatomical reference), characterization, risk stratification, surveillance, assessment of suspected recurrence, and image guidance for biopsy, surgery, focal therapy and radiation therapy.
In 2007, recognizing an important evolving role for MRI in assessment of prostate cancer, the AdMeTech Foundation organized the International Prostate MRI Working Group, which brought together key leaders of academic research and industry. Based on deliberations by this group, a research strategy was developed and a number of critical impediments to the widespread acceptance and use of MRI were identified. Amongst these was excessive variation in the performance, interpretation, and reporting of prostate MRI exams. A greater level of standardization and consistency was recommended in order to facilitate multi-center clinical evaluation and implementation.
In response, the European Society of Urogenital Radiology (ESUR) drafted guidelines, including a scoring system, for prostate MRI known as PI-RADS™ version 1 (PI-RADS™ v1). Since it was published in 2012, PI-RADS™ v1 has been validated in certain clinical and research scenarios.
However, experience has also revealed several limitations, in part due to rapid progress in the field. In an effort to make PI-RADS™ standardization more globally acceptable, the American College of Radiology (ACR), ESUR and the AdMeTech Foundation established a Steering Committee to build upon, update and improve upon the foundation of PI-RADS™ v1. This effort resulted in the development PI-RADS™ v2.
PI-RADS™ v2 was developed by members of the PI-RADS Steering Committee, several working groups with international representation, and administrative support from the ACR using the best available evidence and expert consensus opinion. It is designed to promote global standardization and diminish variation in the acquisition, interpretation, and reporting of prostate mpMRI examinations and it is intended to be a “living” document that will evolve as clinical experience and scientific data accrue. PI-RADS™ v2 needs to be tested and validated for specific research and clinical applications.
PI-RADS™ v2 is designed to improve detection, localization, characterization, and risk stratification in patients with suspected cancer in treatment naïve prostate glands. The overall objective is to improve outcomes for patients. The specific aims are to:
- 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 is not a comprehensive prostate cancer diagnosis document and should be used in conjunction with other current resources. For example, it does not address the use of MRI for detection of suspected recurrent prostate cancer following therapy, progression during surveillance, or the use of MRI for evaluation of other parts of the body (e.g. skeletal system) that may be involved with prostate cancer. Furthermore, it does not elucidate or prescribe optimal technical parameters; only those that should result in an acceptable mpMRI examination.
The PI-RADS Steering Committee strongly supports the continued development of promising MRI methodologies for assessment of prostate cancer and local staging (e.g., nodal metastases) utilizing novel and/or advanced research tools not included in PI-RADS™ v2, such as in-vivo MR spectroscopic imaging (MRSI), diffusion tensor imaging (DTI), diffusional kurtosis imaging (DKI), multiple b-value assessment of fractional ADC, intravoxel incoherent motion (IVIM), blood oxygenation level dependent (BOLD) imaging, intravenous ultra-small superparamagnetic iron oxide (USPIO) agents, and MR-PET. Consideration will be given to incorporating them into future versions of PI-RADS™ as relevant data and experience become available.
2. Section I: Clinical Considerations and Technical Specifications
2.1. Clinical Considerations
2.1.1. Timing of MRI Following Prostate Biopsy
Hemorrhage, manifested as hyperintense signal on T1W, may be present in the prostate gland, most commonly the peripheral zone (PZ) and seminal vesicles, following systematic transrectal ultrasound-guided systematic (TRUS) biopsy and may confound mpMRI assessment. When there is evidence of hemorrhage in the PZ on MR images, consideration may be given to postponing the MRI examination until a later date when hemorrhage has resolved. However, this may not always be feasible or necessary, and clinical practice may be modified as determined by individual circumstances and available resources. Furthermore, if the MRI exam is performed following a negative TRUS biopsy, the likelihood of clinically significant prostate cancer at the site of post biopsy hemorrhage without a corresponding suspicious finding on MRI is low. In this situation, a clinically significant cancer, if present, is likely to be in a location other than that with blood products. Thus, the detection of clinically significant cancer is not likely to be substantially compromised by post biopsy hemorrhage, and there may be no need to delay MRI after prostate biopsy if the primary purpose of the exam is to detect and characterize clinically significant cancer in the gland.
However, post biopsy changes, including hemorrhage and inflammation, may adversely affect the interpretation of prostate MRI for staging in some instances. Although these changes may persist for many months, they tend to diminish over time, and an interval of at least 6 weeks or longer between biopsy and MRI should be considered for staging.
2.1.2. Patient Preparation
At present, there is no consensus concerning all patient preparation issues.
To reduce motion artifact from bowel peristalsis, the use of an antispasmodic agent (e.g. glucagon, scopolamine butylbromide, or sublingual hyoscyamine sulfate) may be beneficial in some patients. However, in many others it is not necessary, and the incremental cost and potential for adverse drug reactions should be taken into consideration.
The presence of stool in the rectum may interfere with placement of an endorectal coil (ERC). If an ERC is not used, the presence of air and/or stool in the rectum may induce artifactual distortion that can compromise DWI quality. Thus, some type of minimal preparation enema administered by the patient in the hours prior to the exam may be beneficial. However, an enema may also promote peristalsis, resulting in increased motion related artifacts in some instances.
The patient should evacuate the rectum, if possible, just prior to the MRI exam.
If an ERC is not used and the rectum contains air on the initial MR images, it may be beneficial to perform the mpMRI exam with the patient in the prone position or to decompress the rectum using suction through a small catheter.
Some recommend that patients refrain from ejaculation for three days prior to the MRI exam in order to maintain maximum distention of the seminal vesicles. However, a benefit for assessment of the prostate and seminal vesicles for clinically significant cancer has not been firmly established.
2.1.3. Patient Information
The following information should be available to the radiologist at the time of MRI exam performance and interpretation:
- Recent serum prostate-specific antigen (PSA) level and PSA history
- Date and results of prostate biopsy, including number of cores, locations and Gleason scores of positive biopsies (with percentage of core involvement when available).
- Other relevant clinical history, including digital rectal exam (DRE) findings, medications (particularly in the setting of hormones/hormone ablation), prior prostate infections, pelvic surgery, radiation therapy, and family history.
2.2. Technical Specifications
Prostate MRI acquisition protocols should always be tailored to specific patients, clinical questions, management options, and MRI equipment, but T2W, DWI, and DCE should be included in all exams. Unless the MRI exam is monitored and no findings suspicious for clinically significant prostate cancer are detected, at least one pulse sequence should use a field-of-view (FOV) that permits evaluation of pelvic lymph nodes to the level of the aortic bifurcation. The supervising radiologist should be cognizant that superfluous or inappropriate sequences unnecessarily increase exam time and discomfort, and this could negatively impact patient acceptance and compliance.
The technologist performing the exam and/or supervising radiologist should monitor the scan for quality control. If image quality of a pulse sequence is compromised due to patient motion or other reason, measures should be taken to rectify the problem and the sequence should be repeated.
2.2.1. Magnetic Field Strength
The fundamental advantage of 3T compared with 1.5T lies in an increased signal-to-noise ratio (SNR), which theoretically increases linearly with the static magnetic field. This may be exploited to increase spatial resolution, temporal resolution, or both. Depending on the pulse sequence and specifics of implementation, power deposition, artifacts related to susceptibility, and signal heterogeneity could increase at 3T, and techniques that mitigate these concerns may result in some increase in imaging time and/or decrease in SNR. However, current state-of-the-art 3T MRI scanners can successfully address these issues, and most members of the PI-RADS Steering Committee agree that the advantages of 3T substantially outweigh these concerns.
There are many other factors that affect image quality besides magnetic field strength, and both 1.5T and 3.0T can provide adequate and reliable diagnostic exams when acquisition parameters are optimized and appropriate contemporary technology is employed. Although prostate MRI at both 1.5 T and 3T has been well established, most members of the PI-RADS Steering Committee prefer, use, and recommend 3T for prostate MRI. 1.5T should be considered when a patient has an implanted device that has been determined to be MR conditional. 1.5T may also be preferred when patients are safe to undergo MRI at 3T, but the location of an implanted device may result in artifact that could compromise image quality (e.g., bilateral metallic hip prosthesis).
The recommendations in this document focus only on 3T and 1.5T MRI scanners since they have been the ones used for clinical validation of mpMRI. Prostate mpMRI at lower magnetic field strengths (<1.5T) is not recommended unless adequate peer reviewed clinical validation becomes available.
2.2.2. Endorectal Coil (ERC)
When integrated with external (surface) phased array coils, endorectal coils (ERCs) increase SNR in the prostate at any magnetic field strength. This may be particularly valuable for high spatial resolution imaging used in cancer staging and for inherently lower SNR sequences, such as DWI and high temporal resolution DCE.
ERCs can also be advantageous for larger patients where the SNR in the prostate may be compromised using only external phased array RF coils. However, use of an ERC may increase the cost and time of the examination, deform the gland, and introduce artifacts. In addition, it may be uncomfortable for patients and increase their reluctance to undergo MRI.
With some 1.5T MRI systems, especially older ones, use of an ERC is considered indispensable for achieving the type of high resolution diagnostic quality imaging needed for staging prostate cancer. At 3T without use of an ERC, image quality can be comparable with that obtained at 1.5 T with an ERC, although direct comparison of both strategies for cancer detection and/or staging is lacking. Importantly, there are many technical factors other than the use of an ERC that influence SNR (e.g., receiver bandwidth, coil design, efficiency of the RF chain), and some contemporary 1.5T scanners that employ a relatively high number of external phased array coil elements and RF channels (e.g., 16 or more) may be capable of achieving adequate SNR in many patients without an ERC.
Credible satisfactory results have been obtained at both 1.5T and 3T without the use of an ERC. Taking these factors into consideration as well as the variability of MRI equipment available in clinical use, the PI-RADS Steering Committee recommends that supervising radiologists to strive to optimize imaging protocols in order to obtain the best and most consistent image quality possible with the MRI scanner used. However, cost, availability, patient preference, and other considerations cannot be ignored.
If air is used to inflate the ERC balloon, it may introduce local magnetic field inhomogeneity, resulting in distortion on DWI, especially at 3T. The extent to which artifacts interfere with MRI interpretation will vary depending on specific pulse sequence implementations, but they can be diminished using correct positioning of the ERC and distention of the balloon with liquids (e.g. liquid perfluorocarbon or barium suspension) that will not result in susceptibility artifacts. When liquid is used for balloon distention, all air should be carefully removed from the ERC balloon prior to placement. Solid, rigid reusable ERCs that avoid the need for inflatable balloons and decrease gland distortion have been developed.
2.2.3. Computer-Aided Evaluation (CAE) Technology
Computer-aided evaluation (CAE) technology using specialized software or a dedicated workstation is not required for prostate mpMRI interpretation. However, CAE may improve workflow (display, analysis, interpretation, reporting, and communication), provide quantitative pharmacodynamic data, and enhance lesion detection and discrimination performance for some radiologists, especially those with less experience interpreting mpMRI exams. CAE can also facilitate integration of MRI data with some forms of MR targeted biopsy systems.
3. Section II: Normal Anatomy and Benign Findings
3.1. Normal Anatomy ( Figure 1 )
From superior to inferior, the prostate consists of the base (just below the urinary bladder), the midgland, and the apex. It is divided into four histologic zones: (a) the anterior fibromuscular stroma, contains no glandular tissue; (b) the transition zone (TZ), surrounding the urethra proximal to the verumontanum, contains 5% of the glandular tissue; (c) the central zone (CZ), surrounding the ejaculatory ducts, contains about 20% of the glandular tissue; and (d) the outer peripheral zone (PZ), contains 70%-80% of the glandular tissue. When benign prostatic hyperplasia (BPH) develops, the TZ will account for an increasing percentage of the gland volume.
Approximately 70%-75% of prostate cancers originate in the PZ and 20%-30% in the TZ. Cancers originating in the CZ are uncommon, and the cancers that occur in the CZ are usually secondary to invasion by PZ tumors.
Based on location and differences in signal intensity on T2W images, the TZ can often be distinguished from the CZ on MR images. However, in some patients, age-related expansion of the TZ by BPH may result in compression and displacement of the CZ. Use of the term “central gland” to refer to the combination of TZ and CZ is discouraged as it is not reflective of the zonal anatomy as visualized or reported on pathologic specimens.
A thin, dark rim partially surrounding the prostate on T2W is often referred to as the “prostate capsule.” It serves as an important landmark for assessment of extra prostatic extension of cancer. In fact, the prostate lacks a true capsule; rather it contains an outer band of concentric fibromuscular tissue that is inseparable from prostatic stroma. It is incomplete anteriorly and apically.
The prostatic pseudocapsule (sometimes referred to as the “surgical capsule”) on T2W MRI is a thin, dark rim at the interface of the TZ with the PZ. There is no true capsule in this location at histological evaluation, and this appearance is due to compressed prostate tissue.
Nerves that supply the corpora cavernosa are intimately associated with arterial branches from the inferior vesicle artery and accompanying veins that course posterolateral at 5 and 7 o’clock to the prostate bilaterally, and together they constitute the neurovascular bundles. At the apex and base, small nerve branches surround the prostate periphery and penetrate through the capsule, a potential route for extraprostatic extension (EPE) of cancer.
3.2. Sector Map (Appendix II)
The segmentation model used in PI-RADS™ v2 was adapted from a European Consensus Meeting and the ESUR Prostate MRI Guidelines 2012. It employs thirty-nine sectors/regions: thirty-six for the prostate, two for the seminal vesicles, and one for the external urethral sphincter. (Appendix II).
Use of the Sector Map will enable radiologists, urologists, pathologists, and others to localize findings described in MRI reports, and it will be a valuable visual aid for discussions with patients about biopsy and treatment options.
Division of the prostate and associated structures into sectors standardizes reporting and facilitates precise localization for MR-targeted biopsy and therapy, pathological correlation, and research. Since relationships between tumor contours, glandular surface of the prostate, and adjacent structures, such as neurovascular bundles, external urethral sphincter, and bladder neck, are valuable information for periprostatic tissue sparing surgery, the Sector Map may also provide a useful roadmap for surgical dissection at the time of radical prostatectomy.
Either hardcopy (on paper) or electronic (on computer) recording on the Sector Map is acceptable.
For information about the use of the Sector Map, see Section III and Appendix II.
3.3. Benign Findings
Many signal abnormalities within the prostate are benign. The most common include:
3.3.1. Benign prostatic hyperplasia (BPH)
Benign prostatic hyperplasia (BPH) develops in response to testosterone, after it is converted to di-hydrotestosterone. BPH arises in the TZ, although exophytic and extruded BPH nodules can be found in the PZ. BPH consists of a mixture of stromal and glandular hyperplasia and may appear as band-like areas and/or encapsulated round nodules with circumscribed margins. Predominantly glandular BPH nodules and cystic atrophy exhibit moderate-marked T2 hyperintensity and are distinguished from malignant tumors by their signal and capsule. Predominantly stromal nodules exhibit T2 hypointensity. Many BPH nodules demonstrate a mixture of signal intensities. BPH nodules may be highly vascular on DCE and can demonstrate a range of signal intensities on DWI.
Although BPH is a benign entity, it may have important clinical implications for biopsy approach and therapy since it can increase gland volume, stretch the urethra, and impede the flow of urine. Since BPH tissue produces prostate-specific antigen (PSA), accurate measurement of gland volume by MRI is an important metric to allow correlation with an individual's PSA level and to calculate the PSA density (PSA/prostate volume).
Hemorrhage in the PZ and/or seminal vesicles is common after biopsy. It appears as focal or diffuse hyperintense signal on T1W and iso-hypointense signal on T2W. However, chronic blood products may appear hypointense on all MR sequences.
A variety of cysts can occur in the prostate and adjacent structures. As elsewhere in the body, cysts in the prostate may contain “simple” fluid and appear markedly hyperintense on T2W and dark on T1W. However, they can also contain blood products or proteinaceous fluid, which may demonstrate a variety of signal characteristics, including hyperintense signal on T1W.
Calcifications, if visible, appear as markedly hypointense foci on all pulse sequences.
Prostatitis affects many men, although it is often sub-clinical. Pathologically, it presents as an immune infiltrate, the character of which depends on the agent causing the inflammation. On MRI, prostatitis can result in decreased signal in the PZ on both T2W and the ADC (apparent diffusion coefficient) map. Prostatitis may also increase perfusion, resulting in a “false positive” DCE result. However, the morphology is commonly band-like, wedge-shaped, or diffuse, rather than focal, round, oval, or irregular, and the decrease in signal on the ADC map is generally not as great nor as focal as in cancer.
Prostatic atrophy can occur as a normal part of aging or from chronic inflammation. It is typically associated with wedge-shaped areas of low signal on T2W and mildly decreased signal on the ADC map from loss of glandular tissue. The ADC is generally not as low as in cancer, and there is often contour retraction of the involved prostate.
Prostatic fibrosis can occur after inflammation. It may be associated with wedge- or band-shaped areas of low signal on T2W.
4. Section III: Assessment and Reporting
A major objective of a prostate MRI exam is to identify and localize abnormalities that correspond to clinically significant prostate cancer, and mpMRI is able to detect intermediate to high grade cancers with volumes ≥0.5cc, depending on the location and background tissue within the prostate gland. However, there is no universal agreement of the definition of clinically significant prostate cancer.
In PI-RADS™ v2, the definition of clinically significant cancer is intended to standardize reporting of mpMRI exams and correlation with pathology for clinical and research applications. Based on the current uses and capabilities of mpMRI and MRI-targeted procedures, for PI-RADS™ v2 clinically significant cancer is defined on pathology/histology as Gleason score ≥7 (including 3+4 with prominent but not predominant Gleason 4 component), and/or volume ≥0.5cc, and/or extra prostatic extension (EPE).
PI-RADS™ v2 assessment uses a 5-point scale based on the likelihood (probability) that a combination of mpMRI findings on T2W, DWI, and DCE correlates with the presence of a clinically significant cancer for each lesion in the prostate gland.
PI-RADS™ v2 Assessment Categories
- PI-RADS 1 – Very low (clinically significant cancer is highly unlikely to be present)
- PI-RADS 2 – Low (clinically significant cancer is unlikely to be present)
- PI-RADS 3 – Intermediate (the presence of clinically significant cancer is equivocal)
- PI-RADS 4 – High (clinically significant cancer is likely to be present)
- PI-RADS 5 – Very high (clinically significant cancer is highly likely to be present)
Assignment of a PI-RADS™ v2 Assessment Category should be based on mpMRI findings only and should not incorporate other factors such as serum prostate specific antigen (PSA), digital rectal exam, clinical history, or choice of treatment. Although biopsy should be considered for PIRADS 4 or 5, but not for PIRADS 1 or 2, PI-RADS™ v2 does not include recommendations for management, as these must take into account other factors besides the MRI findings, including laboratory/clinical history and local preferences, expertise and standards of care. Thus, for findings with PIRADS Assessment Category 2 or 3, biopsy may or may not be appropriate, depending on factors other than mpMRI alone.
It is anticipated that, as evidence continues to accrue in the field of mpMRI and MRI-targeted biopsies and interventions, specific recommendations and/or algorithms regarding biopsy and management will be included in future versions of PI-RADS™.
When T2W and DWI are of diagnostic quality, DCE plays a minor role in determining PIRADS Assessment Category. Absence of early enhancement within a lesion usually adds little information, and diffuse enhancement not localized to a specific T2W or DWI abnormality can be seen in the setting of prostatitis. Moreover, DCE does not contribute to the overall assessment when the finding has a low (PIRADS 1 or 2) or high (PIRADS 4 or 5) likelihood of clinically significant cancer. However, when DWI is PIRADS 3 in the PZ, a positive DCE may increase the likelihood that the finding corresponds to a clinically significant cancer and may upgrade the Assessment Category to PIRADS 4 ( Table 1 ). Likewise when T2W is PIRADS 3 in the TZ, DWI may increase the likelihood that the finding corresponds to a clinically significant cancer and may upgrade the Assessment Category to PIRADS 4 ( Table 2 ).
* “Any” indicates 1-5
* “Any” indicates 1-5
4.1. Reporting (see Appendix I: Report Templates)
4.1.1. Measurement of the Prostate Gland
The volume of the prostate gland should always be reported. It may be determined using manual or automated segmentation or calculated using the formula for a conventional prolate ellipse; (maximum AP diameter) x (maximum transverse diameter) x (maximum longitudinal diameter) x 0.52 ( Figure 2 ).
Prostate volume may also be useful to calculate PSA density (PSA/prostate volume).
4.1.2. Mapping Lesions
Prostate cancer is usually multifocal. The largest tumor focus usually yields the highest Gleason score and is most likely to contribute to extraprostatic extension (EPE) and positive surgical margins.
For PI-RADS™ v2, up to four findings with a PIRADS Assessment Category of 3, 4, or 5 may each be assigned on the Sector Map (Appendix II), and the index (dominant) intraprostatic lesion should be identified. The index lesion is the one with the highest PIRADS Assessment Category. If the highest PIRADS Assessment Category is assigned to two or more lesions, the index lesion should be the one that shows EPE. Thus, a smaller lesion with EPE should be defined as the index lesion despite the presence of a larger tumor with the identical PIRADS Assessment Category. If none of the lesions demonstrate EPE, the largest of the tumors with the highest PIRADS Assessment Category should be considered the index lesion.
If there are more than four suspicious findings, then only the four with the highest likelihood of clinically significant cancer (i.e. highest PIRADS Assessment Category) should be reported. There may be instances when it is appropriate to report more than four suspicious lesions.
Reporting of additional findings with PIRADS Assessment Category 2 or definitely benign findings (e.g. cyst) is optional, but may be helpful to use as landmarks to guide subsequent biopsy or for tracking lesions on subsequent mpMRI exams.
If a suspicious finding extends beyond the boundaries of one sector, all neighboring involved sectors should be indicated on the Sector Map (as a single lesion).
4.1.3. Measurement of Lesions
With current techniques, mpMRI has been shown to underestimate both tumor volume and tumor extent compared to histology, especially for Gleason grade 3. Furthermore, the most appropriate imaging plane and pulse sequence for measuring lesion size on MRI has not been definitely determined, and the significance of differences in lesion size on the various MRI pulse sequences requires further investigation. In the face of these limitations, the PI-RADS Steering Committee nevertheless believes that standardization of measurements will facilitate MR-pathological correlation and research and recommends that the following rules be used for measurements.
The minimum requirement is to report the largest dimension of a suspicious finding on an axial image. If the largest dimension of a suspicious finding is on sagittal and/or coronal images, this measurement and imaging plane should also be reported. If a lesion is not clearly delineated on an axial image, report the measurement on the image which best depicts the finding.
Alternatively, if preferred, lesion volume may be determined using appropriate software, or three dimensions of lesions may be measured so that lesion volume may be calculated (maximum AP diameter) × (maximum transverse diameter) × (maximum cranio-caudal diameter) × 0.52.
In the PZ, lesions should be measured on ADC. In the TZ, lesions should be measured on T2W.
If lesion measurement is difficult or compromised on ADC (for PZ) or T2W (for TZ), measurement should be made on the sequence that shows the lesion best.
In the mpMRI report, the image number(s)/series and sequence used for measurement should be indicated.
4.2. Caveats for Overall Assessment
If both DWI and DCE are inadequate or absent, assessment should be limited to staging for determination of EPE.
5. Section IV: Multiparametric MRI (mpMRI)
5.1. T1-Weighted (T1W) and T2-Weighted (T2W)
Both T1W and T2W sequences should be obtained for all prostate MR exams. T1W images are used primarily to determine the presence of hemorrhage within the prostate and seminal vesicles and to delineate the outline of the gland. T1W images may also useful for detection of nodal and skeletal metastases, especially following intravenous administration of a gadolinium-based contrast agent (GBCA).
T2W images are used to discern prostatic zonal anatomy, assess abnormalities within the gland, and to evaluate for seminal vesicle invasion, EPE, and nodal involvement.
On T2W images, clinically significant cancers in the PZ ( Figure 3 ) usually appear as round or ill- defined hypointense focal lesions. However, this appearance is not specific and can be seen in various conditions such as prostatitis, hemorrhage, glandular atrophy, benign hyperplasia, biopsy related scars, and after therapy (hormone, ablation, etc.).
The T2W features of TZ tumors ( Figure 4 ) include non-circumscribed homogeneous, moderately hypointense lesions (“erased charcoal” or “smudgy fingerprint” appearance), spiculated margins, lenticular shape, absence of a complete hypointense capsule, and invasion of the urethral sphincter and anterior fibromuscular stroma. The more features present, the higher the likelihood of a clinically significant TZ cancer.
TZ cancers may be difficult to identify on T2W images since the TZ is often composed of variable amounts of glandular (T2-hyperintense) and stromal (T2-hypointense) tissue intermixed with each other, thus demonstrating heterogeneous signal intensity. Areas where benign stromal elements predominate may mimic or obscure clinically significant cancer.
Both PZ and TZ cancers may extend across anatomical boundaries. Invasive behavior is noted when there is extension within the gland (i.e. across regional parts of the prostate), into the seminal vesicles, or outside the gland (EPE).
5.1.1. Technical Specifications
Multiplanar (axial, coronal, and sagittal) T2W images are usually obtained with 2D RARE (rapid acquisition with relaxation enhancement) pulse sequences, more commonly known as fast-spin-echo (FSE) or turbo-spin-echo (TSE). In order to avoid blurring, excessive echo train lengths should be avoided.
- Slice thickness: 3 mm, no gap. Locations should be the same as those used for DWI and DCE
- FOV: generally 12-20 cm to encompass the entire prostate gland and seminal vesicles
- In plane dimension: ≤0.7 mm (phase) × ≤0.4 mm (frequency)
3D axial acquisitions may be used as an adjunct to 2D acquisitions. If acquired using isotropic voxels, 3D acquisitions may be particularly useful for visualizing detailed anatomy and distinguishing between genuine lesions and partial volume averaging effects. However, the soft tissue contrast is not identical and in some cases may be inferior to that seen on 2D T2W images, and the in-plane resolution may be lower than their 2D counterpart.
Axial T1W images of the prostate may be obtained with or without fat suppression using spin echo or gradient echo sequences. Locations should be the same as those used for DWI and DCE, although lower spatial resolution compared to T2W may be used to decrease acquisition time or increase anatomic coverage.
|Score||Peripheral Zone (PZ)|
|1||Uniform hyperintense signal intensity (normal)|
|2||Linear or wedge-shaped hypointensity or diffuse mild hypointensity, usually indistinct margin|
|3|| Heterogeneous signal intensity or non-circumscribed, rounded, moderate hypointensity|
Includes others that do not qualify as 2, 4, or 5
|4||Circumscribed, homogenous moderate hypointense focus/mass confined to prostate and <1.5 cm in greatest dimension|
|5||Same as 4 but ≥1.5 cm in greatest dimension or definite extra prostatic extension/invasive behavior|
|Score||Transition Zone (TZ)|
|1||Homogeneous intermediate signal intensity (normal)|
|2||Circumscribed hypointense or heterogeneous encapsulated nodule(s) (BPH)|
|3|| Heterogeneous signal intensity with obscured margins|
Includes others that do not qualify as 2, 4, or 5
|4||Lenticular or non-circumscribed, homogeneous, moderately hypointense, and <1.5 cm in greatest dimension|
|5||Same as 4, but ≥1.5 cm in greatest dimension or definite extraprostatic extension/invasive behavior|
5.2. Diffusion-Weighted Imaging (DWI)
Diffusion-weighted imaging (DWI) reflects the random motion of water molecules and is a key component of the prostate mpMRI exam. It should include an ADC map and high b-value images.
The ADC map is a display of ADC values for each voxel in an image. In most current clinical implementations, it uses two or more b-values and a monoexponential model of signal decay with increasing b-values to calculate ADC values. Most clinically significant cancers have restricted/impeded diffusion compared to normal tissues and, thus, appear hypointense on grey-scale ADC maps. Although ADC values have been reported to correlate inversely with histologic grades, there is considerable overlap between BPH, low grade cancers, and high grade cancers. Furthermore, ADC calculations are influenced by choice of b-values and have been inconsistent across vendors. Thus, qualitative visual assessment is often used as the primary method to assess ADC. Nevertheless, ADC values, using a threshold of 750-900 μm2/sec, may assist differentiation between benign and malignant prostate tissues in the PZ, with ADC values below the threshold correlating with clinically significant cancers.
“High b-value” images utilize a b-value ≥1400sec/mm2. They display preservation of signal in areas of restricted/impeded diffusion compared with normal tissues, which demonstrate diminished signal due to greater diffusion between applications of gradients with different b-values. Compared to ADC maps alone, conspicuity of clinically significant cancers is sometimes improved on high b-value images, especially in those adjacent to or invading the anterior fibromuscular stroma, in a subcapsular location, and at the apex and base of the gland. High b-value images can be obtained in one of two ways: either directly by acquiring a high b-value DWI sequence (requiring additional scan time), or by calculating (synthesizing) the high b-value image by extrapolation from the acquired lower b-value data used to create the ADC map (potentially less prone to artifacts because it avoids the longer TEs required to accommodate the strong gradient pulses needed for high b-value acquisitions). As the b-value increases, the signal-to-noise ratio (SNR) decreases, so that the optimum high b- value may be dependent on magnetic field strength, software, and manufacturer. Thus, there is no currently widely accepted optimal “high b-value”, but if adequate SNR permits, b-values of 1400-2000sec/mm2 or higher seem to be advantageous.
5.2.1. Technical Specifications
Free-breathing spin echo EPI sequence combined with spectral fat saturation is recommended.
- TE: ≤90 msec; TR: ≥3 000 msec
- Slice thickness: ≤4 mm, no gap. Locations should match or be similar to those used for T2W and DCE
- FOV: 16-22 cm
- In plane dimension: ≤2.5 mm phase and frequency
For ADC maps, if only two b-values can be acquired due to time or scanner constraints, it is preferred that the lowest b-value should be set at 50-100 sec/mm2 and the highest should be 800-1000sec/mm2. Additional b-values between 100 and 1000 may provide more accurate ADC calculations and estimations of extrapolated high b-value images (≥1400sec/mm2).
Information regarding perfusion characteristics of tissues may be obtained with additional b-values ranging from 0 to 500 sec/mm2,
Signal intensity in a lesion should be visually compared to the average signal of “normal” prostate tissue in the histologic zone in which it is located.
|Score||Peripheral Zone (PZ) or Transition Zone (TZ)|
|1||No abnormality (i.e., normal) on ADC and high b-value DWI|
|2||Indistinct hypointense on ADC|
|3||Focal mildly/moderately hypointense on ADC and isointense/mildly hyperintense on high b-value DWI.|
|4||Focal markedly hypontense on ADC and markedly hyperintense on high b-value DWI; <1.5 cm in greatest dimension|
|5||Same as 4 but ≥1.5 cm in greatest dimension or definite extraprostatic extension/invasive behavior|
5.2.3. Caveats for DWI
5.3. Dynamic Contrast-Enhanced (DCE) MRI
DCE MRI is defined as the acquisition of rapid T1W gradient echo scans before, during and after the intravenous administration of a low molecular weight gadolinium-based contrast agent (GBCA). As with many other malignancies following bolus injection of a GBCA, prostate cancers often demonstrate early enhancement compared to normal tissue. However, the actual kinetics of prostate cancer enhancement are quite variable and heterogeneous. Some malignant tumors demonstrate early washout, while others retain contrast longer. Furthermore, enhancement alone is not definitive for clinically significant prostate cancer, and absence of early enhancement does not exclude the possibility.
DCE should be included in all prostate mpMRI exams so as not to miss some small significant cancers. The DCE data should always be closely inspected for focal early enhancement. If found, then the corresponding T2W and DWI images should be carefully interrogated for a corresponding abnormality. At present, the added value DCE is not firmly established, and most published data show that the added value of DCE over and above the combination of T2W and DWI is modest. Thus, although DCE is an essential component of the mpMRI prostate examination, its role in determination of PI-RADS™ v2 Assessment Category is secondary to T2W and DWI.
DCE is positive ( Figure 7 ) when there is enhancement that is focal, earlier or contemporaneous with enhancement of adjacent normal prostatic tissues, and usually corresponds to a suspicious finding on T2W and/or DWI. Positive enhancement in a lesion usually occurs within 10 seconds of the appearance of the injected GBCA in the femoral arteries (depending on temporal resolution used to acquire the images, injection rate, cardiac output, and other factors).
The most widely available method of analyzing DCE is direct visual assessment of the individual DCE time-points at each slice location by either manually scrolling or using cine mode. Visual assessment of enhancement may be improved with fat suppression or subtraction techniques (especially in the presence of blood products that are hyperintense on pre-contrast enhanced T1W). Visual assessment of enhancement may also be assisted with a parametric map which color-codes enhancement features within a voxel (e.g., slope and peak). However, any suspicious finding on subtracted images or a parametric map should always be confirmed on the source images.
Considerable effort has gone into “curve typing” (i.e., plotting the kinetics of a lesion as a function of signal vs. time). However, there is great heterogeneity in enhancement characteristics of prostate cancers, and at present there is little evidence in the literature to support the use of specific curve types. Another approach is the use of compartmental pharmacokinetic modeling, which incorporates contrast media concentration rather than raw signal intensity and an arterial input function to calculate time constants for the rate of contrast agent wash-in (Ktrans) and wash-out (kep). Commercial software programs are available that produce “maps” of Ktrans and kep and may improve lesion conspicuity. Although pharmacodynamic (PD) analysis may provide valuable insights into tumor behavior and biomarker measurements for drug development, the PI-RADS Steering Committee believes there is currently insufficient peer reviewed published data or expert consensus to support routine adoption of this method of analysis for clinical use.
Thus, for PI-RADS™ v2, a “positive” DCE MRI lesion is one where the enhancement is focal, earlier or contemporaneous with enhancement of adjacent normal prostatic tissues, and corresponds to a finding on T2W and/or DWI. In the TZ, BPH nodules frequently enhance early, but they usually exhibit a characteristic benign morphology (round shape, well circumscribed). A “negative” DCE MRI lesion is one that either does not enhance early compared to surrounding prostate or enhances diffusely so that the margins of the enhancing area do not correspond to a finding on T2W and/or DWI.
5.3.1. Technical Specifications
DCE is generally carried out for several minutes to assess the enhancement characteristics. In order to detect early enhancing lesions in comparison to background prostatic tissue, temporal resolution should be <10 seconds and preferably <7 seconds per acquisition in order to depict focal early enhancement. Fat suppression and/or subtractions are recommended.
- Although either a 2D or 3D T1 gradient echo (GRE) sequence may be used, 3D is preferred.
- TR/TE: <100msec/ <5msec
- Slice thickness: 3 mm, no gap. Locations should be the same as those used for DWI and T2W.
- FOV: encompass the entire prostate gland and seminal vesicles
- In plane dimension: ≤2 mm X ≤2 mm
- Temporal resolution: ≤10 sec (<7 sec is preferred)
- Total observation rate: ≥2 min
- Dose: 0.1mmol/kg standard GBCA or equivalent high relativity GBCA
- Injection rate: 2-3cc/sec starting with continuous image data acquisition (should be the same for all exams)
5.3.2. PI-RADS Assessment for DCE ( Table 8 )
|Score||Peripheral Zone (PZ) or Transition Zone (TZ)|
|(−)||no early enhancement, or diffuse enhancement not corresponding to a focal finding on T2W and/or DWI or focal enhancement corresponding to a lesion demonstrating features of BPH on T2W|
|(+)||focal, and; earlier than or contemporaneously with enhancement of adjacent normal prostatic tissues, and corresponds to suspicious finding on T2W and/or DWI|
220.127.116.11. Caveats for DCE
6. Section V: Staging
MRI is useful for determination of the T stage, either confined to the gland (≤T2 disease) or extending beyond the gland (≥T3 disease).
The apex of the prostate should be carefully inspected. When cancer involves the external urethral sphincter, there is surgical risk of cutting the sphincter, resulting in compromise of urinary competence. Tumor in this region may also have implications for radiation therapy.
High spatial resolution T2W imaging is required for accurate assessment of extraprostatic extension (EPE), which includes assessment of neurovascular bundle involvement and seminal vesicle invasion. These may be supplemented by high spatial resolution contrast-enhanced fat suppressed T1W.
The features of seminal vesicle invasion include focal or diffuse low T2W signal intensity and/or abnormal contrast enhancement within and/or along the seminal vesicle, restricted diffusion, obliteration of the angle between the base of the prostate and the seminal vesicle, and demonstration of direct tumor extension from the base of the prostate into and around the seminal vesicle.
Imaging features used to assess for EPE include asymmetry or invasion of the neurovascular bundles, a bulging prostatic contour, an irregular or spiculated margin, obliteration of the rectoprostatic angle, a tumor-capsule interface greater than 1.0 cm, breach of the capsule with evidence of direct tumor extension or bladder wall invasion.
The next level of analysis is that of the pelvic and retroperitoneal lymph nodes. The detection of abnormal lymph nodes on MRI is currently limited to size, morphology and shape, and enhancement pattern. In general, lymph nodes over 8 mm in short axis dimension are regarded as suspicious, although lymph nodes that harbor metastases are not always enlarged. Nodal groups that should be evaluated include: common femoral, obturator, external iliac, internal iliac, common iliac, pararectal, presacral, paracaval, and para-aortic to the level of the aortic bifurcation.
Images should be assessed for the presence of skeletal metastases.
Author contributions: Jeffrey C. Weinreb had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Weinreb, Verma, Tempany, Thoeny, Shtern, Margolis, Macura, Haider, Cornud, Choyke, Barentsz.
Acquisition of data: Weinreb, Verma, Tempany, Thoeny, Shtern, Margolis, Macura, Haider, Cornud, Choyke, Barentsz.
Analysis and interpretation of data: Weinreb, Verma, Tempany, Thoeny, Shtern, Margolis, Macura, Haider, Cornud, Choyke, Barentsz.
Drafting of the manuscript: Weinreb, Verma, Tempany, Thoeny, Shtern, Margolis, Macura, Haider, Cornud, Choyke, Barentsz.
Critical revision of the manuscript for important intellectual content: Weinreb, Verma, Tempany, Thoeny, Shtern, Margolis, Macura, Haider, Cornud, Choyke, Barentsz.
Statistical analysis: None.
Obtaining funding: None.
Administrative, technical, or material support: None.
Supervision: Weinreb, Barentsz.
Other (specify): None.
Financial disclosures: Jeffrey C. Weinreb certifies that all conflicts of interest, including specific financial interests and relationships and affiliations relevant to the subject matter or materials discussed in the manuscript (eg, employment/affiliation, grants or funding, consultancies, honoraria, stock ownership or options, expert testimony, royalties, or patents filed, received, or pending), are the following: None.
Funding/Support and role of the sponsor: None.
Report Templates (this section is under construction)
The segmentation model used in PI-RADS™ v2 employs thirty-nine sectors/regions: thirty-six for the prostate, two for the seminal vesicles and one for the external urethral sphincter.
The prostate is divided into right/left on axial sections by a vertical line drawn through the center (indicated by the prostatic urethra), and into anterior/posterior by a horizontal line through the middle of the gland.
The right and left peripheral zones (PZ) at prostate base, midgland, and apex are each subdivided into three sections: anterior (a), medial posterior (mp), and lateral posterior (lp).
The right and left transition zones (TZ) at prostate base, midgland, and apex are each subdivided into two sections: anterior (a) and posterior (p).
The central zone (CZ) is included in the prostate base around the ejaculatory ducts.
The anterior fibromuscular stroma (AS) is divided into right/left at the prostate base, midgland, and apex.
The seminal vesicles (SV) are divided into right/left.
The urethral sphincter (US) is marked in the prostate apex and along the membranous segment of the urethra.
The sector map illustrates an idealized “normal prostate”. In patients and their corresponding MRI images, many prostates have components that are enlarged or atrophied, and the PZ may obscured by an enlarged TZ. In such instances, in addition to the written report, a sector map which clearly indicates the location of the findings will be especially useful for localization.
Atlas (this section is under construction)
Section I: Clinical Considerations and Technical Specifications
Section II: Normal Anatomy and Benign Findings
Section III: Assessment and Reporting
Section V: Staging
SECTOR MAP CREDIT
The prostate sector map was modified by David A. Rini, Department of Art as Applied to Medicine, Johns Hopkins University. It is based on previously published figures by Villers et al (Curr Opin Urol 2009;19:274-82) and Dickinson et al (Eur Urol 2011;59:477-94) with anatomical correlation to the normal histology of the prostate by McNeal JE (Am J Surg Pathol 1988 Aug;12:619-33).
a Yale School of Medicine, New Haven, CT, USA
b Radboudumc, Nijmegen, The Netherlands
c National Institutes of Health, Bethesda, MD, USA
d René Descartes University, Paris, France
e University of Toronto, Sunnybrook Health Sciences Centre, Toronto, Canada
f Johns Hopkins University, Baltimore, MD, USA
g University of California, Los Angeles, CA, USA
h University of Pennsylvania, Philadelphia, USA
i AdMeTech Foundation, Boston, MA, USA
j Harvard University, Boston, MA, USA
k University Hospital of Bern, Bern, Switzerland
l University of Cincinnati, Cincinnati, OH, USA
Corresponding author. Tel. +1 203 785 5913.
These authors are co-first authors.
☆ Please visit www.eu-acme.org/europeanurology to read and answer questions on-line. The EU-ACME credits will then be attributed automatically.
© 2015 Published by Elsevier B.V.