Prostate MRI (an approach)

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Prostate MRI has become an increasingly frequent examination faced in daily radiological practice and is mainly conducted for the detection, active surveillance and staging of prostate cancer. This approach is an example of how to create a radiological report of a prostate MRI with consideration of different imaging features and relevant clinical data.

Recommendations have been given in the Prostate Imaging-Reporting and Data System (PI-RADS) document of which the last version v2.1 has been published by an internationally representative group involving the American College of Radiology (ACR), European Society of Urogenital Radiology (ESUR), and AdMeTech Foundation .

The approach below takes recommendations from those documents and those of other societies as well as personal experiences into consideration .

Clinical information

Clinical information is important and can be paramount in the assessment of more subtle findings and the differentiation of an equivocal finding versus findings of low or high probability. Important clinical information includes the following :

  • age and ethnicity (men of African descent carry a higher risk)
  • prostate specific antigen (PSA)
  • previous systematic biopsy
    • date and results
    • numbers of cores
    • location and Gleason score of positive biopsies (% core involvement)
  • additional relevant history
    • digital rectal examination finding
    • previous surgery or TURP
    • previous radiation therapy
    • medications: androgen deprivation therapy (ADT), alpha-blockers etc.
    • family history
    • previous prostate infections or a history of prostatitis

Unfortunately, the available clinical information will be most often confined to a recent PSA and the details the patient can give you.

Working diagnosis

Risk-stratification of prostate cancer has been based on clinical grounds and clinically significant prostate cancer has been defined as one of the following (biopsy results will not be available in primary detection cases) :

MRI protocol

Standardized imaging acquisition is important.

A multiparametric MRI of the prostate usually consists of the following sequences:

A biparametric MRI of the prostate features the following sequences:

Systematic review

Prostate anatomy is not that complex, however, a systematic review is still essential since the clinical relevant pathology being prostate cancer has different mimickers and can be easily confused with other benign findings, artefacts or anatomical structures.

Scoring systems and measurements

Scoring system used in the assessment of an MRI of the prostate are:

Important measurements include the following:

  • prostate volume
    • measured by automated, semi-automated or manual segmentation methods
    • calculation as cc x ap x rl x 0.52 (ellipsoid formula)
    • craniocaudal diameter (cc): mid-sagittal plane
    • anteroposterior diameter (ap): mid-sagittal plane
    • transverse diameter (rl): from right to left in the mid-axial of the prostate
  • suspicious lesions
    • the largest dimension (as a minimum)
    • plane in which the measurement was conducted
    • MRI usually underestimates the true extent
Signs

The following signs can be used in the evaluation of a prostate MRI:

Step-by-step-approach

"Many ways lead to Rome" is a saying in the western world which basically means that the desired outcome can be reached by different ideas, methods or approaches. Conversely, this means that no singular approach is the only "correct one".

Two approaches are presented below:

Evaluation based on sequence
T1 axial

The following should be assessed for in T1 weighted images:

T2 weighted imaging

T2 weighted imaging is used for the morphological assessment of the prostate. The following structures should be evaluated in T2 weighted images:

  • prostate volume: measurements in mid-sagittal and axial images 
  • prostate zonal anatomy
    • should be evaluated in all planes
    • relation of peripheral versus transitional zone
    • structural and morphological changes within the gland
    • surgical capsule (well-defined/ill-defined)
  • peripheral zone 
    • findings are usually non-specific on T2 weighted images
    • findings are especially important if diffusion-weighted imaging is inadequate
    • usually relatively uniformly hyperintense
    • linear or wedge-shaped hypointensities are usually unsuspicious
    • mild diffuse hypointensities and indistinct margins are usually unsuspicious
    • heterogeneous alterations can be equivocal or ambiguous
    • non-circumscribed lesions can be equivocal or ambiguous
    • focal circumscribed homogenous moderately hypointense lesions are suspicious
  • transitional zone
    • T2WI are the dominant sequence for the transitional zone
    • usually features heterogeneously intermixed glandular and stromal elements
    • also known as “organized chaos”
    • dysplastic nodules should be evaluated  
    • a normal non-hyperplastic zone is rare
    • completely encapsulated nodules are not suspicious
    • mostly encapsulated nodules or atypical nodules are usually not suspicious 
    • as long as they are circumscribed, non-obscured without diffusion restriction
    • suspicious findings e.g. “erased charcoal sign” or “smudged appearance”
  • anterior fibromuscular stroma
    • features a low signal intensity
    • is fairly symmetrical and of crescentic shape
    • deformity irregular margins and bulging of the anterior capsule are suspicious
    • asymmetry and obscured margins versus the adjacent transition zone are suspicious
    • higher signal intensity is suspicious
  • central zone
    • located in the base of the prostate posteromedially around the ejaculatory ducts
    • hypointense in relation to the transitional zone
    • cone or v-shaped on coronal images
  • prostate margins
    • contours should well-delineated with no irregularities or bulges
    • neurovascular bundles should be fairly symmetric
    • obliteration of the recto-prostatic angle
  • seminal vesicles
    • lobulated hyperintense appearance with thin low signal intensity walls
    • intact prostate-seminal vesicle angle
    • focal low signal intensity areas might be suspicious
    • loss of the lobulated architecture is suspicious
    • irregular boundaries can be suspicious
Diffusion-weighted imaging

Diffusion-weighted imaging (DWI) is used as a measure for the mobility of water molecules and diffusion is reduced in hypercellular tissues as prostate cancer.

High b-value diffusion-weighted images and apparent diffusion coefficient (ADC) images are key components of multiparametric and biparametric prostate MRI.

Due to relatively low in-plane resolution, high b-value and ADC images are not quite suitable for the assessment of gland morphology and should be used in conjunction with T1 and T2 weighted images. An abnormality or focal finding is considered as such if they are distinctive from the background and of high signal intensity in the high b-value images and low signal intensity in the ADC map.

The following is assessed with diffusion-weighted imaging:

  • peripheral zone
    • usually no diffusion restriction
    • abnormal linear or wedge-shaped signal alterations are usually not suspicious
    • focal findings of high signal intensity in high b-value images or low signal intensity in ADC images are considered equivocal as long as they are not apparent in both
    • focal findings with obviously restricted diffusion are suspicious 
    • or even highly suspicious if they are greater in size
  • transitional zone
    • focal findings with restricted diffusion apparent on both high b-value DWI and ADC can render less conspicuous findings more suspicious
    • seminal vesicles
    • correlation of suspicious findings in T2 weighted imaging
Dynamic contrast enhancement

Dynamic contrast enhancement is considered a minor feature in the detection of prostate cancer and can be of help in equivocal findings of the peripheral zone if positive. Especially early enhancement is suspicious if focal and should happen within 10 seconds of contrast in the femoral arteries.

It is definitely more useful in a setting after prostatectomy where an enhancing focus will suggest recurrent cancer.

Evaluation based on anatomy
Peripheral zone

The assessment of the peripheral zone is done by correlating findings of DWI with T2WI and T1WI and in case of a multiparametric MRI also with dynamic contrast enhancement. High b-value diffusion-weighted imaging and the apparent diffusion coefficient are considered the most important methods for the evaluation.

Normal or benign findings:

  • uniform high signal intensity on T2 and ADC
  • linear or wedge-shaped low signal on T2 and ADC or linear high signal on high b-value

Equivocal findings:

  • lesions with inconclusive findings on diffusion-weighted imaging (either high signal on high b-value DWI or low signal on ADC) 
  • heterogeneous signal intensity on T2 weighted images

Suspicious or pathological findings:

  • focal hypointense lesions with markedly restricted diffusion and/or early enhancement
  • especially if large
  • broad curvilinear capsular contact, capsular irregularities bulges or breaches
Transitional zone

The assessment of the transitional zone is mainly conducted using T2 weighted images. However, diffusion-weighted imaging is still useful even though less important if compared to the peripheral zone.

Normal or benign findings:

  • well-circumscribed surgical capsule
  • completely encapsulated nodules
  • mostly encapsulated nodules or circumscribed homogeneous nodules without diffusion restriction
  • homogeneous mildly hypointense area between nodules without diffusion restriction
  • heterogeneously intermixed stromal and encapsulated circumscribed glandular elements, also known as “organized chaos”

Equivocal findings:

  • heterogeneous lesions with obscured margins
  • mostly encapsulated nodules or circumscribed homogeneous nodules with diffusion restriction
  • homogeneous mildly hypointense area between nodules with diffusion restriction

Suspicious or pathological findings:

  • larger heterogeneous areas with clearly restricted diffusion
  • non-circumscribed homogeneous moderately hypointense lesions 
    • with an oval, lenticular, or droplet shape
    • with spiculated, irregular or smudgy margins
  • “erased charcoal sign” 
  • invasion of other prostatic zones (e.g. anterior fibromuscular stroma, peripheral or central zone)
  • invasion of other structures e.g. urethral sphincter          
Anterior fibromuscular stroma

Normal or benign findings:

  • symmetrical low signal intensity with a crescentic shape
  • low signal intensity on T2 similar to muscle

Suspicious or pathological findings:

  • asymmetry and deformity 
  • obscured and indistinct margins
  • high signal intensity relative to muscle
  • diffusion restriction
  • early enhancement
Central zone

Normal or benign findings:

  • inverse cone or v-shaped fairly symmetrical
  • low signal intensity on T2
  • mildly high signal on high b-value

Suspicious or pathological findings:

  • asymmetry and deformity 
  • obscured and indistinct margins towards the adjacent peripheral zone
  • diffusion restriction
  • focal asymmetric early enhancement
Prostatic margins

Normal or benign findings:

  • well-delineated contours with no irregularities or bulges or adjacent lesions
  • fairly symmetric neurovascular bundles 
  • preserved fairly symmetrical  bilateral recto-prostatic angles

Suspicious or pathological findings:

  • broad curvilinear tumor-capsule interface > 10mm
  • irregular or spiculated capsular contour or capsular bulge
  • capsular retraction
  • neurovascular bundle asymmetry
  • a frank capsular breach with an invasion of the adjacent tissues
  • obliteration of the recto-prostatic angle
Seminal vesicles

Normal or benign findings:

  • lobulated hyperintense appearance with thin low-signal intensity walls
  • preserved prostate-seminal vesicle angle or space between seminal vesicles and prostate base
  • mild luminal T2 hypointensity with preserved thin low-signal intensity walls
  • luminal T1 hyperintensity (hemorrhage)

Suspicious or pathological findings:

  • loss of the normal lobular architecture
  • tumor extension along the ejaculatory ducts
    • luminal obstruction of the ejaculatory ducts from and above the prostatic base
    • seminal vesicle enlargement and low signal intensity on T2
    • intraluminal diffusion restriction
  • direct tumor extension from the prostatic base
    • loss of the prostate-seminal vesicle angle
    • metachronous tumor deposits
  • intraluminal focal low-signal intensity areas not clearly distinguishable from the walls
    • focal enhancing solid luminal areas
    • focal intraluminal diffusion restriction
Lymph nodes

Regional lymph nodes are located below the common iliac bifurcation and include:

  • internal and external iliac lymph nodes
  • pelvic lymph nodes
  • sacral lymph nodes
  • hypogastric lymph nodes

Normal or benign findings:

  • oval small-sized lymph nodes with a fatty hilum

Suspicious or pathological findings:

  • short axis ≥ 10 mm on all lymph nodes
  • short axis ≥ 8 mm on round lymph nodes
  • irregular margins
  • heterogeneity

Practical points

  • start off with the evaluation of T1 weighted images
    • they feature the greatest coverage
    • it is good to know about hemorrhage and metastatic disease early
  • calculate the prostate volume and PSA density early
    • you get a better feeling of ‘how serious’ the clinical suspicion is
  • assessment of the prostate gland 
    • quickly assess gross prostatic anatomy and morphology before going into detail
    • link sequences in your PACS especially high b-value DWI, ADC and T2W images
    • make use of localizers and crosshair functions of your PACS
    • do not forget the apex and the base
    • do not forget the seminal vesicles
  • describe suspicious findings and use clear terminology
  • as a rule of thumb large suspicious lesions are more likely to be clinically significant
  • be aware of the fact that MRI underestimates the true extent of lesions
  • be aware that very small lesions (e.g. ≤ 5 mm) are hard to target on biopsy but they might require follow up
  • describe typical pitfalls
  • describe benign findings
  • if there is tumor in the anterior fibromuscular stroma state whether this is more likely originating from the transitional or peripheral zone
  • make annotations
    • it is so much easier to reproduce your findings later if you want to biopsy
    • ‘X marks the spot’ –  an arrow or a circle can help a lot

To sum it up

With increasing experience, many radiologists will eventually be doing their evaluation using a mixture of the two above presented strategies.

The radiological report should include at least a description of the following:

  • size and location of the most suspicious focal findings as well as a likelihood rating on a 5-digit scale as recommended by different societies and guidelines
  • any signs of extraprostatic extension and its location including seminal vesicle invasion
  • lymph node involvement or osseous metastases if present

See also