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 (usually mpMRI) 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 latest 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 1,2.
The approach below takes recommendations from those documents and those of other societies as well as personal experiences into consideration 1-4.
On this page:
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 1-4:
age and ethnicity (men of African descent carry a higher risk)
-
prostate specific antigen (PSA)
recent level and history
PSA density (can be calculated if a recent PSA level is available)
free/total PSA ratio (if PSA is between 4-10 ng/mL, no use if PSA >10 ng/mL )
-
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.
MRI protocol
Standardized imaging acquisition is important.
A multiparametric MRI (mpMRI) 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:
-
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 that means the desired outcome can be reached by different ideas, methods or approaches. Conversely, this means that no singular approach is the only "correct one" and two approaches are presented.
Evaluation based on sequence
T1 axial
The following should be assessed for on T1-weighted images:
prostate gland: hemorrhage e.g. hemorrhage exclusion sign
seminal vesicles: hemorrhage
bony pelvis: skeletal metastases
pelvic lymph nodes: enlargement
T2 weighted imaging
T2-weighted imaging is used for the morphological assessment of the prostate. The following structures should be evaluated on 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 non-suspicious
mild diffuse hypointensities and indistinct margins are usually non-suspicious
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 is 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 be well-delineated with no irregularities or bulges
neurovascular bundles should be fairly symmetric
obliteration of the rectoprostatic 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 on 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 on 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 entering 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 >10 mm
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 rectoprostatic 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 artery bifurcation and include:
pelvic lymph nodes
sacral lymph nodes
Normal or benign findings:
oval non-enlarged 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
sometimes it can be helpful to draw a line contouring the extent of the lesion
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