Keywords
multiparametric MRI - prostate cancer - PIRADS
Introduction
Multiparametric magnetic resonance imaging (mp-MRI) has emerged as an important tool
for the detection and characterization of prostatic lesions. It now plays a quintessential
role in the surveillance, diagnosis, and staging of prostate cancer (PCa), as well
as for the detection of local recurrence. As reliance on serum prostate-specific antigen
(PSA) has declined in the recent times, mp-MRI (T2-weighted [T2W] images, diffusion-weighted
imaging [DWI], dynamic contrast-enhanced [DCE] MRI, and magnetic resonance [MR] spectroscopy
[MRS]) has emerged as the go-to tool for urologists all over the world. Hence as radiologists,
it has become necessary to be well versed with the technique, image interpretation,
and fallacies of mp-MRI. This review is aimed at providing a brief overview of prostatic
MRI, its indications, and basics of sequences used, with emphasis on recent updates
in MRI of the prostate.
PCa is one of the most commonly diagnosed malignancies in men. The age-adjusted incidence
rate in India is 37 per 100,000 men per year.[1] Difficulty in accurate staging and prediction of disease progression has made the
management of PCa a complex issue. Traditionally, urologists have relied on digital
rectal examination (DRE), PSA levels, and transrectal ultrasound (TRUS) with sextant
biopsy for the diagnosis of PCa.[2] MRI, in the present scenario, has become indispensable for not only staging of prostatic
cancer but also for its surveillance, detection, and follow-up. Radiologists can be
easily intimidated by prostate MRI examinations. Knowledge of glandular anatomy, the
use and interpretation of various MRI sequences, and, finally, the reporting of correct
findings is challenging for an amateur reader. Although the technical details of imaging
protocols are beyond the scope of this paper, the standard imaging sequences that
constitute multiparametric prostate imaging will be discussed along with their utility.
The aim of this review is to discuss, in brief, the fundamental anatomy of the prostate
and the techniques of mp-MRI and to provide a guide for the interpretation and reporting
of mp-MRI scans.
Normal Anatomy of the Prostate
Normal Anatomy of the Prostate
The prostate is grossly divided into three anatomical regions: the base, the midgland,
and the apex. The base forms the broad and superior most limit of the prostate and
is located below the urinary bladder neck. The narrower apex rests below on the pelvic
floor. Each anatomical region consists of a left side and a right side, thus dividing
the prostate into six parts or sextants. Biopsy samples are obtained from each sextant,
and this organization helps in clinic-radiologic-pathological correlation.
The anterior fibromuscular stroma, the transition zone (TZ), the central zone (CZ),
and the outer peripheral zone (PZ) form the four histological zones.[3] The nonglandular anterior fibromuscular stroma may contribute to up to one-third
of the prostatic mass. It may be replaced by glandular tissue in adenomatous enlargement
of the prostate. However, it is rarely invaded by carcinoma. The TZ surrounds the
urethra proximal to the verumontanum and contains 5% of the glandular tissue. It is
the TZ that accounts for the increase in size of the prostate in benign prostatic
hyperplasia (BPH) by increasing the percentage of the gland volume. It is estimated
that only 20 to 30% of PCas originate in this zone. The CZ expands in a cone shape
around the ejaculatory ducts to the base of the bladder and contains approximately
20% of the glandular tissue. Only 1 to 5% of adenocarcinomas arise in the CZ, although
it may be infiltrated by tumors arising from the PZ. The PZ forms the bulk of the
glandular tissue (70-80%), and 70 to 75% PCas arise from this zone.[4]
On T1-weighted (T1W) images, the entire prostate gland appears uniform with intermediate
signal intensity (SI). Hence, the zonal anatomy of the prostate cannot be made out
on T1W images.2 The location and differences in SI on T2W images often help in distinguishing
TZ from the CZ on MR images ([Fig. 1]). The anterior fibromuscular stroma is seen as dark on both T1 and T2W images. The
PZ, on the other hand, is bright on T2W images, having SI comparable to the adjacent
periprostatic fat. It is bound by a T2 hypointense capsule. Both CZ and TZ tend to
be of lower T2 SI than the PZ possibly because of more compact smooth muscle and lesser
glandular elements. Age-related expansion of TZ seen in BPH, however, may result in
compression and displacement of the CZ.[5]
Fig. 1 Normal anatomy of the prostate.
The“prostate capsule,” which is a thin, dark rim surrounding the prostate gland on
T2W images, serves as an important landmark for the assessment of extraprostatic extension
(EPE) of cancer. It is incomplete anteriorly and apically and contains an outer band
of concentric fibromuscular tissue that is inseparable from the prostatic stroma.
Thus, the prostate lacks a true capsule. The prostatic pseudocapsule (sometimes called
“surgical capsule” when seen at enucleation of an adenoma) on T2W MRI, is a thin,
dark rim at the interface of the TZ with the PZ, which appears so due to compressed
prostate tissue.
The proximal urethra cannot be visualized on MR unless there is a Foley catheter in
situ or a transurethral resection has been performed. The verumontanum appears as
a T2 hyperintense structure in the midline posterior to the urethra. The vas deferens
and seminal vesicles are best seen on axial and coronal images.[2]
The neurovascular bundles (NVBs) to the prostate are formed by the sympathetic and
parasympathetic nerves that supply the corpora cavernosa and the closely associated
arterial branches from the inferior vesicle artery. Together, these course posterolateral
to the prostate bilaterally, at 5 and 7 o'clock positions. Small nerve branches that
surround the prostate periphery and penetrate through the capsule at the apex and
base form a potential route for the EPE of cancer. The NVBs are best seen on axial
images.[2]
Indications for Prostate MRI
Indications for Prostate MRI
MRI has been used for the assessment of the prostate since the 1980s, when it was
used largely for the staging of biopsy-proven prostate carcinoma. The locoregional
spread of the disease was evaluated based on T1W and T2W pulse sequences, which helped
describe the morphology of the prostate.[6]
In the recent times, however, mp-MRI has been developed to significantly improve the
sensitivity and specificity of MRI for carcinoma prostate. mp-MRI integrates anatomical,
functional, and physiological assessments using T2W images, DWI, dynamic contrast
enhancement, and MR proton spectroscopy.
Consequently, prostate MRI is now being used for the detection of tumors and their
localization and characterization and for the prognostication as well as assessment
of suspected recurrent disease. It may even be employed for taking image-guided biopsies
and targeted therapy.[7]
[8]
The extensive use of mp-MRI, however, faced several impediments. The European Society
of Urogenital Radiology (ESUR) drafted guidelines along with a scoring system to standardize
its use. The scoring system is known as Prostate Imaging Reporting and Data System
version 1 (PIRADS v1).[8] This effort generated interest in the use of mp-MRI for the prostate. Moreover,
it provided validation and consistency to imaging findings, which encouraged widespread
use. Nonetheless, the system had limitations in several clinical scenarios. To resolve
the deficiencies, the American College of Radiology, the ESUR, and the AdMeTech Foundation
improved upon PIRADS v1 in the form of PIRADS v2 (2015). PIRADS v2 is designed to
“promote global standardization and diminish variation in the acquisition, interpretation,
and reporting of prostate mp-MRI examinations.”[6]It provides minimum acceptable technical parameters for performing a prostate mp-MRI
and standardizing the terminology and content of radiology reports. The basic aim
of PIRADS is to reduce the variability in imaging interpretation and to enhance communication
between radiologists and the referring clinicians.
Despite the updated version, the limitation of use of PIRADS for the treatment of
naïve prostate glands remains. Thus, its use for the detection of suspected recurrence
or progression is not validated.
Clinical Considerations and Technical Specifications
Clinical Considerations and Technical Specifications
Clinical Considerations
Timing of MRI following Prostate Biopsy
Hemorrhage and postbiopsy changes can possibly confound MRI interpretation. Hemorrhage
appears as bright on the T1W images and is generally seen in the PZ or in the seminal
vesicles following a TRUS-guided biopsy. Two general approaches can be followed to
avoid this potential pitfall. Although hemorrhagic changes can persist for months
following a biopsy, they generally resolve within 6 weeks. Hence, ideally, an MRI
should be performed at least 6 weeks after a biopsy.[6] However, since waiting for 6 weeks may not always be feasible, an initial T1 sequence
may be performed as a screening. If there is evidence of hemorrhage, the MRI may be
postponed after discussion with the urologist. If postponing the MRI is not an option,
it must be remembered that the possibility of finding a malignant lesion at the site
of postbiopsy hemorrhage is low. In such a situation, it is imperative to rule out
malignancy at locations other than those showing hemorrhagic change.[9]
Patient Preparation
The use of enema for rectal emptying is controversial. The presence of air/stool in
the rectum can cause distortion of MR signal and compromise DWI quality. However,
enemas also promote peristalsis, resulting in increased motion-related artifact. Antispasmodics
such as glucagon and scopolamine can reduce these motion-related artifacts; however,
their use must be weighed against the cost and possible adverse effects associated
with their use.
Though there are no guidelines at present, evacuation of the rectum by the patient
prior to the MRI generally obviates the need for enema.
If air is seen on the initial MR images, it may be worthwhile to perform the MRI with
the patient in the prone position or to use a suction catheter for evacuation.[6]
Patient Information
As is repeatedly emphasized, knowledge of the patient's history and clinical findings
is always beneficial while interpreting images. It is important to record the patient's
recent serum PSA level and PSA history. If a biopsy has been performed, the results
of the biopsy including the number and location of positive cores and corresponding
Gleason scores are of immense value to the reporting radiologist.
Technical Specifications of Equipment
Three sequences are essential for a prostate mp-MRI according to the PIRADS v2. These
include T2W, DWI, and DCE. The role of MRS is controversial, but we will still cover
it in this review. Apart from being used for the confirmation and localization of
clinically significant malignancy, mp-MRI also has the potential of being used as
a screening modality. However, cost and time are the two limiting factors. As a corollary,
the supervising radiologist should be vigilant about not including unnecessary sequences
as it not only affects the patient compliance and acceptance but also reduces the
machine throughput.
The field of view should be reduced to include the relevant structures only. However,
at least one pulse sequence should include area up to the aortic bifurcation. This
allows for the evaluation of pelvic nodes.[6]
Field Strength: 3T or 1.5T
With increasing field strength, there is a linear increase in the signal-to-noise
ratio (SNR). Hence, employing a 3T magnetic field instead of a 1.5T one means that
the spatial ratio and temporal resolution both can be increased. The downside is that
increasing the field strength also increases power deposition, artifacts related to
susceptibility, and signal heterogeneity. However, the advantages of 3T scanners over
1.5T significantly outweigh the disadvantages. PIRADS v2 recommends the use of 3T
for prostate MRI. The only indication for using a 1.5T scanner is when a patient has
an implant that is either compatible only with 1.5T or is in a location that could
result in significant artifacts during imaging, such as a hip prosthesis.[6]
Endorectal Coils or Surface Coils
Endorectal coils (ERCs) offer the advantage of increased SNR with the obvious disadvantage
of patient discomfort and poor acceptance apart from possible problems such as deformation
of the gland and artifacts. The increased spatial resolution is especially advantageous
while using lower SNR sequences such as DWI and high temporal resolution DCE. It also
scores over external phased array radiofrequency (RF) coils in obese patients.[6]
At 1.5T, ERC was indispensable for obtaining high-resolution diagnostic quality imaging.
However, at 3T, surface coils can provide equally good quality imaging. Some of the
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 as multiple factors including receiver bandwidth, coil design,
and efficiency of the RF chain.[6]
PIRADS v2 recommends optimization of protocols and coils to obtain the best possible
image quality, taking into account the cost, availability, and patient preference.
No specific recommendation has been made regarding the use of ERC/surface coils.
Computer-Aided Evaluation Technology
Computer-Aided Evaluation Technology
Computer-aided evaluation (CAE) can enhance lesion detection and discrimination performance
of amateur radiologists who have little or no experience in reading prostate MRIs.
However, CAE using specialized software or a dedicated workstation is not required
for prostate mp-MRI interpretation.[6]
Multiparametric MRI
T1- and T2-Weighted Images
T1W images are obtained to detect the presence of hemorrhage within the prostate or
seminal vesicles and to outline the anatomy of the region.
T2W imaging is the best imaging sequence to delineate the prostatic anatomy, for the
detection and categorization of lesions, and for the EPE and nodal assessment.[10]
T2W imaging is the dominant sequence to be assessed in the TZ. Features of malignancy
in the TZ are noncircumscribed, ill-defined, homogeneously hypointense lesions that
have been variously described as erased charcoal or smudgy fingerprint appearance.
Other features of malignancy include spiculated margins, lenticular shape, absence
of a complete hypointense capsule, extension of the lesion into adjacent structures
such as the urethral sphincter, and anterior fibromuscular stroma. Though no single
feature is not diagnostic of malignancy, the likelihood of malignancy increases with
the presence of more number of features.
TZ is composed of variable amounts of glandular (T2 hyperintense) and stromal (T2
hypointense) elements, and this makes the overall signal of the TZ heterogeneous.
Hence, detection of malignancy is especially challenging. Areas where benign stromal
elements predominate can mimic clinically significant malignancy.
In the PZ, clinically significant cancer tends to occur as a well-defined, round,
focal hypointense lesion. However, mimics such as hemorrhage, prostatitis, glandular
atrophy, and benign hyperplasia can mimic malignancy.
A relatively specific sign of malignancy is extension of the lesion across anatomical
boundaries. T2W images can help discern extension of the lesion within the gland (across
regional parts of the prostate) as well as into the seminal vesicles/adjacent fat
and NVB.
Three-dimensional (3D) axial acquisition can help improve the spatial resolution with
isotropic voxels. However, the soft tissue contrast and in-plane resolution tend to
be inferior to routine 2D acquisition. At our institute, we do not routinely acquire
3D datasets.
Diffusion-Weighted Imaging
One sequence that has had a significant role in the increased use and popularity of
mp-MRI is DWI. It is based on the principle of free, random motion of water molecules.
DWI includes an apparent diffusion coefficient (ADC) map and high b-value images ([Table 1]).
Table 1
PIRADS v2 assessment for T2W image
|
Peripheral zone
|
|
Score
|
Lesion characteristics
|
|
Abbreviations: BPH, benign prostatic hyperplasia; PIRADS, Prostate Imaging Reporting
and Data System; T2W, T2-weighted.
|
|
1
|
Uniform hyperintense signal intensity (normal)
|
|
2
|
Linear or wedge-shaped hypointensity or diffuse mild hypointensity, usually indistinct
margin
|
|
3
|
Heterogeneous signal intensity or noncircumscribed,rounded, moderate hypointensity
Includes others that do not qualify as 2, 4, or 5
|
|
4
|
Circumscribed, homogenous moderate hypointense focus/mass confined to the prostate
and < 1.5 cm in greatest dimension
|
|
5
|
Same as 4 but ≥1.5 cm in greatest dimension or definite extraprostatic extension/invasive
behavior
|
|
Transition zone
|
|
Score
|
Lesion characteristics
|
|
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 noncircumscribed, 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
|
The ADC map is a representation of the ADC values of each voxel in the image. It uses
two or more b-values and a monoexponential model of signal decay with increasing b-values to calculate ADC values. Clinically significant cancers have restricted diffusion
as compared with normal tissue and appear hyperintense on high b-value images and hypointense on ADC maps. Quantitatively, ADC values correlate inversely
with the tumor grade, and though attempts have been made to provide cutoffs for ADC
values for predicting malignant lesions, there is considerable overlap between benign
prostatic nodules and low- and high-grade malignant lesions. Though a threshold of
750 to 900 um2/second has been suggested to differentiate benign from malignant prostatic lesions
in the PZ, qualitative visual assessment remains the most commonly employed and used
technique.
DWI of the prostate is unique. It uses high b-value images (>1,400 seconds/m2) that are not routinely used in imaging of other body parts. DWI is highly sensitive
to motion and is a time-consuming imaging technique. As the b-value increases, the spatial resolution decreases. At such high b-values though, only the areas with restricted diffusion show signal and appear bright.
This increases the conspicuity of lesions especially in the subcapsular location at
the base and apex of the gland as well as adjacent to the anterior fibromuscular stroma.
High b-value images can be obtained in two ways: by directly acquiring a high b-value sequence (time consuming) or by extrapolating data acquired at lower b-values to create the ADC maps. The extrapolation time has two advantages: less time
and less artifact. This is achieved by avoiding the longer echo times required to
accommodate the strong gradient pulses required for higher b-value acquisitions.
If only two b-values can be acquired, the lower b-value should be set at 50 to 100 seconds/mm2 and the higher b-value should be 800 to 1,000 seconds/mm2. High b-value images (>1,400 seconds/mm2) can be extrapolated. For more accurate ADC calculation and extrapolation of high
b-value images, additional b-values may be obtained between 100 and 1,000 ([Table 2]).
Table 2
PIRADS v2 assessment for DWI
|
Score
|
Lesion characteristics
|
|
Abbreviations: ADC, apparent diffusion coefficient; DWI, diffusion-weighted imaging;
PIRADS v2, Prostate Imaging Reporting and Data System Version 2.
|
|
1
|
No abnormality 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 hypointense 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
|
Dynamic Contrast-Enhanced MRI
DCE MRI involves acquisition of scans just before, during, and after contrast injection
to study the enhancement characteristics of lesions. Rapid T1W gradient-echo scans
are obtained every few seconds with injection of a bolus of gadolinium-based contrast.
PCas show early enhancement compared with normal tissue with washout. However, prostate
malignancy shows variable and heterogeneous contrast kinetics. Some tumors may show
early washout, whereas others may show contrast retention. Enhancement is neither
a sign of malignancy nor does the absence of enhancement exclude a diagnosis of prostate
malignancy.
The importance of DCE lies in the fact that it can pick up small lesions, which may
show focal early enhancement. Any small lesion showing focal early enhancement should
be carefully looked at in the corresponding T2W images and DWI. However, its role
in the characterization of lesions is limited and subordinate to T2W images and DWI.
Positive early enhancement can be compared with the adjacent normal prostatic tissue.
It usually occurs within 10 seconds of appearance of contrast in the femoral artery.
Contrast enhancement for a lesion is reported as either positive or negative. Positive
DCE refers to focal enhancement of a lesion that is earlier or simultaneous with enhancement
of normal prostatic tissue and corroborating with findings on T2W images and/or DWI.
Negative DCE is defined by the absence of early enhancement or diffuse enhancement
not corresponding to a focal finding on T2 and/or DWI or focal enhancement corresponding
to a lesion demonstrating features of BPH on T2W images.
Diffuse enhancement in the prostate is usually associated with prostatitis. Infiltrating
malignancy may also show diffuse enhancement, but these lesions show corresponding
signal changes on T2W images and DWI as well. At times, histologically sparse PCas
are intermixed with benign prostatic tissues. These tumors tend to be occult on T2W
images and DWI and may be apparent only on DCE. However, these are usually lower grade
malignancies, and the enhancement may be due to concurrent prostatitis.
The easiest and the most commonly used technique for interpretation of DCE is direct
visual assessment. The various DCE time points at each slice location are assessed
either by manual scrolling or using cine mode. It is recommended to use fat suppression
or subtraction technique to improve lesion pickup. Using a parametric map which color-codes
enhancement features within a voxel (such as slope and peak) can also assist image
interpretation. However, any suspicious finding on a parametric map or subtracted
images should be confirmed on the source images.
Another technique used for the interpretation of dynamic contrast enhancement is plotting
the SI of the lesion versus time: curve typing. However, given the heterogeneity in
enhancement characteristics of PCa, this has not proven to be too useful. Perfusion
parameters like Ktrans (wash-in) and Kep (washout) have been used to assist in diagnosis. As of now, not enough published
literature is available to recommend the use of perfusion parameters for the diagnosis
of prostate malignancy ([Table 3]).
Table 3
Magnetic resonance imaging sequence parameters
|
T2W
|
DWI
|
DCE
|
|
Abbreviations: DCE, dynamic contrast enhancement; DWI, diffusion-weighted imaging;
EPI, echo planar imaging; FOV, field of view; FSE, fast spin echo; GRE, gradient echo;
T2W, T2-weighted; TE, echo time; TR, repetition time; TSE, turbo spin echo.
|
|
Sequence
|
TSE/FSE
|
Free-breathing spin-echo EPI with spectral fat saturation (TE ≤90 ms, TR ≥ 3,000 ms)
|
2D/3D T1 GRE sequence TR <100 ms, TE < 5 ms
|
|
Plane of imaging
|
Axial, coronal, sagittal
|
Axial
|
Axial
|
|
Slice thickness
|
3 mm (no gap)
|
≤4 mm
|
3 mm (no gap)
|
|
FOV
|
12-20 cm (to encompass the entire prostate gland and seminal vesicles)
|
16-22 cm
|
Encompass the entire prostate gland and seminal vesicles
|
|
In-plane dimension
|
≤0.7 mm (phase) × ≤0.4 mm (frequency)
|
≤2.5 mm (phase and frequency)
|
≤2 × ≤2 mm
|
|
|
|
Temporal resolution: ≤15 s (<7 s is preferred)Total observation time ≥2 min
|
PIRADS V2 Reporting
PIRADS, like the already established BI-RADS, aims at standardizing assessment and
reporting of mp-MRI for PCa. It uses a 5-point scale to categorize the probability
of malignancy. The aim of PIRADS is to pick up all clinically significant malignancies
while reducing unnecessary biopsies.
Clinically significant PCa has been defined as pathologically proven PCa with Gleason
score ≥ 7 (including 3 + 4 with prominent but not predominant Gleason 4 component),
volume ≥ 0.5 mL, and/or EPE.
The PIRADS assessment is based on a combination of T2W, DWI, and DCE findings, with
the relative importance of these sequences differing according to the zone of the
gland ([Figs. 2]
[3]
[4]
[5]
[6]).
Fig. 2 Uniform high signal intensity T2 and low signal intensity on high b-values DWI and high signal on ADC-PIRADS 1.
Fig. 3 Linear and wedge-shaped T2 hypointensities with indistinct margins without diffusion
restriction-PIRADS 2.
Fig. 4 T2 hypointense lesion with mild hyperintensity on high b-value DWI images with focal mild hypointensity on ADC in the left lateral peripheral
zone (arrows)-PIRADS 3.
Fig. 5 Only an ill-defined T1 and T2 hypointensity in the left lateral peripheral zone.
However, focal markedly hyperintense on high b-value DWI and hypointense area on ADC in left lateral peripheral zone measuring less
than 1.5 cm (arrow)-PIRAD 4.
Fig. 6 A focal well-defined T2 hypointense region in the right peripheral zone, showing
marked diffusion restriction appearing hypointense on ADC and hyperintense on high
b value DWI with early enhancement on DCEI measuring more than 1.5 cm. Features are
consistent with a PIRADS 5 lesion. The lesion shows >1 cm smooth contact with the
adjacent capsule, a sign of EPE. Note the difference between PIRAD 4 (< 1.5 cm) and
PIRAD 5 (>1.5 cm) lesions are only size. Moreover, any capsular or NVB invading lesions
are PIRAD 5 irrespective of size.
Biopsy should be considered for all PIRADS 4 and 5 lesions, whereas PIRADS 1 categorizes
no-touch lesions. Lesions that are PIRADS 2 or 3 require correlation with PSA/DRE
and other clinical details for their management. Depending on the clinical and laboratory
findings as well as the local preferences and technical expertise, these lesions can
either be biopsied or followed up ([Table 4]).
Table 4
PIRADS v2 and the risk of malignancy
|
PIRADS 1
|
Very low (clinically significant cancer is highly unlikely to be present)
|
|
PIRADS 2
|
Low (clinically significant cancer is unlikely to be present)
|
|
PIRADS 3
|
Intermediate (the presence of clinically significant cancer is equivocal)
|
|
PIRADS 4
|
High (clinically significant cancer is likely to be present)
|
|
PIRADS 5
|
Very high (clinically significant cancer is highly likely to be present)
|
Out of the aforementioned three sequences, T2W imaging and DWI are the mainstay for
assessment. DCE plays the role of an adjunct ([Fig. 7]). Early enhancement within a lesion points toward a malignant lesion; however, it
may be disregarded if a finding is a definite PIRADS 1 or 2 lesion based on the T2W
images and DWI. Similarly, lack of early enhancement can be disregarded in lesions
that are PIRADS 4 or 5 based on T2W images and DWI. However, DCE can tilt the scales
in PIRADS 3 lesions where it assumes the role of a referee. For example, in a PIRADS
3 lesion in the PZ according to DWI, a positive DCE increases the likelihood of the
finding being malignant, and the resultant finding can be upgraded to PIRADS category
4 ([Table 5]).
Fig. 7 A focal lesion in the left anterior fibromuscular stroma with extension into the
adjacent central zone showing marked diffusion restriction appearing hyperintense
on DWI, hypointense on ADC map showing early enhancement on DCEI and well-defined
T2 hypointensity (arrows) s/o PIRAD 5 lesion highly suspicious of malignancy.
Table 5
PIRADS v2 final assignment of score
|
Peripheral zone
|
|
DWI
|
DCE
|
Final PIRADS score
|
|
Abbreviations: DCE, dynamic contrast enhancement; DWI, diffusion-weighted imaging;
PIRADS, Prostate Imaging Reporting and Data System; T2W, T2-weighted.
|
|
1
|
Any
|
1
|
|
2
|
Any
|
2
|
|
3
|
-
|
3
|
|
+
|
4
|
|
4
|
Any
|
4
|
|
5
|
Any
|
5
|
|
Transition zone
|
|
T2W
|
DWI
|
Final PIRADS score
|
|
1
|
Any
|
1
|
|
2
|
Any
|
2
|
|
3
|
≤4
|
3
|
|
5
|
4
|
|
4
|
Any
|
4
|
|
5
|
Any
|
5
|
In case the dominant sequence (DWI in the PZ and T2 in the TZ) is technically inadequate,
PIRADS category “X” should be assigned and the sequence should be repeated. In case
that is not possible, assessment can be made with other pulse sequences with a clear
mention of the limitation in the report.
Assessment without Adequate Dynamic Contrast Enhancement
In PZ and TZ, it is determined by DWI assessment category ([Table 6]). If both DWI and DCE are not available, only EPE should be assessed for the purpose
of staging.
Table 6
Assessment of PIRADS v2 without DWI
|
Peripheral zone and transition zone
|
|
T2W
|
DCE
|
Final PIRADS score
|
|
Abbreviations: DCE, dynamic contrast enhancement; PIRADS, Prostate Imaging Reporting
and Data System; T2W, T2-weighted.
|
|
1
|
Any
|
1
|
|
2
|
Any
|
2
|
|
3
|
-
|
3
|
|
+
|
4
|
|
4
|
Any
|
4
|
|
5
|
Any
|
5
|
Measurement of the Prostate Gland
The volume of the prostate gland can either be calculated by automated segmentation
or manually by measuring the maximum dimensions in all three planes using the following
formula:
(Max anteroposterior diameter) × (Max transverse diameter) × (Max longitudinal diameter)
× 0.52.
This helps calculate the PSA density-PSA/prostatic volume.
Mapping Lesions
As prostatic malignancy can be multifocal, up to four findings with a PIRADS assessment
category of 3, 4, or 5 may each be assigned on the sector map, and the index (dominant)
intraprostatic lesion should be identified. The index lesion is the one which has
the highest PIRADS assessment category. PIRADS v2 recommends that up to four findings
with an assessment category of 3, 4, or 5 can be identified and reported. In case
there are more than four lesions with suspicious findings, the four with the highest
PIRADS assessment category should be reported. In case there are two lesions with
the same PIRADS assessment category, the lesion that shows EPE should be the index
lesion even if it is smaller in size. If neither lesions show EPE, then the larger
lesion is considered the index lesion.
It is advised that benign findings such as cysts may be reported but is optional.
They not only help as landmarks to guide subsequent biopsy or in follow-up but also
help provide clarification to clinical colleagues who may see the lesion on films
and have difficulty in interpretation. Ideally, the image number and series on which
the measurement is made should be reported.
Measurement of Lesions
According to the PIRADS v2, the largest dimension of a suspicious finding should be
reported with a mention of the plane (axial/sagittal/coronal) on which the measurement
is made. Furthermore, for PZ lesions, the measurement should be made on ADC images,
and for TZ lesions, measurement should be made on T2W images. However, in case lesion
measurement is difficult on these sequences, it should be made on the sequence that
best shows the lesion.
Staging
To differentiate stage T2 EPE (tumor confined to the gland) from stage T3 EPE, MRI
is a useful investigation. On MRI, it is essential to inspect the apex of the gland
well. Invasion of the external urethral sphincter by cancer leads to a risk of surgically
damaging the sphincter and results in urinary incompetence. Tumor in the apex of the
gland may also need special considerations for radiation therapy.
At times, obvious signs of EPE may be present, such as direct tumor extension into
the bladder base or seminal vesicles with breach of the capsule. However, when there
is no gross EPE, certain surrogate signs can predict EPE. The presence of a prostatic
contour bulge or irregularity of prostatic margins, loss of normal rectoprostatic
angle, asymmetry in the region of the NVBs, and a tumor-capsule contact length more
than 1 cm can indicate EPE. Similarly, extension of malignancy into the seminal vesicle
may be indicated by the features of seminal vesicle invasion including signal abnormality
within the seminal vesicles in the form of T2 hypointensity, which may be focal or
diffuse and/or abnormal contrast enhancement and/or restricted diffusion. Morphologically,
loss of angle between the base of the prostate and the seminal vesicle may also be
an indicator of seminal vesicle invasion.
The other important structures to be analyzed on MRI are the pelvic and retroperitoneal
lymph nodes, that is, the common femoral, obturator, external iliac, internal iliac,
common iliac, pararectal, presacral, and paracaval, and para-aortic lymph nodes. However,
currently, the detection of abnormal lymph nodes is limited to size, morphology, and
enhancement pattern. Although it is known that metastatic lymph nodes are not always
enlarged, lymph nodes over 8 mm in size in the short axis are usually considered suspicious.
One should also assess the images for the presence of skeletal metastases.
Imaging Pearls
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For proper correlation and synchronization between sequences, ensure that the imaging
plane, location, and slice thickness for all sequences (T2W, DWI, and DCE) are the
same.
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All signal abnormalities are not malignancies. In the PZ, signal alteration on T2W/DWI
that is indistinct, linear, lobar, or diffuse, and not rounded may be secondary to
prostatitis rather than malignancy.
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The ability of MRI to reliably detect and characterize malignancy in the PZ is more
than that in the TZ.
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Homogeneous and heterogeneous nodules in the TZ that are round and well circumscribed
are common in men above the age of 40 years. Irrespective of diffusion restriction
and/or enhancement, they are considered to be benign BPH. They may sometimes harbor
a malignancy but the probability is very low.
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Findings on DWI should always be correlated with those on T2W, T1W, and DCE imaging.
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Not all that appears dark on ADC images is malignant. Blood products, areas of fibrosis
or dense fibromuscular stroma, and calcifications can be hypointense on T2 and ADC
maps; however, they tend to be hypointense on high b-value images as well.
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BPH nodules are the most common benign finding that can masquerade as malignancy.
Some of these nodules in the TZ are not clearly encapsulated and may show diffusion
restriction. Also, some of these nodules can get extruded into the PZ and be well
encapsulated and circumscribed with diffusion restriction. This is a limitation of
mp-MRI.
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Signs of extracapsular extension:
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Asymmetric prostate capsular bulge with irregular margins.
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Obliteration of the rectoprostatic angle.
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Asymmetry of NVB.
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Tumor encasement of the NVB.
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Seminal vesicle invasion.
In conclusion, mp–MRI has emerged as an important tool for the detection and characterization
of prostatic lesions. It now plays a quintessential role in the surveillance, diagnosis,
and staging of PCa, as well as for the detection of local recurrence. As reliance
on serum PSA has declined in the recent times, mp–MRI has emerged as the go–to tool
for urologists all over the world.
