An update of pitfalls in prostate mpMRI: a practical approach through the lens of PI-RADS v. 2 guidelines

Abstract

Objectives: The aim of the current report is to provide an update in the imaging interpretation of prostate cancer on multiparametric magnetic resonance imaging (mpMRI), with a special focus on how to discriminate pathological tissue from the most common pitfalls that may be encountered during daily clinical practice using the Prostate Imaging Reporting and Data System (PI-RADS) version 2 guidelines.

Methods: All the cases that are shown in this pictorial review comply with the European Society of Urogenital Radiology (ESUR) guidelines for technical mpMRI requirements.

Results: Despite the standardised manner to report mpMRI (PI-RADS v. 2), some para-physiologic appearances of the prostate can mimic cancer. As such, it is crucial to be aware of these pitfalls, in order to avoid the under/overestimation of prostate cancer.

Conclusions: A detailed knowledge of normal and abnormal findings in mpMRI of the prostate is pivotal for an accurate management of the wide spectrum of clinical scenarios that radiologists may encounter during their daily practice.

Teaching points: • Some para-physiologic appearances of the prostate may mimic cancer. • Knowledge of normal and abnormal findings in prostate mpMRI is pivotal. • Any radiologist involved in prostate mpMRI reporting should be aware of pitfalls.

Keywords: Diagnosis; Magnetic resonance imaging; Pitfalls; Prostate; Prostatic cancer.

Fig. 1

Hypertrophic anterior fibromuscular stroma vs cancer. Axial (a) and coronal (b) T2-weighted images that show an area of homogeneous low signal intensity with a lenticular shape (white arrows), and not significant restriction in the ADC map (c). The red arrows show a small focus of prostate cancer in the anterior right gland (a), corresponding to an area of restricted diffusion in the ADC map (c). These findings were confirmed at final histology, after radical prostatectomy (GS = Gleason score 4 + 3) (d)

Fig. 2

Periprostatic bundle. The arrow in the axial T2-weighted image (a) shows an area of intermediate to low-signal intensity in the right peripheral zone. This corresponds to mild, restricted diffusion on the echo-planar diffusion-weighted sequence (b), due to the slow speed of blood flow and focal enhancement on DCE imaging (c)

Fig. 3

Neurovascular bundle vs cancer. The white arrows show the neurovascular bundle in the axial T2-weighted (a) and DW (b) images, and in the DCE map (c). The high signal intensity on DWI (i.e., restriction) on DWI is due to myelinated nerve fibres. The red arrows show a tumour in the right anterior gland. These findings were confirmed at final histology, after radical prostatectomy (GS = Gleason score 3 + 4) (d)

Fig. 4

The arrows show two median, symmetric, bilateral areas of low signal intensity on axial (a) and coronal (c) T2-weighted imaging, with restricted diffusion in the ADC map (b) and diffuse enhancement on DCE imaging (e). This set of appearances has been called the “moustache sign”

Fig. 5

Axial T2-weighted image (a) of the posterior base. The yellow areas (b) correspond to the protrusion of a large adenoma in the peripheral zone (moustache-sign like), as confirmed at final histology, after radical prostatectomy (c)

Fig. 6

The yellow area is the moustache sign. The arrows show a focal, asymmetric area characterised by low signal intensity on coronal (a) and axial (b) T2-weighted imaging, together with marked restriction on DWI (c). These findings suggest a suspicious lesion in the left peripheral zone (cancer in moustache sign), and this was confirmed at final histology after radical prostatectomy (GS = Gleason score 4 + 4) (d)

Fig. 7

Axial T2-weighted image (a) that shows a hypointense area at the prostate base, in the peripheral zone. This aspect is a variant/extension of the moustache sign, in which the central zone is compressed between the transitional and peripheral zones, adopting a teardrop shape, as represented by the yellow area in the coronal T2-weighted image (b)

Fig. 8

Summary of the different signs (moustache or teardrop) at different levels. CZ = central zone; BPH: benign prostatic hyperplasia; ED = ejaculatory ducts

Fig. 9

The arrows show a focal area with low-signal intensity on axial (a) and coronal (c) T2-weighted imaging, and restricted diffusion on DWI (b) at the prostate apex. The use of the coronal plane is very important to differentiate a pitfall (teardrop) from prostate cancer (cancer in pitfall)

Fig. 10

Axial T2-weighted image (a) that shows a focal area of low signal intensity adjacent to the ejaculatory ducts (white arrow). This corresponds to an area of restricted diffusion on the ADC map (b) and late enhancement on DCE imaging (c) and represents fibrosis, as confirmed at final histology after radical prostatectomy (d)

Fig. 11

The arrows show a focal nodule bulging in the left peripheral zone, characterised by low signal intensity on axial (a) and coronal (c) T2-weighted imaging, with sharply defined margins, and restricted diffusion on the ADC map (b). The presence of tiny bright spots (corresponding to dilated acini) is consistent with a nodule of stromal benign prostatic hyperplasia (BPH), which may sometimes protrude from the central zone

Fig. 12

The red arrows show a focal area in the right peripheral zone with low-signal intensity on axial T2-weighted imaging (a), restricted diffusion in the ADC map (b), avid enhancement in the DCE study (c) and an early wash-in curve (d); these findings are consistent with prostate cancer (Gleason 3 + 3). The white arrows show a diffuse area of decreased signal in the left peripheral zone on T2-weighted imaging (a), and a mild restriction in the ADC map (b) and diffuse contrast uptake (c and d); these findings are consistent with prostatitis

Fig. 13

The first four images show a round-shaped area characterised by intermediate signal intensity on T2-weighted imaging (a), restricted diffusion on DWI (b) and in the ADC map (c), and ring enhancement (d). These findings, together with clinical history, orient towards the diagnosis of abscess. The other four images show a focal nodule in the right anterior peripheral zone, characterised by low signal intensity on axial T2-weighted imaging (e) with sharply defined margins and tiny bright spots, and restricted diffusion on DWI (f) and in the ADC map (g), and homogeneous enhancement on DCE imaging (h). These findings are consistent with an ectopic nodule of BPH

Fig. 14

The arrows show an area of mild, low-signal intensity on T2-weighted imaging (a), with restricted diffusion on DWI (b) and in the ADC map (c). This corresponds to a hyperintense area on pre-contrast T1-weighted imaging (d), which is consistent with the products from the haemoglobin degradation after biopsy, as also supported by the post-contrast subtraction imaging (e) and in the colour DCE map (f). DCE studies have been obtained by gradient-echo sequences (TR: 4,5 ms; TE: 1,5 ms; flip angle: 15°; Average: 4; slice thickness: 2 mm; Matrix: 320 × 320; Scan Time: 3.13 min), using a body-weight adjusted intravenous bolus of gadobutrol (Gadovist, 1 mmol/mL; Bayer Schering Pharma, Berlin, Germany). DWI parameters were: TR ≥ 3000 ms; TE ≤ 90 ms; slice thickness ≤ 4 mm, no gap; field of view:160–220 mm; in plane dimension ≤ 2.5 mm phase and frequency; b values for DWI were 0–500- and values ranging between 1000 and 3000 s/mm². For ADC maps, if only two b values can be acquired, it is preferred that the lowest b value should be set at 50–100 s/mm² and the highest should be 800–1000 s/mm²

Fig. 15

The white arrows show an area mimicking restricted diffusion on DWI (a) and focal enhancement on DCE imaging (b) in the left peripheral zone, very close to the prostatic capsule and adjacent to the endorectal coil surface. This finding could be erroneously interpreted as a lesion, but the axial T2-weighted image (c) does not show this artefact, and therefore prostate cancer can be ruled out. Red arrows show the coil surface