Anatomic Pitfalls

Hypertrophic Anterior Fibromuscular Stroma

According to the McNeal zonal anatomy distinction, AFMS is the most anterior portion of the gland, situated between the two lobes that constitute the TZ, which often appears to be flattened by them.[6] This zone corresponds to a band of tissue formed by muscle cells and connective tissue, which is devoid of glandular tissue. There is variable prominence of AFMS among individuals. Due to the compressive effect of benign prostatic hyperplasia (BPH), it is less prominent in men with increased age and gland size.[7] Owing to its histological composition (muscle cells and connective tissue), it appears homogeneously hypointense on T2W images, which can be mistaken for PCa and also tend to be hypointense ([Fig.?2]). In some cases where hypertrophy of AFMS may mimic PCa on T2W images, signal characteristics on DWI and enhancement on DCE-MRI should be correlated.[8]

|?Fig. 2?Axial T2-weighted imaging (T2WI) showing triangular area of T2-hypointensity (arrows) in midlower prostate, corresponding to hypertrophic anterior fibromuscular stroma (AFS).|

Surgical Capsule

The surgical capsule referred to the connective tissue support that is concentric fibrous tissue normally found separating the TZ and the PZ of the prostate. Due to its fibrous tissue composition, it shows low-signal intensity on T2W images. Normally, it is a linear structure, without a nodular or mass-like configuration ([Fig.?3]). In some cases, asymmetric thickening of the capsule may simulate a suspicious lesion. Multiplanar, high-resolution T2W images and DCE-MRI help us to differentiate it from the malignant lesion.[9] [10]

|?Fig. 3?Axial T2-weighted imaging (T2WI) showing T2-hypointense linear thickening (arrows) of surgical capsule, bulging into the bright peripheral zone (PZ).|

Periprostatic Venous Plexus

The location of the periprostatic venous plexus at the anterolateral margin of the prostate, near the prostate capsule, presents a challenge in assessing lesions of the PZ. Also known as Santorini?s plexus, it is in close proximity to the prostate capsule, and the size of the vessels is variable among patients.[11] Patients with local inflammatory processes often show asymmetric dilation of the venous plexus. It appears rounded on axial images, with variable signal intensity on T2W images (depending on the blood flow inside the vessel and flow voids) and low value on ADC map ([Fig.?4]). To avoid this pitfall, we must know the location of the periprostatic venous plexus, which is found in the anterolateral aspect of the PZ.

|?Fig. 4?Axial T1-weighted (T1W) dynamic contrast-enhanced MR images (a) showing dot like enhancement in the periphery of the prostate, (b) corresponding to perfusion map images showing increased perfusion suggestive of periprostatic venous plexus.|

Neurovascular Bundle

The neurovascular bundle that innervates the corpora cavernosa is located at the posterolateral margin of the prostate, near the capsule, usually at 5 and 7 o?clock position. The preservation of this plexus is critical in postprostatectomy sexual function; however, in infiltrative PCa patients, surgeons are bounded to perform a wider resection. The neurovascular bundle may show a rounded appearance on axial images, with a low signal on T2W and ADC maps ([Fig.?5]). In distorted ADC images, the neurovascular bundle may appear to be in the PZ and mimic a nodule. It is mandatory to evaluate these suspicious nodules on multiplanar, high-resolution T2W images.

|?Fig. 5?Apparent diffusion coefficient (ADC) map showing hypointense dots in posterolateral region of prostate at 5 to 7 o? clock position, which could be erroneously diagnosed as peripheral zone (PZ) cancer, but location of these hypointense dots clinches the diagnosis of neurovascular bundle.|

Benign Prostatic Hyperplasia

Moustache Sign (Bilateral Hyperplasia with Compression of Peripheral Zone)

It is the presence of bilateral symmetrical small adenomatous nodules in the TZ, which determines compression of the peripheral and CZ at the base/ middle of the prostate gland. It is also referred to as ?moustache sign?, which is the presence of bilateral symmetrical homogenous oval-shaped areas of low-signal intensity on T2W images at the base of prostate, lateral to the opening of ejaculatory ducts, which appears like moustache ([Fig.?6]). Presence of symmetry is a crucial feature to determine whether you are dealing with a true ?moustache sign? or a clinically significant suspicious lesion in this same area, especially when evaluating on coronal T2 sequence.[12]

|?Fig. 6?Axial and coronal T2-weighted image (T2WI) of the prostate base (a?and?b) showing bilateral symmetrical oval-shaped areas of low-signal intensity on T2WI (arrows) at the base of the prostate lateral to the opening of ejaculatory ducts. (c) No diffusion restriction is seen in these nodules. (d) Line diagram showing resemblance with a moustache.|

Moustache-Like Sign (Larger Adenoma Against the Peripheral Zone)

Protrusion of large adenomatous nodules in the PZ at the base of the gland shows moustache sign-like appearance ([Fig.?7]).

|Fig. 7Axial T2-weighted image (T2WI) of the prostate base, (a) showing a large adenoma bulging into the peripheral zone (PZ) (arrows) and (b) corresponding line diagram showing resemblance to a moustache, (c) with restricted diffusion in the apparent diffusion coefficient (ADC) map images.|

Cancer in Moustache Sign

The key feature to differentiate a pitfall from PCa is the symmetry of the finding. Asymmetry in the signal intensity with marked T2 hypointensity together with significant early focal enhancement and high grade of diffusion restriction points toward a malignant change?cancer in moustache sign ([Fig.?8]).

|?Fig. 8?MRI prostate showing asymmetrical oval area of low signal on T2-weighted imaging (T2WI) (a) depicting early enhancement on dynamic contrast-enhanced MRI (b), and diffusion restriction (c?and?d) in keeping with prostate cancer.|

Teardrop Sign (Median Posterior Compressed Central Zone)

This corresponds to the fusion and thickening of the prostate-fascia capsule in the midline at the junction of both the lobes surrounding the ejaculatory ducts, also called ?teardrop sign? because of its shape that resembles a teardrop. This pseudolesion is found as a focal nodular hypointense area in the midline at the level of the base-middle third of the gland, as the CZ is compressed between the TZ and PZ ([Fig.?9]).[13]

|?Fig. 9?(a) Axial T2-weighted imaging (T2WI) showing triangular area of hypointensity (arrows) in the peripheral zone (PZ) in the prostate base. (b) Coronal T2WI showing teardrop configuration (arrows) of the compressed central zone (CZ) between the transitional zone (TZ) and PZ. (c) Line diagram showing resemblance with a teardrop.|

Teardrop-like Sign (Protruding Benign Prostatic Hyperplasia Above the Verumontanum)

Verumontanum is an important landmark in distinguishing compressed CZ and protruding BPH nodule. Teardrop sign is due to bulging of CZ above the verumontanum, while teardrop-like sign is due to bulging of BPH nodule above the verumontanum.[14]

Ectopic/Extruded Benign Prostatic Hyperplasia Nodule

Ectopic adenomatous nodules are frequently misdiagnosed by inexperienced radiologists, interpreting it as mp-MRI prostate. These can mimic malignant lesions due to their rounded contour with low signal on T2WI; in some cases, these can show diffusion restriction with early enhancement pattern in perfusion study (DCE). Quantitative analysis of ADC values could improve the diagnostic accuracy in differentiating ectopic BPH nodule from peripheral PCa: the higher the ADC values, the higher the likelihood of ectopic BPH nodule. The crucial step in identification of these nodules is the demonstration of well-defined T2 hypointense capsule. Some of the ectopic BPH nodules may show tiny T2 hyperintense spots, corresponding to dilated acini, and this is a feature that can be used to distinguish them from PZ PCa ([Fig.?10]).

|?Fig. 10?Multiparametric MRI (mp-MRI) prostate showing well-defined encapsulated oval area of T2-hypointensity with internal bright spots within the nodule on axial and coronal T2-weighted imaging (T2WI) (a?and?b). Histopathology (c) showing stromal nodule with slit-like vessels, H and E, ?2 consistent with benign prostatic hyperplasia (BPH) nodule.|

Bacterial Prostatitis (Acute and Chronic)

It comprises the most common category of benign pathologies mimicking PCa. Acute bacterial prostatitis commonly affects young men; however, chronic prostatitis is commonly seen in older patients. Chronic prostatitis tends to be more indolent, and patients are often asymptomatic; however, acute prostatitis patients presents with fever, urinary tract symptoms, and often frequently associated with abscess.[15] Although prostatitis reveals similar signal characteristics as PCa on mp-MRI, the morphology and degree of altered signal intensity with quantitative analysis of perfusion kinetics provide a clue to clinch the diagnosis. Inflammatory lesions exhibit nonmass-like morphology with ill-defined linear rather than rounded margins and diffuse lobar distribution ([Fig.?11]).[16]

|Fig. 11Multiparametric MRI (mp-MRI) prostate showing altered area of signal with fuzzy margins and bulging contour (a), patchy enhancement on postcontrast images (b), and diffusion restriction (c). Histopathological images (d) depicting acute prostatitis with karyorrhectic debris and necrosis, H and E, 10.|

Granulomatous Prostatitis

Most of the cases are idiopathic; however, prior history of intravesical bacillus Calmette?Guerin (BCG) therapy of the urinary bladder, tuberculous prostatitis, and transurethral resection of the prostate is frequently encountered. On mp-MRI, it may appear as mild-to-markedly T2 hypointense lesion, which shows diffusion restriction and early enhancement on DCE-MRI.[17] [18] After antimicrobial therapy, short-term follow-up MRI may be helpful to assess therapeutic response.[9]

Iatrogenic Changes

Biopsy needle-induced damage may determine a transient irregularity of the periprostatic capsule, leading to an overestimation of extraprostatic tumor extension.

Postbiopsy Hemorrhage

Hemorrhage is seen more frequently in the PZ than TZ.[19]

The signal intensity of postbiopsy hemorrhage depends on the state of hemoglobin degradation. In some metabolic states, it can mimic the suspicious lesion due to low signal on T2W and ADC images; however, it characteristically shows a high signal on the T1W sequence ([Fig.?12]). As most of the mp-MRI studies are performed before prostate biopsies, the incidence of this pitfall is low, and if it is performed later, most of the experienced radiologists? advice to wait for at least 6 to 8 weeks postbiopsy. This delay is more advisable if it is required for staging purpose, as staging is more dependent on T2WI. If the primary aim is a diagnosis, then this delay can be avoided, as the likelihood of clinically significant PCa at the site of postbiopsy hemorrhage without a corresponding suspicious finding on MRI is low.

|?Fig. 12?MRI prostate showing an area of mild, low-signal intensity on T2-weighted imaging (T2WI) (a), with corresponding hyperintense area on precontrast T1-weighted imaging (T1WI) (b). Mild restricted diffusion on diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps is seen (c?and?d). This is consistent with the products from the hemoglobin degradation after biopsy.|

Posttreatment Changes

Treatment commonly includes radical prostatectomy, prostate-sparing focal therapy, or radiation therapy (RT), which can include external-beam RT or brachytherapy.[20] There are some common mimics that should be identified in posttreatment cases. Granulation tissue and hemorrhage both will show early enhancement on DCE phase; however, on DWI, they will appear benign with no notable signal abnormalities. Posttreatment appears more hypointense than a recurrent tumor with the absence of early enhancement and presence of delayed enhancement during the venous phase.[20] If metallic clips are used during surgery, the DCE sequence is more reliable than DWI. Early enhancement in DCE is highly suggestive of recurrence. All these changes should be correlated with clinical setting, type of treatment received, and PSA levels.[21]

Artifacts

Mispositioned Endorectal Coil

Use of endorectal is mandatory for the good-quality MRI scanning of prostate on 1.5T scanner. Accurate positioning of a coil is essential; otherwise, it creates artifacts affecting image quality, thus making the diagnosis difficult. Sometimes, it involves adjacent PZ and mimics PCa.

Calcifications

Incidence of prostate calcification is high and found basically due to concreted secretions or corpora amylacea.[22] They are generally detected in the TZ (central gland). Most of the time, they are indolent. They are characteristically hypointense in all sequences like T1, T2, and ADC, with blooming on susceptibility-weighted imaging (SWI), and do not show diffusion restriction ([Fig.?13]). CT is helpful to demonstrate the calcification.

|?Fig. 13?MRI prostate showing hypointense foci on T2-weighted imaging (T2WI) (a), T1-weighted imaging (T1WI) (b), without any diffusion restriction on diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps (c?and?d), suggestive of calcification within the prostate gland.|

Foleys Catheter Tip

Foley?s catheter tip is hypointense on all sequences ([Fig.?14]). Coronal images will show its entire urethral course and its extension into the urinary bladder.

|?Fig. 14?Axial T2-weighted imaging (T2WI) showing artifact (arrow) due to Foley?s catheter within the prostatic tissue.|

Periprostatic Lymph Nodes

Periprostatic lymph nodes are encountered rarely while interpreting mp-MRI prostate. These are commonly located at the base of the prostate gland laterally or posterolaterally. On mp-MRI imaging, these show well-defined rounded morphology with a hypointense signal on T2W sequence and marked diffusion restriction; however, the degree of diffusion restriction does not suggest metastatic involvement. Sometimes on low-resolution DWI/ADC images, periprostatic lymph nodes appeared to be intraprostatic in location. Detailed evaluation on high-resolution, multiplanar T2W images reveals its periprostatic location.[11] [23]

Conclusion

All oncologists and radiologists involved in treating and interpreting mp-MRI prostate should be aware of these fallacies described above. Key points to avoid these pitfalls are illustrated in [Table?3]. Despite the pitfalls described above, biopsy should be considered in men with an equivocal MRI and a PSA density of > 0.15.

Binit Sureka, Associate Professor, MD, DNB, MBA

Department of Diagnostic and Interventional Radiology

All India Institute of Medical Sciences, Jodhpur, Rajasthan, 342005

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