GU – Prostate

“Central Zone: Central zone appears as a symmetric band of tissue between the peripheral and transition zones at the base of the prostate, extending from below the seminal vesicles to the verumontanum and surrounding the ejaculatory ducts. Central zone exhibits decreased T2 signal intensity and decreased ADC relative to the peripheral zone. Depending on the level of the axial slice, the two lobes of the central zone may exhibit a dumbbell-shaped appearance rather than appear as a single confluent structure. On coronal images, the central zone will have a triangular or tear-drop-shaped appearance.”

“Thickening of Surgical Capsule: A ‘surgical’ capsule surrounding the transition zone has also been described. On MRI, the surgical capsule appears as a thin crescentic band of decreased T2 signal intensity surrounding the transition zone. In addition, as with the central zone, the surgical capsule exhibits decreased ADC. When encountered on the ADC map, its location and bandlike or crescentic shape can serve as clues to its cause.”

“Periprostatic Venous Plexus: The normal prostatic veins form a periprostatic venous plexus that courses around the lateral margins of the prostate before communicating with a venous plexus anterior to the prostate and ultimately draining into the internal iliac veins. This plexus forms a site of potential spread of tumor and also has been reported to exhibit a congested appearance in the setting of prostatitis. In addition, Phillips et al. reported in a study using older MRI technology that in some instances it was difficult to separate the periprostatic venous plexus from the normal peripheral zone.”

“Neurovascular Bundle: The NVB has classically been viewed as a discrete structure coursing along the posterolateral margin of the prostate near the prostate capsule at approximately the 5- and 7-o’clock positions and covered by the lateral pelvic fascia. More recent anatomic studies based on surgical specimens from non-nerve-sparing prostatectomy procedures have challenged the historical view of the anatomy of the NVB and show that in approximately half of cases there is no discrete bundle formation and that the nerve trunks are sparsely distributed along both the anterior and posterior lateral aspects of the prostate.”

“The proximity of both the periprostatic venous plexus and NVB to the peripheral zone may create a clinical challenge in assessing for focal peripheral zone lesions. In addition, these structures exhibit decreased T2 signal intensity and a signal void on the ADC map. Generally, the T2-weighted images, given their higher in-plane spatial resolution, are helpful to show that a potential lesion identified on DWI or DCE-MRI represents the normal periprostatic venous plexus or NVB.”

“Postbiopsy Hemorrhage: Hemorrhage may cause decreased T2 signal intensity that can mimic or obscure an area of tumor. The impact of postbiopsy hemorrhage may be most pronounced on T2-weighted imaging, with relatively milder changes encountered on DWI and DCE-MRI. In our experience, a discrete masslike abnormality that shows more marked signal intensity alterations on DWI and DCE-MRI than are observed within the remainder of the peripheral zone should be deemed suspicious for tumor despite the presence of postbiopsy hemorrhage. In addition, extensive postbiopsy hemorrhage has been observed to spare and thus outline the margins of dominant tumors; this “MRI exclusion sign” can be used to potentially take advantage of the distribution of the hemorrhage as a tool to aid tumor localization.”

“Stromal Benign Prostatic Hyperplasia Nodule: 30% of prostate cancers arise from the transition zone. Although glandular BPH exhibits T2 hyperintensity owing to its fluid content and is thus readily distinguished from transition zone tumors, stromal BPH exhibits T2 hypointensity and poses a greater diagnostic dilemma. Stromal BPH nodules are generally rounded or spherical in shape, with discrete margins. On the other hand, a transition zone tumor is suggested in the presence of irregular margins; a lenticular, teardrop, or other unusual shape; and invasive behavior, such as extension into the anterior fibromuscular stroma or adjacent portion of the peripheral zone.”

“Acute and Chronic Prostatitis and Postinflammatory Scars and Atrophy: Within the peripheral zone, a broad spectrum of inflammatory and infectious processes, including acute and chronic prostatitis as well as postinflammatory scars and atrophy, may mimic the presence of tumor. These processes have been reported to cause signal abnormalities on T2-weighted imaging, DWI, DCE-MRI, and MR spectroscopy.”

“A lesion with a more well-defined and nodular appearance should be considered of greater suspicion for tumor. On the other hand, inflammatory lesions are generally less mass-like in morphology, exhibiting margins that are ill defined or linear rather than rounded in appearance.”

“Granulomatous Prostatitis: This uncommon benign inflammatory condition often presents with a firm nodule on digital rectal examination and elevated prostate-specific antigen, thus clinically mimicking prostate cancer. Although possible causes include previous intravesical bacille Calmette-Guérin (BCG) therapy for bladder cancer, tuberculous prostatitis, and previous intervention such as transurethral resection of the prostate, most cases are idiopathic without a specific cause identified. On MRI, granulomatous prostatitis may appear as a discrete mass with markedly abnormal T2 signal intensity and ADC, more pronounced than observed for other inflammatory or infectious processes and thus be deemed of very high suspicion for tumor. Furthermore, there may be associated infiltration of the periprostatic fat by inflammation, thus mimicking extraprostatic tumor extension.”

“Anatomic Distortion of High-b-Value Diffusion-Weighted Images: Additional steps particularly focusing on the impact of rectal gas are warranted. For instance, although anterior-to-posterior phase encoding is most commonly used in abdominal and pelvis MRI given the generally narrower size of the torso in this direction, left-to-right phase encoding may be preferable for routine DWI of the prostate to avoid the propagation of artifacts resulting from rectal gas or stool across the prostate.”

“Lack of Suppression of Benign Prostate Tissue on Standard High-b-Value Diffusion-Weighted Images: Standard DWI protocols for a broad spectrum of organs use a maximal b value in the range of 500– 1000 s/mm2. However, in the prostate, the benign parenchyma continues to exhibit mild hyperintensity at b values in this range. Thus, some focal lesions will be inconspicuous compared with the background prostate parenchyma.”

“Studies have shown greater tumor conspicuity and detection on the ADC map than on the native-acquired DWI, and it is recommended that this image set be reviewed in detail as the primary image set used when performing DWI of the prostate. Expert-panel consensus guidelines prescribe greater weight to findings identified on the ADC map than on the high-b-value images.”

“Suboptimal Windowing of the ADC Map: It is important to recognize that the default window and level of the ADC map generated by the MRI console or PACS system may not be optimal for lesion detection. In particular, if the level is too high, the hypointensity of focal lesions may not be qualitatively appreciated on visual evaluation.”