Section II: Normal Anatomy and Benign Findings
A. Normal Anatomy
From superior to inferior, the prostate consists of the base (just below the urinary bladder), the midgland, and the apex. It is divided into four histologic zones: (a) the anterior fibromuscular stroma, contains no glandular tissue; (b) the transition zone (TZ), surrounding the urethra proximal to the verumontanum, contains 5% of the glandular tissue; (c) the central zone (CZ), surrounding the ejaculatory ducts, contains about 20% of the glandular tissue; and (d) the outer peripheral zone (PZ), contains 70%–80% of the glandular tissue. When benign prostatic hyperplasia (BPH) develops, the TZ will account for an increasing percentage of the gland volume. Approximately 70-75% of prostate cancers originate in the PZ and 20-30% in the TZ. Cancers originating in the CZ are uncommon, and the cancers that occur in the CZ are usually secondary to invasion by PZ tumors. Based on location and differences in signal intensity on T2W images, the TZ can often be distinguished from the CZ on MR images. However, in some patients, age-related expansion of the TZ by BPH may result in compression and displacement of the CZ. Use of the term “central gland” to refer to the combination of TZ and CZ is discouraged as it is not reflective of the zonal anatomy as visualized or reported on pathologic specimens.
A thin, dark rim partially surrounding the prostate on T2W is often referred to as the “prostate capsule”. It serves as an important landmark for assessment of extraprostatic extension of cancer. In fact, the prostate lacks a true capsule; rather it contains an outer band of concentric fibromuscular tissue that is inseparable from prostatic stroma. It is incomplete anteriorly and apically.
The prostatic pseudocapsule (sometimes referred to as the “surgical capsule”) on T2W MRI is a thin, dark rim at the interface of the TZ with the PZ. There is no true capsule in this location at histological evaluation, and this appearance is due to compressed prostate tissue.
Nerves that supply the corpora cavernosa are intimately associated with arterial branches from the inferior vesicle artery and accompanying veins that course posterolateral at 5 and 7 o’clock to the prostate bilaterally, and together they constitute the neurovascular bundles. At the apex and base, small nerve branches surround the prostate periphery and penetrate through the capsule, a potential route for extraprostatic extension (EPE) of cancer.
B. Sector Map (Appendix II)
The segmentation model used in PI-RADS v2 was adapted from a European Consensus Meeting and the ESUR Prostate MRI Guidelines 2012. It employs thirty-nine sectors/regions: thirty-six for the prostate, two for the seminal vesicles and one for the external urethral sphincter.(see Appendix II).
Use of the Sector Map will enable radiologists, urologists, pathologists, and others to localize findings described in MRI reports, and it will be a valuable visual aid for discussions with patients about biopsy and treatment options.
Division of the prostate and associated structures into sectors standardizes reporting and facilitates precise localization for MR-targeted biopsy and therapy, pathological correlation, and research. Since relationships between tumor contours, glandular surface of the prostate, and adjacent structures, such as neurovascular bundles, external urethral sphincter, and bladder neck, are valuable information for periprostatic tissue sparing surgery, the Sector Map may also provide a useful roadmap for surgical dissection at the time of radical prostatectomy. Either hardcopy (i.e. on paper) or electronic (i.e. on computer) recording on the Sector Map.
Either hardcopy (i.e. on paper) or electronic (i.e. on computer) recording on the Sector Map is acceptable.
For information about the use of the Sector Map, see Section III and Appendix II.
C. Benign Findings
Many signal abnormalities within the prostate are benign. The most common include: 1. Benign prostatic hyperplasia (BPH) Benign prostatic hyperplasia (BPH) develops in response to testosterone, after it is converted to di-hydrotesosterone. BPH arises in the TZ, although exophytic and extruded BPH nodules can be found in the PZ. BPH consists of a mixture of stromal and glandular hyperplasia and may appear as band-like areas and/or encapsulated round nodules with circumscribed margins. Predominantly glandular BPH nodules and cystic atrophy exhibit moderate-marked T2 hyperintensity and are distinguished from malignant tumors by their signal and capsule. Predominantly stromal nodules exhibit T2 hypointensity. Many BPH nodules demonstrate a mixture of signal intensities. BPH nodules may be highly vascular on DCE and can demonstrate a range of signal intensities on DWI.
Although BPH is a benign entity, it may have important clinical implications for biopsy approach and therapy since it can increase gland volume, stretch the urethra, and impede the flow of urine. Since BPH tissue produces prostate-specific antigen (PSA), accurate measurement of gland volume by MRI is an important metric to allow correlation with an individual’s PSA level and to calculate the PSA density (PSA/prostate volume).
2. Hemorrhage Hemorrhage in the PZ and/or seminal vesicles is common after biopsy. It appears as focal or diffuse hyperintense signal on T1W and iso-hypointense signal on T2W. However, chronic blood products may appear hypointense on all MR sequences.
3. Cysts A variety of cysts can occur in the prostate and adjacent structures. As elsewhere in the body, cysts in the prostate may contain “simple” fluid and appear markedly hyperintense on T2W and dark on T1W. However, they can also contain blood products or proteinaceous fluid, which may demonstrate a variety of signal characteristics, including hyperintense signal on T1W.
4. Calcifications Calcifications, if visible, appear as markedly hypointense foci on all pulse sequences
5. Prostatitis Prostatitis affects many men, although it is often sub-clinical. Pathologically, it presents as an immune infiltrate, the character of which depends on the agent causing the inflammation. On MRI, prostatitis can result in decreased signal in the PZ on both T2W and the ADC (apparent diffusion coefficient) map. Prostatitis may also increase perfusion, resulting in a “false positive” DCE result. However, the morphology is commonly band-like, wedge-shaped, or diffuse rather than focal, round, oval, or irregular, and the decrease in signal on the ADC map is generally not as great nor as focal as in cancer.
6. Atrophy Prostatic atrophy can occur as a normal part of aging or from chronic inflammation. It is typically associated with wedge-shaped areas of low signal on T2W and mildly decreased signal on the ADC map from loss of glandular tissue. The ADC is generally not as low as in cancer, and there is often contour retraction of the involved prostate.
7.Prostatic fibrosis can occur after inflammation. It may be associated with wedge- or bandshaped areas of low signal on T2W.