T2-weighted imaging of Prostate MRI Exam

T2-weighted fast-spin echo images acquired in multiple planes represent key components of a prostate mpMRI exam. Images should be acquired with high resolution, accomplished by using a small field-of-view and a narrow image thickness of 3 to 4 mm. High-resolution T2-weighted imaging (T2WI) currently provides the best assessment of the prostate’s morphology, margins, and internal structure. The ejaculatory ducts, urethra, and verumontanum are frequently visualized on high-quality T2WI, and these internal landmarks can aid in the correct identification of zonal anatomy, and thus in lesion localization. Differentiation between the peripheral zone (PZ) and central gland (CG) is essential for accurate lesion localization, and making this distinction is typically easy on T2WI. The term “central gland” is often used in prostate MRI reports to refer to the combined central zone (CZ) and transition zone (TZ), which may be difficult to distinguish in a fraction of patients.[12] Vargas et al recently showed that the CZ can be identified as a distinct region in most patients. Appropriate recognition of the CZ is important to avoid its misinterpretation as a potential tumor; however, a very small fraction of tumors may in fact occur within the CZ, and these tend to have more aggressive features compared with CG tumors that only involve the TZ.[12] In terms of staging, the superior morphologic information provided by high-resolution T2WI facilitates assessment of seminal vesicle invasion, extracapsular extension, and invasion of the neurovascular bundles.

The normal glandular tissue in the PZ is predominantly high in signal on T2WI, whereas PCa lesions in the PZ appear as dark (hypointense) foci, possibly round or oval in shape.[13] Unfortunately, some foci of PCa may appear isointense to adjacent benign PZ on T2-weighted images, especially when small or of lower grade. Moreover, in many patients referred for prostate MRI, the PZ is not homogeneously bright, but rather a patchwork of varying signal intensities, secondary to factors such as atrophy, prior episodes of prostatitis, or post-biopsy hemorrhage.[13] Because post-biopsy changes can mimic PCa on T2WI, it is generally recommended to delay mpMRI for at least 8 to 12 weeks after biopsy,[9] and it is also important to evaluate non-contrast T1-weighted images for evidence of biopsy-related hemorrhage.[13]
Several recent publications concerning mpMRI have reported on the diagnostic performance of T2WI for PCa localization.[14-19] In a recent meta-analysis, Tan et al incorporated data from 14 studies with histopathologic correlation and reported an overall sensitivity and specificity for T2WI alone of 0.57−0.62 and 0.74−0.78, respectively.[20] Even when images are acquired using optimal technique, the overall moderate performance of T2WI alone has resulted in the need to incorporate additional functional sequences to facilitate more accurate lesion identification.