MR examination of the prostate needs high field units with no lower than 1.5 T. An optimal MR study for localization and detection of prostate cancer requires the use of an endorectal coil and a pelvic multichannel phased-array coil; endorectal coil is more uncomfortable and makes the examination more expensive. For 3 T units, the need of using an endorectal coil has not been determined yet. If there is a history of previous biopsies, MR examination has to be postponed 8–10 weeks to avoid that glandular hemorrhagic changes or periglandular fibrotic changes that may interfere with the correct interpretation of the study [7].

Morphological information are obtained on T1-weighted and T2-weighted high-resolution images. T1-weighted image allows assessment of pelvic lymph nodes and of the pelvis bone to rule out metastasis and to assess hemorrhagic changes after biopsy (Fig. 48.3) (which can mimic tumor on T2-weighted images) [8]. T2-weighted image of the peripheral gland shows homogeneous high signal intensity in contrast to the heterogeneous low signal intensity of the central-transitional zone, although high-intensity areas of adenoma may be observed in the central zone (Fig. 48.1). The diagnostic criterion for prostate cancer on T2-weighted MR images is the presence of nodular areas of low signal intensity within the normal high signal intensity of the peripheral gland or zone (Fig. 48.4). T2-weighted imaging has limitations for identifying cancer in the transitional zone due to the difficulty to define tumoral zones of low signal intensity within the normal low signal intensity of the central tissue. The findings suggesting cancer in the transitional zone are diffuse low signal intensity with poorly defined margins [9], absence of a margin (commonly seen in adenomas), and disruption of the surgical pseudocapsule (Fig. 48.5). Low signal intensity lesions in the peripheral zone may also be observed in benign lesions such as prostatitis, hyperplasia, fibrosis, calcification, subacute postbiopsy hemorrhage, postradiation therapy changes, and hormone deprivation therapy (Fig. 48.6).

To improve diagnostic accuracy, functional-multiparametric techniques such as diffusion-weighted (DWI) MR imaging, dynamic contrast-enhanced (DCE) MR imaging, and MR spectroscopy imaging (MRSI) have been developed for a better assessment to manage prostate cancer [7, 10].

MR spectroscopic imaging (MRSI) provides information on the gland metabolism. The sequence is acquired using the multivoxel technique, programming a volume on T2-weighted imaging. For three-dimensional spectroscopic acquisition, selective bands are used to achieve water and lipid suppression and to optimize citrate, creatine, and choline detection. Six saturation bands are used to eliminate signals from the adjacent tissue. Field homogenization is automatically calculated. The dataset should be processed using specific software for quantification of the values of the metabolic ratios. Generally, the peaks calculated are for choline, creatine, and citrate. Despite other metabolites can be also identified such as polyamines, they are better detected in 3 T units. Good signal-to-noise ratio needs to be confirmed (>5:1) as well as the absence of contamination by fat or water. Normal prostatic tissue has high citrate values and low choline and creatine values, whereas neoplastic tissue has high choline values and low citrate values [11].

Suspicion criterion is a high [(creatine + choline)/citrate] (CCo/Ci) ratio; however, there is no consensus regarding which metabolic ratio determines the presence of prostate cancer, due to the variability among patients and MR units (Fig. 48.4). Parameters of the metabolic ratio for the central gland have not been established due to the overlap in CCo/Ci ratio between the normal, or hypertrophic, and the neoplastic tissue in the central zone. In any case, the most specific finding for diagnosis of neoplasia in the central zone is the absence or very low values of citrate and the elevation of choline (Fig. 48.5). MR spectroscopy has shown promise in predicting tumor volume, extracapsular extension, and estimation of the aggressiveness of cancer and treatment response. The potential application of MRSI to rule out the presence of prostate cancer in patients with persistently rising PSA value and negative prostate biopsies has been shown. MRSI needs more expertise and longer acquisition time than other functional MR techniques. MRSI has limitations due to artifacts from postbiopsy hemorrhage and suboptimal shimming.

The technological advances of MR have allowed an expansion in the use of the diffusion sequence (DWI) to organs other than the brain. Diffusion sequence provides information about random Brownian motion of the free water molecules in the interstitial space and through the cellular membrane. In general, neoplastic tissue has more restricted diffusion than normal tissue because of its higher cellular density, which hampers normal diffusion of water molecules [12]. Diffusion sequence provides information about cellular density, the tortuosity of the extracellular space, the integrity of the cellular membranes, and the degree of glandular organization. The sequence must include the whole pelvis, prostate, and seminal vesicles in order to simultaneously achieve regional staging and detect a potential extraglandular lesion in the same acquisition [13]. The low mobility of molecules appears as a high signal on DWI MRI; similarly, molecules with high mobility show a loss of signal (Fig. 48.7). Sequence interpretation requires processing and quantification of the diffusion images using the ADC (apparent diffusion coefficient) in the parametric map. For quantification, the 5–10 mm² region of interest (ROI) is placed on the region of choice. Factor b value varies since there is no consensus regarding its optimal value. Nonetheless, values =0 and ≥1,000 s/mm² are recommended for the prostate. The use of higher b values may increase the sensitivity of the sequence by eliminating the high signal intensity of tissues with long T2 relaxation times (edema or fluid due to the high density of their protons), phenomenon known as “T2 shine-through.” Lesions with serious diffusion restrictions appear as low signals on gray-scale ADC maps. ADC quantification via monoexponential fitting is highly recommended for lesion characterization and cancer grading. A bicompartmental model using multiple b values and allowing separating true diffusion without perfusion contamination and the perfusion fraction has demonstrated that low and fast diffusion components are present in the prostate as previously reported in the brain [14]. Fast diffusion dominates the signal at low b values and low diffusion at high b values. With low b values the signal decay is due to the perfusion effect over diffusion, and with higher b values over 100 s/mm², the true diffusion signal decay may be calculated (Fig. 48.7). The fast and slow ADCs of cancer have shown significantly lower values than those of transitional zone and peripheral zone, and the apparent fraction of the fast diffusion component was significantly smaller in cancer than in peripheral zone [15].