Combined 1.5T MRI/MRSI has recognized limitations, including the potential for false positive and false negative results. The limitations include post-biopsy changes, confounding benign pathologies (prostatitis, BPH), mixing of cancer and healthy tissue with small volume (< 0.5 cc) early stage cancer and an insufficient understanding of the histological and biological basis of false positives and false negatives. Post-biopsy hemorrhage results in both over- and under-estimation of tumor extent at MRI and MRSI. (78
). The timing of MRI and MRSI of the prostate following transrectal biopsy is therefore important. Early studies have recommended an interval of 3 weeks between transrectal biopsy and MR imaging (78
). However, there has been a trend towards increased prostate sampling during transrectal biopsy; and currently greater than 6 core biopsies are frequently obtained (81
). The increase in prostate sampling has raised new questions about the optimal timing of MRI and MRSI following transrectal biopsy, and the impact of post-biopsy hemorrhage on interpretation of MR and MRSI. In a recent study of 43 patients with biopsy-proven prostate cancer undergoing endorectal MRI and MRSI prior to radical prostatectomy confirming organ-confined disease, the outcome variables of capsular irregularity and spectral degradation were correlated with the predictor variables of time from biopsy and degree of post-biopsy hemorrhage (82
). The authors found capsular irregularity was unrelated to time from biopsy or to degree of hemorrhage. Spectral degradation was inversely related to time from biopsy (p < 0.01); the mean percentage of degraded peripheral zone voxels was 18.5 % within 8 weeks of biopsy compared to 7 % after 8 weeks. Spectral degradation was unrelated to the degree of hemorrhage. The authors concluded that in organ-confined prostate cancer, capsular irregularity can be seen at any time after biopsy irrespective of the degree of hemorrhage, while spectral degradation is seen predominantly in the first 8 weeks. MR staging criteria and guidelines for scheduling studies after biopsy may require appropriate modification.
Only a few studies have investigated the MRI and MRSI appearances of acute and chronic prostatitis, but these initial reports suggest that in at least some cases prostatitis may mimic cancer. Engelhard et al found that biopsy proven prostatitis correlated with areas of low T2 signal intensity at MRI (83
). Theoretically, the inflammatory process of prostatitis might be expected to reduce the level of citrate seen at MRSI, although Van Dorsten et al failed to find any difference between healthy peripheral zone spectra and peripheral zone spectra from 12 patients thought to have prostatitis (84
). Further studies with better pathological correlation are required to elucidate the true spectroscopic findings in prostatitis. Cancers within the central gland (transition zone and central zone) also have proven to be particularly difficult to discriminate on MRI/MRSI. There is significant overlap of low signal intensity on T2
weighted images and metabolism on MRSI in regions of central gland tumor with predominately stromal Benign Prostatic Hyperplasia (BPH) (85
). Regions of predominately glandular BPH have very elevated levels of citrate and polyamines since they are secretory products of healthy and hyperplastic glandular tissues. While in predominately stromal tissues, such as predominately stromal BPH, citrate and polyamine levels are very low similar to what is observed in cancer. Like cancer, there can also be somewhat elevated choline, since there is increased cellular proliferation in BPH as in cancer.
While prostatitis and stromal BPH are the most common benign confounding factors in over-calling prostate cancer by MRI/MRSI, prostate cancer can also be under-called due to signals arising from surrounding benign tissues masking the metabolic fingerprint of cancer, particularly for small infiltrative disease. Specifically, benign glandular tissues have very high signal intensity on T2 weighted MRI as well as very high levels of polyamines and citrate, and these signals will dominate the prostate spectrum. Predominately mucinogenic prostate cancer is also very difficult to detect on MRI/MRSI. On T2 weighted MRI they have high signal intensity due to the presence of the pockets of mucin. On MRSI, the spectral signal intensity is often very low due to the low density of prostate cancer cells.