Imaging of the prostate

Full text: The main role of imaging in prostatic diseases is for prostate cancer. Transrectal ultrasound (TRUS) and magnetic resonance imaging (MRI) are the most commonly used imaging tools used for the diagnosis of the diseases of the prostate gland. The main indications for TRUS is the evaluation for prostate cancer and guidance for prostate biopsy. On TRUS, the transition zone with a hypoechoic appearance can be differentiated from the peripheral zone, which appears relatively echogenic and homogenous in echotexture. Prostate cancer mainly involves the peripheral zone, though one fifth of the disease can be detected in the transition zone, which is the major site for hyperplastic changes in older men. Color Doppler ultrasound may be helpful for the differentiation of low-risk, hypovascular tumors from high-risk, hypervascular tumors, as the latter group is associated with higher Gleason tumor grades consistent with higher risk for extraprostatic spread. Nevertheless, targeted prostate biopsy solely based on high-frequency color or power Doppler imaging is not recommended, as the technique has inherent risk of missing a significant number of cancers. Although power Doppler ultrasound can enable the operator to perform more accurate sampling of the prostate by determining sites of focal hypervascularity, it has not been found to be superior to color Doppler ultrasound. It has been reported to be useful only for targeted biopsies with limited number of biopsy cores. Microbubble contrast agents may enable better visualization of prostatic microvasculature and cancerous prostate tissue. By means of contrast-enhanced ultrasound (CEUS), the number of cores may be decreased by performing targeted biopsies. Importantly, the detection of the signals reflected by the microbubbles can be enhanced by the phase inversion (pulse-inversion) technology. Prostate cancer appears as a dark zone on elastography representing limited elasticity or compressibility. By means of the technique, cancerous tissue can be differentiated from benign tissues depending on the hardness gradient and degree of elasticity loss. Nevertheless, the technique is not sufficient yet to preclude the requirement for systematic prostate biopsies. TRUS-guided prostate biopsy has been accepted as the 'gold standard' tool for the detection of prostate cancer. Currently, extended sampling protocols involving 10-12 cores with additional laterally directed cores at the aforementioned levels have been used to increase the diagnostic yield. Magnetic resonance imaging (MRI) is useful for prostate cancer detection primarily in patients with persisting suspicion for undisclosed cancer, despite having negative transrectal US and biopsy findings. MRI can also be used for the localization of cancer within the prostate and in local and distant staging of the disease. Technically, fast spin echo imaging preferably with the combination of endorectal and pelvic phased array coils is recommended for an ideal prostate MRI examination. MRI also aids in the local staging of prostate cancer, as it enables the assessment of the capsular penetration, extracapsular spread and local and distant metastasis. Accordingly, the data derived from endorectal MRI can be used with Partin nomograms to predict extracapsular spread of prostate cancer, in intermediate and high risk patients. MRI is a complementary technique to improve tumor detection by the assessment of tumor metabolism. Dynamic contrast-enhanced MRI, on the other hand, enables direct assessment of the angiogenesis associated with prostate cancer and relative peak enhancement has been accepted as the most accurate perfusion parameter for cancer detection. Diffusion- Weighted Imaging is based on the restriction of diffusion and elevation of apparent diffusion coefficient (ADC) in cancerous tissue compared to the normal prostate tissue. However, the diagnostic accuracy of the technique is diminished by the variability of the ADC values