Feature article

Role of MRI in prostate cancer

About 29,430 men in the United States are expected to die from prostate cancer in 2018.1 The prostate gland is a small walnut-shaped gland in a man's abdomen.1,2 When the cells in the prostate gland begin to grow uncontrollably, cancer occurs. Prostate cancer is common in men, but does not always affect them. In fact, many older men who died from other causes had prostate cancer that didn't affect them. Adenocarcinomas, the most common form of prostate cancer, is slow spreading, but other forms spread rapidly. Cancer detection while the cancer is only in the prostate gland has a better chance of successful treatment.

The most common way to screen for prostate cancer is through blood tests, specifically for prostate-specific antigen (PSA.)3 The blood test detects high levels of PSA, which could indicate prostate cancer or any number of other conditions. After an observed high level of PSA, imaging tests and biopsy can be used to diagnose cancer. Imaging methods are used to detect, diagnose, and stage prostate cancer. Ultrasound (US) is the traditional and most common method of imaging the prostate gland.4 US has a relatively minor role in detecting and staging prostate cancer. It provides image guidance for biopsies and brachytherapy (a form of radiation therapy.) US provides inconsistent information about tissue and abnormal growth. Computed tomography (CT) is typically used for pelvic imaging but has little to offer in terms of cancer-specific imaging of the prostate. CT cannot be used to stage cancer. CT is commonly used for assessing metastases, or spreading of cancer beyond the prostate. Magnetic resonance imaging offers a variety of different methods that can be used to assess prostate diseases, including diffusion weighted, dynamic contrast enhanced, and multi-parametric imaging.

Individual methods

Diffusion weighted MR imaging (DW-MRI) is a method that evaluates the Brownian motion (random motion of molecules in a liquid) to present a clear image of the prostate.4 The apparent diffusion coefficient (ADC) measures this change and provides additional information for the radiologists and physicians. DW-MRI has a higher sensitivity than US and CT in regards to prostate cancer, detecting cancer at with greater accuracy. The ADC helps to characterize prostate tissue and determine if a growth is benign or malignant. This is because the ADC shows a significant difference between central gland, stromal hyperplasia and gladular hyperplasia cancers. ADC correlates to Gleason scores (a method of evaluating the prognosis for a man with cancer,) as well as D'Amico clinical risk scores (a method of evaluating the five-year risk of treatment failure.)

DWI is typically done using a single-shot echo planar imaging (SSEPI) sequence.4,6 The SSEPI helps to avoid motion distortion, but causes spatial distortions and low signal-to-noise ratio (SNR.) These spatial distortions can be avoided using reduced field of vision (rFOV) or multi-shot EPI (MSEPI.) Both of these options reduce scan time. The first, rFOV, limits the field of vision, reducing the amount of data attained. The latter obtains more shots, allowing for the correction of motion artifacts that appear on select images through comparison.

Dynamic contrast enhanced (DCE) MRI monitors how a contrast acts after intravenous injection. It highlights different parts of the prostate than conventional MRI and allows the radiologist to image the uptake of the contrast. DCE is more sensitive but less specific than T2-weighted imaging.4 DCE intensity enhancement is due to higher microvessel density and permeability, which are characteristics of tumor tissue.4,6 DCE parameters show different measures between cancer types, just as DWI does.

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Combined methods

Multi-parametric MRI provides functional imaging to supplement anatomical imaging. This is done by obtaining multiple sequences. Multi-parametric MRI combines T2, DW, DCE, and magnetic resonance spectroscopy (MRS) to detect, monitor and stage cancer. T2-weighted imaging provides high spatial resolution and differentiates the zones within the abdomen.4,7,8 DWI and DCE provide information about the tissue and structures in the prostate. MRS helps assess cancer aggressiveness and monitoring therapy response and recurrence. Combining these four methods increases the positive predictive values for detection of cancer in the overall prostate. The 3.0T multi-parametric ability to detect cancer may be beyond what studies have previously presented. MRS is not as commonly used now as it has been in the past, because, even though it has a high specificity in lesion characterization, it is too expensive and requires post-scan input from the radiologist. T2, DWI and DCE combined provide 98% accuracy in cancer diagnosis. Rather than performing a biopsy, mpMRI could help to diagnose cancer in a noninvasive way with accuracy rates that high.

MRI tells a physician more about the prostate and prostate cancer than the primary imaging methods (US and CT.) Additionally, MRI can help diagnose cancer without the need for biopsy. T2-weighted imaging is less likely to show as much information as DWI and DCE individually. If you combine the three methods, MR can more accurately diagnose cancer, leading to a reduction in biopsy risk. MRI could help physicians diagnose the 29,430 men that will be told they have cancer this year. 

1. "About Prostate Cancer." American Cancer Society. 11 March 2016. Web. 16 November 2018. <https://www.cancer.org/cancer/prostate-cancer/about.html>.

2. Mayo Clinic Staff. "Prostate cancer." MayoClinic.org. 9 March 2018. Web. 16 November 2018. <https://www.mayoclinic.org/diseases-conditions/prostate-cancer/symptoms-causes/syc-20353087>.

3. Mayo Clinic Staff. "PSA test." MayoClinic.org. 6 January 2018. Web. 19 November 2018. <https://www.mayoclinic.org/tests-procedures/psa-test/about/pac-20384731>.

4. John V. Hegde, et al. "Multiparametric MRI of Prostate Cancer: An Update on State-of-the-Art Techniques and Their Performance in Detecting and Localizing Prostate Cancer." J Magn Reson Imaging. 1 May 2013. 37(5): 1035-1054. Web. 14 November 2016. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3741996/>.

5. Yuxin Zhang, et al. "Quantitative diffusion MRI using reduced field-of-view and multi-shot acquisition techniques: Validation in phantoms and prostate imaging." Magnetic Resonance Imaging. 2018; 51. Web. 16 November 2018. <https://www.medphysics.wisc.edu/~yzhang/wp-content/uploads/MUSE.pdf>.

6. Piotr Kozlowski, et al. "Combined diffusion-weighted and dynamic contrast-enhanced MRI for prostate cancer diagnosis—Correlation with biopsy and histopthology." JMRI. 9 June 2006. Web. 16 November 2018. <https://onlinelibrary.wiley.com/doi/full/10.1002/jmri.20626>.

7. Tristan Barrett. "What is multiparametric-MRI of the prostate and why do we need it?" Imaging Med. 22 October 2015; 7(1): 13-17. Web. 14 November 2018. <https://www.openaccessjournals.com/articles/what-is-multiparametricmri-of-the-prostate-and-why-do-we-need-it.pdf>.

8. Sangeet Ghai and Masoom A. Haider. "Multiparametric-MRI in diagnosis of prostate cancer." Indian Journal of Urology. July 2015; 31(3): 194-201. Web. 19 November 2018. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4495493/>.