BERKELEY, CA (UroToday.com) - This article provides a rationale for the use of multiparametric MRI in patients at risk for prostate cancer, as well as the technical parameters involved.
The article does not explain how we evolved to this point and where we are likely to go from this point forward. The current state of multiparametric MRI should be understood in its historical context to understand how this examination became so lengthy. If prostate MRI is to become cost-effective, its various components will likely need to be modified or eliminated to reflect patient-specific applications.
The original sequence for prostate MRI was T2 weighted (T2W) imaging performed with an endorectal coil (ERC) where it was performed mainly for staging the prostate capsule and seminal vesicles. T2 weighted imaging has always provided the highest anatomic detail, although not the best contrast resolution for cancer. By the mid-1990s, MR spectroscopy (MRSI) was added to T2W, but it was found difficult to perform routinely outside of academic centers. By the late 1990s dynamic contrast enhancement (DCE) was proposed as a means to improve the diagnosis and staging of prostate cancer. Finally, in the mid-2000s diffusion weighted imaging (DWI), already in common use for neuro applications, was made possible in the body by increasingly sophisticated magnetic gradient corrections. In the early 2010s this was supplemented with “high b” value diffusion images which accentuate areas of cancer but produce very noisy images. Since medical practice is inherently inertial, it was common for the prostate MRI exam to accumulate these additional sequences over time without shedding previous ones, so that the modern multiparametric MRI exam included T2, MRSI, DCE-MRI and DWI, high b value DWI, all performed with an endorectal coil. This created an unwieldy, uncomfortable and expensive hour-long scan. While multiparametric MRI proved its value in detecting and characterizing prostate cancer, it did so at a high cost. The next wave of innovation will be in simplifying and customizing prostate MRI beginning by eliminating the term “multiparametric” as virtually all MRI is multiparametric.
The modern prostate MRI should not be performed as a “one size fits all” exam but rather as a customized evaluation designed to meet the specific needs of the patient. Increasingly, modern prostate MRI can be performed without an endorectal coil, especially in non-obese patients. For instance, MRI is increasingly suggested in the screening setting in conjunction with PSA, in order to decide when and what to biopsy in the prostate. When the PSA is elevated, some centers utilize MRI to decide whether a biopsy is warranted.[2, 3] For this application, a non-endorectal coil T2 and DWI+ high b value image is likely sufficient. This should be a low cost, easy-to-perform tool, to help avoid or defer biopsies in patients with large transition zones due to BPH and no visible peripheral or anterior transition zone lesions. Naturally, such “stripped down” MRIs should be less expensive than the full multiparametric MRI as they require less gantry time. For tumor characterization (i.e., to understand the size and location of a previously diagnosed prostate cancer) we find it useful to perform T2, DWI+ high b value imaging and DCE MRI. This enables the best lesion characterization. An ERC is optional depending on the size of the patient (thinner patients do not require an ERC). For tumor staging of a known cancer in which the status of the capsule and seminal vesicles are most important, the examination can be reduced to a high resolution T2 exam in 3 planes with an endorectal coil. This will provide the highest resolution examination of the prostate and its environs. The patient should be asked to refrain from ejaculation for at least 3 days to fully distend the seminal vesicles and a pelvic MRI should be obtained to assess for enlarged nodes or bony pelvic lesions. However, these preparations may not be needed for the majority of patients undergoing MRI. For patients on active surveillance, a simple non-endorectal coil T2 and DWI+ hi b value imaging exam may suffice. Finally, for patients who undergo treatment such as surgery, focal therapy or radiation therapy, recurrent disease can be detected with an exam that includes an endorectal coil and T2, DWI + high b value and DCE MRI. These strategies are summarized in the table.
Several important caveats should be made. First, when reducing the number or intensity of MRI there is always a risk of missing a tumor. Fortunately, this is a well-known phenomenon in prostate cancer diagnosis and urologists are familiar with this situation. Because prostate cancer is a slow growing process, continued screening with PSA and MRI will detect tumors on subsequent exams. Second, this strategy is predicated on the assumption that MR targeted biopsy (either in gantry or preferably using MR-US fusion biopsy) is available to confirm that the MR-detected lesion is indeed cancer.
Thus, as the field of prostate MRI matures, we predict that the multiparametric exam will become more tailored to each indication and that patients will experience only as much cost, discomfort and inconvenience as is necessary to get the proper exam for the question being asked. The incorporation of MRI into the daily practice of urology represents a large paradigm shift and there is a natural skepticism over whether it is worth the added expense and inconvenience. By modifying the full-fledged “one size fits all” multiparametric exam to a more streamlined, less invasive and less costly “prostate MRI,” it will be easier to incorporate into everyday urologic practice. Thus, we predict the moniker “multiparametric MRI” will disappear and be replaced with its more streamlined descendant, the prostate MRI.
- Kim CK, Park BK, Kim B. High-b-value diffusion-weighted imaging at 3 T to detect prostate cancer: comparisons between b values of 1,000 and 2,000 s/mm2. AJR Am J Roentgenol. 2010 Jan; 194(1):W33-7. Doi:10.2214/AJR.09.3004.
- Petrillo A, Fusco R, Setola SV, et al. Multiparametric MRI for prostate cancer detection: performance in patients with prostate-specific antigen values between 2.5 and 10 ng/mL. J Magn Reson Imaging. 2014 May; 39(5):1206-12
- Jambor I, Kahkonen E, Taimen P, et al. Prebiopsy multiparametric 3T prostate MRI in patients with elevated PSA, normal digital rectal examination, and no previous biopsy. J Magn Reson Imaging. 2014 Jun 23. doi: 10.1002/jmri.24682
- Medved M, Sammet S, Yousef A, et al. MR imaging of the prostate and adjacent anatomic structures before, during, and after ejaculation: qualitative and quantitative evaluation. Radiology. 2014 May; 271(2): 452-60. doi: 10.1148/radiol.14131374
- Johnson LM, Rothwax JT, Turkbey B, et al. Multiparametric magnetic resonance imaging of the prostate aids to detect lesion progression. J Comput Assist Tomogr. 2014 Jul-Aug; 38(4):565-7.
Sandeep Sankineni, MD and Peter Choyke, MD as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Molecular Imaging Program, National Cancer Institute, Bethesda, MD USA
Functional MRI in prostate cancer detection - Abstract