Today's MRI (magnetic resonance imaging) technologies can detect diseases with great accuracy. In the world of prostate cancer (PCa), early detection offers the possibility of minimalist treatments. However, a high degree of certainty is required in scenarios such as this one:
- A man with a suspicious PSA test result may not need a biopsy.
- A man whose PCa has been biopsied may want to undergo active surveillance.
- A man with biopsy-proven CPa may want a subtotal glandular treatment such as hemiablation (just the side of the gland with cancer) or focal therapy (just the tumor + safety margin).
Ultrasound was once the dominant imaging modality for the diagnosis and treatment planning of PCa, but its main limitation is its inability to characterize cancerous tissue as distinct from normal glandular tissue. This means that at least 30% of prostate biopsies are inaccurate. One can be reasonably, but not very confident in treatment decisions.
MRI has revolutionized prostate imaging with the introduction of so-called parameters, or imaging sequences that define various aspects of prostate tissue. The use of two or more parameters is called multiparametric MRI (mpMRI). Today's mpMRI allows a picture of a man's prostate to be drawn in a way that ultrasound cannot. Standard scans include 3 important sequences :
- Anatomical imaging using T1 and T2 weighted sequences to define normal areas of the prostate and any suspicious abnormalities.
- Identify likely cancer sites using diffusion-weighted imaging (DWI), which shows the restriction of movement of water molecules due to tissue architecture (water moves less freely in cancerous tissue than in normal tissue, so it would be analogous to enlarged water in a pond).
- Detecting blood flow in the self-developed blood vessels of a tumor (angiogenesis) using Dynamic Contrast-Enhanced (DCE) imaging
Restriction spectrum imaging (RSI)
Among these three sequences, DWI has often considered the workhorse for revealing the water diffusion qualities in tissues, but it is not used alone because its information is incomplete. It does not provide the geometry of the underlying tissue, so the reader who perceives the restricted movement of water molecules does not know whether the architecture that impedes movement occurs within cells or in the space between cells; in addition, there may be an inherent spatial distortion that degrades the imagery[i]. Although mpMRI with DWI far surpasses prostate ultrasound in terms of performance, its improvement would significantly increase physician and patient confidence in treatment and management decisions.
The latest news in mpMRI is a breakthrough called restriction spectrum imaging. According to McCammack and colleagues (2016), "Restriction Spectrum Imaging (RSI) is an innovative and advanced diffusion sequence that aims to enhance the strengths and fill the gaps in conventional DWI in oncology imaging"[ii][ii].
Like DWI, it is diffusion-based but (excuse the technical language) it "uses data from a wider range of b-values obtained in multiple directions to model a distribution, or spectrum" of prostate tissue geometry to clearly locate the location of impeded water flow within or between cells. In short, cancerous lesions - especially aggressive PCa - becomes conspicuous. In terms of RSI, this is called perceptibility. RSI also corrects the spatial distortion of the DWI.
The McCammack paper cited above reports the results of a study designed to evaluate the clinical efficacy of RSI vs. current mpMRI for PCa detection. 100 men were involved in the study, and all had both mpMRI and RSI scans complete. 33 of the men went on to prostatectomy, and 67 went on to biopsy, all within 6 months of imaging. Images were read by 3 different readers, and all images were compared with post-surgery and post-biopsy pathology. Among the study findings were:
- MRI-MP, combined with RSI, has produced a superior performance for the identification of all PCa as well as high quality PCa
- RSI plus T2 weighted imaging may perform similarly to or better than mpMRI
- RSI can promote greater reader consensus, with agreement increasing when RSI is combined with mRMI and being more consistent when readers use RSI alone.
The authors note that RSI, with its short scan sequence time of approximately 5 minutes, maybe beneficial for patients who cannot tolerate extended scan times, or for those for whom gadolinium-based contrast agents are a hazard due to compromised renal function. Their results also suggest that for at-risk individuals who require a short, targeted screening exam, the combination of weighted RSI plus T2 imaging may be appropriate while providing accurate results.
All of this is good news. The latest addition to prostate MRI is welcome.
NOTE: This content is for informational purposes only and is not a substitute for diagnosis or medical advice. Talk to your doctor if you are experiencing pelvic pain or have other personal medical concerns or questions.