Did you have a beginner’s science kit as a kid? Did it have a microscope? Even if you weren’t science-minded, it was a thrill to put a drop of pond water on a slide and exam it under magnification. What a discovery to see how many things invisible to the naked eye were squirming rapidly around! If the activity suddenly went into slow motion, it would get your attention.
Today’s MRI (magnetic resonance imaging) technologies is detecting disease conditions with great accuracy. In the world of prostate cancer (PCa), early detection offers the potential for minimalist treatments. However, there has to be a high degree of certainty in scenarios like these:
- A man with suspicious PSA test results may not need a biopsy
- A man with biopsy-proven PCa may want to go on Active Surveillance
- A man with biopsy-proven PCa may want a subtotal gland treatment such as a hemiablation (just the side of the gland with cancer) or a focal therapy (just the tumor + safety margin)
Ultrasound used to be the reigning imaging modality when it came to PCa diagnosis and treatment planning, but its chief limitation is its inability to characterize cancerous tissue as distinct from normal gland tissue. This means at least 30% of prostate biopsies are inaccurate. One could be reasonably, but not highly, confident about treatment decisions.
MRI revolutionized prostate imaging with the introduction of what are 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 scan results in a portrait of a man’s prostate gland in a way ultrasound cannot. Standard-of-care scans include 3 important sequences:
- Anatomic imaging using T1 and T2 weighted sequences to define normal prostate zones and any suspicious abnormalities
- Identifying likely areas of cancer based using Diffusion Weighted Imaging (DWI), which depicts restricted movement of water molecules due to tissue architecture (water moves less freely in cancerous tissue than normal tissue, so it would be analogous to the magnified pond water)
- Detecting blood flow in a tumor’s self-developed blood vessels (angiogenesis) by using Dynamic Contrast Enhanced (DCE) imaging
Restriction spectrum imaging (RSI)
Of these three sequences, DWI is often considered the workhorse for revealing diffusion qualities of water in tissue, yet it isn’t used alone because its information is incomplete. It does not obtain the underlying tissue geometry, so the reader perceiving restricted water molecule motion does not know if the architecture hindering motion is occurring within cells or in the space between cells; also, there can be inherent spatial distortion that degrades the imaging.[i] While mpMRI with DWI far outpaces prostate ultrasound in performance, improving it would significantly raise confidence for both physician and patient in making treatment and management decisions.
The latest news in mpMRI is an advance called Restriction Spectrum Imaging. According to McCammack, et al. (2016), “Restriction spectrum imaging (RSI) is an innovative, advanced diffusion sequence that aims to improve upon the strengths and address the shortcomings of conventional DWI in oncologic imaging.”[ii] It was originally developed for imaging in the brain, where it accurately picks up previously undetectable but telltale neurological differences in conditions such as Alzheimer’s disease, Parkinson’s disease, brain cancer, dementia, etc.
Like DWI, it is diffusion based but (with apologies for the technical language) it “uses data from a broader range of b values obtained in multiple directions to model a distribution, or spectrum” of the prostate tissue geometry for clearly discernible location of hindered water flow inside or between cells. In short, cancerous lesions—particularly aggressive PCa—becomes conspicuous. In RSI terms, this is called conspicuity. RSI also corrects for the spatial distortion of 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:
- MP-MRI in combination with RSI produced superior performance in identifying all PCa as well as specifically high-grade PCa
- RSI plus T2 weighted imaging may perform similarly to or better than mpMRI
- RSI may promote greater reader consensus, with agreement increasing when RSI is combined with mpMRI and most uniform when readers use RSI alone.[iii]
The authors point out that RSI, with its short scanning sequence time of about 5 minutes, may benefit patients who can’t tolerate prolonged scan times, or those for whom gadolinium-based contrast agents are a hazard due to compromised kidney function. Their results also suggest that for at-risk individuals who need a short, targeted screening exam, the combination of RSI plus T2 weighted imaging might be appropriate while providing accurate results.
All of this is good news. The latest addition to prostate MRI is a welcome one.
NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you are experiencing pelvic pain, or have any other health concerns or questions of a personal medical nature.
[i] Brunsing R, Schenker-Ahmed NM, White NS, Parsons JK et al. Restriction Spectrum Imaging: An evolving imaging biomarker in prostate magnetic resonance imaging, J Magn Reson Imaging. 2017 Feb; 45(2): 323–336.
[ii] McCammack KC, Schenker-Ahmed NM, White NS, Best SR et al. Restriction spectrum imaging improves MRI-based prostate cancer detection.. Abdom Radiol (NY). 2016 May;41(5):946-53. doi: 10.1007/s00261-016-0659-1.