By: Dan Sperling, MD

A newly developed MRI technology called Restriction Spectrum Imaging is being tested for its accuracy in detecting prostate cancer. If studies bear out, it will prove to be more accurate than Diffusion Weighted Imaging (DWI), one of the parameters used to enhance certain features on MRI scans.

Background

Multiparametric MRI (mpMRI) has already advanced prostate cancer imaging to a point where suspicious lesions seen on scans correlate highly with prostatectomy specimens from the same glands (see earlier article at https://sperlingprostatecenter.com/can-imaging-detect-characterize-prostate-cancer-index-lesion/ ). One particular parameter, Diffusion-Weighted Imaging (DWI-MRI) has been extensively used because of its ability to detect tumors based on the motion (diffusion) of water molecules in different types of tissue. (A brief description of DWI can be found in the article at https://sperlingprostatecenter.com/mri-imaging-prostate-cancer-two-parameters/) However, DWI can be subject to distortion due to magnetic field artifacts. Also, other abnormalities within the prostate, e.g.  inflammation, may be misinterpreted as cancer because of the ambiguous way DWI reveals them.

About four years ago, researcher Nathan S. White, PhD announced the development of RSI for imaging the brain. RSI is a more sophisticated diffusion method that collects “a broader, more extended spectrum of diffusion images, combined with sophisticated modeling of differential water compartments in tissue and correction of spatial distortion.”[i] Tumors have a different cellular structure than normal prostate tissue, so tumor cells impede water molecule motion very specific ways. RSI enhances the signal from this restriction, allowing identification of those cells as cancer. Furthermore, it eliminates the distortion to which DWI is susceptible.

A dramatic example of how RSI detects tissue differences was reported by a clinical team (including White) in a case study of a 52-year old man with incurable brain cancer (glioblastoma).[ii] After surgical removal of the main tumor, the patient had a course of radiation and chemotherapy, as well as a drug that inhibits the cancer’s ability to develop its own new blood vessels. MRI monitoring, including DWI, RSI and contrast-agent MRI that shows blood flow, revealed a spreading area in the patient’s brain. The patient began showing symptoms of cognitive decline. These two factors over the next 18 months (images and symptoms) were interpreted to mean that the tumor was continuing to spread. However, the RSI images were distinct from what would be expected in glioblastoma growth; this correlated with contrast-agent images showing greatly reduced blood volume in the area—a phenomenon that would be almost the opposite of an active tumor’s blood flow. The medical team hypothesized that the suspicious area was comprised of dead tissue (necrosis) related to the drugs, not new tumor growth. The patient’s death occurred 2 years after diagnosis. An autopsy confirmed the RSI findings (necrosis).

RSI-MRI of the prostate

A University of California research team (UCSD, UCLA) has now published the results of a small, non-randomized proof of concept study on a new MRI technology called Restriction Spectrum Imaging (RSI-MRI).[iii]  The authors report encouraging finding.

The study authors enrolled 28 prostate cancer patients who were to undergo radical prostatectomy. Ten of the patients had biopsy-proven T3 disease. The purpose of the study was to compare the performance of MRI and RSI-MRI in detecting extracapsular extension (ECE or tumor breakthrough beyond the prostate capsule. All patients were scanned by both types of imaging before their surgery. For the RSI images, cellularity maps of the restricted water molecular movement were constructed and standardized, and distortion correction was performed. (One patient’s images were not amenable to distortion correction, and therefore this patient’s MRI and RSI were eliminated from the data analysis.)

The team found that of the remaining 27 patients whose images were evaluated and compared with the prostatectomy specimens, the standard MRI correctly identified ECE in 2 of the 9 T3 patients (22%) whereas RSI-MRI identified ECE in 8 of the 9 (89%). RSI-MRI also correctly identified stage T2 disease in the remaining 18 patients.

The authors concluded that RSI-MRI is feasible for prostate imaging and cancer detection, providing measurably more accurate detection of prostate cancer (PCa) than standard MRI. Their study implies that greater localization of the disease, without distortion, can aid treatment planning.  A news story reports, “In another paper, to be published in the journal Frontiers in Oncology, the team reports how RSI-MRI may also be able to predict the grade of a tumor. This is because the amount of water cancer cells can take into their nuclei differs with tumor grade.”[iv]

In summary, RSI-MRI is a promising new advancement of diffusion MRI, and larger randomized trials as well as translational studies are needed to “define its operating characteristics with respect to diagnosis and staging.”[v]

[i] Ibid.

[ii] Farid N, Almeida-Freitas D, White N et al. Restriction-spectrum imaging of bevacizumab-related necrosis in a patient with GBM. Front Oncol. 2013;3:258.

[iii] Rakow-Penner RA, White NS, Parsons JK et al. Novel technique for characterizing prostate cancer utilizing MRI restriction spectrum imaging: proof of principle and initial clinical experience with extraprostatic extension. Prostate Cancer Prostatic Dis. 2015 Jan 6:1-5. doi: 10.1038/pcan.2014.50.

[iv] Paddock, Catharine. “Prostate Cancer ‘Easier to Detect’ with New Type of MRI.” Jan. 7, 2015. http://www.medicalnewstoday.com/articles/287662.php

[v] Rakow-Penner et al. Ibid.