MR spectroscopy could be the ultimate diagnostic tool. Noninvasive, radiation-free, and precise, this adjunct to MR imaging is ripe with potential. Some of that potential has been realized in brain applications, but generally MRS has fallen short of
MR spectroscopy could be the ultimate diagnostic tool. Noninvasive, radiation-free, and precise, this adjunct to MR imaging is ripe with potential. Some of that potential has been realized in brain applications, but generally MRS has fallen short of expectations. GE Healthcare is trying to do something about that.
The company has developed a spectroscopic package called PROSE (PROstate Spectroscopy and imaging Exam) that simultaneously captures image and spectroscopic data of the prostate, providing positional and functional information about abnormalities. The two data sets are coregistered: The image indicates the location of the suspicious tissue, and the spectroscopic data quantitates and graphs levels of chemical in the prostate.
Software precisely controls the application of radio-frequency pulses on 1.5T GE scanners. An endorectal coil tuned for imaging and spectroscopy makes the most of the scan.
The goal is to accurately assess the prostate, characterizing the extent of the tumor visually and then homing in on the chemical data contained in the MR spectrum. The image also indicates the location from which the spectrum was acquired.
MRI/MRS has an advantage over other anatomic/functional hybrids in that the spectroscopic data are obtained simultaneously with the imaging data. There is no difference in the time of acquisition or patient positioning, as may happen with PET/CT.
"We can be sure that the spectrum we get is from the part of the prostate we want," said Lindsey Carver, strategic manager for MR and biosciences at GE Healthcare. "We don't want signals from outside the prostate getting into the spectrum, because that would contaminate what we see."
PROSE provides a chemical fingerprint of the prostate. Creatine, choline, and citrate are critical to the spectroscopic assessment. Creatine and choline volumes tend to go up in cancer, while that of citrate tends to go down. The data for these three biomarkers can be turned into a color-coded map, which can be overlaid on the image to visualize the chemical "hot spots" in context. Specific data can be pulled up from these spots and presented graphically or formulated into ratios.
"The (mathematical pairing) produces a much more repeatable number," Carver said. "And repeatability obviously is what we are looking for here."
Typically, PROSE comes into play after a positive digital exam or prostate-specific antigen test has uncovered a possible problem. Patients with either result usually undergo biopsy with tissue samples taken from several different points in the prostate. PROSE could offer the next step when the results are not definitive.
"If the PSA is very high, but the biopsies come back indicating either normal or not convincingly cancer, the patient doesn't know what to do," she said. "PROSE has been used to make the decision about whether he has cancer or not."
In such cases, PROSE can be helpful in treatment planning. It can also help monitor the effect of therapy, identifying tissue necrosis and watching for the recurrence of cancer. It would be more useful if the numbers could be fine-tuned to determine the stage of the cancer rather than just its presence or absence.
Toward this end, GE is working with the American College of Radiology Imaging Network, which manages clinical trials of imaging technologies related to cancer, to clinically validate the combination of MR and spectroscopy with histologic findings of specific prostate tumors. The goal is to assess the cancer mathematically, matching specific numbers with stages of the disease.
PROSE is being used at only about 20 sites in the U.S. and 70 worldwide, partly because of its complexity. GE runs a training course for new users in collaboration with staff at the University of California, San Francisco, where more than 4000 PROSE cases have been processed.
The difficulty arises not only in applying the technology but in reading the data. Transition to 3T might help. In working with luminaries at UCSF on a 3T prototype of PROSE, GE has found that a stronger signal and reduction in noise spread the peaks of accepted biomarkers in the spectrum, while allowing the documentation of a polyamine peak. This biomarker appears to decrease in the presence of cancer.
"At 3T, we will get better diagnostic capability and even better results than now," Carver said. "This could be something that will change the game."
The use of PROSE to guide biopsies could also boost sales. The endorectal coil has been modified to allow users to biopsy tissue from specific points in the prostate.
One of the greatest hurdles to the widespread adoption of PROSE, however, is not technological but socioeconomic. The use of spectroscopy to assess prostate tumors is not reimbursed by third parties. This last hurdle may be overcome if engineers achieve what they are setting out to do.