Body CT perfusion improves diagnosis, therapy monitoring

CT perfusion techniques, useful in assessing stroke patients, may also demonstrate value elsewhere in the body. Perfusion 3 software from GE Healthcare and syngo Body Perfusion-CT from Siemens Medical Solutions can characterize malignant tumors and track patient response to therapy. Both can be used to assess tumors and other disease processes in areas outside the brain, including the liver, bladder, kidney, and breast.

The two packages have the potential to help characterize tumors as benign or malignant by analyzing differences in perfusion parameters. Tumors are known to enhance and wash out more rapidly than normal tissue. Perfusion software quantitates blood flow on the basis of contrast measurements.

"With CT, you can quantitate real perfusion rates in the number of cubic centimeters per second," said Dr. John Haaga, a professor of radiology at Case Western Reserve University in Cleveland, who uses the syngo Body Perfusion-CT package. "These ready-made programs are wonderful, because before you had them you had to try to do these calculations by hand, and there were all kinds of inaccuracies."

Automated quantitation may help draw a very fine line between abnormalities, extending beyond even cancer and normal tissues. GE reports that users of its Perfusion 3 package have differentiated between pancreatitis and pancreatic cancer.

The clinical value of body perfusion software depends heavily on the interpretive skills of the radiologist, however. Haaga warns of substantial overlap between the response of malignant and healthy tissues.

"Many times you go to biopsy to figure that out," he said.

Perfusion 3 and syngo Body Perfusion-CT list for about $40,000. Their use might ultimately reduce or eliminate the need for tissue biopsies. But for the time being, they are adjuncts to conventional diagnostics, and vendors are careful to say so.

"Body perfusion offers a way of potentially quantifying the process of angiogenesis as a diagnostic indicator and determining the reaction of tumor tissue under treatment earlier than traditional CT, where the only thing you can measure is tumor volume changes," said Bernd Ohnesorge, Siemens vice president of CT.

Siemens' syngo Body Perfusion-CT, released this summer, and GE's Perfusion 3, released over two years ago, calculate blood flow, blood volume, and vessel permeability. The measurements are based on dynamic CT data acquired after the injection of a contrast medium. Both packages offer algorithms optimized for liver analysis, calculating arterial and portal venous components.

"In the liver protocol, we have a number of different maps, such as hepatic arterial fraction, which tells you the percentage of the liver that is supplied by the hepatic artery versus the portal vein," said Karen Procknow, GE product development specialist for functional imaging. "It also calculates the time to enhancement, which shows the first point of enhancement across the liver."

Both companies' software might potentially signal early effects of therapy, documenting changes in parameters compared with baseline studies conducted prior to the start of therapy. Dr. Chaan Ng, an assistant professor of diagnostic radiology at M.D. Anderson Cancer Center, is exploring how perfusion could indicate early response to drug therapy. He believes it might provide a more sensitive indicator than changes in tumor volume, the more widely accepted indicator.

"Morphology has a lot of limitations," Chaan said. "When you are following size, it takes time for changes to become evident. Perfusion has a lot of problems also, but if you pick up perfusion differences, maybe that is telling you something."

Output from CT body perfusion studies includes images and data tables indicating flow rates, vessel permeability, and other parameters that indicate perfusion.

In body perfusion, a specific volume, defined by the target organ or suspicious lesion, is scanned continuously to note differences in contrast enhancement. Gastric processes and respiration present challenges because they may cause the tissue to move, but scanning during breath-hold helps keep respiratory artifacts in check. Anatomic mapping algorithms detect and correct for remaining artifacts, according to GE and Siemens.

Users apply dynamic scan protocols, then postprocess the ensuing data on advanced workstations such as the Siemens Leonardo and GE Advantage.

The users of these technologies are pioneers, as the clinical benefits of the software have yet to be fully documented. Haaga and Chaan are among those trying to develop the rules that define these data.

Haaga has focused on the delivery of vasoactive drugs that affect tumor vessels differently from normal vessels.

"We are trying to modulate the blood flow to set up techniques and methods where we can better distinguish tumor from normal tissue," he said. "The trick is to find drugs that are safe, can be given in a simple fashion, and are specific to the organ we are looking at."

The liver, one of the most common sites for metastatic disease, is one such organ. Haaga is also targeting the breast and prostate.

Documented perfusion techniques that differentiate malignant from benign tissue without biopsy could be available in a few years, he said. Documenting the means for assessing patient response to chemotherapy is at least as far away.