Scientists at M. D. Anderson and Rice have proposed a solution to an imaging challenge that arises when a moving target is imaged using a combined PET/CT scanner. The CT portion of the scanner takes a quick, three-dimensional snapshot of the tumor's anatomical location. The PET scan takes a few minutes to acquire data to generate an image depicting the tumor's uptake of a slightly radioactive glucose-related compound, which provides a measure of the tumor's metabolic activity and thus its health.

In lung cancer, for example, movement caused by a patient's breathing during data acquisition can blur the PET portion of this potent dual-imaging tool, explains Osama Mawlawi, Ph.D., associate professor in M. D. Anderson's Department of Imaging Physics and senior author of a recent paper in the Journal of Nuclear Medicine."PET provides powerful information by reporting the quantity of the injected tracer taken up by the tumor, because uptake is directly correlated to the malignancy of the tumor," Mawlawi says. A blurred image is less accurate and usually underestimates the amount of radioactive tracer in the tumor.

Scientists and physicians adjust for this problem in a number of ways now, including having patients hold their breath during repetitive short PET scans, which many with lung cancer, for example, cannot do. Working with Mawlawi and other scientists at the two institutions, Rice graduate student Guoping Chang designed the hardware, wrote the software and analyzed the data that provides the basis for an automated system to correct for this problem on existing PET/CT scanners. Their method allows the patient to breathe freely during the PET/CT scan while matching the tumor location that was captured during CT with that during PET. "This process is facilitated by automatically selecting PET data that only corresponds to the tumor location captured by CT," Mawlawi explains. This process, which adds 3-10 minutes to the scan, in effect freezes the respiratory motion at the same spot acquired during the CT. 

Patients are often nervous when undergoing a PET/CT scan, and it can take a few minutes for them to relax. Rather than taking the CT immediately after positioning the patient in the scanner, the team proposes conducting the PET scan first on all areas of the body except the area where the tumor lies. With the patient more settled, the CT image is taken followed by a PET scan over the tumor area. This process further ensures matching between the PET and CT scans. 

When the team tested the approach in 13 lung cancer patients, they found that the maximum standardized uptake value increased by 27% and the mean standardized uptake value increased by 28%. Image contrast improved, making the tumor easier to visually inspect. "Now that the numbers are more accurate, we can see the tumor better and the tracer uptake number enables us to assess the tumor's stage more precisely as well as its response to therapy," Mawlawi says. 

While some PET/CT scanners can adjust the PET image based on the phase of the patient's respiratory cycle, none possess the technique honed by the research team, which relies on respiratory amplitude, a measure of the depth of respiration