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Just-Right PET/CT Imaging for Patients of All Sizes

June 05, 2006

Researchers discovered that diagnostic accuracy of images can be improved for people of varying weight and size with use of a fully 3-D, time-of-flight positron emission tomography (PET) with computed tomography (CT) scanner. Their findings were presented during SNM's 53rd Annual Meeting June 3—7 in San Diego.

"This new time-of-flight scanner—used with LYSO detector crystals—will improve the diagnostic accuracy of images, potentially leading to improved sensitivity and specificity in cancer lesion detection tasks in heavy patients, who have traditionally been difficult to image," said Suleman Surti, a research assistant professor at the University of Pennsylvania. "The current generation of PET scanners is limited in the quality of images produced for those patients who are overweight," he said. "This new technology can also reduce scan times for small- and average-size patients without losing diagnostic accuracy," added the co-author of "Imaging Performance of an LYSO-Based TOF PET Scanner."

PET—a unique medical imaging procedure that provides information about the body's chemistry, cell function and location of disease—provides earlier diagnosis, more accurate cancer tumor location and better assessment of patient response to chemotherapy and radiation therapy compared with other imaging techniques. PET can search for cancer in the entire body in a single exam and reveal any metastases (spread) and the primary site. PET involves imaging radioactive events inside a patient that emit two simultaneous and almost coincidental photons, which are detected by the PET scanner, said Surti. "If the exact location and arrival time of these photons in the PET detector are measured, then a precise estimate of the activity distribution within the patient can be obtained; however, in reality, this is not the case," he said. In the past, PET scanners that achieved very good position localization did so at the expense of precise information about the photon arrival time. In recent years, improvements in PET detectors have provided the capability to achieve good localization of position and arrival time of the photons simultaneously. Using this information in image reconstruction translates into improved image quality, which leads to reduced variability in the patient images and better lesion characterization, he detailed.

In a conventional PET system, a radioactive agent is injected into the patient. As each nucleus decays, it releases a positron that immediately collides with an electron, releasing two photons that travel away from the site of interaction at 180 degrees from each other. These pairs of photons are observed by the PET scanner, which uses this information to calculate where the agent is concentrated, thus creating a volumetric image of the patient including the affected area. Although the photons in each pair arrive at slightly different times depending on their origination point, this information has not traditionally been used in the reconstruction of the PET image data. With time-of-flight, this time difference can be measured with reduced uncertainty, enabling the use of this information in image reconstruction and leading to more accurate imaging.

Time-of-flight PET scanners were originally developed in the 1980s and used mainly for research studies. Additionally, the time-of-flight information was obtained at the expense of spatial resolution and sensitivity. Philips Gemini TF is the world's first commercially available time-of-flight PET/CT system that allows this more accurate tracking of photons using time measurements without compromising good characteristics of traditional PET scanners. Image acquisition could be shortened to 10—20 minutes for a whole-body PET scan, depending on patient size. Detector crystals—such as LYSO—perform a critical function in PET scanners. The crystal pixels absorb photons emitted from clinically relevant sites in the patient, converting them into light, which can be routinely processed by conventional electronics and sophisticated software to create an image.

"We performed experiments with this scanner using reproducible phantoms mimicking the presence of lesions in a patient," said Surti. New time-of-flight measurement capability—when used in reconstructing an image—improved image characteristics, which in turn allowed more accurate detection of small lesions, some of which are not visible in a conventional reconstruction without TOF information, he explained.

Research needs to continue, said Surti. "Our data indicate that more robust measures are needed to fully quantify the gains in imaging quality and to better define the relationship of image quality to patient size and timing resolution," he said.

Society of Nuclear Medicine