ORLANDO, FL - One of the promising aspects of PET/CT technology is its potential to deliver diagnostic CT scans simultaneously with functional PET imaging. In practice, most facilities have opted to use the CT portion of the hybrid modality to perform attenuation correction for PET imaging. Generally, if diagnostic-quality CT images are required, a separate scan is ordered.
Researchers from Duke University Medical Center in Durham, NC, have developed and implemented a set of protocols for obtaining diagnostic-quality PET and CT images in one session on a PET/CT scanner. They presented the results of their work at the Academy of Molecular Imaging (AMI) meeting on Wednesday.
The advantages of simultaneous diagnostic-level imaging in the two modalities are numerous, according to Dr. Terence Wong, an assistant professor of radiology, nuclear medicine at Duke, who presented his group's work.
"Patients only have to undergo one imaging session," he said. "Referring physicians are provided both diagnostic CT reports interpreted by radiologists specializing in cross-sectional imaging, as well as PET reports provided by nuclear medicine specialists."
In addition, clinicians receive the highest quality of anatomic definition and the highest degree of image registration possible with the PET/diagnostic CT (PET/DCT) protocol, Wong said. The team, when developing the protocols, set its objectives on producing both CT and PET images with a minimum sacrifice in quality on either modality.
Performing PET/DCT does require more technology and support than conventional PET imaging, Wong noted. The Duke team utilizes a Discovery ST PET/CT scanner (GE Healthcare, Chalfont St. Giles, U.K.) and a power injector (Medrad, Indianola, PA) for CT contrast injection, as well as nursing support and technologists cross-trained in both PET and CT.
The PET imaging protocol calls for oral contrast (Gastrografin) to be administered in 10-ounce doses three times (90 minutes, 60 minutes, and 15 minutes) before the scan. A scout CT is obtained when the patient is positioned, IV contrast is administered, and a 16-second whole-body CT scan is performed at end-expiration by the patient. A PET emission scan is then performed and the images are reconstructed. The nuclear medicine physicians and radiologists at the institution perform their interpretation on a GE AW workstation, Wong said.
The CT protocol calls for 150 cc of Isovue 300 (Bracco Diagnostics, Princeton, NJ) to be administered at 3 cc/sec via the power injector. The 16-slice scan provides a helical acquisition of 3.75-mm slices with a pitch of 1.375:1 at a 0.5-sec rotation and 27.5 mm per rotation. The group uses a GE-developed auto-mA feature of the scanner to automatically calculate a reduced patient radiation dose while adjusting the mAs to compensate for bone and anatomy during the whole-body scan, Wong said.
Rather than use a traditional inspiration breath-hold technique for the CT scan acquisition, the group scans at the end of expiration by the patient. This technique provides for increased basilar atelectasis and the capability for interstitial lung disease comparison between the studies, Wong explained.
"The technique works best if the patient practices it a few times before the scan," he said. "When they do, we have a better than 90% success rate at getting the images on the first pass."
The PET protocol employed by the team is to deliver an 18F-FDG injection at 0.14 mCi per kilogram of patient weight, and conduct a 2D emission scan at two minutes per bed position for patients less than 150 lb, three minutes per bed position for patients between 151-200 lb, and four minutes per bed position for patients greater than 200 lb.
More than 300 PET/DCT studies have been performed on a variety of indications at Duke since February 2004, with more than 60 this past January, according to Wong. The results have received high acceptance from both referring clinicians and their patients, he said.
One of the reasons for the high acceptance by the diagnostic and referring clinicians is that the protocol was revised during its development with feedback from both communities of physicians, Wong noted.
The group is looking to refine the protocol with isotropic voxels for improved reconstruction capabilities, specialized protocols for pediatric and head and neck cases, and further reductions in CT radiation dose. In addition, the group seeks to polish the protocol with an exhaustive list of indications for when and when not to use IV contrast in PET/DCT, Wong said.
"PET/DCT is routinely performed at Duke University Medical Center. It eliminates the need for separate PET and diagnostic CT scans, and it has become the study of choice for many oncologic applications," he said.
By Jonathan S. Batchelor
AuntMinnie.com staff writer
March 24, 2005
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