Swiss researchers optimize CT-PET scanning protocols

2002 02 28 14 45 46 706

VIENNA - A group of Swiss researchers operating one of the first hybrid CT-PET scanners in the world presented early results on their experiences with the system at today's European Congress of Radiology. They found that the combo systems have a measurable impact on the ability of radiologists to identify and locate pathology, while at the same time reducing scan times and increasing patient throughput.

Hybrid CT-PET scanners combine a CT scanner with a PET system in a single gantry in order to collect both anatomical and functional imaging data simultaneously. The technology is believed to be more efficient and accurate than earlier image fusion techniques, which involve image acquisition on separate scanners and fusion via software.

Commercial shipments of hybrid scanners began last year, with Siemens Medical Solutions of Erlangen, Germany, and GE Medical Systems of Waukesha, WI, hitting the market first. Philips Medical Systems of Best, the Netherlands is also bringing a combo CT-PET system to market.

The first GE unit -- called Discovery LS -- was installed at University Hospital in Zurich in March 2001. So far, some 1,200 patients have been scanned on the system, according to Dr. Thomas Hany, who presented the group's work at the ECR meeting.

Hany's presentation focused on the optimization of CT scanning parameters used with the system. The researchers wanted to determine how far they could lower CT dose and still acquire diagnostic images, he said.

The group scanned 53 patients with diagnosed or highly suspected malignancy, primarily lung or ear, nose, and throat tumors, using noncontrast CT. They compared images acquired at four different tube currents -- 10 mA, 40 mA, 80 mA, and 120 mA -- and also compared the hybrid images to PET images alone. The unit's kVp was set to 140, with a tube rotation of 500 ms and 5-mm slice thickness.

The researchers found that adding CT's functional data to PET increased their ability to visualize and locate lesions. With one image Hany displayed, researchers could see FDG uptake in the heart, but weren't exactly sure where it was. Adding the anatomical overlay to the image enabled them to see that the uptake was occurring in the pulmonary vein in the left atrium.

In another case, researchers reviewing the PET image alone classified a lesion as operable. But after adding the CT data, they realized it was inoperable.

The group's accuracy for classifying tumor lesions was 91% for PET alone. Adding CT scanning had the following impact:

Tube current (mA) mAs Diagnostic accuracy
(PET alone) N/A

Based on their experience, the group found that a tube current of 80 mA was the best compromise between image quality and lower radiation dose. (The radiation dose delivered to the patient ultimately is half of the tube-current setting because the scanner completes a rotation in half a second.)

The CT also had other benefits besides better image quality, according to Hany. The researchers used the CT data for transmission attenuation correction, with the scanner taking only 22 seconds to collect the required data. This reduced the total exam time by 33%, he said.

"Before, we were doing one patient an hour," Hany said. "Now we are doing two patients an hour. We are doubling our throughput."

CT-PET images are particularly useful when communicating results to physicians responsible for treating patients, such as surgeons and radiation oncologists. Hybrid images are far easier for these clinicians to read than PET scans alone, Hany said.

Hany's group is particularly interested in cardiac imaging. PET's ability to image perfusion is a major advantage over anatomic modalities. This capability is dovetailing well with recent advances in cardiac CT imaging.

"You have an all-in-one examination," he said.

By Brian Casey staff writer
March 4, 2002

Related Reading

CT/PET scanner, 16-slice CT are Philips RSNA highlights, November 28, 2001

ADM signs with GE for PET and fused imaging systems, November 27, 2001

GE rolls out line of hybrid scanners, June 25, 2001

GE unveils Discovery LS hybrid CT/PET system, June 22, 2001

FDA clears GE combo PET/CT scanner, April 16, 2001

Copyright © 2002

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