Whole-body time-of-flight fluorodeoxyglucose positron emission tomography scans improve lung and liver cancer detection, especially for the heaviest patients, according to a new study.

Conventional PET scans work by picking up gamma rays fired off from radioactive tracers injected in the body. But time-of-flight PET, which uses fast detectors, measures the time it takes for the rays to hit the detector. The result, its enthusiasts claim, is more accurate imaging and a better signal-to-noise ratio.

In the study, published this month in the Journal of Nuclear Medicine, researchers included 100 patients, with a body-mass index ranging from 16 to 45. (According to the World Health Organization, BMIs between 18-30 are within the normal range; over 30 is obese.) The researchers used a wide assortment of body types, as earlier work has suggested that the theoretical improvement from time-of-flight imaging increases with the bulk of the patient.
The researchers then imaged artificial 1-centimeter spheric lesions and added that data to the patient imaging studies, to create fused studies that mimic natural ones. The reason the researchers used artificial, rather than real, lesions, is so they would have greater certainty with the results. Still, they said this was the first study to compare whole-body patient time-of-flight data with conventional PET data.

"In contrast with previously published studies that reported comparison of TOF and non-TOF PET using simulated data or measured data with physical phantoms, this study used whole-body patient data," said co-author Joel S. Karp, a professor of radiologic physics with the University of Pennsylvania in Philadelphia, in a statement.

The fused patient scans - some with normal, some with artificial lesions present - and the same scans without time-of-flight processing were then analyzed by a mathematical model observer, technically called a 3-channel Hotelling observer, which is supposed to predict how a human observer would interpret the scans.

The result: the time-of-flight PET scans, at least for the mathematical observer, bested the conventional PET scans in signal-to-noise ratio for the (artificial) liver and lung tumors. The signal-to-noise ratio for time-of-flight was 8.3 percent higher for liver findings and 15.1 percent higher for the lungs.

Performance was also, as predicted, better at higher BMIs. The signal-to-noise ratio improvement was 9.8 percent in patients with a BMI under 30 (not obese), and 11.1 percent for those 30 or over.

Of note, time-of-flight also showed the highest gains for shortest-acquisition studies and for the lowest lesion contrasts, and the lowest gains for the highest lesion contrasts, the researchers said.

"Over all contrasts and body mass indexes, oncologic TOF PET yielded a significant improvement in lesion detection that was greater for lower lesion contrasts," the authors wrote. "This improvement was achieved without compromising other aspects of PET imaging."

The time-of-flight PET technology, which goes back to early experiments in the 1980s, is currently available on the Ingenuity TF PET/CT, made by Philips Healthcare, and the Discovery PET/CT 690, made by GE Healthcare, among other systems.

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