Longer scan time, not dose increase, produces better PET images for the obese

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The quality of images acquired from heavier patients can be maintained only by scanning for longer periods of time, not by increasing the dose of an injected radiopharmaceutical, according to research published in the June issue of the Journal of Nuclear Medicine.

The quality of 18F-FDG PET/CT images of overweight patients is often degraded. Researchers from Tokyo Women's Medical University and Nagasaki University in Tokyo, Japan, evaluated the effect of optimizing injected dose or acquisition time on the quality of images of overweight patients using lutetium oxyorthosilicate PET/CT with high-performance detector electronics.

They initially retrospectively measured radioactivity concentrations and signal-to-noise ratios (SNRs) in the liver relative to body weight for 80 patients who had undergone 18F-FDG PET/CT according to standard protocol (injected dose, 3.7 MBq/kg; acquisition time, 2 min/bed position).

Patients were grouped (20 per group) according to baseline body weight as G1 (59 kg), G2 (60-69 kg), G3 (70-84 kg) and G4 (85 kg). The researchers then compared the SNRs of G1 with those of G2, G3, and G4 and calculated the ratio squared as a factor to correct the acquisition parameters for overweight patients. They then prospectively enrolled 120 patients according to the same body weight criteria. They multiplied the correction factors to optimize injected doses or acquisition times and defined dose-adjusted groups (20 per group) and time-adjusted groups (20 per group).

According to the results, increasing the dose per kilogram of body weight did not improve the image quality.

Although liver activity did not significantly differ among G1 through G4 irrespective of patient weight, SNR progressively decreased as patient weight increased, ther researchers reported. The liver activity of G2 dose, G3 dose and G4 dose were, respectively, 1.4-, 1.9- and 2.5-fold higher than those of the baseline counterparts. Nevertheless, the increased liver activity of G2 dose, G3 dose, and G4 dose did not significantly affect SNR, compared with the baseline groups. In contrast, the SNR of G4 time was significantly higher than that of G4.

"The major finding of this study was that an extended acquisition time effectively maintained the quality of 18F-FDG PET/CT images of overweight patients. In contrast, an increased dose of up to 2.5-fold higher than 3.7 MBq per kilogram of body weight did not improve image quality," the authors wrote.

There were several limitations to the study, the authors noted, including the fact that image quality at baseline was not compared with that of the time- or dose-adjusted conditions in the same individuals, according to the researchers. Second, as a measure of image quality, they evaluated only SNR in the liver. Lastly, they used the standard deviation (SD) of the pixel values inside the region of interest (ROI) as a marker of image noise. Although the ROI SD is widely used in clinical imaging studies, it has limitations as a marker of noise, the authors noted.