A quantitative approach to a weight-based scanning protocol for PET oncology imaging

The dominant effect on whole-body PET image quality is attenuation, which is determined by patient thickness. Increasing patient thickness exponentially decreases the number of photons detected. This can be partially compensated for by adjusting scan duration. To derive quantitative relationships between patient thickness, scan duration, and lesion detectability we simulated PET acquisitions of an anthropomorphic phantom with spherical target lesions. Three different phantoms were simulated, with effective abdominal diams of 20 cm, 27 cm, and 35 cm. Poisson noise was added to raw sinogram data to simulate scan durations of 1 to 10 min per bed position. Noise levels were based on count rates measured using a countrate phantom with additional attenuation sleeves. Simulated lesions varied from 1.0 to 3.0 cm in diam, and contrast ratios varied from 1.5 to 4.0. Average detectability of each target lesion was calculated from 25 noisy realizations using a non-prewhitening matched filter. Results using model observers demonstrate that detectability was proportional to the square root of the accumulated NEC density for all abdominal thicknesses. For image quality to remain constant for patients with larger cross-sectional areas, acquisition times should be increased accordingly, although in some cases this may not be possible due to practical constraints. In addition we found an inverse-linear relationship between log(diam) and log(contrast) for constant sphere detectability.