NEC Density and Liver ROI S/N Ratio for Image Quality Control of Whole-Body FDG-PET Scans: Comparison with Visual Assessment

Abstract

Patient noise equivalent count (NEC), NEC density, and liver region of interest (ROI) S/N have been proposed as physical indicators of image quality, but have not been thoroughly compared with visual assessments. In this study, those indicators were contrasted with blind visual evaluations for whole-body fluorodeoxyglucose-positron emission tomography (FDG-PET) images acquired under a variety of scanning conditions and body weights. Images were acquired on 15 normal subjects using a SET-3000B/L PET scanner with a continuous bed motion. Body weight ranged from 50.2 to 95.7 kg, with injected activity ranging from 71 to 333 MBq (1.40 to 3.67 MBq/kg) and a scan duration from 10 to 30 min. Patient NEC (PNEC; counts/cm) was calculated as the NEC rate divided by bed speed. NEC density (counts/cm(3)) was defined as the PNEC divided by the cross-sectional area derived from transmission data. Both PNEC and NEC density were averaged from neck to abdomen. Liver S/N was obtained as the pixel mean/SD within the ROI. Blind reviews by 18 professionals were used to visually evaluate image quality. Average visual score correlated with liver S/N, PNEC, and NEC density, with a rank correlation coefficient of 0.81, 0.86, and 0.91, respectively (each p < 0.0003). The "acceptable" quality roughly corresponded to a liver S/N of 10, PNEC of 380 kcounts/cm, and NEC density of 550 counts/cm(3) or more. NEC density, representing count statistics per body volume, reflects the visual image quality assessment and may be utilized for quality control of whole-body FDG-PET images together with the liver ROI S/N ratio.