The aim of this study was to investigate the effects of the pointspread function (PSF) and time-of-flight (TOF) on improving 18 F-FDG PET/CT images in relation to reconstruction parameters and noise-equivalent counts (NEC). Methods: This study consisted of a phantom study and a retrospective analysis of 39 consecutive patients who underwent clinical 18 F-FDG PET/CT. The body phantom of the National Electrical Manufacturers Association and International Electrotechnical Commission with a 10-mm-diameter sphere was filled with an 18 F-FDG solution with a 4:1 radioactivity ratio compared with the background. The PET data were reconstructed with the baseline orderedsubsets expectation maximization (OSEM) algorithm, with the OSEM1PSF model, with the OSEM1TOF model, and with the OSEM1PSF1TOF model. We evaluated image quality by visual assessment, the signal-to-noise ratio of the 10-mm sphere (SNR 10 mm ), the contrast of the 10-mm sphere, and the coefficient of variance in the phantom study and then determined the optimal reconstruction parameters. We also examined the effects of PSF and TOF on the quality of clinical images using the signal-to-noise ratio in the liver (SNR liver ) in relation to the NEC in the liver (NEC liver ). Results: In the phantom study, the SNR 10 mm was the highest for the OSEM1PSF1TOF model, and the highest value was obtained at iteration 2 for algorithms with the TOF and at iteration 3 for those without the TOF. In terms of a postsmoothing filter full width at half maximum (FWHM), the high SNR 10 mm was obtained with no filtering or was smaller than 2 mm for algorithms with PSF and was 4-6 mm for those without PSF. The balance between the contrast recovery and noise is different for algorithms with either PSF or TOF. A combination of PSF and TOF improved SNR 10 mm , contrast, and coefficient of variance, especially with a small-FWHM gaussian filter. In the clinical study, the SNR liver of the low-NEC liver group in the OSEM1PSF1TOF model was compared with that of the high-NEC liver group in conventional OSEM. The PSF1TOF improved the SNR liver by about 24.9% 6 9.81%. Conclusion: A combination of PSF and TOF clearly improves image quality, whereas optimization of the reconstruction parameters is necessary to obtain the best performance for PSF or TOF. Furthermore, this combination has the potential to provide good image quality with either lower activity or shorter acquisition time, thus improving patient comfort and reducing the radiation burden. PET/ CT with 18 F-FDG is useful for the detection and staging of various malignant tumors, monitoring of their response to therapy, and prognostic stratification (1-6). High image quality is required for an accurate diagnosis, although the spatial resolution of PET is relatively low in comparison to other imaging modalities (7). Recently, the information provided by the point-spread function (PSF) and time-of-flight (TOF) has been expected to improve the spatial resolution and signal-to-noise ratio (SNR), respectively, of PET images (8-10).A PET sca...

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ObjectiveThe aim of this study was to evaluate the effect of the point spread function (PSF) and time of flight (TOF) on PET/CT images of overweight patients in relation to the iteration number and the acquisition time.MethodsThis study consisted of a phantom study and a clinical study. The NEMA IEC body phantom and a 40 cm diameter large phantom (LG phantom) simulating an overweight patient were used in this study. Both phantoms were filled with 18F solution with a sphere to background ratio of 4:1. The PET data were reconstructed with the baseline ordered-subsets expectation maximization (OSEM) algorithm, with the OSEM + PSF model, with the OSEM + TOF model and with the OSEM + PSF + TOF model. The clinical study was a retrospective analysis of 66 patients who underwent 18F-FDG PET/CT. The image quality was evaluated using the background variability (coefficient of variance, CVphantom and CVliver) and the contrast (CONTHOT and SNR).ResultsIn phantom study, the CVphantom of the LG phantom was higher than that of the NEMA phantom. The PSF decreased the CVphantom of the LG phantom to the NEMA phantom level. The TOF information accelerated the CVphantom plateau earlier. The best relationship between the CVphantom and the CONTHOT was observed for the OSEM + PSF + TOF. In clinical study, the combination of PSF and TOF decreased the CVliver for overweight patients to that for normal weight patients while it increased the SNR similarly between two patient groups.ConclusionThe combination of the PSF and TOF correction improved the image quality of the LG phantom and overweight patients.

Point-spread function and TOF improved PET/CT image quality for overweight patients who received a lower injected dose. Therefore, the use of PSF and TOF is suggested to maintain the image quality of such patients without extending scanning times. It is greatly beneficial to obtain sufficient image quality for larger patients, especially in delivery institutions where the injection dose cannot be easily increased.

We developed a simple table lookup correction method for the partial volume effect of PET/CT. This new method is considered to be clinically useful for the diagnosis of cervical LN metastasis. Further examination with a greater number of subjects is required to corroborate its clinical usefulness.