The edge artifact in the point-spread function-based PET reconstruction at different sphere-to-background ratios of radioactivity.

OBJECTIVE: The aim of this study was to quantitatively evaluate the edge artifacts in PET images reconstructed using the point-spread function (PSF) algorithm at different sphere-to-background ratios of radioactivity (SBRs). METHODS: We used a NEMA IEC body phantom consisting of six spheres with 37, 28, 22, 17, 13 and 10 mm in inner diameter. The background was filled with (18)F solution with a radioactivity concentration of 2.65 kBq/mL. We prepared three sets of phantoms with SBRs of 16, 8, 4 and 2. The PET data were acquired for 20 min using a Biograph mCT scanner. The images were reconstructed with the baseline ordered subsets expectation maximization (OSEM) algorithm, and with the OSEM + PSF correction model (PSF). For the image reconstruction, the number of iterations ranged from one to 10. The phantom PET image analyses were performed by a visual assessment of the PET images and profiles, a contrast recovery coefficient (CRC), which is the ratio of SBR in the images to the true SBR, and the percent change in the maximum count between the OSEM and PSF images (Δ % counts). RESULTS: In the PSF images, the spheres with a diameter of 17 mm or larger were surrounded by a dense edge in comparison with the OSEM images. In the spheres with a diameter of 22 mm or smaller, an overshoot appeared in the center of the spheres as a sharp peak in the PSF images in low SBR. These edge artifacts were clearly observed in relation to the increase of the SBR. The overestimation of the CRC was observed in 13 mm spheres in the PSF images. In the spheres with a diameter of 17 mm or smaller, the Δ % counts increased with an increasing SBR. The Δ % counts increased to 91 % in the 10-mm sphere at the SBR of 16. CONCLUSIONS: The edge artifacts in the PET images reconstructed using the PSF algorithm increased with an increasing SBR. In the small spheres, the edge artifact was observed as a sharp peak at the center of spheres and could result in overestimation.

Improvement in PET/CT image quality in overweight patients with PSF and TOF.

OBJECTIVE: The 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. METHODS: This 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 (18)F 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 (18)F-FDG PET/CT. The image quality was evaluated using the background variability (coefficient of variance, CVphantom and CVliver) and the contrast (CONTHOT and SNR). RESULTS: In 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. CONCLUSION: The combination of the PSF and TOF correction improved the image quality of the LG phantom and overweight patients.

Accuracy of amplitude-based respiratory gating for PET/CT in irregular respirations.

OBJECTIVE: We evaluated the accuracy of amplitude gating PET (AG-PET) compared with phase gating PET (PG-PET) in relation to respiratory motion patterns based on a phantom analysis. METHOD: We used a NEMA IEC body phantom filled with an (18)F solution with a 4:1 sphere-to-background radioactivity ratio (12.6 and 2.97 kBq/mL). PET/CT scans were acquired in a motionless and moving state on a Biograph mCT. The respiratory movements were simulated by four different waveform patterns consisting of ideal breathing, breathing with a pause period, breathing with a variable amplitude and breathing with a changing baseline. AG-PET selects the narrow bandwidth containing 20 % of the respiratory cycle. PG-PET was reconstructed with five gates. The image quality was physically assessed using the percent contrast (Q H,10mm), background variability (N 10mm) recovery coefficient (RC), and sphere volumes. RESULT: In regular motion patterns with ideal breathing and breathing with a pause period, the Q H,10mm, RC and sphere volumes were not different between AG-PET and PG-PET. In the variable amplitude pattern, the Q H,10mm of AG-PET was higher than that of PG-PET (35.8 vs 28.2 %), the RC of AG-PET was higher than that of PG-PET and sphere volume of AG-PET was smaller than that of PG-PET (6.4 vs 8.6 mL). In the changing baseline pattern, the Q H,10mm of AG-PET was higher than that of PG-PET (42.4 vs 16.7 %), the RC of AG-PET was higher than that of PG-PET and sphere volume of AG-PET was smaller than that of PG-PET (6.2 vs 9.8 mL). The N 10mm did not differ between AG-PET and PG-PET, irrespective of the motion pattern. CONCLUSION: Amplitude gating PET is considered to be more accurate than phase gating PET for examining unstable respiratory motion patterns, such as those involving a variable amplitude or changing baseline.

Impact of Time-of-Flight PET/CT with a Large Axial Field of View for Reducing Whole-Body Acquisition Time.

UNLABELLED: The aim of this study was to evaluate the imaging performance of 39- and 52-ring time-of-flight (TOF) PET/CT scanners. We also assessed the potential of reducing the scanning time using a 52-ring TOF PET/CT scanner. METHODS: PET/CT scanners with 39- and 52-ring lutetium oxyorthosilicate detectors were evaluated. The axial fields of view were 16.2 and 21.6 cm, respectively. We used a National Electrical Manufacturers Association International Electrotechnical Commission body phantom filled with an (18)F solution containing background activity of 5.31 and 2.65 kBq/mL for the studies. The sphere-to-background ratio was 4:1. The PET data were acquired for 10 min in 3-dimensional list mode and then reconstructed with both ordered-subsets reconstruction maximization and ordered-subsets reconstruction maximization plus point-spread function plus time-of-flight algorithms. PET images with different acquisition times were reconstructed (from 1 to 10 min). The image quality was physically assessed using the sensitivity, noise-equivalent counting rate, coefficient of variation of background activity, and relative recovery coefficient. RESULTS: The total system sensitivities of the 39- and 52-ring scanners were 5.6 and 9.3 kcps/MBq, respectively. Compared with the 39-ring scanner, the noise-equivalent counting rate of the 52-ring scanner was 60% higher for both the high-activity and the low-activity models. The recovery coefficient was consistent, irrespective of the number of detector rings. The coefficient of variation of the 52-ring scanner using a 3-min acquisition time was equivalent to that of the 39-ring scanner using a 4-min acquisition time. CONCLUSION: The image quality of the 52-ring scanner is superior to that of the 39-ring scanner. The acquisition time per bed position of the 52-ring system can be reduced by about 25% without compromising image quality. In addition, the number of bed positions required is 25% lower for the 52-ring system. Finally, the examination time required for a whole-body PET scan is considered to be reduced by about 40% if the 52-ring scanner is used.

Influences of point-spread function and time-of-flight reconstructions on standardized uptake value of lymph node metastases in FDG-PET.

PURPOSE: The purpose of this study was to investigate the effects of point-spread function (PSF) and time-of-flight (TOF) on the standardized uptake value (SUV) of lymph node metastasis in FDG-PET/CT. MATERIALS AND METHODS: This study evaluated 41 lymph node metastases in 15 patients who had undergone (18)F-FDG PET/CT. The lesion diameters were 2.5 cm or less. The mean short-axis diameter of the lymph nodes was 10.5 ± 3.7 mm (range 4.6-22.8mm). The PET data were reconstructed with baseline OSEM algorithm, with OSEM+PSF, with OSEM+TOF and with OSEM+PSF+TOF. A semi-quantitative analysis was performed using the maximum and mean SUV of lymph node metastases (SUVmax and SUVmean) and mean SUV of normal lung tissue (SUVlung). We also evaluated image quality using the signal-to-noise ratio in the liver (SNRliver). RESULTS: Both PSF and TOF increased the SUV of lymph node metastases. The combination of PSF and TOF increased the SUVmax by 43.3% and the SUVmean by 31.6% compared with conventional OSEM. By contrast, the SUVlung was not influenced by PSF and TOF. TOF significantly improved the SNRliver. CONCLUSION: PSF and TOF both increased the SUV of lymph node metastases. Although PSF and TOF are considered to improve small-lesion detectability, it is important to be aware that PSF and TOF influence the accuracy of quantitative measurements.

Benefits of point-spread function and time of flight for PET/CT image quality in relation to the body mass index and injected dose.

UNLABELLED: The PET image quality of overweight patients and patients who receive low injected doses deteriorates because of increases in statistical noise. The purpose of this study was to investigate the benefits of the point-spread function (PSF) and time-of-flight (TOF) for PET/CT image quality in such patients. METHODS: The PET images were reconstructed using the baseline ordered-subsets expectation-maximization algorithm (OSEM), OSEM + PSF, OSEM + TOF, and OSEM + PSF + TOF. In the phantom study, we used a National Electrical Manufacturers Association body phantom with different radioactivity concentrations and analyzed image quality using the coefficient of variance in the background (CVphantom). In the clinical study, we retrospectively studied 39 patients who underwent clinical F-FDG PET/CT. The patients were classified into groups based on body mass index and injected dose. Image quality was evaluated using the CV in the liver (CVliver). RESULTS: In the phantom study, PSF and TOF improved the CVphantom, especially in low-activity models. Among all of the reconstructions, the best CVphantom was obtained with OSEM + PSF + TOF. In the clinical study, the CVliver of the low-dose group with OSEM + PSF + TOF was comparable to that of the high-dose group with conventional OSEM. CONCLUSIONS: 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.

Improvement in PET/CT image quality with a combination of point-spread function and time-of-flight in relation to reconstruction parameters.

UNLABELLED: The aim of this study was to investigate the effects of the point-spread 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 ordered-subsets expectation maximization (OSEM) algorithm, with the OSEM+PSF model, with the OSEM+TOF model, and with the OSEM+PSF+TOF 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 OSEM+PSF+TOF 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 OSEM+PSF+TOF model was compared with that of the high-NEC(liver) group in conventional OSEM. The PSF+TOF improved the SNR(liver) by about 24.9% ± 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.