Nobuyoshi Ohya

Improvement in PET/CT Image Quality with a Combination of Point-Spread Function and Time-of-Flight in Relation to Reconstruction Parameters

The aim of this study was to investigate the effects of the point-spread function (PSF) and time-of-flight (TOF) on improving 18F-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 18F-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 18F-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 (SNR10 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 (SNRliver) in relation to the NEC in the liver (NECliver). Results: In the phantom study, the SNR10 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 SNR10 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 SNR10 mm, contrast, and coefficient of variance, especially with a small-FWHM gaussian filter. In the clinical study, the SNRliver of the low-NECliver group in the OSEM+PSF+TOF model was compared with that of the high-NECliver group in conventional OSEM. The PSF+TOF improved the SNRliver 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.

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

&NA; 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 18F-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.

A simple table lookup method for PET/CT partial volume correction using a point-spread function in diagnosing lymph node metastasis

ObjectiveWe evaluated the partial volume effect in PET/CT images and developed a simple correction method to address this problem.MethodsSix spheres and the background in the phantom were filled with F-18 and we thus obtained 4 different sphere-to-background (SB) ratios. Thirty-nine cervical lymph nodes in 7 patients with papillary thyroid carcinoma (15 malignant and 24 benign) were also examined as a preliminary clinical study. First, we developed recovery coefficient (RC) curves normalized to the maximum counts of the 37-mm sphere. Next, we developed a correction table to determine the true SB ratio using three parameters, including the maximum counts of both the sphere and background and the lesion diameter, by modifying the approximation formula of the RC curves including the point-spread function correction. The full width at half maximum in this formula is estimated with the function of the SB ratio.ResultsIn the phantom study, a size-dependent underestimation of the radioactivity was observed. The degree of decline of RC was influenced by the SB ratio. In preliminary clinical examination, the difference in the SUVmax between malignant and benign LNs thus became more prominent after the correction. The PV correction slightly improved the diagnostic accuracy from 95 to 100%.ConclusionsWe 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.

Determination of the optimal acquisition protocol of breath-hold PET/CT for the diagnosis of thoracic lesions.

ObjectiveThe aim of this study was to determine the optimal acquisition scan protocol for deep inspiration breath-hold (BH) fluoro-2-deoxy-D-glucose positron emission tomography (PET) for the examination of thoracic lesions. MethodsWe studied 32 thoracic lesions in 21 patients. Whole-body PET/computed tomography (CT) scanning with free breathing (FB) was performed for 3 min per bed position, followed by a BH-CT and five BH-PET for 20 s each. Summed BH images with total acquisition times of 40, 60, 80 and 100 s were generated (BH×2, BH×3, BH×4 and BH×5, respectively). The displacements between PET and CT images, the lesion volume of the PET image, the maximum standardized uptake value (SUVmax) and the quality of the PET image were assessed in relation to the clinical characteristics of each patient and the summation of the BH-PET images. ResultsBH-PET decreased the tumor volume significantly (FB: 7.23±9.70 cm3, BH×5: 4.71±5.14 cm3, P<0.01) and increased the SUVmax (FB: 6.27±5.41, BH×5: 7.53±6.28, P<0.01). The displacement between the PET and CT images was improved significantly in the BH scans (FB: 0.77±0.53 cm, BH×5: 0.36±0.24 cm, P<0.01). In addition, aging and lung function of patients influenced the reproducibility of BH-PET. The summed BH-PET images, obtained by summation of three or more BH-PET images (total acquisition time of 60 s or more), achieved good image quality. ConclusionBH-PET/CT improved the misregistration between PET and CT images and increased the SUVmax of thoracic lesions. The recommended number of BH-PET images for summation with 20 s of acquisition time is three or more.

Phantom study on three-dimensional target volume delineation by PET/CT-based auto-contouring.

OBJECTIVE The aim of this study was to determine an appropriate threshold value for delineation of the target volume in PET/CT and to investigate whether we could delineate a target volume by phantom studies. METHODS A phantom consisted of six spheres (phi 10-37 mm) filled with 18F solution. Data acquisition was performed PET/CT in non-motion and motion status with high 18F solution and in non-motion status with low 18F solution. In non-motion phantom experiments, we determined two types of threshold value, an absolute SUV (T(SUV)) and a percentage of the maximum SUV (T%). Delineation using threshold values was applied for all spheres and for selected large spheres (a diameter of 22 mm or larger). In motion phantom experiments, data acquisition was performed in a static mode (sPET) and a gated mode (gPET). CT scanning was performed with helical CT (HCT) and 4-dimensional CT (4DCT). RESULTS The appropriate threshold values were aT% = 27% and aT(SUV) = 2.4 for all spheres, and sT% = 30% and sT(SUV) = 4.3 for selected spheres. For all spheres in sPET/HCT in motion, the delineated volumes were 84%-129% by the aT% and 34%-127% by the aT(SUV). In gPET/4DCT in motion, the delineated volumes were 94-103% by the aT% and 51-131% by the aT(SUV). For low radioactivity spheres, the delineated volumes were all underestimated. CONCLUSION A threshold value of T% = 27% was proposed for auto-contouring of lung tumors. Our results also suggested that the respiratory gated data acquisition should be performed in both PET and CT for target volume delineation.