Application to PET Scanners

The aim of the present study was to propose a comprehensive method for positron emission tomography (PET) scanners image quality assessment, by simulation of a thin layer chromatography (TLC) flood source with a previously validated Monte Carlo model. We used the GATE Monte Carlo package (GEANT4 application for tomographic emission) and the reconstructed images were obtained using the software for tomographic image reconstruction (STIR), with cluster computing. The PET scanner used in this simulation study was the General Electric Discovery-ST (USA). The plane source that was used for the image quality assessment was a TLC plate, consisting of an aluminum (Al) foil, coated with a thin layer of silica and immersed in fluorodeoxyglucose (18F-FDG) bath solution (1 MBq). The influence of different scintillating crystals on PET scanner's image quality, in terms of the modulation transfer function (MTF), the normalized noise power spectrum (NNPS) and the detective quantum efficiency (DQE), were also investigated. Modulation transfer function was estimated from transverse slices of the plane source, whereas the NNPS from the corresponding coronal slices. Images were reconstructed by the commonly used 2D filtered back projection (FBP2D), the Kinahan and Rogers FPB3DRP and the maximum likelihood estimation (MLE)-OSMAPOSL algorithms. Images obtained using the OSMAPOSL algorithm were assessed by using 15 subsets and 3 iterations. The PET scanner configuration, equipped with LuAP crystals, exhibited the optimum MTF values in both 2D and 3D FBP image reconstruction, whereas the corresponding configuration with BGO crystals exhibited the optimum MTF values after the iterative algorithm. The scanner equipped with the BGO crystals was also found to exhibit overall the lowest noise levels and the highest DQE values after algorithms. These finding indicate that the GE Discovery ST PET scanner exhibits the optimum image quality parameters, in terms of MTF, NNPS and DQE, with BGO scintillating crystals. Our new method showed that the imaging performance of PET scanners can be fully characterized and further improved by investigation of the imaging chain components through Monte Carlo methods. To this aim, a TLC based plane source was used during the simulation, in order to assess the impact of the scintillating crystal material on PET image quality, with the application of a previously validated Monte Carlo model. The aforementioned plane source can be also useful for the further development of PET and SPET scanners through GATE simulations, for clinical applications.

A positron emission tomography (PET) scanner using a silicon photomultiplier (SiPM PET) in place of a photomultiplier tube significantly improves the spatial and time resolution. It may also improve the evaluation of smaller lesions compared to conventional (non-SiPM) PET scanners. We compared the maximum standardized uptake value (SUVmax), detection sensitivity, and morphological correlation using magnetic resonance imaging (MRI) for primary tongue squamous cell carcinoma between the SiPM PET and non-SiPM PET scanner. We retrospectively reviewed the F-18 fluorodeoxyglucose (FDG) PET/CT features of tongue squamous cell carcinomas in consecutive, newly diagnosed, and pathologically verified patients. Twenty-five of 46 patients were scanned using SiPM PET scanner and the remaining 21 patients were scanned with a non-SiPM PET scanner. We compared the SUVmax and visual evaluation of primary tumor detectability, and the correlation between the PET-based and MRI-based tumor size (long axis, thickness, and volume). Differences in SUVmax and detection sensitivity for the primary tumor were analyzed using Welch's t test and Fisher's exact test, respectively. Correlations among the PET-based, MRI-based tumor size, and SUVmax were assessed using Spearman's rank correlation coefficient. SUVmax of both T1/T2 and T3/T4 primary tumors were significantly higher for the SiPM PET (T1/T2 mean SUVmax: 6.6 ± 4.3, T3/T4 mean SUVmax: 18.2 ± 9.8) than that for the non-SiPM PET (T1/T2 mean SUVmax: 3.4 ± 1.4, T3/T4 mean SUVmax: 10.2 ± 4.9) (P < 0.05). While all cases of T3/T4 primary tumors were detected by both PET scanners, the detection sensitivity for T1/T2 primary tumors was significantly higher for the SiPM PET (80%) than that for the non-SiPM PET (36.4%) (P < 0.05). MRI-based tumor size correlated significantly with SiPM PET-based tumor long axis (ρ = 0.74) and volume (ρ = 0.91), but not with the non-SiPM PET-based tumor long axis and volume in T1/T2 primary lesions. Correlation between MRI-based tumor size and SUVmax was significant in both PET scanners; however, no significant difference was observed between the two scanners. The SiPM PET provides better detection sensitivity and a reliable morphological correlation for the T1/T2 primary tongue tumors than the non-SiPM PET due to its high performance.

Several studies have shown the benefit of an accurate system modeling using Monte Carlo techniques. For state-of-the-art whole-body positron emission tomography (PET) scanners, Monte Carlo-based image reconstruction is associated with a significant computational cost to calculate the system matrix as well as a large memory capacity to store it. In this article, the authors present a simulation-reconstruction framework to solve these problems on the Philips Gemini GS PET scanner. A fast, realistic system matrix simulation module was developed using egs_pet, which is an efficient PET simulation code based on EGSnrc. The generated system matrix was then used in a rotator-based ordered subset expectation maximization (OS-EM) algorithm, which exploits the rotational symmetry of a cylindrical PET scanner. The system matrix was further compressed by using sparse storage techniques. The system matrix simulation took five days on 50 cores of Xeon 2.66 GHz, resulting in a system matrix of 2.01 GB. The entire system matrix could be stored in the main memory of a standard personal computer. The image quality in terms of contrast-noise trade-offs was considerably improved compared to a standard OS-EM algorithm. The image quality was also compared to the clinical software on the scanner using routine parameter settings. The contrast recovery coefficient of small hot spheres and cold spheres was significantly improved. The results indicated that the proposed framework could be used for this PET scanner with improved image quality. This method could also be applied to other state-of-the-art whole-body PET scanners and preclinical PET scanners with a similar shape.

To improve the reliability and convenience of the calibration procedure of positron emission tomography (PET) scanners, we have been developing a novel calibration path based on traceable point-like sources. When using (22)Na sources, special care should be taken to avoid the effects of 1.275-MeV γ rays accompanying β (+) decays. The purpose of this study is to validate this new calibration scheme with traceable point-like (22)Na sources on various types of PET scanners. Traceable point-like (22)Na sources with a spherical absorber design that assures uniform angular distribution of the emitted annihilation photons were used. The tested PET scanners included a clinical whole-body PET scanner, four types of clinical PET/CT scanners from different manufacturers, and a small-animal PET scanner. The region of interest (ROI) diameter dependence of ROI values was represented with a fitting function, which was assumed to consist of a recovery part due to spatial resolution and a quadratic background part originating from the scattered γ rays. The observed ROI radius dependence was well represented with the assumed fitting function (R (2) > 0.994). The calibration factors determined using the point-like sources were consistent with those by the standard cross-calibration method within an uncertainty of ±4 %, which was reasonable considering the uncertainty in the standard cross-calibration method. This novel calibration scheme based on the use of traceable (22)Na point-like sources was successfully validated for six types of commercial PET scanners.

Positron emission tomography (PET) scanners require periodic monitoring in order to maintain scanner performance. The aim of the present study was to examine the deterioration of PET scanner performance caused by aging. We retrospectively examined PET scanner performance alterations in terms of sensitivity, spatial resolution, false coincidences due to scatter and random coincidences based on 13 years of follow-up data, including data when the PET scanner underwent an overhaul at the 10th year after installation. Sensitivity and scatter fraction were calculated by using cross calibration factor (CCF) measurement data, which are collected routinely. Efficacy of the examining the sensitivity and scatter was confirmed by NEMA measurements. Trans-axial resolution was measured as full width at half-maximum (FWHM) and full width at tenth-maximum (FWTM) at 0-20 cm offset from the field of view (FOV) center at the time of installation, 8 years after installation, and immediately after the overhaul. Random coincidence rate fraction was measured in a wide range of count rates before and after the overhaul. The results indicated that the total reduction of sensitivity during the first 10 years was 41% of the initial value in terms of NEMA measurement, and that the annual reduction of sensitivity progressed at a rate of 4.7% per year in terms of CCF measurement data. The changes in sensitivity can be calculated using CCF measurement data. Regarding the spatial resolution, mean FWHM and FWTM values were increased by 1.7 and 3.6%, respectively, in 8 years after installation. The relative scatter fraction was significantly increased compared with that before the overhaul. The random fraction decreased by 10-15% after the overhaul within a certain range of random count rates (1-120 kcps). In the case of our scanner, the parameter that displayed the largest change was the sensitivity, and this change was thought to be caused by the reduction of photomultiplier tube (PMT) gain, although the changes in PMT gain can cause various types of performance deterioration, as investigated in this study. We observed that the sensitivity of our PET scanner generally deteriorated due to aging. Sensitivity monitoring using CCF measurements can be an easy and useful method for monitoring and maintaining the performance of PET scanners against aging. Since the data were obtained from a single scanner, the authors would encourage the initiation of a follow-up study involving various scanners.

Anorgan-specific Positron emission tomography (PET) scanner can achieve the same sensitivity with much fewer detectors as compared to a whole-body PET scanner, thereby substantially reducing the system cost. It can also achieve much better spatial resolution as compared to a whole-body PET scanner since the photon noncollinearity effect is reduced by using smaller detector ring diameter. Consequently, the development of organ-specific PET scanners with high spatial resolution, high sensitivity, and low cost has been a major focus of international research in PET instrument development for many years. The focus of this work is to develop high-resolution depth encoding PET detectors with high timing resolution and a reduced number of signal processing electronic channels. Consequently, PET scanners tailored for specific organs can be developed with high spatial and timing resolutions, enhanced sensitivity, and affordable cost. An 8×8 silicon photomultiplier (SiPM) array with a pixel size of 3×3 mm2 and a multiplexed signal readout circuit is coupled to one end of the lutetium yttrium orthosilicate (LYSO) array with a glass light guide between them to achieve a good crystal identification of small crystals by using only four position-encoding energy signals. A 4×4 SiPM array with a pixel size of 6×6 mm2 and an individual readout circuit is coupled to the other end of the crystal array without a light guide to achieve a good coincidence timing resolution (CTR). The depth of interaction (DOI) of the detector is measured by ratio of the energies measured by the two SiPM arrays and can be used to correct the depth dependency of the timing. The flood histograms, energy resolutions (ERs), DOI resolutions, and CTRs of two detectors utilizing LYSO arrays with different crystal sizes were measured with each of the two SiPM arrays alternately placed at the front of the detectors. A better flood histogram was obtained by placing the 8×8 SiPM array in front of the detector. The depth dependency of timing was larger when the 4×4 SiPM array was placed at the front of the detector. A better CTR was obtained by placing the 4×4 SiPM array at the back of the detector as compared to placing it at the front of the detector if the depth-dependent timing correction was not performed. If the depth-dependent timing correction was performed, a better CTR can be obtained by placing the 4×4 SiPM array at the front of the detector. The first detector using a 12×12 LYSO crystal array with a crystal size of 1.95×1.95×20 mm3 provided a flood histogram with all crystals clearly resolved, an ER of 11.7%, a DOI resolution of 2.9mm, and a CTR of 275ps with the depth-dependent timing correction. The second detector using a 23×23 LYSO crystal array with a crystal size of 0.95×0.95×20 mm3 provided a flood histogram with all but the edge crystals clearly resolved, an ER of 17.6%, a DOI resolution of 2.3mm, and a CTR of 293ps with the depth-dependent timing correction. PET detectors with small crystal cross-sectional sizes, good DOI and timing resolutions and a reduced number of electronics channels were developed. The detectors can be used to develop high performance organ-specific PET scanners.

Since the early 2000s, many types of positron emission tomography (PET) scanners dedicated to breast imaging for the diagnosis of breast cancer have been introduced. However, conventional performance evaluation methods developed for whole-body PET scanners cannot be used for such devices. In this study, we developed phantom tools for evaluating the quantitative accuracy of positron emission mammography (PEM) and dedicated-breast PET (dbPET) scanners using novel traceable point-like 68Ge/68Ga sources. The PEM phantom consisted of an acrylic cube (100 × 100 × 40mm) and three point-like sources. The dbPET phantom comprised an acrylic cylinder (ø100 × 100mm) and five point-like sources. These phantoms were used for evaluating the fundamental responses of clinical PEM and dbPET scanners to point-like inputs in a medium. The results showed that reasonable recovery values were obtained based on region-of-interest analyses of the reconstructed images. The developed phantoms using traceable 68Ge/68Ga point-like sources were useful for evaluating the physical characteristics of PEM and dbPET scanners. Thus, they offer a practical, reliable, and universal measurement scheme for evaluating various types of PET scanners using common sets of sealed sources.

Traditionally, the figures of merit used in designing a positron emission tomography (PET) scanner are spatial resolution, noise equivalent count rate, noise equivalent sensitivity, etc. These measures, however, do not directly reflect the lesion detectability using the PET scanner. Here the author proposes to optimize PET scanner design directly for lesion detection. The signal-to-noise ratio (SNR) of lesion detection can be easily computed using the theoretical expressions that the author has previously derived. Because no time-consuming Monte Carlo simulation is needed, the theoretical expressions allow evaluation of a large range of parameters. The PET system parameters can then be chosen to achieve the maximum SNR for lesion detection. The simulation study shown in this paper was focused on a single ring PET scanner without depth of interaction measurement. It can be extended to multiring (two- or three-dimensional) PET scanners and detectors with depth of interaction measurement.

We propose and evaluate the performance of an improved preclinical positron emission tomography (PET) scanner design, referred to as Polaroid-PET, consisting of a detector equipped with a layer of horizontal Polaroid to filter scintillation photons with vertical polarization. This makes it possible to improve the spatial resolution of PET scanners based on monolithic crystals. First, a detector module based on a lutetium-yttrium orthosilicate monolithic crystal with 10 mm thickness and silicon photomultipliers (SiPMs) was implemented in the GEANT4 Monte Carlo toolkit. Subsequently, a layer of Polaroid was inserted between the crystal and the SiPMs. Two preclinical PET scanners based on ten detector modules with and without Polaroid were simulated. The performance of the proposed detector modules and corresponding PET scanner for the two configurations (with and without Polaroid) was assessed using standard performance parameters, including spatial resolution, sensitivity, optical photon ratio detected for positioning, and image quality. The detector module fitted with Polaroid led to higher spatial resolution (1.05 mm FWHM) in comparison with a detector without Polaroid (1.30 mm FHWM) for a point source located at the center of the detector module. From 100% of optical photons produced in the scintillator crystal, 65% and 66% were used for positioning in the detectors without and with Polaroid, respectively. Polaroid-PET resulted in higher axial spatial resolution (0.83 mm FWHM) compared to the scanner without Polaroid (1.01 mm FWHM) for a point source at the center of the field of view (CFOV). The absolute sensitivity at the CFOV was 4.37% and 4.31% for regular and Polaroid-PET, respectively. Planar images of a grid phantom demonstrated the potential of the detector with a Polaroid in distinguishing point sources located at close distances. Our results indicated that Polaroid-PET may improve spatial resolution by filtering the reflected optical photons according to their polarization state, while retaining the high sensitivity expected with monolithic crystal detector blocks.

The current positron emission tomography (PET) design is aimed toward establishing an entire-body PET scanner. An entire-body PET scanner is a scanner whose axial field of view (FOV) covers the whole body of a patient, whereas whole-body PET scanner can be of any axial FOV length, but was designed for a whole-body scan. Despite its high production cost, an entire-body depth-of-interaction PET scanner offers many benefits, such as shorter and dynamic PET time acquisition, as well as higher sensitivity and count rate performance. This PET scanner may be cost-effective for clinical PET scanners with high scan throughput. In this work, we evaluated the sensitivity and count rate performance of a 2-m-long PET scanner with conventional data acquisition (DAQ) architecture, using Monte Carlo simulation, and we evaluated two ring diameters (60 and 80 cm) to reduce the scanner cost. From simulation of scanning with a 2-m axial FOV, the sensitivity for a 2-m-long PET scanner of 60 and 80-cm diameter is around 80 and 68 times higher, respectively, than that of the conventional PET scanner. In addition, for the 2-m-long PET scanner with 60-cm diameter, the peak noise equivalent count rate (NECR) was 843 kcps at 125 MBq, whereas the peak for the 80-cm diameter was 989 kcps at 200 MBq. This shows gains of 15.3 and 17.95, respectively, in comparison with that of the conventional PET scanner. The 2-m-long PET scanner with 60-cm ring diameter could not only reduce the number of detectors by 21 %, but also had a 17 % higher sensitivity compared to that with an 80-cm ring diameter. On the other hand, despite the higher sensitivity, the NECR of the 60-cm ring diameter was smaller than that of the 80-cm ring diameter. This results from the single data loss due to dead time, whereas grouping of axially stacked detectors was used in the conventional DAQ architecture. Parallelization of the DAQ architecture is therefore important for the 2-m-long PET scanner to achieve its optimal performance.

We have estimated count rate properties of three-dimensional (3-D) positron emission tomography (PET) scanners based on a Gd/sub 2/SiO/sub 5/:Ce (GSO) detector with depth of interaction (DOI) capability using a large-area position-sensitive photomultiplier tube (PS-PMT). The proposed detector unit consists of 64 crystal blocks with four stages of 2/spl times/2 GSO arrays coupled to a 52-mm square PS-PMT which has small dead space. With appropriate light control in the crystal block, DOI information can be obtained using simple Anger-type positioning logic. Thus, dead-time factors can be calculated using a count rate model with standard acquisition architecture. Compton and photoelectric interactions in the scintillator and uniform cylindrical phantoms were tracked by Monte Carlo simulation programs. Since the DOI detector can provide high resolution throughout the entire field of view, 3-D PET scanners with a large solid angle covered by the detectors with relatively small ring diameters were simulated. The preliminary results suggest that, compared to current PET scanners, high noise equivalent count rate can be obtained by the proposed scanner designs despite the relatively large size of the detector module. The count rate performance can be improved by the reduction of single events that cause block dead-time losses at the cost of a slight decrease in sensitivity.

A prototype brain positron emission tomography (PET) scanner using semiconductor detectors and depth of inleraclion (DOI) information has been developed to achieve high spatial resolulion and reduced scalier fraction. At the first step of the development, we created a two-dimensional prololype PET scanner composed of a single-slice full-ring detector unit to confirm the feasibility of the basic technologies that are necessary lo realize a semiconductor PET scanner. Through phantom and small-animal studies, the feasibility of the semiconductor PET was confirmed and the results showed that the semiconductor PET could produce quantitative imaging with high spatial resolution. Based on these achievements, a prototype brain PET scanner was developed to demonstrate the high spatial resolution and quantitative imaging capability required in human imaging.

The axial field of view (AFOV) of a positron emission tomography (PET) scanner greatly affects the quality of PET images. Although a total-body PET scanner (uEXPLORER) with a large AFOV is more sensitive, it is more expensive and difficult to widely use. Therefore, we attempt to utilize high-quality images generated by uEXPLORER to optimize the quality of images from short-axis PET scanners through deep learning technology while controlling costs. The experiments were conducted using PET images of three anatomical locations (brain, lung, and abdomen) from 335 patients. To simulate PET images from different axes, two protocols were used to obtain PET image pairs (each patient was scanned once). For low-quality PET (LQ-PET) images with a 320-mm AFOV, we applied a 300-mm FOV for brain reconstruction and a 500-mm FOV for lung and abdomen reconstruction. For high-quality PET (HQ-PET) images, we applied a 1940-mm AFOV during the reconstruction process. A 3D Unet was utilized to learn the mapping relationship between LQ-PET and HQ-PET images. In addition, the peak signal-to-noise ratio (PSNR) and structural similarity index measure (SSIM) were employed to evaluate the model performance. Furthermore, two nuclear medicine doctors evaluated the image quality based on clinical readings. The generated PET images of the brain, lung, and abdomen were quantitatively and qualitatively compatible with the HQ-PET images. In particular, our method achieved PSNR values of 35.41 ± 5.45dB (p < 0.05), 33.77 ± 6.18dB (p < 0.05), and 38.58 ± 7.28dB (p < 0.05) for the three beds. The overall mean SSIM was greater than 0.94 for all patients who underwent testing. Moreover, the total subjective quality levels of the generated PET images for three beds were 3.74 ± 0.74, 3.69 ± 0.81, and 3.42 ± 0.99 (the highest possible score was 5, and the minimum score was 1) from two experienced nuclear medicine experts. Additionally, we evaluated the distribution of quantitative standard uptake values (SUV) in the region of interest (ROI). Both the SUV distribution and the peaks of the profile show that our results are consistent with the HQ-PET images, proving the superiority of our approach. The findings demonstrate the potential of the proposed technique for improving the image quality of a PET scanner with a 320mm or even shorter AFOV. Furthermore, this study explored the potential of utilizing uEXPLORER to achieve improved short-axis PET image quality at a limited economic cost, and computer-aided diagnosis systems that are related can help patients and radiologists.

We are developing a positron emission tomography (PET) scanner based on avalanche photodiodes (APD), monolithic LYSO:Ce scintillator crystals and a dedicated readout chip. All these components allow operation inside a magnetic resonance imaging (MRI) scanner with the aim of building a PET/MRI hybrid imaging system for clinical human brain studies. Previous work verified the functional performance of our first chip (VATA240) based on a leading edge comparator and the principle of operation of our radiation sensors, which are capable of providing reconstructed images of positron point sources with spatial resolutions of 2.1 mm FWHM. The new VATA241 chip presented in this work has been designed with the aim of reducing the coincidence window of our final PET scanner by implementing an on-chip constant fraction discriminator (CFD), as well as providing a better robustness for its implementation in the full-scale PET scanner. Results from the characterization of the VATA241 chip are presented, together with the first results on coincidence performance, validating the new design for our application.

Earlier investigations with BGO positron emission tomography (PET) scanners showed that the scatter correction technique based on multiple acquisitions with different energy windows are problematic to implement because of the poor energy resolution of BGO (22%), particularly for whole-body studies. We believe that these methods are likely to work better with NaI(TI) because of the better energy resolution achievable with NaI(TI) detectors (10%). Therefore, we investigate two different choices for the energy window, a low-energy window (LEW) on the Compton spectrum at 400-450 keV, and a high-energy window (HEW) within the photopeak (lower threshold above 511 keV). The results obtained for our three-dimensional (3-D) (septa-less) whole-body scanners [axial field of view (FOV) of 12.8 cm and 25.6 cm] as well as for our 3-D brain scanner (axial FOV of 25.6 cm) show an accurate prediction of the scatter distribution for the estimation of trues method (ETM) using a HEW, leading to a significant reduction of the scatter contamination. The dual-energy window (DEW) technique using a LEW is shown to be intrinsically wrong; in particular, it fails for line source and bar phantom measurements. However, the method is able to produce good results for homogeneous activity distributions. Both methods are easy to implement, are fast, have a low noise propagation, and will be applicable to other PET scanners with good energy resolution and stability, such as hybrid NaI(TI) PET/SPECT dual-head cameras and future PET cameras with GSO or LSO scintillators.