In this study, we explore quantitative PET imaging without CT as a solution to true low-dose PET scan. The proposed reconstruction algorithm is particularly suitable when prior attenuation information, i.e. previous CT data, is available. Gaussian mixture models (GMM) were used to define the distribution of attenuation coefficients within the patient body using the prior CT data. Incorporating the background radiation from lutetium in LSO as ‘transmission data’ along with predefined GMM, a so-called TOF-MLAA-GMM was developed. In the proposed algorithm, attenuation maps from LSO background radiation are employed to provide information about non-radioactive objects in the FOV (such as patient bed), create suitable initial conditions and scatter data, and minimize the scaling problem in TOF-MLAA. A NEMA NU-2-like phantom and 18 F-FDG patient data from a long-axial FOV scanner, Biograph Vision Quadra PET/CT (Siemens Healthineers), were used to evaluate the developed algorithm. For both the phantom and patient data, PET images from TOF-MLAA-GMM were compared against those from TOF-MLAA and OSEM using CT-based attenuation and scatter corrections. Studying both the phantom and patient PET images, it was seen that the developed TOF-MLAA-GMM algorithm outperforms TOF-MLAA. Analysis of the mean SUV across various organs of the patient showed a mean quantification error of -14.80% (range: -27.20 to -6.93%) in the PET images using TOF-MLAA and 10.64% (range: 2.20% to 14.55%) when using TOF-MLAA-GMM attenuation maps. The proposed method can therefore be used in low-dose PET studies where previous attenuation data are available to avoid repeated CT scans.

Data-driven Time Alignment (TA) is currently part of scanner Quality Control (QC) procedure, where dedicated cylindrical phantom is used. An object is reconstructed based on non-TOF data (or imperfect TOF data). Based on the object knowledge, TOF modeled data can be subsequently generated and compared against measured data to estimate detectors time offsets (TO). The distinguished feature of data-driven TA is that no assumption about scanned object is needed.LSO background radiation was suggested for monitoring scanner properties. Currently long acquisition overnight disrupts the clinical service; detector energy properties, such as 511 keV energy spectrum potential drift, are indirectly monitored. The disadvantage is that LSO emits lower than 511 keV energies photons, requiring a different scanner setup and resulting in potentially different detector responses. The advantage is that no specially designed sources are used, and LSO total activity is large enough in long axial filed-of-view (LAFOV) scanners. In addition, detector time properties can be checked during patient scans.The goal of this work is to investigate the LSO background radiation detection timing properties in connection to 511 keV photons detection. The data-driven TA is applied on LSO background distribution and results are compared against dedicated source TA. The reconstruction of LAFOV Biograph Vision Quadra scanner LSO activity revealed non-uniform block pattern, different from one observed with 511 keV coincidences. The LSO TA time offsets were relatively close to TOs, derived from standard QC.

AbstractPurposeLong‐axial field‐of‐view (FOV) positron emission tomography (PET) scanners have gained a lot of interest in the recent years. Such scanners provide increased sensitivity and enable unique imaging opportunities that were not previously feasible. Benefiting from the high sensitivity of a long‐axial FOV PET scanner, we studied a computed tomography (CT)–less reconstruction algorithm for the Siemens Biograph Vision Quadra with an axial FOV of 106 cm.MethodsIn this work, the background radiation from radioisotope lutetium‐176 in the scintillators was used to create an initial estimate of the attenuation maps. Then, joint activity and attenuation reconstruction algorithms were used to create an improved attenuation map of the object. The final attenuation maps were then used to reconstruct quantitative PET images, which were compared against CT‐based PET images. The proposed method was evaluated on data from three patients who underwent a flurodeoxyglucouse PET scan.ResultsSegmentation of the PET images of the three studied patients showed an average quantitative error of 6.5%–8.3% across all studied organs when using attenuation maps from maximum likelihood estimation of attenuation and activity and 5.3%–6.6% when using attenuation maps from maximum likelihood estimation of activity and attenuation correction coefficients.ConclusionsBenefiting from the background radiation of lutetium‐based scintillators, a quantitative CT‐less PET imaging technique was evaluated in this work.

Many modern PET scanners feature lutetium-based scintillators, such as LSO, which emit background radiation due to the existence of the natural occurring radioisotope 176 Lu. It has been shown that the background radiation from LSO can be used to reconstruct an estimate of patient attenuation maps which can then be used in a joint attenuation and activity reconstruction algorithm such as TOF-MLAA to create enhanced attenuation maps. Following this methodology, the current work explores CT-less reconstruction methods that are particularly suitable for hybrid PET/MRI scanners. Anatomical information from MR images can be used to create regions with unknown linear attenuation coefficients. The unknown attenuation coefficients in each region are first estimated using the attenuation maps from the background radiation and then enhanced in a modified TOF-MLAA algorithm which will be referred to as TOF-MLAA-Reg. In the absence of TOF PET/MR data with background radiation, the developed TOF-MLAA-Reg algorithm was examined using pseudo-MR images derived from CT of a PET/CT scanner and compared against PET images from OSEM using CT-based attenuation and scatter correction and TOF-MLAA. Phantom and patient data were used to evaluate the performance of the developed TOF-MLAA-Reg algorithm incorporating background radiation. Results suggest that TOF-MLAA-Reg significantly minimizes quantification error in PET images when compared to TOF-MLAA. Using a segmented anatomical map of the studied patient, TOF-MLAA showed an average organ SUV error of - 14.80% (range: -27.20% to -6.93%) while the proposed TOF-MLAA-Reg algorithm reduced the error down to only 3.32% (range: -8.10% to 5.18%) across various organs. In conclusion, the proposed method can be a solution to quantitative PET imaging in modern TOF PET/MRI scanners with lutetium-based scintillators.

In PET scanners with lutetium-based scintillators, such as LSO, the background radiation from radioisotope 176Lu can be used to create an estimate of the patient attenuation maps. While these attenuation maps suffer from noise due to low count statics, they can be improved through a joint reconstruction of activity and attenuation. In this work, we propose a CT-less reconstruction framework in which an initial estimate of the attenuation maps is first created using the background radiation. The initial attenuation maps are then used to estimate scatter sinograms and fed into a time-of-flight (TOF) maximum likelihood activity and attenuation (MLAA) algorithm in which activity and attenuation images are sequentially updated. The proposed algorithm was evaluated using the data obtained from a Siemens Biograph Vision scanner and a NEMA NU-2 image quality phantom. Comparing the reconstructed images using MLAA and LSO background radiation and CT-driven attenuation maps suggests that the developed TOF-MLAA algorithm is a potential solution to stand-alone PET scans.

Energy resolution is an important parameter which affects the capability of scanner to distinguish scatter counts from trues count. A better energy resolution can reduce the amount of scatter in the acquired data. In this article we propose a new method to measure energy resolution using the NEMA NEC phantom. This method is evaluated by comparing the energy resolution obtained from proposed method with that obtained using a line-source in air at 45 mm offset from the center of scan FOV. The measurements are performed on Siemens next generation PET/CT SiPM PET/CT and Biography mCT scanners. For Biograph mCT scanner, at low count rate, energy resolution of 12.2% was obtained using line-source in air and 12.1% was obtained using line-source in NEC phantom. For prototype SiPM scanner, at low count rate, energy resolution of 9.4 % was obtained using both methods. At low to moderate count rates, an energy resolution using line-source in NEC phantom after proper corrections, and line-source in air are similar.

LSO and LYSO are today the most common scintillators used in positron emission tomography. Lutetium contains traces of 176Lu, a radioactive isotope that decays β - with a cascade of γ photons in coincidence. Therefore, Lutetium-based scintillators are characterized by a small natural radiation background. In this paper, we investigate and characterize the 176Lu radiation background via experiments performed on LSO-based PET scanners. LSO background was measured at different energy windows and different time coincidence windows, and by using shields to alter the original spectrum. The effect of radiation background in particularly count-starved applications, such as 90Y imaging, is analysed and discussed. Depending on the size of the PET scanner, between 500 and 1000 total random counts per second and between 3 and 5 total true coincidences per second were measured in standard coincidence mode. The LSO background counts in a Siemens mCT in the standard PET energy and time windows are in general negligible in terms of trues, and are comparable to that measured in a BGO scanner of similar size.

The key performance parameter of a Time-of-flight (TOF) positron emission tomography (PET) system is the time resolution, which has a direct effect on the TOF signal-to-noise ratio (SNR) gain. TOF gain has been modeled, simulated and measured in past literature, and advanced models have been proposed. In his work we are able to compare models and simulations with experimental data below 250ps time resolution, using a next generation Siemens SiPM PET/CT prototype scanner. TOF gain is assessed using uniform cylindrical phantoms of different diameters (20cm, 37cm). Filtered back projection (FBP) reconstruction was used for the reconstruction, in TOF and nonTOF version.A SNR TOF gain up to 3.9, equivalent to a TOF sensitivity gain of about 15.1, was measured in this preliminary experiment. In addition, results at variable random fraction are presented.

Time resolution is an important performance parameter. High time offlight resolution leads to better signal to noise ratio and contrast resolution but currently there is no standard method to compare time resolution between scanners. Since the time resolution varies based upon location of radioactive source, count-rate, and the setup used to acquire the data, a new method [1] was proposed to measure time-resolution using the NEMA NEC or scatter phantom [2]. This method with some modifications is likely to become part of 2017 NEMA publication. In this article we evaluate this method by comparing the time resolution measurements obtained using this method with those obtained from line-source at 45 mm offset from the center of FOV for a Siemens next generation SiPM PET/CT prototype scanner with high time resolution and Biograph mCT PET/CT scanner. Time-resolution measured at low count rate was 213 ps for the SiPM scanner with the NEMA NEC phantom and 220 ps with line-source in air. For Biograph mCT scanner those numbers were respectively 540 ps and 534 ps respectively.

Scintillation materials that lack intrinsic luminescence centers must be doped with optically active ions in order to provide luminescent centers that radiatively de-excite as the final step of the scintillation process.Codoping, on the other hand, can be defined as the incorporation of additional specific impurity species usually for the purpose of modifying the scintillation properties, mechanical properties, or the crystal growth behavior.In recent years codoping has become an increasingly popular approach for engineering scintillators with optimal performance for targeted applications.This report reviews several successful examples and its effect on specific properties.

During the reconstruction process, scatter correction and normalization needs to be applied to correct raw PET data. Normalization corrects for all the variations between the detector responses and typically consists of various components each representing specific physical effect. Only one set of normalization components is typically estimated for the clinical energy window. However, some of the components might change depending on the energy of the detected photons within the clinical imaging window. In this work, we investigated the need for accounting for such dependency in clinically used energy window.

Prompt emission of gamma radiation degrades the quantitative accuracy and image quality on I-124 PET studies. This prompt gamma generates a relatively flat background compared to object scatter and makes tail fitting of single scatter simulation difficult. Single scatter simulation with tail fitting will be overestimated if this background due to prompt gamma is not accounted for. Several solutions have been investigated to estimate this prompt gamma background such as Monte-Carlo simulation. However, Monte-Carlo simulations take too long to be used in clinical practice. Another approach assumes this background is relatively flat, and uses total measured singles data to approximate it. This assumption was successfully implemented for prompt gamma ray in the cases of Rb-82 and Ga-68 because the branching ratios are small compared to the amplitude of single scatter for tail fitting procedure. However, the larger branching ratio of I-124 generates a non-uniform background which produces error in the single scatter scaling with scatter tail fitting. In this paper, we propose a novel method to generate the non-uniform prompt gamma background and compare it to Monte-Carlo simulations and experimental measurement on I-124 phantom.

The sensitivity of a PET scanner is a function of the solid angle of coverage from the scintillation material to the object being scanned and the stopping power of the scintillation material being used. PET scanners also operate with an energy window centered on the 511 keV annihilation photons, wide enough to account for the energy resolution of the scanner's scintillator. The energy window reduces the amount of object scatter accepted, but also eliminates detector scatter events that do not experience object scatter. By adding lower energy windows, an un-scattered, full energy deposition event can be put in coincidence with a lower energy event, creating a high angle scatter line of response, or a detector scattered event that represents an object un-scattered line of response. Using a normalization specifically calculated for the coincidences with the new, lower energy windows and scatter correction of the high angle object scattered events, the data is reconstructed separately from the simultaneously collected PET data acquired with the traditional energy windows. Preliminary results showed that multi energy data reconstruction images can be created, recovering unscattered true events present across all data.

Lutetium Oxyorthosilicate (LSO) is an attractive scintillator for positron emission tomography (PET) imaging, due to its relatively high light output, short decay time, and high stopping power. LSO has a 2.6% natural abundance of Lu-176, a long-lived (3.78×10 10 years) radioisotope that generates an intrinsic background. This low background rate has been capitalized on to perform PET quality checks and measure transmission data. In this work, LSO was exposed to the neutrons produced by the O-18(p, n)F-18 reaction in a Siemens Eclipse Cyclotron to selectively alter the intrinsic background rate. The cyclotron was run for a total of 1827 microamp-hours over two weeks, with mean proton energy of 11 MeV. Ten LSO pixels (4×4×20 mm 3 ) were placed in locations selected for a desired neutron energy range. The range of neutron energies produced permitted the Lu-175(n,γ)Lu-176 and Lu-176(n,γ)Lu-177 reactions. Three of the pixels were placed behind polyethylene to further moderate the neutrons. The pixel's scintillation properties were measured to determine if they had been damaged by the irradiation. Gamma ray spectroscopy was performed for several weeks following the irradiation to evaluate the transmutations engendered. Despite an increase in trapping centers, scintillation properties were not changed, and the expected Lu-177 activation occurred.

Ga-68 Prostate Specific Membrane Antigen also known as PSMA is currently used in prostate cancer PET imaging. The resulting images show high uptakes in kidney and bladder which could produce a photopenic artifact (halo) and potentially mask tumor lesions or bone metastasis at the level of kidney or bladder. The measured contrasts between these organs and background could be as high as 200:1 and 50:1 for kidney and bladder respectively. The correct quantification in these areas requires precise scatter correction which needs to account for the effect of prompt gamma. Ga-68 has a prompt gamma at 1077 keV with a branching ratio of 3.2%. An unscattered prompt gamma ray of 1077 keV in the object has a small probability to be detected. An object scattered prompt gamma has a higher detection probability. When the contrast is low, more accurate quantification can be achieved. On the contrary, when the contrast is very high, halo artifact can be observed around high uptake organs. The purpose of this work is to evaluate the effect of Ga-68 prompt gamma in clinical PSMA studies. The halo artifact around kidney and bladder is strongly reduced by applying a Prompt Gamma Correction. Selected studies were performed on a Siemens mCT and acquired by Technische Universität München, Germany.