Modern Positron Emission Tomography Scanners Research Articles

Harmonisation of quantitative assessment between different generation of PET/CT: Biograph mCT and Biograph Vision

The usage of modern positron emission tomography scanners, in particular with digital detectors, allows obtaining images with better quality, increases the detection of small pathological lesions, reduces scanning time and the activity administered to the patient which leads to a decrease of patient dose as well. However, the values of the quantitative image parameters shift upward, which can lead to significant differences with the quantitative assessment obtained on the previous generation device. In order to compare quantitative assessments obtained on different generations of PET/CT, it is necessary to harmonise quantitative image parameters and perform regular quality control. The aim of current work is the comparison of different methods for harmonization of quantitative image parameters on the example of harmonisation of two PET/CT: Biograph mCT 128 and Biograph Vision 600. NEMA IEC Body phantom filled with 18F solution was scanned in Listmode in two bed positions with overlap in the sphere area during five minutes per bed position. Recovery coefficient used for harmonisation was measured for each sphere of the phantom. Harmonisation between Vision and mCT was performed using two methods: choosing of harmonised reconstruction parameters and EQ.PET technology. The acceptable divergence range between the recovery coefficients for Vision and for mCT is ±10% (20% range). The recovery coefficients measured for reconstruction: 4 iterations and 5 subsets, ToF+PSF, Gaussian 7 mm, matrix 220x220 completely fit within the 20% range. The recovery coefficients measured using EQ = 6 mm (optimal value) fit within the 20% range except the spheres with a diameter of 10 and 13 mm. Both harmonisation methods allow to approximate the quantitative assessment/ However, EQ.PET has limitations for the small lesions. Choosing harmonised reconstruction parameters is the mostwidely used harmonisation method; the EQ.PET allows to harmonise quantitative assessment without the use of multiple reconstruction protocols and losses in visualization ability

Offset compensated baseline restoration and computationally efficient hybrid interpolation for the brain PET

Abstract The acquisition of positron emission tomography (PET) pulses introduces artifacts and limits the performance of the scanner. To minimize these inadequacies, this work focuses on the design of an offset compensated digital baseline restorer (BLR) along with a two-stage hybrid interpolator. They respectively treat the incoming pulse offsets and limited temporal resolution and improve the scanner performance in terms of calculating depth of interactions and line of responses. The offset of incoming PET pulses is compensated by the BLR and then their interesting parts are selected. The selected signal portion is up-sampled with a hybrid interpolator. It is composed of an optimized weighted least-squares interpolator (WLSI) and a simplified linear interpolator. The processes of calibrating the WLSI coefficients and characterizing the BLR and the interpolator modules are described. The functionality of the proposed modules is verified with an experimental setup. Results have shown that the devised BLR effectively compensates a dynamic range of bipolar offsets. The signal selection process allows focusing only on the relevant signal part and avoids the unnecessary operations during the post-interpolation process. Additionally, the hybrid nature allows improving the signal temporal resolution with an appropriate precession at a reduced computational complexity compared to the mono-interpolation-based arithmetically complex counterparts. The component-level architectures of the BLR and the interpolator modules are also described. It promises an efficient integration of these modules in modern PET scanners while using standard and economical analog-to-digital converters and field-programmable gate arrays. It avoids the development of high-performance and expensive application-specific integrated circuits and results in a cost-effective realization.

Feasibility of 15O-water PET studies of auditory system activation during general anesthesia in children

Background15O-Water positron emission tomography (PET) enables functional imaging of the auditory system during stimulation via a promontory electrode or cochlear implant, which is not possible using functional magnetic resonance imaging (fMRI). Although PET has been introduced in this context decades ago, its feasibility when performed during general anesthesia has not yet been explored. However, due to a shift to earlier (and bilateral) auditory implantation, the need to study children during general anesthesia appeared, since they are not able to cooperate during scanning. Therefore, we evaluated retrospectively results of individual SPM (statistical parametric mapping) analysis of 15O-water PET in 17 children studied during general anesthesia and compared them to those in 9 adults studied while awake. Specifically, the influence of scan duration, smoothing filter kernel employed during preprocessing, and cut-off value used for statistical inferences were evaluated. Frequencies, peak heights, and extents of activations in auditory and extra-auditory brain regions (AR and eAR) were registered.ResultsIt was possible to demonstrate activations in auditory brain regions during general anesthesia; however, the frequency and markedness of positive findings were dependent on some of the abovementioned influence factors. Scan duration (60 vs. 90 s) had no significant influence on peak height of auditory cortex activations. To achieve a similar frequency and extent of AR activations during general anesthesia compared to waking state, a lower cut-off for statistical inferences (p < 0.05 or p < 0.01 vs. p < 0.001) had to be applied. However, this lower cut-off was frequently associated with unexpected, “artificial” activations in eAR. These activations in eAR could be slightly reduced by the use of a stronger smoothing filter kernel during preprocessing of the data (e.g., [30 mm]3).ConclusionsOur data indicate that it is feasible to detect auditory cortex activations in 15O-water PET during general anesthesia. Combined with the improved signal to noise ratios of modern PET scanners, this suggests reasonable prospects for further evaluation of the method for clinical use in auditory implant users. Adapted parameters for data analysis seem to be helpful to improve the proportion of signals in AR versus eAR.

LEVELS OF PATIENTS EXPOSURE AND A POTENTIAL FOR OPTIMIZATION OF THE PET DIAGNOSTICS IN THE RUSSIAN FEDERATION

This study presents an overview of the most common positron emission tomography examinations in Russia, as well as the acquisition protocols and patient doses. The data collection was performed in 2012–2017 in 19 positron emission tomography departments in 12 regions of the Russian Federation by questioning the staff. The majority of the Russian positron emission tomography departments were equipped by modern positron emission tomography scanners combined with computed tomography. In each investigated department, data on all types of positron emission tomography examinations, radiopharmaceuticals, administered activities used for standard patient (body mass 70±5 kg) and parameters of computed tomography protocols was collected. The effective doses of patients from combined positron emission computed tomography examinations were estimated as a sum of the dose from the internal exposure (injected radiopharmaceutical) and the external exposure (computed tomography scan). Whole body positron emission tomography examinations in Russia were commonly performed with 18F-fluorodeoxyglucose ( 18 F-FDG), 18 F-choline, 11 С-choline, 68 GaPSMA, 68 Ga-DOTA-TATE, 68 Ga-DOTA-NOC, brain examinations – 18 F-FDG, 11 С-metionine, 18 F-choline, 18 F-tyrosine, myocardial perfusion – 13 N-ammonie. The highest patient effective doses (about 17 mSv) were observed for whole-body positron emission computed tomography examinations; for brain examinations – 3,4 – 4,8 mSv; for myocardial perfusion – 2,8 mSv. The computed tomography scan contributes up to 65 – 95% to the total patient effective dose for whole body examinations; 20 – 30% for head examinations. For the multiphase computed tomography scan effective doses may be increased to: 15 mSv for head examinations, 25 – 30 mSv for whole body examinations and 35 – 40 mSv for myocardial examinations. A standardization of acquisition and processing protocols is necessary for optimization of positron emission tomography examinations in Russia and for the intercomparison of results obtained in different positron emission tomography departments. Low dose computed tomography protocols, justification of diagnostic and multiphase computed tomography protocols, application of tube current modulation system and modern reconstruction algorithms, education and training of the staff in the field of radiation protection should be used for optimization of radiation protection of patient.

Optimisation of scatter correction based on the use of partially transparent beam stoppers in PET

To improve the counting efficiency, modern positron emission tomography scanners generally perform 3D data acquisition with the septa retracted. As a consequence, the scatter fraction can surge to as high as 40%. Our previous method proposed the use of a beam stoppers (BS) device to directly measure the scatter and to remove it from the sinogram. The BS partially attenuate the primary radiation of certain lines of response (LOR) without affecting the scattering events. To measure the scatter, dual scans are performed with and without the device in place. This study proposes an improved method. The dual scans are performed with the device placed at two different locations. The proposed method reduces the effective sampling distance without the need to actually increase the stopper's number. The scatter component at the LOR blocked by each stopper can be estimated. Assuming that the scattered radiation has a spatially slow-varying distribution, the whole scattered sinogram can be recovered using interpolation from these local measurements. Once the scatter component is estimated, the sinogram of the primary radiation can be obtained by subtracting the scatter component from the original data. The scatter free image is then reconstructed from the primary sinogram. This method was applied to scanned data of two digital phantoms using the Simulation System for Emission Tomography simulation package to demonstrate its effectiveness. Preliminary results showed that this method has slightly better performance than the previous BS method and the conventional single scatter simulation technique with smaller mean squared error and less noise in the corrected image.

Novel scatter correction for three-dimensional positron emission tomography by use of a beam stopper device

Modern positron emission tomography scanners generally perform three-dimensional data collection without septa, which increases not only true coincidences but also scattered coincidences. In this study, a beam stopper (BS) device which is made of lead was used to estimate the scatter component in the PET sinogram. Scattered events were then directly measured from those lines of response blocked by stoppers. By assuming that the scatter distribution is a spatially slow-varying function, the scatter component can be recovered using the cubic-spline interpolation from the local measurements. Monte Carlo simulations of an abdomen phantom and the Zubal phantom were performed. Preliminary results demonstrated that the proposed BS method can improve image contrast and quantitative accuracy. Among different configurations, the BS device consisting of twelve lead stoppers with 3 mm radius yielded the optimal result compared to the other BS configurations. This BS method also outperformed the dual-energy window method up to 40% based on the mean squared error. The results indicated that the proposed BS method permits a direct, fast, and accurate scatter correction.

Capillaries within compartments: microvascular interpretation of dynamic positron emission tomography data

Measurement of exchange of substances between blood and tissue has been a long-lasting challenge to physiologists, and considerable theoretical and experimental accomplishments were achieved before the development of the positron emission tomography (PET). Today, when modeling data from modern PET scanners, little use is made of earlier microvascular research in the compartmental models, which have become the standard model by which the vast majority of dynamic PET data are analysed. However, modern PET scanners provide data with a sufficient temporal resolution and good counting statistics to allow estimation of parameters in models with more physiological realism. We explore the standard compartmental model and find that incorporation of blood flow leads to paradoxes, such as kinetic rate constants being time-dependent, and tracers being cleared from a capillary faster than they can be supplied by blood flow. The inability of the standard model to incorporate blood flow consequently raises a need for models that include more physiology, and we develop microvascular models which remove the inconsistencies. The microvascular models can be regarded as a revision of the input function. Whereas the standard model uses the organ inlet concentration as the concentration throughout the vascular compartment, we consider models that make use of spatial averaging of the concentrations in the capillary volume, which is what the PET scanner actually registers. The microvascular models are developed for both single- and multi-capillary systems and include effects of non-exchanging vessels. They are suitable for analysing dynamic PET data from any capillary bed using either intravascular or diffusible tracers, in terms of physiological parameters which include regional blood flow.

Evaluation of a stereotactic frame for repositioning of the rat brain in serial positron emission tomography imaging studies

For serial imaging studies of the rat brain with positron emission tomography (PET), reproducible positioning of the head can facilitate spatial alignment of images and quantitative analysis. To achieve this aim, we constructed a plastic head frame and tested the positioning reproducibility on a high-resolution small-animal PET scanner, microPET. Two sets of ear bars, with tapers of either 18° (sharp) or 45° (blunt), were evaluated for their relative precision in securing the animal to the frame. For sequential positioning of an animal, average distances from the mean position of 0.51 mm (SD 0.41 mm) and 0.91 mm (SD 0.48 mm) were measured with the sharp and blunt ear bars, respectively. These results show that a rat brain can be reproducibly positioned using the frame, with a variation of position less than the spatial resolution of modern animal PET scanners. Brain regions of interest defined on one scan and copied across subsequent scans of a frame-repositioned animal resulted in an average coefficient of variation of 5.4% (SD 2.7%) using the sharp ear bars and 6.8% (SD 2.5%) using the blunt ear bars. This methodology has the potential to improve quantitative assessment for serial PET studies.

Positron emission tomography (PET) scanning in breast cancer.

primary diagnosis, more accurate non-invasive Quantitative analysis techniques include dynamic staging, and more effective methods of following studies which generate absolute glucose metabolic therapy and establishing prognosis. A large number rates and simple relative quantitative ratios, known of new or improved imaging techniques have been as standardized uptake values or SUVs [3, 6]. developed in recent years yet most patients still Simple qualitative visual analysis of the images has require multiple examinations of various organs, also shown high accuracy [6, 7]. including ultrasound of the liver, isotope bone Several studies have examined the initial detecscans, mammograms and CT scans. Furthermore, tion and staging of breast cancer using FDG–PET the current sensitivities and/or specificities of these scans. Wahl and colleagues detected 10 out of 10 methods leave much room for improvement. primary breast lesions and, although these were Positron emission tomography (PET) imaging has generally large lesions (3.2–12 cm), 2 of the patients recently shown promise as a single technique that had dense glandular breast tissue with negative might have equivalent or better accuracy than the mammograms [4]. In a series of 28 patients with other current methods combined [1–4]. 35 suspect breast masses, 27 of which were found The greatest developments in PET imaging to be malignant, Adler et al reported a sensitivity during the 1990s have been in oncology. A number of 90% and specificity of 100% with primary of PET radiopharmaceuticals have been utilized lesions as small as 0.9 cm being correctly identified for oncological investigations including labelled as tumour [8]. In their quantitative analysis, the hormones or their analogues, labelled amino acids, malignant lesions had a mean FDG uptake value labelled hypoxia agents and labelled thymidine [2, (SUV) of 12.8, while benign solid lesions averaged 3]. However, the vast majority of the work to date 1.8 and benign cysts 0.2. Furthermore, there was a has centred on the use of fluorodeoxyglucose labsignificant correlation between normalized FDG elled with Fluorine-18 (18FDG). The 2 h half life uptake and the histological nuclear grade of the not only makes whole body imaging practical but primary lesion. Tse et al from the University of allows for multicentre distribution of tracer. The California at Los Angeles also reported 100% high aerobic glycolytic rate of most tumours means specificity but a false negative rate of 16% [9]. that there is a much higher uptake of FDG in One of their false negative studies had diffuse malignant tissues than in normal tissue. These high microscopic foci and the other had a low grade glycolytic rates are most likely the result of tumour with microscopic foci of invasion. Lesions increased glucose transporter expression and less than 1 cm were detected. While mammography increased hexokinase activity in tumours [2, 3, 5]. was less specific, it was complementary in detecting Moderate levels of hypoxia also increase tumour a FDG–PET negative non-palpable lesion. The glucose uptake rates by 30% or more [5]. Most following year, Nieweg et al from the MD modern PET scanners have large fields of view, Anderson Cancer Center reported a series of 20