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Positron emission tomography (PET) plays an increasingly important role in research and clinical applications, catalysed by remarkable technical advances and a growing appreciation of the need for reliable, sensitive biomarkers of human function in health and disease. Over the last 30 years, a large amount of the physics and engineering effort in PET has been motivated by the dominant clinical application during that period, oncology. This has led to important developments such as PET/CT, whole-body PET, 3D PET, accelerated statistical image reconstruction, and time-of-flight PET. Despite impressive improvements in image quality as a result of these advances, the emphasis on static, semi-quantitative 'hot spot' imaging for oncologic applications has meant that the capability of PET to quantify biologically relevant parameters based on tracer kinetics has not been fully exploited. More recent advances, such as PET/MR and total-body PET, have opened up the ability to address a vast range of new research questions, from which a future expansion of applications and radiotracers appears highly likely. Many of these new applications and tracers will, at least initially, require quantitative analyses that more fully exploit the exquisite sensitivity of PET and the tracer principle on which it is based. It is also expected that they will require more sophisticated quantitative analysis methods than those that are currently available. At the same time, artificial intelligence is revolutionizing data analysis and impacting the relationship between the statistical quality of the acquired data and the information we can extract from the data. In this roadmap, leaders of the key sub-disciplines of the field identify the challenges and opportunities to be addressed over the next ten years that will enable PET to realise its full quantitative potential, initially in research laboratories and, ultimately, in clinical practice.

Count rate loss in singles transmission scans due to dead time can decrease the accuracy of attenuation correction. In addition, for very low transmission source activities measured attenuation may be affected by noise and background radiation emitted by radioactive lutetium-176 in the scintillator material. The aims of this study were to investigate count rate performance in singles transmission scanning for the microPET Focus 220 animal scanner, to examine the effect of energy window width on noise, scatter and dead time, and to determine the optimal range of activity for accurate attenuation correction. Transmission measurements were performed with a decaying [sup.99m]Tc point source and using wide (21%) and narrow (4%) energy windows. We observed count rate losses at the detector level and in the front end electronics during energy discrimination. Further losses emerged due to multiplexing at the detector head interface for activities higher than 150 MBq for the wide energy window. Substantial count rate losses during transfer of list mode data from the microPET to the PC were also observed for this condition. Lutetium-176 background caused a significant deviation of measured attenuation for activities lower than 10 MBq for both energy windows. No significant bias due to scatter was introduced when using the wide energy window. We conclude that accurate attenuation data can be obtained with single photon transmission source activity in the range 10 to 150 MBq without requiring deadtime correction. Index Terms--Attenuation correction, microPET, positron emission tomography, small animal imaging.

This document is a correction notice for an article titled "Virtual cylindrical PET for efficient DOI image reconstruction with sub-millimetre resolution." The authors of the article acknowledge that they omitted a reference to a previous study by Scheins et al. Although Scheins et al. did not specifically address depth-of-interaction (DOI) PET reconstruction, their work on virtual cylinders laid the groundwork for the implementation of virtual cylinders in DOI PET reconstruction. The authors provide the correct reference and emphasize the importance of acknowledging Scheins et al.'s contribution in this context. [Extracted from the article]

In a previous simulation study, we demonstrated the feasibility of using coded apertures together with pixelated detectors for small animal SPECT. In this paper, we further explore the potential of this approach with a prototype detector and simulated multipinhole apertures. We also investigated the effect of multiplexing due to overlapped projections on convergence properties, image signal-to-noise ratio (SNR) and spatial resolution. The detector comprises a 48 x 44 array of NaI(TI) crystals, each 1 mm x 1 mm x 5 mm on a 1.25-mm pitch. The crystal array is directly coupled to a Hamamatsu R3941 8 cm position sensitive photomultiplier tube. Multipinhole apertures were simulated by performing repeated SPECT acquisitions of the same object with a single tungsten pinhole translated to different positions in the aperture plane. Image reconstruction is based on a three-dimensional ray driven projector which is an extension of a method described for single pinhole SPECT with a displaced center of rotation. Image estimates are updated using the maximum likelihood expectation maximization (ML-EM) algorithm. The effect of multiplexing was to slow convergence and reduce the achievable SNR by approximately 15% compared with nonmultiplexed data (but the result may be achieved in a fraction of the time). The reconstructed resolution obtained with a resolution phantom was 1.5-mm full width at half maximum and there was no appreciable difference between the resolution of multiplexed and nonmultiplexed data. These results encourage us to develop a prototype coded aperture system for high sensitivity, high resolution small animal SPECT. Index Terms--Coded aperature, iterature reconstruction, single photon emission computed tomography (SPECT), small animal imaging.

Accurate determination of the input function is essential for absolute quantification of physiological parameters in positron emission tomography and single-photon emission computed tomography imaging, but it requires an invasive and tedious procedure of blood sampling that is impractical in clinical studies. We previously proposed a technique that estimates simultaneously kinetic parameters and the input function from the tissue impulse response functions and requires two blood samples. A nonlinear least squares method estimated all the parameters in the impulse response functions and the input function but failed occasionally due to high noise levels in the data, causing an ill-conditioned cost function. This paper investigates the feasibility of applying a Monte Carlo method called simulated annealing to estimate kinetic parameters in the impulse response functions and the input function. Time-activity curves of teboroxime, which is very sensitive to changes in the input function, were simulated based on published data obtained from a canine model. The equations describing the tracer kinetics in different regions were minimized simultaneously by simulated annealing and nonlinear least squares. We found that the physiological parameters obtained with simulated annealing are accurate, and the estimated input function more closely resembled the simulated curve. We conclude that simulated annealing reduces bias in the estimation of physiological parameters and determination of the input function. Index Terms--Impulse response function, input function, kinetic parameter estimation, nonlinear least squares, simulated annealing, single-photon emission computed tomography (SPECT), teboroxime.

Quantitative positron emission tomography (PET) studies provide in vivo measurements of dynamic physiological and biochemical processes in humans. A limitation of PET is an inability to provide precise anatomic localization due to relatively poor spatial resolution when compared to magnetic resonance (MR) imaging. Manual placement of region-of-interest (ROI) is commonly used in clinical and research settings in analysis of PET datasets. However, this approach is operator dependent and time-consuming. A semi- or fully-automated ROI delineation (or segmentation) method offers advantages by reducing operator error/subjectivity and thereby improving reproducibility. In this work, we describe an approach to automatically segment dynamic PET images using cluster analysis and we validate our approach with a simulated phantom study and assess its performance with real dynamic PET data. Our preliminary results suggest that cluster analysis can automatically segment tissues in dynamic PET studies and has the potential to replace manual ROI delineation for some applications. Index Terms--Cluster analysis, functional imaging, positron emission tomography (PET), segmentation.

Methods capable of correcting for head motion in all six degrees of freedom have been proposed for positron emission tomography (PET) brain imaging but not yet demonstrated in human studies. These methods rely on the accurate measurement of head motion in relation to the reconstruction coordinate frame. We present methodology for the direct calibration of an optical motion-tracking system to the reconstruction coordinate frame using paired coordinate measurements obtained simultaneously from a PET scanner and tracking system. We also describe the implementation of motion correction, based on the multiple acquisition frame method originally described by Picard and Thompson, using data provided by the motion tracking system. Effective compensation for multiple six-degree-of-freedom movements is demonstrated in dynamic PET scans of the Hoffman brain phantom and a normal volunteer. We conclude that reduced distortion and improved quantitative accuracy can be achieved with this method in PET brain studies degraded by head movements. Index Terms--Motion correction, motion tracking, positron emission tomography (PET).

3D PET has higher sensitivity than 2D PET. Sensitivity is dependant on detector properties such as density, stopping power, photofraction, etc, but for a given detcetor material the sensitivity of a tomograph is primarily determined by two components: the geometric solid angle for detection, and the fractional dead time, i.e., the time for which the detector is unavailable for accepting events. The loss in overall sensitivity as a function of radioactivity concentration due to these factors for 3D PET has been characterised by a parameter, the effective sensitivity, which combines absolute sensitivity and noise equivalent count rates. This parameter includes scatter, system sensitivity, dead time, and random coincidence rates, and permits comparisons between different tomographs as well as the same tomograph under different conditions. Effective sensitivity decreases most rapidly for larger, open 3D tomographs. The loss in effective sensitivity with increasing count rate suggests that new faster scintillation detectors will be needed to realize the sensitivity gain of 3D PET over a wide dynamic range of radioactivity concentrations.

We investigated potential sources of bias due to simultaneous emission and transmission (SET) scanning and their effect on parameter estimation in dynamic positron emission tomography (PET) oncology studies. The sources of bias considered include: i) variation in transmission spillover (into the emission window) throughout the field of view, ii) increased scatter arising from rod sources, and iii) inaccurate deadtime correction. Net bias was calculated as a function of the emission count rate and used to predict distortion in [18F]2-fluoro-2-deoxy-D-glucose (FDG) and [11C]thymidine tissue curves simulating the normal liver and metastatic involvement of the liver. The effect on parameter estimates was assessed by spectral analysis and compartmental modeling. The various sources of bias approximately cancel during the early part of the study when count rate is maximal. Scatter dominates in the latter part of the study, causing apparently decreased tracer clearance which is more marked for thymidine than for FDG. The irreversible disposal rate constant, [K.sub.i], was overestimated by 30% for thymidine. We conclude that SET has a potential role in dynamic FDG PET but is not suitable for 11C-labeled compounds. Index Terms - Oncology, positron emission tomography, transmission scanning.