IEEE Nuclear Science Symposium conference record. Nuclear Science Symposium最新文献

We are conducting a two-phase study, which aims to select design parameters of a long-axis positron emission tomography (PET) scanner that are a compromise between detection performance and cost. In this first phase, we examine the effects of axial length, detector thickness and collimator geometry on the noise equivalent count rate per axial length (λNEC) and noise equivalent count rate per slice (dNEC). We use these metrics as approximate, but quickly computed, indicators of a PET-scanner's performance at a detection task. From this first phase, we select a subset of scanner designs for which we can conduct a detailed study of tumor detectability and quantitation accuracy in whole-body PET imaging.

The addition of accurate system modeling in PET image reconstruction results in images with distinct noise texture and characteristics. In particular, the incorporation of point spread functions (PSF) into the system model has been shown to visually reduce image noise, but the noise properties have not been thoroughly studied. This work offers a systematic evaluation of noise and signal properties in different combinations of reconstruction methods and parameters. We evaluate two fully-3D PET reconstruction algorithms: (1) OSEM with exact scanner line of response modeled (OSEM+LOR), (2) OSEM with line of response and a measured point spread function incorporated (OSEM+LOR+PSF), in combination with the effects of 4 post filtering parameters and 1-10 iterations. We used a modified NEMA IQ phantom, which was filled with 68Ge and consisted of 6 hot spheres of different sizes with a target/background ratio of 4:1. The phantom was scanned 50 times in 3D mode on a clinical system to provide independent noise realizations. Data were reconstructed with OSEM+LOR and OSEM+LOR+PSF using different reconstruction parameters. With access to multiple realizations, 4 metrics are adopted to quantify the noise characteristics in the reconstructed images. Image roughness and the standard deviation image are measures of the pixel-to-pixel variation, while NEMA and ensemble noises quantify the region-to-region variation. In addition to 4 noise metrics, we also evaluate signal to noise performance with accepted signal strength measures (recovery coefficient, SNR for quantitation), and study the relations between different metrics. From the analysis results, a linear correlation is observed between NEMA noise and ensemble noise for all different combinations of reconstruction methods and parameters, suggesting that NEMA style noise is a reasonable surrogate for ensemble noise when multiple realizations of scans are not available in practice. At the same number of iterations, the addition of PSF reduces image roughness for unfiltered images by roughly 35%, while the addition of PSF does not reduce NEMA style or ensemble noise. When noise is measured across realizations, the PSF based method offers slightly improved ( 7%) signal to noise performance across a range of reconstruction parameters.

We are developing a prototype monolithic scintillation camera with optical sensors on the entrance surface (SES) for use with statistically-estimated depth-of-interaction in a continuous scintillator. We opt to use Geiger-Müller mode avalanche photodiodes (GM-APDs) for the SES camera since they possess many desirable properties; for the intended application (SES and PET/MR imaging), they offer a thin attenuation profile and an operational insensitivity to large magnetic fields. However, one issue that must be addressed in using GM-APDs in an RF environment (as in MR scanners) is the thermal dissipation that can occur in this semiconductor material.Signals of GM-APDs are strongly dependent on junction temperature. Consequently, we are developing a temperature-controlled GM-APD-based PET camera whose monitored temperature can be used to dynamically account for the temperature dependence of the output signals. Presently, we aim to characterize the output-signal dependence on temperature and bias for a GM-APD-based scintillation camera.We've examined two GM-APDs, a Zecotek prototype MAPD-3N, and a SensL commercial SPMArray2. The dominant effect of temperature on gain that we observe results from a linear dependence of breakdown voltage on temperature (0.071 V/°C and 0.024 V/°C, respectively); at 2.3 V excess bias (voltage above breakdown) the resulting change in gain with temperature (without adjusting bias voltage) is -8.5% per °C for the MAPD-3N and -1.5 % per °C for the SPMArray2. For fixed excess bias, change in dark current with temperature varied widely, decreasing by 25% to 40% as temperature was changed from 20 °C to 10 °C and again by 20% to 35% going from 10 °C to 0 °C. Finally, using two MAPD-3N to read out a pair of 3.5-by-3.5-by-20 mm(3) Zecotek LFS-3 scintillators in coincidence, we observe a decrease from 1.7 nsec to 1.5 nsec in coincidence-time resolution as we lowered temperature from 23 °C to 10 °C.

Many methods have been proposed for generating an image-derived input function (IDIF) exclusively from PET images. The purpose of this study was to assess the viability of a multimodality approach utilizing registered MR images. 3T-MR and HRRT-PET data were acquired from human subjects. Segmentation of both the left and right carotid arteries was performed in MR images using a 3D level sets method. Vessel centerlines were extracted by parameterization of the segmented voxel coordinates with either a single polynomial curve or a B-spline curve fitted to the segmented data. These centerlines were subsequently re-registered to static PET data to maximize the accurate classification of PET voxels in the ROI. The accuracy of this approach was assessed by comparison of the area under the curve (AUC) of the IDIF to that measured from conventional automated arterial blood sampling.Our method produces curves similar in shape to that of blood sampling. The mean AUC ratio of the centerline region was 0.40±0.19 before re-registration and 0.69±0.26 after re-registration. Increasing the diameter of the carotid ROI produced a smooth reduction in AUC. Thus, even with the high resolution of the HRRT, partial volume correction is still necessary. This study suggests that the combination of PET information with MR segmented regions will demonstrate an improvement over regions based solely on MR or PET alone.

Our laboratory is developing a high-resolution PET detector capable of providing depth-of-interaction information (dMiCE) by tailoring the light sharing between two adjacent detector elements. Each detector element in the prototype system has a 2×2 mm(2) cross section and is directly coupled to a micro-pixel avalanche photodiode (MAPD). In this setup the distribution of the ratio of light shared between two adjacent detector elements can be expressed as a function of the depth of interaction. The three-dimensional points of interaction of a coincidence pair of photons within the detector module is estimated by numerical calculation of an expectation of the points of interaction conditioned on the signals measured by the MAPDs (Bayesian estimate). This conditional expectation is computed from estimates of the probability density function of the light collection process and a model of the kinetics of photon interactions in the detector module. Our algorithm is capable of handling coincidences where each photon interacts any number of times within the detector module before being completely absorbed or escaping. In the case of multiple interactions our algorithm estimates the position of the first interaction for each of the coincidence photons.

Flying-focal-spot (FFS) technique has been used for improving the sampling condition in advanced clinical CT by collecting multiple cone-beam data sets with the focal-spot at different locations at each "projection view". It has been demonstrated that the increased sampling rate in FFS scans can substantially reduce aliasing artifacts in reconstructed images. However, the increase of the sampling density through multiple illuminations at each view can result in the increase of radiation dose to the imaged subject. In this work, we have applied a compressive-sensing (CS)-based algorithm to image reconstruction from data acquired in FFS scans. The results of the study demonstrate that aliasing artifacts observed images reconstructed by use of analytic algorithms can be suppressed effectively in images reconstructed with this CS-based algorithm from only data acquired at one FFS scan.

We have developed a flexible x-ray micro-CT system, named FaCT, capable of changing its geometric configuration and acquisition protocol in order to best suit an object being imaged for a particular diagnostic task. High-performance computing technologies have been a major enabling factor for this adaptive CT system in terms of system control, fast reconstruction, and data analysis. In this work, we demonstrate an adaptive procedure in which a quick, sparse-projection pre-scan is performed, the data are reconstructed, and a region of interest is identified. Next, a diagnostic-quality scan is performed where, given the region of interest, the control computer calculates an illumination window for on-line control of an x-ray source masking aperture to transmit radiation only through the region of interest throughout the scan trajectory. Finally, the diagnostic scan data are reconstructed, with the region of interest being clearly resolved. We use a combination of a multi-core CPU and a pair of NVIDIA Tesla GPUs to perform these tasks.

The University of Washington developed a Firewire based data acquisition system for the MiCES small animal PET scanner. Development work has continued on new imaging scanners that require more data channels and need to be able to operate within a MRI imaging system. To support these scanners, we have designed a new version of our data acquisition system that leverages the capabilities of modern field programmable gate arrays (FPGA). The new design preserves the basic approach of the original system, but puts almost all functions into the FPGA, including the Firewire elements, the embedded processor, and pulse timing and pulse integration. The design has been extended to support implementation of the position estimation and DOl algorithms developed for the cMiCE detector module. The design is centered around an acquisition node board (ANB) that includes 65 ADC channels, Firewire 1394b support, the FPGA, a serial command bus and signal lines to support a rough coincidence window implementation to reject singles events from being sent on the Firewire bus. Adapter boards convert detector signals into differential paired signals to connect to the ANB.

Positron emission mammography (PEM) uses two opposing gamma-ray imagers and limited-angle tomography techniques to image radiotracer distributions within the breast. Due to their smaller size and closer proximity to the source, dedicated PEM cameras can provide better spatial resolution and count sensitivity than whole-body positron emission tomographs. We performed several clinical imaging tests on a commercially available PEM camera, the PEM Flex Solo II. This system is comprised of two opposing 6 cm × 16.4 cm detectors that scan in unison to cover up to a 24 cm × 16.4 cm field of view (FOV). We measured spatial resolution, uniformity, recovery coefficients (RC), and quantification using the system clinical software. Image linearity and coefficient of variation (CV) at the edge of the FOV were also characterized. Anecdotal examples of clinical patient data are presented. Spatial resolution is 2.4 mm FWHM for image planes parallel to the detector faces; background variability is 6%; quantification and RC varied within the FOV; positioning linearity began at ~ 13 mm from the edge of the detector housing; CV increased rapidly at the edge of the FOV due to limited sampling in these image planes.

We report on a high resolution, monolithic crystal PET detector design concept that provides depth of interaction (DOI) positioning within the crystal and is compatible for operation in a MRI scanner to support multimodal anatomic and functional imaging. Our design utilizes a novel sensor on the entrance surface (SES) approach combined with a maximum likelihood positioning algorithm. The sensor will be a two-dimensional array of micro-pixel avalanche photodiodes (MAPD). MAPDs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). In addition, they can be operated in high magnetic fields to support PET/MR imaging. Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics for a variety of detector configurations. The crystal was always modeled as a 48.8 mm by 48.8 mm by 15 mm monolithic slab of a lutetium-based scintillator. The SES design was evaluated via simulation for three different two-dimensional MAPD array sizes: 8×8 with 5.8×5.8 mm(2) pads; 12×12 with 3.8×3.8 mm(2) pads; and 16×16 with 2.8×2.8 mm(2) pads. To reduce the number of signal channels row-column summing readout was explored for the 12×12 and 16×16 channel array devices. The intrinsic spatial resolution for the 8×8 MAPD array is 0.88 mm FWHM in X and Y, and 1.83 mm FWHM in Z (i.e., DOI). Comparing the results versus using a conventional design with the photosensors on the backside of the crystal, an average improvement of ~24% in X and Y and 20% in Z is achieved. The X, Y intrinsic spatial resolution improved to 0.66 mm and 0.65 mm FWHM for the 12×12 and 16×16 MAPDs using row-column readout. Using the 12×12 and 16×16 arrays also led to a slight improvement in the DOI positioning accuracy.