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We designed a dedicated multi-detector multi-pinhole brain SPECT scanner to generate images of higher quality compared to general-purpose systems. The system, AdaptiSPECT-C, is intended to adapt its sensitivity-resolution trade-off by varying its aperture configurations allowing both high-sensitivity dynamic and high-spatial-resolution static imaging. The current system design consists of 23 detector heads arranged in a truncated spherical geometry. In this work, we investigated the axial and angular sampling capability of the current stationary system design. Two data acquisition schemes using limited rotation of the gantry and two others using axial translation of the imaging bed were also evaluated concerning their impact on image quality through improved sampling. Increasing both angular and axial sampling in the current prototype system resulted in quantitative improvements in image quality metrics and qualitative appearance of the images as determined in studies with specifically selected phantoms. Visual improvements for the brain phantoms with clinical distributions were less pronounced but presented quantitative improvements in the fidelity (normalized root-mean-square error (NRMSE)) and striatal specific binding ratio (SBR) for a dopamine transporter (DAT) distribution, and in NRMSE and activity recovery for a brain perfusion distribution. More pronounced improvements with increased sampling were seen in contrast recovery coefficient, bias, and coefficient of variation for a lesion in the brain perfusion distribution. The negligible impact of the most cranial ring of detectors on axial sampling, but its significant impact on sensitivity and angular sampling in the cranial portion of the imaging volume-of-interest were also determined.

Single photon emission tomography imaging of dopamine transporters (DATs) in the brain is a widely utilized study to improve the diagnosis of Parkinsonian syndromes, where conventional (parallel-hole and fan-beam) collimators on dual-head scanners are commonly employed. We have designed a multipinhole (MPH) collimator to improve the performance of DAT imaging. The MPH collimator focuses on the striatum and hence offers a better tradeoff for sensitivity and spatial resolution than the conventional collimators within this clinically most relevant region for DAT imaging. Our original MPH design consisted of 9 pinholes with a background-to-striatal (Bkg/Str) projection multiplexing of 1% only. In this simulation study, we investigated whether further improvements in the performance of MPH imaging could be obtained by increasing the number of pinholes, hence by enhancing the sensitivity and sampling, despite the ambiguity in reconstructing images due to increased multiplexing. We performed analytic simulations of the MPH configurations with 9, 13, and 16 pinholes (aperture diameters: 4–6 mm) using a digital phantom modeling DAT imaging. Our quantitative analyses indicated that using 13 (Bkg/Str: 12%) and 16 (Bkg/Str: 22%) pinholes provided better performance than the original 9-pinhole configuration for the acquisition with two or four angular views, but a similar performance with 8 and 16 views.

Stationary single photon emission computed tomography (SPECT) systems offer numerous advantages over rotating-head systems, but have one notable drawback in that the array of relatively smaller cameras contains more edges and gaps than clinical two headed systems. Scintillation events occurring at the edges of traditional photomultiplier-tube (PMT)-based SPECT cameras lose spatial resolution due to loss of light-sampling. Simulations using customized non-sequential raytracing scripts to model and analyze mean detector response functions (MDRFs) showed significant improvement in spatial resolution for hybrid-sensor cameras employing both silicon photomultipliers (SiPMs) and PMTs. The results inform the hardware design of AdaptiSPECT-C: a stationary clinical whole-brain SPECT imager with adaptive apertures for selective dynamic or high spatial resolution imaging. Its modular hybrid cameras use SiPMs to augment the PMTs and improve spatial resolution for position estimation tasks. SiPMs, having a small pitch and efficient fill factor, are employed as a border around the edges of each detector area. PMTs, being low cost and reliable, are packed in the center. The front end electronics are split into two main boards: one to drive and provide signal conditioning for the PMTs, and the other performing a similar function for the SiPMs. Ultimately, 81 total signal channels leave each camera as negative voltage pulses. AdaptiSPECT-C will have two equatorial rings of 10 cameras each and a quasi-vertex ring of 4 cameras, totaling 24. Modularity is the guiding design principle for the mechanical components of the cameras and ensures ease of assembly and field service in the completed system.

An adaptive-stationary-modular multi-pinhole (MPH) brain SPECT, AdaptiSPECT-C is being developed by the University of Arizona and University of Massachusetts Medical School to meet static and dynamic brain SPECT imaging needs. Salient features of the ASC include the use of adjustable pinhole apertures to dynamically adapt to imaging task needs, improved light measuring around the edge of the scintillator crystal, plus motion tracking and correction with attenuation correction enabled by usage of depth-sensing (DS)-cameras. For a target system spatial resolution of 8 mm at the focal point of the apertures, selected to enable comparison to current 2-headed commercial (2HC) SPECT imaging, we report investigation of aperture layout designs for a system with 3 rings of 18.4 cm flat square detector modules. We investigated sensitivity at the focal point in comparison to 2HC for usage of 1 versus 5 apertures per module, and variation in the extent of truncation and multiplexing of the irradiation fields by adjustment of the aperture location between the detector and focal point. For a system with one aperture per module and minor truncation we determined a sensitivity of 2.7x that of 2HC; whereas, with use of 4 oblique apertures with minor truncation and moderate multiplexing we determined the sensitivity was 4.6x, and with all 5 apertures resulting in significant multiplexing the sensitivity was 5.7x. We also determined through simulation better visualization of the rods of a Derenzo phantom, and perfusion distribution of XCAT brain phantom with the 5 pinhole design, using solely the 4 oblique pinholes. We thus believe that this design with 5 pinholes per detector module is an excellent candidate for use in construction of the AdaptiSPECT-C system.

We are designing AdaptiSPECT-C, a novel, multiple-detector, adaptive-aperture single-photon emission, tomographic (SPECT) system dedicated to brain imaging. This system is designed to change sensitivity and spatial resolution in realtime to address the needs of dynamic imaging[1]. The aim of this work is to document the creation of a manufacturable aperture design including hardware, electronics and software that effectively adapt in real-time spatial resolution and sensitivity functions to the needs of a dynamically changing subject. We accomplish these goals through metal printing of apertures and with custom control boards based on the Arduino microcontroller. With this design we are able to precisely control each aperture motion with a step resolution of 0.20 millimeters, which is within our required tolerances.

A next-generation multi-pinhole system dedicated to brain SPECT imaging is being constructed by our research team, which we call AdaptiSPECT-C. The system prototype used herein consists of 25 square detector modules and a total of 100 apertures grouped by 4 per module. The system is specifically designed for multi-purpose brain imaging and capable of adapting in real-time each aperture size and whether it is open or shuttered closed. The use of such system would provide optimum high-performance patient-personalized imaging for a wide range of brain imaging tasks. In this work we investigated the effect of pinhole diameter variation on spherical tumor quantification for the promising brain tumor imaging agent 123I-CLINDE. To assess the quality of the images reconstructed for the different aperture sizes, we used a customized multiple-sphere tumor phantom derived from the XCAT software with a tumor size of 1 cm in diameter. Our results suggest through quantification and visual inspection that an aperture diameter in the range of 2 to 5 mm in diameter for the adaptive AdaptiSPECT-C system is likely the most suited for high performance brain tumor 123I-CLINDE imaging. In addition, our study concludes that a 4 mm pinhole diameter given its excellent spatial-resolution-to-sensitivity trade-off is promising for scout acquisition in localizing target tumor regions within the brain. We have initiated a task-based performance on the tumor detection and localization accuracy for a range of simulated tumor sizes using the channelized non-pre-whitening (CNPW) matched-filter scanning-observer.

With brain-dedicated multi-detector systems employing pinhole apertures the usage of detectors facing the top of the patient’s head (i.e. quasi-vertex (QV) views) can provide the advantage of additional viewing from close to the brain for improved detector coverage. In this paper, we report the results of simulation and reconstruction studies to investigate the impact of the QV views on the imaging performance of AdaptiSPECT-C, a brain-dedicated stationary SPECT system under development. In this design, both primary and scatter photons from regions located inferior to the brain can contribute to SPECT projections acquired by the QV views, and thus degrade AdaptiSPECT-C imaging performance. In this work, we determined the proportion, origin, and nature (i.e. primary, scatter, and multiple-scatter) of counts emitted from structures within the head and throughout the body contributing to projections from the different AdaptiSPECT-C detector rings, as well as from a true vertex view detector. We simulated phantoms used to assess different aspects of image quality (i.e. uniform activity concentration sphere, and Derenzo), as well as anthropomorphic phantoms with different count levels emulating clinical 123I activity distributions (i.e. DaTscan and perfusion). We determined that attenuation and scatter in the patient’s body greatly diminish the probability of the photons emitted outside the volume of interest reaching to detectors and being recorded within the 15% photopeak energy window. In addition, we demonstrated that the inclusion of the residual of such counts in the system acquisition does not degrade visual interpretation or quantitative analysis. The addition of the QV detectors improves volumetric sensitivity, angular sampling, and spatial resolution leading to significant enhancement in image quality, especially in the striato-thalamic and superior regions of the brain. Besides, the use of QV detectors improves the recovery of clinically relevant metrics such as the striatal binding ratio and mean activity in selected cerebral structures. Our findings proving the usefulness of the QV ring for brain imaging with 123I agents can be generalized to other commonly used SPECT imaging agents labelled with isotopes, such as 99mTc and likely 111In.

SPECT imaging of the dopamine transporter (DAT) is used for diagnosis and monitoring progression of Parkinson's disease (PD), and differentiation of PD from other neurological disorders. The diagnosis is based on the DAT binding in the caudate and putamen structures in the striatum. We previously proposed a relatively inexpensive method to improve the detection and quantification of these structures for dual-head SPECT by replacing one of the fan-beam collimators with a specially designed multipinhole (MPH) collimator. In this paper, we developed a realistic model of the proposed MPH system using the GATE simulation package and verified the geometry with an analytic simulator. Point source projections from these simulations closely matched confirming the accuracy of the pinhole geometries. The reconstruction of a hot-rod phantom showed that 4.8-mm resolution is achievable. The reconstructions of the XCAT brain phantom showed clear separation of the putamen and caudate, which is expected to improve the quantification of DAT imaging and PD diagnosis. Using this GATE model, point spread functions modeling physical factors will be generated for use in reconstruction. Also, further improvements in geometry are being investigated to increase the sensitivity of this base system while maintaining a target spatial resolution of 4.5-5 mm.

Application of multi-pinhole collimator in pinhole-based SPECT increases detection sensitivity. The presence of multiplexing in projection images due to the usage of multiple pinholes can further improve the sensitivity at the cost of adding data ambiguity. We are developing a next-generation adaptive brain-dedicated SPECT system –AdaptiSPECT-C. The AdaptiSPECT-C can adapt the multiplexing level and system sensitivity using adaptable pinhole modules. In this study, we investigated the performance of 4 data acquisition schemes with different multiplexing levels and sensitivities on cerebral SPECT imaging. Schemes #1, #2, and #3 have

AdaptiSPECT-C (ASC) is being designed as an adaptive multi-pinhole SPECT imaging system with multiple detectors arranged in sphere-like geometry in three rings (Caudal, Middle, and Quasi-Vertex (QV)). Herein we investigate the usage of rotated square detectors for the first two rings, and hexagonal detectors for QV ring. We compare designs for different levels of temporal shuttering of the multiple apertures irradiating each detector. We assess image quality for a variety of phantoms assessing uniformity, axial sampling, and brain perfusion imaging. We determine that rotated square detectors can provide good uniformity, the axial sampling we have observed thus far based on reconstruction of a tailored Defrise Phantom and reconstructions of the brain phantom modeling perfusion distribution.

We introduce a generic analytic simulation and image reconstruction software platform for multi-pinhole (MPH) SPECT systems. The platform is capable of modeling common or sophisticated MPH designs as well as complex data acquisition schemes. Graphics processing unit (GPU) acceleration was utilized to make a high-performance computing software. Herein, we describe the software platform and provide verification studies of the simulation and image reconstruction software.

We report our investigation of system designs and 3D reconstruction for a dedicated brain-imaging SPECT system using multiple square or square and hexagonal detector modules. The system employs shuttering to vary which of multiple pinhole apertures are enabled to pass photons through to irradiate the detectors. Both multiplexed and nonmultiplexed irradiation by the pinholes are investigated. Sampling is assessed by simulated imaging of a uniform activity concentration in a spherical tub filling the VOI and a tailored Defrise phantom consisting of a series of activity containing slabs aligned axially. Potential image quality for clinical imaging is assessed through simulated imaging of an XCAT brain phantom with an activity distribution simulating perfusion imaging.

The use of accurate system response modeling has been proven to be an essential key of SPECT image reconstruction, with its usage leading to overall improvement of image quality. The aim of this work was to investigate the imaging performance using an XCAT brain perfusion phantom of two modeling strategies, one based on analytic techniques and the other one based on GATE Monte-Carlo simulation. In addition, an efficient forced detection approach to improve the overall simulation efficiency was implemented and its performance was evaluated. We demonstrated that accurate modeling of the system matrix generated by Monte-Carlo simulation for iterative reconstruction leads to superior performance compared to analytic modeling in the case of clinical I-123 brain imaging. It was also shown that the use of the forced detection approach provided a quantitative and qualitative enhancement of the reconstruction.

Multi-pinhole (MPH) collimators are known to provide better trade-off between sensitivity and resolution for preclinical, as well as for smaller regions in clinical SPECT imaging compared to conventional collimators. In addition to this geometric advantage, MPH plates typically offer better stopping power for penetration than the conventional collimators, which is especially relevant for I-123 imaging. The I-123 emits a series of high-energy (>300 keV, similar to 2.5% abundance) gamma photons in addition to the primary emission (159 keV, 83% abundance). Despite their low abundance, high-energy photons penetrate through a low-energy parallel-hole (LEHR) collimator much more readily than the 159 keV photons, resulting in large downscatter in the photopeak window. In this work, we investigate the primary, scatter, and penetration characteristics of a single pinhole collimator that is commonly used for I-123 thyroid imaging and our two MPH collimators designed for I-123 DaTscan imaging for Parkinson's Disease, in comparison to three different parallel-hole collimators through a series of experiments and Monte Carlo simulations. The simulations of a point source and a digital human phantom with DaTscan activity distribution showed that our MPH collimators provide superior count performance in terms of high primary counts, low penetration, and low scatter counts compared to the parallel-hole and single pinhole collimators. For example, total scatter, multiple scatter, and collimator penetration events for the LEHR were 2.5, 7.6 and 14 times more than that of MPH within the 15% photopeak window. The total scatter fraction for LEHR was 56% where the largest contribution came from the high-energy scatter from the back compartments (31%). For the same energy window, the total scatter for MPH was 21% with only 1% scatter from the back compartments. We therefore anticipate that using MPH collimators, higher quality reconstructions can be obtained in a substantially shorter acquisition time for I-123 DaTscan and thyroid imaging.

We are developing a multi-detector pinhole-based stationary brain-dedicated SPECT system: AdaptiSPECT-C. In this work, we introduced a new design prototype with multiple adaptable pinhole apertures for each detector to modulate the multiplexing by employing temporal shuttering of apertures. Temporal shuttering of apertures over the scan time provides the AdaptiSPECT-C with the capability of multiple-frame acquisition. We investigated, through analytic simulation, the impact of projection multiplexing on image quality using several digital phantoms and a customized anthropomorphic phantom emulating brain perfusion clinical distribution. The 105 pinholes in the collimator of the system were categorized into central, axial, and lateral apertures. We generated, through simulation, collimators of different multiplexing levels. Several data acquisition schemes were also created by changing the imaging time share of the acquisition frames. Sensitivity increased by 35% compared to the single-pinhole-per-detector base configuration of the AdaptiSPECT-C when using the central, axial, and lateral apertures with equal acquisition time shares within a triple-frame scheme with a high multiplexing scenario. Axial and angular sampling of the base configuration was enhanced by adding the axial and lateral apertures. We showed that the temporal shuttering of apertures can be exploited, trading the sensitivity, to modulate the multiplexing and to acquire a set of non-multiplexed non-truncated projections. Our results suggested that reconstruction benefited from utilizing both non-multiplexed projections and projections with modulated multiplexing resulting in a noticeably reduction in the multiplexing-induced image artefacts. Contrast recovery factor improved by 20% (9%) compared to the base configuration for a Defrise (hot-rod) phantom study when the central and axial (lateral) apertures with equal time shares were combined. The results revealed that, as an overall trend at each simulated multiplexing level, lowest normalized root-mean-square errors for the brain gray-matter regions were achieved with the combined usage of the central apertures and axial/lateral apertures.

Simulation studies have been essential for development of SPECT imaging systems. GATE is one of the most commonly used simulation toolkits in nuclear medicine. This software package allows the users to build system geometries and phantoms based on primitive objects such as cylinder, sphere, and cube. However, modeling systems with complex geometry is challenging, if not impossible using these primitive volumes. The latest GATE release addressed this issue by allowing the users to import surface meshes created in a computer aided design software thus enabling accurate simulation of complex system or phantom geometries. In this study we present our GATE mesh-based simulations of a next-generation multi-pinhole SPECT system for the clinical brain imaging, called AdaptiSPECT-C. An additional challenge with the AdaptiSPECT-C is that the volume of the standard voxelized XCAT phantom overlaps with the spherical collimator plate. In order to address this issue, we developed a mesh modeling of the XCAT human phantom by directly using the native XCAT nurbs data, which also provided a more accurate representation of the anatomy. Two approaches for simulating mesh-based activity source were developed and evaluated. The first method consisted of using an acceptance/rejection criterion confining a cubical source into the mesh object and the second one was based on a conversion of a mesh-based volume into a voxelized object. Although the two strategies led to very similar results, the voxelized-mesh approach was significantly faster in computation time. We successfully imported and simulated in GATE a complete SPECT acquisition incorporating an STL representation of system, phantom anatomy, and activity source.

Collimator penetration, down-scatter related to 123I high energy photons, and scatter within detectors can significantly degrade the imaging performance of any system. Precise selection of pinhole and collimator parameters using simulation studies has the potential to considerably reduce these effects. This type of investigation, thus represents an essential step in system development. An innovative multi-pinhole system, AdaptiSPECT-C, dedicated to clinical brain SPECT imaging, is currently under development at the universities of Massachusetts and Arizona. The aim of this work was to determine the system parameters which considerably improve AdaptiSPECT- C imaging performance for the criteria of sensitivity and relative amounts of scatter and penetration. A 20 mm thick, tungsten alloy collimator leads to the best trade-off between performance and price in case of 123I imaging. Tungsten alloy provided performance relatively close to that of gold in terms of stopping power as compared to lead alloy. A pinhole center distance of 0.5 cm to the aperture entry port led to the best compromise for locating the aperture within the aperture plate in terms of sensitivity and relative amounts of scatter and penetration.

AdaptiSPECT-C, a multi-pinhole, multi-detector, and stationary brain SPECT imaging system is being developed by the University of Massachusetts Medical School and the University of Arizona. During AdaptiSPECT-C acquisitions, the patient’s head will be positioned entirely inside a hemi-spherical collimator assembly. A popular method to mitigate head motion during the protracted acquisitions is to use external cameras for tracking the motion of the head throughout the acquisition. This motion estimate is then used during SPECT reconstruction to compensate for the motion. Unfortunately, the close proximity of the bore to the patient will occlude the motion tracking camera’s view of the patient from outside the bore, thereby posing a significant challenge to using such methods. Therefore, we propose to use small-baseline stereo depth-sensing cameras placed inside the AdaptiSPECT-C bore, to track patient’s head motion. In this study, using an imaging phantom, we measure the manually generated phantom motion at a distance of about 15 cm from small baseline stereo depth sensing cameras. The motion is measured simultaneously with Optitrack motion tracking system, a commercially available marker based motion tracking system to provide a reference ground truth. Finally, we showed that at close range, we can utilize the 3D surface estimates generated by the small baseline stereo depth sensing cameras, for aligning a preexisting CT to the patient’s SPECT acquisition in order to provide attenuation correction during SPECT reconstruction. Future work will include reconstruction simulations that incorporate motion correction based on actual motion data from the tested cameras as well as providing CT alignment for CT-based attenuation correction.

PURPOSE: Single-photon emission computed tomography (SPECT) is a noninvasive imaging modality, used in myocardial perfusion imaging. The challenges facing the majority of clinical SPECT systems are low sensitivity, poor resolution, and the relatively high radiation dose to the patient. New generation systems (GE Discovery, DSPECT) dedicated to cardiac imaging improve sensitivity by a factor of 5–8. This improvement can be used to decrease acquisition time and/or dose. However, in the case of ultra-low dose (~3 mCi) injections, acquisition times are still significantly long, taking 10–12 min. The purpose of this work is to investigate a new gamma camera design with 21 hemi-ellipsoid detectors each with a pinhole collimator for cardiac SPECT for further improvement in sensitivity and resolution and reduced patient exposures and imaging times. METHODS: To evaluate the resolution of our hemi-ellipsoid system, GATE Monte-Carlo simulations were performed on point-sources, rod-sources, and NCAT phantoms. For average full-width-half-maximum (FWHM) equivalence with base flat-detector, the pinhole-diameter for the curved hemi-ellipsoid detector was found to be 8.68 mm, an operating pinhole-diameter nominally expected to be ~3 times more sensitive than state-of-the-art systems. Rod-sources equally spaced within the region of interest were acquired with a 21-detector system and reconstructed with our multi-pinhole (MPH) iterative OSEM algorithm with collimator resolution recovery. The results were compared with the results of a state-of-the-art system (GE Discovery) available in the literature. The system was also evaluated using the mathematical anthropomorphic NCAT (NURBS-based Cardiac Torso; Segars et al. IEEE Trans Nucl Sci. 1999;46:503–506) phantom with a full (clinical)-dose acquisition (25 mCi) for 2 min and an ultra-low dose acquisition of 3 mCi for 5.44 min. The estimated left ventricle (LV) counts were compared with the available literature on a state-of-the-art system (DSPECT). FWHM of the LV wall on MPH-OSEM-reconstructed images with collimator resolution recovery was estimated. RESULTS: On acquired rod-sources, the average resolution (FWHM) after reconstruction with resolution recovery in the entire region of interest (ROI) for cardiac imaging was on the average 4.44 mm (±2.84), compared to 6.9 mm (±1 mm) reported for GE Discovery (Kennedy et al., J Nucl Cardiol. 2014:21:443–452). For NCAT studies, improved sensitivity allowed a full-dose (25 mCi) 2-min acquisition (Ell8.68mmFD) which yielded 3.79 M LV counts. This is ~3.35 times higher compared to 1.13 M LV counts acquired in 2 min for clinical full dose for state-of-the-art DSPECT. The increased sensitivity also allowed an ultra-low dose acquisition protocol (Ell8.68 mmULD), 3 mCi (eight times less injected dose) in 5.44 min. This ultra-low dose protocol yielded ~1.23 M LV counts which was comparable to the full-dose 2-min acquisition for DSPECT. The estimated NCAT average FWHM at the LV wall after 12 iterations of the OSEM reconstruction was 4.95 and 5.66 mm around the mid-short-axis slices for Ell8.68mmFD and Ell8.68mmULD, respectively. CONCLUSION: Our Monte-Carlo simulation studies and reconstruction suggest using (inverted wineglass sized) hemi-ellipsoid detectors with pinhole collimators can increase the sensitivity ~3.35 times over the new generation of dedicated cardiac SPECT systems, while also improving the reconstructed resolution for rod-sources with an average of 4.44 mm in region of interest. The extra sensitivity may be used for ultra-low dose imaging (3 mCi) at ~5.44 min for comparable clinical counts as state-of-the-art systems. Key words: curved detectors, high-performance cardiac SPECT, multi-pinhole cardiac SPECT

Most brain SPECT imaging procedures are currently being performed using general-purpose systems which are unable to fully take advantage of the use of clinically available agents. We are designing a novel multi-detector, multi-pinhole modular dedicated brain SPECT imaging system called AdaptiSPECT-C to improve sensitivity and resolution, and address the static and dynamic imaging needs. The aim of this study was to evaluate the axial and angular sampling sufficiency of a preliminary design of the system using simulation of the XCAT and a customized Defrise phantom. The simulator as well as image reconstruction projector is based on analytical modeling. The results provided an insight into the axial and angular sampling of the region-ofinterest of the AdaptiSPECT-C system and possible approaches to enhance the image quality in this regard showing that the application of approaches for increasing axial and angular samples including multipinhole shattering concept can enhance the quality of the reconstructed images.

A next-generation, adaptive, brain-imaging, single photon emission computed tomography system is currently under development at the Universities of Arizona and Massachusetts. The proposed multi-pinhole based modular gamma camera configuration enables the acquisition of “quasi-vertex” views i.e., views close to the vertex. This study is concerned with understanding how activity inferior to the brain will influence these views and ultimately, the reconstruction of the volume of interest. Analytical models can provide some measure of the detected primary gamma radiation originating in an organ or tissue outside the region of interest. Scattered gamma radiation will be detected as well but given the difficulty in modeling such phenomena, Monte Carlo simulations are used to quantify its effect. Using computer generated phantoms, the influence of activity from organs and tissues in the thorax, neck and lower head as compared to that from brain structures in the quasivertex views, will be detected, identified and quantified. The simulation consists of two components: detectors and sources. The detectors were simulated gamma camera modules consisting of a tungsten pinhole collimator, an air gap, and a NaI(Tl) scintillator surrounded by lead shielding. The sources were phantoms with activity (I-123, primary gamma photons at 159 keV) set in the liver, lungs, striatum, salivary glands and the thyroid.

Herein we report on the mathematical modeling of the simulated point spread functions (PSFs) of pinhole apertures for clinical I-123 DaTscan imaging on a dual-head SPECT system consisting of fan and multi-pinhole (MPH) collimators on separate heads. The PSFs can be measured sparsely by translating a point source within the volume of interest (VOI). These PSFs were generated using GATE Monte Carlo simulation software and were then modeled using standard 2D Gaussian having 6 parameters, and three other models using higher order polynomial terms as well as cross terms in the exponential. The goal is to efficiently store the parameters of the modeled PSF, measured across the VOI and then interpolate them on the fly during reconstruction. It has been shown that MPH reconstruction can be improved with accurate modeling of the PSF. However, for our application it has been determined that improved accuracy in PSF modeling (reduced NRMSE) can be obtained by incorporating more polynomial terms in the exponential than employed by the standard 2D Gaussian, especially with increased pinhole angulations. In this paper we introduce higher order polynomial terms (degree 3 and 4) as an extension to the Gaussian model and observe that these added terms could significantly reduce the NRMSE.

A mechanical design is reported for constructing a multi-pinhole (MPH) collimator that will be used with a fanbeam collimator for high spatial resolution and sensitivity imaging of the striatal region of patients suspected of having Parkinson's disease. The design strategy was to alter an existing commercial pinhole collimator by adding the MPH components while keeping the portion of the collimator that attaches to the head of the gamma-camera. The MPH array consists of nine apertures focused at a common point at the center of the striatal region of the brain with their entrance and exit ports such that no overlapping occurs of their projections on the detector of the gamma-camera. A shutter mechanism is included at the side of the aperture plate towards the detector which will allow opening an additional eight apertures to image with increased sensitivity, but with the projections multiplexed with those of the original nine pinholes. Testing of the material properties of the alloys used in construction of the MPH collimator provided validated values. These were used in a SolidWorks simulation of material deformation due to gravity during imaging to ensure safety and imaging accuracy. The SolidWorks CAD design of the MPH collimator was imported into the GATE Monte Carlo package to simulate brain imaging and to test striatal imaging with the model geometry.

Earlier, we proposed an inexpensive method to improve the performance of the conventional dual-head SPECT systems for I-123 dopamine transporter (DAT) imaging of Parkinson Disease. In this approach, the collimator on one head is replaced with a multi-pinhole (MPH) collimator while retaining the conventional collimator on the other head, thus enabling combined MPH/Parallelbeam (or Fanbeam) acquisition. The original MPH design consisted of 9 apertures with rectangular entrance/exit ports covering a cylindrical volume of interest (VOI) around the striatum with minimal multiplexing. Our main goal in this work is to increase the sensitivity of this system by adding more pinholes and allowing multiplexing. For a more efficient usage of the available detector coverage, we also proposed the usage of pinholes with circular apertures and elliptical ports referred to as “lofthole” herein. Circular apertures are desired because of the uniform spatial resolution, more accurate point spread function modeling and fabrication while the elliptical ports provide more flexibility in the detector coverage compared to circular ports. In this preliminary work, we present a comparison of 9, 13, 17 and 16 pinhole configurations for conic-holes (circle bases) and loftholes (circular and elliptic bases) through projections obtained from GATE Monte Carlo software.

SPECT is primarily used in the clinic for noninvasive cardiac myocardial perfusion imaging. However, sensitivity is impaired due to the need for collimation in SPECT. New second generation systems including those with multi-pinhole collimation has shown high sensitive acquisitions. This has enabled faster or lower dose “stress-first” acquisitions, but not both. Here a third generation dedicated Cardiac SPECT system is proposed which yields higher sensitivity by a factor of ∼3.1 compared to second generation systems (and more than order of magnitude over Generation I systems), by using hemi-Ellipsoid shaped detectors with pinhole collimation. This may ultimately enable ultra-low-dose imaging (∼3mCi) in less than 4 min.

Breast CT is an emerging imaging technique that can portray the breast in 3D and improve visualization of important diagnostic features. Early clinical studies have suggested that breast CT has sufficient spatial and contrast resolution for accurate detection of masses and microcalcifications in the breast, reducing structural overlap that is often a limiting factor in reading mammographic images. For a number of reasons, image quality in breast CT may be improved by use of an energy resolving photon counting detector. In this study, the authors investigate the improvements in image quality obtained when using energy weighting with an energy resolving photon counting detector as compared to that with a conventional energy integrating detector.Using computer simulation, realistic CT images of multiple breast phantoms were generated. The simulation modeled a prototype breast CT system using an amorphous silicon (a-Si), CsI based energy integrating detector with different x-ray spectra, and a hypothetical, ideal CZT based photon counting detector with capability of energy discrimination. Three biological signals of interest were modeled as spherical lesions and inserted into breast phantoms; hydroxyapatite (HA) to represent microcalcification, infiltrating ductal carcinoma (IDC), and iodine enhanced infiltrating ductal carcinoma (IIDC). Signal-to-noise ratio (SNR) of these three lesions was measured from the CT reconstructions. In addition, a psychophysical study was conducted to evaluate observer performance in detecting microcalcifications embedded into a realistic anthropomorphic breast phantom.In the energy range tested, improvements in SNR with a photon counting detector using energy weighting was higher (than the energy integrating detector method) by 30%-63% and 4%-34%, for HA and IDC lesions and 12%-30% (with Al filtration) and 32%-38% (with Ce filtration) for the IIDC lesion, respectively. The average area under the receiver operating characteristic curve (AUC) for detection of microcalcifications was higher by greater than 19% (for the different energy weighting methods tested) as compared to the AUC obtained with an energy integrating detector.This study showed that breast CT with a CZT photon counting detector using energy weighting can provide improvements in pixel SNR, and detectability of microcalcifications as compared to that with a conventional energy integrating detector. Since a number of degrading physical factors were not modeled into the photon counting detector, this improvement should be considered as an upper bound on achievable performance.

Breast CT (BCT) using a photon counting detector (PCD) has a number of advantages that can potentially improve clinical performance. Previous computer simulation studies showed that the signal to noise ratio (SNR) for microcalcifications is higher with energy weighted photon counting BCT as compared to cesium iodide energy integrating detector (CsI-EID) based BCT. CsI-EID inherently weighs the incident x-ray photons in direct proportion to the energy (contradicting the information content) which is not an optimal approach. PCD do not inherently weigh the incident photons. By choosing optimal energy weights, higher SNR can be achieved for microcalcifications and hence better detectability. In this simulation study, forward projection data of a numerical breast phantom with microcalcifications inserted were acquired using CsI-EID and PCD. The PCD projections were optimally weighed, and reconstructed using filtered back-projection. We compared observer performance in identifying microcalcifications in the reconstructed images using ROC analysis. ROC based results show that the average area(s) under curve(s) (AUC) for AUC PCD based methods are higher than the average AUC CsI-EID method.

Photon counting x-ray detectors (PCD) promise to provide substantial improvements in computed tomography (CT) as compared to Cesium Iodide based energy integrating x-ray detectors (CsI-EID). This paper investigates the possible improvements in signal-to-noise ratio (SNR) obtained by using a cadmium zinc telluride (CZT) crystal based PCD with energy weighting (EW) in low dose breast CT (BCT).

In an effort to improve the early stage detection and diagnosis of breast cancer, a number of research groups have been investigating the use of x-ray computerized tomography (CT) systems dedicated for use in imaging the breast. For a number of reasons, the performance of energy integrating detectors are sub-optimal for use in CT imaging of the breast. It is expected that the next generation of x-ray detectors for digital radiography and CT will have the capability of counting individually measured photons and recording their energy. In this paper, we used computer simulations to evaluate improvements in image quality that can be attained using energy weighting photon counting detectors for breast CT and a lower kVp settings. Results from this study suggest that improvements in SNR performance can be attained with photon counting detectors as compared to energy integrating detectors.

Purpose: Breast CT is an emerging imaging technique that can portray the breast in 3D and improve visualization of important diagnostic features. Early clinical studies have suggested that breast CT has sufficient spatial and contrast resolution for accurate detection of masses and microcalcifications in the breast, reducing structural overlap that is often a limiting factor in reading mammographic images. For a number of reasons, image quality in breast CT may be improved by use of an energy resolving photon counting detector. In this study, the authors investigate the improvements in image quality obtained when using energy weighting with an energy resolving photon counting detector as compared to that with a conventional energy integrating detector. Methods: Using computer simulation, realistic CT images of multiple breast phantoms were generated. The simulation modeled a prototype breast CT system using an amorphous silicon (a-Si), CsI based energy integrating detector with different x-ray spectra, and a hypothetical, ideal CZT based photon counting detector with capability of energy discrimination. Three biological signals of interest were modeled as spherical lesions and inserted into breast phantoms; hydroxyapatite (HA) to represent microcalcification, infiltrating ductal carcinoma (IDC), and iodine enhanced infiltrating ductal carcinoma (IIDC). Signal-to-noise ratio (SNR) of these three lesions was measured from the CT reconstructions. In addition, a psychophysical study was conducted to evaluate observer performance in detecting microcalcifications embedded into a realistic anthropomorphic breast phantom. Results: In the energy range tested, improvements in SNR with a photon counting detector using energy weighting was higher (than the energy integrating detector method) by 30%–63% and 4%–34%, for HA and IDC lesions and 12%–30% (with Al filtration) and 32%–38% (with Ce filtration) for the IIDC lesion, respectively. The average area under the receiver operating characteristic curve (AUC) for detection of microcalcifications was higher by greater than 19% (for the different energy weighting methods tested) as compared to the AUC obtained with an energy integrating detector. Conclusions: This study showed that breast CT with a CZT photon counting detector using energy weighting can provide improvements in pixel SNR, and detectability of microcalcifications as compared to that with a conventional energy integrating detector. Since a number of degrading physical factors were not modeled into the photon counting detector, this improvement should be considered as an upper bound on achievable performance.

Measurement of environmental endotoxin exposures is complicated by variability encountered using current biological assay methods arising in part from lot-to-lot variability of the Limulus-amebocyte lysate (LAL) reagents. Therefore, we investigated the lot-to-lot repeatability of commercially available recombinant Factor C (rFC) kits as an alternative to LAL. Specifically, we compared endotoxin estimates obtained from rFC assay of twenty indoor dust samples, using four different extraction and assay media, to endotoxin estimates previously obtained by Limulus amebocyte lysate (LAL) assay and amounts of 3-hydroxy fatty acids (3-OHFA) in lipopolysaccharide (LPS) using gas-chromatography mass spectroscopy (GC-MS). We found that lot-to-lot variability of the rFC assay kits does not significantly alter endotoxin estimates in house dust samples when performed using three of the four assay media tested and that choice of assay media significantly altered endotoxin estimates obtained by rFC assay of house dust samples. Our findings demonstrate lot-to-lot reproducibility of rFC assay of environmental samples and suggest that use of rFC assay performed with Tris buffer or water as the extraction and assay medium for measurement of endotoxin in dust samples may be a suitable choice for developing a standardized methodology.