On Artifact-Free Projection Overlaps in Multi-Pinhole Tomographic Imaging

Dopamine Transporter SPECT with 12-Minute Scan Duration Using Multiple-Pinhole Collimators

This study evaluated the potential to reduce the scan duration in dopamine transporter (DAT) SPECT when using a second-generation multiple-pinhole (MPH) collimator designed for brain SPECT with improved count sensitivity and improved spatial resolution compared with parallel-hole and fanbeam collimators. Methods: The retrospective study included 640 consecutive clinical DAT SPECT studies that had been acquired in list mode with a triple-head SPECT system with MPH collimators and a 30-min net scan duration after injection of 181 ± 10 MBq of [123I]FP-CIT. Raw data corresponding to scan durations of 20, 15, 12, 8, 6, and 4 min were obtained by restricting the events to a proportionally reduced time interval of the list-mode data for each projection angle. SPECT images were reconstructed iteratively with the same parameter settings irrespective of scan duration. The resulting 5,120 SPECT images were assessed for a neurodegeneration-typical reduction in striatal signal by visual assessment, conventional specific binding ratio analysis, and a deep convolutional neural network trained on 30-min scans. Results: Regarding visual interpretation, image quality was considered diagnostic for all 640 patients down to a 12-min scan duration. The proportion of discrepant visual interpretations between 30 and 12 min (1.2%) was not larger than the proportion of discrepant visual interpretations between 2 reading sessions of the same reader at a 30-min scan duration (1.5%). Agreement with the putamen specific binding ratio from the 30-min images was better than expected for 5% test-retest variability down to a 10-min scan duration. A relevant change in convolutional neural network-based automatic classification was observed at a 6-min scan duration or less. Conclusion: The triple-head SPECT system with MPH collimators allows reliable DAT SPECT after administration of about 180 MBq of [123I]FP-CIT with a 12-min scan duration.

Improvement in sampling and modulation of multiplexing with temporal shuttering of adaptable apertures in a brain-dedicated multi-pinhole SPECT system

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.

Performance evaluation of a novel multi-pinhole collimator for dopamine transporter SPECT

There is a tradeoff between spatial resolution and count sensitivity in SPECT with conventional collimators. Multi-pinhole (MPH) collimator technology has potential for concurrent improvement of resolution and sensitivity in clinical SPECT of 'small' organs. This study evaluated a novel MPH collimator specifically designed for dopamine transporter (DAT) SPECT with a triple-head SPECT camera. Count sensitivity was measured with a ^99mTc point source placed on the lattice points of a 1 cm grid covering the whole field-of-view (FOV). Spatial resolution was assessed with a Derenzo type hot rod phantom. An anthropomorphic striatum phantom was scanned with total activity representative of a typical patient scan and different striatum-to-background activity concentration ratios. Recovery of striatum-to-background contrast was assessed by the contrast-recovery-coefficient. Measurements were repeated with double-head SPECT with fan-beam or low-energy-high-resolution-high-sensitivity (LEHRHS) collimators. A patient referred to DAT SPECT because of suspicion of Parkinson's disease was scanned with both LEHRHS and MPH collimators after a single tracer injection. The axial MPH sensitivity profile was approximately symmetrical around its peak, although it was shifted 7 cm towards the patient to simplify positioning. Peak sensitivity of the triple-head MPH system in the center of the FOV was 620 cps MBq^-1 compared to 225 cps MBq^-1 for the double-head fan-beam system. Sensitivity of the MPH system decreased towards the edges of the FOV. The full width of the sensitivity profile at 200 cps MBq^-1 was 21 cm transaxially and 11 cm axially. In MPH SPECT of the Derenzo phantom all rods with ≥ 5 mm diameter were clearly visible. MPH SPECT improved striatal contrast recovery by ≥ 20% compared to fan-beam SPECT. The patient scan demonstrated good image quality of MPH SPECT with almost PET-like delineation of putamen and caudate nucleus. SPECT with dedicated MPH collimators provides considerable improvement of the resolution-sensitivity tradeoff in DAT SPECT compared to SPECT with fan-beam or LEHRHS collimators.

A noise smoothing origin ensemble algorithm based on local filtering

An origin ensemble (OE) image reconstruction algorithm can be used for the fast reconstruction of unconventional geometrical images, e.g. in a Compton camera (CC) system. Due to the low-count rate in the emission data, the reconstructed image is often noisy and inhomogeneous in density. In this study, we propose a way to smooth out the noise in the OE algorithm. During the OE reconstruction, the algorithm stochastically modifies the current location to a random new voxel along the probable corresponding curve of each event depending on the relative event density of the new and old locations. In the original OE technique, the event density is simply the number of events in the voxel. In the proposed method, the event density is estimated from the filtering of a kernel window centered on the voxel. Incorporating the regional filtering is similar to performing an OE algorithm on a smoothed image at each iteration and enables the reconstruction of a smoother image. A Flangeless Esser PET phantom and a multi-activity phantom are used to study the property of the new reconstruction algorithm. The results indicate that the proposed method performs better than a conventional OE algorithm in terms of normalized mean square error (NMSE) and structural similarity (SSIM). Both contrast noise ratio (CNR) and reconstruction accuracy of the new method are better than the conventional OE algorithm and their performances improve with the increase of object size. The median-OE possesses the highest overall image quality and recovery rate among the three filter-OE algorithms and is the method of choice for image reconstruction. Comparing to conventional post-smoothing OEs, the NMSE of median-OE improves 57.6% (46.9%) and the SSIM increased by 73.2% (51.1%) for the Esser (multi-activity) phantom. The proposed OE algorithm is simple and efficient for noise smoothing without complex calculations and highly suited for low-count cases.

Theory and realization of a 2D high resolution and high sensitivity SPECT system with an angle-encoding attenuator pattern

The camera of the conventional SPECT system requires a collimator to allow incoming photons from a specific range of incident angle to reach the detector. It is the major factor that determines the spatial resolution of the camera. Moreover, it also greatly reduces the number of detected photons and hence increases statistical fluctuations in the acquired image data. The goal of this paper is to propose a theory and design for a novel high resolution and high sensitivity SPECT system without conventional collimators. The key is to resolve the incident photons from all directional angles and detected by every detector bin. Special 'attenuators' were designed to 'encode' the incoming photons from different directions similar to coded aperture to form projection data for image reconstruction. Each encoded angular pattern of detected photons was recorded as one measurement. Different angular patterns were achieved by changing the configurations of the attenuators so that angular pattern of different measurements or measurement matrix (MM) is invertible, which guarantee a unique reconstructed image. In simulation, the attenuators were fitted on a virtual full-ring gamma camera, as an alternative to the collimators in conventional SPECT systems. To evaluate the performance of the new SPECT system, analytical simulated projection data in 2D scenario were generated from the XCAT phantom. Noisy simulation using 100 noise realizations suggests that the new attenuator design provides much improved image quality in terms of contrast-noise trade-offs (~30% improvement). The results suggest that the new design of using attenuators to replace collimator is feasible and could potentially improve sensitivity without sacrificing resolution in today's SPECT systems.

An extension to artifact-free projection overlaps

PURPOSE

In multipinhole single photon emission computed tomography, the overlapping of projections has been used to increase sensitivity. Avoiding artifacts in the reconstructed image associated with projection overlaps (multiplexing) is a critical issue. In our previous report, two types of artifact-free projection overlaps, i.e., projection overlaps that do not lead to artifacts in the reconstructed image, were formally defined and proved, and were validated via simulations. In this work, a new proposition is introduced to extend the previously defined type-II artifact-free projection overlaps so that a broader range of artifact-free overlaps is accommodated. One practical purpose of the new extension is to design a baffle window multipinhole system with artifact-free projection overlaps.

METHODS

First, the extended type-II artifact-free overlap was theoretically defined and proved. The new proposition accommodates the situation where the extended type-II artifact-free projection overlaps can be produced with incorrectly reconstructed portions in the reconstructed image. Next, to validate the theory, the extended-type-II artifact-free overlaps were employed in designing the multiplexing multipinhole spiral orbit imaging systems with a baffle window. Numerical validations were performed via simulations, where the corresponding 1-pinhole nonmultiplexing reconstruction results were used as the benchmark for artifact-free reconstructions. The mean square error (MSE) was the metric used for comparisons of noise-free reconstructed images. Noisy reconstructions were also performed as part of the validations.

RESULTS

Simulation results show that for noise-free reconstructions, the MSEs of the reconstructed images of the artifact-free multiplexing systems are very similar to those of the corresponding 1-pinhole systems. No artifacts were observed in the reconstructed images. Therefore, the testing results for artifact-free multiplexing systems designed using the extended type-II artifact-free overlaps numerically validated the developed theory.

CONCLUSIONS

First, the extension itself is of theoretical importance because it broadens the selection range for optimizing multiplexing multipinhole designs. Second, the extension has an immediate application: using a baffle window to design a special spiral orbit multipinhole imaging system with projection overlaps in the orbit axial direction. Such an artifact-free baffle window design makes it possible for us to image any axial portion of interest of a long object with projection overlaps to increase sensitivity.

Design of a Multi-Pinhole Collimator for I-123 DaTscan Imaging on Dual-Headed SPECT Systems in Combination with a Fan-Beam Collimator

For the 2011 FDA approved Parkinson's Disease (PD) SPECT imaging agent I-123 labeled DaTscan, the volume of interest (VOI) is the interior portion of the brain. However imaging of the occipital lobe is also required with PD for calculation of the striatal binding ratio (SBR), a parameter of significance in early diagnosis, differentiation of PD from other disorders with similar clinical presentations, and monitoring progression. Thus we propose the usage of a combination of a multi-pinhole (MPH) collimator on one head of the SPECT system and a fan-beam on the other. The MPH would be designed to provide high resolution and sensitivity for imaging of the interior portion of the brain. The fan-beam collimator would provide lower resolution but complete sampling of the brain addressing data sufficiency and allowing a volume-of-interest to be defined over the occipital lobe for calculation of SBR's. Herein we focus on the design of the MPH component of the combined system. Combined reconstruction will be addressed in a subsequent publication. An analysis of 46 clinical DaTscan studies was performed to provide information to define the VOI, and design of a MPH collimator to image this VOI. The system spatial resolution for the MPH was set to 4.7 mm, which is comparable to that of clinical PET systems, and significantly smaller than that of fan-beam collimators employed in SPECT. With this set, we compared system sensitivities for three aperture array designs, and selected the 3 × 3 array due to it being the highest of the three. The combined sensitivity of the apertures for it was similar to that of an ultra-high resolution fan-beam (LEUHRF) collimator, but smaller than that of a high-resolution fan-beam collimator (LEHRF). On the basis of these results we propose the further exploration of this design through simulations, and the development of combined MPH and fan-beam reconstruction.

Enhanced PET resolution by combining pinhole collimation and coincidence detection

Spatial resolution of clinical PET scanners is limited by detector design and photon non-colinearity. Although dedicated small animal PET scanners using specialized high-resolution detectors have been developed, enhancing the spatial resolution of clinical PET scanners is of interest as a more available alternative. Multi-pinhole 511 keV SPECT is capable of high spatial resolution but requires heavily shielded collimators to avoid significant background counts. A practical approach with clinical PET detectors is to combine multi-pinhole collimation with coincidence detection. In this new hybrid modality, there are three locations associated with each event, namely those of the two detected photons and the pinhole aperture. These three locations over-determine the line of response and provide redundant information that is superior to coincidence detection or pinhole collimation alone. Multi-pinhole collimation provides high resolution and avoids non-colinearity error but is subject to collimator penetration and artifacts from overlapping projections. However the coincidence information, though at lower resolution, is valuable for determining whether the photon passed near a pinhole within the cone acceptance angle and for identifying through which pinhole the photon passed. This information allows most photons penetrating through the collimator to be rejected and avoids overlapping projections. With much improved event rejection, a collimator with minimal shielding may be used, and a lightweight add-on collimator for high resolution imaging is feasible for use with a clinical PET scanner. Monte Carlo simulations were performed of a (18)F hot rods phantom and a 54-pinhole unfocused whole-body mouse collimator with a clinical PET scanner. Based on coincidence information and pinhole geometry, events were accepted or rejected, and pinhole-specific crystal-map projections were generated. Tomographic images then were reconstructed using a conventional pinhole SPECT algorithm. Hot rods of 1.4 mm diameter were resolved easily in a simulated phantom. System sensitivity was 0.09% for a simulated 70-mm line source corresponding to the NEMA NU-4 mouse phantom. Higher resolution is expected with further optimization of pinhole design, and higher sensitivity is expected with a focused and denser pinhole configuration. The simulations demonstrate high spatial resolution and feasibility of small animal imaging with an add-on multi-pinhole collimator for a clinical PET scanner. Further work is needed to develop geometric calibration and quantitative data corrections and, eventually, to construct a prototype device and produce images with physical phantoms.

Evaluation of Fisher Information Matrix-Based Methods for Fast Assessment of Image Quality in Pinhole SPECT

The accurate determination of the local impulse response and the covariance in voxels from penalized maximum likelihood reconstructed images requires performing reconstructions from many noise realizations of the projection data. As this is usually a very time-consuming process, efficient analytical approximations based on the Fisher information matrix (FIM) have been extensively used in PET and SPECT to estimate these quantities. For 3D imaging, however, additional approximations need to be made to the FIM in order to speed up the calculations. The most common approach is to use the local shift-invariant (LSI) approximation of the FIM, but this assumes specific conditions which are not always necessarily valid. In this paper we take a single-pinhole SPECT system and compare the accuracy of the LSI approximation against two other methods that have been more recently put forward: the non-uniform object-space pixelation (NUOP) and the subsampled FIM. These methods do not assume such restrictive conditions while still increasing the speed of the calculations considerably. Our results indicate that in pinhole SPECT the NUOP and subsampled FIM approaches could be more reliable than the LSI approximation, especially when a high accuracy is required.

Review of SPECT collimator selection, optimization, and fabrication for clinical and preclinical imaging

In single photon emission computed tomography, the choice of the collimator has a major impact on the sensitivity and resolution of the system. Traditional parallel-hole and fan-beam collimators used in clinical practice, for example, have a relatively poor sensitivity and subcentimeter spatial resolution, while in small-animal imaging, pinhole collimators are used to obtain submillimeter resolution and multiple pinholes are often combined to increase sensitivity. This paper reviews methods for production, sensitivity maximization, and task-based optimization of collimation for both clinical and preclinical imaging applications. New opportunities for improved collimation are now arising primarily because of (i) new collimator-production techniques and (ii) detectors with improved intrinsic spatial resolution that have recently become available. These new technologies are expected to impact the design of collimators in the future. The authors also discuss concepts like septal penetration, high-resolution applications, multiplexing, sampling completeness, and adaptive systems, and the authors conclude with an example of an optimization study for a parallel-hole, fan-beam, cone-beam, and multiple-pinhole collimator for different applications.

Longitudinal in vivo evaluation of bone regeneration by combined measurement of multi-pinhole SPECT and micro-CT for tissue engineering

Over the last decades, great strides were made in the development of novel implants for the treatment of bone defects. The increasing versatility and complexity of these implant designs request for concurrent advances in means to assess in vivo the course of induced bone formation in preclinical models. Since its discovery, micro-computed tomography (micro-CT) has excelled as powerful high-resolution technique for non-invasive assessment of newly formed bone tissue. However, micro-CT fails to provide spatiotemporal information on biological processes ongoing during bone regeneration. Conversely, due to the versatile applicability and cost-effectiveness, single photon emission computed tomography (SPECT) would be an ideal technique for assessing such biological processes with high sensitivity and for nuclear imaging comparably high resolution (<1 mm). Herein, we employ modular designed poly(ethylene glycol)-based hydrogels that release bone morphogenetic protein to guide the healing of critical sized calvarial bone defects. By combined in vivo longitudinal multi-pinhole SPECT and micro-CT evaluations we determine the spatiotemporal course of bone formation and remodeling within this synthetic hydrogel implant. End point evaluations by high resolution micro-CT and histological evaluation confirm the value of this approach to follow and optimize bone-inducing biomaterials.

The evaluation of data completeness and image quality in multiplexing multi-pinhole SPECT

Multi-pinhole collimators are often used in pre-clinical SPECT systems because they have a better resolution-sensitivity tradeoff than parallel hole collimators when imaging small objects. Most multi-pinhole collimators are designed to allow no or only a limited amount of overlap between the different pinhole projections because the ambiguity introduced by multiplexing pinholes can result in artifacts. The origin of these artifacts is still not fully understood, but previous research has already shown that data incompleteness could be part of the explanation. Therefore, we developed a method to investigate data completeness in multiplexing multi-pinhole systems and showed that a certain activity distribution can be successfully reconstructed when the nonmultiplexed data is complete or when the overlap can be sufficiently de-multiplexed. We validated this method using computer simulated phantom data of different multiplexing systems. We also studied contrast-to-noise and nonprewhitening matched filter signal-to-noise ratio (NPW-SNR) to compare the image quality in a single pinhole system with multiplexing systems. We found that our method can indeed be used to evaluate data completeness in multiplexing systems and found no artifacts in the systems that had complete data. Sensitivity increased significantly with multiplexing but we found only small, nonsignificant differences in contrast-to-noise ratio. However, the NPW-SNR did slightly improve in the multiplexing setups. We conclude that more multiplexing does not necessarily result in more artifacts and that even a high amount of multiplexing can still result in artifact-free images if the nonmultiplexed data is complete or when the overlap can be sufficiently de-multiplexed.

Reducing multiplexing artifacts in multi-pinhole SPECT with a stacked silicon-germanium system: a simulation study

In pinhole single photon emission computed tomography (SPECT), multi-pinhole collimators can increase sensitivity but may lead to projection overlap, or multiplexing, which can cause image artifacts. In this work, we explore whether a stacked-detector configuration with a germanium and a silicon detector, used with 123I (27-32, 159 keV), where little multiplexing occurs in the Si projections, can reduce image artifacts caused by highly-multiplexed Ge projections. Simulations are first used to determine a reconstruction method that combines the Si and Ge projections to maximize image quality. Next, simulations of different pinhole configurations (varying projection multiplexing) in conjunction with digital phantoms are used to examine whether additional Si projections mitigate artifacts from the multiplexing in the Ge projections. Reconstructed images using both Si and Ge data are compared to those using Ge data alone. Normalized mean-square error and normalized standard deviation provide a quantitative evaluation of reconstructed images' error and noise, respectively, and are used to evaluate the impact of the additional nonmultiplexed data on image quality. For a qualitative comparison, the differential point response function is used to examine multiplexing artifacts. Results show that in cases of highly-multiplexed Ge projections, the addition of low-multiplexed Si projections helps to reduce image artifacts both quantitatively and qualitatively.