Optimization of geometrical calibration in pinhole SPECT

Previously, we developed a method to determine the acquisition geometry of a pinhole camera. This information is needed for the correct reconstruction of pinhole single photon emission computed tomography images. The method uses a calibration phantom consisting of three point sources and their positions in the field of view (FOV) influence the accuracy of the geometry estimate. This paper proposes two particular configurations of point sources with specific positions and orientations in the FOV for optimal image reconstruction accuracy. For the proposed calibration setups, inaccuracies of the geometry estimate due to noise in the calibration data, only cause subresolution inaccuracies in reconstructed images. The calibration method also uses a model of the point source configuration, which is only known with limited accuracy. The study demonstrates, however, that, with the proposed calibration setups, the error in reconstructed images is comparable to the error in the phantom model.     

Perturbative refinement of the geometric calibration in pinhole SPECT

The paper investigates the geometric calibration of a rotating gamma camera for pinhole (PH) single photon emission computed tomography (SPECT) imaging. Most calibration methods previously applied in PH-SPECT assume that the motion of the camera around the object belongs to a well-defined class described by a small number of geometric parameters, for instance seven parameters for a circular acquisition with a single pinhole camera. The proposed new method refines an initial parametric calibration by applying to each position of the camera a rigid body transformation that is determined to improve the fit between the measured and calculated projections of the calibration sources. A stable estimate of this transformation can be obtained with only three calibration sources by linearizing the equations around the position estimated by the initial parametric calibration. The performance of the method is illustrated using simulated and measured micro-SPECT data.     

Maximum likelihood reconstruction for pinhole SPECT with a displaced center-of-rotation

The authors describe the implementation of a maximum likelihood (ML) algorithm using expectation maximization (EM) for pinhole SPECT with a displaced center-of-rotation. A ray-tracing technique is used in implementing the ML-EM algorithm. The proposed ML-EM algorithm is able to correct the center of rotation displacement which can be characterized by two orthogonal components. The algorithm is tested using experimentally acquired data, and the results demonstrate that the pinhole ML-EM algorithm is able to correct artifacts associated with the center-of-rotation displacement.     

System calibration and statistical image reconstruction for ultra-high resolution stationary pinhole SPECT

For multipinhole single-photon emission computed tomography (SPECT), iterative reconstruction algorithms are preferred over analytical methods, because of the often complex multipinhole geometries and the ability of iterative algorithms to compensate for effects like spatially variant sensitivity and resolution. Ideally, such compensation methods are based on accurate knowledge of the position-dependent point spread functions (PSFs) specifying the response of the detectors to a point source at every position in the instrument. This paper describes a method for model-based generation of complete PSF lookup tables from a limited number of point-source measurements for stationary SPECT systems and its application to a submillimeter resolution stationary small-animal SPECT system containing 75 pinholes (U-SPECT-I). The method is based on the generalization over the entire object to be reconstructed, of a small number of properties of point-source responses which are obtained at a limited number of measurement positions. The full shape of measured point-source responses can almost be preserved in newly created PSF tables. We show that these PSFs can be used to obtain high-resolution SPECT reconstructions: the reconstructed resolutions judged by rod visibility in a micro-Derenzo phantom are 0.45 mm with 0.6-mm pinholes and below 0.35 mm with 0.3-mm pinholes. In addition, we show that different approximations, such as truncating the PSF kernel, with significant reduction of reconstruction time, can still lead to acceptable reconstructions.      

Molecular imaging of small animals with a triple-head SPECT system using pinhole collimation

Pinhole collimation yields high sensitivity when the distance from the object to the aperture is small, as in the case of imaging small animals. Fine-resolution images may be obtained when the magnification is large since this mitigates the effect of detector resolution. Large magnifications in pinhole single-photon emission computed tomography (SPECT) may be obtained by using a collimator whose focal length is many times the radius of rotation. This may be achieved without truncation if the gamma camera is large. We describe a commercially available clinical scanner mated with pinhole collimation and an external linear stage. The pinhole collimation gives high magnification. The linear stage allows for helical pinhole SPECT. We have used the system to image radiolabeled molecules in phantoms and small animals.     

Determination of mechanical and electronic shifts for pinhole SPECT using a single point source

The effects of uncompensated electronic and mechanical shifts may compromise the resolution of pinhole single photon emission computed tomography. The resolution degradation due to uncompensated shifts is estimated through simulated data. A method for determining the transverse mechanical and axial electronic shifts is described and evaluated. This method assumes that the tilt of the detector and the radius of rotation (ROR) are previously determined using another method. When this assumption is made, it is possible to determine the rest of the calibration parameters using a single point source. A method that determines the electronic and mechanical shifts as well as the tilt has been previously described; this method requires three point sources. It may be reasonable in most circumstances to calibrate tilt much less frequently than the mechanical shifts since the tilt is a property of the scanner whereas the mechanical shift may change every time the collimator is replaced. An alternative method for determining the ROR may also be used. Lastly, we take the view that the transverse electronic shift and the focal length change slowly and find these parameters independently.     

Simulated annealing image reconstruction method for a pinhole aperture single photon emission computed tomograph (SPECT)

A series of computer experiments was performed to determine the relative performance of simulated annealing, quenched annealing, and a least-squares iterative technique for image reconstruction for single photon emission computed tomography (SPECT). The simulated SPECT geometry was of the pinhole aperture type, with 32 pinholes and 128 or 512 detectors. To test the robustness of the reconstruction techniques upon arbitrary geometries, a 360-detector geometry with a random pixel-detector-factor matrix was tested. Eight computer-simulated, 10-cm-diameter planar phantoms were used with 1961 2-mm(2) reconstruction bins and a range of 3000 to 50,000,000 detected photon counts. Reconstruction quality was measured by a normalized, squared error picture distance measure. Over a wide range of noise, the simulated annealing method had slightly better reconstruction quality than the iterative method, although requiring greater reconstruction time. Quenched annealing was faster than simulated annealing, with comparable reconstruction quality. Methods of efficiently controlling the simulated annealing algorithm are presented.     

Optimized acquisition time and image sampling for dynamic SPECT ofTl-201

With the recent development in scatter and attenuation correction algorithms, dynamic single photon emission computerized tomography (SPECT) can potentially yield physiological parameters, with tracers exhibiting suitable kinetics such as thallium-201 (Tl-201). A systematic way is proposed to investigate the minimum data acquisition times and sampling requirements for estimating physiological parameters with quantitative dynamic SPECT. Two different sampling schemes were investigated with Monte Carlo simulations: (1) Continuous data collection for total study duration ranging from 30-240 min. (2) Continuous data collection for first 10-45 min followed by a delayed study at approximately 3 h. Tissue time activity curves with realistic noise were generated from a mean plasma time activity curve and rate constants (K₁-k₄) derived from Tl-201 kinetic studies in 16 dogs. Full dynamic sampling schedules (DynSS) were compared to optimum sampling schedules (OSS). The authors found that OSS can reliably estimate the blood flow related K₁ and V_d comparable to DynSS. A 30-min continuous collection was sufficient if only K₁ was of interest. A split session schedule of a 30-min dynamic followed by a static study at 3 h allowed reliable estimation of both K₁ and V_d avoiding the need for a prolonged (>60-min) continuous dynamic acquisition. The methodology developed should also be applicable to optimizing sampling schedules for other SPECT tracers     

Characterization issues of gate geometry in multifinger structurefor RF-SOI MOSFETs

In this letter, we propose that f_T and f_max properties in partially depleted SOI devices were analyzed in terms of gate electrode layout, body instability, and body resistance of multifinger structure. The speed characteristics were a strong function of the number of fingers and gate types such as T-gate and H-gate structures and were evaluated by g_m variation (Δg_m ), extra parasitic capacitance (ΔC_gs), and ac body instability     

X ray scanning pinhole microscope

A simple absorption-type scanning pinhole X ray microscope is presented. Owing to its simple structure, it may find applications in industrial environments. The resolution is determined by the size of pinholes used.     

Modeling and simulation of pinhole channels

This article presents a general pinhole channel reference model based on the cause of pinhole effect. On the base of this reference model, a deterministic simulation model is developed by keeping the difference of correlation properties between reference and simulation model as smaller as possible. The correlation properties include temporal autocorrelation function (ACF), two-dimensional (2-D) space cross-correlation function (CCF) and frequency correlation function (FCF). The results show that although pinhole channel has good correlation properties, the channel capacity is very low because of the low rank of channel transform matrix.     

Resolution-effective diameters for asymmetric-knife-edge pinhole collimators

The effects of penetration are included in the formulas for the prediction of the resolution of pinhole collimators through the use of effective diameters. Expressions of the resolution-effective diameter for pinholes with a double-knife-edge (DKE) profile are available in the literature. In this paper the expressions applicable to asymmetric-knife-edge (AKE) profiles, which include the important case of the single-knife-edge (SKE), are presented. Results indicate that the simplest methods that are still accurate in the calculation of DKE effective diameters do not produce in general formulas with similar accuracy for AKE profiles, due to increased susceptibility to penetration. Especially at high energy (365 keV), for the SKE case more advanced formulas are necessary and were, therefore, derived.     

Single Event Transient acquisition and mapping for space device Characterization

It is necessary for space applications to evaluate the sensitivity of electronic devices to radiations. It was demonstrated that radiations can cause different types of effects to the devices and possibly damage them [1] [2]. The interest in the effect of Single Event Transient (SET) has recently risen because of the increased ability of parasitic signals to propagate through advanced circuit with gate lengths shorter than 0.65 nm and to reach memory elements (in this case they become Single Event Upset (SEUs)). Analog devices are especially susceptible to perturbations by such events which can induce severe consequences, from simple artifacts up to the permanent fail of the device. This kinds of phenomena are very difficult to detect and to acquire, because they are not periodical. Furthermore, they can vary a lot depending on different parameters such as device technology and biasing. The main obstacle for the analysis is due to the maximum frequency of these signals, which is unknown. It is consequently difficult to set a correct sample frequency for the acquisition system. In this document a methodology to evaluate SETs in analog devices is presented. This method allows to acquire automatically these events and to easily study the sensitivity of the device by analyzing a “SETs cartography”. The advantages are different: it allows to easily acquire and analyze the SETs in an automatic way; the obtained results allow the user to accurately characterize the device under test; and, finally, the costs due to the implementation of the tests are lower than a classical analysis performed by a particle accelerator.     

Characterization of a distortion-corrected Nd:YAG laser with aself-conjugating loop geometry

A detailed experimental and theoretical characterization of a self-adaptive solid-state laser is presented. The system uses a saturable gain medium (Nd:YAG amplifier) as the adaptive element in an externally injected self-intersecting loop geometry. We demonstrate energy output >300 mJ, high energy reflectivity >10⁴, low input energy threshold of ~5 μJ, and phase-conjugate properties of the system that compensate for both intracavity and extracavity phase distortions. The spatial output beam size is compared to a Gaussian mode analysis based on ABCD ray transfer matrices. The temporal, spectral, energy, and threshold characteristics are compared to one-dimensional analytical and transient numerical simulations     

A pinhole model for metal-insulator-semiconductor solar cells

Based upon a consideration of the standard theory of MIS solar cells and the experimental results on such devices, the concept of tunneling through the thin insulating layer as the controlling mechanism is rejected. In place of the tunneling mechanism the concept of parallel Schottky diodes through pinholes in the insulator is proposed. The mathematical formulation fits this proposal. The characteristics and limits of efficiency expected for such a pinhole MIS are explored and found to be in good accord with existing results on experimental diodes. The equations needed to specify the density and size of the pinholes needed for optimal efficiency are formulated but not solved. A suggestion to test the hypothesis by artificially producing pinhole devices is proposed.     

Automatic construction of 3D-ASM intensity models by simulating image acquisition: application to myocardial gated SPECT studies

Active shape models bear a great promise for model-based medical image analysis. Their practical use, though, is undermined due to the need to train such models on large image databases. Automatic building of point distribution models (PDMs) has been successfully addressed and a number of autolandmarking techniques are currently available. However, the need for strategies to automatically build intensity models around each landmark has been largely overlooked in the literature. This work demonstrates the potential of creating intensity models automatically by simulating image generation. We show that it is possible to reuse a 3D PDM built from computed tomography (CT) to segment gated single photon emission computed tomography (gSPECT) studies. Training is performed on a realistic virtual population where image acquisition and formation have been modeled using the SIMIND Monte Carlo simulator and ASPIRE image reconstruction software, respectively. The dataset comprised 208 digital phantoms (4D-NCAT) and 20 clinical studies. The evaluation is accomplished by comparing point-to-surface and volume errors against a proper gold standard. Results show that gSPECT studies can be successfully segmented by models trained under this scheme with subvoxel accuracy. The accuracy in estimated LV function parameters, such as end diastolic volume, end systolic volume, and ejection fraction, ranged from 90.0% to 94.5% for the virtual population and from 87.0% to 89.5% for the clinical population.      

Analytic determination of pinhole collimator sensitivity with penetration

Pinhole collimators are widely used to image small organs and animals. The sensitivity of knife-edge pinhole collimators has been previously estimated using an "effective diameter" formulation and experimentally described using a sin(x) theta fit, where theta is the angle between the line segment from the center of the aperture to the photon source and its projection onto the plane of the aperture. An analytic form of the sensitivity of the pinhole collimator is derived in this paper. A numerical formula for predicting the sin(x) theta form of the sensitivity is calculated from the analytic form. Experimental data are compared with the theoretical estimate and the sin(x) theta prediction. The agreement is excellent.