Detector Response Model Research Articles

Evaluation of 3D Monte Carlo-Based Scatter Correction for 201Tl Cardiac Perfusion SPECT

(201)Tl-Chloride ((201)Tl) is a myocardial perfusion SPECT agent with excellent biochemical properties commonly used for assessing tissue viability. However, cardiac (201)Tl SPECT images are severely degraded by photons scattered in the thorax. Accurate correction for this scatter is complicated by the nonuniform density and varied sizes of thoraxes, by the additional attenuation and scatter caused by female patients' breasts, and by the energy spectrum of (201)Tl. Monte Carlo simulation is a general and accurate method well suited to modeling this scatter. Statistical reconstruction that includes Monte Carlo modeling of scatter was compared with statistical reconstruction algorithms not corrected for scatter. In the ADS method, corrections for attenuation, detector response, and scatter (Monte Carlo-based) were implemented simultaneously via the dual-matrix ordered-subset expectation maximization algorithm with a Monte Carlo simulator as part of the forward projector. The ADS method was compared with the A method (ordered-subset expectation maximization with attenuation correction) and with the AD method (a method like the A method but with detector response modeling added). A dual-head SPECT system equipped with two (153)Gd scanning line sources was used for simultaneously acquiring transmission and emission data. Four clinically realistic phantom configurations (a large thorax and a small thorax, each with and without breasts) with a cardiac insert containing 2 cold defects were used to evaluate the proposed reconstruction algorithms. We compared the performance of the different algorithms in terms of noise properties, contrast-to-noise ratios, the contrast separability of perfusion defects, uniformity, and robustness to anatomic variations. The ADS method provided images with clearly better visual defect contrast than did the other methods. The contrasts achieved with the ADS method were 10%-24% higher than those achieved with the AD method and 11%-37% higher than those achieved with the A method. For a typical contrast level, the ADS method exhibited noise levels around 27% lower than the AD method and 34% lower than the A method. Compared with the other 2 algorithms, the ADS reconstructions were less sensitive to anatomic variations and had better image uniformity in the homogeneously perfused myocardium. Finally, we found that the improvements that can be achieved with Monte Carlo-based scatter correction are stronger for (201)Tl than for (99m)Tc imaging. Our results indicate that Monte Carlo-based scatter correction is suitable for (201)Tl cardiac imaging and that such correction simultaneously improves several image-quality metrics.

TH‐D‐VaIB‐03: Unified Algorithm for KV and MV Scatter and Beam‐Hardening Correction Using the Convolution‐Superposition Method

Purpose: Quantitative cone beam CT (CBCT) is essential for advanced radiation oncology (RO) applications such as portal image‐based 3D dose reconstruction. Quantitative CT requires accurate modeling of scatter, beam‐hardening and detector response. Scatter correction methods are typically semi‐empirical in nature and are designed to reduce visible artifacts while incurring low computational cost. In contrast, Monte Carlo (MC) methods are accurate but impractically slow. Convolution‐superposition (CS) scatter models offer a good balance between accuracy and computational complexity. We show how CS can be employed to implement a unified correction method that enables quantitative kV and MV imaging. Method and Materials: (1) We perform detailed MC modeling of the kV and MV cone beam imaging systems. (2) Using MC, we generate calibration data that map intensities recorded on the flat panel imagers to water‐equivalent thicknesses (WETs). (3) The MC models are used to generate pencil beam kernels for water cylinders of varying thickness. (4) Scattergrams are generated from acquired projection images via the CS method using these kernels indexed by the WET at each pixel. (5) Scattergrams are iteratively refined using a multiplicative correction formula that ensures that the estimated primary image remains non‐negative even when scatter‐to‐primary ratios are very high. (6) The FDK reconstruction algorithm is applied directly to the thickness maps corresponding to the estimated primary images. Results: The algorithm is able to reduce maximum non‐uniformity in the reconstruction of a 16cm cylindrical homogeneous tissue equivalent phantom from 11.7% to 1.5%. When applied to a challenging 35cm × 22.5cm oblong water phantom, a non‐uniformity reduction in from 36% to 2.5% is achieved. A dataset of 200 1024×1024 projections can be processed in 25 seconds. Conclusions: CS methods can be used at both kV and MV energies to enable reconstruction of quantitative CBCT images. Conflict of Interest: Supported by Siemens.

Determining total I-131 activity within a VoI using SPECT, a UHE collimator, OSEM, and a constant conversion factor

Accurate determination of activity within a volume of interest is needed during radiopharmaceutical therapies. Single-photon emission computed tomography(SPECT) is employed but requires a method to convert counts to activity. We use a phantom-based conversion; that is, we image an elliptical cylinder containing a sphere that has a known amount of 131-I activity inside. The regularized space alternating generalized expectation (SAGE) algorithm employing a strip-integral detector-response model was employed for reconstruction in previous patient evaluations. With that algorithm and a high-energy collimator, the estimates for sphere activity varied with changes in: 1) the level of uniform background activity in the cylinder; 2) the image resolution due to different values of the radius of rotation R; and 3) the volume of the sphere. When one used those to convert reconstructed counts within a patient tumor into an activity estimate, the resultant value may have been in error because of patient-phantom mismatch. As a potential remedy, in this paper, we use an ordered subsets expectation maximization (OSEM) algorithm with a 3-D depth-dependent detector-response model and an ultra-high-energy collimator. Results after 100 OSEM iterations and using a maximum counts registration show the estimates for sphere activity: 1) have a dependence on the level of background activity with a slope whose absolute magnitude is typically only 0.37 times that with SAGE; 2) are independent of R; and 3) are independent of sphere volume down to and including a sphere volume of 20 cm/sup 3/. We conclude that using a global-average conversion factor to relate counts to activity and no volume-based correction might be reasonable with OSEM. For a test of that conclusion, target activity is estimated for an anthropomorphic phantom containing a 100 cm/sup 3/ spherical tumor centrally located inferior to the lungs. With OSEM-based quantification, using: 1) a global-average conversion factor and 2) no volume-based correction, mean bias in the simulated-tumor activity estimate over 20 realizations is -7.37% (relative standard deviation =5.93%). With SAGE-based quantification using: 1) the conversion factor corresponding to the experimental estimate of background and 2) volume-based correction, the mean bias is -10.7% (relative standard deviation =2.37%). The mean bias is smaller in a statistically significant way and relative standard deviation is not more than a factor of 2.5 bigger with OSEM compared to SAGE. In addition, with OSEM, a patient image apparently shows more highly resolved features, and the activity estimates for two tumors are increased by an average of 10%, relative to results with SAGE.

Prospects for quantitative computed tomography imaging in the presence of foreign metal bodies using statistical image reconstruction.

X-ray computed tomography (CT) images of patients bearing metal intracavitary applicators or other metal foreign objects exhibit severe artifacts including streaks and aliasing. We have systematically evaluated via computer simulations the impact of scattered radiation, the polyenergetic spectrum, and measurement noise on the performance of three reconstruction algorithms: conventional filtered backprojection (FBP), deterministic iterative deblurring, and a new iterative algorithm, alternating minimization (AM), based on a CT detector model that includes noise, scatter, and polyenergetic spectra. Contrary to the dominant view of the literature, FBP streaking artifacts are due mostly to mismatches between FBP's simplified model of CT detector response and the physical process of signal acquisition. Artifacts on AM images are significantly mitigated as this algorithm substantially reduces detector-model mismatches. However, metal artifacts are reduced to acceptable levels only when prior knowledge of the metal object in the patient, including its pose, shape, and attenuation map, are used to constrain AM's iterations. AM image reconstruction, in combination with object-constrained CT to estimate the pose of metal objects in the patient, is a promising approach for effectively mitigating metal artifacts and making quantitative estimation of tissue attenuation coefficients a clinical possibility.

Atomic spectrometry update. X-ray fluorescence spectrometry

This annual review of X-ray fluorescence covers developments over the period 2001–2002 in instrumentation and detectors, matrix correction and spectrum analysis software, X-ray optics and microfluorescence, synchrotron XRF, TXRF and portable XRF as assessed from the published literature. The review also covers a survey of applications, including sample preparation, geological, environmental, archaeological, forensic, biological, clinical, thin films, chemical state and speciation studies. During the current review period, publications have demonstrated the elegant and (almost) comprehensive description of a detector response model for Si(Li) detectors, matching theory with practice, by the Guelph group, and the continuing growth in use of EDXRF in place of WDXRF in a range of applications, particularly the analysis of environmental samples. A notable application, which is as much a comment on the state of society as on science, is the study of blood lead concentrations of gunshot victims. Finally, it is pleasing to note the increasing number of third generation synchrotron facilities becoming available, so extending the facilities for specialised XRF research.

X-ray spectral variability of the Seyfert galaxy NGC 4051

We report on the X-ray spectral variability of the Seyfert 1 galaxy NGC 4051 observed with the Rossi X-ray Timing Explorer (RXTE) during a 1000 day period between May 1996 and March 1999. The spectra were obtained as part of monitoring observations and from two long observations using the RXTE Proportional Counter Array (PCA). During the monitoring period the 2-10 keV flux of NGC 4051 varied between 10E-12 and 7x 10E11 (cgs). We re-analysed RXTE PCA observations from a distinct low state in May 1998 using the latest background and detector response models. The RXTE and BeppoSAX observations of NGC 4051 during the low state show a very hard spectrum with a strong unresolved fluorescence line. This emission, probably due to reflection from a molecular torus, is likely to be constant over long time-scales and is therefore assumed as an underlying component at all flux states. By subtracting the torus component we are able to determine the spectral variability of the primary continuum. In the variable component we observe a strong anti-correlation of X-ray flux and spectral hardness in the PCA energy band. We show that the changes in hardness are caused by slope variability of the primary power law spectrum rather than by changing reflection or variable photoelectric absorption. The primary spectral index varies between Gamma=1.6 for the faintest states and Gamma=2.3 during the brightest states, at which level the spectral index approaches an asympotic value. We find that the response of the flux of the 6.4 keV iron fluorescence line to changes in the continuum flux depends on the timescale of the observation. The profile of the line is very broad and indicates an origin in the innermost regions of the accretion disk.

Absolute quantitation of a spherical I-131 activity distribution using a high-resolution rotating collimator: a phantom study

SPECT data were acquired using a high-resolution rotating parallel-hole collimator designed for imaging of medium-energy radionuclides. The authors reconstructed images of I-131 activity distributions for a 2.2 cm-diameter sphere inside a water-filled cylinder, which approximates the activity distribution and attenuation medium encountered in the I-131 monoclonal antibody treatment of brain tumors. Reconstructions were performed using FBP, OSEM and MAP/ICD methods including detector response modeling in the iterative algorithms. The MAP/ICD with an edge-preserving convex prior and OSEM reconstructions provided the more accurate absolute activity distributions with better resolution (FWHM of 0.5 cm). Once the image resolution was accounted for in the determination of volumes of interest, the sphere's activity and activity density were reproduced to within /spl plusmn/5% of their known values. The activity estimation based on MAP/ICD and OSEM reconstructions showed relatively little dependence on the size of the volume of interest compared to FBP images.

Detector response models for statistical iterative image reconstruction in high resolution PET

One limitation in a practical implementation of statistical iterative image reconstruction is to compute a transition matrix accurately modeling the relationship between projection and image spaces. Detector response function (DRF) in positron emission tomography (PET) is broad and spatially-variant, leading to large transition matrices taking too much space to store. In this work, the authors investigate the effect of simpler DRF models on image quality in maximum likelihood expectation maximization reconstruction. The authors studied 6 cases of modeling projection/image relationship: tube/pixel geometric overlap with tubes centered on detector face; same as previous with tubes centered on DRF maximum; two different fixed-width Gaussian functions centered on DRF maximum weighing tube/pixel overlap; same as previous with a Gaussian of the same spectral resolution as DRF; analytic DRF based on linear attenuation of /spl gamma/-rays in detector arrays weighing tube/pixel overlap. The authors found that DRF oversimplification may affect visual image quality and image quantification dramatically, including artefact generation. They showed that analytic DRF yielded images of excellent quality for a small animal PET system with long, narrow detectors and generated a transition matrix for 2-D reconstruction that could be easily fitted into the memory of current stand-alone computers.

Measurement of theWboson mass

This paper presents a measurement of the mass of the W boson using data collected with the CDF detector during the 1992--1993 collider run at the Fermilab Tevatron. A fit to the transverse mass spectrum of a sample of 3268 W\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu} events recorded in an integrated luminosity of 19.7 ${\mathrm{pb}}^{\mathrm{\ensuremath{-}}1}$ gives ${\mathit{M}}_{\mathit{W}}^{\mathrm{\ensuremath{\mu}}}$=80.310\ifmmode\pm\else\textpm\fi{}0.205 (stat)\ifmmode\pm\else\textpm\fi{}0.130 (syst) GeV/${\mathit{c}}^{2}$. A fit to the transverse mass spectrum of a sample of 5718 W\ensuremath{\rightarrow}e\ensuremath{\nu} events recorded in 18.2 ${\mathrm{pb}}^{\mathrm{\ensuremath{-}}1}$ gives ${\mathit{M}}_{\mathit{W}}^{\mathit{e}}$=80.490\ifmmode\pm\else\textpm\fi{}0.145 (stat)\ifmmode\pm\else\textpm\fi{}0.175 (syst) GeV/${\mathit{c}}^{2}$. Combining the muon and electron results, accounting for correlated uncertainties, yields ${\mathit{M}}_{\mathit{W}}$=80.410\ifmmode\pm\else\textpm\fi{}0.180 GeV/${\mathit{c}}^{2}$.

Mean-variance analysis of block-iterative reconstruction algorithms modeling 3D detector response in SPECT

We study the statistical convergence properties of two fast iterative reconstruction algorithms, the rescaled block-iterative (RBI) and ordered subset (OS) EM algorithms, in the context of cardiac SPECT with 3D detector response modeling. The Monte Carlo method was used to generate nearly noise-free projection data modeling the effects of attenuation, detector response, and scatter from the MCAT phantom. One thousand noise realizations were generated with an average count level approximating a typical T1-201 cardiac study. Each noise realization was reconstructed using the RBI and OS algorithms for cases with and without detector response modeling. For each iteration up to twenty, we generated mean and variance images, as well as covariance images for six specific locations. Both OS and RBI converged in the mean to results that were close to the noise-free ML-EM result using the same projection model. When detector response was not modeled in the reconstruction, RBI exhibited considerably lower noise variance than OS for the same resolution. When 3D detector response was modeled, the RBI-EM provided a small improvement in the tradeoff between noise level and resolution recovery, primarily in the axial direction, while OS required about half the number of iterations of RBI to reach the same resolution. We conclude that OS is faster than RBI, but may be sensitive to errors in the projection model. Both OS-EM and RBI-EM are effective alternatives to the EVIL-EM algorithm, but noise level and speed of convergence depend on the projection model used.

High-resolution 3D Bayesian image reconstruction using the microPET small-animal scanner

A Bayesian method is described for reconstruction of high-resolution 3D images from the microPET small-animal scanner. Resolution recovery is achieved by explicitly modelling the depth dependent geometric sensitivity for each voxel in combination with an accurate detector response model that includes factors due to photon pair non-collinearity and inter-crystal scatter and penetration. To reduce storage and computational costs we use a factored matrix in which the detector response is modelled using a sinogram blurring kernel. Maximum a posteriori (MAP) images are reconstructed using this model in combination with a Poisson likelihood function and a Gibbs prior on the image. Reconstructions obtained from point source data using the accurate system model demonstrate a potential for near-isotropic FWHM resolution of approximately 1.2 mm at the center of the field of view compared with approximately 2 mm when using an analytic 3D reprojection (3DRP) method with a ramp filter. These results also show the ability of the accurate system model to compensate for resolution loss due to crystal penetration producing nearly constant radial FWHM resolution of 1 mm out to a 4 mm radius. Studies with a point source in a uniform cylinder indicate that as the resolution of the image is reduced to control noise propagation the resolution obtained using the accurate system model is superior to that obtained using 3DRP at matched background noise levels. Additional studies using pie phantoms with hot and cold cylinders of diameter 1-2.5 mm and FDG animal studies appear to confirm this observation.

Material Requirements for a Boron Phosphide Thermal Neutron Counter

AbstractElectrical characterization (current versus voltage and capacitance versus voltage) of nonstoichiometric amorphous boron phosphide Schottky diodes for neutron detection is presented. These results are incorporated in a Monte Carlo model of detector response to determine material requirements for a boron phosphide neutron counter.

Improved SPECT quantitation using fully three-dimensional iterative spatially variant scatter response compensation

The quality and quantitative accuracy of iteratively reconstructed SPECT images improves when better point spread function (PSF) models of the gamma camera are used during reconstruction. Here, inclusion in the PSF model of photon crosstalk between different slices caused by limited gamma camera resolution and scatter is examined. A three-dimensional (3-D) projector back-projector (proback) has been developed which models both the distance dependent detector point spread function and the object shape-dependent scatter point spread function of single photon emission computed tomography (SPECT). A table occupying only a few megabytes of memory is sufficient to represent this scatter model. The contents of this table are obtained by evaluating an analytical expression for object shape-dependent scatter. The proposed approach avoids the huge memory requirements of storing the full transition matrix needed for 3-D reconstruction including object shape-dependent scatter. In addition, the method avoids the need for lengthy Monte Carlo simulations to generate such a matrix. In order to assess the quantitative accuracy of the method, reconstructions of a water filled cylinder containing regions of different activity levels and of simulated 3-D brain projection data have been evaluated for technetium-99m. It is shown that fully 3-D reconstruction including complete detector response and object shape-dependent scatter modeling clearly outperforms simpler methods that lack a complete detector response and/or a complete scatter response model. Fully 3-D scatter correction yields the best quantitation of volumes of interest and the best contrast-to-noise curves.

A 3D model of non-uniform attenuation and detector response for efficient iterative reconstruction in SPECT

A 3D physical model for iterative reconstruction in SPECT. has been developed and applied to experimental data. The model incorporates nonuniform attenuation using reconstructed transmission CT data and distance-dependent detector response based on response function measurements over a range of distances from the detector. The 3D model has been implemented in a computationally efficient manner with practical memory requirements. The features of the model that provide efficiency are described including a new region-dependent reconstruction (RDR) technique. With RDR, filtered backprojection is used to reconstruct areas of the image of minimal clinical importance, and the result is used to supplement the iterative reconstruction of the clinically important areas of the image. The 3D model was incorporated into the maximum likelihood-expectation maximization (ML-EM) reconstruction algorithm and tested in three phantom studies - a point source, a uniform cylinder, and an anthropomorphic thorax - and a patient 99Tcm sestamibi study. Reconstructed images with the 3D method exhibited excellent noise and resolution characteristics. With the sestamibi data, the RDR technique produced essentially the conventional ML-EM estimate in the cardiac region with substantial time savings.

Coupled gas chromatography–atomic absorption spectrometry using semi-enclosed tubular atomizers: theoretical analysis of detector performance characteristics

For the determination of specific organometallic compounds in various environmental samples, coupled gas chromatography–atomic absorption spectrometry (GC–AAS) has proved to be a useful instrumental combination. However, to enable the optimum dimensions and operating conditions of the atomizer to be identified requires consideration of the theoretical aspects of GC–AAS detector performance. In this work, expressions are derived to describe the relative values of the detector sensitivity (peak height), and chromatographic efficiency and resolution in terms of the ratio between the residence time of the analyte in the atomizer and the standard deviation of the (assumed) Gaussian peak eluting from the GC system (τR/σG). The greatest detector sensitivity is obtained at high τR/σG values, but at the expense of degraded separation capabilities. Recommendations are given for the design of GC–AAS systems providing the best possible performance characteristics. A detector response model is also presented for predicting characteristic mass values (peak height and peak area) provided by GC–AAS systems and verified using literature data and experimental results.

Lag times associated with fire detection and suppression

The effectiveness of fire detection systems and fire mitigation strategies can be related to three distinct time lags associated with building fires: a transport time lag, a detection time lag, and a suppression time lag. The impacts of these lag periods on fire detection and suppression are developed. Transport lag periods are considered in terms of available correlations of fire plume and ceiling jet data, detection lag periods in terms of available heat detector response models that use these data correlations. Suppression lags are developed in terms of expected response times for automatic and manual suppression. Example calculations are presented.

Gamma-ray response functions for scintillation and semiconductor detectors

A gamma-ray detector response function model has been developed which enables computation of spectra for a variety of scintillation or high purity germanium detectors over the energy range 50 keV to 3 MeV. A set of 32 empirically determined parameters are used to describe the detector efficiency and interaction event probabilities. The model also computes the effects of scattered radiation, attenuation, coincidence event detection, and random pileup. The accuracy and general applicability of this model for a variety of detector types and configurations surpasses capabilities existing heretofore. The ability to compute accurate spectral templates is a requirement for radiochemical analysis of low resolution spectra using linear regression. An example analysis of a sodium iodide measurement of fallout from the Chernobyl nuclear reactor accident is presented. The detector response model also provides a means of computing the response of a detector to an arbitrary photon flux profile such as leakage spectrum computed by a radiation transport code.

Evaluation of Non-Linear FPD Response

Abstract A new model of Flame Photometric Detector (FPD) response has been developed and applied for FPD signal evaluation. It has been shown that the second order polynomial fit quantitatively describes the detection mechanism and that this is the only relevant procedure for detector signal evaluation. Detector characteristics, such as sensitivity, linearity, lower detection limit and selectivity have been quantitatively described and their limited meaning shown. Selectivity of detector depends on its construction and should be selected as high as possible. The same holds for separation efficiency of the gas chromatographic system used. It is not recommended to use FPD as a monitor of the sulphur content in the air.