Proc. SPIE. 7306, Optics and Photonics in Global Homeland Security V and Biometric Technology for Human Identification VI
KEYWORDS: Imaging systems, Sensors, Calibration, Crystals, Field programmable gate arrays, Personal digital assistants, Gamma radiation, Data communications, Signal detection, Global Positioning System
The GE Intelligent Personal Radiation Locator (IPRL) system consists of multiple hand held radiation detectors and a
base station. Each mobile unit has a CZT Compton camera radiation detector and can identify isotopes and determine the
direction from which the radiation is detected. Using GPS and internal orientation sensors, the system continuously
transforms all directional data into real-world coordinates. Detected radiation is wirelessly transmitted to the base station
for system-wide analysis and situational awareness. Data can also be exchanged wirelessly between peers to enhance the
overall detection efficiency of the system. The key design features and performance characteristics of the GE IPRL
system are described.
The output response characteristics of an X-ray photon counting detector are measured experimentally and
simulated using a Monte Carlo method in order to quantify the loss of statistical information due to pile-up. The
analysis is applied to idealize counting detector models, but is adaptable to realistic event processing that is not
amenable to analytic solution. In particular, the detective quantum efficiency (DQE) is calculated as a function of flux
rate and shown to have an intermediate zero for the paralyzable case at the maximum periodic rate. The progressive
degradation of the spectral response as a function of increasing flux rate is also modeled. Analogous metrics to DQE
are defined in regards to the detector's ability to resolve atomic number and enhance image contrast based on atomic
number differentiation. Analytic solutions are provided for the output and linearized response statistics and these
interpolate well across the Monte Carlo and experimental results.
A 4π direction-sensitive gamma imager is presented, using a 1 cm3 3D CZT detector from Yinnel
Tech and the RENA-3 readout ASIC from NOVA R&D. The measured readout system electronic noise is
around 4-5 keV FWHM for all anode channels. The measured timing resolution between two channels
within a single ASIC is around 10 ns and the resolution is 30 ns between two separate ASIC chips. After
3D material non-uniformity and charge trapping corrections, the measured single-pixel-event energy
resolution is around 1% for Cs-137 at 662 keV using a 1 cm3 CZT detector from Yinnel Tech with an 8 x 8
anode pixel array at 1.15 mm pitch. The energy resolution for two pixel events is 2.9%. A 10 uCi Cs-137
point source was moved around the detector to test the image reconstruction algorithms and demonstrate
the source direction detection capability. Accurate source locations were reconstructed with around 200
two-pixel events within a total energy window ±10 keV around the 662 keV full energy peak. The angular
resolution FWHM at four of the five positions tested was between 0.05-0.07 steradians.
The material specificity of computed tomography is quantified using an experimental benchtop imaging system
and a physics-based system model. The apparatus is operated with different detector and system configurations each
giving X-ray energy spectral information but with different overlap among the energy-bin weightings and noise
statistics. Multislice, computed tomography sinograms are acquired using dual kVp, sequential source filters or a
detector with two scintillator/photodiodes layers. Basis-material and atomic number images are created by first
applying a material decomposition algorithm followed by filtered backprojection. CT imaging of phantom materials
with known elemental composition and density were used for model validation. X-ray scatter levels are measured with a
beam-blocking technique and the impact to material accuracy is quantified. The image noise is related to the intensity
and spectral characteristics of the X-ray source. For optimal energy separation adequate image noise is required. The
system must be optimized to deliver the appropriate high mA at both energies. The dual kVp method supports the
opportunity to separately engineer the photon flux at low and high kvp. As a result, an optimized system can achieve
superior material specificity in a system with limited acquisition time or dose. In contrast, the dual-layer and sequential
acquisition modes rely on a material absorption mechanism that yields weaker energy separation and lower overall
A convolution model of scatter that is adaptable to rapid simulation and correction algorithms is tested against the measured scatter profiles. In the simple case of a uniform acrylic sheet, the convolution approach yields about 10% absolute agreement with the measured scatter profile. However, significant qualitative differences are demonstrated for phantoms with non-uniform thickness or composition. For example, the scatter profile is dependent on a bone's vertical position in the phantom whereas the primary is unchanged. Similarly, a cusp shape in the scatter profile observed near the abrupt edge of an acrylic sheet is not produced in the convolution model. An alternate approach that calculates the scatter as a 3D integral over the object volume can reproduce this behavior.
The purpose of this paper is to investigate the use of electron-beam Computed Tomography (EBCT) dual energy scanning for improved differentiation of calcified coronary arteries from iodinated-contrasted blood, in fast moving cardiac vessels. The dual energy scanning technique can lead to an improved cardiac examination in a single breath hold with more robust calcium scoring and better vessel characterization. Dual energy can be used for material discrimination in CT imaging to differentiate materials with similar CT number, but different material attenuation properties. Mis-registration is the primary source of error in a dual energy application, since acquisitions have to be made at each energy, and motion between the acquisitions causes inconsistencies in the decomposition algorithm, which may lead to artifacts in the resultant images. Using EBCT to quickly switch x-ray source peak voltage potential (kVp), the mis-registration of patient anatomy is minimized since acquisitions at both energy spectra are completed in one study at the same cardiac phase. Two protocols for scanning the moving heart using EBCT were designed to minimize registration issues. Material basis function decomposition was used to differentiate regions containing calcium and iodine in the image. We find that this protocol is superior to CT imaging at one energy spectrum in discriminating calcium from contrast-enhanced lumen. Using dual energy EBCT scanning can enable accurate calcium scoring, and angiography applications to be performed in one exam.
In addition to a conventional Computed Tomography (CT) image, dual energy (dual kVp) imaging can be used to generate an image of the same anatomy that represents the equivalent density of a particular material, for example, calcium, iodine, water, etc. This image can be used to improve the differentiation of materials as well as improve the accuracy of absolute density measurements in a cross-sectional image. It is important to understand the certainty of the estimation of the density of the material. Both simulations and measurements are used to quantify these errors. Data are acquired using a flat-panel based volumetric CT system, by taking two scans and adjusting the maximum energy of the source spectrum (kVp). Physics based simulations are used to compare with the measurements. After validating the simulation algorithms, the accuracy of the dual kVp method is determined using the simulations in a perturbation study.
Dose is becoming increasingly important for computed tomography clinical practice. It is of general interest to understand the impact that system design can have on dose and image quality. This study addresses the effect of bowtie shape on the dose and contrast-to-noise across the field of view. Simulation of the CT acquisition is used to calculate the energy deposition throughout a numerical phantom for a set of relevant system operating parameters and bowtie shapes. Mean absorbed dose is calculated by summing over the phantom volume and is compared with other typical dose specifications. A more aggressive attenuation profile of the bowtie which offers higher attenuation in the periphery of the field of view can offer the benefit of lower dose but at the expense of reduced contrast-to-noise at the edge of the cross-sectional image.