Calculated response of intrinsic germanium detectors to narrow beams of photons with energies up to ∼300 keV

Optimization of a table-top x-ray fluorescence computed tomography (XFCT) system

Pencil beam x-ray fluorescence computed tomography (XFCT) has typically used a single spectrometer and prohibitively long scan times. However, detecting backscattered fluorescent x-rays from gold nanoparticles (AuNPs) using multiple spectrometers greatly reduces image noise and scan time. The arrangement of eight spectrometers for combined K-shell and L-shell XFCT was investigated along with a variety of conditions. A 2.5 cm-diameter cylindrical water phantom containing 4 mm-diameter vials with 0.1%-2% AuNP concentrations by weight was modeled by TOPAS, a GEANT4-based Monte Carlo software. The phantom was irradiated to 30 mGy by a 0.5 mm Pb-filtered 120 kVp and 1 mm Al-filtered 30 kVp 1 mm² x-ray pencil beam to yield respective Au K-shell and L-shell fluorescent x-rays, with 50 0.5 mm translation and 2-degree rotation steps. Eight CdTe and silicon drift detector (SDD) spectrometers were placed 2.25 cm away from the isocentre. The respective energy resolution was applied to the detected energy spectra and the spectra were corrected for detector response before extracting the fluorescence signal. Three CdTe and SDD spectrometer configurations (isotropic/backscattered grid/backscattered row arrangements), two CdTe crystal sizes (9 mm²/25 mm²), two scanning techniques (moving/stationary spectrometers) and five vial-edge depths (0-4 mm) were considered in optimizing the contrast-to-noise ratio (CNR) for each XFCT image reconstructed with a maximum-likelihood expectation maximization (MLEM) algorithm. The isotropic spectrometer arrangement had AuNP detection sensitivities of 0.106% for K-shell and 0.132% for L-shell XFCT at 4 mm depth. Comparatively, the backscattered grid arrangement had the best AuNP sensitivity of 0.055% and 0.095%. The highest K-shell (0.044%) and L-shell (0.004%) AuNP sensitivities were found for vials at 0 mm depth. Using stationary spectrometers or the 9 mm² CdTe crystal compromised the CNR. For the best-case arrangement, L-shell XFCT is superior at vial-edge depths less than 3.0 mm. This work demonstrated the importance of spectrometer arrangement and vial depth for improving AuNP sensitivity and will guide the design for our table-top XFCT system.

Consistency of photon emission intensities for efficiency calibration of gamma-ray spectrometers in the energy range from 20keV to 80keV

The efficiency calibration for different high-purity germanium detectors in the low-energy range was established by the conventional method, using standard radioactive sources. The peak shapes were carefully analysed taking account of natural linewidth, full-energy width at half maximum and scattering. Complementary information was obtained by Monte Carlo simulation using the PENELOPE code, after optimization of the geometrical parameters. This was used to measure photon emission intensities of some low-energy emitting radionuclides, including ¹³³Ba, and compared to the tabulated values.

New National Air-Kerma Standard for Low-Energy Electronic Brachytherapy Sources

The new primary standard for low-energy electronic brachytherapy sources for the United States is described. These miniature x-ray tubes are inserted in catheters for interstitial radiation therapy and operate at tube potentials of up to about 50 kV. The standard is based on the realization of the air kerma produced by the x-ray beam at a reference distance in air of 50 cm.

Monte Carlo simulation of a computed tomography x-ray tube

The dose delivered to patients during computed tomography (CT) exams has increased in the past decade. With the increasing complexity of CT examinations, measurement of the dose becomes more difficult and more important. In some cases, the standard methods, such as measurement of the computed tomography dose index (CTDI), are currently under question. One approach to determine the dose from CT exams is to use Monte Carlo (MC) methods. Since the patient geometry can be included in the model, Monte Carlo simulations are potentially the most accurate method of determining the dose delivered to patients. In this work, we developed a MC model of a CT x-ray tube. The model was validated with half-value layer (HVL) measurements and spectral measurements with a high resolution Schottky CdTe spectrometer. First and second HVL for beams without additional filtration calculated from the MC modelled spectra and determined from attenuation measurements differ by less than 2.5%. The differences between the first and second HVL for both filtered and non-filtered beams calculated from the MC modelled spectra and spectral measurements with the CdTe detector were less than 1.8%. The MC modelled spectra match the directly measured spectra. This works presents a first step towards an accurate MC model of a CT scanner.

Measurement and validation of benchmark-quality thick-target tungsten X-ray spectra below 150 kVp

Pulse-height distributions of two constant potential X-ray tubes with fixed anode tungsten targets were measured and unfolded. The measurements employed quantitative alignment of the beam, the use of two different semiconductor detectors (high-purity germanium and cadmium-zinc-telluride), two different ion chamber systems with beam-specific calibration factors, and various filter and tube potential combinations. Monte Carlo response matrices were generated for each detector for unfolding the pulse-height distributions into spectra incident on the detectors. These response matrices were validated for the low error bars assigned to the data. A significant aspect of the validation of spectra, and a detailed characterization of the X-ray tubes, involved measuring filtered and unfiltered beams at multiple tube potentials (30-150 kVp). Full corrections to ion chamber readings were employed to convert normalized fluence spectra into absolute fluence spectra. The characterization of fixed anode pitting and its dominance over exit window plating and/or detector dead layer was determined. An Appendix of tabulated benchmark spectra with assigned error ranges was developed for future reference.

Towards a proper modeling of detector and source characteristics in Monte Carlo simulations

We performed a systematic study on the influence of different source configurations on the reliability of Monte Carlo calculations for the response of Ge detectors. Calculated full-energy peak efficiencies are compared to experimental values in the energy range 46-1800 keV. Setups with different characteristics are considered, from point-like to volume sources. Among the latter there are filters and aqueous matrices and sediments in Petri boxes or Marinelli beakers. The analysis of the deviations between experimental and calculated results for the different configurations enables us to detect inaccuracies in the description of detector and source characteristics and to improve them. By means of this procedure satisfactory results of the efficiencies were obtained in the whole energy range. For all setups, the deviations average 1.5%, except for the sediment sources where they are up to 3.3%.

Monte Carlo simulation of source-excited in vivo x-ray fluorescence measurements of heavy metals

This paper reports on the Monte Carlo simulation of in vivo x-ray fluorescence (XRF) measurements. Our model is an improvement on previously reported simulations in that it relies on a theoretical basis for modelling Compton momentum broadening as well as detector efficiency. Furthermore, this model is an accurate simulation of experimentally detected spectra when comparisons are made in absolute counts; preceding models have generally only achieved agreement with spectra normalized to unit area. Our code is sufficiently flexible to be applied to the investigation of numerous source-excited in vivo XRF systems. Thus far the simulation has been applied to the modelling of two different systems. The first application was the investigation of various aspects of a new in vivo XRF system, the measurement of uranium in bone with 57Co in a backscatter (approximately 180 degrees) geometry. The Monte Carlo simulation was critical in assessing the potential of applying XRF to the measurement of uranium in bone. Currently the Monte Carlo code is being used to evaluate a potential means of simplifying an established in vivo XRF system, the measurement of lead in bone with 57Co in a 90 degrees geometry. The results from these simulations may demonstrate that calibration procedures can be significantly simplified and subject dose may be reduced. As well as providing an excellent tool for optimizing designs of new systems and improving existing techniques, this model can be used in the investigation of the dosimetry of various XRF systems. Our simulation allows a detailed understanding of the numerous processes involved when heavy metal concentrations are measured in vivo with XRF.

Image quality control for digital mammographic systems: initial experience and outlook

This report presents (1) a broad topical review and a tutorial of the possibilities for image quality control (IQC) with digital systems, and (2) results and initial experience for IQC with two commercial digital imaging systems, but with limited discussion on any particular method. Digital imaging systems used for mammographically guided digital stereotactic breast biopsy were evaluated extensively at the University of Arizona. Measurements were made of linearity, sensitivity, signal-to-noise ratio, and square-wave modulation. Images of phantoms such as the American College of Radiology Accreditation Phantom and the contrast detail mammography Phantom were evaluated as well as images of the x-ray source's focal spot. The evaluation also included the cathode ray tubes for the imaging systems. The data collected show that digital imaging systems have an important advantage over film-screen systems because they provide a digital signal as output that can be used for quantitative analysis. As a result, IQC can become a much more quantitative discipline than presently practiced, providing more information on the imaging systems under evaluation, and providing better control over their properties during actual operation.

Energy imparted to water slabs by photons in the energy range 5-300 keV. Calculations using a Monte Carlo photon transport model

In diagnostic examinations of the trunk and head, the energy imparted to the patient is related to the radiation risk. In this work, the energy imparted to laterally infinite, 10-300 mm thick water slabs by 5-300 keV photons is calculated using a Monte Carlo photon transport model. The energy imparted is also derived for energy spectra of primary photons relevant to diagnostic radiology. In addition to values of energy imparted, values of backscattered and transmitted energies, quantities primarily obtained in the transport calculations, are reported. Assumptions about coherent scattering are shown to be important for values of backscattered and transmitted energies but unimportant with respect to values of energy imparted. Comparisons are made with other Monte Carlo results from the literature. Discrepancies of 10-20% in some calculated quantities can be traced back to the use of different tabulations of interaction cross-sections by various authors.

A Monte Carlo program for photon transport using analogue sampling of scattering angle in coherent and incoherent scattering processes

A computer program was developed for the Monte Carlo simulation of photon transport. The program was designed for photon transport simulation in geometries occurring in diagnostic radiology and especially for the investigation of scattered radiation. A method is described for the analogue sampling of scattering angle in coherent and incoherent scattering processes. The two scattering processes are treated separately, and the influence of coherent scattering, an often neglected process, can be estimated quantitatively. The program can also be used for the calculation of the energy imparted to water slabs and fluorescent screens.