Transmitted X-ray Spectrum Research Articles

Evaluation of Transmitted X-ray Spectrum, Lead Equivalent, and Uniformity of Radiation Protective Clothing Made of Lead-containing and Lead-free Materials

The purpose of this study was to evaluate the protective performance of several new radiation-protective clothing and to clarify issues of quality control. The composition of the shielding elements was analyzed using X-ray fluorescence analysis, and the energy spectrum of transmitted X-rays was measured. Furthermore, the lead equivalent and uniformity were measured from the transmitted X-ray doses according to Japanese industrial standards (JIS). Uniformity was evaluated by transmitting X-ray images of each radiation protective clothing in addition to the conventional method. The energy spectrum showed K-absorption edges of lead, bismuth, tin, etc., which were detected in the composition analysis. The multi-layered protective material maintained higher shielding ability at high tube voltages. In addition, X-ray images of the radiation-protective clothing showed uneven density and dots, and the differences in uniformity measurement methods and points that didn't meet the required shielding capacity were seen. The current JIS does not allow accurate evaluation of the lead equivalent and uniformity, so visual evaluation of X-ray images is important. It is necessary to establish standardized standards for quality control performed by each facility.

Design of Layered CdZnTe Sensor for X-ray Absorptiometry via Monte Carlo N-Particle Simulations

We report on the possibility that a new dual-energy X-ray absorptiometry can potentially be designed by Monte Carlo N-Particle (MCNP) simulations to determine the optimal conditions for the thickness of double-layered CdZnTe detector layers and the X-ray tube voltages. The optimal thicknesses of the front and the rear CdZnTe in a detector and an intermediate filter have been determined from transmitted X-ray spectra. In addition, the performance has been tested with a virtual phantom composed of bone and soft tissue. Two types of CdZnTe detectors, one with a single and the other with a 2x2 square array, were simulated with a pencil-type beam of X-rays having spectra generated using the SRS 78 program for 80, 90, 100, 110, and 120 kV. According to the analysis of the relative detection efficiency for X-ray photons for both CdZnTe detectors, the optimum detector-layer thicknesses were found to be 0.5 mm and 3 mm for the front and the rear CdZnTe detectors, respectively, at 120 kV. The X-ray spectra collected from the front and the rear CdZnTe detectors show considerable variability depending upon the locations the beam passes through; the center of a phantom, the boundary between bone and soft tissue, and a soft-tissue-only region. From these results-based MCNP simulations, we suggest that a new bone mineral densitometry sensor based on this double-layered CdZnTe detector has a very promising configuration.

An FPGA-based algorithm to correct the instability of high-resolution and high-flux X-ray spectroscopic imaging detectors

The emergence of CdTe Photon Counting Detectors (PCD) with energy discrimination capabilities opens up new perspectives in X-ray imaging. Medical and security applications require very high fluxes and consequently a very fast shaper in order to limit dead time losses due to pile-up. However, if the shaper is faster than the collection of the charges in the semiconductor, there is a loss of charge called ballistic deficit. Moreover, variations of the electrical field profile in the detector over time causes a change in the collection time of the charges. According to this, the response of the detector will be affected by these variations. The instability of the response is visible over time as a channel shift of the spectra, resulting in a false information of the photon energy. The aim of this work is to develop real-time digital algorithms implementable in an FPGA in order to correct these effects and to give a stable response of the detector at very high fluxes. The method has been tested with a 4×4 pixels detector (CdTe) of 3 mm thickness and 800 μm pitch which is able to measure transmitted X-ray spectra in the energy range of 20–160 kV on 256 energy bins. The detector is coupled with an innovative custom-designed digital readout electronics able to perform a fast digital signal processing for very high-fluxes measurements. The developed method was initially tested at low count rate with a 57Co and 241Am γ-ray sources. After this first validation, it was tested with an X-ray beam, with a counting rate up to ∼2⋅106 c/s. With the characterization and the validation of this innovative algorithm we prove its ability in providing a stable response of the detector over time without losing on the energy resolution (∼8% at 122 keV) and on the dead time (∼70 ns).

Subpixel x-ray imaging with an energy-resolving detector.

The detector pixel size can be a severe limitation in x-ray imaging of fine details in the human body. We demonstrate a method of using spectral x-ray measurements to image the spatial distribution of the linear attenuation coefficient on a length scale smaller than one pixel, based on the fact that interfaces parallel to the x-ray beam have a unique spectral response, which distinguishes them from homogeneous materials. We evaluate the method in a simulation study by simulating projection imaging of the border of an iodine insert with [Formula: see text] in a soft tissue phantom. The results show that the projected iodine profile can be recovered with an RMS resolution of 5% to 34% of the pixel size, using an ideal energy-resolving detector. We also validate this method in an experimental study by imaging an iodine insert in a polyethylene phantom using a photon-counting silicon-strip detector. The results show that abrupt and gradual transitions can be distinguished based on the transmitted x-ray spectrum, in good agreement with simulations. The demonstrated method may potentially be used for improving visualization of blood vessel boundaries, e.g., in acute stroke care.

Application of a CdTe Detector for Measurements of Mammographic X-ray Spectra

This work aims to characterize mammographic x-ray beams incident and transmitted by breast phantoms (from 0 to 45 mm) composed from known proportion of glandular and adipose tissue-equivalent materials. This study was performed for mammographic x-ray beams generated by a mammography equipment using different target/filter combinations (Mo/Mo, Mo/Rh and W/Rh). It was studied the modification of spectra shape of the beams transmitted through different thicknesses of these materials. It was also evaluated the penetrability of these transmitted beams by its correlations to the HVL, which were experimentally estimated and derived from the x-ray spectra measured using a spectrometry system with a CdTe detector. The x-ray spectra transmitted by the phantom with higher density presented lower intensity than those transmitted by those with lower density, as expected. The differences between the HVL values derived from the spectra and those estimated using air kerma measurements are lesser than 6% for about 88% of the spectra measured in this work. The expected spectra variations with phantom thickness, revealed by the measured transmitted x-ray spectra, were also confirmed by HVL measurements and agree with the estimated attenuation curves.The motivation of the study was related to the robustness of the spectra as a descriptor of radiation beams and the possibility of using these transmitted spectra for dose assessment related to mammographic procedures. We can conclude that developed method is able to characterize mammographic x-ray beams making it possible the use of this kind of data for dose assessment in mammography.

Multicolor spectral photon-counting computed tomography: in vivo dual contrast imaging with a high count rate scanner

A new prototype spectral photon-counting computed tomography (SPCCT) based on a modified clinical CT system has been developed. SPCCT analysis of the energy composition of the transmitted x-ray spectrum potentially allows simultaneous dual contrast agent imaging, however, this has not yet been demonstrated with such a system. We investigated the feasibility of using this system to distinguish gold nanoparticles (AuNP) and an iodinated contrast agent. The contrast agents and calcium phosphate were imaged in phantoms. Conventional CT, gold K-edge, iodine and water images were produced and demonstrated accurate discrimination and quantification of gold and iodine concentrations in a phantom containing mixtures of the contrast agents. In vivo experiments were performed using New Zealand White rabbits at several times points after injections of AuNP and iodinated contrast agents. We found that the contrast material maps clearly differentiated the distributions of gold and iodine in the tissues allowing quantification of the contrast agents’ concentrations, which matched their expected pharmacokinetics. Furthermore, rapid, repetitive scanning was done, which allowed measurement of contrast agent kinetics with high temporal resolution. In conclusion, a clinical scale, high count rate SPCCT system is able to discriminate gold and iodine contrast media in different organs in vivo.

Measurements of ionization states in warm dense aluminum with betatron radiation

Time-resolved measurements of the ionization states of warm dense aluminum via K-shell absorption spectroscopy are demonstrated using betatron radiation generated from laser wakefield acceleration as a probe. The warm dense aluminum is generated by irradiating a free-standing nanofoil with a femtosecond optical laser pulse and was heated to an electron temperature of ∼20-25 eV at a close-to-solid mass density. Absorption dips in the transmitted x-ray spectrum due to the Al^{4+} and Al^{5+} ions are clearly seen during the experiments. The measured absorption spectra are compared to simulations with various ionization potential depression models, including the commonly used Stewart-Pyatt model and an alternative modified Ecker-Kröll model. The observed absorption spectra are in approximate agreement with these models, though indicating a slightly higher state of ionization and closer agreement for simulations with the modified Ecker-Kröll model.

Estimation of Basis Line-Integrals in a Spectral Distortion-Modeled Photon Counting Detector Using Low-Order Polynomial Approximation of X-ray Transmittance.

Photon counting detector (PCD)-based computed tomography exploits spectral information from a transmitted x-ray spectrum to estimate basis line-integrals. The recorded spectrum, however, is distorted and deviates from the transmitted spectrum due to spectral response effect (SRE). Therefore, the SRE needs to be compensated for when estimating basis line-integrals. One approach is to incorporate the SRE model with an incident spectrum into the PCD measurement model and the other approach is to perform a calibration process that inherently includes both the SRE and the incident spectrum. A maximum likelihood estimator can be used to the former approach, which guarantees asymptotic optimality; however, a heavy computational burden is a concern. Calibration-based estimators are a form of the latter approach. They can be very efficient; however, a heuristic calibration process needs to be addressed. In this paper, we propose a computationally efficient three-step estimator for the former approach using a low-order polynomial approximation of x-ray transmittance. The low-order polynomial approximation can change the original non-linear estimation method to a two-step linearized approach followed by an iterative bias correction step. We show that the calibration process is required only for the bias correction step and prove that it converges to the unbiased solution under practical assumptions. Extensive simulation studies validate the proposed method and show that the estimation results are comparable to those of the ML estimator while the computational time is reduced substantially.

Evaluation of mean conversion coefficients from air-kerma to H*(10) using secondary and transmitted x-ray spectra in the diagnostic radiology energy range

Ambient dose equivalent H*(10) is an operational quantity recommended by the IAEA to establish dose constraints in area monitoring for external radiation. The direct measurement of H*(10) is not common due to the complexity in the calibration procedures of radiation monitors involving the use of expanded and aligned radiation fields. Therefore, conversion coefficients are used to assess H*(10) from the physical quantity air-kerma. Conversion coefficients published by international commissions, ICRU and ICRP, present a correlation with the radiation beam quality. However, Brazilian regulation establishes 1.14 Sv Gy−1 as unique conversion coefficient to convert air-kerma into H*(10), disregarding its beam quality dependence. The present study computed mean conversion coefficients from secondary and transmitted x-ray beams in order to improve the current assessment of H*(10). The weighting of conversion coefficients corresponding to monoenergetic beams with the spectrum energy distribution in terms of air-kerma was used to compute the mean conversion coefficients. In order to represent dedicated chest radiographic facilities, an anthropomorphic phantom was used as scatter object of the primary beam. Secondary x-ray spectra were measured in the diagnostic energy range at scattering angles of 30°, 60°, 90° 120° and 150° degrees. Barite mortar plates were used as attenuator of the secondary beam to produce the corresponding transmitted x-ray spectra. Results show that the mean conversion coefficients are about 43% higher than the recommended value accepted by Brazilian regulation. For secondary radiation measured at 100 kV the mean coefficient should be 1.46 Sv Gy−1, which represent the higher value in the mean coefficient set corresponding to secondary beams. Moreover, for transmitted x-ray beams at 100 kV, the recommended mean conversion coefficient is 1.65 Sv Gy−1 for all barite mortar plate thickness and all scattering angles. An example of application shows the discrepancy in the evaluation of secondary shielding barriers in a controlled area when the shielding goals is evaluated. The conclusion based on these results is that a unique coefficient may not be adequate for deriving the H*(10).

Evaluation of conversion coefficients relating air-kerma to H*(10) using primary and transmitted x-ray spectra in the diagnostic radiology energy range

According to the International Commission on Radiation Units and Measurements (ICRU), the relationship between effective dose and incident air-kerma is complex and depends on the attenuation of x-rays in the body. Therefore, it is not practical to use this quantity for shielding design purposes. This correlation is adopted in practical situations by using conversion coefficients calculated using validated mathematical models by the ICRU. The ambient dose equivalent, H*(10), is a quantity adopted by the IAEA for monitoring external exposure. Dose constraint levels are established in terms of H*(10), while the radiation levels in radiometric surveys are calculated by means of the measurements of air-kerma with ion chambers. The resulting measurements are converted into ambient dose equivalents by conversion factors. In the present work, an experimental study of the relationship between the air-kerma and the operational quantity ambient dose equivalent was conducted using different experimental scenarios. This study was done by measuring the primary x-ray spectra and x-ray spectra transmitted through materials used in dedicated chest radiographic facilities, using a CdTe detector. The air-kerma to ambient dose equivalent conversion coefficients were calculated from these measured spectra. The resulting values of the quantity ambient dose equivalent using these conversion coefficients are more realistic than those available in the literature, because they consider the real energy distribution of primary and transmitted x-ray beams. The maximum difference between the obtained conversion coefficients and the constant value recommended in national and international radiation protection standards is 53.4%. The conclusion based on these results is that a constant coefficient may not be adequate for deriving the ambient dose equivalent.

Proposed New Accelerator Design for Homeland Security X-Ray Applications

Two goals for security scanning of cargo and freight are the ability to determine the type of material that is being imaged, and to do so at low radiation dose. One commonly used technique to determine the effective Z of the cargo is dual-energy imaging, i.e. imaging with different x-ray energy spectra. Another technique uses the fact that the transmitted x-ray spectrum itself also depends on the effective Z. Spectroscopy is difficult because the energy of individual x rays needs to be measured in a very high count-rate environment. Typical accelerators for security applications offer large but short bursts of x-rays, suitable for current-mode integrated imaging. In order to perform x-ray spectroscopy, a new accelerator design is desired that has the following features: 1)increased duty factor in order to spread out the arrival of x-rays at the detector array over time; 2)x-ray intensitymodulation from one delivered pulse to the next by adjusting the accelerator electron beam instantaneous current so as to deliveradequate signal without saturating the spectroscopic detector; and 3)the capability to direct the (forward peaked) x-ray intensity towards high-attenuation areas in the cargo (“fan-beam-steering”). Current sources are capable of 0.1% duty factor, although usually they are operated at significantly lower duty factors (∼0.04%), but duty factors in the range 0.4-1.0% are desired. The higher duty factor can be accomplished, e.g., by moving from 300 pulses per second (pps) to 1000 pps and/or increasing the pulse duration from a typical 4μs to 10μs. This paper describes initial R&D to examine cost effective modifications that could be performed on a typical accelerator for these purposes, as well as R&D for fan-beam steering.

Laser wakefield generated X-ray probe for femtosecond time-resolved measurements of ionization states of warm dense aluminum

We have developed a laser wakefield generated X-ray probe to directly measure the temporal evolution of the ionization states in warm dense aluminum by means of absorption spectroscopy. As a promising alternative to the free electron excited X-ray sources, Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure. The X-ray probe system consists of an adjustable Kirkpatrick-Baez (KB) microscope for focusing the Betatron emission to a small probe spot on the sample being measured, and a flat Potassium Acid Phthalate Bragg crystal spectrometer to measure the transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines. An X-ray focal spot size of around 50 μm was achieved after reflection from the platinum-coated 10-cm-long KB microscope mirrors. Shot to shot positioning stability of the Betatron radiation was measured resulting in an rms shot to shot variation in spatial pointing on the sample of 16 μm. The entire probe setup had a spectral resolution of ~1.5 eV, a detection bandwidth of ~24 eV, and an overall photon throughput efficiency of the order of 10(-5). Approximately 10 photons were detected by the X-ray CCD per laser shot within the spectrally resolved detection band. Thus, it is expected that hundreds of shots will be required per absorption spectrum to clearly observe the K-shell absorption features expected from the ionization states of the warm dense aluminum.

Measurement of spectra for intra-oral X-ray beams using biological materials as attenuator

Abstract In diagnostic radiology, the radiation interaction probability in matter is a strong function of the X-ray energy. The knowledge of the X-ray energy spectral distribution allows optimizing the radiographic imaging system in order to obtain high quality images with as low as reasonably achievable patient doses. In this study, transmitted X-ray spectra through dentin and enamel that are existing materials in intra-oral radiology were experimentally determined in an X-ray equipment with 40–70 kV variable range. Dentin and enamel samples with 0.4–3.8 and 0.6–2.6 mm thick were used as attenuators. X-ray transmitted spectra were measured with XR-100T model CdTe detector and half-value layers (HVL) were determined. Characteristics of both dentin and enamel transmitted spectra showed that they have differences in the penetration power in matter and in the spectrum distribution. The results will be useful for phantom developments based on dentin and enamel for image quality control in dental radiology.

ON THE PROPERTIES OF THERMAL DISK WINDS IN X-RAY TRANSIENT SOURCES: A CASE STUDY OF GRO J1655–40

We present the results of hydrodynamical simulations of the disk photosphere irradiated by strong X-rays produced in the inner most part of the disk. As expected, the irradiation heats the photosphere and drives a thermal wind. To apply our results to the well-studied X-ray transient source GRO J1655-40, we adopted the observed mass of its black hole, and the observed properties of its X-ray radiation. To compare the results with the observations, we also computed transmitted X-ray spectra based on the wind solution. Our main finding is: the density of the fast moving part of the wind is more than one order of magnitude lower than that inferred from the observations. Consequently, the model fails to predict spectra with line absorption as strong and as blueshifted as those observed. However, despite the thermal wind being weak and Compton thin, the ratio between the mass-loss rate and the mass accretion rate is about seven. This high ratio is insensitive to the accretion luminosity, in the limit of lower luminosities. Most of the mass is lost from the disk between 0.07 and 0.2 of the Compton radius. We discovered that beyond this range the wind solution is self-similar. In particular, soon after it leaves the disk, the wind flows at a constant angle with respect to the disk. Overall, the thermal winds generated in our comprehensive simulations do not match the wind spectra observed in GRO J1655-40. This supports the conclusion of Miller et al. and Kallman et al. that the wind in GRO J1655-40, and possibly other X-ray transients, may be driven by magnetic processes. This in turn implies that the disk wind carries even more material than our simulations predict and as such has a very significant impact on the accretion disk structure and dynamics.

Monte Carlo simulation of x-ray spectra in mammography

A model for generating x-ray spectra in mammography is presented. This modelused the ITS version 3 Monte Carlo code for simulating the radiationtransport. Various target/filter combinations such as tungsten/aluminium,molybdenum/molybdenum, molybdenum/rhodium and rhodium/rhodium were used in thesimulation. Both bremsstrahlung and characteristic x-ray production wereincluded in the model. The simulated x-ray emission spectra were compared withtwo sets of spectra, those of Boone et al (1997 Med. Phys. 241863-74) and IPEM report 78. The χ2 test was used for the overallgoodness of fit of the spectral data. There is good agreement between thesimulated x-ray spectra and the comparison spectra as the test yielded aprobability value of nearly 1. When the transmitted x-ray spectra for specifictarget/filter combinations were generated and compared with a measuredmolybdenum/rhodium spectrum and spectra generated in IPEM report 78, closeagreement is also observed. This was demonstrated by the probability value forthe χ2 test being almost 1 for all the cases. However, minordifferences between the simulated spectra and the `standard' ones are observed.