X-ray Beam Spectrum Research Articles

Reference dosimetry at the Australian Synchrotron's imaging and medical beamline using free‐air ionization chamber measurements and theoretical predictions of air kerma rate and half value layer

Novel, preclinical radiotherapy modalities are being developed at synchrotrons around the world, most notably stereotactic synchrotron radiation therapy and microbeam radiotherapy at the European Synchrotron Radiation Facility in Grenoble, France. The imaging and medical beamline (IMBL) at the Australian Synchrotron has recently become available for preclinical radiotherapy and imaging research with clinical trials, a distinct possibility in the coming years. The aim of this present study was to accurately characterize the synchrotron-generated x-ray beam for the purposes of air kerma-based absolute dosimetry. The authors used a theoretical model of the energy spectrum from the wiggler source and validated this model by comparing the transmission through copper absorbers (0.1-3.0 mm) against real measurements conducted at the beamline. The authors used a low energy free air ionization chamber (LEFAC) from the Australian Radiation Protection and Nuclear Safety Agency and a commercially available free air chamber (ADC-105) for the measurements. The dimensions of these two chambers are different from one another requiring careful consideration of correction factors. Measured and calculated half value layer (HVL) and air kerma rates differed by less than 3% for the LEFAC when the ion chamber readings were corrected for electron energy loss and ion recombination. The agreement between measured and predicted air kerma rates was less satisfactory for the ADC-105 chamber, however. The LEFAC and ADC measurements produced a first half value layer of 0.405 ± 0.015 and 0.412 ± 0.016 mm Cu, respectively, compared to the theoretical prediction of 0.427 ± 0.012 mm Cu. The theoretical model based upon a spectrum calculator derived a mean beam energy of 61.4 keV with a first half value layer of approximately 30 mm in water. The authors showed in this study their ability to verify the predicted air kerma rate and x-ray attenuation curve on the IMBL using a simple experimental method, namely, HVL measurements. The HVL measurements strongly supports the x-ray beam spectrum, which in turn has a profound effect on x-ray dosimetry.

A new gas attenuator system for the ID17 biomedical beamline at the ESRF

On the biomedical beamline ID17 at the ESRF a gas attenuator system has been installed to complement and protect the standard solid state attenuators (graphite, Al and Cu) against fatigue and damage due to the very high heat load from the beamline's wiggler source. This series of attenuators defines the flux (dose rate) and the X-ray beam spectrum for the Microbeam Radiation Therapy (MRT) research at ID17 which is currently under development towards clinical application. For this, the attenuators at MRT will be crucial elements to guarantee beam- and dose rate characteristics and the new gas attenuator will become a radiation therapy safety device. The installed gas attenuator and its test results will be presented.

A Hybrid Angular/Energy Dispersive Method to Improve Some Characteristics of Laboratory X-Ray Diffraction

The Energy Dispersive X-ray Diffraction, generally referred as EDXD, has shown to be a valid alternative to the conventional Angular Dispersive X-ray Diffraction, the ADXD. EDXD exhibits several advantages to its AD counterpart, mainly related to the properties of the polychromatic X-ray beam utilized for diffracting, such as higher signal intensities, a wider accessible region of the reciprocal space, a greater transparency of samples, and a parallel data collection of the q-points in the diffraction pattern acquisition. However, the main drawback of poly-chromaticity lays in the fact that the quantities that modulate the scattered intensity in a diffraction measurement depend on the energy. These quantities are the primary X-ray beam spectrum, polarization, and X-ray absorption, the last producing by far the most critical effect because it rapidly changes as a function of energy. Therefore, a detailed knowledge of the energy dependence of all these quantities is required in EDXD in order to process the data correctly and prevent systematic errors. The difficulty in handling the energy-dependent factors complicates the experimental procedure and may make the measurements unreliable. In the present paper, a hybrid method between the ED and AD X-ray Diffraction is proposed to maintain the advantages of the polychromatic nature of the radiation utilized in EDXD, while preventing the problems produced by the energy-dependent quantities.

Effect of spectral shape in the relative efficiency of LiF: Mg,Ti exposed to 20 keV effective energy X-rays

Abstract This work is aimed at studying the possible dependence of the thermoluminescent response of LiF:Mg,Ti (TLD-100) on the X-ray beam spectrum. TLD-100 chips were irradiated with 20 keV effective energy filtered tungsten X-ray spectra, generated by a tube operated at 30, 40, 50, 60, 70 and 80 kV, and the response was measured relative to the response to 60Co gammas. TLDs were exposed in air at air kerma values between 50 and 300 mGy. Glow curve deconvolution into 8 peaks (3, 4, 5, 6a, 6b, 7, 8 and 9) was performed in order to evaluate the response of individual peaks and regions of interest in the glow curve. Results show that all peaks display a linear response, at least up to 200 mGy, for all X-ray beam qualities. The relative response of the total glow curve area, the dosimetric region (peaks 3 + 4 + 5) and peaks 5 to 9 is statistically constant for beams generated by voltages from 30 to 70 kV; for 80 kV, the values are lower by about 20% for peaks 8 and 9 and 9% for the rest. The relative efficiency for the glow curve area, the dosimetric region and peaks 5 (1.15, 1.10 and 1.13), peaks 8 and 9 (2.05 and 1.55) do not depend on the beam spectrum between 30 and 70 kV, while for 80 kV the efficiency is lower. For peaks 6a, 6b and 7, relative efficiencies (1.91, 2.94 and 2.60) are voltage independent.

Exploring the useful exposure range limits of three intraoral image receptors for various tube potential, tube current and exposure time settings

Objectives: To determine the useful exposure range limits of three intraoral image receptors of different technology when exposed to different X-ray beam spectra, dose and dose rate levels. Study Design: A dental X-ray unit offering a wide range of tube potential, tube current and exposure time settings was used to expose a dental quality control phantom. The receptors that were used to capture the radiographic images of the phantom were: the Kodak Insight, the Kodak RVG-6000 and the Duerr Vistascan system. The images that were produced over a wide range of exposure factor settings were evaluated in terms of diagnostic quality by three experienced radiologists. Results: The number of images with acceptable diagnostic quality was in total 1257; 310 with Insight, 331 with RVG 6000 and 616 with Vistascan. At 60 kV, diagnosable images were produced with doses ranging from 0.44-1.56 mGy for the Insight film 0.44-2.82 mGy for the RVG 6000 and 0.22-4.93 mGy for the Vistascan system. At 70 kV, the respective ranges were 0.39-1.28 mGy for the Insight film 0.31-2.55 mGy for the RVG6000 and 0.30-3.46 mGy for the Vistascan system. Conclusions: The Vistascan exhibited the widest useful exposure range and required the least exposure to produce a diagnosable image at almost all tube potential settings. The RVG 6000 exhibited a slightly wider useful exposure range than the Insight film, with almost the same dose requirements especially in higher Kv settings.

Performance comparison between three intraoral image receptors of different technology at a variety of tube potential, tube current and exposure time settings using a stepwedge phantom

Purpose: To comparatively evaluate the per-formance of three intraoral image receptors of different technology when exposed to different X-ray beam spectra, dose and dose rate levels using a stepwedge phantom. Materials and methods: The intraoral radiographic receptors evaluated were: the Kodak Insight F speed class film, the Kodak RVG 6000, and the Duerr Vis-tascan Combi PSP system. A dental quality control phantom made of Plexiglas, containing an aluminium stepwedge with 12 steps and 7 holes drilled in each step was radiographed using a dental X-ray unit offering a wide range of tube potential, tube current and exposure time settings. The visibility of the holes in the images produced with each one of the three receptors was assessed by three independent observers. For each image the total image quality score (TS) was derived from the summation of the number of visible holes in each step. The numbers of perceptible holes in each experimental condition (TSs) were statistically analyzed through use of analysis of variance. Intraobserver and interobserver agree-ment was also measured. Results: Vistascan ex-hibited the most extended useful exposure range, followed by RVG 6000 and Insight. RVG 6000 exhibited the largest TS values in all tube potential settings except 70 kV where the Vistascan performed better. Insight performed better than Vistascan only at 60 and 63 kV. Vistascan performed better at 66 and 70 kV, Insight at 60 and 66 kV, whereas RVG performed equally well at all tube potential settings, except than at 52 and 70 kV. For the Insight the largest TS values were obtained with the smallest ESAK values whereas with the Vistascan the largest TS were obtained with ESAK values that where the largest observed. Conclusions: The performance of all receptors tested was greatly dependent on the exposure parameters and mainly on the kV settings. Overall, the RVG 6000 offered the best image quality at doses somewhere in between those required by the Insight and the Vistascan.

An investigation of dose changes for therapeutic kilovoltage x‐ray beams with underlying lead shielding

Kilovoltage x-ray beams are used to treat cancer on or close to the skin surface. Many clinical cases use high atomic number materials as shielding to reduce dose to underlying healthy tissues. In this work, we have investigated the effect on both the surface dose and depth doses in a water phantom with lead shielding at depth in the phantom. The EGSnrc Monte Carlo code was used to simulate the water phantom and to calculate the surface doses and depth doses using primary x-ray beam spectra derived from an analytical model. The x-ray beams were in the energy range of 75-135 kVp with field sizes of 2, 5 and 8 cm diameter. The lead sheet was located beneath the water surface at depths ranging from 0.5-7.5 cm. The surface dose decreased as the lead was positioned closer to the water surface and as the field size was increased. The variation in surface dose as a function of x-ray beam energy was only small but the maximum reduction occurred for the 100 kVp x-ray beam. For the 8 cm diameter field with the lead at 1 cm depth and using the 100 kVp x-ray beam, the surface dose was reduced to 0.898 of the surface dose in the water phantom only. Measured surface dose changes, using a Farmer-type ionization chamber, agreed with the Monte Carlo calculated doses. Calculated depth doses in water with a lead sheet positioned below the surface showed that the dose fall-off increased as the lead was positioned closer to the water surface as compared to the depth dose in the water phantom only. Monte Carlo calculations of the total x-ray beam spectrum at the water surface showed that the total fluence decreased due to a reduction in backscatter from within the water and very little backscatter from the lead. The mean energy of the x-ray spectrum varied less than 1 keV, with the lead at 1 cm beneath the water phantom surface. As the Monte Carlo calculations showed good agreement with the measured results, this method can be used to verify surface dose changes in clinical situations where measurements are difficult. The clinical impact of the use of lead must be considered in the dose prescription for patients being treated with kilovoltage x-ray beams.

Management of pediatric radiation dose using Philips fluoroscopy systems DoseWise: perfect image, perfect sense

Although image quality (IQ) is the ultimate goal for accurate diagnosis and treatment, minimizing radiation dose is equally important. This is especially true when pediatric patients are examined, because their sensitivity to radiation-induced cancer is two to three times greater than that of adults. DoseWise is an ALARA-based philosophy within Philips Medical Systems that is active at every level of product design. It encompasses a set of techniques, programs and practices that ensures optimal IQ while protecting people in the X-ray environments. DoseWise methods include management of the X-ray beam, less radiation-on time and more dose information for the operator. Smart beam management provides automatic customization of the X-ray beam spectrum, shape, and pulse frequency. The Philips-patented grid-controlled fluoroscopy (GCF) provides grid switching of the X-ray beam in the X-ray tube instead of the traditional generator switching method. In the examination of pediatric patients, DoseWise technology has been scientifically documented to reduce radiation dose to <10% of the dose of traditional continuous fluoroscopy systems. The result is improved IQ at a significantly lower effective dose, which contributes to the safety of patients and staff.

Improvement of the radiation hardness of a directly converting high resolution intra-oral X-ray imaging sensor

The radiation tolerance of a directly converting digital intra-oral X-ray imaging sensor reported in Spartiotis et al. [Nucl. Instr. and Meth. A 501 (2003) 594] has been tested using a typical dental X-ray beam spectrum. Radiation induced degradation in the performance of the sensor which consists of CMOS signal readout circuits bump bonded to a high resistivity silicon pixel detector was observed already before a dose (in air) of 1 krad. Both increase in the leakage current of the pixel detector manufactured by Sintef, Norway and signal leakage to ground from the gate of the pixel input MOSFETs of the readout circuit were observed and measured. The sensitive part of the CMOS circuit was identified as the protection diode of the gate of the input MOSFET. After removing the gate protection diode no signal leakage was observed up to a dose of 5 krad (air) which approximately corresponds to 125.000 typical dental X-ray exposures. The radiation hardness of the silicon pixel detector was improved by using a modified oxidation process supplied by Colibrys, Switzerland. The improved pixel detectors showed no increase in the leakage current at dental doses.

Resolution degradation due to K-characteristic photon absorption in dual phosphor image receptors.

Image receptors based on the absorption of x rays by phosphor screens have a system resolution described predominantly by the modulation transfer function (MTF) of the phosphor screens utilized. This ideal MTF is modified by the presence of secondary effects within the imaging receptor such as scatter from the receptor cover, visible light crossover (in the case of screen film cassettes) and scatter generated within the phosphor itself, presumed to be almost entirely due to photoelectric interactions. In this paper the MTF characteristics resulting from the absorption of K-characteristic x rays emitted by one screen of a pair in an image receptor and absorbed by the adjacent screen (referred to as cross-fluorescence hereafter) are examined. This effect has been previously modeled [Med. Phys. 23, 1253 (1996)] and recently a computer program has been written to calculate intensity of the radial component of a point spread function (PSF) describing the above process. From this, the MTF of cross-fluorescence under varying conditions can be calculated. The model was applied to a dual screen experiment described in the literature [Med. Phys. 23, 871 (1996)], from which it was possible to show good agreement between calculated results and those derived from the measurement. The effect of screen thickness, screen separation, and phosphor material on the MTF of cross-fluorescence was investigated using simulated x-ray beam spectra in association with phosphor screen parameters. Results showed that the calculated MTF falls to below 0.1 at 1 cy/mm in all cases examined. This MTF was reduced further as the separation between the screens increased from that commonly used in a screen film cassette. Beam energy, phosphor thickness, and phosphor material had a minimal or no effect on MTF. The effect of cross-fluorescence on total image receptor MTF is to reduce the high-frequency components of that function by an amount that is in proportion to the scatter fraction of the effect.

DETECT-dual energy transmission estimation CT-for improved attenuation correction in SPECT and PET

Accurate attenuation correction is a vital component of any quantitative emission CT (ECT) study, especially /sup 131/I-mIBG SPECT where accurate activity measurements are required for dosimetry calculations. Conventional CT is often used to obtain an attenuation map, which is then scaled to the /sup 131/I emission energy (364 keV). Scaling inaccuracies due to the increased photoelectric component of the attenuation coefficients of bone at CT energies lead to an overestimate of activity. Dual energy CT is more accurate but must be justified against increased cost scan time and dose to the patient. We propose a method (DETECT) of obtaining two attenuation maps from a single CT scan using a conventional CT scanner operating at 120 and 140 kVp for alternate slices. Each map's missing slices are reconstructed using weighted linear interpolation. The effect of the X-ray beam spectrum has been investigated by simulated scaling using the CT spectra and by a phantom study. Assuming a perfect emission detector, segmented abdominal CT data predicts a mean activity overestimate of 2/spl plusmn/3% for DETECT compared to 12/spl plusmn/5% for the standard single energy (120 kVp) CT method when scaled to 364 keV, suggesting that DETECT can offer reliable attenuation correction with no dose or user penalty.

The AAPM/RSNA physics tutorial for residents. X-ray generators.

The x-ray generator delivers the electrical power to energize the x-ray tube and permits the selection of x-ray energy, x-ray quantity, and exposure time. Major internal components of the generator include transformers, diodes and rectifier circuits, filament and stator circuits, timer switches, and kilovolt and milliampere meters. Single-phase, three-phase, high-frequency, and constant potential generators produce different voltage waveforms (ripple) and x-ray beam spectra. Phototimer and automatic brightness control subsystems measure radiation exposure incident on the image receptor to give instantaneous feedback for optimal radiographic film densities and fluoroscopic image brightness, respectively. At the generator control console, the operator sets the tube voltage, tube current, exposure time, phototimer film density, spot film acquisition, and fluoroscopic parameters. Selection of generator power and options depends on the intended clinical use. X-ray tube focal spot size and power loading capability should be matched to the x-ray generator and clinical imaging requirements. Single and multiple exposure rating charts as well as anode and housing thermal characteristic charts indicate power input and dissipation rates specific to a generator and x-ray tube target and housing.

A correlated noise reduction algorithm for dual-energy digital subtraction angiography.

It has long been recognized that the problems of motion artifacts in conventional time subtraction digital subtraction angiography (DSA) may be overcome using energy subtraction techniques. Of the variety of energy subtraction techniques investigated, non-k-edge dual-energy subtraction offers the best signal-to-noise ratio (SNR). However, this technique achieves only 55% of the temporal DSA SNR. Noise reduction techniques that average the noisier high-energy image produce various degrees of noise improvement while minimally affecting iodine contrast and resolution. A more significant improvement in dual-energy DSA iodine SNR, however, results when the correlated noise that exists in material specific images is appropriately cancelled. The correlated noise reduction (CNR) algorithm presented here follows directly from the dual-energy computed tomography work of Kalender who made explicit use of noise correlations in material specific images to reduce noise. The results are identical to those achieved using a linear version of the two-stage filtering process described by Macovski in which the selective image is filtered to reduce high-frequency noise and added to a weighted, high SNR, nonselective image which has been processed with a high-frequency bandpass filter. The dual-energy DSA CNR algorithm presented here combines selective tissue and iodine images to produce a significant increase in the iodine SNR while fully preserving iodine spatial resolution. Theoretical calculations predict a factor of 2-4 improvement in SNR compared to conventional dual-energy images. The improvement factor achieved is dependent upon the x-ray beam spectra and the size of blurring kernel used in the algorithm.(ABSTRACT TRUNCATED AT 250 WORDS)

A source of spurious peaks in a multi-crystal X-ray spectrometer

The authors demonstrate that, under certain conditions, the spectrum of a monochromatic X-ray beam analysed by a multi-crystal spectrometer includes several spurious peaks. These erroneous peaks are the result of the angular separation of dynamically and kinematically diffracted radiation. The above separation of the diffracted radiation occurs when the diffraction takes place near, instead of on, a reciprocal lattice point, at one of the spectrometer's crystals.

A comparison of water equivalent thickness measurements: CT method vs. heavy ion beam technique

The purpose of this study was to evaluate existing X-ray CT methods for determining water-equivalent path length from body surface to a tumour site. In the method, the CT numbers are used to obtain linear attenuation coefficients which provide a measure of the electron density. These numbers are averaged over the energy spectrum of the diagnostic X-ray beam and other parameters which have a dependence on energy. From range measurements with heavy charged particles, it is also possible to obtain an independent and direct measure of electron density along the beam path. In the results reported here, the beam path electron density or water-equivalent path length was measured with charged particle beams, using radiation sensitive diodes as target markers. To minimise error which would be introduced by motion of the target volume, a frozen dog cadaver was used. Comparison was made between the water-equivalent path length measured with high energy particle beams, and the water-equivalent path length estimated from an X-ray CT image of the same target volume by the methods presently used in charged particle therapy (Chen et al. 1979). There was good agreement between the values determined directly with neon or helium ion beams, but when these values were compared with estimated path lengths derived from X-ray CT data, it was observed that the CT range could be in error by as much as 11% for adverse conditions of marked inhomogeneity and the presence of high atomic number bone. Under the best conditions of moderate inhomogeneity and absence of bone, the derived CT range values agreed reasonably well with the direct measurements.

Central-axis depth-dose data for diagnostic radiology

Depth-dose data have been measured for a range of irradiation conditions relevant to diagnostic radiology in order to provide a basis for the calculation of patient dosage arising from X-ray examinations. In general, data are required for larger field sizes, longer source-surface distances and greater tube filtrations than are commonly encountered in radiotherapy. Principles and techniques of measurement are discussed with particular attention to the specification of X-ray beam quality. It is recommended that both peak tube potential and first half-value thickness should be specified, especially for the determination of doses at depths of 5 cm or more. With the appropriate specification of quality, good agreement is found between these measurements and other compilations of depth-dose data for radiotherapy use. Zero-area depth-doses have also been measured and compared with calculated values derived from theoretical considerations of X-ray beam spectra.

Some Considerations on the Excitation Efficiency in X-Ray Spectrochemical Analysis

AbstractMost considerations of excitation conditions either have ignored the target material of the X-ray tube and the resulting spectrum of the primary X-ray beam or have dealt with it in very general terms, making a few broad assumptions regarding its effect on the final result. This is quite understandable since any attempt to handle the quantitative relationship between the primary spectrum and the resulting fluorescent spectrum results in extremely complex and unwieldy mathematical expressions. However, with the number of different primary sources now available, it is possible to evaluate these complex expressions in terms of actual data and permit more reliable values to be placed on the relative contributions of the components in the primary spectrum.A discussion is included of results obtained with X-ray tubes with chromium, molybdenum, tungsten, and platinum targets together with a few typical applications to actual X-ray spectrochemical procedures. Also discussed are the other variables which play a part in the excitation of fluorescent X-ray spectra, such as the primary X-ray tube voltage and the window of the X-ray tube.