A modified x-ray spectra reconstruction technique

We have previously reported on a four-parameter Laplace transform pair model that accurately reconstructs x-ray spectra from attenuation data. However, for some spectra, the model exhibits non-physical characteristics at the higher x-ray energies. This results when one of the fitted parameters, v, is less than or equal to 0.6. Simply limiting the parameter to values greater than 0.6 produces an unsatisfactory result. It is shown that improved accuracy is obtained when the model is applied to a more filtered segment of the attenuation curve. The resulting spectrum is satisfactory for the more filtered beam. One can then reliably construct the original spectrum by mathematically correcting for the additional filtration.     

Application of a Laplace transform pair model for high-energy x-ray spectral reconstruction

A Laplace transform pair model, previously shown to accurately reconstruct x-ray spectra at diagnostic energies, has been applied to megavoltage energy beams. The inverse Laplace transforms of 2-, 6-, and 25-MV attenuation curves were evaluated to determine the energy spectra of these beams. The 2-MV data indicate that the model can reliably reconstruct spectra in the low megavoltage range. Experimental limitations in acquiring the 6-MV transmission data demonstrate the sensitivity of the model to systematic experimental error. The 25-MV data result in a physically realistic approximation of the present spectrum.     

Measurement of diagnostic x-ray spectra using a silicon photodiode

Diagnostic x-ray spectra (66-103 kV, 1-2 mA) were measured without pinhole collimator or liquid nitrogen cooling by using a silicon p-i-n photodiode. An x-ray count rate was low (approximately 400 photons/s) at distances of 2-3 m away from an x-ray target, because the i layer of the photodiode was very small (1.5 mm2 in area and 75 microns in thickness). The junction reverse current of the photodiode was 50-70 pA at room temperature and energy resolution for 59.5-keV gamma rays was 2.0 keV (full width at half-maximum), which was limited by the electric noise of a preamplifier. Measured spectra were corrected for detector distortion. The corrected spectra were similar to those determined with a Ge detector. The i-layer thickness of the photodiode was an important parameter for the correction. In particular, when the thickness was thin, the fraction of photoelectron escape became large; this fraction was estimated analytically for various i-layer thicknesses. The angular dependence of the full-energy peak efficiency was measured; for the photons whose energies were above 17.8 keV the angular dependence was isotropic except for angles greater than 60 degrees off the photodiode axis. This method is therefore applicable to the measurement of scattered x rays.     

Reconstruction of diagnostic x-ray spectra by numerical analysis of transmission data

Numerical reconstruction of x-ray spectra from narrow-beam transmission data was tested with published spectra in the range from 45 to 100 kVp. Transmission curves were calculated from the spectra to simulate measured data. Spectra were reconstructed from these transmission curves with use of an iterative numerical analysis. Comparison of the calculated spectra with the original spectra shows good agreement, including the tungsten characteristic x rays. This demonstrates the potential usefulness of measured transmission data for deducing x-ray spectra in the diagnostic energy range.     

Determination of diagnostic x-ray spectra with characteristic radiation using attenuation analysis

An analytical method to compute x-ray spectra from attenuation data utilizing the Laplace transformation has been extended to include characteristic radiation. It is based on an a priori technique of determining the ratio of characteristic radiation exposure to the total radiation exposure in diagnostic spectra. The technique is shown to produce characteristic intensities in good agreement with experimentally determined values.     

Simulation of the effect of anode surface roughness on diagnostic x-ray spectra

To simulate the effect of the condition of the anode surface on x-ray output the surface roughness of anode fragments of replaced clinical x-ray tubes was determined. A mean roughness of 1.32-5.22 microm giving effective tungsten absorption layers of up to 18 microm thickness in the direction of the x-ray beam was determined. X-ray spectra were then simulated using the measured surface profiles. The most prominent effect was the reduction of the kerma output of the tube with increasing roughness (-33% at 50 kV tube voltage and 5 microm roughness). The effects of beam hardening are less pronounced. The corresponding changes in the spectral parameters are generally small but largest for tube voltages up to 70 kV. For higher voltages the increased photon attenuation above the K-edge of tungsten reduces the effects of roughness on spectral parameters. The maximum shifts in mean photon energy of about 1 keV and in HVL of about 0.2 mm Al were obtained for an anode roughness of 5 microm.     

Effects of voltage ripple and current mode on diagnostic x-ray spectra and exposures

The voltage-ripple dependence relationship on x-ray energy-spectral values (energy fluence per unit interval of photon energy) and exposures at 70-kV peak were obtained theoretically by using the semiempirical formula of emission spectra given by Birch and Marshall [Phys. Med. Biol. 24, 505 (1979)]. The calculations were performed with and without various thicknesses of aluminum. As the ripple increases, the energy-spectral values decrease as expected. When the ripple is large, however, energy-spectral values (per mAs) take the minimum values; therefore, the exposure (per mAs) also reaches the minimum value for the unsaturating current modes, contrary to expectation. The reasons for this phenomenon were clarified. Exposures clearly take the minimum value in 2-pulse units. This phenomenon was experimentally verified.     

A silicon diode in a thimble-type mount for measurement of diagnostic x-ray spectra

A silicon detector which can be used like a thimble chamber has been constructed. The silicon diode chip used here is very small (1.7 x 1.7 x 0.1 mm3), and the construction materials of the detector mount are thin and have low atomic numbers. Thus, the characteristic x-rays from the mount give no distortion on the obtained spectra, and the photons incident on the back surface of the silicon chip can be measured. Furthermore, the perturbation of the x-ray field is small. The energy resolution of the detector is 2.0 keV (full width at half maximum) for 59.5 keV photons at room temperature. Bremsstrahlung spectra (60-100 kV, 1-2 mA) have been measured without using a pinhole collimator; the obtained spectra are in good agreement with those obtained with a Ge detector. The spectrum of the radiation in a water phantom irradiated with 90 kV x-rays has also been measured by inserting the detector in the phantom; the obtained scattered spectrum has been corrected for the angular dependence of detector efficiency. The increase in total photon number due to this correction has been found to be below 2%.     

The use of cadmium telluride detectors for the qualitative analysis of diagnostic x-ray spectra

A method is introduced for the evaluation of x-ray spectra from x-ray machines operating in the range 50-100 kVp using a cadmium telluride (CdTe) detector with low detection efficiency. The pulse height distribution obtained with this kind of detector does not represent the true photon spectra owing to the presence of K-escape, Compton scattering, etc.; these effects were evaluated using a Monte Carlo method. A stripping procedure is described for implementation on a Univac 1100/82 computer. The validity of our method was finally tested by comparison with experimental results obtained with a Ge detector and with data from the literature; the results are in good agreement with published data.     

Monte carlo simulation of the compton scattering technique applied to characterize diagnostic x-ray spectra

The quality control of x-ray tubes for medical radiodiagnostic services is very important for such devices. Therefore, the development of new procedures to characterize the x-ray primary beam is highly interesting in order to obtain an accurate assessment of the actual photon spectrum. The Compton scattering technique is very useful to determine x-ray spectra (in the 10-150 kVp range), avoiding a pile-up effect in the detector since a large room is not usually available to apply other techniques. In this work, this process has been simulated using a Monte Carlo code, MCNP 4C. Some geometrical models have been developed and different techniques have been studied in order to improve statistics and accuracy in the acquisition of Pulse Height Distribution (PHD). The effect of both the collimation of the primary beam and the scattering angle of the spectrometer has been analyzed. Results obtained using simulation models have been compared with experimental measurements.     

Off-axis x-ray spectra: a comparison of Monte Carlo simulated and computed x-ray spectra with measured spectra

The off-axis x-ray spectra from a constant potential x-ray generator were measured with a high purity germanium spectrometer cooled to liquid nitrogen temperature. The measured spectra were compared with off-axis x-ray spectra calculated using a code based on the semiempirical model developed by Tucker et al. and Monte Carlo simulated x-ray spectra using the EGS4 code system. In this study, both the Tucker model, and the EGS4 code system, were found to produce off-axis bremsstrahlung x-ray spectra which agreed well with the spectra measured at three emerging angles. In the measured and the EGS4 generated spectra the total K-characteristic peaks were in increasing order, as observed in the anode to cathode direction, whereas the Tucker model produced maximum total K-characteristic peaks at the 6 degrees anode side, and lesser amounts at the central axis and the 6 degrees cathode side. Large differences in the total K-characteristic lines is seen among the three different methods. The EGS4 code system was able to produce x-ray spectra for a combination of target materials.     

Assessment of different computational models for generation of x-ray spectra in diagnostic radiology and mammography

Different computational methods based on empirical or semi-empirical models and sophisticated Monte Carlo calculations have been proposed for prediction of x-ray spectra both in diagnostic radiology and mammography. In this work, the x-ray spectra predicted by various computational models used in the diagnostic radiology and mammography energy range have been assessed by comparison with measured spectra and their effect on the calculation of absorbed dose and effective dose (ED) imparted to the adult ORNL hermaphroditic phantom quantified. This includes empirical models (TASMIP and MASMIP), semi-empirical models (X-rayb&m, X-raytbc, XCOMP, IPEM, Tucker et al., and Blough et al.), and Monte Carlo modeling (EGS4, ITS3.0, and MCNP4C). As part of the comparative assessment, the K x-ray yield, transmission curves, and half value layers (HVLs) have been calculated for the spectra generated with all computational models at different tube voltages. The measured x-ray spectra agreed well with the generated spectra when using X-raytbc and IPEM in diagnostic radiology and mammography energy ranges, respectively. Despite the systematic differences between the simulated and reference spectra for some models, the student's t-test statistical analysis showed there is no statistically significant difference between measured and generated spectra for all computational models investigated in this study. The MCNP4C-based Monte Carlo calculations showed there is no discernable discrepancy in the calculation of absorbed dose and ED in the adult ORNL hermaphroditic phantom when using different computational models for generating the x-ray spectra. Nevertheless, given the limited flexibility of the empirical and semi-empirical models, the spectra obtained through Monte Carlo modeling offer several advantages by providing detailed information about the interactions in the target and filters, which is relevant for the design of new target and filter combinations and optimization of radiological imaging protocols.     

Segmentation-free statistical image reconstruction for polyenergetic x-ray computed tomography with experimental validation

This paper describes a statistical image reconstruction method for x-ray CT that is based on a physical model that accounts for the polyenergetic x-ray source spectrum and the measurement nonlinearities caused by energy-dependent attenuation. Unlike our earlier work, the proposed algorithm does not require pre-segmentation of the object into the various tissue classes (e.g., bone and soft tissue) and allows mixed pixels. The attenuation coefficient of each voxel is modelled as the product of its unknown density and a weighted sum of energy-dependent mass attenuation coefficients. We formulate a penalized-likelihood function for this polyenergetic model and develop an iterative algorithm for estimating the unknown density of each voxel. Applying this method to simulated x-ray CT measurements of objects containing both bone and soft tissue yields images with significantly reduced beam hardening artefacts relative to conventional beam hardening correction methods. We also apply the method to real data acquired from a phantom containing various concentrations of potassium phosphate solution. The algorithm reconstructs an image with accurate density values for the different concentrations, demonstrating its potential for quantitative CT applications.     

Monte carlo simulation of x-ray spectra in diagnostic radiology and mammography using MCNP4C

The general purpose Monte Carlo N-particle radiation transport computer code (MCNP4C) was used for the simulation of x-ray spectra in diagnostic radiology and mammography. The electrons were transported until they slow down and stop in the target. Both bremsstrahlung and characteristic x-ray production were considered in this work. We focus on the simulation of various target/filter combinations to investigate the effect of tube voltage, target material and filter thickness on x-ray spectra in the diagnostic radiology and mammography energy ranges. The simulated x-ray spectra were compared with experimental measurements and spectra calculated by IPEM report number 78. In addition, the anode heel effect and off-axis x-ray spectra were assessed for different anode angles and target materials and the results were compared with EGS4-based Monte Carlo simulations and measured data. Quantitative evaluation of the differences between our Monte Carlo simulated and comparison spectra was performed using student's t-test statistical analysis. Generally, there is a good agreement between the simulated x-ray and comparison spectra, although there are systematic differences between the simulated and reference spectra especially in the K-characteristic x-rays intensity. Nevertheless, no statistically significant differences have been observed between IPEM spectra and the simulated spectra. It has been shown that the difference between MCNP simulated spectra and IPEM spectra in the low energy range is the result of the overestimation of characteristic photons following the normalization procedure. The transmission curves produced by MCNP4C have good agreement with the IPEM report especially for tube voltages of 50 kV and 80 kV. The systematic discrepancy for higher tube voltages is the result of systematic differences between the corresponding spectra.     

Portable x-ray diagnostic equipment

Scientific-Industrial Association “Spektr”, Moscow. Translated from Meditsinskaya Tekhnika, No. 4, pp. 22–28, July–August, 1992.     

Ionometric x-ray diagnostic devices

Scientific-Research Institute for Introscopy, Moscow. Moscow Scientific-Research Institute for Diagnostics and Surgery. Translated from Meditsinskaya Tekhnika, No. 3, pp. 29–31, May–June, 1995.