Diagnostic X-ray Spectra Research Articles

Shaping X-ray spectra with filters in X-ray diagnostics

The influence on image contrast, tube load and patient mean absorbed dose of different ways of shaping diagnostic X-ray spectra by placing filters in the beam is derived fro two radiographic models (abdominal screen-film radiography and intra-oral, dental radiography) using a computational model. The filters are compared at either equal tube load (keeping tube potential constant) or equal contrast (adjusting the tube potential with the different filters), but always at equal energy imparted per unit area to the image receptor. Compared at equal tube load and relative to standard aluminum filtration, reductions in the mean absorbed dose in the patient of 15-25% can be achieved using filters of Cu, Ti, W and Au (increasing the tube load by 30-40% compared with standard aluminium filtration). However, contrast is also reduced by 7%. Compared at equal contrast, the dose reductions are smaller, about 10%. Filters of copper are generally recommended, as are filters of aluminium. The use of bandpass filters (K-edge filters) should be restricted to examinations where the need for substantial variation in tube potential from patient to patient is small. The benefit of using thicker filters than those commonly used today (increasing tube load by factors of 1.4-2.0 compared with no added filter) is small as the dose reduction is most rapid for small initial values of added filters, and the increase in tube load increases steadily with increasing filter thickness.

Experimental separation of diagnostic X-ray spectra into scatter and primary components.

Experimental separation of diagnostic X-ray spectra into scatter and primary components.

Calculation of Characteristic X-rays in Diagnostic X-ray Spectrum

We examined the experimental data of Fewell et al. [ Handbook of Computed Tomography X-Ray Spectra (HHS Publication, Rockville, 1981) No. 81–8162], which exhibited the variation of the characteristic radiation yield with the beam angle, but did not confirm any model proposed by previous workers to explain its variation. We developed a new method of calculating the intensity of \\hyphenationbrems-strah-lung the characteristic radiation for the X-ray tube used in medical diagnosis. Our method was based on the photoelectric interactions of the bremsstrahlung photons with the target material, where the bremsstrahlung spectra used were calculated taking into account the electron energy distributions in a target material described in our previous paper. Our method did not include any adjustment parameters to fit with experimental data. Our calculated results are in good agreement with the experimental results of Fewell et al. In particular, the dependence of the intensity of the characteristic radiation on the beam angle, calculated using our method, agreed with the experiment results of Fewell et al., while the methods proposed by previous workers led to the reverse dependence.

Composite Materials for X-ray Protection

We have developed and tested a radiation protection material that provides similar attenuation for diagnostic x-ray spectra to that of conventional Pb apron materials with approximately 30% reduced weight. By combining a number of elements with different K absorption energies, such as Ba, W, and Pb, energy attenuation for given spectra can be optimized with respect to total cross-sectional mass loading. Alternatively, garments with much higher protective factors at equivalent weight to conventional garments could be produced. The reduction in the amount of Pb used also reduces problems associated with the toxicity of the material during manufacture and disposal. Back strain can be reduced for personnel performing special radiological procedures that require wearing protective garments for long periods of time.

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*1.7*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.

Photoelectron escape from a silicon X-ray detector: Monte Carlo study

Photoelectron escape fractions for small X-ray detectors used in the measurement of diagnostic X-ray spectra are calculated using a Monte Carlo method. The calculation is carried out for detectors 50 to 500 mu m in thickness and at several different diameters with incident photons in the energy range 10-300 keV. When a photoelectron escapes, the partial energy of an incident photon is absorbed in the detector; the spectra of the absorbed energy are shown. The calculated escape fractions are compared with experimental data, and a good agreement is obtained. The diagnostic X-ray spectra measured with a silicon detector 95 mu m in thickness is corrected for detector distortion using the calculated escape fraction; with this correction, a spectrum similar to that determined with a Ge detector is obtained. For an easy calculation of the escape fractions, an empirical equation is presented; this equation gives the escape fractions for any size of detector. The escape fraction depends strongly on detector size and on photon energy; the detector size suitable for measured X-ray energy is also discussed.

Effects of Voltage Ripple and Current Mode on Diagnostic X-ray Spectra and Mean Energies

Effects of Voltage Ripple and Current Mode on Diagnostic X-ray Spectra and Mean Energies

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.

Laplace reconstruction of experimental diagnostic x-ray spectra.

This paper displays the results of a blind study used to determine the capability of a Laplace transform pair model to accurately reconstruct diagnostic x-ray spectra from experimental attenuation data. Spectra reconstructed from attenuation measurements are compared to experimental spectra obtained on the same unit using an intrinsic germanium spectrometer system. The results show that when pure attenuation materials are used, good agreement is obtained between the experimental and computed spectra. If an alloy attenuator like 1100 aluminum is used, the proportion of contaminants must be included in the Laplace formulation for accurate reconstruction.

A method for the reconstruction of diagnostic X-ray spectra from measurements of attenuation

The method presented here uses tables of published X-ray spectra which can be adjusted by computer until good agreement is obtained between the measured attenuations and attenuations calculated using the computer-generated spectra. Allowance is made for the presence of characteristic X-rays by the choice of spectral tabulation and the effects of K-edge filters are included by using an appropriate exponential factor to modify the tabulated data. The fitted X-ray spectra are therefore assured of being realistic in shape because of their close relationship to known, tabulated spectra. The fit to the attenuation data is obtained by varying the peak voltage and adding (non-K-edge) filtration to the computer-generated spectrum. The technique therefore provides a means of estimating peak voltages and filtration for the X-ray set under investigation relative to a standardised set of X-ray spectra.

Diagnostic X-ray spectra measurements using a silicon surface barrier detector

A silicon surface barrier detector is used to analyse diagnostic X-ray spectra. This detector, usually employed to detect charged particles, has a very low efficiency for X-ray. This characteristic is advantageous in overcoming experimental problems caused by the high fluence rates typical of diagnostic X-ray beams. The pulse height distribution obtained with silicon surface barrier detectors is very different from the true photon spectra because of the presence of escaped Compton photons and the fact that detection efficiency falls abruptly when photon energy increases. A detailed analysis of the spurious effects involved in detection is made by applying a Monte Carlo method. A stripping procedure is described for implementation on a personal computer. The validity of this method is finally tested by comparison with the experimental results obtained with a Ge detector. The spectra obtained with the Si detector are in fairly good agreement with the analogous spectra measured with a Ge detector. The advantages of using Si as opposed to Ge detectors in X-ray spectrometry can be summarised as: its simplicity of use, its greater economy for use in routine diagnostic X-ray spectroscopy and the possibility that the stripping procedure can be implemented on a personal computer.

Resolution limits of a low-pressure ionography chamber

The sources of resolution loss are presented for an ionography chamber operated at low gas pressure (1-2*105 Pa). The alignment tolerances imposed by the spherical geometry have been calculated and are shown to be easily attainable. Monte Carlo simulations, calculating the blurring effect of photoelectron ranges, have been carried out for several diagnostic X-ray spectra and with Freon 13B1 as the chamber gas. The resulting line spread functions have been Fourier transformed to give the modulation transfer function. It is seen that for all of the X-ray beam qualities presented in this paper, the limiting resolution is in excess of 2 cycles/mm. Finally, the effect of ante-charge collection upon the resolution is discussed and a technique is proposed whereby this effect may be reduced.

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 the authors' 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.

Theoretical and experimental studies of the influence of air kerma rate on threshold contrast in diagnostic fluoroscopy systems

Experimental measurements of threshold contrast (CT) as a function of air kerma rate at the input plane of the image intensifier have been made for several diagnostic fluoroscopy units in clinical use. Threshold contrasts are determined by viewing a test object containing holes of fixed diameter and various depths under defined irradiation conditions. Kerma rate variations are effected by introducing aluminum sheets into the X-ray beam at fixed values of tube potential and current. At low kerma rates where quantum noise dominates, low tube potentials (60 kVp) usually yield lower values of CT than do higher potentials (100 kVp). At higher kerma rates the opposite is often true. A simple theoretical model for noise propagation in fluoroscopic imaging systems using models of diagnostic X-ray spectra lends qualitative support to the experimental findings. The often-quoted suggested upper limit of 100 mu R s-1 (0.87 mu Gy s-1) at the input phosphor would seem to be justified under the test conditions since little improvement in CT is usually observed at higher kerma rates. However, application to clinical practice would ideally require the use of more realistic phantom studies.

Effect of collimators on the measurement of diagnostic x-ray spectra

One of the difficulties in the measurement of diagnostic x-ray spectra is the high fluence rate of the photons, which may exceed the capability of the spectrometer system. Pinhole collimators are often used which reduce the count rate to an acceptable level. The measured spectrum, however, may be distorted due to the penetration of photons through the collimator material. In this study, the effects of collimators on the measured spectra were evaluated. As an indicator of collimator performance, the authors determined the transmission equivalent aperture (TEA), through which a portion of the incident photons impinge on the detector. The dependence of the TEA on the geometry of collimation, pinhole area, collimator material and thickness, detector area and incident x-ray energy was investigated. The penetration correction factors (PCF) for the measured spectra were also derived. The authors obtained good correspondence between the theoretical spectra and the measured spectra which were corrected for collimator penetration by means of the calculated PCFs. These results indicate that diagnostic x-ray spectra generated at high tube current can be measured if one uses a collimator with a small aperture together with its PCF.

Effect of tungsten absorption edge filter on diagnostic x-ray spectra, image quality and absorbed dose to the patient

The x-ray spectra from a tungsten-target diagnostic tube were measured with a lithium-drifted silicon detector. Four characteristic x-ray peaks were clearly observed. When a 0.05-0.3 mm thick tungsten absorption edge filter was added to the tube, the number of photons in the spectra above the K-absorption edge decreased dramatically. The effect of the absorption edge filter on image quality and on the absorbed dose were investigated by both measurement and Monte Carlo calculation. The absorbed dose to the patient is reduced without image quality being degraded.

Rapid computation of diagnostic X-ray bremsstrahlung spectra.

A method is described for rapid and accurate computation of diagnostic x-ray spectra. Accuracy limitations of Kramers' equation are overcome by providing intensity correction factors derived from published measured data. Use of a parameter based on the energy of the Kramers spectrum intensity peak permits deriving master factor curves which are remarkably independent of kVp, waveform and filtration. Computed and measured spectra generally agree to better than +/- 1 keV for beams generated at 100 kVp and below. Possible application of the method to higher energy diagnostic beams is discussed.

Correction for beam hardening in computed tomography

The author investigates how one can estimate the total attenuation, p, of a polyenergetic X-ray beam what the total attenuation, m, of a monoenergetic beam would have been along the same ray. He finds that for beams with typical diagnostic X-ray spectra passing through the human body one can find a simple function f such that f(p) is a sufficiently close estimate of m to allow good reconstructions. He also finds that m cannot be accurately estimated from p based on the assumption that the human body consists of water alone. The author's results are demonstrated by reconstructions of a mathematical model of a cross-section of the human thorax. The article is self-contained and includes in its Appendices a detailed discussion of the mathematical nature of the problem of beam hardening in computed tomography.

Measured x-ray spectra from 25 to 110 kVp for a typical diagnostic unit.

Diagnostic x-ray spectra were measured with a NaI(T1) spectrometer system. A digital computer was used to correct the observed spectrum and to find the true spectrum by solving a matrix equation. X-ray spectra for various diagnostic x-ray units were measured under different conditions of focal spot size, duration of exposure, tube current, and kVp. Half-value layer, effective photon energy, and mean energy were measured and compared for similar x-ray units. The principal variation in x-ray spectra was related to differences in kVp and tube filtration. Focal spot size and mAs variation did not significantly influence the measured x-ray spectra.