Robert G. Waggener

Correction for spectral artifacts in cross-sectional reconstruction from x rays.

A monoenergetic response correction is described which, along with adequate filtration, may be used to remove the spectral shift artifact encountered in three-dimensional reconstruction from x rays. Reconstructions were carried out by means of a convolution algorithm for simulated data using this method. These are compared with reconstructions obtained using fixed-length water-bath scans as a remedy for the special artifact. These studies suggest that the spectral artifact can be successfully eliminated from computerized cross-sectional scans without resorting to the use of the water bath while, at the same time, improving quantum statistics and/or permitting operation at a lower tube current.

X-ray spectra estimation using attenuation measurements from 25 kVp to 18 MV

Attenuation measurements for primary x-ray spectra from 25 kVp to 18 MV were made using aluminum filters for all energies except for orthovoltage where copper filters were used. An iterative perturbation method, which utilized these measurements, was employed to derive the apparent x-ray spectrum. An initial spectrum or pre-spectrum was used to start the process. Each energy value of the pre-spectrum was perturbed positively and negatively, and an attenuation curve was calculated using the perturbed values. The value of x-rays in the given energy bin was chosen to minimize the difference between the measured and calculated transmission curves. The goal was to derive the minimum difference between the measured transmission curve and the calculated transmission curve using the derived x-ray spectrum. The method was found to yield useful information concerning the lower photon energy and the actual operating potential versus the nominal potential. Mammographic, diagnostic, orthovoltage, and megavoltage x-ray spectra up to 18 MV nominal were derived using this method. The method was validated using attenuation curves from published literature. The method was also validated using attenuation curves calculated from published spectra. The attenuation curves were then used to derive the x-ray spectra.

Experimental and calculated bremsstrahlung spectra from a 25‐MeV linear accelerator and a 19‐MeV betatron

The bremsstrahlung spectra from a 25‐MeV linear accelerator and a 19‐MeV betatron have been measured using a NaI(Tl) spectrometer system. The spectra show a low energy cut‐off at 0.6 and 0.4 MeV, respectively, and the maximum photon energies were 26.8 and 19 MeV, respectively. Cross sections, thin‐ and thick‐target photon spectra and total electron energy losses (collision and radiative) were computed using a digital computer for tungsten ( Z = 74 ) and platinum ( Z = 78 ) targets. The measured spectra were compared to the calculated spectra for thin and thick targets using electron kinetic energies of 26.75 and 19 MeV. The photon spectra from the two therapy units are different; however, depth dose data (10 × 10 cm, 100 cm TSD) are approximately the same. In addition, narrow‐beam attenuation coefficients in lead for the two machines were measured. The effective energy derived from the first HVL was 8.1 MeV for the linear accelerator and 7.8 MeV for the betatron.

Calculated mammographic spectra confirmed with attenuation curves for molybdenum, rhodium, and tungsten targets

A model for calculating mammographic spectra independent of measured data and fitting parameters is presented. This model is based on first principles. Spectra were calculated using various target and filter combinations such as molybdenum/molybdenum, molybdenum/rhodium, rhodium/rhodium, and tungsten/aluminum. Once the spectra were calculated, attenuation curves were calculated and compared to measured attenuation curves. The attenuation curves were calculated and measured using aluminum alloy 1100 or high purity aluminum filtration. Percent differences were computed between the measured and calculated attenuation curves resulting in an average of 5.21% difference for tungsten/aluminum, 2.26% for molybdenum/molybdenum, 3.35% for rhodium/rhodium, and 3.18% for molybdenum/rhodium. Calculated spectra were also compared to measured spectra from the Food and Drug Administration [Fewell and Shuping, Handbook of Mammographic X-ray Spectra (U.S. Government Printing Office, Washington, D.C., 1979)] and a comparison will also be presented.

Spectral effects on three‐dimensional reconstruction from x rays

Continuous bremsstrahlung spectra were calculated for 120 kVp for constant and sinusoidal potentials. Fluorescent radiation for the tungsten target was added to the bremsstrahlung, and the spectra were attenuated through various filter materials. A drawing of an object to be scanned was divided into an array of small squares in which the composition was assumed to be constant. Transmission data for 120 rays at each of 120 angles spanning a range of 180 degrees were calculated. Two algorithms for the reconstruction of attenuation coefficients from projection data, an algebraic reconstruction technique (ART) and the convolution method, were utilized to reconstruct effective coefficients. The effect of spectral filtration on the quality of the reconstruction was evaluated. Lightly filtered x-ray beams give rise to severe distortions in image quality, with values of the reconstructed coefficients rising toward the periphery of the object. Highly filtered beams give rise to images with less pronounced distortion.

Measurement of primary bremsstrahlung spectrum from an 8-MeV linear accelerator

The bremsstrahlung spectrum from an 8-MeV linear accelerator has been measured using a NaI(T1) spectrometer system. The spectrum shows a low-energy cutoff at 0.4 MeV and the maximum photon energy to be approximately 6% greater than the nominal energy. The maximum emission of energy fluence was 1.6 and 1.8 MeV for measured and calculated values, respectively. The fast neutron dose in the photon beam was approximately 0.09% of the x-ray dose. The weighted mean energy was 2.3 MeV, measured value, and 2.4 MeV, calculated value.

Measured X-Ray Spectra from 25 to 110 kVp for a Typical Diagnostic Unit

Abstract 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.

Half-value layer and intensity variations as a function of position in the radiation field for film-screen mammography.

Differences in half-value layer (HVL) and radiation intensity are investigated as a function of position in the mammographic radiation field. Sources of systematic variation include the heel effect, the inverse square law, and differential photon path lengths through thicknesses of inherent and added filtration. The combination of these effects can increase the HVL by as much as 9% and reduce intensity by as much as 40% along the cathode-anode axis. To the left and right of the x-ray field central axis, reductions in radiation intensity of up to 9% and minor increases in HVL are noted as well. Optical density variations as a function of position in the field correlate well with the measured radiation intensity changes.

Estimation of chemical composition and density from computed tomography carried out at a number of energies.

A method is presented by whichcomputed tomography scans carried out at a number of energies may be utilized to obtain cross-sectional images of density and atomic number in addition to the conventional array of linear attenuation coefficients. This type of analysis has been carried out for various substances of biological relevance. Computer simulated reconstructions of clinical situations suggest that the method shows promise for providing additional diagnostic information and might dispense to some extent with the necessity of injecting contrast agents into the patient.

Comparison of voltage-divider, modified Ardran-Crooks cassette, and Ge(Li) spectrometer methods to determine the peak kilovoltage (kVp) of diagnostic x-ray units.

This report describes several different techniques that have been used to determine the peak kilovoltage (kVp) of single- and three-phase diagnostic x-ray units operating in both the radiographic and fluoroscopic mode, from 60 to 110 kVp, using (1) a voltage divider with oxcilloscope display; (2) a voltage divider and special summing amplifier with digital display of the voltage utilizing a multichannel analyzer; (3) a version of the Ardran-Crooks cassette technique; and (4) a Ge(Li) spectrometer method. Each technique presents a distinct advantage: Method (1) enables the waveform to be viewed directly; method (2) is probably the most accurate and reproducible technique; method (3) has the greatest ease of operation; and method (4) provides the ability to visualize the x-ray spectrum.