Thomas R. Fewell

Molybdenum, rhodium, and tungsten anode spectral models using interpolating polynomials with application to mammography

Computer simulation is a convenient and frequently used tool in the study of x-ray mammography, for the design of novel detector systems, the evaluation of dose deposition, x-ray technique optimization, and other applications. An important component in the simulation process is the accurate computer-generation of x-ray spectra. A computer model for the generation of x-ray spectra in the mammographic energy range from 18 kV to 40 kV has been developed. The proposed model requires no assumptions concerning the physics of x-ray production in an x-ray tube, but rather makes use of x-ray spectra recently measured experimentally in the laboratories of the Center for Devices and Radiological Health. Using x-ray spectra measured for molybdenum, rhodium, and tungsten anode x-ray tubes at 13 different kVs (18, 20, 22, ..., 42 kV), a spectral model using interpolating polynomials was developed. At each energy in the spectrum, the x-ray photon fluence was fit using 2, 3, or 4 term (depending on the energy) polynomials as a function of the applied tube voltage (kV). Using the polynomial fit coefficients determined at each 0.5 keV interval in the x-ray spectrum, accurate x-ray spectra can be generated for any arbitrary kV between 18 and 40 kV. Each anode material (Mo, Rh, W) uses a different set of polynomial coefficients. The molybdenum anode spectral model using interpolating polynomials is given the acronym MASMIP, and the rhodium and tungsten spectral models are called RASMIP and TASMIP, respectively. It is shown that the mean differences in photon fluence calculated over the energy channels and over the kV range from 20 to 40 kV were -0.073% (sigma = 1.58%) for MASMIP, -0.145% (sigma = 1.263%) for RASMIP, and 0.611% (sigma = 2.07%) for TASMIP. The polynomial coefficients for all three models are given in an Appendix. A short C subroutine which uses the polynomial coefficients and generates x-ray spectra based on the proposed model is available on the World Wide Web at http:/(/)www.aip.org/epaps/epaps.html.

Photon energy distribution of some typical diagnostic x-ray beams.

A high-purity germanium spectrometer system was used to determine primary x-ray spectra over the 45--90-kVp region. Methods were devised for producing and examining spectra stimulating diagnostic conditions without operating the x-ray generator at high current levels. The techniques used to correct the experimental data and produce a photon fluence spectrum are discussed. The results, presented graphically and in tables, have been normalized to yield the relative number of photons per 2-keV interval. Methods for converting a normalized spectrum into a photon fluence spectrum that will produce an exposure of 1 R are presented. The analytical model and procedures used to calculate the K-escape fraction are discussed.

ATTENUATION PROPERTIES OF DIAGNOSTIC X-RAY SHIELDING MATERIALS

Single- and three-phase broad-beam x-ray attenuation data have been obtained using lead, steel, plate glass, gypsum wallboard, lead acrylic, and wood. Tube voltages of 50, 70, 100, 125, and 150 kVp were employed and the resulting curves were compared to transmission data found in the literature. To simplify computation of barrier requirements, all data sets were parametrized by nonlinear least-squares fit to a previously described mathematical model. High attenuation half value layers and the lead equivalence of the alternate materials were also determined.

Evaluation of x-ray sources for mammography

A computational approach is being developed for the evaluation of mammographic imaging system performance. This approach takes into account both the spatial frequency properties and the x-ray spectral characteristics of the system being evaluated. The initial version of the program that implements the approach has been used to evaluate a conventional mammography source assembly for several breast thicknesses, and to compare the conventional tube and filter combination to alternatives that have been suggested for the imaging of breasts that are thicker or more dense than average. It has also been used to study the effect of varying the thickness of the molybdenum filter in the conventional system. The parameters calculated include contrast, average glandular dose, tube load, and a figure of merit, SNR2/Dose. The calculations confirm the strong dependence of system performance on both tube potential and breast thickness for the standard system, and indicate that alternative designs can improve performance in the imaging of thicker or more dense breasts. The study of filter thickness shows that, of the four parameters calculated, only tube load is strongly affected by filter thickness.

A Comparison of Mammographic X-Ray Spectra

X-ray spectra produced by mammographic systems are compared to spectra from conventional diagnostic x-ray systems. Some systems use special anode materials and beam filters to produce x-ray spectra more suitable for mammography. The data show that the spectra produced by some systems are unique; in fact, one using molybdenum for both an anode and beam filtering element can produce an x-ray spectrum having more than 80% of the photons below 20 KeV. Using some typical breast phantom materials as attenuators, the primary x-ray spectra incident upon the imaging system were simulated and displayed. Implications of spectral shaping to image quality and patient dose are discussed.

Precision fabrication of two-dimensional antiscatter grids

Creatv MicroTech is developing two-dimensional, air-core, anti-scatter grids that have the potential to significantly reduce scatter-to-primary ratio and increase primary transmission in mammography. The fabrication method uses x-ray lithography and electroplating, which allows the fabrication of high aspect ratio metal parts. Two unfocused nickel grids were fabricated, one 1.5 cm X 1.5 cm and the other 1.44 cm X 1.44 cm. The grids have 20 micron thick walls and a period of 300 microns. Monte Carlo simulations were performed to predict their performance. The x-ray source was a 30 kVp Mo-anode spectrum and 30 microns of added Mo filtration. Preliminary calculations for a 2 mm-high grid and a 4 cm lucite phantom indicate that a scatter-to-primary ratio less than 3% can be achieved even at 3 cm from the center of the grid. Experiments to test the performance of the grids have been conducted at FDA using a Mo target, 30 micron Mo filter at 30 kVp and a 4 cm thick lucite phantom. A germanium detector was used. Data from a mammographic grid made by Smit Rontgen was taken as a reference. These Ni grids with grid ratios of 6.4 and 7.1 reduce scatter and increase primary transmission compared to the conventional reference grid. This fabrication method is capable of producing focused grids. The demonstration of larger, focused grids is the next step.

Storage phosphor-based digital mammography using a low-dose x-ray system optimized for screen-film mammography

We are examining the feasibility of performing digital mammography by combining a storage- phosphor image receptor with a highly efficient x-ray system. The image receptor consists of Fuji series HR-V high resolution imaging plates and a Fuji 9000 reader. The x-ray system was developed using multiparameter optimization techniques, with the goal of reducing patient dose as much as possible while retaining acceptable imaging performance. We have measured sensitometric properties, modulation transfer function (MTF), and noise power spectrum (NPS) of the Fuji plates with low-energy x-ray spectra. We have used the measurements, along with information about the x-ray system, to estimate signal-to-noise ratios (SNRs) for objects in a contrast-detail (C-D) phantom. We present the results of our measurements on the Fuji plates, comparisons of calculated and observed C-D diagrams for this system and a conventional system, and comparisons of phantom images and doses for this system to images and doses for a conventional system. We conclude that digital mammography with the system studied is at least feasible since phantom image quality is comparable to that of a conventional system at dose levels that are somewhat lower.

The Measurement Of Diagnostic X-Ray Spectra With A High Purity Germanium Spectrometer

This paper describes a system which approaches the state-of-the-art in energy dispersive x-ray spectrometry. Special methods for obtaining x-ray spectra from machines operating at high current levels and other phenomena which could cause spectral distortion are discussed. Entrance and exit spectra from some typical diagnostic procedures are shown. It is demonstrated that the exit spectra can be closely approximated by replacing the phantom with aluminum filtration. Since the sensitometric performance of imaging systems is influenced by beam quality, this technique can provide x-ray beams similar to those used in diagnostic radiology. The usefulness of the spectrometer for accurately measuring kVp and determining unknown elements of an intensifying screen is also illustrated.

Radiation protection requirements for medical x‐ray film

Previous darkroom shielding requirements for medical x-ray film-assumed that the film should not be exposed to diagnostic x-ray radiation levels greater than 2 microGy (0.2 mR) for the life of the film. Modern medical x-ray films are much less sensitive to ionizing radiation, with most films showing at least an order of magnitude less sensitivity than previously assumed. Conversely, these same films when loaded in cassettes using modern intensifying screens exhibit an order of magnitude greater sensitivity when these cassettes are exposed to ionizing radiation. These data suggest that protection of modern medical x-ray film, stored in a darkroom, may require less shielding than previously assumed. Conversely, film loaded in a cassette will require greater shielding.

Selection of Technique Factors for Mobile Capacitor Energy Storage X-Ray Equipment

The technique factors of capacitor energy storage x-ray equipment influence the x-ray beam quality and quantity differently than those encountered with conventional single-phase or constant potential equipment. This is due to the nature of the high voltage waveform applied to the x-ray tube in capacitor energy storage systems. A lack of understanding of this difference can lead to excess patient exposure through inappropriate selection of technique factors by either the manufacturers of these systems or uninformed technologists using them. From analysis of exposure measurements made with a Masonite phantom, a method has been developed for determining whether a selected technique may result in unnecessary patient exposure. In addition, the distinction between technique factors for conventional high tension transformer and capacitor energy storage systems is reviewed.