Physical and clinical evaluation of new high-strip-density radiographic grids

The imaging performance of new high-strip-density (HSD) grids having 57 lines/cm was compared with that of conventional low-strip-density (LSD) grids having 33 or 40 lines/cm. The unique advantage of HSD grids is that, under most standard radiographic conditions, the grid lines are not noticeable on the final image, even if the grid is stationary. This is due to the combined effect of the high fundamental spatial frequency of HSD grids, the modulation transfer function of screen-film systems and of the human visual system, and scattered radiation. Monte Carlo simulation studies, phantom images, and clinical evaluation indicate that HSD grids can provide contrast improvement factors and Bucky factors that are comparable to or slightly better than those obtained with LSD grids. Therefore, it may now be possible to eliminate moving Bucky trays from radiographic tables and fluoroscopic devices.

Energy and angular dependence of x-ray absorption and its effect on radiographic response in screen--film systems

The relationship between the film density and the x-ray energy absorbed in the screens, and the effect of the components of a screen--film system on x-ray absorption in the screen phosphor has been studied. It is concluded that, to a good approximation, the total absorbed x-ray energy can be calculated ignoring components other than the phosphor, and that the absorbed energy is related to the film density through the characteristic curve of the film. This simple relationship provides a basis for the theoretical prediction of the radiographic performance of an imaging system. Analytical expressions have been derived for the energy absorbed in model screens containing one or two high-atomic-number elements. For incident energies above the K-edge of the high-Z elements, the contribution of the K x-rays to energy absorption was included by use of a K-reabsorption factor. This factor was determined for eight screen pairs as a function of incident photon energy and incident angle.

Evaluation of cassette performance: physical factors affecting patient exposure and image contrast

Physical properties of four radiographic cassettes were investigated in combination with various screen/film systems. These properties included (a) transmittance of monoenergetic x rays over 14-96 keV and comparison with predictions based on attenuation coefficients; (b) relative patient exposure from 60 to 120 kV (from phantom measurements) and correlation with predictions based on measured transmittance as well as average energies and angles of incident radiation; and (c) amounts of scatter from various cassette materials and comparison with Monte Carlo calculations. The results provide a model of performance based on quantitation of cassette effects on system speed and image contrast. Carbon-fiber cassettes required the lowest patient exposure, produced the least scatter, and (in low-scatter situations) were capable of increased image contrast.

The validity of Monte Carlo simulation in studies of scattered radiation in diagnostic radiology

A computer program using Monte Carlo methods has been developed for the simulation of photon scattering in tissue-equivalent phantoms for incident x-rays in the diagnostic energy range. The study verified that the sampling schemes used in the program can produce random samples according to the theoretical probability distribution functions which describe the photon-scattering process. The Monte Carlo program was applied to determinations of the scatter fractions and edge responses for various phantom sizes, X-ray energies, and recording systems. These quantities were also measured experimentally under comparable imaging conditions. Excellent agreement was obtained between the predicted and experimental results. This investigation established the validity of our Monte Carlo calculations for studies of the physical characteristics of scattered radiation in diagnostic radiology.

X-ray tube focal spot sizes: comprehensive studies of their measurement and effect of measured size in angiography

Thirty-two focal spot sizes of four x-ray tubes were measured by the pinhole, star pattern, slit, and root-mean-square (RMS) methods under various exposure conditions. The modulation transfer functions (MTFs) and line spread functions (LSFs) were also determined. The star pattern focal spot sizes agreed with the effective sizes calculated from the frequencies at the first minimum of the MTF within 0.04 mm for large focal spots and within 0.01 mm for small focal spots. The focal spot size determined by the slit method was approximately equal to the width of the LSF at the cutoff level of 0.15 +/- 0.06 of the peak value. The RMS method provided the best correlation between the measured focal spot sizes and the corresponding image distributions of blood vessels. The pinhole and slit methods tended to overestimate the focal spot size, but the star pattern method tended to underestimate it. For approximately 90% of the focal spots, the average of the star and slit (or pinhole) focal spot sizes agreed with the RMS focal spot size within +/- 0.1 mm.

Investigation of the performance of antiscatter grids: Monte Carlo simulation studies

Monte Carlo methods have been applied to the study of the performance of antiscatter grids in radiography. Photon histories for imaging with a 20 cm thick water phantom irradiated with an 80 kV x-ray spectrum were generated by Monte Carlo simulation. The scattering and absorption of the photons transmitted from the phantom were traced in the grid. Grids were evaluated with strip densities ranging from 10 to 67 lines/cm, grid ratios from 6 to 15, and lead-to-interspace ratios from 1/9 to 1/2. The contrast improvement factor and the Bucky factor were used as the benefit and cost indicators, respectively, for evaluation of the grids. The dependence of these factors on the grid ratio, lead-to-interspace ratio, strip density, lead content, and interspace material was investigated. The results indicate that high-strip-density grids with thin lead strips and high grid ratios can provide higher contrast improvement factors than are achieved with low-strip-density grids, without an increase in patient exposure. This theoretical approach is useful for the prediction and development of optimal antiscatter grids for radiographic procedures.

Monte Carlo simulation studies of backscatter factors in mammography

Experimentally determined backscatter factors in mammography can contain significant systematic errors due to the energy response, dimensions, and location of the dosimeter used. In this study, the Monte Carlo method was applied to simulate photon-scattering in tissue-equivalent media and to determine backscatter factors without the interference of a detector. The physical processes of measuring backscatter factors with a lithium fluoride thermoluminescent dosimeter (TLD) and an "ideal" tissue-equivalent detector were also simulated. Computed results were compared with the "true" backscatter factors and with measured values reported by other investigators. It was found that the TLD method underestimated backscatter factors in mammography by as much as 10% at high energies with TLD-100 (3.2- x 3.2- x 0.89-mm3) crystals.

Monte Carlo simulation studies of detectors used in the measurement of diagnostic x-ray spectra

Accurate determination of diagnostic x-ray spectra requires, among many other technical considerations, correction of the experimental data for the energy-dependent response of the x-ray detector. In this study, we investigated the energy response of three types of detectors (intrinsic germanium, lithium-drifted silicon, and sodium iodide) commonly used in the diagnostic energy range. We used a Monte Carlo simulation technique to compute photopeak efficiencies, escape peak losses, and Compton fractions as a function of incident x-ray energy for a variety of detector thicknesses and diameters. Calculated responses were compared with experimental results obtained for the Ge and Si(Li) detectors. The measurements, in which we used fluorescent x ray to simulate the monoenergetic beams assumed for the calculations, indicate good agreement between theoretical and experimental approaches. Techniques for correction of measured spectra for the known response of the detector were also evaluated.

Evaluation of absorbed dose in mammography: monte carlo simulation studies

Computer programs for calculation of the backscatter factor and absorbed dose in breast phantoms are developed based on Monte Carlo simulation. Rad/roentgen conversion factors are calculated for water, fat, a mixture of 50% water and 50% fat, and lucite irradiated with monoenergetic and polyenergetic x rays from a tungsten or molybdenum anode x-ray tube. These factors can be used to estimate skin dose, average depth dose, or average integral dose in a breast when the beam quality, exposure in air at the skin position, and composition of the breast are known. Calculated backscatter factors are considerably greater than measured values reported previously.

Determination of radiographic screen-film system characteristic curve and its gradient by use of a curve-smoothing technique

One of the important factors affecting radiographic image quality is the film contrast of the recording system, which may be expressed by the gradient of its H & D curve. Experimental determinations of H & D curves are subject to relatively small error, which can nevertheless cause large fluctuations and uncertainties in the calculated gradient. To reduce this fluctuation, we wrote a computer program for orthogonal-polynomial (discrete Legendre polynomials) curve fitting by means of least-squares estimation of parameters. This procedure facilitates curve smoothing and gradient computation following sensitometric measurements. The program was tested against a large number of measured characteristic curves. A brief description of the program is given, with examples which illustrate its applications.