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

Reduction of patient dose in medical radiography by utilizing scattered X-rays: relation between permissible limit of scatter fraction, viewer brightness, and perceptibility of vision

This paper proposes a new technique for reducing the patient dose when employing medical radiographs prepared by using screen-film systems. In this technique the patient dose can be reduced by employing scattered X-rays in order to obtain the same film density as that realized without the use of scattered X-rays. The minimum perceptible thickness difference ΔX(min), which can be recognized by liminal vision, was psychophysically calculated by considering the energy spectrum of incident X-ray, sensitivity spectrum of the screen layer, and the perception capability of human vision. From the calculated ΔX(mins) in various conditions, the permissible upper limit of scatter fraction for obtaining the same ΔX(min) for three kinds of luminances, and the fraction of reduction in the primary X-rays were determined. As an example of the results, when the object size required for perception is 1.3 mm, a scatter fraction up to 42% can be permitted at a density D of 1.0 for a luminance of 2548 cd m(-2). When we increase the luminance of the viewer from 478 cd m(-2) to 2548 cd m(-2), the upper limit of the permitted scatter fraction varies from 30% to 42% at a D of 1.0, i.e., the patient dose can be reduced by 17% under the same perceptibility of ΔX(min) by utilizing scattered X-rays. This reduction can be successfully achieved by changing the lead content of the grid from 0.45 to 0.38 g cm(-2).

Blurring artifacts in megavoltage radiography with a flat-panel imaging system: comparison of Monte Carlo simulations with measurements

Originally designed for use at medical-imaging x-ray energies, imaging systems comprising scintillating screens and amorphous Si detectors are also used at the megavoltage photon energies typical of portal imaging and industrial radiography. While image blur at medical-imaging x-ray energies is strongly influenced both by K-shell fluorescence and by the transport of optical photons within the scintillator layer, at higher photon energies the image blur is dominated by radiation scattered from the detector housing and internal support structures. We use Monte Carlo methods to study the blurring in a notional detector: a series of semi-infinite layers with material compositions, thicknesses, and densities similar to those of a commercially available flat-panel amorphous Si detector system comprising a protective housing, a gadolinium oxysulfide scintillator screen, and associated electronics. We find that the image blurring, as described by a point-spread function (PSF), has three length scales. The first component, with a submillimeter length scale, arises from electron scatter within the scintillator and detection electronics. The second component, with a millimeter-to-centimeter length scale, arises from electrons produced in the front cover of the detector. The third component, with a length scale of tens of centimeters, arises from photon scatter by the back cover of the detector. The relative contributions of each of these components to the overall PSF vary with incident photon energy. We present an algorithm that includes the energy-dependent sensitivity and energy-dependent PSF within a ray-tracing formalism. We find quantitative agreement (approximately 2%) between predicted radiographs with radiographs of copper step wedges, taken with a 9 MV bremsstrahlung source and a commercially available flat-panel system. The measured radiographs show the blurring artifacts expected from both the millimeter-scale electron transport and from the tens-of-centimeters length scale arising from the scattered photon transport. Calculations indicate that neglect of the energy-dependent blurring would lead to discrepancies in the apparent transmission of these wedges of the order of 9%.

Resolution degradation due to K-characteristic photon absorption in dual phosphor image receptors

Image receptors based on the absorption of x rays by phosphor screens have a system resolution described predominantly by the modulation transfer function (MTF) of the phosphor screens utilized. This ideal MTF is modified by the presence of secondary effects within the imaging receptor such as scatter from the receptor cover, visible light crossover (in the case of screen film cassettes) and scatter generated within the phosphor itself, presumed to be almost entirely due to photoelectric interactions. In this paper the MTF characteristics resulting from the absorption of K-characteristic x rays emitted by one screen of a pair in an image receptor and absorbed by the adjacent screen (referred to as cross-fluorescence hereafter) are examined. This effect has been previously modeled [Med. Phys. 23, 1253 (1996)] and recently a computer program has been written to calculate intensity of the radial component of a point spread function (PSF) describing the above process. From this, the MTF of cross-fluorescence under varying conditions can be calculated. The model was applied to a dual screen experiment described in the literature [Med. Phys. 23, 871 (1996)], from which it was possible to show good agreement between calculated results and those derived from the measurement. The effect of screen thickness, screen separation, and phosphor material on the MTF of cross-fluorescence was investigated using simulated x-ray beam spectra in association with phosphor screen parameters. Results showed that the calculated MTF falls to below 0.1 at 1 cy/mm in all cases examined. This MTF was reduced further as the separation between the screens increased from that commonly used in a screen film cassette. Beam energy, phosphor thickness, and phosphor material had a minimal or no effect on MTF. The effect of cross-fluorescence on total image receptor MTF is to reduce the high-frequency components of that function by an amount that is in proportion to the scatter fraction of the effect.

The use of carbon fibre material in radiographic cassettes: estimation of the dose and contrast advantages

A Monte Carlo simulation has been used to estimate the dose and contrast advantages of replacing radiographic cassette fronts fabricated from aluminium with cassette fronts fabricated from low atomic number material (carbon fibre). The simulation used a realistic imaging geometry and calculations were made both with and without an anti-scatter grid. Account was taken of the scatter generated in the cassette front and the effect of beam hardening on primary contrast. Dose and contrast were evaluated for a range of cassette front thicknesses and tube potentials (60-150 kV) as well as for four examinations representative of situations with varying amounts of scatter. The results with an anti-scatter grid show a clear dose and contrast advantage in all cases when an aluminium cassette front is replaced with a low attenuation cassette front. The contrast advantage is dependent upon the examination and is generally greater for imaging bony structures than for imaging soft tissue. If a 1.74 mm aluminium cassette front is compared with a 1.1 mm carbon fibre cassette front, then the dose advantages are 16%, 9%, 8% and 6% and the contrast advantages are 10%, 7%, 4% and 5% for the AP paediatric pelvis examination at 60 kV, the anteroposterior (AP) lumbar spine examination at 80 kV, the lateral lumbar spine examination at 100 kV and the posteroanterior (PA) chest examination at 150 kV, respectively. The results without an anti-scatter grid show an increased dose advantage when a low attenuation cassette front is used, but the contrast advantage is small and in some situations negative.

Scatter-to-primary ratio and absorption efficiency in screen-film and computed radiography systems

PURPOSE

To determine whether the scatter-to-primary (s/p) ratio, measured on a screen-film or computed radiographic (CR) system, varies with chemical composition or primary attenuation properties.

MATERIALS AND METHODS

The s/p ratios of four Gd2O2S screens, three CaWO4 screens and a CR screen, were determined under uniform irradiation conditions. One simulated low scatter conditions, such as in lung imaging, and the other simulated high scatter conditions as in abdominal imaging. The absorption efficiency of each screen was measured and calculated.

RESULTS

The measured s/p ratio varied within each set of screens. The s/P ratio was related to the absorption efficiency of primary radiation for the screen-film and CR system. No difference in the s/p ratio was seen between screen-film and CR systems when normalised for the primary absorption efficiency.

CONCLUSIONS

Absorption efficiency of the imaging system is a major indicator of s/p ratio, for both screen-film and CR systems.