Kenneth E. Huff

Analysis of the detective quantum efficiency of a radiographic screen-film combination.

Detective quantum efficiency provides a useful measure of the imaging efficiency of imaging systems. Methods for measuring the exposure and the spatial-frequency dependence of the contrast transfer function, the noise power spectrum, and the detective quantum efficiency are developed for x-ray imaging systems. These are applied to a high-resolution screen-film combination exposed to a 30-kV-peak x-ray spectrum. The major component sources of screen-film noise in this system are identified and quantified. These are interpreted in terms of a simple model to predict the screen-film noise power spectrum and detective quantum efficiency. Reasonable agreement is found between model predictions and experimental measurements.

Evaluation of a cassette-screen-film combination for radiation therapy portal localization imaging with improved contrast

A traditional limitation with radiation therapy portal images is low image contrast, due in part to the low attenuation of the exposing radiation by the tissues being imaged, and the contrast capabilities of the image receptor. We have developed, and have clinically evaluated, a cassette-screen-film combination for portal localization imaging, which features a copper front screen plus Gd2O2S:Tb fluorescent screens and a slow-speed, fine grain, film emulsion with inherently high contrast coated on both sides of a 7 mil Estar base. The film can be processed in a conventional rapid-process film processor. Sensitometric data indicate that the film contrast (average gradient) for the new combination is approximately 3.5 times higher than the conventional portal localization systems in current use. The new combination has been clinically compared with two conventional systems. The required monitor unit settings were found to be similar. Initial clinical results indicate portal images made with the new combination are superior to those obtained with the conventional combinations. The images have much higher contrast, subjective impressions of lower noise, show clearer definition of structures, and are much easier to read.

Development of a novel high-contrast cassette/film/screen system for radiation therapy portal localization imaging

Radiation therapy portal images have traditionally exhibited poor discrimination of areas of interest, due to low subject contrast of anatomical parts being imaged at megavoltage energies, and the contrast capabilities of the image receptors. As a result of this low contrast, positioning of the radiation beam and placement of shielding blocks can be difficult. A novel, high-contrast cassette/film/screen system has ben developed and clinically evaluated for portal imaging. This system features a copper front screen, a gadolinium oxysulfide, terbium activated intensifying screens and a slow speed film with inherently high contrast. Very high film contrast is achieved by narrow grain size distribution and metal ion doping of the silver halide microcrystals. This high-contrast film is exposed by light from the intensifying screen, further increasing contrast. Sensitometric data indicates this new system to have 3.5X greater contrast than conventional portal localization imaging systems at comparable monitor units. Initial clinical evaluation indicates this new system to yield significantly superior images showing clearer definition of structures and was much easier to read and interpret.

Sources Of Noise In High-Resolution Screen-Film Radiography

The contrast transfer function (CTF), noise power spectrum (NPS), noise equivalent quanta (NEQ), and detective quantum efficiency (DQE) of a high-resolution screen-film combination have been measured for exposure to a 30 kVp x-ray spectrum. In addition to these overall system characteristics, selected component sources of noise in the screen-film combination have been determined experimentally. These data are interpreted in terms of a simple model which is used to predict screen-film NPS, NEQ, and DQE. Reasonable agreement between model predictions and experimental measurements has been found.

Comparison Of Theory And Experiment For The Dqe Of A Radiographic Screen-Film System

Models have been developed for the DQE of a screen-film system that, although based on a number of simplifying assumptions, include most of the significant parameters. At the same time an increasing number of experimental measurements are available for the complete DQE characteristics as a function of exposure and spatial frequency for practical screen-film systems. A systematic set of DQE measurements is presented here, which we examine from the viewpoint of current models. This reveals both areas of agreement and aspects that require further theory and experimentation.

Signal-To-Noise-Ratio Measurements On Two High-Resolution Screen-Film Systems

Measurements of the performance of two high-resolution radiographic screen-film systems at 50 and 30 kVp are given in terms of the noise equivalent number of recorded x-ray quanta (NEQ) and the detective quantum efficiency (DQE). Additionally, the MTF, contrast transfer function (CTF), and noise power spectra (NPS) of these imaging systems are presented. The NEQ, DQE, CTF, and NPS are shown to be strong functions of both spatial frequency and exposure. The shapes of these surfaces are significantly different from those published for general-purpose radiographic systems; these data provide a useful context within which screen-film system performance can be interpreted.

Status Report Of ANSI PH2-31 Task Force On Sensitometry Of Screen-Film-Processing Combinations

The American National Standard Method for the Sensitometry of Medical and Dental X-ray Films, PH2.9, provides standard methods for measuring the speed and average gradient of medical and dental x-ray films. The most recent revision of this standard was completed this year. While the response of a film can be measured by this standard method, there are no provisions for measuring the response of an intensifying screen or of a screen-film combination. Therefore, the sensitometric parameters of the film are of limited utility to users of screen-film combinations. In an attempt to correct this situation, a task force was established last year by Subcommittee PH2-31 of the American National Standards Institute (ANSI) to pursue the establishment of a new and separate standard providing a method for the direct measurement of the response of screen-film-processing combinations. This paper is a report on the status of the work of that task force.

Image recording system characteristics for radiation therapy: portal localization and verification

Radiotherapy portal imaging is the process of producing images using a radiation treatment linear accelerator or cobalt 60 unit. Portal images are used to evaluate the position of the radiation treatment beam and placement of shielding blocks with respect to the patients anatomy. This paper discusses types, characteristics and clinical use of radiation therapy portal imaging systems.