Arthur G. Haus

SCREEN-FILM AND DIGITAL MAMMOGRAPHY: Image Quality and Radiation Dose Considerations

Factors affecting image quality and patient dose in screen-film and digital mammography have been discussed. Some proposed parameters for judging image quality and breast exposure measurements and dose calculations relating to changes in image quality factors have been reviewed. It is important to remember that the goal in making a mammogram is to obtain as much diagnostic information as possible at the lowest dose compatible with that information. As noted previously, this necessitates compromises (i.e., an optimization of factors that affect image quality). These include beam quality, compression, imaging geometry, grids, receptor characteristics, processing of the film or digital image, and display and viewing conditions. If this is done correctly, a high-quality mammogram can be obtained at a reasonably low dose to the patient. The goal is not simply to use as low a dose as possible, because if this is done there is a large risk of degrading the performance of mammography in detecting or accurately characterizing small, node-negative cancers.

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.

Evaluation of mammographic viewbox luminance, illuminance, and color

Twenty-three viewboxes were evaluated in six mammography facilities. Luminance and illuminance measurements were made with a recently calibrated photometer. Color temperatures were measured with a Minolta color meter. The average luminance for the 23 viewboxes was 2920 nit (lumen/Sr/m2), the lowest value was 1610 nit, and the highest value was 3630 nit. The average illuminance was 40 lux (lumen/m2), with 6 lux as the lowest value, and 97 lux as the highest. The average color temperature was 8400 K with the lowest value of 4900 K, and the highest of 10,900 K.

Problems associated with simulated light sensitometry for low‐crossover medical x‐ray films

Over the past ten years the evolution of medical x-ray films has been toward films with reduced intensifying-screen light crossover in order to reduce blur and obtain higher spatial resolution. For films with very low crossover, misleading and incorrect sensitometric data may be obtained for film contrast evaluation and processor control if a simulated light sensitometer with a single-sided, light-exposing device is used. Screen light exposures were made using an inverse square, intensity-scale sensitometer. Simulated light exposures were made using a widely used single-sided, simulated-light sensitometer commonly used for film processor quality control, and a new simulated-light sensitometer capable of producing either single- or double-sided sensitometric exposures. The films used included one single-emulsion film and three double-emulsion medical x-ray films with light-crossover values ranging from approximately 3% to 30%. Sensitometric data showed a significant distortion (bump) in the characteristic curve for the 3% light-crossover film exposed with the single-sided, simulated-light sensitometer.

Processor quality control in laser imaging systems.

Sensitometric techniques for performing processor quality control in laser imaging systems are analyzed in this study. The sensitivity of conventional x-ray films using simulated screen-light sensitometry is compared with helium-neon (HeNe) laser film exposed with a simulated red-light sensitometer, a standalone (reference) laser sensitometer, an experimental (unstable) laser sensitometer, and laser printers. Infrared (IR) laser film exposed with an IR laser diode and a simulated IR sensitometer are also evaluated. It is demonstrated that laser-generated step tablets provide an easy and reliable method of performing laser film processor quality control.

Method of simulated screen sensitometry for asymmetric, low crossover medical x‐ray films

Recognition of the importance of performing simulated screen-light sensitometry of medical x-ray films for the purpose of processor quality control has increased over the past several years. As a result there is a greater need to provide new techniques for performing simulated screen-light sensitometry. Medical films with reduced intensifying screen-light crossover intended to achieve reduced blur and higher spatial resolution pose particular problems in doing simulated screen-light sensitometry if care is not taken to choose a proper simulated light sensitometer with the capability of simultaneous double-sided exposures. Misleading and incorrect sensitometric data can be obtained for film contrast evaluation if a single side exposure is used. Asymmetric, near-zero crossover films pose even greater problems as proper orientation of the film and proper degree of light output asymmetry need be achieved in order to obtain correct sensitometry. The films used in this study were three double emulsion films varying in crossover from 3% to 24%. Of the two very-low-crossover films, one had symmetric emulsion layers while the second featured emulsion layers which were asymmetric in terms of contrast and speed. Sensitometric data show several curve shapes with significant distortions, depending on orientation, for the asymmetric, low-crossover film when exposed using a single-sided exposure. Only by using a double-sided exposure and an appropriate neutral density filter to simulate the degree of screen-light asymmetry in this system could one achieve a characteristic curve comparable to that achieved by inverse square sensitometry.

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.

Screen-Film Mammography Update: X-Ray Units, Breast Compression, Grids, Screen-Film Characteristics, And Radiation Dose

Screen-film mammography is growing rapidly. The impor-tance of using dedicated x-ray units with appropriate beam quality and breast compression for screen-film mammography is emphasized. The present status of grids for mammography is presented. Mammographic screen-film characteristics, in terms of contrast, light diffusion, and noise, are reviewed. Radiation dose in mammography is discussed in terms of measurement, calculation, and theoretical risk.

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.

Physicists in mammography--a historical perspective.

Medical physicists and engineers, working with radiologists and technologists, have made significant contributions in the design of mammographic x-ray units and image receptors, as well as in the development of methods for evaluating mammographic image quality and procedures for quality control. More accurate methods of measuring radiation exposure in the energy range of mammography and more relevant calculations of radiation dose to breast tissue at risk have also been realized. This article will discuss some of the major contributions made by medical physicists for the benefit of mammography. Contributions of radiologists in mammography have been published elsewhere [Bassett, Gold, and Kimme-Smith (1994)]. All contributions cited in this article are based on referenced publications and citations in the following: Medical Physics; Radiology; NCRP Report No. 85; Quality Determinants in Mammography; AAPM Report No. 29; Reduced Dose Mammography, W. W. Logan and E. P. Muntz (editors); RSNA Categorical Course: Technical Aspects of Breast Imaging, A. Haus and M. Yaffe (editors); Film Processing in Medical Imaging, A. G. Haus (editor); Screen-Film Mammography: Imaging Considerations in Medical Physics, G. T. Barnes and G. Donald Frey (editors). The article is divided into six sections: (1) x-ray equipment and receptor development, (2) image quality, (3) radiation dose, (4) phantoms, (5) quality assurance, (6) digital mammography, and (7) reports and committees.