Robert E. Dickerson

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.

Objective performance characteristics of a new asymmetric screen‐film system

A study of the objective imaging characteristics of a new asymmetric screen-film system is presented herein. The system is characterized by high x-ray absorption asymmetric screens, and a low-noise, high-contrast asymmetric film having near-zero crossover. Comparisons are made with the imaging characteristics of two widely used conventional screen-film systems. Sensitometry, modulation transfer function, and noise power spectra were measured using standard methods. Granularity, noise equivalent quanta, and detective quantum efficiency were computed from these. The new screen-film system has an average gradient at lung-field densities between the two conventional systems studied, while the mediastinum contrast exceeds both conventional systems. The lung-field modulation transfer function half bandwidth of the new asymmetric system exceeds that of both conventional systems by 60%. At mid exposures the detective quantum efficiency of the new asymmetric system is comparable to those of the conventional systems studied. However, the exposure range over which detective quantum efficiency remains high is substantially wider.

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.

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.

Design and performance features of a new mammographic film/screen system

Most current research in mammographic detector development has been focused on digital detectors. However, the vast majority of clinical practice still uses film/screen systems. This paper will report on the design and performance features of a new film/screen system for mammography that has the potential to become the new gold standard in image quality for this demanding application. This new system includes a new film and new intensifying screens. The new film has unique emulsions on each side of the base. It features novel fine-grain, metal ion-doped silver halide microcrystals. It is exposed with a single intensifying screen. Two different intensifying screens have been developed to create systems with different speeds. The new screens use an improved phosphor and new coating structures. The highfrequency MTF has been boosted. The system provides higher contrast, lower noise, and sharper images.

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.