Charles E. Willis

An exposure indicator for digital radiography

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

Performance evaluation of computed radiography systems.

Recommended methods to test the performance of computed radiography (CR) digital radiographic systems have been recently developed by the AAPM Task Group No. 10. Included are tests for dark noise, uniformity, exposure response, laser beam function, spatial resolution, low-contrast resolution, spatial accuracy, erasure thoroughness, and throughput. The recommendations may be used for acceptance testing of new CR devices as well as routine performance evaluation checks of devices in clinical use. The purpose of this short communication is to provide a tabular summary of the tests recommended by the AAPM Task Group, delineate the technical aspects of the tests, suggest quantitative measures of the performance results, and recommend uniform quantitative criteria for the satisfactory performance of CR devices. The applicability of the acceptance criteria is verified by tests performed on CR systems in clinical use at five different institutions. This paper further clarifies the recommendations with respect to the beam filtration to be used for exposure calibration of the system, and the calibration of automatic exposure control systems.

An exposure indicator for digital radiography: AAPM Task Group 116 (Executive Summary)

Digital radiographic imaging systems, such as those using photostimulable storage phosphor, amorphous selenium, amorphous silicon, CCD, and MOSFET technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital postprocessing of the acquired image data. Overexposure and underexposures are not readily recognizable. As a result, patient dose has a tendency to gradually increase over time after a department converts from screen/film-based imaging to digital radiographic imaging. The purpose of this report is to recommend a standard indicator which reflects the radiation exposure that is incident on a detector after every exposure event and that reflects the noise levels present in the image data. The intent is to facilitate the production of consistent, high quality digital radiographic images at acceptable patient doses. This should be based not on image optical density or brightness but on feedback regarding the detector exposure provided and actively monitored by the imaging system. A standard beam calibration condition is recommended that is based on RQA5 but uses filtration materials that are commonly available and simple to use. Recommendations on clinical implementation of the indices to control image quality and patient dose are derived from historical tolerance limits and presented as guidelines.

Preliminary assessment of computed tomography and satellite teleradiology from operation desert storm

Computed tomography at military transportable hospitals was used for the first time during the recent Operation Desert Storm in the Saudi Arabian desert. Scan quality was excellent and the scans proved clinically important in patient management. A teleradiology link via satellite to the U.S. mainland was also successfully employed. The objectives of the teleradiology link were to validate the concept distant interpretation of images obtained on the battlefield and to provide specialty radiology consultation. This technology shows great promise for future applications, both for combat casualty care and for civilian disaster medical support operations.

Measurement of CT radiation profile width using CR imaging plates

This paper describes the procedure for using a Fuji computed radiography (CR) imaging plate (IP) for the measurement of computed tomography (CT) radiation profiles. Two sources of saturation in the data from the IP, signal and quantization, were characterized to establish appropriate exposure and processing conditions for accurate measurements. The IP generated similar profiles compared to those obtained from digitized ready-pack films, except at the profile edges, where the exposure level is low. However, when IP pixel values are converted to exposure, CR and digitized film profiles are in agreement. The full width at half maximum (FWHM) of the CT radiation profile was determined from the relationship between pixel value and exposure and compared to FWHM of the digitized optical density profile from film. To estimate the effect of scattering by the cassette material, radiation profiles were acquired from IPs enclosed in a cassette or in a paper envelope. The presence of the cassette made no difference in the value determined for FWHM. With proper exposure and processing conditions, the FWHM of 5, 10, and 15 mm collimated beams were measured using IPs to be 7.1, 11.9, and 17.0 mm and using film to be 7.2, 12.2, and 16.8 mm, respectively. Our results suggest that, under appropriate conditions, the estimation of the width of the CT radiation profile using Fuji CR is comparable to the measurement from film density described in American Association of Physicists in Medicine (AAPM) Report No. 39. Although our experiment was conducted using Fuji CR, we anticipate that CR plates from other vendors could be successfully used to measure CT beam profiles because of similar empirical relationships between pixel value and exposure.

Evolution of teleradiology in the defense medical establishment.

The Medical Diagnostic Imaging Support (MDIS) System is a four-year contract to install large-scale picture archival and communications systems (PACS) and teleradiology in Army and Air Force medical treatment facilities. MDIS specifications were based on the results of three years of tri-service research and development through the Digital Imaging Network Systems Project and the Tactical Air Command Teleradiology Project. At the time of the governments request for proposals, MDIS functional specifications represented the most comprehensive understanding of the requirements for large-scale PACS and teleradiology complied by a composite team of radiologists, physicists, clinical engineers, hospital administrators, technologists, and computer systems engineers. As MDIS sites become operational, a better understanding of the capabilities and limitations of teleradiology is emerging. This paper reviews functions and subsystems common to all teleradiology systems, MDIS specifications for teleradiology, installation planning, and the status of Army and Air Force Teleradiology with special emphasis on early installations that validate routine teleradiology operations.

Optimization and quality control of computed radiography

Computed radiography (CR) is a relatively new technique for projection radiography. Few hospitals have CR devices in routine service and only a handful have more than one CR unit. As such, the clinical knowledge base does not yet exist to establish quality control (QC) procedures for CR devices. Without assurance that CR systems are operating within nominal limits, efforts to optimize CR performance are limited in value. A complete CR system includes detector plates that vary in response, cassettes, an electro-optical system for developing the image, computer algorithms for processing the raw image, and a hard copy output device. All of these subsystems are subject to variations in performance that can degrade image quality. Using CR manufacturer documentation, we have defined acceptance protocols for two different Fuji CR devices, the FCR 7000 and the AC1+, and have applied these tests to ten individual machines. We have begun to establish baseline performance measures and to determine measurement frequencies. CR QC is only one component of the overall quality control for totally digital radiology departments.

Objective measures of quality assurance in a computed radiography-based radiology department

Parameters are needed to assess quality assurance in a radiology department where computed radiography (CR) is the principal means of image acquisition. Laser-printed computed radiographs were collected for all patients examined over a period of several days. A sample of 1200 was sorted by subject anatomy and the associated exam information was entered into an EXCEL spreadsheet. Sensitivity (S) numbers were sorted into histogram and analyzed using standard descriptive statistics. Each film was over-read by a board-certified radiologist to assess whether the image was diagnostic and to determine if there were pathologic findings. A significant proportion of images were acquired using inappropriate menu codes. The histogram of S numbers for a given menu code describes a log normal distribution. The S number depends on the technologists ability to control the technique. A significant proportion of the images were deemed non diagnostic, and many correlated to excessive S numbers. Some were a result of mispositioning. The S number is a valid retrospective measure of radiographic quality assurance. Departments using CR should strive for control on menu codes selected and S numbers produced. Such data should be available from PACS databases.

Logistic representation of the sensitometric response of screen–film systems: Empirical validation

An empirical logistic model that linearizes the sensitometric response data of screen-film systems over the entire dynamic range of exposures is presented. This linearization is evident when the net optical densities are scaled as fractions of the net saturation density and plotted on commercial logit, probit, or double-log paper, except under conditions of reciprocity law failure. Weighted linear regression analysis shows that the intercept, but not the slope, depends on the screen-film system used. Previous work indicates that the slope is also independent of development time and photon energy. The model is verified through an analysis of tabulated sensitometric data published by the Center for Devices and Radiological Health.

Consequences of modern anthropometric dimensions for radiographic techniques and patient radiation exposures

Radiographic techniques are devised on the basis of anatomic dimensions. Inaccurate dimensions can cause radiographs to be exposed inappropriately and patient radiation exposures to be calculated incorrectly. The source of anatomic dimensions in common usage dates back to 1948. The objective of this study was to compare traditional and modern anthropometric data, use modern dimensions to estimate potential errors in patient exposure, and suggest modified technique guidelines. Anthropometry software was used to derive modern anatomic dimensions. Data from routine annual testing were analyzed to develop an x-ray generator output curve. Published tabulated data were used to determine the relationship between tissue half-value layer and kilovoltage. These relationships were used to estimate entrance skin exposure and create a provisional technique guide. While most anatomic regions were actually larger than previously indicated, some were similar, and a few were smaller. Accordingly, exposure estimates were higher, similar, or lower, depending on the anatomic region. Exposure estimates using modern dimensions for clinically significant regions of the trunk were higher than those calculated with traditional dimensions. Exposures of the postero-anterior chest, lateral chest, antero-posterior (AP) abdomen, male AP pelvis, and female AP pelvis were larger by 48%, 31%, 54%, 52%, and 112%, respectively. The dimensions of bony regions of the anatomy, such as the joints and skull, were unchanged. These findings are consistent with the idea that anatomic areas where fat is deposited are larger in the modern U.S. population than they were in previous years. Exposure techniques for manual radiography and calculations of patient dose for automatic exposure control radiography should be adjusted according to the modern dimensions. Population radiation exposure estimates calculated in national surveys should also be modified appropriately.