J. Thomas Payne

Patient Dosage in Computed Tomography

The maximum surface dosage in most clinical CT scans seems to range from 2–10 rads/study but much larger dose per study values seem possible with both rotate-translate and rotary geometry designs. The CT scanner type in itself does not significantly reduce doses. Secondary radiation dose values were measured for critical organs and indicate that dosage from secondary radiations may be reduced significantly by external shielding. Dose values in the vicinity of most CT scanners are typically 1–2 mrad/scan at 1 meter at the parameters of a typical clinical scan.The maximum surface dosage in most clinical CT scans seems to range from 2-10 rads/study but much larger dose per study values seem possible with both rotate-translate and rotary geometry designs. The CT scanner type in itself does not significantly reduce doses. Secondary radiation dose values were measured for critical organs and indicate that dosage from secondary radiations may be reduced significantly by external shielding. Dose values in the vicinity of most CT scanners are typically 1-2 mrad/scan at 1 meter at the parameters of a typical clinical scan.

Performance Evaluation and Quality Assurance of Computed Tomography Scanners, with Illustrations from the EMI, ACTA, and Delta Scanners

Performance evaluation of equipment for computed tomography (CT) involves the integration of: (a) establishing performance criteria; (b) designing and implementing test procedures; and (c) reconciling test results in terms of desired performance. Precision (noise), contrast scale, linearity, accuracy, spatial independence, spatial resolution, artifacts, reproducible performance, and patient exposure are several parameters discussed, as are problems of measurement with regard to non-water bath scanners. Performance and quality control tests for the ACTA, Delta, and EMI scanners are outlined. Guidance for the prospective purchaser of CT equipment is presented as a summary of the ideas discussed.

X-ray-transmission computed tomography.

The immediate goal of clinically based x-ray-transmission computed tomography (CT) is to provide a measurement of the x-ray linear attenuation coefficient in cross section with the ultimate goal of impacting on patient management and care. To do this with the accuracy needed for clinical goals requires the careful integration of x-ray physics, detector technology, and mathematical reconstruction theory. Performance evaluation and quality assurance are necessary adjuncts to a CT scanning program. A number of investigative studies are underway.

Regional myocardial blood flow measurements in the evaluation of patients with coronary artery disease

Myocardial imaging with 133-Xe and a gamma camera was employed to evaluate total and regional myocardial blood flow. The technique detected vasodilatation after injection of papaverine or diatrizoate. Contrast medium caused transient vasodilatation with return to baseline flow within five minutes. Myocardial tissue flow tended to decrease as coronary artery stenosis became more severe. There was overlap of flow measurements in patients with and without coronary artery disease. Coronary flow measurements made at rest are not considered to be an essential clinical tool. gpreater diagnostic benefit is obtained from the scintigram which distinguishes between akinesia caused by ischemia and akinesia due to extensive scarring.

CT determination of parameters for inhomogeneity corrections in radiation therapy of the esophagus.

Accurate dose prediction for megavoltage photon therapy of carcinoma of the esophagus requires information on tumor depth, lung thickness, and lung density. The authors found that CT localization of internal and external contours is accurate within +/- 1 mm. Lung density can be measured with an error of less than 0.02 g/cm3 in the range 0.25-1.00 g/cm3. Variance between predicted and measured dosage was less than 3% in all patients and in most RANDO phantom measurements. Accurate radiation therapy planning is possible with CT information from a commercial scanner.

Comparison and Performance of Anger Cameras

Measurements of sensitivity, spatial resolution, dead time, and field uniformity, as well as imaging of phantoms, provide a satisfactory means of evaluating and comparing Anger camera systems. The authors recommend that these parameters be measured periodically to detect deterioration of performance. A clinical evaluation of images depends upon the type of cathode-ray tube display and associated film response. Thus, given an optimized camera system, an appropriate display format is also required for best results.

Protocol for a computed tomography scanner performance repository

An argument is presented for organization of a Computed Tomography Scanner Performance Repository. Performance data on CT scanners would be distributed in statistical form to the radiological community. A Protocol is presented for the uniform collection of data. Results of exposure, resolution and visibility measurements made by the authors are presented for the EMI 5005/U, Pfizer 0200 FS and General Electric CT/T scanners.

Limitations of Available Gamma Camera Oscilloscopes for Whole-Body Bone Image or Minified Multi-Image Display1

The limits of image minification on conventional cathode-ray tubes were determined for two commercially available Anger cameras. For medium intensity settings, it was found that minification of more than 8 to 1 (Pho/Gamma HP) and 6 to 1 (Nuclear Data 60), respectively, would result in degradation of system resolution.

Acceptance Testing of a Computerized Tomographic Scanner

Computerized tomographic scanners have gained quick and widespread acceptance in diagnostic radiological practice. The cost of such units is currently about a half million dollars. Technologically, they are one of the most complicated pieces of equipment to be found in a radiology department. Because of the cost and complexity, it seems logical to set up performance specifications, acceptance tests, and a quality assurance program for a CT scanner. Pertinent performance specifications are herein described and discussed. In order to assure that the CT unit does meet specifications, appropriate acceptance tests are likewise discussed. Finally, a basic quality assurance program is outlined with an indication of the tests to be performed and their time frequency.

Quantitative Photographic Method For Determining Anger Camera Uniformity

In Anger camera imaging, probably the most important parameter of camera performance is field uniformity. For the majority of camera users, field uniformity is evaluated in a subjective manner by simply viewing polaroid field floods. Due to film lim-itations, viewing conditions and statistical limitations, it is questionable if count density differences less than 15-20% can really be identified. To shift from a qualitative to quantitative method of field uniformity determination, a photographic method using high count density 70mm images has been developed. For the majority of Nuclear Medical imaging procedures, the current instrument of choice is the Anger camera. Of the An camera performance parameters, probably the most important is field uniformity. For those few institutions with dedicated computers interfaced to an Anger camera, field uniformity can be assessed and even corrected by the computer. However, for the majority of camera users, no quantitative method of uniformity evaluation exists. To shift from a qualitative to quantitative method of field uniformity determination without an interfaced dedicated computer, a phogographic method using high count density 70mm images has been developed; similar to film isodore methods in radiation therapy.