Deceptive debris. (Technical Query).
They were subtle changes at first and could be ignored easily. But Richard Russell, a radiologic technologist with more than 10 years' experience, soon realized he had a problem and began a persistent investigation. These subtle soft-tissue artifacts at times mimicked the appearance of tumor pathologies. (See Fig. 1.) A year into the investigation the department changed its film-screen system, but the artifacts remained. As the artifact problem became worse, Mr. Russell noted an important fact--the artifacts only appeared on Bucky work; they were not apparent on tabletop examinations. The question remained: What was the cause?
[FIGURE 1 OMITTED]
It took the efforts of a field engineer to find the solution. After viewing radiographs exposed to low exposure factors (60 kVp and 1.2 mAs, see Fig. 2), the service engineer discovered that the cause was the ionization chambers of the automatic exposure control (AEC) unit. After more than 10 years of use, the ionization chambers had deteriorated.
[FIGURE 2 OMITTED]
The ionization chambers are situated between the grid of the Bucky device and the cassette. They have 2 electrodes separated by a thin layer of air. All are enclosed within an aluminum shell approximately the size of a credit card. When x-rays penetrate the thin aluminum shell and strike the the air molecules, electrons are released. This ionization results in an electric current that is stored in a capacitor or condenser. After a maximum amount of electricity is stored, any additional electricity will cause the capacitor to discharge, terminating the exposure. There are usually 3 ionization chambers (2 laterals and a center cell), and the radiographer selects the optimum chamber for the exam. Generally, the kVp range used in most diagnostic radiographic examinations is sufficient to overpenetrate the ionization chambers, thus preventing them from casting unwanted artifacts on the image.
When Mr. Russell took the old ionization chambers apart, he found plastic debris that caused the artifacts. With the new ionization chamber, the department is now artifact free. (Incidentally, the field engineer said that he had only encountered this problem once before in his 25-year career.)
Thanks to Richard Russell, R.T.(R)(T), of St. Petersburg, Fla, for submitting this query.
Technical Query is a troubleshooting column that covers image acquisition and processing. This issue's column is by Olive Pearl, M.S., R.T.(R)(M), a clinical instructor at the Stamford Hospital School of Radiography in Stamford, Conn. Ms. Pearl is author of Spanish for Professionals in Radiology and Appleton & Lange Mammography Review.
Submissions for the "Technical Query" column are welcomed and should be mailed to Radiologic Technology, c/o "Technical Query," 15000 Central Ave. SE, Albuquerque, NM 87123-3917.