AbstractID: 6644 Title: A New Test Object for Evaluation of Enhanced Contrast Portal
Radiograph Image Quality and Establishment of Technique Charts
Enhanced Contrast Localization (EC-L) film-screens, introduced by Eastman Kodak Co. in
1996, produce excellent portal localization radiographs. However, the range of absorbed dose
yielding acceptable optical densities is as small as 1 cGy. Technique charts relating anatomic
thickness, cassette distance from the anatomy and from the radiation source, and the
absorbed dose (monitor units) to the film are required to yield consistent radiographs.
Teletherapy devices (Co-60 units with high activity (13000 Ci) sources) produce sufficient
absorbed dose that EC-L radiography cannot be performed even with the least time (0.01
min) setting. To establish consistent localization radiography a new quality assurance
phantom has been built to measure and test portal radiograph image contrast and resolution.
The Plexiglas test phantom is 9.8 in by 9.8 in by 2.8 in thick with a series of holes (air) and
polyvinylchloride (to simulate bone) inserts from 0.25 in to 1 in diameter with thicknesses
from 0.25 in to 2.5 in. Used with Plexiglas slabs of the same area but varying thicknesses
(0.5 in, 1 in, etc.) with the test phantom at the center of the stack, patient anatomy can be
simulated and radiographs obtained at different distances. Technique charts for both linear
accelerators and cobalt-60 units have been established and will be presented with
radiographs. The methodology improves contrast and image quality of the acceptable
radiographs and reduces the number of poor quality radiographs rejected. Methods of
attenuating the cobalt-60 beam to obtain acceptable radiography with the EC-L film-screen
system are presented.