Assessment of fluoroscopic systems with a simple test object.

Abstract

A non-invasive test protocol and simple to use test object (the Westmead test object) has been devised to evaluate image quality of the complete fluoroscopic system, including the grid, under automatic brightness control (ABC). The new protocol tests the overall fluoroscopic system rather than individual components such as the image intensifier (II) tube, using the kVp and dose rate as determined by the ABC. A radiographic absorber is required to simulate clinical conditions by giving patient equivalent scatter, beam quality and exit beam intensity. Twenty (20) cm of water was used as an absorber and compared to commonly used copper sheet. It was found that no single thickness of copper gave both patient equivalent beam hardness and patient equivalent absorption under ABC conditions. The two absorbers, 1 mm copper and 20 cm water, were also trialed with the new test object in a survey of 23 fluoroscopic systems using the ABC protocol. Results for two test object features, Threshold Contrast and Contrast-Detail (Low Contrast Resolution), indicated an increased spread in the results for the water absorber when compared to the copper absorber. Of 15 units having a 23 cm field size, 10 units gave better Contrast-Detail results using a copper rather than a water absorber, with 5 results equivocal. Similarly, ten (10) units gave better Threshold Contrast results with copper, while 4 units were equivocal and 1 unit gave a poorer result with copper absorber when compared to a water absorber. It is therefore concluded that the test object and protocol should be used with a patient equivalent absorber (such as 20 cms water) for acceptance tests. Good image quality discrimination between units can then be obtained by correlating x-ray input dose with test object results, such as is given using the Contrast-Detail feature under ABC conditions.