"Focal spot condition" is a general term that was coined to serve as a heading for the section of the regulations that includes tests that yield information on the focal spot. One of these tests involves a measurement of focal spot dimensions, the other assesses the system resolution. The focal spot measurements provide information about focal spot sizes, whereas the system resolution test provides an evaluation of the performance of the entire system (focal spot measurement evaluates only one component of the system). Thus the system resolution is an outcome-based test and has a greater value in image quality evaluation. Therefore, FDA made the system resolution test mandatory effective October 28, 2002.
The regulations do not prohibit the use of focal spot measurements in addition to the system resolution test. FDA believes if the system resolution test indicates a problem, focal spot measurements should be performed to determine if the focal spot is the cause of the problem. In many cases, the focal spot will not be the cause of the system resolution test failure, and other factors in the imaging chain will have to be evaluated to identify the actual problem.