SN 4840 Bad Pixel Calibration Failures Due To Tube Problems

SN 4840 Bad Pixel Calibration Failures Due To Tube Problems SN4840 Bad Pixel Calibration Failures Due To Tube Problems Rev. 1 5326371-8EN PURPOSE/SYMPTOM Bad Pixel Calibration is failing with error messages referring to too many Bad Pixels or Bad lines, while the image is clinically OK (i.e. there are no visible artifacts) and IQST checks are passing. Detector alignment or replacement does not resolve the issue. PROBLEM At very low tube power levels used for e.g. some of the Bad Pixel Calibration images’ acquisition, some tubes may emit X-ray from outside the regular Focal Spot, i.e. behaving as having double Focal Spots. This results in a double image of the collimator blades at the detector, one shifted vs. the other. Eventually, there will be semi-dark bands at two sides of the image (illuminated by one or the other of the “Focal Spot”-s only), and higher brightness over the center of the image (sum of brightnesses, where the image of the two “Focal Spots” overlap) – see Illustration 1. for details. The semi-dark border is detected then as a group of Bad Pixels by the calibration application. By the time of compiling the Service Note, three occurrences of the issue were known. Both cases, the phenomenon was limited to very low power levels, i.e. not having a direct impact on clinical Image Quality in most applications. Also, the issue is blocking the execution of Preventive Maintenance. There is a specific test procedure in place to detect the issue (see section “Procedure”, subsection “Detection of the issue” for details). SOLUTION Contact OLC and Engineering. The actual solution needs to be decided based on the magnitude of the issue and the types of exams performed on the system. PROCEDURE 1 DETECTION OF THE ISSUE 1.1 Detection Procedure 1) Remove the table from the X-ray field, set 30 fps “Low” for Fluoro, and put the 20 mm Aluminum plate (found in the IQST Suitcase) on the tube cover. 2) Select FOV20, close the collimator so that ~30% of the image in the center is still visible (i.e. not covered by the collimator blades – see Illustration 1. for reference) 3) Make a short preparatory Fluoro, then acquire a ~5 sec long fluoro sequence (30fps, Low!). Store the sequence using fluoro store. 4) Remove the Aluminum plate and acquire another sequence with the same settings (30 fps “Low”) the same way, i.e. short preparatory shot, then a ~5 sec long sequence. Store the sequence with Fluoro Store. Check whether you can see any "collimator blade shadow" appearing on this second sequence as indicated in the picture below. If the “collimator blade shadow” was visible, save the fluoro store images and contact OLC and/or Engineering. Illustration 1.