Yusuf Emre Erdi DSc, Presenter: Nothing to Disclose

Usman Mahmood MS, Abstract Co-Author: Nothing to Disclose

It has been a well known fact that x-ray tube output decreases over time due to tungsten buildup on the exit window or anode surface roughening. However, the amount of output reduction has not been quantitated before. This work aims to measure CT x-ray tube output during its life-cycle.

Twenty-eight (20 General Electric (Waukesha, WI) and 8 Philips (Andover, MA)) CT scanners have been included in this analysis. GE scanners (LS-16, VCT, HD 750) were mainly used for diagnostic radiology and PET/CT scanning with high patient loads (15-30 patients/day). Philips scanners (Brilliance, Precedence, Brightview) were in radiation therapy planning and SPECT/CT with low patient loads (<10 patients a day). The x-ray tube outputs have been followed since acceptance testing and, with a tube change, the measurement cycle re-starts. Dose was measured using a PMMA head CT phantom manufactured by Radcal (RadCal, Monrovia, CA). The measurement system was a 9095 multipurpose analyzer with 10x9-3CT ion chamber both from Radcal. The ion chamber is inserted into the peripheral and central axis locations, and CTDIvol is calculated as weighted average of doses at those locations. All dose measurements were performed in axial mode.

None of the 40 GE tubes made it into their 5th year and only 1 Philips tube lasted after 4 years of clinical service.Dose measurements performed 1 year after tube replacement showed that both GE (1.89%) and Philips (1.52%) tubes had less than a 2% decrease in tube output. For the tubes that lasted at least 2 years, the decrease in radiation output was 5.15% for GE and 1.78% for Philips. Four years after tube replacement, the radiation output decreased by 10% for GE and by 3% for Philips tubes. A Philips tube, which has been in service for 7 years, has not shown any decrease in radiation output.

Depending on the CT x-ray tube age, radiation output can be reduced by up to 10%, which will translate into the increase of noise in clinical images unless compensated by an AEC system. For optimum clinical practice, it helps to know the age of the tube and increase tube current or scan time accordingly to compensate for the reduced x-ray tube output.

CT x-ray tube output decreases over time. When quantitated, it is possible to compensate this decrease by increasing the tube output so image quality stays constant.  

Erdi, Y, Mahmood, U, CT X-ray Tube Output Changes Over Time.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14045427.html