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International Journal of Cancer Therapy and Oncology 
www.ijcto.org 

Copyright © Stauduhar et al.                                                                                   ISSN 2330-4049 

Are output measurements always necessary after CT tube 

replacement?   

Paul J Stauduhar, Aaron Kyle Jones  
 

The University of Texas M. D. Anderson Cancer Center, Houston, TX, USA. 
 

Received March 19, 2014; Published Online April 08, 2014 

[Presented at the Young Investigator’s Symposium at the 2014 Annual Meeting of  
Southwest Chapter of American Association of Physicists in Medicine (AAPM) in San Antonio, Texas, USA] 

Conference Proceeding 

Abstract 

Purpose: TX regulations and the ACR require that CT radia-

tion output be measured within 30 days of major service. 

The most common major service is tube replacement. We 

hypothesized that historical QC data could be used instead to 

determine if output measurements are necessary, reducing 

the need for costly output measurements. 

 

Methods: We reviewed 66 records of tube replacements to 

determine with what frequency output falls outside specifi-

cations. We also conducted an experiment to verify that 

clinically significant output changes could be identified by 

comparing image noise in historical QC data with the same 

data after tube replacement. We used 30 days of historical 

QC data to establish a baseline noise level and 95% confi-

dence interval (CI) for individual noise measurements. To 

simulate output changes, we acquired phantom images with 

our QC protocol while manually changing output (mA). We 

acquired 10 images using the baseline output and 10 images 

at each different “output”. We evaluated individual images 

and subsets of images at each “output” to determine if the 

system was within the manufacturer’s specifications.  

 

Results: None of the 66 tube replacements resulted in an 

output change that exceeded specifications. Analysis of 30 

days of historic QC data for our experimental system indi-

cated a mean noise of 5.4 HU with 95% CI of 5.1 ‒ 5.7 HU. 
When using the mean noise of 10 images acquired at each of 

the varying outputs, we were able to identify, with 100% 

accuracy, images acquired at outputs outside manufacturer’s 

specifications. 

 

Conclusion: The results of our review of historical tube re-

placement data indicated the likelihood of output falling 

outside manufacturer’s specifications is low. Considering 

this, it is likely that by using QC data from programs re-

quired by regulation and the ACR physicists can reliably 

verify radiation output stability remotely instead of making 

physical measurements. 

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Presenting author: Paul J Stauduhar; The University of Texas M. 

D. Anderson Cancer Center, Houston, TX, USA. 

 

Cite this article as: 

P Stauduhar, A Jones. Are output measurements always necessary 

after CT tube replacement? Int J Cancer Ther Oncol 2014; 
2(2):020238. DOI: 10.14319/ijcto.0202.38 

 

http://ijcto.org/index.php/IJCTO/index
http://dx.doi.org/10.14319/ijcto.0202.38


 2 Stauduhar et al.                                                             International Journal of Cancer Therapy and Oncology 
                      www.ijcto.org 

Copyright © Stauduhar et al.                                                                                   ISSN 2330-4049 

 

 

 
FIG.1: Plot showing output after a CT Xray tube change. Output specifications (0%) and limits (red and green dashed lines) are from manu-

facturer technical specifications. The limit for Siemens was +/-20%, GE publishes an expected variation of +/-15% and a maximum variation of 
+/- 40%. In the current study (66 instances), we found no records of X�ray tubes failing to meet manufacturer’s specifications when OEM 
replacement tubes are used. 

 

 

 

 
*Image noise is mean from 10 scans  

†15% output change corresponds to 3.9% change in image noise, 95% CI for image noise = [5.1, 5.7]. CI was calculated by using the CI for 

baseline noise and applying scaling factor to calculate CI at +/� 15% output deviation from baseline. The lower limit of the CI for �15% output 
and the upper limit of the CI for +15% output were used in the final CI. 

 
FIG. 2: Results of QC experiment demonstrate that it is possible to detect changes in post�replacement tube output using historical QC data. 
Values in the top row highlighted green indicate output changes that are within +/-15% manufacturer limits, values highlighted in red fall 
outside limits. “Pass” or “Fail” values in bottom row indicate results of comparison of image noise between “post�change” images, simulated by 
changing mA, and “pre�change” historical QC data. One output value that was within specified limits (-15%) was indicated as failing by our 
analysis. It was noted that changes in image noise were larger than expected as output decreased, while changes were at the expected level as 

output was increased. One possible explanation for this result is increasing contribution of electronic noise as output was reduced. 


