Artifact in Head CT Images Due to Air Bubbles in X-Ray Tube Oil Poster No.: C-0671 Congress: ECR 2016 Type: Educational Exhibit Authors: H. Patel 1, W. Liu 2, J. DeSanto 2, S. Meagher 2, M. Zagardo 2, K. Keywords: DOI: Fraser 2, L. Wang 2, J. Maksimovic 2 ; 1 Peoria Heights, Il/US, 2 Peoria/ US Equipment, Physics, Education, CT, Neuroradiology brain, Radiation physics, CNS, Artifacts, Education and training 10.1594/ecr2016/C-0671 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. www.myesr.org Page 1 of 10
Learning objectives Investigate a type of CT artifact in cerebral white matter simulating pathology. Page 2 of 10
Background There are two types of x-ray tube arcing. The first type is caused by the accumulation of tungsten gas buildup over the years. The tungsten gas attracts the high voltage electron beam as it travels toward the rotating anode. This artifact appears as a streak-like pattern or random noise [1]. The second type of tube arcing is caused by air-bubbles in the x-ray tube oil. If there is a buildup of air bubbles the x-ray tube may not fire at all [1]. However, when the amount of air is small the x-ray tube may still function, but cause artifact. We could not find any report of this artifact in head CT images. At our hospital, we have observed this type of artifact on several occasions. It presented as attenuated subcortical white matter of the supratentorial brain similar to the appearance caused by leukoariosis or leukoencephalopathy. Page 3 of 10
Findings and procedure details Several actions were taken in order to better understand this artifact. First, the patients were sent for follow-up MRI. All demonstrated normal brain parenchyma. Figures 1-3 include examples of patients with artifact on their head CT that appears as patchy hypoattenuation of the subcortical white matter. MRI T2 FLAIR images in the same patients revealed no abnormality to correspond to the areas of hypoattenaution on CT. Age and gender matched controls are included for each patient. Secondly, to rule out the influence of skull, the skull density and width of these patients with artifact were measured and compared to those of other non-artifact patients. No significant difference was found. Figure 4 illustrates the locations of where the skull mass density and calvarium width measurements were obtained in the patients. A t-test was performed which revealed no significant difference between the patients and controls. The data is provided in Table 1. Thirdly, the CT manufacturer was contacted for consultation. The manufacturer recognized that it came from air-bubbles in the x-ray tube and an air removal procedure was arranged. Page 4 of 10
Images for this section: Fig. 1: Standard CT brain window (W:80 L:40) and narrow window (W:30 L30) were provided for the artifact and control groups. MRI T2 FLAIR images were provided for the artifact group. Patients 1 demonstrate patchy white matter hypoattenuation on standard brain windows (arrow), which is more conspicuous on narrow windows. Subsequent MRI demonstrated normal white matter on T2 FLAIR. Age and sex matched CT control is provided for comparison. Department of Radiology, OSF Saint Francis Medical Center, University of Illinois College of Medicine at Peoria/USA 2014 Page 5 of 10
Fig. 2: Standard CT brain window (W:80 L:40) and narrow window (W:30 L:30) were provided for the artifact and control groups. MRI T2 FLAIR images were provided for the artifact group. Patient 2 demonstrate patchy white matter hypoattenuation on standard brain windows (arrow), which is more conspicuous on narrow windows. Subsequent MRI demonstrated normal white matter on T2 FLAIR. Age and sex matched CT control is provided for comparison. Department of Radiology, OSF Saint Francis Medical Center, University of Illinois College of Medicine at Peoria/USA 2014 Page 6 of 10
Fig. 3: Standard CT brain window (W:80 L:40) and narrow window (W:30 L:30) were provided for the artifact and control groups. MRI T2 FLAIR images were provided for the artifact group. Patient 3 demonstrate patchy white matter hypoattenuation on standard brain windows (arrow), which is more conspicuous on narrow windows. Subsequent MRI demonstrated normal white matter on T2 FLAIR. Age and sex matched CT control is provided for comparison. Department of Radiology, OSF Saint Francis Medical Center, University of Illinois College of Medicine at Peoria/USA 2014 Fig. 4: Locations where calvarium width and mass density measurements (HU) were obtained. Measurements for Patient 1 are provided. Department of Radiology, OSF Saint Francis Medical Center, University of Illinois College of Medicine at Peoria/USA 2014 Page 7 of 10
Table 1: The mean mass density (HU), width (mm), and density x width are listed for the patient and control subgroups. The t-test values comparing the artifact and control subgroups have been provided. The mean of width x HU is greater for the patient than the control only in subgroup 1. Thus, we do not think the density and width are related to the cause of the artifact. - Peoria Heights/US Page 8 of 10
Conclusion The artifact caused by air-bubbles in the x-ray tube may bring misdiagnosis or confusion. To our knowledge, the artifact has not been previously described. It is unlikely that the artifact is caused by the influence of the skull density and its width. The manufacturer recognized that air-bubbles in the x-ray tube oil were a potential cause for artifact. A QC scan was performed by the manufacturer on two scanners in which the artifact was noted. The QC scan confirmed the presence of air-bubbles within the x-ray tube oil. A field engineer removed the air-bubbles from the x-ray tube oil, which eliminated the artifact. A pathology-like artifact in head CT images caused by air-bubbles in the x-ray tube oil is reported. On head CT, the artifact appears as patchy white matter hypoattenuation. Early recognition of this artifact can avoid potentially delayed or misdiagnosis and unnecessary additional workup. A manufacturer specific QC scan can be performed to confirm the presence of air-bubbles in the x-ray tube oil. Page 9 of 10
References [1] "Physics and Instrumentation Study Guide - Comprehensive Overview." CTtutor.com- Computed Tomography Review Online. Radtutor Educational Services, 2014. Web. 1 Dec. 2014. <http://www.cttutor.com/ct-anatomy-study-exam.htm>. Page 10 of 10