Thursday Case of the Day Physics Author: Timothy P. Szczykutowicz Ph.D, DABR University of Wisconsin Madison, Departments of Radiology, Medical Physics, and Biomedical Engineering History: An adult head is scanned using a helical/spiral mode and the resulting images exhibit hypo and hyper intensities at the brain/bone interface and a blurry appearance throughout the slice. What is causing these artifacts? Motion Too high of a helical pitch Oil in the x-ray tube housing Failure to turn on a beam hardening correction Figure 1: Axial 5 mm CT image of the head. Image shown at a ww/wl of 80/25 HU. Blue arrow depicts one instance of the artifact present throughout the image.

Diagnosis: A. Motion artifact.

Advance to the next slides to see detailed image based examples of each of the proposed solutions. Solution Detail: There is a fundamentally different appearance of motion artifacts depending on scan mode. In axial/sequential scan modes, motion usually induces streak like artifacts, while in helical/spiral scan modes motion induces the artifacts emanating locally from regions of high CT number changes aligned along a single direction. Too high of a helical pitch will cause artifacts, especially near bony/soft tissue interfaces. The artifacts from high pitch scanning, however, do not produce the blurry appearance observed in this case. Artifacts from high pitch scanning will produce “windmill” streaks emanating from bony structures or air in the axial plane as opposed to the more global streaks produced by head motion in an axial/sequential scan. Oil in the x-ray tube housing produces low frequency hypo and hyper intensities throughout an image of a low frequency “blotchy” appearance. These artifacts appear intermittingly, and seemingly randomly as the bubble moves about the housing. The bubbles only cause artifacts when they travel between the focal spot and the x-ray exit window. Failure to turn on a beam hardening algorithm would not produce the blurry appearance observed in this case. Failure to activate a beam hardening algorithm would produce shading artifacts connecting regions of high density bone and may blur the brain/bone interface.

Discussion: Axial/sequential versus helical/spiral motion artifact appearance Helical Spiral scan artifact appearance Figure 2: (Left) The blue arrow points out the motion induced artifact in the axial plane from a helical/spiral scan. (Middle) More obviously, the yellow arrow shows how the sphenoid bone is “doubled”, an obvious sign that the scan suffers from motion. (Right) The red arrows on this sagittal slice show how the motion artifacts appear to “shift” slabs of the head. The regions of the apparent “breaks” in the skin line correspond to the places where the scanner was when the patient moved. Axial Sequential artifact appearance Figure 3: Motion artifacts in axial/sequential mode appear as global streaks (i.e. not localized just to certain regions of the head) across the head in one direction. The streaks do not exhibit any “bending”, the streaks are straight across the image. (left) and (right) show two different patients both suffering from motion.

By the end of the scan, the head has moved Discussion: Axial/sequential versus helical/spiral motion artifact appearance Helical images contain data from multiple detector channels and view angles acquired at different time points acquired over time periods > 1 rotation Helical interpolation scheme, from “Computed Tomography” by Willi A. Kalender. Publicis Corporate Publishing 2005 Figure 4: In a helical/spiral data acquisition, data is usually needed from 360 degrees about the patient, and that data is interpolated from view angles mostly out of plane of the reconstructed slice. When motion corrupts some of the projection data, that corrupted data is spread throughout the image volume because of the width of the filter used to combine data across the detector and view angle. The corrupted data will produce artifacts strongest for the view angle/s over which the motion occurred and to a lesser and lesser amount as one moves to view angles acquired during no/lesser motion as more and more non-motion corrupted data is used in the interpolation. This is why the motion induced artifacts do not form along straight lines. By the end of the scan, the head has moved The view angles at the start and end of the scan “see” the head has moved by the biggest amount. Since the views at the start and end contain information that is not consistent, we see artifacts projected through the image along that view angle Figure 5: In an axial/sequential scan mode, the two projections having the largest disagreement are the ones at the start and the end of the scan. This is why we see motion artifacts in an axial/sequential scan mode occurring all along the same line. That line corresponds to where the CT scanner started/ended acquiring projection data. On some scanners, this angle is always the same, on other scanners, it will be random. The starting angle being fixed or being random is vendor/model dependent.

Phantom used in this example Discussion: High pitch artifacts appearance High pitch Low pitch Phantom used in this example Figure 6: High pitch (>1) axial and coronal images shown on the left exhibit more helical artifact relative to low pitch (~0.5) images shown on the right. Blue arrows denote artifacts observed in the high pitch example that are not present in the low pitch example. The form of these high pitch artifacts is unique from the motion induced artifacts of both axial and helical modes shown in Figures 2 and 3. High pitch artifacts emanate from locations within the scan volume where large changes in CT number occur in the z direction. Often, they are referred to as “windmill” artifacts owing to the fact they rotate as one progresses through an axial image volume.

Discussion: Oil in the x-ray tube artifact appearance This artifact is arguably the most serious issue one can have with their CT scanner It mimics pathology It occurs randomly Figure 7: Examples of a routine head scan suffering from bubble artifacts (yellow circles). These artifacts mimic a wide variety of pathologies. Figure 8: Two QA images taken 30 seconds apart, the artifact only appears in one of them! Bubbles in the cooling oil float around inside the tube housing. When they happen to be located between the focal spot and exit window, we see issues.

Discussion: Beam hardening correction artifact appearance Inserting additional slides for Discussion: Before filling in this slide, click on “Insert” in the top bar and select “Duplicate Slide”. Then return to previous slide. Discussion: Beam hardening correction artifact appearance Phantom used in this example Beam hardening correction on Beam hardening correction off Figure 9: Examples of head image quality with and without a full beam hardening correction applied. All images in this figure have a soft tissue beam hardening correction, but only the left column has an additional bone beam hardening correction applied. One can see how the additional bone correction improves the brain bone interface (blue oval) and removes the hyper intensity between the posterior fossa (i.e. “Hounsfield bar”, shown by the blue arrow).

References/Bibliography: General Artifact articles Barrett, Julia F., and Nicholas Keat. "Artifacts in CT: recognition and avoidance." Radiographics 24.6 (2004): 1679-1691. Oil Bubble article Trieu, Nelson, et al. "Artefact on CT brain images caused by the presence of air bubbles in the cooling oil of the X‐ray tube." Journal of medical imaging and radiation oncology 61.2 (2017): 197-203.