Thursday Case of the Day Physics Authors Robert

Thursday Case of the Day Physics Authors: Robert Mac. Dougall, M. Sc. , DABR Boston Children’s Hospital History: A new CT scanner was installed in the ED with a lightweight, low power (72 k. W) x-ray tube without a secondary (rebound) electron collection mechanism. Dark lines were observed by radiologists in coronal and sagittal reformats of CT head scans performed in axial (non-helical) mode. The likely cause of the artifact is: Figure 1: Coronal and sagittal reformats showing radiolucent (dark) “line” artifacts A. Patient Motion B. Improper Cupping Correction C. Off-Focus Radiation combined with heel effect causing HU gradient at bonebrain interface across detector rows. D. Beam Hardening

Findings: All detector calibrations were performed and found to be within manufacturer acceptance limits. The mechanical stability of the table was verified to be operating at specification by a service engineer. Addition tests and measurements were performed to isolate the cause of the artifact: 1. From clinical scans, the distance between artifact “lines” was confirmed to match the nominal beam collimation by a medical physicist. 10 mm Figure 2: Artifact “period” (i. e. distance between artifact lines) was found to match nominal beam collimation (10 mm).

Findings: 2. A stationary head phantom was scanned and the artifact was found to be reproducible in a motionless phantom. The artifact was also more severe for wider beam collimations. 3. The artifact was not observed in transaxial images. Figure 3: Transaxial image with no visible artifact

Diagnosis: C. Off-Focus Radiation combined with heel effect causing HU gradient at bone-brain interface across detector rows.

Discussion: Off-focus radiation is created by rebound electrons that strike the target away from the focal spot. If these rebound electrons are not collected by a secondary electron capture mechanism, the radiation produced will lead to reduced contrast at in the bone/brain interface. Figure 4. The Physics of Off-focal radiation

Discussion: The heel effect makes the effect of off-focal radiation detector-row dependent and as a result the bone/brain interface will have higher contrast in the anode direction (less off-focal radiation). an Figure 4: Impact of heel effect of off-focal radiation and bone/brain interface at different detector-row locations.

Discussion: Since the effect of off-focal radiation is z-axis dependent, it is most visible in multi-planar reformats such as coronal and sagittal views where there is a sharp border between contiguous beam collimations and hence detector rows. The artifact will not be visible in individual transaxial reformats. The artifact is also much less visible in helical mode acquisition where interpolated detector data is used for image reconstruction. Figure 5: From left – Transaxial, Coronal and Sagittal reformats. The artifact is only visible in the coronal and sagittal images

References/Bibliography: • Johns HE, Cunningham JR. Physics of Radiology, Fourth Edition. 4 Sub edition. Springfield, Ill. , U. S. A: Charles C Thomas Pub Ltd; 1983. • Curry TS, Dowdey JE, Murry RC. Christensen’s Physics of Diagnostic Radiology. Lippincott Williams & Wilkins; 1990 (pg. 25) • Bushberg JT. The Essential Physics of Medical Imaging. Lippincott Williams & Wilkins; 2002. • Barrett JF, Keat N. Artifacts in CT: Recognition and Avoidance. Radio. Graphics. 2004 Nov 1; 24(6): 1679– 91

Teaching Points: • Off-focus radiation is rarely considered in the diagnosis of CT artifacts because tubes have traditionally been built with rebound electron capture mechanism (e. g. a metal envelope at ground potential surrounding the anode). • Lighter tubes are being introduced at lower power levels that do not incorporate a rebound electron capture mechanism. • Off-focus radiation negatively impacts image quality, particularly at the border of the skull/brain interface in neuro CT and can lead to artifacts in MPR images. • Typically, only axial scan modes are affected by off-focal radiation • The intended scan modes and the x-ray tube design should be carefully considered in the purchase of a new CT scanner.