Thursday Case of the Day
Physics
Authors: Robert MacDougall, M.Sc., DABR
Boston Children’s Hospital
History:
A new CT scanner was installed in the ED with a lightweight, low
power (72 kW) 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 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. RadioGraphics. 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.