Transcript Conference Category

Head Holder Image Artifact:
Shadows in the midst of Light,
and How We Fixed It.
Fan Yang, M.D., Ph.D.
Ichiro Ikuta, M.D., M.M.Sc.
34 Maple Street
Norwalk, CT 06856 U.S.A.
[email protected]
Twitter: @radiology_ninja
Disclosures
Fan Yang, M.D, Ph.D.: No disclosures.
Ichiro Ikuta, M.D., M.M.Sc.: No disclosures.
Purpose
We will elucidate the etiology of a head CT image artifact which may hinder image
interpretation, and how to avoid creation of this image artifact.
A noncontrast head CT was ordered in the setting of head trauma to assess for acute
traumatic injury such as intracranial hemorrhage. While this clinical scenario is
commonplace and the imaging indicated, we began to notice a recurrent image
artifact in the midst of the axial images that was occurring for multiple head CT
examinations.
A review of the physics of x-ray beam attenuation assists in the understanding of
this phenomenon.
Case Report
• A single slice in the axial stack of images had uniform low density values
compared to immediately adjacent slices. While sometimes this darkened image
occurred near the floor of the anterior fossa or middle fossa, sometimes it would
occur in the middle of the fronto-parietal parenchyma without a significantly
attenuating calvarial landmark within the image or any adjacent image.
• Multiple maintenance efforts were attempted, but the image artifact persisted.
Finally, it was determined that the artifact was most prominent at certain gantry
angles, and when the head was located near the base of the CT head holder. It
was determined that the x-ray beams were traversing through a thicker part of
the CT head holder holder material (carbon fiber) on affected axial slices,
resulting in an attenuated image.
• To avoid this image artifact, our protocols now call for the patient to be
positioned with the head as far into the head holder as possible, avoiding the
head holder all together on head CT examinations.
Cephalad slice
Attenuated slice
Caudal slice
The middle slice demonstrates diffuse attenuation, while the left image (cephalad)
and right image (caudal) lack this attenuation. The image artifact is most prominent
within the midline as the CT head holder attenuates both the projected x-ray beam
and x-rays intended to be detected.
Image post-processing with reconstructed multiplanar images demonstrate the x-ray
beam attenuation. A surface shaded display also demonstrates focal abrupt increased
attenuation.
Attenuation by
carbon fiber head
holder of
approximately 15%
Attenuation by
carbon fiber head
holder of
approximately 15%
At this one slice, the X-ray
beam traverses tangentially
through a thick portion of the
base of the head holder,
thereby resulting in
attenuation of 81%.
CT head holder
CT head holder artifact at
the slice shown
CT head holder base
Physics Discussion
• CT head holders are designed to be radiolucent.
• X-ray attenuation proportional to the atomic number cubed (Z3).
• Carbon in Carbon fiber has a Z of 6, which is much lower than the Z of
commonly used metals.
• Zcarbon = 6, Zaluminum = 13, Ziron = 26
• Given the same thickness in design, an aluminum head holder would have
10x the attenuation compared to carbon fiber, and an iron/steel head
holder would have 81x the attenuation of carbon fiber.
• Zcarbon3 = 216, Zaluminum3 = 2197, Ziron3 = 17576,
• X-ray attenuation is also proportional to the thickness and density of the material
used.
• The relation of attenuation to thickness has an exponential relationship
that depends on the linear attenuation coefficient (μ) and thickness of the
material used.
• Attenuated intensity /original beam intensity = e (μ * thickness)
• Although the carbon fiber head holder has minimal x-ray attenuation
relative to metals, the shape/design of the CT head holder results in differing
attenuation values based on the thickness of the carbon fiber that the x-ray
beam must traverse.
• CT head holder thickness is approximately 5mm, which attenuates the beam
by approximately 15%. An oblique x-ray beam through estimated 5cm of
carbon fiber (at certain gantry angles) at the base of the CT head holder
attenuates the beam by up to 81%.
• Linear attenuation coefficient for carbon at 100KeV, u = 0.335 cm-1.
• Attenuated intensity at 0.5cm thickness = e (-.335 * 0.5) = 0.856 (15%)
• Attenuated intensity at 5.0cm thickness = e (-.335 * 0.5) = 0.187 (81%)
• This artifact therefore results in increased x-ray attenuation on a single
slice or a few consecutive slices.
• Studies in radiation therapy using high energy values have found a similar
phenomenon in cases where high gantry angles are associated with x-ray
attenuation values from 2% normally to 9% when at an oblique angle1.
1. McCormack S1, Diffey J, Morgan A. The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert. Med Phys. 2005
Feb;32(2):483-7.
Conclusion
Patient positioning is critical to avoid image artifacts from the head holder.
Educating the CT technologists to position the patient with the head at the maximal
cephalad position within the head holder will help avoid this image artifact.
CT head holder artifact at the
slice shown in red, due to patient
positioning and gantry tilt
Properly positioned
patient, without gantry
tilt.
Suggestions
The following are suggestions to minimize this artifact:
• Position the patient's head cephalad relative to the angled base of the head
holder to move the artifact outside the field of view, however this would not
be ideal if patient were to also obtain a CT of the cervical spine.
• Scan axial slices of CT brain without tilting of the gantry to minimize the xray beam traversing through the base of the CT head holder.
• Reduce hyper-extension of the head if possible.
• Educate the CT technologists of these protocol procedures.
References

JT Bushberg, JA Seibert, EM Leidholdt and JM Boone, The Essential
Physics of Medical Imaging. Williams and Wilkins, Baltimore, MD 1994

Barrett JF1, Keat N. Artifacts in CT: Recognition and Avoidance.
Radiographics. 2004 Nov-Dec;24(6):1679-91.

McCormack S1, Diffey J, Morgan A. The effect of gantry angle on
megavoltage photon beam attenuation by a carbon fiber couch insert. Med
Phys. 2005 Feb;32(2):483-7.
Head Holder Image Artifact:
Shadows in the midst of Light,
and How We Fixed It.
Fan Yang, M.D., Ph.D.
Ichiro Ikuta, M.D., M.M.Sc.
34 Maple Street
Norwalk, CT 06856 U.S.A.
[email protected]
Twitter: @radiology_ninja