Fan Yang, M.D., Ph.D. Ichiro Ikuta, M.D., M.M.Sc. Head Holder Image Artifact: Shadows in the midst of Light, and How We Fixed It. 34 Maple Street Norwalk, CT U.S.A.

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.

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. Cephalad sliceAttenuated slice Caudal slice

Image post-processing with reconstructed multiplanar images demonstrate the x-ray beam attenuation. A surface shaded display also demonstrates focal abrupt increased attenuation.

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%. Attenuation by carbon fiber head holder of approximately 15% CT head holder artifact at the slice shown CT head holder CT head holder base

Physics Discussion CT head holders are designed to be radiolucent. X-ray attenuation proportional to the atomic number cubed (Z 3 ). Carbon in Carbon fiber has a Z of 6, which is much lower than the Z of commonly used metals. Z carbon = 6, Z aluminum = 13, Z iron = 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. Z carbon 3 = 216, Z aluminum 3 = 2197, Z iron 3 = 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 = cm -1. Attenuated intensity at 0.5cm thickness = e (-.335 * 0.5) = (15%) Attenuated intensity at 5.0cm thickness = e (-.335 * 0.5) = (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 angle McCormack S1, Diffey J, Morgan A. The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert. Med Phys 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. Properly positioned patient, without gantry tilt. CT head holder artifact at the slice shown in red, due to patient positioning and 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 x- ray 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 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 Nov-Dec;24(6): Barrett JF1, Keat N. Artifacts in CT: Recognition and Avoidance. Radiographics Nov-Dec;24(6): McCormack S1, Diffey J, Morgan A. The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert. Med Phys Feb;32(2): McCormack S1, Diffey J, Morgan A. The effect of gantry angle on megavoltage photon beam attenuation by a carbon fiber couch insert. Med Phys Feb;32(2):483-7.

Fan Yang, M.D., Ph.D. Ichiro Ikuta, M.D., M.M.Sc. Head Holder Image Artifact: Shadows in the midst of Light, and How We Fixed It. 34 Maple Street Norwalk, CT U.S.A.