CT Artifacts

Artifacts of CT studies (Figure 9.5A-G) include high density foreign bodies such as a shotgun pellet in the right orbit and face causing streak artifact (Figure 9.5C). More significant metallic streak artifacts can be seen from dental fillings (Figure 9.5A). By viewing the image at bone window, most of that artifact can be reduced (Figure 9.5B). Circular or ringtype artifacts are usually related to detector problems. Figure 9.5D is an axial CT scan that was performed on a machine with a detector failure. Motion can degrade the quality of imaging and introduce artifacts at the time of scanning (Figure 9.5G). Positioning artifacts can also be misleading. On occasion, one may get the impression of proptosis due to an artifact. For example, a positioning artifact (Figure 9.5E,F) makes the left globe appear to be proptotic. The normal position of the globe in adults is 9.4 mm behind the interzygomatic line (range, 5.9-12.8 mm).18 Other artifacts include partial voluming, where the scanning voxel will include tissues of two widely different densities such as cerebrospinal fluid and brain. When these two densities are averaged together they may create the illusion of a lesion. Beam-hardening artifact (Hounsfield artifact), which is mostly seen in the posterior fossa in the middle cranial fossa and orbit, represents increased attenuation of the x-ray beam due to the density and thickness of bone in these areas. The beam-hardening artifacts present a problem, particularly in the apex of the orbit and the optic canal.

part two: diagnosis of orbital tumors part two: diagnosis of orbital tumors

FIGURE 9.2. Axial orbital CT images with contrast (A) at level of the optic nerve and (B) showing the optical canal. (C) Parasagittal reconstruction of orbital CT along the course of the optic nerve. (D)

Orbital coronal reconstruction of the orbits at midlevel. (E) Coronal orbital reconstruction with contrast at the equator level of the globe.

FIGURE 9.2. Axial orbital CT images with contrast (A) at level of the optic nerve and (B) showing the optical canal. (C) Parasagittal reconstruction of orbital CT along the course of the optic nerve. (D)

Orbital coronal reconstruction of the orbits at midlevel. (E) Coronal orbital reconstruction with contrast at the equator level of the globe.

0 0

Post a comment