41 Computed Tomography Artifacts

10.1055/b-0040-176877

41 Computed Tomography Artifacts

Clark W. Sitton and Kaye D. Westmark

41.1 Computed Tomography Angiography

41.1.1 Case Presentation: CTA Artifacts Mimicking Carotid Artery Dissection

History: A 35-year-old man admitted to the neurosurgical service for management of a subdural hematoma suffered during motor vehicle collision. A CT angiography (CTA) of the neck was performed as routine screening based on the mechanism of the injury ▶ Fig. 41.1. The radiology report describes a dissection of the left carotid artery origin (see ▶ Table 41.1).

**Fig. 41.1** Maximum-intensity projection (MIP) sagittal **(a)** and axial source **(b)** images from a CTA demonstrate a faint linear filling defect originating at the junction of the carotid bulb with the common carotid artery (*arrows*). On the axial images, the defect fails to make contact with the wall of the vessel.

**Table 41.1** Differential diagnosis for flaplike filling defects at the origin of the internal carotid artery
Differential diagnosis	Comments
Flow/admixture artifact	Faintly seen linear or curvy linear area of reduced density originating at the posterior margin of the bulb and extending into the vessel lumen, often forming a curvilinear shape
Atherosclerotic dissection	Filling defect, surrounded by contrast, on contrast images and dynamic angiography with clear attachment to the vessel wall. Associated with atherosclerotic changes in the vessel walls such as calcification and irregularity
Traumatic dissection	Traumatic dissections of the internal carotid artery occur in two locations, between the origin and skull base, where this styloid ligament passes, and at or just beneath the skull base, where the artery is tethered by the carotid canal. Traumatic injuries elsewhere are extremely rare except in the case of penetrating trauma. Dissections will extend from the aortic arch into the common, and internal carotid arteries
Carotid web	1- to 2-mm-thick shelf of soft tissue arising from the posterior wall of the carotid not associated with atherosclerotic change

Imaging impression: Normal carotid bifurcation; the apparent filling defect is artifactual.

Cause: Artifactual hypodensity secondary to turbulence related to the carotid bifurcation, resulting in heterogeneous mixture of contrast and nonopacified blood.

Additional testing to consider in difficult cases:

Recommended: Delayed contrast examination, such as routine contrast-enhanced CT of the neck. This admixture phenomenon is extremely transient and relates to the high speed of modern multidetector scanners.
Not recommended: Two-dimensional time-of-flight MR angiography. If there is turbulence at the bulb significant enough to produce this artifact on CT, it will almost certainly produce a similar artifact on MRI, although the physics is completely different.

Other common phenomenon that may simulate acute dissection on CTA:

CT beam hardening artifact: Streaklike artifact related to high-density material such as dental amalgam or other radiopaque foreign bodies as well as high-density contrast in the venous system. The resulting hypodense linear bands projected from the dense object traverse the vessel in the axial plane, resulting in linear filling defects on the reconstructed images, and sometimes a flaplike appearance may be present even on the source images.
Pulsation artifact: Motion of the carotid artery during the CT acquisition results in the superimposition of the carotid density on adjacent structures, resulting in a Venn diagram type of overlap appearance on the axial image. This can then result in irregularity in the contour of the vessel lumen or the appearance of linear flaplike filling defects on reconstructed images. This primarily occurs in in two locations: at the thoracic junction, where the carotid artery pulsatility is more marked, and, in children and younger patients, in the internal carotid artery where it lies immediately adjacent to the internal jugular vein.
FMD: Thickening of the vessel wall can result in a beaded, irregular appearance very similar to arterial dissection. Often bilateral, it can involve the vertebral arteries as well. It is important to recognize that FMD is associated with intracranial aneurysms and real dissections.