Artifacts in Spiral CT

MISREGISTRATION ARTIFACTS

Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. The use of positioning aids is sufficient to
prevent voluntary movement in most patients. However, in some cases it may be necessary to immobilize the patient by means of sedation. Using as short a scan time as possible helps minimize artifacts when scanning regions prone to movement. Respiratory motion can be minimized if patients are able to hold their breath. Manufacturers minimize motion artifacts by using overscan and underscan modes, software correction, and cardiac gating.In this image a misregistration artifact seen on attenuation-corrected image (left) but not on non–attenuation-corrected image (right). Artifact is result of patient motion between CT and PET acquisitions



SCALLOPING

Scalloping Artifact are caused because the slice sensitivity profile is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping can occur in skull CTs, in slices in which the skull diameter quickly changes its axial direction. This can be corrected by reducing the pitch.

BANDING

Band artifacts are sometimes seen in the endocardium as dark bands or horizontal shifts in multiplanar reformatted (MPR) or three-dimensional (3-D) images. This is caused by motion.




STAIR-STEPPING

Stair step artifacts appear around the edges of structures in multiplanar and
three-dimensional reformatted images when wide collimations and nonoverlapping reconstruction intervals are used. They are less severe with helical scanning, which permits reconstruction of overlapping sections without the extra dose to the patient
that would occur if overlapping axial scans were obtained. Stair step artifacts are virtually eliminated in multiplanar and three-dimensional reformatted images from thin-section data obtained with today’s multisection scanners.



PITCH EFFECT

In single CT spiral scanner, the pitch is the table movement per tube rotation/slice collimation. In multi-sliceCT spiral scanners, the definition is table movement per
rotation/single slice collimation. If pitch is increased, than the table speed increases, mAs decreases, patient dose decreases, and either the effective slice
width increases or the image noise increases. So for reducing the artifacts due to spiral rotation, we should decrease pitch. (No Image)