Bolus Tracking

Bolus tracking is a technique used in CT angiography to visualize vessels more clearly. This method individualizes the timing of a volume of contrast media delivery to a region of interest. The image is scanned once the region of interest reaches a certain level of contrast. This method of imaging is primarily used to evaluate arteries, such as the aorta, pulmonary artery, cerebral and carotid arteries. Images acquired from a bolus track, can be manipulated into a maximum intensity projection (MIP) or a volume rendered image (VRI).

Maximum Intensity Projection (MIP)

MIP is a three dimensional processing method used in CT and MRI. Blood vessels in CT angiography data sets are best viewed as MIP images. This method of reformation takes the brightest pixels in the image volume and projects them into the final MIP. Since blood usually has a higher pixel intensity than other structures in the background, the blood vessels are displayed brighter than the background.

Segmentation

Segmentation is a process used to simplify and/or change the representation of an image into something that is more meaningful and easier to analyze. A label is assigned to every pixel in an image such that pixels with the same label share certain visual characteristics. Desired tissue is imaged while undesireable tissue can be removed.

Banding Artifacts

In transmission computed tomography, relative x-ray attenuation measurements are made at various angles around a patient's body. These input projection data are reconstructed to yield a cross-sectional view of internal structure. If the body section contains material that severely attenuates the x-ray beam, high-density bands that obliterate internal structure will be produced in the process of image reconstruction. A technique has been developed that removes this imaging artifact. The approach views the affected projection data as misinformation. These data are assigned new values, and image reconstruction is performed without changing existing hardware or software.

Pitch Effect

In spiral CT, errors may arise with an excessively high pitch factor. If pitch is increased while holding kVp, mA, and beam collimation constant, then 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 the pitch.

Scalloping Artifact

Scalloping artifact is due to the fact that the slice sensitivity profile is increased in spiral CT so that the partial volume artifacts become stronger. This type of artifact can arise in skull CT, where the skull diameter quickly changes its axial direction. Reducing the pitch factor makes the artifact less noticeable.

Misregistration Artifacts

Patient motion can cause misregistration artifacts within the image. Small motions can cause blurring and larger physical displacements produce artifacts that appear as double images or ghost images. Proper patient stabilization can prevent voluntary motion however, some involuntary motion may be unavoidable.