ARTIFACTS IN SPIRAL CT

CT image of an acrylic sphere showing windmill artifact

A higher pitch tends to cause helical artifacts, degradation of section sensitivity profile (slice broadening) and decrease in spatial resolution. Because spiral scanning requires an interpolation process to recover the consistent projections of individual slices, additional artifacts may be produced. Appearance and severity of spiral artifacts depend on scanning pitch and the type of interpolation algorithm. The typical windmill-like appearance of such artifacts are due to the fact that several rows of detectors intersect the plane of reconstruction during the course of each rotation. As helical pitch increases, the number of detector rows intersecting the image plane per rotation increases and the number of "vanes" in the windmill artifact increases. Z-filter interpolators are commonly used on multisection scanners to reduce the severity of windmill artifacts.

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Scalloping artifact is due to the fact that the slice sensitivity profile (SSP) is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping can occur in skull CTs, particularly in slice positions in which the skull diameter quickly changes its axial direction. The data structure looks like a cut radish. This image error can be corrected by reducing the pitch factors.

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Ct image of the head shows motion artifacts

Misregistration artifacts occur when the same anatomy is not registered in the same pixel of the image matrix. Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. Voluntary patient motion can be prevented by giving good patient instructions, using positioning aids, using immobilization aids including sedation, and using as short a scan time as possible. Built-in features by manufacturers for minimizing motion artifacts are: overscan and underscan modes, software correction, and cardiac gating.

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Coronal (left) and sagittal (right) reformatted images of the heart obtained from CT data show banding artifacts (arrowheads).

Banding artifacts in cardiac CT scans are horizontal shifts in the MPR or 3-D images. They result from increased heart rate during the scan. The most frequent causes of banding artifacts are arrhythmia, no breath hold, and alterations in heart rate during acquisition. However, the occurrence of banding artifacts cannot be predicted prior to scanning in most cases. Long acquisition time is associated with increased frequency of alteration in heart rate during data acquisition. Thus, multi-detector row CTs, with its fast scanning capability, reduces the prevalence and degree of alterations in heart rate during data acquisition.

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Stair step artifacts appear around the edges of structures in multiplanar and 3-D reformatted images when wide collimations and nonoverlapping reconstruction intervals are used. Stair step artifacts are virtually eliminated in multiplanar and 3-D reformatted images from thin-section data obtained with today's multisection scanners.

BOLUS TRACKING

Because the newer MDCT scanners have shorter scan times it was necaessary to re-evaluate the injection protocols for CTA. Bolus tracking is a method that individualizes the timing of contrast media (CM) delivery to a region of interest (ROI). An injection of CM is tracked or observed until it reaches the desired Hounsfield Units (HU) threshold for the ROI. When that threshold is met wait a few seconds and then scan to obtain the optimal image enhancement.

This method of imaging is used primarily to produce images of arteries, such as the aorta, pulmonary arteries, cerebral and carotid arteries. The image shown above illustrates this technique on a sagittal MPR (multi planar reformat). The image is demonstrating the blood flow through an abdominal aortic aneurysm or AAA. The bright white on the image is the contrast. You can see the lumen of the aorta in which the contrast is contained, surrounded by a grey 'sack', which is the aneurysm. Images acquired from a bolus track, can be manipulated into a MIP (maximum intensity projection) or a volume rendered image. (http://www.wikipedia.com/)

MAXIMUM INTENSITY PROJECTION

CT Abdominal Aortogram MIP projection, gray-scale inverted image

MIP is a post processing technique that reconstructs an image by selecting the highest value pixels along any arbitrary line through the data set and exhibiting only those pixels. It creates a 3-D image from multislice 2-D data sets; it's the simplest form of 3-D imaging. MIP images are widely used in CTA because they can be reconstructed very quickly. (Bushong, p.453-454)

SEGMENTATION

Left: Coronal slice from CT volume Middle: Same slice after segmentation/labeling Right:Isosurface plot of the femur and pelvis after segmentation

Segmentation is a step in processing an object model into a simulated 3-D image. Segmentation is a processing technique used to identify the structure of interest in a given scene. It determines which voxels are a part of the object and should be displayed and which are not and should be discarded. (Seeram, p. 346)