PITCH ARTIFACTS IN MULTISECTION SCANNING

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. One of the benefits of z-filter interpolators is that they reduce the severity of windmill artifacts, especially when the image reconstruction width is wider than the detector acquisition width. Artifacts may also be slightly reduced by using noninteger pitch values relative to detector acquisition width, such as pitches of 3.5 or 4.5 on a four-section scanner. This is because z-axis sampling density is optimized for noninteger pitches.

PITCH EFFECT

A TYPICAL WINDMILL-LIKE APPEARANCE OF SUCH ARTIFACTS IS DUE TO THE FACT THAT SEVERAL ROWS OF DETECTORS INTERSECT THE PLANE OF RECONSTRUCTION DURING THE COURSE OF EACH ROTATION.

BOLUS TRACKING

The newer MDCT scanners have shorter scan times it was necessary 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 artery, cerebral and carotid arteries. The image shown below illustrates this technique on a sagittal MPR (multiplanar 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.

BOLUS TRACKING

MAXIMUM INTENSITY PROJECTION

Maximum Intensity Projection (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.

MAXIMUM INTENSITY PROJECTION

CT Abdominal Aortogram MIP projection, gray-scale inverted image.

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.