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.

SEGMENTATION

LEFT: coronal slice from CT volume

MIDDLE: same slice after segmentation/labeling

RIGHT: Isosurface plot of the femur and pelvis after segmentation

ARTIFACTS IN SPIRAL CT

The following artifacts are listed on this blog.

Misregistration
Scalloping
Banding
Stair-Stepping
Pitch Effect

MISREGISTRATION ARTIFACTS

Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image. Steps can be taken to prevent voluntary motion, but some involuntary motion may be unavoidable during body scanning. However, there are special features on some scanners designed to minimize the resulting artifacts.

MISREGISTRATION ARTIFACTS

Patient motion causing misregistration artifacts on this CT head image.

MISREGISTRATION ARTIFACTS

Motion artifact simulating the appearance of dissection. Postcontrast image through the ascending aorta reveals an irregular line extending across the aorta (curved arrow), simulating aortic dissection. This line was not seen on images immediately above and below this level (not shown). Motion artifacts are most prevalent and severe near the base of the heart. They may be recognized by their non-anatomic configuration and their inconstant appearance on sequential images.

SCALLOPING ARTIFACTS

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. This image error can be corrected by reducing the pitch factors.

Unable to locate any Computed Tomography (CT) Scalloping Artifact Images .

SCALLOPING ARTIFACTS

SCALLOPING OF THE DIAPHRAGM

Note: Multiple arcuate elevations of the right hemi-diaphragm scalloping is seen in about 10% of normal chest x-rays.

BANDING ARTIFACTS

Band artifacts are produced when the projection reading of a single channel or a group of channels consistently deviate from the truth. That can be the result of defective detector cells of DAS (Data Acquisition System) channels, deficiencies in system calibration, or a suboptimal image-generation process. This is predominately a third-generation CT scanner phenomenon. The detector channel reading is always mapped to a straight line that is at a fixed distance to the isocenter of the system, a defective reading forms a ring pattern during the back-projection process.


Note: Band or Ring Artifacts are produced in the same way.

BANDING ARTIFACT

Left Image: CORONAL

Right Image: SAGITTAL

Reformated image of the heart obtained from CT data showing Banding Artifacts (arrowheads).

Pitfalls in 16-detector row CT of the coronary arteries.

STAIR-STEP ARTIFACT

CT COLONOGRAPH

Stair-Step Artifacts in the rectum in a 58 year old asymptomatic women. 3-D Endoluminal CT Image (3-mm section thickness) shows a series of concentric rings (arrows) around the Colonic Lumen.

STAIR STEP ARTIFACTS

High Resolution CT and CT Angiography of Pheripheral Pulmonary Vascular Disorder.
Stair-Step Artifact in oblique branches (arrowheads) due to insufficient spatial resolution.

STAIR-STEP ARTIFACT

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.

STAIR-STEPPING

SPIRAL CT PITCH EFFECT

PITCH EFFECT:

CT pitch is generally defined as the table travel per rotation divided by the collimation of the x-ray beam. This beam-pitch definition can also be refered to as table travel per rotation divided by effective detector row thickness. Thus, a beam-pitch of 1.0 facilitates an acquisition with no overlap or gap, a beam-pitch of less than 1.0 facilitates an overlapping acquisition, and a beam-pitch of greater than 1.0 facilitates an interspersed acquisition. Pitch has a smaller effect on image quality with use of multi–detector row CT scanners than it does with use of single–detector row CT scanners.

PITCH EFFECT