Artifacts

1. Misregistration
The severity in misregistration comes more from fluctuations of theheart rate more than the use of Burst or Burst Plus. The higher heartrates are handled very well with the sector mode of scanning especiallywhen that rate is very steady. If the heart rate is making frequent andwide variations then the likelihood of misregistration increases. As tothe effect of pitch on misregistration, the lower the pitch the moreoverlap in the 40 mm acquisition is possible, reducing the artifactpotential.In response to a patient scheduled for a pulmonary vein ablation study,for a patient in active atrial fibrillation the recommendation would beto ignore the wildly fluctuating heart rate and set the system to scanin the segment mode with your lowest pitch capability (0.16 on aVCT--type in 35 for the heart rate to set this low) in order to have asmuch overlap in the data as possible. Second option would be to do thestudy as an ungated study and simply capture the heart in a helicalacquisition using your fastest rotation speed for routine scanning ( 0.4sec ) in order to reduce as much of the cardiac motion as possible. Thecardiac structure itself would be seen very well and 3D post processingto use navigator views of the Ostia of the pulmonary vessels and atrialappendage, as well as vessel analysis to lay out the pulmonary vesselscan still be achieved. In regards to regular CCTA exams, once again, a slower, or lower,pitch will provide more overlap reducing the potential formisregistration. The thing to remember is a lower pitch means more timein the scan field which translates to increased dose possibilities.

FIGURE 4. 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.

2. Scalloping
Scalloping is due to the fact that the slice sensitivity profile is increased in spiral CT so that partial volume artifacts also become stronger. Scalloping is a phenomenon arising, for example, in skull tomographies, particularly in slice positions in which the skull diameter quickly changes its axial direction.

Figure 9. Fibrous dysplasia (16 x 0.625 mm). (A and B) Note the characteristic ground-glass appearance on the axial oblique reformatted images. (C and D) Minimal expansion and endosteal scalloping are well seen on the 3D renderings.

3. Banding
Band artifacts due to bulk motion were investigated in images acquired with fast gradient echo sequences. A simple analytical calculation shows that the width of the artifacts has a square-root dependence on the velocity of the imaged object, the time taken to acquire each line of k-space and the field of view in the phase-encoding direction. The theory furthermore predicts that the artifact width can be reduced using parallel imaging by a factor equal to the square root of the acceleration parameter. The analysis and results are presented for motion in the phase-and frequency-encoding directions and comparisons are made between sequential and centric ordering. The theory is validated in phantom experiments, in which bulk motion is simulated in a controlled and reproducible manner by rocking the scan table back and forth along the bore axis. Preliminary cardiac studies in healthy human volunteers show that dark bands may be observed in the endocardium in images acquired with nonsegmented fast gradient echo sequences. The fact that the position of the bands changes with the phase-encoding direction suggests that they may be artifacts due to motion of the heart walls during the image acquisition period. One of problems with the current cardiac CT imaging is the banding artifacts, i.e., horizontal shifts in multiplanar reformatted (MPR) or three-dimensional (3-D) images.

Figure 6a. CT images of the posterior fossa show the dark banding that occurs between dense objects when only calibration correction is applied (a) and the reduction in artifacts when iterative beam hardening correction is also applied (b). (Reprinted, with permission, from reference 1.)

4. Stair-Stepping
The stair-step artifact associated with surfaces or object borders inclined relative to the table translation direction (8,9). Stair-step artifacts characteristically deteriorate the appearance of two-dimensional reformation and 3D-rendered objects and may affect the accuracy of volume or diameter measurements of structures within the scanned.
PURPOSE: Stair-step artifacts in helical computed tomography (CT) are associated with inclined surfaces in longitudinal sections. The authors investigated the origin and the characteristics of the artifacts. MATERIALS AND METHODS: A cone phantom and a skull were dry-scanned with a helical CT scanner, and images were reconstructed by using the half- scan interpolation algorithm with combinations of detector collimation (1 and 5 mm), table feed (1, 2, 5, and 10 mm), and reconstruction interval (1, 2, 5, and 10 mm). RESULTS: Stair-step artifacts were perceived in most instances. Stair-step artifacts arose from two sources: large reconstruction intervals and asymmetric helix interpolation, forming isoclosed curves and spirallike patterns in three-dimensional axial views, respectively. CONCLUSION: To eliminate the stair-step artifacts, both the collimation and the table feed should be less than the longitudinal dimension of the important feature on inclined surfaces, and the reconstruction interval should be less than the table feed. Adaptive interpolation may correct the artifacts. Figure 1. Schematic depicts measurement of stair-step artifacts in a 45° inclined acrylic rod phantom (attenuation, 130 HU) immersed in vegetable oil (attenuation, -100 HU). A profile of 360 radial distance (r) measurements were obtained between the computed centerline path ( ) and the surface of the rod, with use of an attenuation threshold of 0 HU. SDr in the coronal plane served as the measure of artifactual surface distortion of the rod.

5. Pitch Effect
Pitch refers to the ratio between the rate at which the table the patient lies on moves through the scanner and the rate at which the scanner spins. Pitch can be increased by about half to reduce radiation dose by about a third, without any loss of image quality. With single-slice helical CT, an increased pitch can decrease the radiation dose to the patient if all other parameters are constant.
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