ARTIFACTS IN SPIRAL CT

Misregistration

Patient motion can cause misregistration artifacts, which usually appear as shading or streaking in the reconstructed image (Fig 16). 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.

Avoidance of Motion Artifacts by the Operator.—
The use of positioning aids is sufficient to prevent voluntary movement in most patients.
However, in some cases (e.g, pediatric patients), it may be necessary to immobilize the patient by means of sedation. Using as short as scan time as possible helps minimize artifacts when scanning regions prone to movement. Respiratory motion can be minimized if patients are able to hold their breath for the duration of the scan. The sensitivity of the image to motion artifacts depends on the orientation of the motion. Therefore, it is preferable if the start and end position of the tube is aligned with the primary direction of motion, for example, vertically above or below a patient undergoing a chest scan. Specifying body scan mode, as opposed to head scan mode, may automatically incorporate some motion artifact reduction in the reconstruction.

Built-in Features for Minimizing Motion Artifacts.—
Manufacturers minimize motion artifacts by using overscan and underscan modes, software correction, and cardiac gating. Overscan and underscan modes: The maximum discrepancy in detector readings occurs between views obtained toward the beginning and end of a 360° scan. Some scanner models use overscan mode for axial body scans, whereby an extra 10% or so is added to the standard 360° rotation. The repeated projections are averaged, which helps reduce the severity of motion artifacts. The use of partial scan mode can also reduce motion artifacts, but this may be at the expense of poorer resolution. Software correction: Most scanners, when used in body scan mode, automatically apply reduced weighting to the beginning and end views to suppress their contribution to the final image. However, this may lead to more noise in the vertical direction of the resultant image, depending on the shape of the patient. Additional, specialized motion correction is available on some scanners. The effectiveness of one such technique in correcting artifacts due to motion of a fluid interface is demonstrated in Figure 17.
Cardiac gating: The rapid motion of the heart can lead to severe artifacts in images of the heart and to artifacts that can mimic disease in associated structures, for example, dissected aorta. To overcome these difficulties, techniques have been developed to produce images by using data from just a fraction of the cardiac cycle, when there is least cardiac motion. This is achieved by combining electrocardiographic gating techniques with specialized methods of image reconstruction (4).


PURPOSE: To determine the frequency and patterns of respiratory-induced misregistration artifact seen on spiral CT of the liver. MATERIALS AND METHODS: Two hundred patients with hepatic mass underwent spiral CT, and arterial phase images were compared with those of the portal phase in all cases and or of the delayed phase in 138. The patterns of misregistration artifact were divided into two groups: skipping, where at least two slices in the craniocaudal length of the mass were missed, and the partial volume veraging artifact thus excluded; and overlapping, where the same or reversed images were seen in succeeding sequences. We reviewed the location and size of the masses, and the presence or absence, and patterns of the misregistration artifact. RESULTS: Fourteen (7%) of 200 spiral CT scans demonstrated the misregistration artifact; in five of these there was skipping (involving a hepatic mass larger than 2 cm in two cases, and one smaller than 2 cm in three cases), and in nine there was overlapping (six masses larger than 2 cm, and three smaller than this). A lipiodol-laden mass measuring 5 mm was completely missed during the arterial phase. and in one case the spleen sequence was reversed. Thirteen (93%) of fourteen masses were located in the right lobe. CONCLUSION: Two patterns of misregistration artifact, skipping and overlapping, were observed, and their combined frequency was 7%. So as not to miss small hepatic masses or overestimate their size, careful respiratory control is therefore needed.

A)Topographic 3D displays of helical CT PET with a mild-to-moderate anterior and lateral defect (top row) that is not present on cine CT PET (bottom row). White indicates the highest myocardial uptake of 82Rb, reflecting the highest myocardial perfusion, with red being the next highest and progressively lower perfusion indicated by color gradations from red to yellow, green, and blue. (B) For same patient as in A, misregistration on helical CT-PET fusion images in transaxial (top) and coronal (bottom) views. Arrows indicate heart borders on helical CT and PET emission images as unmatched, with region of misregistration corresponding to area of artifactual defect. Magnified inset illustrates quantification of misregistration in transaxial view—here, 12 mm—using an electronic caliper on the screen. (C) For same patient, cine CT-PET fusion images show good coregistration associated with no defect and a normal scan.

Stair-stepping Artifacts


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 (Fig 28). Stair step artifacts are virtually eliminated in multiplanar and three-dimensional reformatted images from thin-section data obtained with today’s multisection scanners

Stair-step artifacts result from data reconstruction and misregistration. With loss of data along the z-axis, apparent areas of both decreased and increased attenuation occur. This apparent decrease in attenuation of the vessel lumen on multiple images can mimic the appearance of PE. On axial images, the vessel lumen is generally affected on alternating images with normal intervening vessel enhancement. On reconstructed images, the "stair-step" appearance can be appreciated (figure 11). Decreasing the slice thickness and overlapping images will reduce the amount of artifact along the z-axis; however, when slice thickness is too thin, quantum mottle, scan time, and radiation dose to the patient can provide significant problems.

Stair-step artifact. (A) Axial CT shows central low attenuation in the right lower lobe pulmonary artery raising suspicion for pulmonary embolus (arrow). (B) Coronal reformatted image shows linear low attenuation across the vessel from data misregistration along z-axis


Stair step artifact is associated with inclined surfaces in
reformatted slices
Causes
• Large reconstruction interval
• Asymmetric helical interpolation
Correction
• Collimation and feed less than feature sizes,
and small reconstruction interval
• Adaptive interpolation

Pitch Effect





In general the same artifacts are produced in spiral and conventional scanning. Meanwhile, because the 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. In single CT spiral scanner, the pitch is the table movement per tube rotation/slice collimation. For a typical 1 second rotation scanner a pitch of 2 means the table traveled 10 mm with a 5 mm slice width or collimation. In multi-slice CT spiral scanners, the definition is table movement per rotation/single slice collimation. With a 1 sec scanner there is 1 rotation per second. So if the table travels 4 mm in a second and a 1 mm collimator is used then the pitch would be 4. Fig.
6 shows a spiral scanning and the pitch for this scanning. If pitch is increased while holding kVp, mA, and beam collimation constant, then the 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 pitch. Fig. 7 shows the effect of reducing pitch for a multi-slice spiral scanner.

To understand the effect of pitch on raw data interpolation in multi-slice spiral/helical CT, and provide guidelines for scanner design and protocol optimization. Multi-slice spiral CT is mainly characterized by the three parameters: the number of detector arrays, the detector collimation, and the table increment per X-ray source rotation. The pitch in multi-slice spiral CT is defined as the ratio of the table increment over the detector collimation. In parallel to the current framework for studying longitudinal image resolution, the central fan- beam rays of direct and opposite directions are considered, assuming a narrow cone-beam angle. Generally speaking, sampling in the Radon domain by the direct and opposite central rays is non-uniform along the longitudinal axis. Using a recently developed methodology for quantifying the sensitivity of signal reconstruction from non-uniformly sampled finite points, the effect of pitch on raw data interpolation is analyzed in multi-slice spiral CT. Unlike single-slice spiral CT, in which image quality deceases monotonically as the pitch increases, the sensitivity of raw data interpolation in multi-slice spiral CT increases in an alternating way as the pitch increases, suggesting that image quality does not decrease monotonically in this case. The most favorable pitch can be found from the sensitivity-pitch plot for any given set of multi-slice spiral CT parameters. An example for four-slice spiral CT is provided. The study on the pitch effect using the sensitivity analysis approach reveals the fundamental characteristics of raw data interpolation in multi-slice spiral CT, and gives insights into interaction between pitch and image quality. These results may be valuable for design of multi-slice spiral CT scanners and imaging protocol optimization in clinical applications.

Pitch dependence of longitudinal sampling and aliasing effects in multi-slice helical computed tomography (CT).

In this work, we investigate longitudinal sampling and aliasing effects in multi-slice helical CT. We demonstrate that longitudinal aliasing can be a significant, complicated, and potentially detrimental effect in multi-slice helical CT reconstructions. Multi-slice helical CT scans are generally undersampled longitudinally for all pitches of clinical interest, and the resulting aliasing effects are spatially variant. As in the single-slice case, aliasing is shown to be negligible at the isocentre for circularly symmetric objects due to a fortuitous aliasing cancellation phenomenon. However, away from the isocentre, aliasing effects can be significant, spatially variant, and highly pitch dependent. This implies that measures more sophisticated than isocentre slice sensitivity profiles are needed to characterize longitudinal properties of multi-slice helical CT systems. Such measures are particularly important in assessing the question of whether there are preferred pitches in helical CT. Previous analyses have generally focused only on isocentre sampling patterns, and our more global analysis leads to somewhat different conclusions than have been reached before, suggesting that pitches 3, 4, 5, and 6 are favourable, and that half-integer pitches are somewhat suboptimal.

Scalloping

Intensity based registration (e.g., mutual information) suffers from a scalloping artifact giving rise to local maxima and sometimes a biased global maximum in a similarity objective function. Here, we demonstrate that scalloping is principally due to the noise reduction filtering that occurs when image samples are interpolated. Typically at a much smaller scale (100 times less in our test cases), there are also fluctuations in the similarity objective function due to interpolation of the signal and to sampling of a continuous, band-limited image signal. Focusing on the larger problem from noise, we show that this phenomenon can even bias global maxima, giving inaccurate registrations. This phenomenon is readily seen when one registers an image onto itself with different noise realizations but is absent when the same noise realization is present in both images. For linear interpolation, local maxima and global bias are removed if one filters the interpolated image using a new constant variance filter for linear interpolation (cv-lin filter), which equalizes the variance across the interpolated image. We use 2D synthetic and MR images and characterize the effect of cv-lin on similarity objective functions. With a reduction of local and biased maxima, image registration becomes more robust and accurate. An efficient implementation adds insignificant computation time per iteration, and because optimization proceeds more smoothly, sometimes fewer iterations are needed.
New spiral-related reconstruction artifacts arise with this CT method, which are mainly due to interpolation inaccuracies in combination with the selection of the pitch factor. In principle, one finds here as many image error types as interpolation method. In this section, the interpolation problem cannot be discussed in detail, but the so-called scalloping artifact should be mentioned, which 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 phenomenon arising, for example, in skull tomographies, particularly in slice positions in which the skull diameter quickly changes its axial direction. Two slices with different curvatures in axial direction have been selected in a skull phantom. For comparison, the slices with a thickness of 1 mm were measured for both positions conventionally, i.e., without a continuous table feed.
The virtually different thickness of the skull in two slice position is due to the fact that the angle between the corresponding reconstruction layer and the local skull surface normal vector varies.

Endosteal scalloping demonstrated on CT scan. Axial CT through the mid humerus clearly demonstrates endosteal erosion (arrow) resulting from a thyroid carcinoma metastatic lesion.