After craniotomy for aneurysmal clipping, the intracranial vessels are examined for any residual neck, parent artery compromise, vasospasm, or aneurysmal regrowth. As the standard tool for intracranial vascular examination, digital subtraction angiography is a time-consuming method that carries risks of thromboembolic and puncture-site complications. In comparison, computerized tomographic angiography (CTA) has several advantages, including low-contrast dose, low radiation exposure, short acquisition time, and low invasiveness. Moreover, CTA can be used to obtain parenchymal structural information that cannot be determined by catheter angiography. These advantages make CTA an attractive alternative to catheter angiography, and its diagnostic use has increased with improvements in multidetector computerized angiography (CT) techniques4,9,12).

However, clip artifacts limit the visualization of the vessels in the vicinity of the clip, particularly in patients in whom cobalt alloy clips have been used11-13,16). Several CT acquisition settings and modes have been proposed to reduce clip artifacts5,6,7,12). However, from a clinical perspective, switching to an artifact-reducing acquisition mode might not always be possible because aneurysmal clipping is frequently performed on an emergent basis.

In our daily clinical practice, we have observed that clip artifacts vary, even under the same imaging acquisition settings, according to clip orientation. These findings have been verified by van der Schaaf et al.12) and Pechlivanis et al.10) using CT scans in a phantom model. Those studies were performed in vitro using discrete numeric variables. To substantiate the effect of clip orientation and other factors on the degree of clip artifacts in vivo, we applied mathematical analysis using continuous numeric variables to retrospectively analyze the quantitative relationship between clip artifacts and related factors in real patients with cobalt alloy clips.

The mean (±standard deviation, range) of the clip blade length was 10.79 mm (±2.41, 5-16). The means of angles a and b were 41.65° (±19.83, 0-90) and 19.75° (±21.62, 0-80), respectively. The average d_max value of the white artifacts was 13.2 mm (±5.47, 1-29). The average angle and number of streak artifacts were 73.92° (±89.99, 5-360) and 3.83 (±2.39, 2-12), respectively. The average d_max of the artifacts in 3-D volume-rendered images was 15.80 mm (±13.14, 0-44).

In correlation analysis, angle b was negatively correlated with the d_max of the white artifacts (r=-0.589, p<0.001) and positively correlated with the angle (r=0.636, p<0.001) and number of streak artifacts (r=0.505, p<0.001). The clip blade length was positively correlated with the diameter of clip artifacts in the 3-D volume-rendered images (r=0.454, p=0.004) (Fig. 2). Angle a had no statistically significant relationship with the clip artifacts (data not shown).

Linear regression analysis revealed that angle b was positively correlated to the d_max of the white artifacts (ß=-0.108, p=0.005), the angle of the streak artifacts (ß=2.745, p<0.001) and the number of streak artifacts (ß=0.070, p<0.001). The clip blade length was linearly associated with the number of artifacts in the 3-D volume-rendered images (ß=1.918, p=0.033) (Table 1). In the multiple linear regression analysis, both angle b and the clip blade length were positive predictors of the degree of each clip artifact in the axial scan (white artifacts, and angle and number of streak artifacts) and in the 3-D volume-rendered images, respectively (Table 2).

Since 1937 when Dandy2) first introduced the V-shaped silver clip, various materials have been used for intracranial aneurysmal clips. However, cranial metallic implants, particularly cobalt alloy clips, interfere with CT evaluation in the same anatomic location due to clip-induced artifacts. Since the introduction of titanium alloy clips, the occurrence of metallic artifacts adjacent to the clip has greatly decreased, resulting in widespread usage of titanium alloy clips. Although clips made from titanium alloy permit better visualization in the clip vicinity than those made from cobalt alloy3,11,12,14,16), the latter still play a large clinical role due to several advantages, including high tensile strength, high closing force, and extra-large and variously fenestrated clip profiles8). Therefore, we wanted to determine whether there were factors other than the clip property itself influencing the cobalt clip artifacts that could be easily managed.

Previous researchers have investigated various approaches for minimizing clip artifacts in post-aneurysmal clipping CT scans from patients. Some authors have proposed the use of ideal image acquisition parameters, such as pitch, kVp, reconstruction algorithm, and type or concentration of contrast material6,12,13). Others have suggested the use of ideal acquisition modes, including nonhelical6,7), gated multidetector CT5), and the orbital synchronized helical scan technique15). These previous studies have several limitations. For example, many were performed in phantom models6,7). The environment in the human brain after aneurysmal clipping is significantly different from the phantom environment due to the presence of bony structures, blood, brain, and soft tissues adjacent to the clip. Finally, the studies that have analyzed clip artifacts in patients had very small sample sizes1,5) or were not quantitative1,11,12,16). In 1999, Brown et al.1) first reported the effect of clip orientation on the number of clip artifacts. In their report on four patients, they proposed that lateral tilting of the head during CT acquisition could reduce the number of clip artifacts by projecting the aneurysm clip away from the remainder of the circle of Willis. In 2006, van der Schaaf et al.12) used a phantom model to evaluate different clip orientations and their effect on artifact volume, showing that positioning of the clip perpendicular to the scan plane led to significantly fewer artifacts. Those findings were quite in agreement with the findings of the present study, although the previous study dealt with the observation regarding clip orientation in a comparatively simpler fashion with selected clip-gantry angles of 0 degree, 45 degrees, and 90 degrees. To the best our knowledge, this study is the first to demonstrate a clear linear relationship between clip artifacts and the CT scan gantry angle in vivo. In addition to clip orientation, the authors attempted to use statistical analyses of clip blade length as a variable for clip artifacts based on the hypothesis that the amount of metallic component might influence the amount of clip artifacts. The study by van der Schaaf et al.12) dealt with the correlation between clip artifacts and kVp/pitch and proposed that 140 kVp and less than 0.6 pitch was an ideal setting for the reduction of metallic artifacts. Our study was conducted in a fixed setting of 120 kVp and 0.96 pitch, focusing on the clip orientation and clip property itself.

In summary, all clip artifacts that could be measured in a routine axial scan (white artifacts, and the angle and number of streak artifacts) were linearly correlated with the degree of angle b, whereas the artifacts in 3-D volume-rendered images were influenced by the clip blade length.

Of these factors, the scanning gantry angle is the only factor that can be modified during postoperative scanning. Indeed, the physician and technician could easily modulate this angle by altering the patient's head positioning (i.e., through head flexion or extension), or by altering the scan angle according to the CT lateral scout view in which the clip orientation is clearly visible. White artifacts interfere with the visualization of adjacent structures more than streak artifacts. The diameter of white artifacts in the CT scan could be reduced by changing the patient's head position or the gantry angle, with the clip blade being placed as perpendicularly as possible to the scan plane on the lateral scout view

This study has several limitations. Unfortunately, the correlation between the clip artifacts and clip orientation in the frontal scout view could not be analyzed because of the limitation of this retrospective analysis. Although the clip orientation on the lateral view is the most important contributing factor for the degree of clip artifacts in axial CT images, this is not the case with CTA reconstruction images. Three dimensional CTA might be associated with various technical factors, including patient motion and timing of contrast injection, thus it might be difficult to determine the mechanism underlying the amount of clip artifacts in these images. Compared with measuring artifacts in axial CT conducted under a constant two-dimensional field, measuring artifacts in CTA is more inconstant in 3-D space. Moreover, artifacts in 3-D CTA with a large d_max but small volume might be oversized in the measurements, because the artifacts are measured by diameter rather than volume. Because the use of a complex clip type, including a fenestrated or angled blade, might interfere with the results, we attempted to avoid this issue by including only cases with a straight clip blade. In spite of this effort, there is still a possibility that the clip spring portion might influence the pattern or amount of clip artifacts. Multiple clips or those with a complex shape preclude the clinical implication of artifact-reducing strategies based on clip orientation.

Clip artifacts generated by a cobalt alloy clip in a 64-channel CT scan may be influenced by the clip length and the angle between the clip and the scan plane. The number of white artifacts increased when the orientation of the angle between the clip blade and the scan plane was parallel. These findings suggest that clip artifacts after aneurysmal clipping could be minimized by simple maneuvers, such as minimal changes in the scan gantry angle on the lateral scout view if the clip blade is located parallel to the scan plane.