Introduction
Conventional angiography using the digital subtraction technique is the gold standard for evaluating intracranial aneurysms after surgical clipping.10) However, conventional angiography is invasive and time consuming. The risk of an ischemic neurological complication associated with angiography has been reported to range from 0.4 to 1.0%4)7)25) and the rate of silent embolism on diffusion magnetic resonance (MR) imaging has been reported to be 18%.6) Furthermore, MR angiography is not a reasonable alternative for the evaluation of clipped aneurysms, as clip-induced signal losses prevent evaluations in the vicinity of a clip. On the other hand, computed tomography angiography (CTA) as a diagnostic tool in the postoperative setting, is also problematic. In particular, metallic implants induce huge artifacts in CT images that deteriorate image quality and often prevent the visualization of structures of interest, such as, the aneurysm neck and the surrounding vasculature.
The aim of this study was to compare 16- and 64-row multislice CTA with respect to titanium and cobalt-alloy clip-induced artifacts.
Materials and methods
Forty aneurysms in 37 patients (9 men and 28 women; mean age, 58.9 [36~83] years) treated with cobalt alloy clips (Sugita Aneurysm Clip; Mizuho, Tokyo) or titanium clips (Yasargil titanium aneurysm clip; Aesculap, T?ttlingen, Germany) and who underwent postclipping CTA were enrolled in this study. CTA was performed using a 16-row (Somatom, Siemens, Erlangen, Germany) or a 64-row multislice CT (Somatom) using the following parameters; collimation 0.75mm, helical pitch 11/16, tube voltage 120kv, tube current 200mAs, rotation time 0.5 seconds, reconstruction section width 1.0mm and reconstruction interval 0.7mm. A single clip was used in 36 aneurysms, two clips in 3 and three clips in 1. The locations of the clipped aneurysms are provided in Table 1.
Sixteen-row multislice CTA was performed on 12 (30%) aneurysms and 64-row on 28 (70%). In the 16-row multislice CTA group, 8 aneurysms (6 patients) were treated with cobalt-alloy clips and 4 aneurysms (4 patients) with titanium clips. In the 64-row multislice CTA group, 14 aneurysms (13 patients) were treated by cobalt-alloy clips and 14 aneurysms (14 patients) were treated with titanium clips.
Clip induced artifacts were composed of white and black components (Fig. 1A). The white component consisted of a local halo which surrounded clips and a variable number of white streaks radiating in all directions (Fig. 1B), whereas the black component was band-like and emanated from the ends of clips (Fig. 1C). Artifact sizes were measured the mm2 for white and black components and clip components in CTA images. Total clip induced artifact sizes were calculated by summing the areas of white and black components. When measuring areas of black components, the display window level was set at -31 and window width was set at zero and when measuring areas of white components, the display window level was set at 95 and window width was set at zero.20)
All measurements were performed using commercially available software (Maroview, Infinitt, Seoul).
Statistical analysis was performed using the Mann- Whitney U-test and p-values of less than 0.05 were considered statistically significant.
Results
Titanium clips (634.9±308.44mm2) were found to produce significantly smaller artifacts than cobalt alloy clips (2,797.4±3,121.98mm2) on CTA source images (p=0.006). Regarding CTA modalities, artifacts induced by titanium alloy clips on 64-row multislice CTA images (544.0±68.77mm2) were found to be significantly smaller than those on 16-row images (953.3±279.95mm2) (p=0.026, Table 2). However, both CTA modalities were similarly affected by artifacts caused by cobalt alloy clips (64-row multislice CTA, 2,191.5±2,072.86mm2 versus 16-row multislice CTA, 3,857.6±4,386.56mm2, p=0.246).
Discussion
Follow-up evaluations are important after the surgical clipping of cerebral aneurysms. The purposes of postoperative evaluations are to detect residual aneurysms and parent vessel compromises.11) Incomplete treatment may result in bleeding or regrowth and thus, the recognition of a residual aneurysm is clinically important and the accuracy of postoperative imaging is critical.2)5)14) Recently, CTA has been widely used as a noninvasive means of evaluating cerebral aneurysms after surgical clipping,3)13)17)18)21) and has substantially replaced conventional angiography for the postoperative evaluation of aneurysms.1)8)22)23) However, clip-induced artifacts on CTA images can affect image quality depending on the clip material and the orientation, shape and number of clips.16)20)
The present study shows that titanium clips produce smaller artifacts than cobalt-alloy clips on CTA images. Sagara et al. reported that CTA is limited when cobalt clips are used, because the extensive artifacts produced prevent proper evaluations.16) Accordingly, the present study confirms that titanium clips offer an advantage over in terms of clip-induced artifacts.
Furthermore, the present study shows 64-row multislice CTA reduces artifacts induced by titanium clips as compared with 16-row multislice CTA. Modern multislice CT technology offers reduced image degradation and improved image quality and the faster speed of rotation of the higher order multidetector row scanners reduce scanning times and allow the acquisition of more slices,9)12)19)24) which reduce artifacts and result in the of pure arterial phases with less associated venous contamination. Lee et al. found that 16-channel multidetector CTA was superior to 4-channel in terms of its ability to discriminate surgical clips and parent vessels.13) In the present study, increasing detector row number was found to reduce titanium clip induced artifacts. However, it failed to overcome the artifacts induced by cobalt alloy clips. Therefore, despite the above-mentioned technologic advances, conventional angiography remains the most accurate tool for the postoperative assessments of aneurysms treated with cobalt alloy clips.
Some limitations of this study should be borne in mind. Its major limitations are the small number of patients enrolled and its retrospective nature. Furthermore, clip angles in the scanning plane and the number of clips used, which were not examined, could have influenced artifacts.15)16)20)
Conclusion
Our quantitative analyses shows titanium clips produce smaller artifacts than cobalt-alloy clips, and that titanium clip-induced artifacts are smaller in 64-row multislice CTA images than in 16-row images. However, cobalt-alloy clip induces artifacts were huge and were unresponsive to increasing row number. Accordingly, the findings of this study suggest that multislice CTA with a large number of detector rows be used for the postoperative evaluation and follow-up of aneurysms treated with titanium clips, but not for aneurysms treated with cobalt alloy clips.
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