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Neuroradiology |
1 From the Departments of Radiology (M.S., W.J.v.R., M.J.S.) and Neurosurgery (D.W.), St Elisabeth Ziekenhuis, Hilvarenbeekseweg 60, 5022 GC Tilburg, the Netherlands; and Department of Electrical Engineering, Mathematics and Computer Science, University of Twente, the Netherlands (J.O.B., C.H.S.). Received March 24, 2003; revision requested May 28; final revision received October 6; accepted November 20. Address correspondence to M.S. (e-mail: radiol@knmg.nl).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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MATERIALS AND METHODS: The volumes of 145 aneurysms that were treated with coils were calculated with biplanar angiographic images and a custom-designed method. Partially thrombosed aneurysms were excluded. Packing was defined as the ratio between the volume of the inserted coils and the volume of the aneurysm and was calculated for all 145 aneurysms. Results at 6-month follow-up angiography were dichotomized into presence or absence of compaction.
RESULTS: Aneurysm volume, packing, and compaction at 6-month follow-up were closely related. Large aneurysm volume was associated with low packing and frequent compaction. High packing prevents compaction. If the aneurysm volume was packed for 24% or more with coils, compaction did not occur in aneurysms with a volume of less than 600 mm3. In small aneurysms with volumes of less than 200 mm3, compaction did not occur when packing was above 20%.
CONCLUSION: The common practice of inserting as many coils as possible in cerebral aneurysms is sensible in trying to avoid compaction. In aneurysms with packing of 24% or more, no compaction occurred at 6-month angiographic follow-up. In aneurysms with a volume of more than 600 mm3, high packing could not be achieved, which resulted in compaction in the majority of aneurysms.
© RSNA, 2004
Index terms: Aneurysm, cerebral, 17.38 • Aneurysm, therapy, 17.1269
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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We developed a method to retrospectively assess the volume of cerebral aneurysms with biplanar angiographic images. Thus, the purpose of our study was to assess the relation between aneurysm volume, packing, and compaction in cerebral aneurysms treated with coils.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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From January 1998 until January 2001, 218 cerebral aneurysms in 198 consecutive patients were treated with detachable coils. Thirty-six patients with 43 aneurysms did not undergo follow-up angiography for the following reasons: procedural death (n = 5), death due to sequelae of subarachnoid hemorrhage (n = 11), unrelated death (n = 3), surgical clip placement after incomplete coil placement (n = 3), and refusal (n = 14).
For this study, 30 additional patients with 30 aneurysms were excluded for various reasons. We excluded patients with partially thrombosed aneurysms, since reopening of these aneurysms may be caused by factors other than compaction, such as thrombus resolution or migration of the whole coil mesh into the thrombus. Partial thrombosis of the aneurysm was appreciated at magnetic resonance (MR) imaging, computed tomography (CT), and/or angiography before treatment in 10 patients. In another six patients, the coil mesh itself was unchanged at follow-up angiography, but it was displaced, with subsequent partial reopening of the lumen. We assumed this displacement had been caused by previously unsuspected partial thrombosis of the aneurysm, and these patients were excluded as well. Three patients with aneurysms that were treated with coil placement after failed clip placement were excluded, since the clip hindered volume measurement. Two patients, both with an arteriovenous malformation and a flow aneurysm that was treated with coils, were excluded. Two patients had a trilobular aneurysm and were excluded because of problems with volume measurement. Three patients with procedural rupture were excluded, as coils were placed outside the aneurysmal sac. Finally, images were not available for four patients.
The present study group consisted of 132 patients with 145 aneurysms that were treated with coils. There were 95 women and 37 men with a mean age of 53 years (median, 52 years; range, 32–76 years). The mean age of the men was 54 years (median, 54 years; range, 32–70 years), and the mean age of the women was 52 years (median, 52 years; range, 33–76 years). Of the 145 aneurysms, 113 were ruptured and 32 were not. Seventeen of these unruptured aneurysms were additional aneurysms; 10 were truly incidental, three were treated with coils after failed surgery, and two had symptoms of mass effect. The aneurysms were located in the anterior communicating artery (n = 41), basilar tip (n = 32), posterior communicating artery (n = 22), ophthalmic artery (n = 11), pericallosal artery (n = 11), middle cerebral artery (n = 7), superior cerebellar artery (n = 5), posterior inferior cerebellar artery (n = 5), anterior choroidal artery (n = 3), carotid bifurcation (n = 3), posterior cerebral artery (n = 3), and basilar trunk (n = 2). From estimation of maximum diameter, 93 aneurysms were classified as small (<10 mm), and 52 were classified as large (
10 mm). All 145 aneurysms underwent angiographic follow-up after a mean interval of 6.7 months (median, 6 months).
Coil Placement Procedure
Coil placement was performed by one of two physicians (M.S. or W.J.v.R.) with general anesthesia and systemic heparinization at a biplanar angiographic unit (Integris V3000; Philips Medical Systems, Best, the Netherlands). Coil placement was performed with Guglielmi detachable coils (GDC-10 and/or -18; Boston Scientific/Target Therapeutics, Fremont, Calif) in 136 aneurysms; nine large aneurysms were treated with mechanically detachable coils (Detach 18; Cook, Copenhagen, Denmark). The aim of coil placement was to pack the aneurysm as densely as possible. Coils were inserted into the aneurysmal sac until no more could be delivered.
Volume Assessment of Aneurysms
During the study period, three-dimensional rotational angiography with volume measurement software was not available. For this study, a method was developed to retrospectively assess the volume of aneurysms by using two-dimensional hard copies obtained with biplanar digital subtraction angiography (J.O.B. and C.H.S.). This method is based on the premise that the aneurysm, when imaged with biplanar angiography, is in or close to the isocenter of the C arms. The distance between the x-ray source and the image intensifier was maximal (117 cm and 130 cm for the frontal and the lateral C arms, respectively) to ensure there would be enough space for anesthetic devices and (re)positioning of the C arms during the procedure. The entrance diameter of the image intensifier was known for all angiograms. The two-dimensional hard copies obtained with biplanar digital subtraction angiography of all aneurysms treated with coils were scanned on a transparent film scanner, and images of approximately 1,000 x 1,000 pixels were obtained. These digitized images were calibrated for magnification and angle of entrance of the x-ray beams and were subsequently cropped.
Tresholding and contour detection were used to segment the aneurysm manually. After segmentation of the aneurysm in the two projections, a computer program was used to perform three-dimensional reconstruction of the aneurysm as a stack of ellipses (Fig 1). Numeric integration of this stack provides the estimated volume of the aneurysm. The algorithm was validated with three detachable latex balloons (Goldvalve; Nycomed, Paris, France) that were filled with a contrast agent and served as phantoms. The volume of the contrast agent (Omnipaque 300; Nycomed) inside the balloons was calculated by weighing the balloon before and after filling. Subtracting both magnitudes resulted in the weight of the contrast agent, and multiplication of this weight by the density provided the volume of the contrast agent. The balloons were imaged with biplanar digital subtraction angiography, and the volume was assessed as described previously. The errors in volume calculation in the three balloons were 0.2% (inserted volume, 477.6 mm3; assessed volume, 476.7 mm3), 1.6% (inserted volume, 1,360.0 mm3; assessed volume, 1,338.7 mm3) and 2.4% (inserted volume, 327.1 mm3; assessed volume, 319.4 mm3).
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Six-month follow-up angiograms were reviewed, and compaction was dichotomized as absent or present by three authors (M.S., W.J.v.R., and D.W., in consensus). Compaction was defined as a decrease in volume of the coil mesh that led to more contrast material filling of the aneurysm at follow-up angiography, as compared with the immediate postembolization angiogram.
All 145 aneurysms, which were labeled as having or not having compaction, were displayed in a packing versus volume graph to show the interrelation between volume, packing, and compaction. A threshold of minimal packing, above which no compaction occurred, was sought. Relative risks for compaction were calculated for the following factors: age greater than median age, sex, and ruptured aneurysms.
The relation between aneurysm volumes versus packing was further clarified by dividing aneurysm volumes into three groups that best highlighted the relation between these two variables. The same was done for the relation between aneurysm volumes and compaction. The relation between packing and compaction was similarly studied by dividing the aneurysms into three packing groups that best clarified the relation between packing and compaction.
Statistical Analysis
Statistical analysis was performed by using SPSS 10.0 software (SPSS, Chicago, Ill). For the relationships of volume versus packing and volume versus compaction, the correlation coefficient was calculated by using the Pearson test.
We calculated relative risks for compaction at 6-month follow-up with corresponding 95% CIs for the following factors: age greater than median age, sex, and rupture of the aneurysm. These relative risks were estimated by calculating the odds ratios with logistic regression.
The relationship between high packing percentage and no compaction and between large aneurysm volume and compaction was assessed with logistic regression analysis. P values that were less than .05 were considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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There was no difference in the relative risk of compaction for age (1.585; 95% CI: 0.763, 3.291), sex (0.948; 95% CI: 0.421, 2.138), and ruptured aneurysms (1.237; 95% CI: 0.505, 3.035).
The aneurysms were subdivided into three groups: those with a volume smaller than 100 mm3 (n = 68), those with a volume between 100 mm3 and 600 mm3 (n = 60), and those with a volume larger than 600 mm3 (n = 17). Corresponding spherical diameters for aneurysms with volumes of 100 mm3 and 600 mm3 were 5.8 mm and 10.4 mm, respectively. The cutoff point in aneurysms with volumes greater than 600 mm3 was chosen because packing of 24% or more was not reached. The cutoff point in aneurysms with a volume of 100 mm3 was chosen because the number of these aneurysms was close to that of aneurysms between 100 mm3 and 600 mm3. Mean and median packing values that were achieved in these aneurysm volume groups are shown in Figure 3a. The percentages of compacted aneurysms at 6-month follow-up angiography in the three groups are displayed in Figure 3b.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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The known fact that large aneurysms treated with coil placement frequently compact (3,8) was confirmed in this study. We found that high packing protects against compaction; if 24% or more of the aneurysm volume was packed with coils, compaction did not occur in aneurysms with volumes smaller than 600 mm3. In aneurysms packed between 20% and 23.9%, compaction did not occur if aneurysm volume was smaller than 200 mm3. Seventy-five of 145 aneurysms (53%) were packed accordingly. Aneurysms with volumes larger than 600 mm3 could not be packed sufficiently, and this may explain why compaction frequently occurs in large aneurysms. The question of whether or not compaction would occur if large aneurysms were packed more than 24% remains unanswered. Future research should aim at trying to obtain high packing in these large aneurysms as well, possibly with the use of thicker coils. Since the volume of coils is defined as 
r2 times coil length, a small increase in coil thickness will lead to a relatively large increase in inserted coil volume.
The relation between packing and compaction in cerebral aneurysms that were treated with coils has been studied previously, but these studies were conducted with a small number of patients (4–7). The results of these studies are, however, remarkably similar to ours, in that packing of more than 20%–25% was found to protect against compaction.
Our study has a retrospective design with inherent shortcomings, and the results need to be confirmed with prospective studies. If our results are confirmed, this may lead to obviation of follow-up angiography in selected patients with sufficiently packed aneurysms. Currently, we are conducting a prospective study in which aneurysm volume is calculated before coil placement with calibrated three-dimensional data sets obtained with rotational angiography.
For this retrospective study, we developed a method to estimate the volume of aneurysms with two-dimensional hard copies obtained with biplanar digital subtraction angiography. This method has several inherent limitations that may introduce errors in volume assessment, such as malpositioning of the aneurysm in the isocenter, the use of only two projections for the reconstruction of a three-dimensional image, and manual segmentation. On the other hand, our method is more accurate for volume assessment than methods in which aneurysm volume is calculated with mathematic equations in which the aneurysm is assumed to be ellipsoid and calibration is performed with the aid of a ruler (4–7). In these methods, the error in volume assessment will be substantial if the aneurysm shape is not ellipsoid. If the two projections are not exactly parallel and orthogonal to the longest axis of the aneurysm, an additional error is introduced. Our method corrects for both projection and nonellipsoid aneurysm shape. Moreover, the reconstructed aneurysm in our method is defined by 200–400 measured points (depending on the size of the aneurysm) instead of by the six measured points that are defined in existing methods (Fig 1).
Another limitation of the present study is the exclusion of partially thrombosed aneurysms. In these aneurysms, sufficient packing does not preclude reopening of the aneurysm because factors other than compaction—including thrombus resolution, migration of the coil mesh into the thrombus, or both—can cause the aneurysm to reopen over time (9). If partial thrombosis is apparent at the time of initial coil placement, this possible reopening may be anticipated by performing earlier follow-up angiography and, if necessary, additional coil placement. In six of the 16 partially thrombosed aneurysms in the present study, however, partial thrombosis only became apparent after 6 months of follow-up angiography. While an intraluminal thrombus in large and giant aneurysms can be detected with MR imaging or contrast-enhanced CT, the accuracy of these imaging modalities in small aneurysms is unknown. In our opinion, future research should be aimed at detection of the presence or absence of intraluminal thrombus in these cerebral aneurysms. This problem of undetected partial thrombosis of aneurysms may reflect one of the most fundamental differences between clip and coil placement: occlusion of the aneurysm from the outside versus occlusion of the lumen of the aneurysm from the inside.
The exact mechanism that leads to a stable occlusion of aneurysms in humans after coil placement is unknown (10–13). Results in animal studies suggest that the insertion of coils facilitates the organization of the occluded aneurysm by creating a cessation of flow, which induces thrombus formation and early endothelialization and is followed by formation of a neointima that covers the orifice of the aneurysm. After the initial thrombus formation, there might be a critical time period in which the neointimal layer should be formed. Whether or not this occurs might be dependent on the density of packing and local flow characteristics. If the neointimal layer fails to form, the aneurysmal sac remains open, and the coil mesh is exposed to the water hammer effect of the pulsatile blood flow, which results in compaction (14–16). The fact that we found a lower limit of packing above which reopening does not occur may indicate that a dense coil mesh is a prerequisite to ensure that the subsequent biologic mechanisms will be effective.
If our results are confirmed in prospective studies, they may have important implications for daily practice and future scientific research. In aneurysms without partial thrombosis and with packing of more than 24%, one could refrain from follow-up angiography, which would make coil placement as effective as clip placement, for which follow-up angiography is not routinely performed. In future comparative studies on the effectiveness of newly designed coils (17–21), such as those with complex three-dimensional shapes or those coated with biologically active or ß-radiation emitting substances, we believe it is mandatory to take initial packing into account. For instance, if an aneurysm that is packed more than 24% with a new coil shows stability over time, this finding will prove little about the effectiveness of such a new coil. Moreover, in comparative studies between new and traditional coils, volumetric assessment of aneurysms enables selection of those aneurysms at risk for compaction, thus increasing statistical power and limiting the number of patients that need to be included.
In conclusion, the common practice of inserting as many coils as possible in cerebral aneurysms is sensible in trying to avoid compaction. Over time, packing plays a critical role in the compaction of aneurysms that were treated with coils, and packing of 24% or more is associated with stability of nonthrombosed aneurysms with volumes of less than 600 mm3 (spherical diameter, 10.4 mm). Future research should be aimed at the development of coils that allow denser packing of larger aneurysms.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTSDISCUSSIONREFERENCES |
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= compaction. 
= no compaction.
