The study from Goddard et al1 in the September 2005 issue of AJNR entitled “Absent Relationship Between the Coil-Embolization Ratio in Small Aneurysms Treated with a Single Detachable Coil and Outcomes” is, in our opinion, an example of how poor methodology leads to a wrong conclusion.
The authors concluded that 25 small aneurysms (2–8 mm) achieved satisfactory stability despite having a low average packing attenuation of 8.2%. Their results contradict 2 larger previous studies conducted by us and comprising 145 and 144 aneurysms.2, 3 The volumes of aneurysms in our studies were either assessed by a custom-designed computer program that reconstructed 3D aneurysms from 2D angiographic images or from 3D rotational angiographic datasets; both methods were validated with phantoms. We found mean packing densities of 23% and 30% for all aneurysm sizes, much higher than the reported 8.2% by the authors. Moreover, a firm relationship between packing attenuation and aneurysm volume was found in both studies: Packing is inversely related to aneurysm volume, or in other words, in smaller aneurysms, higher packing densities are obtained than in larger aneurysms. As we review our data base of 445 small aneurysms of 2–8 mm and 176 larger aneurysms, we find a significantly higher packing in small aneurysms than in large aneurysms (24.6%, SD 8.0, range 5%–65% versus 21.9%, SD 5.8, range 8%–40%, t test, P = .0001).
The conclusion of Goddard et al1 that “small aneurysms achieved satisfactory stability despite having a low average packing attenuation of 8.2%” is based on erroneous methodology of aneurysm-volume calculation, leading to structural overcalculation of aneurysm volumes and hence lower packing densities.
First of all, aneurysm size was assessed by comparing aneurysm diameter with the estimated size of internal controls such as the internal carotid artery or the basilar artery. This is an inadequate method because diameters of these arteries vary widely in individuals and estimation errors as small as 1 mm in a small aneurysm result in large volume errors. For example, a 3-mm spheric aneurysm has a volume of 14.1 mm3, and a 4-mm aneurysm, 33.5 mm3. Second, “largest” aneurysm dimension was used in the formula V = 4/3πr3 to calculate aneurysm volume, which invariably results in overestimation of aneurysm volume because a sphere is the largest possible volume of a given diameter. For instance, the real volume of an aneurysm of 2 × 2 × 6 mm is 12.6 mm3, whereas their method calculated a volume of 113 mm3. Therefore, the authors are euphemistic when they state, “This may have led to over calculation of the aneurysm volume and therefore lower packing.” This point is illustrated in Table 1, in which aneurysm volumes are displayed for 382 aneurysms from our data base with estimated maximal diameters of 2–8 mm, assessed in the same way as described by Goddard et al.1 Aneurysms of the same estimated maximal size vary 6–14 times in volumes.
Several data from the table in study of Goddard et al1 are questionable and should have alerted the authors (and reviewers) to their erroneous methodology. For example, patient 4 has a 7-mm aneurysm (volume, 179.6 mm3), and a 1.02-mm3 coil is inserted (equal to the volume of a 2-cm GDC-10 Ultrasoft coil [Boston Scientific Corp, Natick, Mass]), resulting in a packing of 0.6%. This aneurysm did not show recurrence at a follow-up of 52 weeks. Imagine the angiographic picture of a 7-mm spheric aneurysm with a 2-cm coil in it. The aneurysm would not have been occluded at all, and “no aneurysm recurrence at 52 weeks” does not make any sense.
The reported low-mean packing of 8.2% in aneurysms of 2–8 mm by Goddard et al1 in coiling is the result of structural overestimation of aneurysm volume. The statement that there is no relationship between packing and outcome in small aneurysms is simply not true and may even have serious consequences in daily practice. After reading this article, some operators may be satisfied with unacceptable low packing densities in coiling of small aneurysms with inherent risks of rebleeding and reopening with time.
The calculation of aneurysm volume is difficult: Aneurysm shape is often irregular and measurements of dimensions on 2D images need to be adjusted for largely unknown magnifications. Volume measurements from 3D angiographic datasets are more accurate but still depend on manual aneurysm segmentation and image threshold settings. Recently we developed a method to overcome the problem of manual threshold setting by using gradient edge detection to define the contours of aneurysms and validated this method with phantoms.4
Volume ranges of 382 aneurysms with estimated largest diameters of 2–8 mm
Packing Density in Coiling of Small Intracranial Aneurysms: Reply
Reply:
We read the letter of Drs. Willem Jan van Rooij and Menno Sluzewski criticizing our study.1A We freely admitted within our article that using the formula 4/3πr3A overestimates the volume in some instances. However, the behavior of single coils on deployment within the small aneurysms has indicated that they were appropriately sized. This behavior has confirmed the use of vessel references in these circumstances because the single coil conformed to the confines of these small aneurysm sacs without excessive movement on deployment and detachment. As most interventionalists do, we tend to undersize the coil in ruptured aneurysms. That a single small coil in the neck of a 7-mm aneurysm resulted in obliteration, we consider fortunate.
There is significant variation in aneurysm volume in van Rooij and Sluzewski’s table and in their other cited publications.2A–4A In their submitted table, the mean aneurysm volume was larger for the 2- to 5-mm aneurysms than ours using the volume of a sphere. They also had a significant range in their calculated mean volume, which used biplane angiography, rotational angiography, and a custom computer program. These were all larger than that measured with our technique. In addition, there was a significant range in each of their measured volumes. We did not have any 6-mm aneurysms, and the 2 7-mm aneurysms in our study had a calculated volume significantly larger than the mean volume of van Rooij and Sluzewski. However, this volume was still smaller than the upper range of the measured volumes of the 7-mm aneurysms in their table.
Their technique is quite sophisticated, requiring rotational angiography and a custom computer program, which are not available to all. The technique that we used for our study is simple, practical, and demonstrates excellent results in these small aneurysms. Our experience and that of others including van Rooij and Sluzewski is that the greater amount of coil deposited within an aneurysm, the less risk of coil compaction or aneurysm recurrence. However, we also believe that efforts to achieve some arbitrary packing attenuation in small aneurysms may lead to aggressive attempts at placing additional coils that may be dangerous. We wish to communicate that for many small aneurysms, a single coil may be curative.
References
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