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Logo of interneuroInterventional Neuroradiology
Interv Neuroradiol. 2015 February; 21(1): 50–54.
Published online 2015 February 1. doi:  10.15274/INR-2014-10105
PMCID: PMC4757206

Ultra-small diameter coils for treatment of intracranial aneurysms


This study reports our initial clinical experience treating very small intracranial aneurysms using only Target® Nano™ coils.

Retrospective angiographic and clinical analysis was performed on a non-randomized single arm registry of all intracranial aneurysms treated with only Target® Nano™ coils (1 mm and 1.5 mm diameter only) during a 12 month period at two academic hospitals.

Fourteen patients with 14 intracranial aneurysms were treated. The maximum diameter of saccular aneurysms treated ranged from 1.5 to 3.5 mm; minimum aneurysm diameter was 1.1 to 2 mm. The immediate complete aneurysm occlusion rate was 86% (12/14), and a small residual within the aneurysm was seen in 14% (2/14) of cases.

Packing density from coils ranged between 24% and 83% (mean 51%). The immediate complication rate was 0% (0/14). The angiographic/MR angiography follow-up period was 22 to 70 weeks (mean 37 weeks) with an overall complete occlusion rate of 9/11 (81%), recurrence in 18% (2/11), and lack of follow-up in three cases, two due to death during hospitalization and one procedure not yet due for imaging follow-up. Both patients who died presented with brain aneurysm ruptures prior to treatment. Both recurrences were retreated with repeat coiling procedures.

Our initial results using only Target® Nano™ coils for the endovascular treatment of very small intracranial aneurysms have demonstrated initial good safety and efficacy profiles.

Keywords: Very small intracranial aneurysms, ultra-small coils


Endovascular coiling of intracranial aneurysms has become a well-established and successful alternative to craniotomy and surgical clipping since its inception in the early 1990s. Small and very small intracranial aneurysms were considered challenging to treat from an endovascular perspective but have been treated commonly with endovascular techniques in recent years with good overall safety and efficacy profiles, according to recent reports.14 Although no consensus exists regarding the definition of small or very small intracranial aneurysms, prior reports on the topic have designated aneurysms under 3 mm in maximal diameter as “very small;” we use the same criterion for defining very small aneurysms in this report.14 The introduction of new endovascular devices has enabled treatment of such lesions previously considered untreatable by endovascular techniques.

One advancement is the increasing availability of small, complex shaped microcoils such as the very small diameter (1 mm and 1.5 mm) Target® 360 Ultrasoft coils (Stryker-Neurovascular, Kalamazoo, MI, USA), referred to as Target® Nano™ coils. This line of coils was introduced in the United States for use in intracranial aneurysms in 2012. The complex design and soft nature of these very small diameter coils may allow for and/or assist in the endovascular treatment of very small brain aneurysms. We present our initial results using Target® Nano™ coils alone to treat very small intracranial aneurysms.

Materials and Methods

This was a non-randomized, retrospective study of treatment of intracranial aneurysms performed at two institutions over a one-year period. Inclusion criteria included all intracranial aneurysms treated with only 1 mm and 1.5 mm diameter Target® 360 Ultrasoft Nano™ coils during the study period. Exclusion criterion was any prior treatment of the index lesion. No other inclusion or exclusion criteria, including aneurysm morphology, were used.

Endovascular Treatment

All patients had diagnostic cerebral angiograms performed in multiple angiographic projections, including three-dimensional angiography, during the initial patient workup to characterize the morphology of the target aneurysm. Tri-axial endovascular access consisting of 80 cm, 6 French sheaths was used in most cases; 6 French guide catheters were used in all cases. All aneurysms were treated using a single microcatheter which was navigated into the aneurysm sac. Balloon assistance was used in four of 14 cases. Stent assistance and flow diversion were not utilized in any case. All aneurysms were treated until angiographic occlusion was achieved, or until no further coils could be deployed safely within the aneurysm sac.

Although the aneurysms in this series were not similar in shape, a simple volume calculation was used for each aneurysm:

Aneurysmvolume = (π × width × depth × height)/6

The volume of the coils was calculated by the following formula:

Coilvolume = π(radius)2 × (lengthofcoil)

The aneurysm’s packing density was then calculated by using this formula:

Packingdensity    = (coilvolume/aneurysmvolume) × 100%

Immediate angiographic imaging post treatment and follow-up imaging were evaluated by two interventional neuroradiologists (GJ and DG). Discrepancies were mitigated by consensus. Follow-up imaging consisted of digital subtraction angiography (DSA) or contrast-enhanced MR angiography (CE-MRA).


Between August 2012 and August 2013, 14 patients (12 women, 2 men) with a total of 14 intracranial aneurysms were treated using 1 mm and 1.5 mm diameter Target® 360 Ultrasoft coils alone at two university medical centers. All cases were saccular aneurysms treated with intra-aneurysmal coiling. Indications for treatment were as follows: index aneurysm rupture in 9/14 cases (64%), prior history of other brain aneurysm rupture in four (28%) cases, irregular aneurysm morphology in one (7%) case. Maximal saccular aneurysm diameter ranged from 1.5 mm to 3.5 mm, while the smallest diameters ranged from 1.1 mm to 2.5 mm.

A complex coil shape was used as the initial framing coil in every case. The coils were placed successfully in all patients without difficulty in deployment. The length of each coil was 2 cm, 3 cm, or 4 cm. Packing density ranged from 24% to 83% (mean 51%). In four of 14 patients, other coil types from other manufacturers with coil diameters of 1.5 mm and 2 mm were attempted to be placed but could not be deployed safely and were therefore removed.

The immediate complete aneurysm occlusion rate was 86% (12/14), a small residual at the neck of the aneurysm with approximately 80% volumetric occlusion of the aneurysm was seen in 7% (1/14) of cases, and residual aneurysmal filling with 90% volumetric occlusion of the aneurysm was seen in 7% (1/14) of cases in which there was stagnation of flow within the dome of the aneurysm on immediate post-coiling angiography.

Two patients died during hospitalization after presenting with brain aneurysm ruptures. No re-hemorrhage of the target aneurysm was detected on serial head CT imaging during the hospital course in these patients. One patient was coiled without yet being due for imaging follow-up.

The 22 to 70 week (mean 37 weeks) follow-up imaging in the remaining 11 patients demonstrated that 9/11 (81%) aneurysms remained completely occluded. There were two recurrences in 11 follow-up cases (18%) requiring treatment with a second coiling procedure in one case and multiple additional coiling procedures (once with a stent) in one case. The results are summarized in Table 1.

Table 1.
Summary of patients treated.


There were no recorded technical or clinical complications. All coil loops were contained within the aneurysm sac in all. There were no thromboembolic complications during coiling nor during the follow-up period. Although two patients died during hospitalization after presenting with brain aneurysm ruptures, serial head CT imaging did not demonstrate evidence of new intracranial hemorrhages in these patients in the hours and days prior to death.


Size can pose a significant therapeutic challenge in the endovascular repair of very small brain aneurysms, defined as less than 3 mm in maximal diameter.2,3,58 Such lesions provide a relatively small margin for error in regard to catheter, wire, and coil placement within the lumen of the aneurysm. Current small microcatheter diameters are nearly 1 mm in size which may be only slightly larger than the smallest dimension of the aneurysm itself. Only a small amount of the microwire can be placed into the aneurysm lumen, providing relatively little support or “purchase” over which the microcatheter can be delivered. The microcatheter can herniate out of these lesions relatively easily, and safely reaccessing such lesions which may be partially coiled or even otherwise with a microcatheter can be difficult. Occasionally, endovascular treatment may need to be aborted in such cases and procedural-related complications have been reported to be relatively high.3,6,9 Given the increased risk of complications related to endovascular occlusion of small ruptured cerebral aneurysms, there is an argument for maintaining a low threshold for microsurgical repair if endovascular occlusion is expected to be challenging. It seems, therefore, that there is room for improvement in our endovascular techniques and devices for such lesions. The advent of new endovascular devices such as flexible distal access catheters, low profile microcatheters, and soft complex microcoils has helped facilitate the safe treatment of these lesions in recent years.14 Reports on the endovascular treatment of very small intracranial aneurysms are limited in number but have shown good angiographic and clinical outcomes.1,3,6,914

For many years after the advent of coil occlusion of brain aneurysms, there remained both a paucity of and limited availability of small complex-shaped microcoils. The introduction of new coil types such as the Target® 360 Ultrasoft line of coils known as Nano™ coil represents an evolution of available endovascular technology in this regard. We report our initial experience using only Target® Nano™ coils (currently available in only 1 mm and 1.5 mm diameter) for treating these lesions, and our initial data, although small in power, demonstrate favorable initial safety and efficacy profiles. Our initial complete occlusion rate of 70%, small residual aneurysmal neck remnant rate of 20%, packing densities, and recanalization rate of 2/8 (25%) compare favorably with those of other reports on the treatment of very small brain aneurysms.1,3,6,911 (Figure 1)

Figure 1.
a) Anteroposterior (AP) projection digital subtraction angiographic (DSA) image of the head upon right internal carotid artery (ICA) contrast material injection demonstrates a small, 2 mm, saccular, right middle cerebral artery bifurcation aneurysm ...

Small brain aneurysms have been associated with relatively high rates of intraoperative rupture.3,6,9,1518 Brinjikji et al. performed a meta-analysis of small aneurysm data (n = 422), in addition to 71 patients treated at their institution, and found a procedural rupture rate of 8.3% with a mortality rate of 2.4%.6 Nguyen et al. similarly reported a relatively high intraprocedural rupture rate of 11.7% in aneurysms less than 3 mm in diameter (n = 60) vs an intraprocedural rupture rate of 2.3% in aneurysms greater than 3 mm (n = 622).15 Chung et al. reported an 8.3% risk of intra-procedural rupture for ruptured aneurysms less than 3 mm in size.19 Chung et al.’s series had a significantly higher rate of intraprocedural rupture for aneurysms less than 3 mm in size (n = 72) compared to those larger than 3 mm (n = 300), and the overall rate of intraprocedural complications for small ruptured aneurysms was nearly three times that of larger aneurysms.19 Intraprocedural complications in that study were found to have contributed directly to morbidity and mortality resulting either in dependent care or inpatient mortality.19 We experienced no intraoperative aneurysm ruptures in our series and no known deaths directly attributable to coiling. The average aneurysmal volume of approximately 4 mm3 in our series is amongst the smallest aneurysmal size reported in series on similar topics in the medical literature.1,3,6,914

Aneurysm rupture at presentation also serves as an independent predictor of intraoperative aneurysm rupture in endovascular series. Some authors have included both ruptured and unruptured small cerebral aneurysms in their analyses, and the intraprocedural rupture rate in the unruptured cohort is less than that in the ruptured cohort.14,17 The meta-analysis by Brinjikji et al. reported an intraprocedural rupture rate during embolization of small ruptured cerebral aneurysms as approximately double that of small unruptured aneurysms (10.7% vs 5%).6 Although the majority of patients in our series (10/14) presented after index aneurysm rupture, we experienced no intraoperative aneurysm rupture.

Limitations of our series include its small power and retrospective, non-randomized design. Follow-up in our series is also limited both in the number of patients and time period for follow-up. More experience with these coils, especially in the framing and filling of aneurysms in larger numbers of patients will be necessary before conclusions can be drawn and comparisons with other coil types can be made. Nevertheless, we have been encouraged by our initial results and have started a prospective multicenter registry utilizing only Target Ultrasoft coils for the treatment of small brain aneurysms with a longer follow-up period. A subset of patients in this registry is expected to receive only Target Nano coils.


Our small, multicenter data set demonstrates the initial safe deployment of Target® Nano™ coils in ruptured and unruptured brain aneurysms without major complications and with a high rate of occlusion on short-term follow-up imaging. More clinical data should be collected to further validate the results of this series. Treatment for small brain aneurysms should continue to be individually tailored after careful clinical assessment of the patient and the lesion.


This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

Conflict of interest

The authors declare no conflict of interest.


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