Use of CT Angiography in Comparison with Other Imaging Techniques for the Determination of Embolus and Remnant Size in Experimental Aneurysms Embolized with Hydrogel Filaments

Experimental Design

The rabbits were cared for in accordance with Austrian regulations governing animal experiments, and the experimentation was approved by the committee for animal experiments from Land Salzburg, Austria. Experimental bifurcation aneurysms were created in 12 New Zealand white rabbits (mixed sex, 2.5–3.5 kg). Three weeks postcreation, the aneurysms were embolized with detachable hydrogel filaments. Six aneurysms were embolized as completely as possible. Six aneurysms were embolized incompletely to ensure blood filling in the neck. Three aneurysms from each group underwent DSA, CTA, and MRA follow-up and harvest at 4 and 13 weeks.

Aneurysm Creation

Experimental bifurcation aneurysms were created in 12 New Zealand white rabbits. The microsurgical construction of the carotid bifurcation aneurysms was performed according to previously described methods.¹⁶ Both CCAs were exposed, and a permanent ligature was placed proximal to the left CCA bifurcation and transected. With single-knot 10-0 Prolene sutures (Ethicon, Cincinnati, Ohio), an end-to-side anastomosis of the left-to-right CCA was performed, and the aneurysm sac was constructed with a venous graft pouch from the left external jugular vein. For all operative procedures, the animals were anesthetized by using an intramuscular injection of 20–30 mg/kg of ketamine hydrochloride and 0.2 mL of 2% xylazine hydrochloride, followed by maintenance anesthesia with intravenous injection of a saline solution of ketamine and xylazine (5:1:5; 0.5–1 mL/h/kg).

Embolic Devices

Detailed descriptions of the preparation, characterization, and physical evaluation of the hydrogel filaments is given elsewhere.¹⁷ Briefly, the hydrogel filaments comprised 66% barium sulfate, 23% trimethylolpropane ethoxylate triacrylate, 8% tert-butyl acrylate, 2% 2-hydroxyethyl methacrylate, and 1% N,N′-methylenebisacrylamide. Filaments of various secondary diameters (2–6 mm and straight) and lengths (4–20 cm) were used to embolize the aneurysm sac. The hydrogel filaments underwent a 1.4-fold increase in volume on hydration.

Aneurysm Embolization and Follow-Up

With the rabbit under anesthesia and with sterile conditions, the right femoral artery was surgically exposed and a 5F vascular sheath was placed. Following intravenous heparin (100 U/kg) administration, a 5F guiding catheter was advanced into the right CCA. The aneurysm dimensions were measured by using DSA with a radiopaque ball bearing (7.9 mm) as a sizing reference. A microcatheter was advanced into the aneurysm sac.

Embolization was performed under fluoroscopic guidance. Six aneurysms were embolized as completely as possible while minimizing protrusion of embolic devices into the parent artery. The distal half of the remaining 6 aneurysms was embolized, ensuring blood filling in the neck of the aneurysm. After performing DSA to document contrast filling into the aneurysm sac, we removed all catheters and the sheath. The proximal aspect of the femoral artery was ligated with 6-0 silk suture.

At the time of follow-up, the rabbits were anesthetized and a 5F sheath was placed in the left femoral artery. Follow-up angiography was performed by using a 5F guiding catheter positioned in the right CCA. Following DSA, the rabbits were imaged by using CT and MR imaging. They were sacrificed by a lethal injection of sodium pentobarbital or an overdose of the anesthetic agents and 2 mL of T61 euthanasia solution.

CTA

CTA was performed by using a 10-multisectional helical CT machine (Brilliance 10; Philips Healthcare, Best, the Netherlands). With a power injector, contrast (10 mL, 80% Visipaque 270:20% 0.9% saline; GE Healthcare, Piscataway, New Jersey) was administered through a cannula in the ear vein of the rabbit at a rate of 0.5 mL/s. CTA was performed (0.9–120 kV, 250 mAs, 10 × 0.8 mm) by using a bolus-triggered system with 110-HU threshold through a transverse section at the level of the heart. Postprocessing was performed on a workstation (Brilliance, Philips Healthcare) for viewing multiplanar reformatted images. Using the DSA image as a reference, we prepared 2D maximum intensity projection views.

MRA

Initial unenhanced MRA was performed by using a 3T scanner (Magnetom Trio; Siemens, Erlangen, Germany) with the rabbit placed inside a 32-channel head coil (Siemens). The aneurysms were imaged by using a 3D time-of-flight sequence with a TR of 23 ms, a TE of 4.37 ms, a flip angle of 18°, a matrix of 304 × 384, an FOV of 150 × 150 mm, and a voxel size of 0.4 × 0.4 × 0.6 mm. MRAs were prepared on a workstation (syngo MR B15, Siemens) by using the DSA image as a reference.

Histologic Processing

The aneurysm−parent artery complex was first rinsed in situ with saline solution and then perfusion-fixed with 10% neutral buffered formalin. After surgical excision, the specimen was placed in fresh fixative. To preserve the hydrogel filament−tissue interface, we embedded 6 of the aneurysms in methyl methacrylate. The aneurysms were sawed longitudinally through the neck and 6-μm sections were prepared by using a rotary microtome. To permit immunohistochemical analyses, the remaining 6 aneurysms were embedded in paraffin and then bisected longitudinally through the neck of the aneurysm. Five-micrometer sections were prepared by using a rotary microtome. The methacrylate and paraffin sections were stained with hematoxylin and eosin and Movat pentachrome stains.

Evaluation Criteria

Statistical Analysis

Statistical analyses were performed by using JMP 7.0 (SAS Institute, Cary, North Carolina). For the procedural data, differences in continuous and discrete data were assessed by using analysis of variance and χ² tests, respectively. The embolic mass and remnant areas were compared by using paired t tests. Statistical significance was accepted at P ≤ .05.