High-resolution three-dimensional 3 T magnetic resonance angiography for the evaluation of experimental aneurysm in the rabbit

Anatomical Variations of the Common Carotid Arteries and Neck Structures of the New Zealand White Rabbit and Their Implications for the Development of Preclinical Extracranial Aneurysm Models

BACKGROUND

Rabbit models involving neck arteries are of growing importance for the development of preclinical aneurysm models. An optimal understanding of the anatomy is primordial to allow the conception of models while minimizing mortality and morbidity. The aim of this study is to give reliable anatomical landmarks to allow a standardized approach to the neck vessels.

METHODS

We performed a necropsy on nine specimens from ongoing experimental studies. We measured the distance between the origins of the right and left common carotid artery (rCCA/lCCA) and between the rCCA and the manubrium sterni (MS). The structures at risk were described.

RESULTS

Female New Zealand White rabbits (NZWR) weighing 3.7 ± 0.3 kg and aged 25 ± 5 weeks were included. The rCCA origin was located 9.6 ± 1.2 mm laterally and 10.1 ± 3.3 mm caudally to the MS. In all specimens, the lCCA originated from the aortic arch, together with the brachiocephalic trunk (BCT), and 6.2 ± 3.1 mm proximally to the rCCA origin. The external and internal jugular veins, trachea and laryngeal nerve were the main structures at risk.

CONCLUSIONS

The data help to localize both CCAs and their origin to guide surgical approaches with the manubrium sterni as a main landmark. Special attention has to be paid to the trachea, jugular veins and laryngeal nerves.

New mechanical thrombectomy model in the rabbit: A feasibility study

BACKGROUND AND PURPOSE

Thrombolytic therapy represented the gold standard for the treatment of ischemic stroke. Its drawbacks include increased bleeding risk and low recanalization rates. Mechanical thrombectomy is a new promising therapy option. Devices used for this procedure were substantially improved during recent years. New devices require extensive preclinical invivo testing. We therefore sought to translate the commonly used pig model to a simplified and cheaper rabbit model.

MATERIALS AND METHODS

We performed thromboembolisation in eight intubated and sedated adult female New Zealand white rabbits. The thrombus was created by careful rotation of autologous blood in a silicone tube and addition of barium sulfate for radiopacity. We injected the artificial thrombus via a catheter through the cannulated femoral artery. After thromboembolisation, 2D-DSA was performed to evaluate location and thrombus dimensions.

COMPARISON WITH EXISITING METHOD(S)

None.

RESULTS

No complications or mortality were observed in our series. In seven cases (87.5%) the location of the thromboembolism was the maxillary artery and in one case (12.5%) the thrombus reached the occipital artery. The radiopaque thrombus had a length of 7.0±4.55mm invivo. Vessel diameters in angiographic evaluation were 2.44±0.21mm for the common carotid artery and 2.1±0.16mm for the maxillary artery.

CONCLUSIONS

The novel small animal model for mechanical thrombectomy in rabbit is technically feasible and cheap. It offers comparable vessel diameters to cranial arteries and closely mimics human coagulation system.

From bench to bedside: utility of the rabbit elastase aneurysm model in preclinical studies of intracranial aneurysm treatment

Preclinical studies are important in helping practitioners and device developers improve techniques and tools for endovascular treatment of intracranial aneurysms. Thus an understanding of the major animal models used in such studies is important. The New Zealand rabbit elastase induced arterial aneurysm of the common carotid artery is one of the most commonly used models in testing the safety and efficacy of new endovascular devices. In this review we discuss: (1) the various techniques used to create the aneurysm, (2) complications of aneurysm creation, (3) natural history of the arterial aneurysm, (4) histopathologic and hemodynamic features of the aneurysm, (5) devices tested using this model, and (6) weaknesses of the model. We demonstrate how preclinical studies using this model are applied in the treatment of intracranial aneurysms in humans. The model has similar hemodynamic, morphological, and histologic characteristics to human aneurysms, and demonstrates similar healing responses to coiling as human aneurysms. Despite these strengths, however, the model does have many weaknesses, including the fact that the model does not emulate the complex inflammatory processes affecting growing and ruptured aneurysms. Furthermore, the extracranial location of the model affects its ability to be used in preclinical safety assessments of new devices. We conclude that the rabbit elastase model has characteristics that make it a simple and effective model for preclinical studies on the endovascular treatment of intracranial aneurysms, but further work is needed to develop aneurysm models that simulate the histopathologic and morphologic characteristics of growing and ruptured aneurysms.

Very large and giant microsurgical bifurcation aneurysms in rabbits: Proof of feasibility and comparability using computational fluid dynamics and biomechanical testing

BACKGROUND

Giant aneurysms are challenging lesions with unacceptable high rates of aneurysm recanalization and rerupture following embolization. Reliable in vivo models are urgently needed to test the performance of new more efficient endovascular devices.

MATERIALS AND METHODS

Aneurysms were created in 11 New Zealand white rabbits (4.5-5.5kg): A long venous pouch (length 25-30mm) mimicking the aneurysm sac was derived from the external jugular vein and sutured into a microsurgically created bifurcation between both common carotid arteries. After 4 weeks the rabbits underwent 3T Magnetic resonance angiography (3T-MRA). Exemplary computational fluid dynamics (CFD) simulations were performed to compare the flow conditions of giant rabbit and human aneurysms. We used species-related boundary conditions, in particular, we measured blood viscosity values. Biaxial mechanical tests were performed for the mechanical characterization and comparison.

COMPARISON WITH EXISITING METHOD(S)

None.

RESULTS

No peri- or postoperative mortality was observed. 3T-MRA showed aneurysm patency in 10 out of 11 aneurysms (90.9%). Aneurysm lengths ranged from 21.5-25.6mm and aneurysm necks from 7.3-9.8mm. CFD showed complex flow profiles with multiple vortices in both, rabbit and human aneurysms. Lower blood viscosity values of the rabbit (3.92mPas vs. human 5.34mPas) resulted in considerable lower wall shear stress rates (rabbit 0.38Pa vs. human 1.66Pa). Mechanical tests showed lower stiffness of rabbit aneurysms compared to unruptured human aneurysms.

CONCLUSIONS

The proposed model showed favorable aneurysm patency rates, low morbidity and good hemodynamic comparability with complex flow patterns. Biomechanical testing suggests that experimental aneurysms might be even more fragile compared to human aneurysms.

Aneurysm wall thickness measurements of experimental aneurysms: in vivo high-field MR imaging versus direct microscopy

BACKGROUND

Thin cerebral aneurysm wall thickness (AWT) is connected to high aneurysm rupture risk. MR imaging of AWT leads to overestimations. The aim of the present study was to quantify MR inaccuracy by comparison with accurate light microscopic measurements.

METHODS

In 13 experimental microsurgical bifurcation aneurysms in rabbits, 3 Tesla (3 T)-MR imaging using contrast-enhanced T1 Flash sequences (resolution: 0.4 × 0.4 × 1.5 mm³) was performed. The aneurysms were retrieved immediately after MR acquisition, cut longitudinally, and calibrated photographs were obtained. AWT (dome, neck) and parent vessel thickness (PVT) were measured on the MR images and microscopic photographs by independent investigators. All parameters were statistically compared (Wilcoxon test, Spearman correlation).

RESULTS

AWT and PVT could be imaged and measured in all aneurysms with good quality. Comparison with the "real" light microscopic measurements showed a progressive tendency of MR AWT overestimation with smaller AWT: AWT at the dome (0.24 ± 0.06 mm vs. MR 0.30 ± 0.08 mm; p = 0.0078; R = 0.6125), AWT at the neck (0.25 ± 0.07 mm vs. MR 0.29 ± 0.07 mm; p = 0.0469; R = 0.7451), and PVT (0.46 ± 0.06 mm vs. MR 0.48 ± 0.06 mm; p = 0.5; R = 0.8568).

CONCLUSION

In this experimental setting, 3 T-MR imaging of cerebral AWT showed unacceptable inaccuracies only below the image resolution threshold. Theoretically, AWT for clinical usage could be classified in ranges, defined by the maximum image resolution.

Comparison between routine cylindrical cerebral aneurysm volume approximation and three-dimensional volume measurements in experimental aneurysms

OBJECTIVES

Aneurysm volume is routinely approximated calculating cylindrical volumes. Exact aneurysm volume assessment is crucial for liquid polymer embolization. The aim of this study was to compare simple cylindrical volume approximations with direct multiplanar reconstruction (MPR) segmentational volumetry in a saccular/complex experimental rabbit bifurcation aneurysm model.

METHODS

In 12 female New Zealand white rabbits, saccular, broad-based, bilobular, and bisaccular aneurysms (three of each) were created using the rabbit venous pouch bifurcation model. Contrast-enhanced magnetic resonance angiography (CE-MRA) was performed, and maximal intensity projection (MIP) reconstructions as well as an MPR dataset were acquired. Aneurysm width and length were measured in MIP images, and the volume was approximated calculating cylindrical volumes. Three-dimensional (3D) segmentational volumetry using the MPR dataset was performed in a semi-automated manner.

RESULTS

Maximal intensity projection cylindrical volumes ranged from 53·6 to 503·5 mm(3) (mean 186·5±118 mm(3)). Multiplanar reconstruction segmentation-based volumes ranged from 74·7 to 581·0 mm(3) (mean 202·2±133 mm(3)). The mean relative difference between MIP cylindrical and MPR segmentation volume calculation was 24·7% (range -77·5 to +50·8%). Only 4 of 12 MPR segmentational volumes were within a 10% range of results calculated for MIP cylindrical volume, and 3 of those were in broad-based aneurysms.

CONCLUSION

This descriptive study demonstrates that estimated MIP cylindrical volumes differ from those measured by MPR segmentation volumetry. With the increasing acquisition of 3D data as 3D-MRA and the increasing need for exact volume determination, studies on the accuracy of computational segmentational volumetry of CE-MRA are necessary.

Evaluation of cerebral aneurysm wall thickness in experimental aneurysms: comparison of 3T-MR imaging with direct microscopic measurements

BACKGROUND

Thin aneurysm wall thickness (AWT) is thought to portend an elevated risk of intracranial aneurysm rupture. Magnetic resonance imaging (MRI) is biased by AWT overestimations. Previously, this suspected bias has been qualitatively described but never quantified. We aimed to quantify the overestimation of AWT by MRI when compared to the gold standard of AWT as measured by light microscopy of fresh aneurysm specimens (without any embedding procedure). This analysis should help to define the clinical potential of MRI estimates of AWT.

METHODS

3-Tesla (3T) MRI (contrast-enhanced T1 Flash sequences; resolution: 0.4 x 0.4 x 1.5 mm(3)) was performed in 13 experimental aneurysms. After MR acquisition, the aneurysms were retrieved, longitudinally sectioned and calibrated micrographs were obtained immediately. AWT at the dome, AWT at the neck and parent vessel wall thickness (PVT) were measured on precisely correlated MR-images and histologic micrographs by blinded independent investigators. Parameters were statistically compared (Wilcoxon test, Spearman's correlation).

RESULTS

AWT was assessed and reliably measured using MRI. Interobserver variability was not significant for either method. MR overestimation was only significant below the image resolution threshold: AWT at the dome (0.24 ± 0.06 mm vs. MR 0.30 ± 0.08 mm; p = 0.0078; R = 0.6125), AWT at the neck (0.25 ± 0.07 mm vs. MR 0.29 ± 0.07 mm; p = 0.0469; R = 0.7451), PVT (0.46 ± 0.06 mm vs. MR 0.48 ± 0.06 mm; p = 0.5; R = 0.8568).

CONCLUSION

In this experimental setting, MR overestimations were minimal (mean 0.02 mm) above the image resolution threshold. When AWT is classified in ranges defined by the MR resolution threshold, clinical usage may be beneficial. Further quantitative and comparative experimental and human studies are warranted to confirm these findings.

3D computerized occlusion rating of embolized experimental aneurysms using noninvasive 1.5T MR imaging

BACKGROUND AND PURPOSE

For embolized cerebral aneurysms, the initial occlusion rate is the most powerful parameter to predict aneurysm rerupture and recanalization. However, the occlusion rate is only estimated subjectively in clinical routine. To minimize subjective bias, computer occlusion-rating (COR) was successfully validated for 2D images. To minimize the remaining inaccuracy of 2D-COR, COR was applied to 1.5T 3D MR imaging.

MATERIALS AND METHODS

Twelve experimental rabbit aneurysms were subjected to stent-assisted coil embolization followed by 2D DSA and 3D MR imaging. Subjective occlusion-rate (SOR) was estimated. Linear parameters (aneurysm length, neck width, parent vessel diameter) were measured on 2D DSA and 3D MR imaging. The occlusion rate was measured by contrast medium-based identification of the nonoccluded 2D area/3D volume in relation to the total aneurysm 2D area/3D volume. 2D and 3D parameters were statistically compared.

RESULTS

There were no limiting metallic artifacts by using 3D MR imaging. Linear parameters (millimeters) were nearly identical on 2D DSA and 3D MR imaging (aneurysm length: 7.5 ± 2.6 versus 7.4 ± 2.5, P = .2334; neck width: 3.8 ± 1.0 versus 3.7 ± 1.1, P = .6377; parent vessel diameter: 2.7 ± 0.6 versus 2.7 ± 0.5, P = .8438), proving the high accuracy of 3D MR imaging. COR measured on 3D MR imaging was considerably lower (61.8% ± 26.6%) compared with the following: 1) 2D-COR (65.6% ± 27.1%, P = .0537) and 2) 2D-SOR estimations (69.2% ± 27.4%, P = .002). These findings demonstrate unacceptable bias in the current clinical standard SOR estimations.

CONCLUSIONS

3D-COR of embolized aneurysms is easily feasible. Its accuracy is superior to that of the clinical standard 2D-SOR. The difference between 3D-COR and 2D-COR approached statistical significance. 3D-COR may add objectivity to the ability to stratify the risk of rerupture in embolized cerebral aneurysms.

Complex bilobular, bisaccular, and broad-neck microsurgical aneurysm formation in the rabbit bifurcation model for the study of upcoming endovascular techniques

BACKGROUND AND PURPOSE

Despite rapid advances in the development of materials and techniques for endovascular intracranial aneurysm treatment, occlusion of large broad-neck aneurysms remains a challenge. Animal models featuring complex aneurysm architecture are needed to test endovascular innovations and train interventionalists.

MATERIALS AND METHODS

Eleven adult female New Zealand rabbits were assigned to 3 experimental groups. Complex bilobular, bisaccular, and broad-neck venous pouch aneurysms were surgically formed at an artificially created bifurcation of both CCAs. Three and 5 weeks postoperatively, the rabbits underwent 2D-DSA and CE-3D-MRA, respectively.

RESULTS

Mortality was 0%. We observed no neurologic, respiratory, or gastrointestinal complications. The aneurysm patency rate was 91% (1 aneurysm thrombosis). There was 1 postoperative aneurysm hemorrhage (9% morbidity). The mean aneurysm volumes were 176.9 ± 63.6 mm(3), 298.6 ± 75.2 mm(3), and 183.4 ± 72.4 mm(3) in bilobular, bisaccular, and broad-neck aneurysms, respectively. The mean operation time was 245 minutes (range, 175-290 minutes). An average of 27 ± 4 interrupted sutures (range, 21-32) were needed to create the aneurysms.

CONCLUSIONS

This study demonstrates the feasibility of creating complex venous pouch bifurcation aneurysms in the rabbit with low morbidity, mortality, and high short-term aneurysm patency. The necks, domes, and volumes of the bilobular, bisaccular, and broad-neck aneurysms created are larger than those previously described. These new complex aneurysm formations are a promising tool for in vivo animal testing of new endovascular devices.

Improved microsurgical creation of venous pouch arterial bifurcation aneurysms in rabbits

BACKGROUND AND PURPOSE

The choice of the experimental aneurysm model is essential for valid embolization-device evaluations. So far, the use of the rabbit venous pouch arterial bifurcation aneurysm model has been limited by demanding microsurgery, low aneurysm patency rates, and high mortality. This study aimed to facilitate microsurgery and to reduce mortality by optimized peri-/postoperative management.

MATERIALS AND METHODS

Aneurysms were created in 16 New Zealand white rabbits under general intravenous anesthesia. Using modified microsurgical techniques, we sutured a jugular vein pouch into a bifurcation created between both CCAs. Aggressive anticoagulation (intraoperative intravenous: 1000-IU heparin, 10-mg acetylsalicylic acid/kg; postoperative subcutaneous: 14 days, 250-IU/kg /day heparin) and prolonged postoperative anesthesia (fentanyl patches: 12.5 μg/h for 72 hours) were applied. Angiographic characteristics of created experimental aneurysms were assessed.

RESULTS

The reduced number of interrupted sutures and aggressive anticoagulation caused no intra-/postoperative bleeding, resulting in 0% mortality. Four weeks postoperation, angiography showed patency in 14 of 16 aneurysms (87.5%) and Ohshima type B bifurcation geometry. Mean values of parent-artery diameters (2.3 mm), aneurysm lengths (7.9 mm), and neck widths (4.1 mm) resulted in a mean 1.9 aspect ratio.

CONCLUSIONS

Compared with historical controls, the use of modified microsurgical techniques, aggressive anticoagulation, and anesthesia resulted in higher aneurysm patency rates and lower mortality rates in the venous pouch arterial bifurcation aneurysm model. Gross morphologic features of these aneurysms were similar to those of most human intracranial aneurysms.