Abstract
Purpose
Conventional intra-arterial digital subtraction angiography, which requires surgical exposure and ligation of the femoral or carotid artery, is a limited method of evaluating elastase-induced aneurysms in New Zealand white rabbits. The purpose of this study was to assess aneurysm morphology, occlusion rates and complications after flow diverter treatment comparing intravenous and intra-arterial digital subtraction angiography.
Methods
We previously published a preclinical study in which we evaluated the occlusion rates of elastase-induced aneurysms after treatment with a prototype flow diverter, by using intra-arterial digital subtraction angiography at three months (n = 9) and six months (n = 9). In addition to that intravenous digital subtraction angiography before treatment, after one month (early follow-up group) and after three months (late follow-up group) was performed. Occlusion rates were compared within the two groups by means of residual contrast filling.
Results
Baseline aneurysm characteristics revealed no significant differences between intra-arterial digital subtraction angiography and intravenous digital subtraction angiography. Aneurysm occlusion rates in both follow-up groups using intravenous digital subtraction angiography were significantly higher compared to intra-arterial digital subtraction angiography (early follow-up group: intravenous digital subtraction angiography (one month) versus intra-arterial digital subtraction angiography (three months); p = 0.03 and late follow-up group: intravenous digital subtraction angiography (three months) versus intra-arterial digital subtraction angiography (six months); p = 0.04). Intravenous digital subtraction angiography is feasible to detect and reproduce device occlusions, in-stent stenosis and post-stent stenosis.
Conclusion
Intravenous digital subtraction angiography can not give a sufficient statement on the aneurysm occlusion process compared to intra-arterial digital subtraction angiography and is therefore not recommended for imaging follow-up after flow diverter treatment in rabbits. Regarding untreated aneurysms and complications like device occlusions, in-stent stenosis and post-stent stenosis intravenous digital subtraction angiography proofed to be a good alternative to intra-arterial digital subtraction angiography in our study.
Keywords: Animal studies, flow diverter, aneurysm, digital subtraction angiography
Introduction
The elastase-induced aneurysm model in rabbits was first described in 1999 by Cloft et al.1 Since the aneurysms are similar in morphology and size to those seen in humans, this animal model is widely applied in preclinical research to test new endovascular devices.2–5
In recent years flow diverters (FDs) have provided a promising alternative in the treatment of complex intracranial aneurysms, such as giant, wide-necked and fusiform aneurysms. Several preclinical and clinical studies have demonstrated acceptable rates of aneurysm occlusion, morbidity, and mortality.2,3,6–8 While in clinical practice follow-up of treated intracranial aneurysms is more frequently performed using magnetic resonance (MR) imaging, the standard technique to assess aneurysm morphology and occlusion rates in this animal model is intra-arterial digital subtraction angiography (IA-DSA) and histological assessment. However in the animal model used, IA-DSA requires surgical exposure and ligation of either the femoral artery or the carotid artery. Since the right carotid artery is used for aneurysm induction and the right femoral artery for aneurysm treatment of all sorts, intra-arterial follow-up angiography is limited. Many studies have already shown that intravenous digital subtraction angiography (IV-DSA) through ear vein puncture is a good alternative for imaging untreated elastase-induced aneurysms.9,10
The purpose of this study was to assess and compare occlusion rates and complications of elastase-induced aneurysms using IV-DSA and IA-DSA after treatment with a prototype flow diverter. In vivo in-depth performance analysis of the prototype FD used as well as occurring complications were addressed in our previous studies.4,5,11
Material and methods
Animal experiment
All the experiments were approved by the animal protection committee of our university and conducted in accordance with the guidelines for animal experiments. Elastase-induced aneurysms were created in 18 New Zealand female white rabbits (body weight 4–6 kg), as first described by Cloft et al.1 and randomly divided into two groups. The groups have been divided regarding their follow-up intervals in an early and a late follow-up group. Within the early follow-up group IV-DSA was performed after one month and IA-DSA after three months. Within the late follow-up group IV-DSA was performed after three months and IA-DSA after six months. All the animals received premedication of aspirin (10 mg/kg orally) and clopidogrel (10 mg/kg orally) from three days prior to placement of the FD until their sacrifice. Treatment was performed 4–6 weeks after aneurysm induction described in detail in our previous study.4 All procedures and angiograms were performed by means of a single-arm angiographic system (Ziehm Vision Imaging, Nuremberg, Germany).
IA-DSA
Arterial follow-up intervals were chosen in accordance with those in clinical practice and previous preclinical studies.2,3,6 In both groups IA-DSA was performed immediately before implantation of the prototype FD and after a follow-up period of three and six months. The early follow-up group (n = 9) was evaluated after three months and the late follow-up group (n = 9) after six months. IA-DSA at final follow-up was performed via the left carotid artery using a 4F sheath, as previously described.4
Imaging was performed in posterior-anterior projection at two frames/s, with a hand injection of 5 ml of iodinated contrast agent (Ultravist 300; iopromide; Bayer Schering Pharma, Berlin, Germany) at a rate of approximately 3 ml/s.
IV-DSA
In general anaesthesia a 24-gauge cannula was inserted into the left ear vein and equipped with a three-way valve. Imaging was performed in posterior-anterior projection at two frames/s, with a hand injection of 8 ml of iodinated contrast agent (Ultravist 300; iopromide; Bayer Schering Pharma, Berlin, Germany) as quickly as possible. IV-DSA was performed immediately before implantation of the prototype FD, within the early follow-up group after one month (n = 9) and within the late follow-up group after six months (n = 9).
Angiographic evaluation
Two interventional neuroradiologists analysed angiograms regarding aneurysm morphology and aneurysm occlusion rates. Angiograms were processed using the software OsiriX. Aneurysm morphology was determined as width, height and neck diameters. Measurements were performed with an external sizing device as reference (one-cent coin). The aneurysm neck was determined as the distance between the proximal and distal origin of the aneurysm sac. The mid-portion of this line to the top of the aneurysm dome was defined as aneurysm height. Aneurysm width was determined as the maximum diameter of the aneurysm sac. Accurate aneurysm area was measured using the pencil tool of OsiriX by edging the contrast filled aneurysm, including the above described parameters. Aneurysm area indicated in mm2 was measured using the defined area of the sizing device (one-cent coin). Contrast agent density within the vessels was measured using the oval region of interest tool of OsiriX and was indicated as pixel density (pixels per inch (PPI)). To determine contrast agent dilution in IA-DSA and IV-DSA, density peak was measured within the aortic arch and compared to the reference value in the left carotid artery (IA-DSA) and the left jugular vein (IV-DSA). Differences of contrast agent densities between IA-DSA and IV-DSA at both time-points were compared with each other.
Statistical analysis
Continuous variables are expressed as means ± standard deviations. Categorical variables are presented as absolute and relative frequencies, unless stated otherwise. Fisher’s exact tests were performed for the comparison of categorical variables between the groups. Continuous variables were tested for normal distribution. Differences between the groups were compared with Student’s t-tests. Differences between venous and arterial measurement in each group were compared with paired t-test. Statistical significance was accepted at a two-sided p value of <0.05. All data analyses were performed using SPSS Statistics 22TM (IBM Inc., Chicago, Illinois, USA).
Ethics
The experiments were approved by the local animal protection committee of the Saarland University and conducted in accordance with the European legislation on the protection of animals (22/11) and the NIH guidelines on the care and use of laboratory animals (NIH publication #85-23 Rev. 1985).
Results
IA-DSA contrast agent density within the aortic arch compared to the left carotid artery revealed a dilution of 4.3 ± 0.7% in the early follow-up group and 4.3 ± 0.8% in the late follow-up group. IV-DSA contrast agent density within the aortic arch compared to the left jugular vein revealed a dilution of 27.8 ± 6.5% in the early follow-up group and 27.3 ± 6.8% in the late follow-up group. We observed a significantly greater dilution of contrast agent in IV-DSA compared to IA-DSA at both time-points (p < 0.001).
Aneurysm morphology including width, height, and aneurysm neck as well as aneurysm area revealed no significant differences when comparing IV-DSA with IA-DSA of untreated elastase-induced aneurysms (Table 1 and Figure 1).
Table 1.
Comparison of aneurysm morphology represented as mean ± standard deviation.
| Angiography | Neck (mm) | Width (mm) | Height (mm) |
|---|---|---|---|
| IV-DSA | 2.68 ± 0.72 | 2.72 ± 0.84 | 5.53 ± 1.45 |
| IA-DSA | 2.66 ± 0.63 | 2.73 ± 0.83 | 5.50 ± 1.40 |
| p-Value | 0.945 | 0.967 | 0.950 |
IA-DSA: Intra-arterial digital subtraction angiography; IV-DSA: Intravenous digital subtraction angiography.
Figure 1.
Aneurysm morphology as area (mm2) before treatment, comparing intravenous digital subtraction angiography (IV-DSA) with intra-arterial digital subtraction angiography (IA-DSA). Each dot is one measurement. (a) Early follow-up group and (b) late follow-up group.
Comparing IV-DSA and IA-DSA within the two follow-up groups, aneurysm occlusion rates measured by aneurysm area were significantly higher after IV-DSA performed earlier (Figure 2). In the early follow-up group, four cases revealed no residual filling of the aneurysm with an occlusion rate of 100% when controlled with IV-DSA after one month. After three months, IA-DSA of these aneurysms showed an occlusion rate of 91.5 ± 1.5%. Only in one case did IA-DSA show a higher occlusion rate (39.1%) than IV-DSA (34.5%). IA-DSA of two aneurysms revealed almost no aneurysm occlusion of 13.1% and 19.1%; in these aneurysms IV-DSA also showed a higher occlusion rate of 16.7% and 39.4% (Figure 3(a) and (b)).
Figure 2.
Occlusion rate (%) after flow diverter treatment, comparing intravenous digital subtraction angiography (IV-DSA) with intra-arterial digital subtraction angiography (IA-DSA) at both time points. Before-after, symbols and lines. Each dot is one measurement. (a) Early follow-up group, one month compared with three months; (b) late follow-up group, three months compared with six months.
Figure 3.
(a) Intravenous digital subtraction angiography (IV-DSA) at one-month follow-up shows an insufficient aneurysm occlusion measuring 39.4%. the white arrow indicates low-grade in-stent stenosis (ISS). (b) Intra-arterial digital subtraction angiography (IA-DSA) after three-month follow-up revealing an occlusion rate of 19.1%. Furthermore, the white arrow indicates severe progress of ISS.
In the late follow-up group, IV-DSA after three months as well as IA-DSA after six months showed complete occlusion of 100% in five cases. In three cases IV-DSA revealed an occlusion rate of 100% while IA-DSA only showed an occlusion rate of 82.1 ± 6.9% (Figure 4(a) and (b)).
Figure 4.
(a) Intravenous digital subtraction angiography (IV-DSA) at three-month follow-up shows a completely occluded aneurysm with no clear residual filling. The white arrow indicates a post-stent stenosis (PSS) of approximately 30% with poststenotic dilatation. (b) Intra-arterial digital subtraction angiography (IA-DSA) after six-month follow-up showing a small residual filling of the aneurysm neck with an occlusion rate of 70% (white arrow). The white arrowhead indicates an increased PSS of approximately 40%.
Device occlusions were observed in a total of three cases (two in the early follow-up group and one in the late follow-up group). IV-DSA already revealed the occlusion after one month in the early follow-up group and after three months in the late follow-up group (Figure 5(a) and (b)).
Figure 5.
(a) Intra-arterial digital subtraction angiography (IA-DSA) after six-month follow-up confirms the distal device occlusion to a comparable extent seen after three months. (b) Intravenous digital subtraction angiography (IV-DSA) at three-month follow-up shows a distal device occlusion with a large collateral artery.
We detected a total of three instances of in-stent stenosis (ISS), two in the early follow-up group and one in the late follow-up group. In the early follow-up group IV-DSA revealed an ISS of 28.4 ± 14.9% and IA-DSA showed an ISS of 57.8 ± 35.1% (Figure 3(a) and (b)). In the late follow-up group the ISS seen after IV-DSA measured 35.5% and 61.4% after IA-DSA. Comparing the observed ISS after IV-DSA and IA-DSA revealed no statistically significant differences (p = 0.147).
In the late follow-up group we experienced five cases of post-stent stenosis (PSS) measuring 18.7 ± 13.9% with IV-DSA after three months and 27.3 ± 13.2% with IA-DSA after six months (Figure 4(a) and (b)). In one case IA-DSA showed a very mild stenosis of 7.2%, which was not seen after IV-DSA. There was no statistically significant difference when comparing the seen PSS after IV-DSA with IA-DSA (p = 0.198).
Discussion
In our previous studies we described the in vivo performance and occurring complications of flow-diverting devices in the treatment of elastase-induced aneurysms.4,5,11 Our previous work has shown that aneurysm occlusion after FD treatment is a slow advancing process and takes a certain time.4 Many clinical studies have also shown that complete aneurysm occlusion can take up to 12 months or even longer.12–16 The purpose of this study was to see if additionally performed IV-DSA (after one and three months) is helpful to evaluate ongoing and estimate duration of aneurysm occlusion and therefore more invasive IA-DSA can be avoided.
Our study has shown no differences between morphology measurements of untreated aneurysm using IV-DSA and IA-DSA. When comparing IV-DSA performed earlier with final IA-DSA after FD treatment we observed significantly higher aneurysm occlusion rates in both follow-up groups. Only one case in the early follow-up group revealed a higher occlusion rate at final IA-DSA. Furthermore, in five cases in the late follow-up group IV-DSA and IA-DSA were similar, since complete occlusion of the aneurysm was noticed. In a total of seven cases IV-DSA led us to believe that complete aneurysm occlusion was reached, while IA-DSA performed later revealed that there was still a small residual filling of the aneurysm neck. Since we experienced no complete aneurysm occlusion in the early follow-up group we believe that complete occlusion seen in the late follow-up group occurred between three and six months. Even in two cases where IV-DSA showed a minor occlusion of the aneurysm dome, IA-DSA revealed that there was almost no occlusion. It is already known that the quality of IV-DSA is inferior to IA-DSA17 and that IV-DSA is more susceptible to motion artifacts.10 Despite these facts we believe that two main aspects further explain the paradoxical occlusion rates comparing IV-DSA and IA-DSA, as seen in our study: the dilution of contrast agent and the different pressure ratio, which exists between IV-DSA and IA-DSA. Density measurements of both angiographies have shown a significant dilution of contrast agent during IV-DSA by primarily passing through the pulmonary circulation before reaching the aneurysm. Furthermore, we believe that pressure rates of IV-DSA using a 24-gauge cannula are not similar to IA-DSA using a 4F sheath, and that the contrast agent during IV-DSA also experiences a decrease in velocity during passage of the pulmonary circulation. Unfortunately we were not able to perform pressure measurements of the injected contrast agents during IV-DSA and IA-DSA in order to substantiate our assumption. We have considered the fact that a recurrence after six months of previously completely occluded aneurysms could be possible, but believe it is highly unlikely, since such cases are not reported in literature and the time period of three months is very short.
From the results of our study we believe that the quality of IV-DSA is not sufficient enough to become an inherent part in the imaging follow-up after FD treatment in the used animal model. We also believe that the quality of IV-DSA, at least when using a non-state-of-the-art angiography system, cannot be further improved to an extent where this is imaginable. Alternative imaging methods like computed tomography angiography (CTA) and MR angiography (MRA) have shown promising results in the follow-up of untreated aneurysms in the same animal model.18 For further studies it would be interesting to investigate the efficacy of CTA and MRA in evaluating the occlusion rates of aneurysms after FD treatment in rabbits.
Nevertheless, with IV-DSA we were able to detect previously described complications5,11 such as ISS, PSS, and device occlusions after only one month. As demonstrated before, we were able to correlate severe ISS, seen in both follow-up groups, to wire fractures.11 In these cases IV-DSA revealed the stenosis at an early stage, while comparison with IA-DSA performed later showed an increase in the course of time. Previously described PSS was only seen in the late follow-up group and could be seen after only three months with IV-DSA. In these cases, final IA-DSA also revealed a progression of the PSS in the course of time. Unfortunately, neither the increased ISS nor PSS after IA-DSA could be statistically verified. We believe that the reason for this was the small number of cases seen in our study.
Previously we experienced three distal device occlusions, presumably due to a distinct tapering of the subclavian artery in the rabbits, leading to extreme oversizing of the devices.4 The exact pathomechanism of the observed occlusions still remains uncertain. Even extensive evaluation in our former studies was unable to answer this question. In these cases IV-DSA already revealed the occlusion and its collateralization after one month and after three months. Furthermore, previously described PSS were also reproduced with IV-DSA, the earliest at three months, and revealed an increase at six months IA-DSA. Considering these facts we believe that the device occlusions most likely were promoted by an in-stent thrombosis and not by a PSS, especially since the extent of the PSS seen after three months was minor.
Limitations
Our preclinical study has one major limitation. We did not perform an additional IV-DSA at the same time as the final IA-DSA since the observed occlusion rates were initially promising when judging the IV-DSA.
Conclusion
In our experience IV-DSA is an alternative method to evaluate untreated aneurysms, especially when considering the minor invasiveness compared to IA-DSA in the animal model used. Our study has shown that IV-DSA is also suitable to evaluate complications after FD treatment regarding device occlusions, in-stent-stenosis and post-stent-stenosis. With IV-DSA we were only able to get an impression of the ongoing occlusion process of the treated aneurysms. In terms of precise assessment of aneurysm occlusion, IV-DSA was revealed to be insufficient compared to IA-DSA since we experienced paradoxical occlusion rates in our study. This led us to believe that complete aneurysm occlusion was reached earlier and to expect greater occlusion rates than actually observed. In summary, with IV-DSA a precise statement on aneurysm occlusion rates is not possible and therefore not recommended for imaging follow-up after FD treatment in the used animal model.
Acknowledgements
The authors would like to thank Admedes Schuessler GmbH, Pforzheim, Germany, for providing the prototype FDs. They would also like to thank the head of department M. Menger and staff of the Department of Experimental Surgery for supporting this study. The following author contributions were made: A Simgen: interventions, angiographic evaluation and manuscript writing; T Tomori: animal care and manuscript writing; H Bomberg: statistical analysis; U Yilmaz: angiographic evaluation; C Roth: angiographic evaluation; W Reith: project development, interventions, angiographic evaluation and manuscript writing; R Mühl-Benninghaus: Interventions, angiographic evaluation and manuscript writing.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The authors received no financial support for the research, authorship and/or publication of this article.
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