This simulation study analyzed blood flow stagnation in the ICA in these case scenarios (bypass blood flow rates of 0, 25, 45, 50, 75, and 100 ml/min) based on a patient's preoperative condition data.
This study was approved by the institutional review board of Nagoya University Graduate School of Medicine (approval number: 2016 − 0437). All procedures performed were in accordance with the ethical standards of the Declaration of Helsinki. All experiments were performed in accordance with relevant named guidelines and regulations and informed concent was obtained from the patient’s families.
Our institute’s policy is essentially based on BTO with a hypotensive challenge (Fig. 1) . In cases in which retrograde dome filling from collateral flow was observed during BTO, a parent artery occlusion (PAO) was conducted. Otherwise, proximal ICA ligation of the cervical portion was performed with no retrograde dome filling from the collateral flow during BTO. Treatment for contralateral CCA was based on the second BTO’s results. A high-flow bypass with PAO was administered if a patient exhibited a neurologic deficit during a 20-minute BTO. An STA–MCA bypass with PAO was performed in patients who demonstrated neurologic deficits during a BTO test within a 15-minute hypotensive challenge. Without bypass, PAO was performed in patients who did not demonstrate neurologic deficits during the BTO test within 15 min of a hypotensive challenge.
Digital subtraction angiography
Three-dimensional digital subtraction angiography (3D-DSA) was performed by femoral catheterization using the Seldinger technique with a biplane DSA unit with rotational capabilities (Axiom Artis dTA; Siemens Healthcare, Erlangen, Germany). Typically, 4–10 ml of nonionic contrast medium (Iopamidol, 300 mg of iodine per ml; Teva Takeda Pharma, Nagoya, Japan) was used per acquisition. The spatial resolution was 0.32 × 0.32 mm. Standard anteroposterior and lateral DSA images were obtained with the catheter in the four significant arteries (the common carotid arteries and vertebral arteries). The single 3D rotational angiographic acquisition is typically performed to evaluate aneurysm shape and surrounding blood vessels. Images were reconstructed using a 256 × 256 matrix. Rotational angiographic data were transferred to an independent workstation (Syngo Workplace; Siemens Healthcare, Erlangen, Germany) to generate 3D reformatted images. 3D-DSA data were immediately sent to an adjacent 3D workstation (Siemens Medical Solutions, Erlangen, Germany). An experienced endovascular staff reviewer evaluated the conventional DSA and discussed the operative strategy with the surgical intervention staff.
The CFD model
Postoperative blood flow characteristics were evaluated by CFD analysis. The vascular geometry was reconstructed from the 3D-DSA images, and the surface was smoothed using a 3D slicer, which is an image-computing platform (ver. 4.10.2, The Slicer Community, Harvard, USA). Five layers of prism meshes were created near the vessel wall, and the remaining geometry was filled with polyhedral meshes. The total number of meshes was approximately 600,000. Blood was treated as an incompressible Newtonian fluid with a density of 1,060 kg/m3 and viscosity of 0.004 Pa∙s.
Flow simulation in the 3D domain of the vascular geometry obtained from the 3D-DSA images was conducted using scFlow (MSC software, Japan). The flow was assumed steady. A no-slip condition was applied to the wall, and the vessel wall was assumed to be rigid.
Bypass surgery of the left MCA with clipping of the left ICA was simulated by providing inflow to the left MCA. Previous reports have revealed low-flow bypass grafts, such as the STA permit 15–50 ml/min flow velocity [2–4]. High-flow bypass grafts, such as a radial artery or saphenous vein graft, have flow rates of 50–150 ml/min [2, 5]. Flow simulations were implemented at various intracranial blood flows of the bypass graft (bypass blood flow of 0, 25, 45, 50, 75, and 100 ml/min). Based on the patient's own DSA data, inflows of 235, 5, and 114 ml/min were administered to the right ICA, basilar artery (BA), and left ophthalmic artery, respectively. These blood flow velocities were obtained from the patients’ data. A 0D blood flow model was assigned to the rest of the blood vessels, numerically coupled with blood flow in a 3D domain.
For comparison, ICA clipping without bypass surgery was also simulated. The boundary condition was the same as in the cases with bypass surgery, except for the left MCA. In this case, the 0D blood flow model was assigned to the left MCA instead of defining the flow rate.
Particle residence time
The particle residence time (PRT) was calculated for the quantitative evaluation of particle tracking. The trajectories of massless and volumeless particles were calculated using the predictor–corrector method considering the flow velocity fields.
A 64-year-old woman visited an ophthalmologist for diplopia for one month. She was referred to our department for abductor nerve palsy. Brain magnetic resonance imaging revealed a 25-mm left giant ICA cavernous segment aneurysm. A 3D-DSA was performed (Fig. 2); the left common carotid artery angiography (CCAG) suggested a left giant ICA cavernous segment aneurysm. The right CCAG with a left ICA balloon occlusion indicated adequate perfusion of the contralateral MCA territory via the anterior communicating artery (Acom). Left external carotid artery angiography with left ICA balloon occlusion indicated retrograde blood flow to the aneurysm via the left ophthalmic artery. No neurological deficit was observed with left BTO, although aphasia occurred during the BTO test with a hypotensive challenge. Computed tomography (CT) perfusion imaging during the BTO test indicated no change in cerebral blood flow (CBF), a slight increase in cerebral blood volume (CBV), and an extension of mean transit time. This CT perfusion image suggested that peripheral arterial dilation led to preserving the left CBF.
The patient was in supine, and her head was rotated toward the right side at approximately 30°. A left frontotemporal craniotomy was performed followed by cervical ICA ligation and a double STA–MCA bypass surgery. Intraoperative indocyanine green video angiography revealed good bypass flow. Sensory-evoked potential and motor-evoked potential monitorings were normal during the surgery.
After the operation, the patient woke up with good consciousness. The patient was transferred to the intensive care unit with no remarkable neurological deficits. Approximately 12 h postoperatively, contralateral hemiplegia, hemianesthesia, and homonymous hemianopsia occurred with occlusion of the anterior choroidal artery. Brain magnetic resonance angiography revealed an acute thrombus formation at the C8 segment of the ICA with good bypass patency (Fig. 3). Despite systemic anticoagulation and antiplatelet therapy, the thrombus formation progressed. On postoperative day 4, the patient died of hemorrhagic cerebral infarction.