High-frequency optical coherence imaging can guide neurointerventional surgery to achieve optimal deployment of flow diverters, stents and intrasaccular devices, write Matthew Gounis and Ajit Puri. Here, they discuss the evolution of the technology, and detail their institution’s experience developing the novel modality, which they have designed specifically for application to the neurovasculature.
In a recently published article Nature Communications volume 11, article number: 2851 (2020), we have introduced a technology designed for high-resolution intravascular imaging of the cerebrovasculature. The foundational technology is optical coherence tomography (OCT), which uses nearinfrared light to interrogate the arterial lumen and reconstruct cross-sectional images of the blood vessel from the backscattered light.
More than a decade has passed since this technology was introduced to image coronary artery disease, providing detailed information about atherosclerotic plaques with approximately 18µm spatial resolution, allowing precise sizing of stents during treatment.
As clinical evidence has mounted, OCT has been shown to affect clinical decisions in nearly 50% of coronary interventions, as well as reduce major in-hospital cardiac adverse events and mortality. However, the existing platform cannot reliably be used in the intracranial circulation.
Although there are scattered case reports that describe OCT in neurovascular applications, the design of the system is not optimised to allow routine use in the highly tortuous intracranial vasculature due to the large profile of the device, relatively rigid construction with transitions at the wrong locations, and most importantly— nonuniform rotation of the lens due to contact friction of the torque wire with the encasing catheter, which can cause image distortion or, more commonly, catastrophic failure of the device.
In collaboration with Gentuity LLC (Sudbury, Middlesex County, USA), the University of Massachusetts Medical School (Worcester, USA) has engaged in a National Institutes of Health funded programme to develop a novel highfrequency optical coherence imaging (HF-OCT) technology designed specifically for application to the neurovasculature.
With a profile of 0.0155” (1.2 Fr), the HF-OCT imaging catheter has similar performance characteristics as a microguidewire, including a shapable coil spring tip. The described experiments entailed patient-specific, whole circle of Willis vascular phantom testing to ensure that contrast flushing protocols deployed would not differ from standard protocols used for rotational angiography.
In order to image the arterial lumen, blood has to be momentarily cleared so as not to attenuate the laser. Since the device acquires images at 250 frames per second, it is possible to acquire 8cm of vascular imaging within approximately 2–3 seconds. We found that the standard contrast injection protocols deployed clinically for rotational angiography provided sufficient blood clearance for optimal imaging. Subsequently, in vivo modelling in a porcine brachial artery with elevated tortuosity demonstrated that imaging with uniform illumination was possible and the individual layers of the arterial wall could be reliably visualised.
Neurovascular devices, such as flow diverters and stents were deployed in the animal model and interrogated with HF-OCT at a resolution approaching 10µm, standard digital subtraction angiography, as well as high-resolution cone-beam CT. We found that inter-rater reliability to assess flow diverters and stent malapposition or acute thrombus formation along the surface of the device was significantly superior with HFOCT as compared to the other imaging modalities.
Finally, cadaveric intracranial atherosclerosis specimens were imaged with HF-OCT and compared with histological analysis from a blinded pathologist. Agreement on plaque characterisation (fibrotic, necrotic lipid core, or micro-calcifications) was concordant between HF-OCT and histopathological analysis.
Unlike the coronary and peripheral circulations, in situ analysis to study vascular disease is currently not possible for the intracranial circulation. Thus, longitudinal data for plaque development, vasculopathies, or degeneration of the wall in aneurysms does not exist. This technology, which is nearing clinical introduction, can shine light on cerebrovascular pathology.
With sizing of devices for specific lesions approaching the resolution of our angiography equipment, highresolution intra-vascular imaging can provide precise sizing for personalised intervention. HF-OCT can guide neurointerventional surgery to achieve optimal deployment of flow diverters, stents and intrasaccular devices, and provide clinically viable information to prevent complications and potentially prognosticate clinical/angiographic outcomes. Mirroring the role of OCT in coronary artery interventions, clinical research exploring novel devices will certainly benefit with HF-OCT.
Matthew Gounis is the director of the New England Center for Stroke Research and Professor of the Department of Radiology at UMass Memorial Medical Center, Worcester, USA.
Ajit Puri serves as co-director of the New England Center for Stroke Research, Chief of NeuroInterventional Radiology at UMass Memorial Medical Center (Worcester, USA), and is an Associate Professor of Radiology at the University of Massachusettes Medical School.