The Better to See With
By Beth W. Orenstein
Radiology Today
Vol. 25 No. 6 P. 18
New Imaging Probe Helps Diagnose Neurovascular Disease
In recent years, endovascular interventions have become the preferred approach for the treatment of strokes and other endovascular conditions, including brain aneurysms, ischemic occlusions, dissections, and intracranial atherosclerotic disease (ICAD). Thanks to recent advances in imaging technology, patients with endovascular conditions that may have previously been considered inoperable have undergone successful interventions. However, the problem of accurately diagnosing intracranial artery pathology has persisted. X-ray techniques, such as digital subtraction angiography, CT, cone beam CT (CBCT), and MRI, are used to assess aneurysms and arterial wall disease. Still, these clinical imaging modalities have limited contrast and spatial resolution and can have metal-induced artifacts.
“Interpreting these images can be ambiguous and insufficient,” says Vitor M. Pereira, MD, a neurointerventionalist specializing in minimally invasive procedures of intracranial circulation at St Michael’s Hospital in Toronto.
A new neuro optical coherence tomography (nOCT) probe by Spryte Medical in Boston could help clinicians better see the pathology associated with neurovascular disease and help improve the treatment of stroke and related conditions. This miniaturized OCT imaging probe has proven to enhance visualization of the patient’s arterial pathology, allowing more accurate procedural decision making. The probe is designed to navigate the tortuous paths of cerebrovascular circulation and provide high-resolution imaging in situ. Spryte Medical, an intravascular imaging and AI company, published its first-in-human study of its nOCT technology in May in Science Translational Medicine. Pereira was the lead author.
The study involved 32 patients. Using nOCT, researchers imaged the anterior and posterior circulation of their brains, including distal segments of the internal carotid and middle-cerebral arteries, as well as the vertebral, basilar, and posterior cerebral arteries. They were able to capture a broad spectrum of neurovascular pathologies, including brain aneurysms, ischemic stroke, arterial stenoses, dissections, and ICAD. The artifact-free images were high-resolution visualizations of intracranial artery pathology and therapeutic devices.
Because the study was designed to include a limited number of participants, Pereira chose patients with known underlying pathology and interactions with treatment devices. “I also imaged patients with nOCT at different time points, including preprocedural diagnostic or treatment planning, intraprocedural treatment evaluation, and posttreatment diagnostic cases,” he says. When Pereira has access to more devices, he intends to incorporate nOCT into his neurointerventional daily practice for many applications.
Small Wonder
Giovanni J. Ughi, PhD, senior director of software and advanced development of Spryte Medical and a part-time faculty member at UMass Chan Medical School in Worcester, Massachusetts, explains that nOCT works similarly to echography and ultrasound; it reconstructs an image by measuring the intensity of reflected light. Because the speed of light is incredibly fast, the “time of flight” for light cannot be measured directly, “So we use interferometric techniques,” Ughi says. A miniaturized, flexible optical probe was developed to navigate cerebral arteries. Inside the probe, there is a minutely thin optical fiber—about the size of a human hair—that is used to project the near-infrared light inside the arteries. A miniaturized lens on the tip of the fiber illuminates the arterial wall and collects the reflected light in situ. nOCT is a high-speed imaging modality, acquiring hundreds of images per second, visualizing a long, threedimensional segment of the cerebrovascular arteries in two seconds. The images are displayed in near real-time during the acquisition and are readily available for review.
A large team of engineers has been working on this technology for several years in close collaboration with clinical experts and medical advisors, Ughi says. Several technical challenges needed to be overcome. A team of mechanical engineers collaborated closely with optical engineers on the fiber-optic probe design, he says. Similarly, optical, electrical, and software engineers worked together to develop the imaging console. The main challenge was miniaturizing a flexible imaging probe—approximately one-third of a millimeter in diameter— that can safely and effectively navigate the human cerebrovasculature.
“At the same time, the console features a new generation OCT optical engine, capable of acquiring OCT images at a much higher speed and with a significantly larger field of view than previous generations of OCT technology, as it is required for neurovascular applications,” Ughi says.
High-Resolution Images
For the study, the researchers explored several different applications in neurovascular medicine in the 32 patients. nOCT was used to aid the diagnosis and treatment of brain aneurysms, as well as stroke caused by stenotic lesions and thrombotic complications such as large vessel occlusions due to clots, Ughi says. The study’s results show strong potential in all these conditions, showing how nOCT intravascular imaging provided information not available on standard-of-care X-ray angiography.
Ughi believes nOCT has the potential to prevent complications of endovascular treatment of cerebrovascular disease, provide procedural guidance, and improve individual patient management. The nOCT images visualize the microstructure of arterial wall disease in high resolution, revealing details about the pathology not currently available to any other clinical modality, he says. Similarly, they visualize the features of endovascular therapeutic devices at a spatial resolution approaching 10 microns and illustrate how a device is interacting with the arterial wall and the disease, elucidating whether device implantation was successful or incomplete. They can visualize small clots and the onset of thromboembolic and other complications. At follow-up, nOCT can provide a clear indication of device and disease healing progression and visualize the efficacy of medical therapies.
“We believe that nOCT will open new avenues on how to optimize patient neuroendovascular treatments, improve our understanding of intracranial artery biology and neurovascular disease, and improve current therapies and treatments,” Ughi says.
Pereira says the device enabled his team to see many details that they would not have otherwise seen. For example, he says, they saw thrombus in a vessel that they thought might be ICAD. “There is no other way to determine thrombus/ICAD like this,” he says. “If this patient had been treated as if it was ICAD, this might have led to unnecessary medications and procedures.”
The researchers also saw the wall thickness of an extremely small aneurysm (just 30 micrometers thick), which they would not have seen using any not normally treated, but an indicator of the rupture risk is the thickness of the aneurysm wall, which is [typically] unable to be seen,” Pereira says. “Most unbelievably, really, was seeing that a stent placed in a vessel eight years previously was still not endothelialized because it was sticking out into the middle of the artery.”
Workflow Integration
Ughi says that those familiar with neurovascular studies would have no difficulty learning to use it. nOCT was designed to integrate with standard-of-care neurovascular workflow, he adds. “Our study results showed that it can be easily integrated into clinical practice.”
nOCT imaging procedures were quick and appeared safe and effective, even when applied multiple times during the same procedure, Ughi says. No additional medications are required. “nOCT images are clear and in high resolution, unequivocally depicting the arterial state,” he says.
Pereira says the nOCT probe has been designed to navigate the cerebrovascular tortuosity through the normal, extremely thin microcatheters that physicians use every day for some devices. He did not have any difficulty using the device or the console system.
For patients, nOCT acquisitions require a brief injection of contrast (as routinely done for X-ray angiography imaging), Ughi says. The total duration of an nOCT acquisition is two seconds, so it may go unnoticed by patients. In this first-in-human study, the procedure was well tolerated by all participants, Ughi notes.
The new device is not seen as a replacement for X-ray angiography, the primary imaging modality used in real-time to guide neuroendovascular treatments. However, Ughi says, nOCT provides important information not available with X-ray angiography. It will be used as an adjunct modality to enhance the diagnosis and visualization of the arterial wall pathology and anatomy, free of artifacts and in high resolution, as well as in therapeutic devices. It will augment interventionalists’ understanding of the disease they are treating and help them see how therapeutic devices interact with arteries during implantation, Ughi says.
There is an AI component to the technology. The use of AI is focused on the automated analysis, processing, and classification of the nOCT and X-ray angiography images obtained during procedures. “These intracranial image data have never been previously available and represent an extremely important and valuable training dataset,” Ughi says. “This will help extracting quantitative information from nOCT datasets, automating the image review workflow. This will facilitate the use of nOCT in clinical practice and clinical research.”
Promise for the Future
Intravascular OCT is a well-established modality for intravascular imaging in cardiology, Ughi says. It has been used for the imaging of coronary arteries with several benefits to patients for more than 15 years. The images are clear, in high resolution, and of easy interpretation to interventionalists. The same type of image quality was found and reported in the 32 patients in this study. The high-resolution image quality of nOCT also facilitates the use of AI/machine learning for automated analysis, streamlining image review in clinical settings, Ughi says.
Neurointerventionists interpreting the images from nOCT is probably the most challenging area “as we haven’t been exposed to OCT for the brain before,” Pereira says. “The main difficulty is understanding how to translate what you are seeing on the nOCT to what is being seen on the angiogram. As the nOCT takes a picture within the vessel, we cannot determine the orientation of the vessel itself in the patient.” However, he adds, the difficulty in interpreting the images can be overcome as other technologies, such as cone beam CT, have done through education and, ultimately, use. Spryte Medical is working to develop a comprehensive “atlas” to help educate the neurointerventional community to understand the images, he says.
Pereira does not currently have clinical access to nOCT, and other OCT technology is not designed to navigate the brain’s vasculature. “I have the latest angiography equipment from both Philips and Siemens, which has amazing resolution and features to see the contrast. We also use cone beam CT regularly in our practice,” he says. “CBCT provides a hint of what is happening, but it cannot provide the resolution that nOCT can provide.”
Based on his experience with nOCT, Pereira is anxious for it to be made available and would recommend its use. “This technology will enable neurointerventionists to truly see the pathology they are treating, the interaction of their devices on that pathology, and the patient’s normal anatomy,” he says. “As we have seen, the safety profile is good, and the usability is excellent.”
— Beth W. Orenstein of Northampton, Pennsylvania, is a freelance medical writer and regular contributor to Radiology Today.
