Optical Coherence Tomography as a Potential Tool For Extravascular Imaging in Vascular Neurosurgery

Abstract

Intraoperative imaging can affect decision making and outcomes in neurovascular surgery. Current modalities have limitations: Duplex ultrasonography is associated with poor definition; continuous-wave Doppler fails at quantifying the degree of vessel stenosis; and invasive catheter angiography, the gold standard, is time-consuming and carries hemorrhagic and thromboembolic risk. Optical coherence tomography (OCT), a new optical imaging technique that uses infrared light to produce real-time cross-sectional images similar to ultrasonography but with significantly higher resolution (up to 5 to 10 μm), has been used successfully on endovascular catheters to image vessel walls.1 OCT has been shown to resolve arterial wall structures differentiating between intima, media, and adventitia.2 It can even delineate different types of tissue in an atheroma such as lipid, calcium, and fibrous cap.3 Furthermore, OCT permits 4-dimensional (3 spatial dimensions plus time) reconstruction of the vessel lumen. These advantages have allowed endovascular OCT to emerge as an adjuvant vascular intraoperative imaging technique; its utility, however, remains limited by the need for percutaneous arterial access and its short optimal imaging time (2 seconds). Using OCT extravascularly could have several benefits in open vascular neurosurgery. To evaluate extravascular OCT as a high-resolution noninvasive imaging modality, Wicks et al4 assessed the ability of a custom-built OCT device to image the antiatherosclerotic effect of pravastatin therapy in a murine model of carotid atherosclerosis. The experimenters randomized wild-type mice and apolipoprotein E (ApoE)-deficient mice (susceptible to plaque development) to 3 treatment groups: wild-type mice (n = 13) on standard diet; ApoE-deficient mice (n = 13) on high-fat, atherosclerotic diet; and ApoE-deficient mice (n = 13) on high-fat, atherosclerotic diet and daily pravastatin. Their hypothesis was that atherosclerotic changes that should be observed mainly in groups 2 and 3 should be detectable by OCT. To test this hypothesis, the authors conducted 2 independent studies. Study 1, a validation study, aimed to determine the optimal time for carotid plaque development and the validity of the surgical procedure. OCT imaging was performed at 8 weeks (n = 1 per group), 10 weeks (n = 1 per group), and 14 weeks (n = 3 per group). After the mice underwent surgical carotid exposure and OCT imaging, they were either kept alive for second imaging or euthanized and perfused for in situ carotid artery imaging with the aim of comparing plaque morphology with the in vivo images. Study 2 aimed to establish the sensitivity and specificity of OCT imaging in determining the presence and size of atherosclerotic plaques in the 3 groups. For this purpose, mice (n = 8 per group) were administered their designated diet and treatment for 1 week. They then underwent in vivo OCT imaging and postperfusion in situ OCT imaging. After resection of the carotid arteries, the authors compared histological sections with OCT images (Figure).Figure: Murine carotid artery lumen patency percentage as calculated by live optical coherence tomography (OCT) imaging, postperfusion OCT imaging, and histology. ApoE KO, apolipoprotein E knockout; HFD, high-fat diet. Reprinted with permission from Wicks RT, Huang Y, Zhang K, et al. Extravascular optical coherence tomography: evaluation of carotid atherosclerosis and pravastatin therapy. Stroke. 2014;45(4):1123-1130.The validation study (study 1) showed that plaque development was initially seen with OCT imaging at 8 weeks and optimally developed at 14 weeks in all ApoE-deficient mice on atherosclerotic diet. In that study, ApoE-deficient mice on pravastatin therapy had a lesser degree of plaque development. In study 2, OCT imaging demonstrated the presence of carotid plaques in all ApoE-deficient mice on high-fat diet. Furthermore, OCT had 100% sensitivity and specificity for plaque detection in comparisons of the 3 groups. This interesting study reveals that OCT offers the potential for a real-time, high-resolution vessel lumen evaluation with simultaneous 4-dimensional reconstruction. This promising technology has the potential to analyze vessel wall characteristics, which could influence revascularization and clipping strategies. The technology may also prove useful during carotid endarterectomies. Integration of imaging technologies into microscopes and image guidance systems are necessary steps for developing the next-generation operating room.