Introduction: Microsurgery for treatment of cerebrovascular aneurysms and atriovenous malformations demands comprehension of the three-dimensional vascular anatomy in the surgical field despite access to only a small range of viewing angles intraoperatively. State-of-the-art modalities including indocyanine green angiography, digital subtraction angiography, micro-doppler ultrasonography, neuronavigation, and neurophysiological monitoring are used adjunctively and help overcome the limitations of direct microscope visualization. However, none of these permit real-time 3D visualization of the microvascular architecture. Photoacoustic imaging uses non-ionizing pulsed laser excitation to create transient thermoelastic expansion detected as an ultrasonic emission. Pulsed lasers tuned within the near-infrared spectrum can take advantage of endogenous substrates, such as blood, based on differential absorbance relative to surrounding tissue. Traditional ultrasonography is limited near the skull base because of complex reverberations of the pulse echo signal upon the skull surface and the poor ultrasound scattering of blood. The photoacoustic technique combines the advantages of selective optical excitation with the established 3D reconstruction techniques available in ultrasonography. Blood absorbs well in the near-infrared spectrum which is advantageous since these wavelengths are capable of penetration deep (>1cm) into biological tissues.
Methods: A benchtop photoacoustic rig utilizing a pulsed 800nm laser excitation source and a commercial ultrasound scanner with a motorized scanning platform was used to create 3D volumetric scans of a vascular phantom model filled with oxygenated bovine whole blood (Figure 1A).
Results: 3D photoacoustic angiograms could be acquired at depths >2cm from the ultrasound probe head after infra-red excitation (41mJ/cm2). Larger, more superficial structures simulating engorged aneurysms allowed sufficient infrared transmittance and subsequent acoustic emission such that even 0.3mm ‘vessels’ could be seen deep to these obstructions from a single vantage-point without advanced image processing (Figure 1B).
Conclusions: This phantom-derived data indicates that photoacoustic imaging may represent effective means of real-time intraoperative 3D angiography.
Patient Care: Post-operative angiography studies have revealed as much as a 10% rate of major vessel occlusion postoperatively. Additionally, 12% of clipped aneurysms result in incomplete obliteration or a visible residual neck, suggesting that a higher degree of surgeon confidence regarding the surrounding vasculature could improve clip placement.
Learning Objectives: By the conclusion of this session, participants should be able to identify near-infrared photoacoustic imaging technology as an emerging tool for intraoperative 3D angiography.
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3. Siasios I, Kapsalaki EZ, Fountas KN. The role of intraoperative micro-doppler ultrasound in verifying proper clip placement in intracranial aneurysm surgery. Neuroradiology. 2012;54(10):1109-1118