Quantification of Internal Carotid Artery Flow with Digital Subtraction Angiography: Validation of an Optical Flow Approach with Doppler Ultrasound

BACKGROUND AND PURPOSE:Digital subtraction angiography is the reference standard technique to evaluate intracranial vascular anatomy and used on the endovascular treatment of vascular diseases. A dedicated optical flow-based algorithm was applied to DSA to measure arterial flow. The first quantification results of internal carotid artery flow validated with Doppler sonography are reported.MATERIALS and METHODS:We included 22 consecutive patients who underwent endovascular procedures. To assess the sensitivity of the algorithm to contrast agent-blood mixing dynamics, we acquired high-frame DSA series (60 images/s) with different injection rates: 1.5 mL/s (n = 19), 2.0 mL/s (n = 18), and 3.0 mL/s (n = 13). 3D rotational angiography was used to extract the centerline of the vessel and the arterial section necessary for volume flow calculation. Optical flow was used to measure flow velocities in straight parts of the ICAs; these data were further compared with Doppler sonography data. DSA mean flow rates were linearly regressed on Doppler sonography measurements, and regression slope coefficient bias from value 1 was analyzed within the 95% confidence interval.RESULTS:DSA mean flow rates measured with the optical flow approach significantly matched Doppler sonography measurements (slope regression coefficient, b = 0.83 ± 0.19, P = .05) for injection rate = 2.0 mL/s and circulating volumetric blood flow <6 mL/s. For injection rate = 1.5 mL/s, volumetric blood flow <3 mL/s correlated well with Doppler sonography (b = 0.67 ± 0.33, P = .05). Injection rate = 3.0 mL/s failed to provide DSA–optical flow measurements correlating with Doppler sonography because of the lack of measurable pulsatility.CONCLUSIONS:A new model-free optical flow technique was tested reliably on the ICA. DSA-based blood flow velocity measurements were essentially validated with Doppler sonography whenever the conditions of measurable pulsatility were achieved (injection rates = 1.5 and 2.0 mL/s).

Even though digital subtraction angiography has traditionally been confined to standard anatomy assessment, quantification of blood flow based on DSA is becoming an important topic that could help neurointerventionists in making adequate peri-procedural decisions. Some reports have described new techniques based on DSA that are able to assess flow or flow changes during treatment of stented aneurysms.^1,2 However, the development of clinically useful tools based on the integration of engineering, hemodynamic and physiologic knowledge still requires improved translation of biofluid mechanical information into clinical applications.³ X-ray video densitometry, based on the detection of the displacement of radiopaque contrast material through the vascular system, has been studied since the early 1960s and has been divided into 2 main classes: tracking and computational methods.⁴ Sarry et al⁵ estimated the flow by using an inverse advection model. Bogunović and Loncarić⁶ proposed the combination of DSA and 3D rotational angiography (3DRA), using an analysis of the time-attenuation curves. Rhode et al⁷ developed a model-based and weighted-optical field (OF) approach to improve already existing techniques and compared the results with simulation data, while Imbed et al^8,9 developed a similar approach on the femoral artery and simulated angiographic data. Waechter et al¹⁰ developed a model-based approach to measure flow in the cerebral arteries.These approaches were all limited in case of fast flows, required longer straight-vessel segments, and were affected by low signal-to-noise ratios. A dedicated algorithmic scheme was developed to reduce instabilities due to temporal and spatial noise and to cope with fast flows, to address these issues.¹¹ Essentially, we used only the modulation by the cardiac cycle to extract flow-velocity values. The OF principle was then applied to the pulsating component of dye-concentration signal.¹¹ In this article, the first clinical results of the proposed algorithm applied to the ICA are reported. The clinical implementation and verification against Doppler ultrasound (USD) data in a consecutive cohort of patients are described, and the limits of the current technique are discussed.