Editorial for “Assessment of Complex Flow Patterns in Patients With Carotid Webs, Patients With Carotid Atherosclerosis, and Healthy Subjects Using 4D Flow MRI ”
Year: 2023
Type: editorial
Abstract: Vascular imaging has primarily focused on lumen imaging to identify severe stenoses that reduce blood flow. This emphasis on stenosis severity was reinforced by the landmark North American Carotid Endarterectomy trial (NASCET)showing substantial benefit from surgery for stenoses of 70% or greater,1 a critical threshold often corresponding to the point where pressure gradients can be measured at catheter angiography. It is also reinforced by the Poiseulle's Law for viscous Newtonian flow in a tube showing that flow varies directly with radius to the fourth power2; once a critical flow limiting percent narrowing threshold is reached, small changes in radius have enormous impact on maximum flow. Historically, an emphasis on lumen geometry was necessitated by the inherent technological limitations of catheter angiography which only visualizes the lumen especially when performed with background subtraction. MR Angiography has often been compared to catheter angiography, as the gold standard of reference, with MR Angiography being criticized for having lower resolution, offsetting the MRA advantage of being safe and noninvasive.3 But, although the lower spatial resolution of MRA makes precise measurement of percent stenosis more challenging, MR is three-dimensional and sees beyond the lumen, also interrogating the vessel wall4 and flow dynamics.5 In particular, four-dimensional flow has ushered in a plethora of flow features such as particle tracings, streamlines, viscous energy loss, turbulent kinetic energy, pressure mapping, wall shear stress, pulse wave velocity, and others that can be evaluated, which may be more important than lumen geometry in determining clinical significance of vascular lesions. In this article, El Sayed et al. used four-dimensional flow MRA to analyze 10 patients with symptomatic carotid webs with a mean lumenal narrowing of 32% comparing to eight patients with a similar degree of atherosclerotic narrowing without web and 10 healthy volunteers.6 These lesions are well below the threshold of importance based upon the 70% NASCET stenosis severity threshold; yet, they were all symptomatic, likely periodically showering emboli downstream. Flow analysis shows all these carotid web lesions had low time averaged wall shear stress (TAWSS) and high oscillatory shear index (OSI) parameters that indicate stagnant flow, which can predispose to thrombus formation. Wall shear stress is the shear rate at the vessel wall and is important for determining how blood interacts with the endothelium. High shear helps to break up platelet aggregates/microthrombi and disperses pro-coagulant factors before the emboli forming cascade gets underway. Thus, it makes sense that low shear is a sign of increased embolic risk. Shear varies over the cardiac cycle, high in systole and low in diastole, which is addressed here by measuring time averaged wall shear stress (TAWSS). OSI is a more complex concept corresponding to the negative wall shear rate divided by the sum of negative and positive wall shear rates.7 This reflects the relative amount of forward flow (low OSI = 0 corresponds to completely positive forward flow) vs. oscillatory flow (high OSI = 0.5 corresponds to back and forth flow only with no net positive flow). High OSI has been associated with release of platelet microparticles and procoagulants,8 which likely contribute to formation of the emboli causing symptoms in these carotid web patients. Although MRA cannot attain the degree of two-dimensional spatial resolution possible with catheter X-ray angiography, by more fully characterizing all the features of a stenosis including the underlying wall and its impact on blood flow, a more complete picture of patient risk and outcomes may emerge. One can imagine a future in which MRA is primarily searching the vascular system for regions of flow disturbances and then further examining each of these flow disturbance sites with detailed mapping of mural anatomy and lumen geometry. Any limitation MRA has regarding spatial resolution will be more than compensated by flow details, vessel wall imaging and the safety advantages of being noninvasive and free of ionizing radiation.
DOI: 10.1002/jmri.29011
Author: Martin R. Prince
Language: English
Subfield: Pulmonary and Respiratory Medicine
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W4386623995