Image-Based Evaluation of Vascular Function and Hemodynamics

Abstract

The noticeable characteristics of the blood vascular structure are the inconsistent viscosity of blood and the stiffness of the vascular wall. If we can control these two factors, we can solve more problems related to hemodynamics and vascular wall function. Understanding the properties of hemodynamics and vascular wall function may provide more information applicable to clinical practice for cardiovascular disease. The bedside techniques evaluating vascular function usually measure indirect parameters. In contrast, some medical imaging techniques provide clear and direct depictions of functional cardiovascular characteristics. In this review, image-based evaluation of hemodynamic and vascular wall functions is discussed from the perspective of blood flow velocity, flow volume, flow pattern, peripheral vascular resistance, intraluminal pressure, vascular wall stress, and wall stiffness.

Keywords: Arterial wall stiffness; Flow velocity; Flow volume; Peripheral vascular resistance; Wall shear stress.

Fig. 1

Doppler ultrasonography. a Color Doppler ultrasonography consisting of a B-mode ultrasonographic image and an overlaid color map of velocity. Incoming and outgoing flows can be mapped as red or blue colors. A higher velocity shows more extreme colors on the color bar. b By locating a sample volume within the color Doppler zone, the real-time continuous time-velocity spectrum can be acquired. White dots within the spectrum indicate velocities of each ultrasonic reflector, blood cells. By numerical calculation of the quantitative data, time-averaged mean velocity (TAM) and other velocity derivatives can be acquired. PSV = Peak systolic velocity; EDV = end-diastolic velocity; HR = heart rate. Colors refer to the online version only.

Fig. 2

Velocity-encoded MRPC imaging. a Basic fast gradient-echo image (magnitude image) can be acquired in high temporal resolution and can be seen as a cine image. With this image, the anatomical location of the blood flow can be identified. Setting ROIs can be done on this magnitude image since the edge detection is easier than in a phase image. b By adding an initial bipolar pulse on the fast gradient-echo sequence, protons within the static tissue and dynamic flow should show the nulled and bipolar phases of precession. The difference in phase implies the velocity of the proton, which is displayed in grey scale by its direction across the imaging plane. Ascending and descending aortas show opposite grey scale since the flow direction is opposite. c By locating the ROI on the magnitude image, the flow velocity data can be acquired from the phase image. Although they are not in real time, time-velocity curves can be drawn by post-processing the acquired data. The descending aortic flow shows a negative value by the opposite flow direction to the ascending aortic flow. A = Ascending aorta; D = descending aorta.

Fig. 3

Flow volume analysis in cerebral arteries of a 70-year-old asymptomatic male during a health screening. a By using the flow diameter and time-averaged mean flow velocity (TAM), the volume flow can be estimated. The left internal carotid artery shows a normal flow pattern and velocities. b The left internal carotid artery is completely occluded showing no flow and a calcified plaque within the lumen. c The left vertebral artery shows a normal flow pattern and velocities. Due to cardiac arrhythmia, the time-velocity spectrum shows irregular pulse intervals. d The right vertebral artery is completely occluded without any flow signal. e A comprehensive cerebral blood flow analysis chart displays the flow volume of the bilateral internal and external carotid and vertebral arteries. In this patient, the right internal carotid and vertebral arteries show no flow (green rectangles). The trace values are due to wall vibration artifacts. However, the total cerebral blood flow volume is within the normal range due to a compensatory overflow in the left internal carotid artery (red rectangle). PSV = Peak systolic velocity; EDV = end-diastolic velocity; HR = heart rate. Colors refer to the online version only.