Contrast Enhanced Ultrasound Perfusion Imaging in Skeletal Muscle

Abstract

The ability to accurately evaluate skeletal muscle microvascular blood flow has broad clinical applications for understanding the regulation of skeletal muscle perfusion in health and disease states. Contrast-enhanced ultrasound (CEU) perfusion imaging, a technique originally developed to evaluate myocardial perfusion, is one of many techniques that have been applied to evaluate skeletal muscle perfusion. Among the advantages of CEU perfusion imaging of skeletal muscle is that it is rapid, safe and performed with equipment already present in most vascular medicine laboratories. The aim of this review is to discuss the use of CEU perfusion imaging in skeletal muscle. This article provides details of the protocols for CEU imaging in skeletal muscle, including two predominant methods for bolus and continuous infusion destruction-replenishment techniques. The importance of stress perfusion imaging will be highlighted, including a discussion of the methods used to produce hyperemic skeletal muscle blood flow. A broad overview of the disease states that have been studied in humans using CEU perfusion imaging of skeletal muscle will be presented including: (1) peripheral arterial disease; (2) sickle cell disease; (3) diabetes; and (4) heart failure. Finally, future applications of CEU imaging in skeletal muscle including therapeutic CEU imaging will be discussed along with technological developments needed to advance the field.

Keywords: Contrast ultrasound; Microbubbles; Skeletal muscle perfusion.

Figure 1. Contrast specific imaging of skeletal muscle. (A) 2D-B mode harmonic imaging of calf muscle (soleus and gastrocnemius). (B) Contrast specific imaging of calf muscle prior to contrast microbubble administration. (C) Contrast specific imaging of calf muscle during contrast microbubble infusion. Note the near complete loss of tissue signal compared to 2D using contrast specific imaging. The residual ultrasound signal is from interfaces of tissue planes within skeletal muscle that can produce strong non-linear acoustic signal. Accordingly, background subtraction using imaging frames obtained immediately after a destructive pulse sequence is recommended to permit analysis of signal produced by microbubbles alone.

Figure 2. Microvascular blood volume: (A) Relationship between microbubble concentration and ultrasound video intensity. At lower concentrations, signal intensity is within the portion of the ultrasound dynamic range where the relationship between microbubble concentration and intensity is nearly linear (red portion). Intermediate concentrations result in ultrasound signal saturation (blue portion) whereby signal intensity is maximal. High microbubble concentrations attenuate the ultrasound pulses causing a decrease in the video intensity (green portion). (B) Post-destruction color coded CEU images of skeletal muscle in systole and diastole. Kinetic models of microvascular flux rate: (C) Time-intensity curve (TIC) analysis of contrast tissue transit rate kinetics after bolus injection. Parameters such as time to peak (TTP) and peak intensity (VI peak) are measured directly while maximum slope (m) and mean transit time (MTT) are calculated. (D) Time-intensity curve for contrast destruction-replenishment kinetics during continuous infusion of microbubbles. After transiently destroying microbubbles within the imaging sector (yellow bolt), video intensity recovery over time is fit to the function y = A (1 - e^βt). The product of microvascular blood volume (A-value which is the plateau of the curve) and the blood flux rate (β or rate constant of replenishment) is calculated to determine tissue blood flow.

Figure 3. Contrast enhanced ultrasound perfusion imaging of skeletal muscle at rest and during exercise in a healthy control. (A) Color coded background-subtracted contrast enhanced ultrasound (CEU) images at various time intervals after high-power destruction in calf muscle of a normal control subject at rest and immediately after cessation of contractile exercise. Note the differences in both time to peak intensity and absolute peak video intensity between rest and stress. (B) Time-intensity data for CEU destruction-replenishment perfusion imaging at rest and immediately following exercise. BG: background.

Figure 4. Contrast enhanced ultrasound perfusion imaging in patient with peripheral artery disease (PAD) versus healthy control. (A) Color coded background-subtracted contrast enhanced ultrasound (CEU) images at time intervals following a destructive pulse in control and PAD patient during exercise. (B) Time-intensity data for CEU destruction-replenishment perfusion imaging at rest and immediately following exercise in a patient with moderate PAD versus a healthy control. At rest, perfusion imaging does not differentiate between the PAD patient with effort induced claudication and a healthy control. During exercise, microvascular blood flow (A × B) is significantly reduced in the PAD patient compared to the control. By parametric analysis, the reduction in blood flow in the PAD patient is mediated by a markedly reduced β function, describing the microvascular flux rate.

Figure 5. Maximal intensity projection (MIP) imaging in a murine model of hindlimb ischemia. Contrast perfusion imaging on day 5 following unilateral femoral artery ligation in a mouse hindlimb. MIP image 4 seconds after microbubble destruction demonstrating homogeneous (A) and heterogeneous (B) spatial distribution of vessels during angiogenic remodeling. MIP long axis image immediately after microbubble destruction (C) and 2 seconds after microbubble destruction (D) demonstrating the architecture of the inflow vessels.