Dr. Guy Van Camp
The main objective of this session was to understand the basics of contrast echocardiography and instrumentation, with a specific emphasis on the safety of the technique.
Leda Galiuto started with a description of the interaction between the contrast microbubbles and ultrasound. Ultrasound microbubbles are pure intravascular markers and have the same behaviour in the coronary microcirculation as red blood cells. Depending on the power of the ultrasound beam these bubbles will behave differently when they enter an ultrasound field. At low mechanical index (MI), linear behaviour of the bubbles predominates and the reflected signals can be visualised in the fundamental frequency domain. However, the poor signal to noise ratio in respect to the echos from the tissue results in insufficient imaging of the myocardial perfusion and of left ventricular opacification. On the contrary, increasing the power leads to a non linear response of the bubbles resulting in bright US reflections from the bubbles (harmonics) and, with adequate imaging solutions, reflections from the tissue can be minimised making perfusion of the myocardium together with left ventricular opacification possible. Increasing the energy even more will result in the destruction of the bubbles also resulting in a bright reflection of US followed by the destruction of the bubbles with disappearance of the echo signals from the bubbles until new bubbles have entered the US beam. Sanjiv Kaul explained the reasons that MCE can result in imaging perfusion heterogeneity within the myocardium in significant coronary artery stenosis. Not the coronary stenosis on itself but rather the pressure differences between the aorta and the right atrium dictate the regulation of the coronary microcirculation. It will be the interplay between the arterioles, the capillaries and the venules which will define the myocardial blood volume (image we obtain with MCE) with the arterioles playing a dynamic role in constriction and dilatation and the capillaries only able to be recruited or de-recruited and with a systolic emptying of blood from the arterioles and the venules. Using destruction (flash imaging) of the bubbles followed by refilling of the myocardium, MCE can be used to obtain quantitative data of microbubble velocity (β) and myocardial blood volume (plateau A) and the product of both parameters is a measurement of myocardial blood flow. Leda Galiuto explained in her second talk the different imaging settings and protocols for contrast echocardiography. Using high mechanical index, the signal of bubble destruction can be used to image myocardial perfusion but bubble destruction implicates that these techniques need intermittent imaging, giving the bubbles the time between imaging to refill the ultrasound beam. Using low mechanical index imaging, real time contrast imaging is possible but specific imaging modalities are needed to exclude the harmonic signal response from the tissue (pulse inversion, power modulation,…). Combining these techniques with stress echocardiography (vasodilatation or dobutamine) results in imaging the dynamics of the microcirculation in coronary artery disease. Finally, Sanjiv Kaul illustrated clearly that MCE is a safe technique. Anaphylactic reaction linked to complement activation is rarely seen, but the incidence of this reaction during MCE seems to be lower than exercise and other stress testing techniques. Every echolab using MCE must have the necessary equipment to act adequately if this rare condition occurs.
Basics and Instrumentation
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