Echocardiography

Chapter 5


Echocardiography





1. How does echocardiography work?


    Echocardiography uses transthoracic and transesophageal probes that emit ultrasound directed at cardiac structures. Returning ultrasound signals are received by the probe, and the computer in the ultrasound machine uses algorithms to reconstruct images of the heart. The time it takes for the ultrasound to return to the probe determines the depth of the structures relative to the probe because the speed of sound in soft tissue is relatively constant (1540 m/sec). The amplitude (intensity) of the returning signal determines the density and size of the structures with which the ultrasound comes in contact.


    The probes also perform Doppler ultrasonography, which measures the frequency shift of the returning ultrasound signal to determine the speed and direction of moving blood through heart structures (e.g., through the aortic valve) or in the myocardium itself (tissue Doppler imaging).


    Appropriateness criteria for obtaining an echocardiogram are given in Box 5-1.



Box 5-1   APPROPRIATENESS CRITERIA FOR ECHOCARDIOGRAPHY






Modified from Douglas PS, Khandheria B, Stainback RF, et al: ACCF/ASE/ACEP/ASNC/SCAI/SCCT/SCMR 2007 appropriateness criteria for transthoracic and transesophageal echocardiography. J Am Coll Cardiol 50:187-204, 2007.


2. What is the difference between echocardiography and Doppler?


    Echocardiography usually refers to two-dimensional (2-D) ultrasound interrogation of the heart in which the brightness mode is used to image cardiac structures based on their density and location relative to the chest wall. Two-dimensional echocardiography is particularly useful for identifying cardiac anatomy and morphology, such as identifying a pericardial effusion, left ventricular aneurysm, or cardiac mass.


    Doppler refers to interrogation of the movement of blood in and around the heart, based on the shift in frequency (Doppler shift) that ultrasound undergoes when it comes in contact with a moving object (usually red blood cells). Doppler has three modes:



image Pulsed Doppler (Fig. 5-1, A), which can localize the site of flow acceleration but is prone to aliasing



image Continuous-wave Doppler (Fig. 5-1, B), which cannot localize the level of flow acceleration but can identify very high velocities without aliasing


image Color Doppler (Fig. 5-2), which uses different colors (usually red and blue) to identify flow toward and away from the transducer, respectively, and identify flow acceleration qualitatively by showing a mix of color to represent high velocity or aliased flow



Doppler is particularly useful for assessing the hemodynamic significance of cardiac structural disease, such as the severity of aortic stenosis (see Fig. 5-1), degree of mitral regurgitation (see Fig. 5-2), flow velocity across a ventricular septal defect, or severity of pulmonary hypertension. The great majority of echocardiograms are ordered as echocardiography with Doppler to answer cardiac morphologic and hemodynamic questions in one study (e.g., a mitral stenosis murmur); 2-D echo to identify the restricted, thickened, and calcified mitral valve (Fig. 5-3); and Doppler to analyze its severity based on transvalvular flow velocities and gradients.



3. How is systolic function assessed using echocardiography?


    The most commonly used measurement of left ventricular (LV) systolic function is left ventricular ejection fraction (LVEF), which is defined by:

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Jun 5, 2016 | Posted by in CARDIOLOGY | Comments Off on Echocardiography

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