Chapter Outline
Basic Physics 160
Imaging Modalities 163
One-Dimensional Echocardiography—A-, B-, and M-Mode Echocardiography 163
Two-Dimensional Echocardiography 163
Doppler Echocardiography 163
Three-Dimensional Echocardiography 164
Workstation and the Probe 165
Preprocessing Functions 168
Postprocessing Functions 168
Image Optimization 168
Artifacts 168
Maintenance of Equipment 168
Ultrasound Safety 169
Cleaning and Disinfection 169
Conclusion 169
Transesophageal echocardiography (TEE) is now frequently used as a monitor and diagnostic tool in the operating room and it is becoming increasingly important that anesthesiologists learn to acquire and interpret basic images. The diagnosis of certain life-threatening conditions, such as cardiac tamponade, aortic dissection, hypovolemia, and severe left ventricular dysfunction, can be made with even limited training. A competent echocardiographer needs to have a working knowledge of certain theoretical and practical aspects of TEE for it to be of use clinically. These include the basic physics of ultrasound, navigating the computer workstation, important safety issues, and general maintenance of equipment.
A basic TEE machine requires a system for ultrasound beam generation, a system capable of receiving the returning ultrasound beams, a system to process these received signals and last, but not least, a way to display these images so the human eye and brain can appreciate it.
Basic Physics
TEE uses ultrasound (sound above the audible frequency in humans), to formulate images of cardiovascular structures. A transducer at the tip of the TEE probe transmits and receives ultrasonic waves, which are then computer processed and displayed in real time on a monitor. Sound waves typically demonstrate a sinusoidal pattern ( Figure 11–1 ). The wavelength (λ) is the distance between successive peaks (or troughs) of the wave, and the frequency (F) is the number of wavelengths per unit of time measured in hertz (cycles per second). When used in medical ultrasound, the wavelength is related to the resolution of the displayed images and the frequency influences the depth of penetration. The maximum depth of penetration is usually about 200 to 400 times the wavelength. Typically, ultrasound waves with frequencies between 2.5 and 7.5 MHz with wavelengths of 0.2 to 0.6 mm are used in TEE. This correlates to a resolution of 0.4 to 1.2 mm and a depth of penetration of up to 24 cm.
The speed or propagation velocity (C) of sound is determined by the medium in which it travels and is approximately 1540 m/s in heart tissue. The mathematical relationship of frequency, wavelength, and propagation velocity is as follows:
C = λ × F
Since the speed of sound in cardiac tissue is constant, frequency and wavelength are inversely proportional. Thus improved resolution at higher frequencies is obtained at the expense of decreased tissue penetration, and vice versa. Also, the distance of a structure of interest from the transducer may be determined by calculating the time a transmitted wave takes to return to the probe.
When an ultrasound wave reaches an interface of two substances with differing acoustic impedances , or densities, part of it is reflected back toward the transducer and part is transmitted through. The degree to which a wave is reflected is directly proportional to the size of the difference in acoustic impedance. An example of this is when air bubbles enter the heart. Blood and air differ significantly in density, so air bubbles reflect the vast majority of the ultrasound and are seen as very bright white specks. The type of reflection a sound wave undergoes is classified as either specular or scattered ( Figure 11–2 ). Specular reflection occurs when a wave hits an object with a large, smooth surface. It will be strongly reflected at an angle equal and opposite to the angle of incidence. The greatest degree of reflection occurs when the angle of incidence is 90 degrees. Therefore, manipulating the TEE probe to align the ultrasound beam perpendicular to a structure of interest will aid in visualization.
Scattering reflection occurs with substances that possess uneven surfaces. Only a portion of the ultrasound waves return to the receiver and the remainder are reflected in diverging directions. These structures will appear darker on the screen as a result. Both specular and scattering reflection play a role in the acquisition of images in TEE. Probe position and multiplane angle should be directed in such a way as to obtain optimal visualization of the structure of interest.
The concept of refraction deals with the portion of ultrasound that is transmitted through at the interface of two different substances and is involved in the production of artifacts. The TEE machine assumes that the transmitted beam will continue to travel linearly throughout its course. However, the direction of the beam is actually altered as it passes from one medium to the next. The magnitude of the change is proportional to the difference in density of the two media, and the acuity of the angle of incidence ( Figure 11–3 ). Visualization of the pulmonary artery catheter in the aorta is a frequent occurrence and is an example of artifact secondary to refraction.
Attenuation refers to the progressive loss in amplitude of an ultrasound signal as a function of the distance traveled within a certain medium, usually due to the processes of absorption and scattering . Absorption is the conversion of sound energy into heat, and scattering refers to the progressive divergence of sound waves from their original course as they travel further away from their source. The intensity of an attenuated ultrasound signal can be augmented by increasing the gain controls on the TEE machine. Images on the screen will appear brighter as the gain is increased and certain structures may be easier to visualize. However, the amount of noise will also be amplified and the borders of other structures may appear fuzzy and difficult to differentiate ( Figure 11–4 , A and B ).
Depth controls the distance at which objects are visualized on the TEE monitor. Increasing the depth will result in a decrease in size and resolution of the displayed images, but potentially more structures will be able to be seen. Normally a depth of approximately 15 cm is an adequate starting point at which to begin an examination. A greater depth is appropriate in certain circumstances such as an assessment of a pericardial or pleural effusion, whereas it is best to perform planimetry or caliper functions of a structure with the view as close as possible ( Figure 11–5 ).
A focus function is available that enables the echocardiographer to concentrate the area of highest resolution on a structure of particular interest, such as on the mitral valve in the midesophageal four chamber views. The image of this area appears as slightly higher quality than that of the surrounding structures. The TEE machine performs this automatically by manipulating certain dimensions of the transmitted ultrasound beam. It is important to be cognizant of the location of the focus sector when evaluating any ultrasound image.
Two-dimensional echocardiography is excellent for visualizing the structure and motion of the heart but is unable to provide information about blood flow within the cardiovascular system. Doppler echocardiography was developed for this purpose and is an integral part of a comprehensive TEE examination. It is useful to assess intracardiac velocities and in the gradation of valvular stenosis and regurgitation. The direction and speed of blood flow are determined by the changes in frequency and wavelength of an ultrasound wave as it is reflected off of moving red blood cells, with the transducer serving as the reference point. The Doppler shift describes the changes in frequency of an ultrasonic wave as it is reflected off moving targets ( Figure 11–6 ). It is mathematically expressed as follows:
Δ F = V × cos × 2 F t / C
