Sound is the vibration of a physical medium. In clinical echocardiography, a mechanical vibrator, known as the transducer, is placed in contact with the esophagus (TEE), skin (transthoracic echocardiography [TTE]), or the heart (epicardial echocardiography) to create tissue vibrations. The resulting tissue vibrations create a longitudinal wave with alternating areas of compression and rarefaction (Fig. 26-1).

The amplitude of a sound wave represents its peak pressure and is appreciated as loudness. The level of sound energy in an area of tissue is referred to as intensity. The intensity of the sound signal is proportional to the square of the amplitude and is an important factor regarding the potential for tissue damage with ultrasound. Since levels of sound pressure vary over a large range, it is convenient to use the logarithmic decibel (dB) scale:

where A is the measured sound amplitude and A_r is a standard reference sound level; I is the intensity and I_r is a standard reference intensity. The Food and Drug Administration limits the intensity output of cardiac ultrasound systems to be <720 W/cm² because of concerns of potential tissue injury.¹²

Sound waves are also characterized by their frequency (f), or pitch, expressed in cycles per second, or Hertz (Hz), and by their wavelength (λ). These attributes significantly impact the depth of penetration of a sound wave in tissue and the image resolution of the ultrasound system.

The propagation velocity of sound (v) is determined solely by the medium through which it passes. In soft tissue, the speed of sound is approximately 1,540 m/s. As the product of wavelength and frequency equals velocity: V = λ × f, it becomes apparent that the wavelength and frequency are inversely related: λ = v × 1/f and that λ = (1,540 m/s)f. High-frequency, short-wavelength ultrasound is more easily focused and directed to a specific target location. Image resolution also increases with short-wavelength
sound waves; for these reasons, ultrasonic frequencies of 2 to 10 MHz are preferred in clinical echocardiography.

The propagation of a sound wave through the body is markedly affected by its interactions with the various tissues encountered. These interactions result in reflection, refraction, scattering, and attenuation of the ultrasound signal and determine the resulting appearance of the two-dimensional image.

Echocardiographic imaging relies on the transmission and subsequent reflection of ultrasound energy back to the transducer. A sound wave propagates smoothly through uniform tissue until it encounters the interface between two tissues varying in acoustic impedance (a property largely related to the density [ρ] of the tissue and the speed that ultrasound travels). A large interface oriented perpendicular to the sound beam will produce a mirror-like reflection of sound back toward the transducer with only a portion of the signal passing through the interface. Since cardiac structures are detected by their reflected echocardiography signal, echocardiographers adjust the position of the TEE transducer so that the direction of its beam is perpendicular to the cardiac structure of interest.

Refraction causes a change in direction of propagating sound and occurs when an interface lies oblique to the sound beam. Refraction is an important factor in the formation of artifacts as the transducer mistakenly interprets a reflection from the refracted beam as originating from a cardiac structure located within the intended scanning field.

Scattering reflections occur when an ultrasound beam encounters small or irregularly shaped surfaces, such as red blood cells. These reflectors scatter ultrasound energy in all directions, so that far less energy is reflected back to the transducer. This type of reflection is the basis of the Doppler analysis of blood flow (see following).

Even when traveling through uniform tissue, sound undergoes a steady loss (i.e., attenuation) in intensity as a consequence of dispersion and absorption. Attenuation results in less energy returning to the transducer and low-quality images with poor signal-to-noise ratios. To combat attenuation, echocardiographers select better penetrating low-frequency signals (e.g., 2.5 instead of 7.5 MHz) and choose an imaging window that is close to the structure of interest. Adjusting the gain controls to amplify the weak returning signals makes their appearance brighter on the display. Unfortunately, this increases the brightness of artifactual noise, which negatively impacts the image appearance.

To convert echoes into images, the returning ultrasound pulses are received, electronically processed, and displayed. The oscillator repeatedly cycles the transducer from a brief transmission to a relatively long receive mode. During the receive phase, the reflected echocardiography signals are captured and converted to electrical signals by the piezoelectric crystal. The echocardiography system employs a series of controls including system gain, time gain compensation, compression, and postprocessing settings (not unlike those available with digital imaging software) to optimize the signal for display. Adjustments are used, for example, to emphasize edge detection versus tissue texture, or to improve the delineation of weaker reflectors. The choice of settings is dictated by the examination and the preferences of the echocardiographer.

Ultrasonic imaging is based on the amplitude and time delay of the reflected signals. Since the velocity in tissue is a relatively constant 1,540 m/s, only the distance of the structure from the transducer alters the time required for the ultrasound wave to travel to and from the reflected structure. So by timing the interval between transmission and return of the reflections, the echocardiography system can precisely calculate the distance of a structure from the transducer.

Current imaging is based on brightness mode or B-mode technology. With B-mode, the amplitude of the returning echoes from a single pulse determines the display brightness of the representative pixels. M-mode or motion mode adds temporal information to B-mode by displaying a series of sequentially collected B-mode images. M-mode echocardiography provides a one-dimensional, single-beam view through the heart but updates the B-mode images at a very high rate, providing dynamic real-time imaging. M-mode remains the best technique for examining the timing of cardiac events (Fig. 26-3).

Two-dimensional (2-D) echocardiography is a modification of B-mode echocardiography and the mainstay of the echocardiographic examination. Instead of repeatedly firing ultrasound pulses in a single direction, the transducer in 2-D echocardiography sequentially directs the ultrasound pulses across a sector of the cardiac anatomy. In this way, 2-D imaging displays a tomographic section of the cardiac anatomy, and unlike M-mode, reveals the shape and lateral motion (Fig. 26-4).

Two-dimensional scanning is achieved using phased array technology, which sequentially activates each crystal in the array and thereby steers the beam without the transducer itself being moved. The two commonly used electronic scanning systems in medical ultrasound are the linear scanners and sector scanners.

The linear scanner uses a long linear array. Groups of crystals are activated sequentially from one end of the transducer to the other. The firing of each group of crystals creates an image of the structures directly in front of them. With sequential firing, the anatomic features from one end of the transducer to the other are imaged (Fig. 26-5). The disadvantage of this approach is that the transducer face must be large to cover a broad anatomic area. The linear array is commonly used to guide vascular access and regional anesthetic procedures.

The phased array sector scanner is the most commonly used in echocardiography. Here the ultrasound scan is sequentially directed in a fan-like arc. The resulting sector image known as a frame is similar in shape to that covered by a windshield wiper. The 2-D scanner then repeats the entire process to update the image and capture motion.

Expert echocardiographers select machine settings to optimize particular image qualities for the examination at hand. As discussed in the following sections, these selections will determine whether sector size, spatial resolution, or dynamic motion is best displayed.

The pulse repetition frequency is the rate at which sound pulses are triggered. The greater the pulse repetition frequency, the greater the number of scan lines that are emitted in a given period of time. This enhances motion display. Unfortunately, sector depth must be reduced because pulse repetition frequency is inversely related to the sector depth as a longer period of time is required for the ultrasound to travel the increased distances.

The frame rate is the frequency at which the sector is rescanned. A high frame rate improves the capture of movement. Typically, frame rates >30 per second are desired. The frame rate is critically dependent on the sector depth, which determines the time required for each scan line to be received, and the sector width, which increases the number of scan lines that must be transmitted. Consequently, increases in sector size and depth come at the cost of a decreased frame rate and poor motion imaging.

The number of scan lines per degree of the sector (scan line density) greatly affects the image resolution. Doubling the scan lines essentially doubles the lateral resolution. However, the cost is a decrease in the frame rate and motion imaging.

The echocardiographer must thoughtfully select among settings that will often have opposing effects between the size of the imaging field, the imaging resolution, and the frame rate. A common approach is to focus each part of the examination on a given structure of interest and select the imaging plane that best delineates the structure in the near field. Motion display can then be enhanced without costs in lateral resolution by decreasing the sector angle and depth. In situations in which the maximal frame rate is desired, M-mode is chosen.

The TEE probe is inserted in the anesthetized patient in a manner similar to insertion of an orogastric tube. For improved image quality, the stomach is emptied of gastric contents and air prior to probe insertion. The jaw is lifted with the left hand and the TEE probe, well lubricated, is inserted with the right hand by applying gentle but constant pressure. If significant
resistance is encountered, additional force should be strictly avoided as oropharyngeal or esophageal injury may result. Rather, a decrease in neck extension and/or use of a laryngoscope to visualize the oropharyngeal structures often will allow easy passage of the probe. The TEE probe is advanced beyond the larynx and the cricopharyngeal muscle (around 25 to 30 cm from teeth) until a loss of resistance is appreciated. At this point, the TEE probe lies in the upper esophagus and the first cardiovascular images are seen. Extrinsic compression of the esophagus (e.g., osteophytes or an aortic aneurysm) may impede probe placement.⁹,¹⁰

TEE is a semi-invasive procedure. When performed by qualified operators, TEE has a low incidence of complications. A retrospective study performed on 846 patients who underwent TEE described the following complications: Three patients with pharyngeal abrasions, one patient with a chipped tooth, and few patients with transient vocal cord paresis.¹⁴ Another retrospective study performed on a large case series of 7,200 patients showed that the morbidity associated with TEE placement is 0.2% and the mortality is 0%.¹⁵ The most common complaint (0.1%) was postoperative odynophagia. Various studies have suggested an association between swallowing dysfunction after cardiac surgery and the use of intraoperative TEE.¹⁶,¹⁷ This fact is important as postoperative swallowing dysfunction is associated with pulmonary complications.¹⁶

To maintain the safety profile of TEE, each patient should be evaluated before the procedure for signs, symptoms, or history of esophageal pathology. Amongst the most feared complications of TEE are esophageal or gastric perforation.¹⁸ For skilled practitioners, this complication is extremely rare. Patients with extensive esophageal and gastric diseases are at highest risk of perforation. Contraindications to TEE probe placement are represented by esophageal stricture, rings or webs, esophageal masses (especially malignant tumors), recent bleeding of esophageal varices, Zenker’s diverticulum, status post radiation to the neck, and recent gastric bypass surgery.¹³,¹⁸ In the rare case in which TEE is essential and is the only alternative, placement of the TEE probe can be performed under direct visualization with a combined gastroscopic and echocardiographic examination.¹³

Image acquisition depends on precise manipulation of the TEE probe. By advancing the shaft of the probe, the probe position can be moved from the upper esophagus to the midesophagus and into the stomach. The shaft can also be manually rotated to the left or to the right. By using the large knob on the probe handle, the head of the probe can be anteflexed (turning the knob clockwise) and retroflexed (turning the knob counter clockwise). The smaller knob, located on top of the large knob, is used to tilt the head of the probe to the right or to the left. Using the electronic switch on the probe handle, the operator can rotate the ultrasound beam from 0 (transverse plane) to 180 degrees in 1-degree increments.

The previously mentioned controls allow the experienced echocardiographers to perform comprehensive cardiac imaging. However, the diversity of imaging planes can confuse the less experienced echocardiographers, leaving them unable to recognize the various anatomic structures presented. Thus, an understanding of the basic rules of imaging orientation is essential to echocardiographic interpretation.

The ultrasound beam is always directed perpendicular to the probe face. The 2-D TEE image is displayed as a sector scan. The apex of the sector is in close proximity to the TEE probe and the structures seen in this area will be the posterior ones (e.g., left atrium). The arc of the sector will display the more distal and thereby more anterior structures. The angle of rotation of the imaging array determines the right and left orientations. An easy way to understand this orientation is to place your right hand in front of your chest with the palm facing down, the thumb oriented left and the fingers oriented anterior right. The scan lines that generate the TEE image start at your fingers and sweep toward the thumb. Consequently, the right anatomic structures will be displayed on the left side of the monitor (similar to chest x-ray orientation; Fig. 26-6). Increasing the imaging plane angle produces clockwise rotation of the sector scan. This can be visualized by rotating your hand in a clockwise fashion. For example, at the 90-degree imaging plane the left side of the screen now displays posterior structures (note position of fingers) and the right side of the screen anterior structures (note the position of the thumb; Fig. 26-7).

Each TEE examination is performed with the goal that no important diagnosis is missed. For this reason, a comprehensive evaluation is preferred with each cardiac chamber and valve imaged in at least two orthogonal planes. However, in an emergency situation, such examination may not be possible. In these cases, most echocardiographers will focus the TEE examination to those views most likely to provide a diagnosis, that is, in a hypotensive patient the transgastric short-axis view of the left ventricle is examined for diagnosing hypovolemia, coronary ischemia, or acute heart failure.

To achieve the goals of the intraoperative TEE examination, the Society of Cardiovascular Anesthesiologists together with the American Society of Echocardiography has published guidelines for performing a comprehensive intraoperative TEE examination.¹⁹ These guidelines include 20 standardized 2-D echocardiographic views. Each TEE examination should be recorded (video tapes or digital media) along with a detailed report of the examination. Miller et al.²⁰ proposed a shortened version of the comprehensive examination that would meet the goals established by these guidelines for basic intraoperative TEE proficiency and is particularly useful when time constraints preclude a more extensive examination. The sequence in which the views are acquired differs among echocardiographers. In the following section, we detail the acquisition and anatomic features of the most commonly used intraoperative views.

In order to better conceptualize the morphology and pathology of the heart, three-dimensional (3-D) image presentation has been developed. The recent introduction of a real-time 3-D TEE probe makes this goal a reality for intraoperative echocardiographers. This technology is capable of acquiring full volumes of the left ventricle, of visualizing heart valves in three dimensions (Fig. 26-27), and assessing the synchrony of LV contraction.²¹

Uses of 3-D TEE are just emerging. The utility of 3-D imaging of the MV for an MV repair surgery is of particular interest (Fig. 26-28).²² The capacity of this probe to assess LV contraction synchrony in patients undergoing resynchronization therapy with biventricular pacing may offer a means to maximize their cardiac output. Additional intraoperative applications have emerged, involving percutaneous procedures (transcutaneous aortic valve insertion, noninvasive mitral repair, repair of paraprosthetic leaks, closure of ASD) and open surgical procedures. Guidelines on the use of 3-D echocardiography have also been published.²³

Two Doppler techniques, pulsed wave (PW) and continuous wave (CW), are commonly used to evaluate blood flow. A thorough understanding of the advantages and disadvantages of each technique is critical in selecting the one most appropriate for the clinical setting at hand.²⁵,²⁶ In clinical practice, PW and CW Dopplers are frequently used in conjunction with 2-D imaging. The 2-D image is used to identify the area of interest and guide the echocardiographer in precisely localizing the sampling volume in a PW study or in directing the beam in a CW study.

CFD provides a dramatic display of both blood flow and cardiac anatomy by combining 2-D echocardiography and Doppler (Fig. 26-31). The PW Doppler used for CFD differs from that previously discussed in two important ways. CFD performs multiple sample volume recordings along each scan line as the beam is swept through the sector. This approach provides flow data at each location in the sector, which can be overlaid on the structural data obtained by 2-D imaging. The Doppler velocity data from each sample volume are color-coded and superimposed on top of the gray scale 2-D image. In the most widely accepted color code, red hues indicate flow toward the transducer and blue hues indicates flow away from the transducer. The ability to provide a real-time, integrated display of flow and structural information makes CFD useful for assessing valvular function, aortic dissection, and congenital heart abnormalities. However, an important caveat to its use in the clinical setting must be noted. Since it relies on PW Doppler measurements, CFD is susceptible to alias artifacts. Aliasing in the color flow map is illustrated in Figure 26-32. This alias pattern
can be useful to calculate blood flow in mitral valve disease using the proximal isovelocity surface area (PISA) method (Fig. 26-32).