Ultrasound

Matthew Abrahams

Jorge Pineda

▪ INTRODUCTION

Anesthesiologists have recently started using ultrasound (US) for a variety of purposes. US can be used to evaluate a patient (diagnostic) or during procedures (interventional). US is a powerful tool that allows anesthesiologists to see structures beneath the skin, is noninvasive, and does not produce harmful radiation. Compared to other types of imaging, US is relatively portable and inexpensive. In addition, US images are generated nearly instantly, providing real-time information at the bedside. US equipment may appear complex at first, but by understanding a few basic principles and becoming familiar with your US machine’s basic controls, you can quickly become comfortable using it.

Anesthesiologists may use US to determine the status of a patient’s medical condition. For example, US can be used to perform a detailed examination of the heart (echocardiography) either externally (transthoracic) or by placing a special transducer in the patient’s esophagus (transesophageal). US can also be used to evaluate the patient’s blood vessels for narrowing or blockage or to determine the patient’s volume status (do they have sufficient intravascular volume). It can be used to examine the patient’s internal organs or to look for fluid collections in the patient’s skin, muscle, or body cavities. US is also commonly used during procedures such as vascular access or nerve blocks. Specific uses of US are discussed in more detail in other chapters. This chapter focuses on the basic principles of US: the physics of US image formation, US machine controls, basic US terminology, storage of US images, tips for optimizing conditions during US exams and procedures, and proper use and maintenance of US equipment. In order to illustrate the concepts we discuss, we have included pictures showing various US machine controls and sample US images. We made them using equipment available at our institution. This is not intended to be a comprehensive user’s manual for every US machine currently available. Different models of US machines have different types of controls, and the images may have a different “look.” It is important to become familiar with the machine(s) you will be using and apply the concepts in this chapter to understand how to use them.

▪ BASIC US PHYSICS/MACHINE CONTROLS

The basic underlying principle of US physics is that sound waves are emitted and received by the US transducer. A piezoelectric element (vibrates when an electrical current is applied) in the transducer emits sound waves that are reflected by the patient’s tissues. The reflected waves are received by the transducer, which measures the timing and strength of the reflected waves. The waves are emitted and received in a very thin beam, which is a flat plane about as thick as a piece of paper. The US machine then processes this information to produce the image on the US screen. The image produced is based on how fast the US waves move through various tissues and how much of the US energy the tissues reflect. US waves are above the range of normal human hearing (higher frequency) and so are not audible. The US transducer spends the majority of time receiving reflected US waves and is only emitting US energy about 0.1% (1/1000th) of the time. There are no known harmful effects to live tissue from exposure to US waves in frequencies used clinically.

Frequency of US Waves

The behavior of US waves is governed by the simple equation: λ = v/f. This describes the relationship between the frequency of US waves (f)
and their wavelength (λ). The velocity of US waves through tissue (v) is relatively constant at approximately 1600 m/s, though this varies slightly depending on the water content of the tissue. From this equation, you can see that the frequency and wavelength are inversely proportional, meaning that as the frequency increases, the wavelength becomes shorter. Conversely, as the frequency decreases, the wavelength gets longer. US waves’ ability to travel through tissue and the resolution (sharpness) of the US image also depend somewhat on the frequency and wavelength (Fig. 38.1). The resolution of the US image is approximately two wavelengths, so a shorter wavelength (higher frequency) will improve the resolution of the US image.

The usual range of US waves emitted/received by US transducers commonly used by anesthesiologists is 2-15 megahertz (mHz) or 2-15 million cycles per second. Unfortunately, higher-frequency US waves do not penetrate tissue well and are not capable of imaging deeper structures (Fig. 38.1). So it may be helpful to use a lower frequency to image these deeper structures, but the image will not be as sharp (lower resolution).

Gain

The brightness of the US image can be adjusted using the gain controls on the US machine (Fig. 38.2). The gain is the amplitude of the US waves. Adjusting the overall gain control is similar to turning up the volume on a stereo. This makes the entire image brighter, but this may not improve the quality of the image, just as turning up the volume on a stereo may make the music louder but not improve the quality of the song. Fine gain controls such as time-gain compensation (TGC) or lateral gain compensation (LGC) can be used to brighten or darken specific areas of the image (Fig. 38.2). This is similar to adjusting equalizer settings on a stereo. To compensate for the effect of depth on signal strength, the TGC sliders are often arranged in a diagonal pattern to increase the brightness lower in the image (deeper). The most important point about TGC and LGC controls is that they should be checked to prevent overadjusting the image as this can produce significant artifacts and make US image interpretation more challenging. When in doubt, it is probably best to leave the sliders in a neutral position. Slight adjustments can then be made to “fine-tune” the US image as necessary.

Depth

The depth of the US image can be adjusted on nearly every US machine (Fig. 38.3). It is important to adjust the depth properly so that the structure(s) of interest is on the screen. Too much depth is not helpful as it makes the target structure(s) smaller in the US image, while deeper structures may not be relevant to the procedure being performed. In general, it is usually ideal to have the entire target structure(s) in the US image and adjust the depth so that the target(s) is roughly centered in the image. This allows the anesthesiologist to see other structures in the area that he or she may wish to avoid puncturing unintentionally, and to help visualize the needle if it is directed deeper than intended (Fig. 38.3).

Doppler/Color

Another useful feature of most US machines commonly used by anesthesiologists is known as Doppler (or color) imaging. This uses a physical phenomenon known as the Doppler shift to measure movement. The Doppler shift describes the effect of an object’s direction and velocity on the sound emitted by the object as detected by the receiver. For example, as a train moves toward you, the sounds it makes have a higher pitch, while the sounds it makes have a lower pitch as it moves away from you. The faster the object is moving, the greater the change in pitch. The US transducer acts as the receiver. Objects moving toward the transducer reflect US waves at a higher pitch than emitted and those moving away at a lower pitch than emitted (Fig. 38.4). This phenomenon can be used to measure flow through blood vessels.

The Doppler effect is displayed on the US screen as color, and the color scale can be used to show how fast blood (or any other substance) is moving toward or away from the US transducer. If the direction of flow is perpendicular to the transducer, flow is neither toward nor away from the transducer, and there may not be color on the US screen even though there is blood flowing through the vessels. Tilting the US transducer (discussed later) toward or away from the direction of flow may improve the color signal.

Focus

The US waves emitted from the transducer are shaped like an hourglass (Fig. 38.5). The

narrowest portion of the beam is known as the “focal zone.” In this area, there is the least interference among US waves and the US image is clearest. In general, the focus depth should be adjusted so that it is centered over the target structure(s). If multiple US beams are being used simultaneously (see below), the number of focal zones can be adjusted as well. If multiple beams are used, the focal zone can be made wider or narrower to include more or less of the US image.

▪ FIGURE 38.1 A: Schematic showing the relationship between frequency of US waves and tissue penetration. Red dots represent tissues/structures reflecting US waves (arrows). This shows that high-frequency waves are more likely to be reflected, preventing them from penetrating deeper. This is known as tissue attenuation. B: Controls for adjusting frequency on Sonosite (left) and Philips (middle, right) US machines. The Sonosite does not specify the actual frequency (in MHz) but uses a “Gen, Res, Pen” system. “Gen” is a midrange (general) frequency, “Pen” is a lower (penetrating) range, and “Res” is a high-frequency (resolution) range. The Philips machine also uses the “P,R,G” nomenclature on the US screen. C: US images of the same superficial area using high (left) and low (right) frequencies. The image on the left has better resolution.

▪ FIGURE 38.1 (Continued) D: US images of the same deep area using high (left) and low (right) frequencies. The image on the right shows the nerve better though the resolution is lower.

▪ FIGURE 38.2 A: Pictures of gain controls for Sonosite (left) and Philips (middle, right) US machines. The Sonosite allows adjustment of overall gain and separate adjustment of gain in the near and far areas of the US image. The Philips unit allows for adjustment of overall gain as well as at different levels of the US image (time-gain compensation or TGC, top sliders, move side -to side) as well as on the edges of the US image (lateral-gain compensation or LGC, lower sliders, move up or down). The TGC and LGC sliders are arranged haphazardly in the middle, and more conventionally in the image on the right. B: Same US image with gain adjusted. On the left, the gain is too low, making the image dark and hard to interpret. On the right, the gain is too high, making the image too bright and again hard to interpret.

▪ FIGURE 38.2 (Continued) C: Same US image with TGC and LGC adjusted. The image on the left shows how improper use of the TGC controls can give the US image a “striped” appearance, making interpretation difficult. The image on the right shows how improper adjustment of the LGC controls can make one side of the US image too bright or too dark, again making interpretation difficult.

▪ FIGURE 38.3 A: Depth controls for the Sonosite (left) and Philips (right) US machines. B: US images of the median nerve showing too little (left) and too much (right) depth. The nerve is not seen in the image on the left as the field of view is too shallow. The nerve appears very small in the image on the right as the field of view is too deep.

▪ FIGURE 38.4 A: Schematic showing the effect of movement of an object on the frequency of sound waves emitted or reflected by an object. The object (dot) is moving in the direction of the arrow (left) effectively compressing the sound waves moving in the same direction as the object (higher frequency) and dilating the sound waves moving in the opposite direction (lower frequency). This is known as the Doppler effect. B: Color imaging controls for the Sonosite (left) and Philips (right) US machines. C: Still image showing use of color imaging. The color denotes flow through blood vessels. In this image, the vein is blue and the artery is orange/red. This is due to the color scale seen in the upper right area of the US image. Veins may not always appear blue and arteries may not always appear orange/red depending on the color scale selected and the orientation of the US transducer relative to the vessels in the US image.

Multibeam

Many newer US transducers are capable of emitting multiple US beams simultaneously (Fig. 38.6). These may be oriented in slightly different directions or use frequencies that are

slightly different (harmonic imaging). This may reduce artifacts in the US image and may give the US image a “smoother” appearance. This also requires more processing of the image, and so the frame acquisition rate (number of times per second the US image is changed) may decrease. This can make movement within the image appear choppy. Multibeam imaging can usually be turned on or off easily, and there is usually a marker (varies depending on manufacturer) on the image to show if this feature is being used or not.

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