- Ultrasound is a noninvasive diagnostic tool used to complement other imaging modalities.
- The degree to which the ultrasound beam penetrates the patient and the image resolution obtained depend on the frequency of the transducer used.
- Artifacts can be beneficial or detrimental to image interpretation.
Ultrasonography is the use of high-frequency sound waves to generate an image. Because ultrasonography is relatively safe and noninvasive, it has become a useful diagnostic tool in veterinary medicine.1 Veterinary technicians, especially those who wish to learn how to perform ultrasound examinations, should have a basic understanding of ultrasonography: how sound waves are produced and interact with tissue, what types of images can be obtained, how to get the best image, and how to identify common artifacts.
Ultrasound examinations complement other imaging modalities, such as radiography, and allow more definitive diagnostic tests (e.g., biopsy, fine-needle aspiration) to be conducted. However, ultrasonography is limited by the fact that it is user dependent.2,3 This means that the quality of the images obtained and their accurate interpretation depend on the experience and knowledge of the sonographer.
Sound is a wave of energy that, unlike x-rays, must be transmitted through a medium. Sound waves can be described by their frequency, wavelength, and velocity. The frequency is the number of cycles or waves that are completed every second, and the wavelength is the distance needed to complete one wave cycle. The frequency of the sound waves used in ultrasonography is well above the limit of the human ear (20,000 kHz) — usually in the range of 2 to 12 MHz (2 to 12 million Hz).4
An inverse relationship exists between the frequency and the wavelength of a sound wave: the higher the frequency, the shorter the wavelength. This relationship affects the choice of frequency used in each patient undergoing ultrasonography. Higher-frequency ultrasound waves create higher-resolution images, but their shorter wavelength makes them unable to penetrate deeper tissues. Lower-frequency waves have better penetrating power, but because of their longer wavelengths, their resolution is lower. Weighing the need for higher resolution versus more penetrating power is always a consideration when selecting a transducer frequency.
The velocity of an ultrasound wave is independent of the frequency. However, it changes depending on the medium through which the wave is traveling. For example, the velocity of sound is 331 m/sec in air and 4,080 m/sec in bone.2 Within the soft tissues of the body, it is considered to be steady at about 1,540 m/sec.3 This medium-dependent variation affects the ultrasound image produced (discussed below).
The following equation demonstrates the relationship between frequency, wavelength, and velocity:
Velocity (m/sec) = Frequency (cycles/sec) x Wavelength (m)
Two basic principles need to be understood regarding how ultrasound is generated and an image is formed. The first is the piezoelectric effect, which explains how ultrasound is generated from ceramic crystals in the transducer.4 An electric current passes through a cable to the transducer and is applied to the crystals, causing them to deform and vibrate. This vibration produces the ultrasound beam. The frequency of the ultrasound waves produced is predetermined by the crystals in the transducer.
The second key principle is the pulse-echo principle, which explains how the image is generated.5 Ultrasound waves are produced in pulses, not continuously, because the same crystals are used to generate and receive sound waves, and they cannot do both at the same time. In the time between the pulses, the ultrasound beam enters the patient and is bounced or reflected back to the transducer. These reflected sound waves, or echoes, cause the crystals in the transducer to deform again and produce an electrical signal that is then converted into an image displayed on the monitor. The transducer generally emits ultrasound only 1% of the time; the rest of the time is spent receiving the returning echoes.4
Interaction with Tissue
Ultrasound produced by the transducer interacts with different tissues in a variety of ways that may help or hinder image formation. Attenuation and refraction are the two major types of tissue interaction.
Attenuation is the gradual weakening of the ultrasound beam as it passes through tissue. Attenuation can be caused by reflection, scattering, or absorption of the sound waves and is compensated for by use of specific controls, discussed below.5
Reflection takes place when ultrasound waves are bounced back to the transducer for image generation. The portion of the ultrasound beam that is reflected is determined by the difference in acoustic impedance between adjacent structures.5 Acoustic impedance is the product of a tissue's density and the velocity of the sound waves passing through it; therefore, the denser the tissue, the greater the acoustic impedance. The large differences in density and sound velocity between air, bone, and soft tissue create a correspondingly large difference in acoustic impedance, causing almost all of the sound waves to be reflected at soft tissue-bone and soft tissue-air interfaces. On the other hand, because there is little difference in acoustic impedance between soft tissue structures, relatively few echoes are reflected to the transducer from these areas.
Scattering refers to the redirection of ultrasound waves as they interact with small, rough, or uneven structures.5 This tissue interaction occurs in the parenchyma of organs, where there is little difference in acoustic impedance, and is responsible for producing the texture of the organ seen on the monitor. Scattering increases with higher-frequency transducers, thus providing better detail or resolution.
Absorption occurs when the energy of the ultrasound beam is converted to heat. This occurs at the molecular level as the beam passes through the tissues.5
Refraction occurs when the ultrasound beam hits a structure at an oblique angle. The change in tissue density produces a change in velocity, and this change in velocity causes the beam to bend, or refract.2,5 This type of tissue interaction can also cause artifacts that need to be recognized by the sonographer.
Information generated from an ultrasound examination can be displayed in a variety of ways, called modes. The mode used for display depends on the type of ultrasound unit used, the information to be obtained, and the organ being examined.
A (Amplitude) Mode
In A mode, the returning echoes are displayed on the monitor as spikes originating from a single vertical or horizontal baseline.5 The depth of the echo is determined by the position of the spike on the axis, with the top or left side of the monitor being the most superficial and the bottom or right side being farther away. The height of the spike correlates to the amplitude of the echo. This mode is not frequently used other than in ophthalmology.
B (Brightness) Mode
In B mode, echoes are represented by dots on a line that form the basis of a two-dimensional image.5 The brightness of each dot indicates the amplitude of the returning echo. Its location relative to the transducer is displayed along the vertical axis of the monitor, with the top of the monitor representing the transducer. The returning echo's location along the axis is based on the amount of time it takes for the ultrasound wave to be transmitted from the transducer and reflected back. Echoes arising from structures in the near field (close to the transducer) take less time than those coming from the far field (farther away from the transducer) because they travel a shorter distance.
Real-time B mode ultrasonography allows a complete, two-dimensional, cross-sectional image to be generated by using multiple B-mode lines.5 In real-time B mode, the transducer sweeps the ultrasound beam through the patient many times a second. With each pass of the ultrasound beam, multiple lines of dots are generated on the monitor, producing a complete image. These B-mode lines remain on the monitor until the next sweep of the ultrasound beam. Because several beam sweeps are performed per second, a moving, changing, "real-time" image is generated. This is the mode most commonly used in veterinary practice.
M (Motion) Mode
M mode is used in echocardiography and allows the sonographer to measure the heart to assess cardiac function and chamber size. M mode uses a single B-mode line, with the amplitude of the echoes indicated by the brightness of the displayed dots. The difference is that the information obtained from that single line is constantly swept across the monitor so that the motion of the body part being investigated is displayed along the horizontal axis.6
To obtain good-quality images, the sonographer must know what type and size of transducer to use and how to use the available ultrasound controls. There are many transducers or probes from which to choose, and selection of the appropriate one depends on the location of structures to be imaged and the size of the patient.
Transducers are first classified as linear or sector, according to the arrangement (array) of the crystals and the shape of the imaging field produced on the monitor. In a linear transducer, the crystals are oriented in a straight line, producing a rectangular image in which both the near and far fields are wide. Linear transducers provide superior resolution of near-field structures and therefore are commonly used in equine reproduction and tendon examinations.5 However, their large footprint can limit their use in cardiac and abdominal studies, where it may be difficult to fit the probe between the ribs.
Sector transducers contain curvilinear arrays of crystals that produce a fan- or pie-shaped image with a narrow near field and wider far field, which is helpful in imaging deeper structures. These transducers have a rounded or convex curve to their surface and are considered common multipurpose transducers.5 They generally have a small footprint, which makes them useful for small animals and in performing cardiac examinations. Phased-array transducers have crystals that are stacked in a pyramid shape. This allows them to be small yet capable of producing a sector-shaped field.5
Sector transducers may be further classified by whether they sweep the ultrasound beam through the patient mechanically or electronically. Mechanical sector scanners work by oscillating or rotating one or more crystals in the transducer. These transducers are prone to wear from the moving parts, and because other transducers have become more economical, mechanical sector scanners are now less popular.5 Electronic sector scanners move the ultrasound beam through the patient by firing multiple crystals at precise times. These transducers come in a number of configurations depending on how the crystals are arranged.
To obtain the best resolution, it is recommended to use the highest-frequency transducer that will penetrate to the desired depth. In general, a 3.5-MHz probe will be needed in large dogs, a 5.0-MHz probe in medium dogs, and a 7.5- to 10-MHz probe in small dogs and cats, depending on the type of study being conducted.5
During the examination, the sonographer must know how to manipulate the controls on the ultrasound unit to obtain a useful image. Ultrasound machines come with a variety of controls to alter the image, but only a few major ones are discussed here.
The control that alters the intensity of the ultrasound beam generated from the transducer is often referred to as the power control. However, different manufactures may have slightly different names for the same control (e.g., intensity control, output control5), so it is important to know the specific term for each unit. The power control alters the amount of voltage that is delivered to the piezoelectric crystals and thus the intensity of the ultrasound beam and the returning echoes. To boost the signal of the echoes without creating unwanted artifacts, it is recommended to keep the power set as low as possible and instead adjust the amplification of the echoes. This can be done by adjusting the gain or time gain compensation control.
The gain control uniformly alters the brightness of all the echoes on the monitor regardless of their location. The time gain compensation control allows the sonographer to adjust the amplification of returning echoes at various depths. This control typically appears as a column of sliders or knobs. Each slider correlates to a particular section on the monitor, with the first slider altering the most superficial echoes and each successive slider controlling deeper or more distal echoes. Time gain compensation controls are usually arranged diagonally from the upper left corner to the bottom right with the goal of producing an image of uniform intensity. The first few sliders are pushed to the left to suppress strong echoes returning from the near field that could cause reverberation artifact; the remaining sliders are pushed progressively to the right to compensate for progressive attenuation of the ultrasound beam as it travels into deeper tissue.
A depth control allows the sonographer to control the depth of the image display. It may be necessary to change the depth control to place the structure of interest in the middle of the monitor to optimize its visualization. If the depth control is set too deep, the resulting image will be small and near the top of the screen. A shallow setting enlarges the structure of interest, possibly to the point where it is no longer seen on the monitor.
Artifacts are features of the ultrasound-generated image that do not truly represent the area being examined.7 It is important for the sonographer to be able to recognize common artifacts and understand how and why they occur so that, if necessary, they can be eliminated through adjustment of the imaging technique. Some artifacts can be helpful, aiding in the diagnostic potential of ultrasonography.
Acoustic shadowing occurs when the ultrasound beam encounters an area of gas or mineralization.4,7 The gas or mineralized structure inhibits passage of the beam, which either bounces back to the transducer or is absorbed. Because the ultrasound beam cannot penetrate this area, an anechoic shadow appears on the monitor distal to the area. Acoustic shadowing is often seen with urinary calculi or gas within the gastrointestinal tract. This artifact aids in the identification of calculi but also prevents examination of deeper structures.
Acoustic shadowing can sometimes be overcome by repositioning the patient or finding another acoustic window but cannot be corrected by increasing the power or gain.
Also called through transmission, acoustic enhancement is an area of increased echogenicity distal to structures with low attenuation of ultrasound waves.8 As noted above, ultrasound waves are attenuated as they pass through tissues and thus become weaker. Using the time gain compensation controls, the sonographer can adjust for this by increasing the strength of echoes returning from deeper areas. When the deeper area is distal to a structure that attenuates few ultrasound waves (e.g., fluid-filled structures such as the gallbladder and bladder), the corresponding area of the image becomes hyperechoic. This artifact is useful in differentiating a fluid-filled structure from an anechoic solid structure (e.g., a lymph node). If an anechoic structure does not produce acoustic enhancement, the sonographer should first check the time gain compensation control. If this control is set too low, acoustic enhancement can be masked.
Edge shadowing is an artifact that appears as an anechoic area extending distally from the lateral margins of round, fluid-filled structures like the bladder, gallbladder, and kidneys. It is created by the change in velocity and resulting refraction of the ultrasound beam as it passes through the fluid-filled structure.8 It cannot be corrected.
Side Lobe Artifact
Side lobe artifact occurs when echoes are produced from sound waves that are not traveling in the same direction as the primary ultrasound beam.9 These echoes are inaccurately placed in the image as if they came from the primary beam. They are usually associated with round structures that have highly reflective interfaces, such as the bladder and gallbladder, and they appear as faint echoes within these anechoic structures. Side lobe artifact can generally be eliminated by reducing the gain or adjusting the depth of the ultrasound beam.
Often seen in fluid-filled structures (e.g., bladder, gallbladder, cysts), slice thickness artifact is produced when part of the ultrasound beam lies outside of the structure being examined.7 This part of the ultrasound beam interacts with adjacent tissue and is erroneously displayed within the lumen of the anechoic structure. It will often take on the appearance of sediment or a mass and is often referred to as pseudosludge when associated with the gallbladder.10 However, slice thickness artifact can be differentiated from true sediment. True sediment has a flat surface and moves to the dependent side of the patient regardless of position, whereas slice thickness artifact has a curved surface and remains perpendicular to the ultrasound beam.10 The sonographer can try to adjust the transducer so that the entire beam is within the fluid-filled structure, eliminating the artifact. Using a higher-frequency transducer also reduces slice thickness artifact.
Reverberation occurs when the ultrasound beam bounces back and forth several times between a highly reflective tissue interface and the transducer. Each time the returning echo hits the transducer, the reflecting surface appears twice as deep on the screen. This results in an image of evenly spaced lines extending from the near field into the far field. Reverberations may be classified as external or internal depending on where they arise. External reverberation is associated with air between the skin and the transducer; internal reverberation is seen with bone, gas, or metallic objects inside the patient. External reverberation can often be corrected by ensuring that the patient's hair is closely clipped and by using ample acoustic coupling gel.
Mirror image artifact is often seen at rounded, highly reflective interfaces and leads to the erroneous placement of structures on the sonographic image. It is most commonly seen during abdominal ultrasonography when the ultrasound beam travels through the liver and gallbladder to the diaphragm.8 Some of the returning echoes are reflected back into the liver and gallbladder. However, the echoes that return from this second pass are assumed by the ultrasonography machine to have traveled in a straight line. Because they take longer to return, they are interpreted as being twice as deep as the original echoes. The result is that the liver and gallbladder appear on the image on both sides of the diaphragm. This image may be incorrectly interpreted as a diaphragmatic hernia if the sonographer is unaware of mirror image artifact. Scanning the patient at a different angle or through a different acoustic window may eliminate mirror image artifact and misinterpretation.
Veterinary technicians who have a good understanding of ultrasonography, including what ultrasound is, how it is produced, and how an image is generated, can more effectively educate clients and other members of the veterinary health care team and, ultimately, increase the quality of care. Technicians may also wish to expand on a basic knowledge of ultrasonography as part of their own education and training to increase their abilities and responsibilities. Just as ultrasonography is now conducted by technicians in human medicine, it is likely that this imaging technique will one day be a common skill for veterinary technicians, yielding more time for the veterinarian to interpret the images and treat the patient.