Chapter 3. Basic Principles of Ultrasonography

Emergency Physicians (EPs) have performed bedside ultrasound (US) for more than three decades. Today, US is ubiquitous in our specialty. Each year new physicians learn the skill, fresh evidence is brought to light supporting the practice, and novel indications are explored. Technological advances have delivered smaller machines with improved image quality that are less expensive than ever before. Several US manufacturers have developed machines targeted to Emergency Medicine (EM), taking into consideration our specific indications and less-than-forgiving work environment. US training is an important component of Emergency Medicine residency programs and a required core competency procedure of the Accreditation Council for Graduate Medical Education Residency Review Committee.1 There are numerous opportunities for supplementary training in emergency US, ranging from local courses to established fellowship training programs.

Safety considerations have also contributed to the acceptance of bedside US. Sonography is noninvasive, safe in pregnancy, and does not require contrast agents that risk kidney failure, subcutaneous extravasation, and allergic reactions.2 With increasing concern in the medical community over the long-term effects of ionizing radiation, US is recognized as an attractive alternative.3 Additionally, the U.S. Department of Health and Human Services' Agency for Healthcare Research and Quality has highlighted US guidance for central line insertion as 1 of their top 10 recommended practices.4

The intent of this chapter is to provide an introduction to bedside US for the EP. The physician-sonographer should have a general understanding of the physics underlying the properties of an US wave. “Knobology” is a colloquial term used to describe the study of the buttons, dials, switches, and, of course, knobs, on the console of an US machine. It is important that all users have a good sense of their machine's operational functions. Typical machines and transducers used in the ED will be described.

The Paradigm of Bedside US

Bedside US is a relatively new paradigm in clinical medicine. Traditional radiology US involves a break in the patient–physician encounter. The multistep process involves the EP evaluating the patient and determining that an US is warranted, the radiologic technologist performing the study in an US suite, the radiologist reviewing the images and generating a report, and the EP ultimately correlating that information back to the patient (Figure 3-1).

Bedside US crafts a direct and immediate relationship between patient and EP (Figure 3-1). The technology is placed in the EP's hand for both image acquisition and interpretation. The EP can immediately synthesize US findings with clinical and laboratory data to paint a more complete diagnostic picture. A patient's condition can change on a moment's notice in the ED, and, fittingly, US is a dynamic tool that can be used swiftly and serially throughout a patient's course.

In the early years of emergency US, EPs often used large tank-like machines that were not conducive to a fast-paced work environment. The evolution of US has delivered small portable machines that produce high-quality images. Many US devices are the size of laptop computers that fit on maneuverable carts. “Pocket-sized” machines, not much larger than mobile phones, are being introduced and offer even greater portability (Signostics, Palo Alto, CA; GE Corporation, Fairfield, CT). The notion of the “sono-stethoscope” promises devices as small, easy to use, and available as the stethoscope, but with the superior insight of sonography. This device speaks to a true paradigm shift in clinical medicine.

The primary indications of emergency US have traditionally included: cardiac US for the presence of pericardial fluid and for cardiac activity; abdominal US for the identification of free peritoneal fluid; aortic US for abdominal aneurysms; biliary US for the detection of gallstones and cholecystitis; renal US for hydronephrosis and nephrolithiasis; and pelvic US for the identification of an intrauterine pregnancy and the exclusion of an ectopic pregnancy (Table 3-1). Basic emergency US has operated on the binary premise that emergencies are to be included or excluded by this diagnostic tool. As the field has developed, the boundaries between primary and secondary indications have blurred, and the yes/no equation for the evaluation of emergencies has matured.

The list of indications for present-day emergency US is long and continually expanding (Table 3-1). Other common indications include the evaluation for deep vein thrombosis. Lung US can be used to diagnose pneumothorax, pleural fluid, and interstitial lung diseases. US evaluation for abscesses, primarily subcutaneous but also peritonsillar, deep-space, and abdominal is common. Ocular US can be used to identify a retinal detachment, lens dislocation, vitreous hemorrhage, foreign bodies, and optic nerve sheath assessment for increased intracranial pressure. Patients in shock or hypotensive may be evaluated by US of the inferior vena cava and internal jugular veins to estimate central venous pressure. Gastrointestinal US is used for the identification of appendicitis, pyloric stenosis, and other conditions. Male and female genitourinary US is used to assess for testicular and ovarian torsion, intrauterine pregnancy, and ectopic pregnancy. Musculoskeletal US is used to diagnose joint effusions, tendinopathy, and fractures. Procedural US is used for guided assistance of vascular access, paracentesis, thoracentesis, pericardiocentesis, arthrocentesis, lumbar puncture, foreign body removal, nerve blocks, and other procedures.

History

Bats and toothed whales have used sound for echolocation for millions of years. Dolphins produce a series of clicks that pass through the lipid-rich melon on their heads, an acoustical lens of sorts that focuses the sound waves into a beam. Returning echoes are processed to determine the location of objects for navigational and hunting purposes.5 Sonar (acronym for sound navigation and ranging) is a maritime technique that uses sound waves for identifying oceanic objects. It was initially introduced in response to the sinking of the Titanic in 1912.6 It was further developed and employed in World War I for submarine detection.6 In the early 1950s, a radiologist named Douglas Howry and a team of other physicians introduced the first diagnostic US machine using a water-bath immersion tank. In the 1960s, direct contact (probe to patient) scanners were developed, and the contemporary saga of US as a viable diagnostic modality commenced.7

Sound

Sound is a variation in pressure traveling through a medium and it is described as a wave. We commonly understand the sound of our vocal communication to travel through air, but sound may also travel through fluid and solid structures. Pressure variations produced by sound waves mechanically displace or oscillate the particles of the medium. This oscillation produces cycles of higher and lower densities, or compression and rarefaction, respectively. Sound waves differ from water waves in that they are longitudinal, meaning the cycles of compression and rarefaction travel in the same direction as the wave. Imagine an analogous spring coil with a force exerted into its length. A wave, or region of compression, will pass down through the coil in a longitudinal or parallel fashion (Figure 3-2). It is important to have an understanding of the terms that describe sound waves as described in the following sections.

Frequency and Period

Frequency is the number of cycles of pressure variation per 1 second. A single cycle begins at a baseline of absent sound, increases to a maximum value (compression), decreases to a minimum value (rarefaction), and returns to baseline (Figure 3-2). Frequency is measured in hertz with a unit of cycles per second and corresponds to the pitch or tone of a sound. The upper acoustical frequency of human hearing is approximately 20,000 Hertz (Hz), beyond which is considered “US.” Diagnostic US frequencies are generally in the range of 1 to 10 million Hz or MHz. US transducers operate on either the “low” or “high” end of this frequency spectrum, although for each transducer frequency can be adjusted within a limited range. Frequency is a principal determinant of the resolution and penetration of an image.

Period is the time required for one cycle to occur. Thus, it is the inverse of frequency (Figure 3-3). In ultrasonography, period is typically measured in microseconds (μs).

Wavelength and Propagation Speed

While period measures the duration in time of a single cycle, wavelength measures the distance in space of a single cycle (Figure 3-3). Wavelength is related to the propagation speed and the frequency of a wave. It is represented by the following equation: wavelength = propagation speed/frequency.

Propagation speed is the velocity at which sound travels through tissue. An average speed through soft tissues of 1540 m/s is generally assumed in basic US. Newer US machines incorporate the propagation speeds of different tissues to provide enhanced imaging.

Amplitude, Output, and Bioeffects

Amplitude is a measure of the height or maximum variation of a wave from baseline (Figure 3-3). While frequency corresponds to the pitch or tone of a sound, amplitude is the “loudness” or volume of a sound. In US, this correlates to the brightness of the image. Adjustments of the machine's output affect the amplitude, but changing the gain produces a similar effect on received echoes. It is preferable to alter brightness on the back end with gain, rather than subject the patient to increased output. While diagnostic US has proven to be exceedingly safe, concerns do exist over possible thermal and mechanical adverse bioeffects.8 EPs should strive to perform studies in the shortest time frame and at the lowest output possible, in line with the safety acronym ALARA (as low as reasonably achievable).

Pulsed US

The earliest US machines produced a continuous stream of US waves. Today's machines release pulses or packets of waves, that is, a few cycles of US at a time. The repetitive pulses are separated by gaps of no sound. Machines can generate pulses of varying duration, frequency, and fraction of time with respect to the soundless gaps. Pulsed US has been essential in the development of advanced imaging.

Axial Resolution

Axial resolution refers to the ability of the US machine to distinguish two separate structures that lie on top of one another and in a parallel plane to the US beam (Figure 3-4). As viewed on the screen, this is one structure on top of another. The structures must produce two separate echoes for each to be recognized as distinct. The smaller the wavelength of the transmitted US beam, the closer in position can be the two tiny distinguishable structures. The size of the wavelength can be decreased by increasing the frequency of the US beam. Increasing the frequency delivers greater axial resolution. But a sacrifice is seen in penetration because of increased attenuation at higher frequencies.

Lateral Resolution and Focus

Lateral resolution refers to the machine's ability to distinguish two separate structures that lie side-by-side in a plane perpendicular to the US beam (Figure 3-5). As seen on the screen, this is one structure to the side of another. Lateral resolution is a function of the beam width. The narrower the beam, the greater is the ability to produce separate echoes for two adjacent objects. Focusing narrows an US beam at specific depths. The focal zone is the narrowest part of the hourglass-shaped US beam. Focusing can be achieved automatically by the machine or manually by the operator. One or multiple focal zones can be set at specific depths to narrow the beam for optimal lateral resolution.

Temporal Resolution

Continuous US scanning is actually a collection of still frames displayed rapidly over time. US machines produce numerous frames per second as the scan beam is transmitted over and over again through the tissue. The number of frames per second is known as the frame rate. The greater the frame rate, the better the temporal resolution, and the smoother the moving image appears. High frame rates are particularly important for scanning moving structures such as the heart. Employing additional functions such as Doppler scanning may limit the machine's ability to produce high frame rates.

Echoes

Diagnostic US is predicated on sound waves not only transmitting through tissues but also reflecting back to the transducer (Figure 3-6). At an interface of two different tissues, the proportion of transmission and reflection is determined by the acoustic impedance of the tissues. Acoustic impedance is a measure of a tissue's resistance to sound penetration (density × propagation speed). Acoustic impedance is high in “hard” tissues such as bone, lower in visceral organs, and negligible in fluid. Reflection of an echo is a function of the difference between the acoustic impedance of two adjacent tissues. For instance, when a sound wave travels from soft tissue to bone, a significant proportion of the wave will reflect back to the probe, and a minimal amount will transmit through for imaging of deeper structures.

When an US beam travels through parenchymal tissue without interfaces (of homogenous acoustic impedance), such as the liver, imaging is generated not by reflection but rather by scatter. Scattering occurs when a sound wave encounters particles smaller than its wavelength or objects with rough and irregular surfaces. An analogous effect is seen when light is shone through fog. There is some transmission and reflection of the light, but it is the scattering of light that reveals the mass of fog.

In order for an US machine to generate an image, it must determine not only the intensity of the returning echo but also the location of the reflecting structure (aka reflector). With a given propagation speed (velocity of sound through tissue), the machine uses the travel time of the sound wave to calculate the distance the reflector sits from the transducer. The longer the travel time, the deeper is the reflector or structure.

Attenuation

Sound weakens or attenuates as it propagates through a medium (Figure 3-6). Attenuation is a result of absorption, reflection, refraction, and scattering of the US wave. Absorption is the conversion of sound to heat as the wave passes through tissue and accounts for the vast majority of attenuation. Absorption has very limited clinical effect for diagnostic US but is the basis of therapeutic US. Reflection is the “echoing” of the wave back to the transducer. Refraction and scattering are the redirection of waves on encountering certain interfaces.

There are many types of transducers (also called probes) that vary in size, shape, construction, transmitted frequency, and function (Table 3-2). Transducers can be expensive and are at risk for damage in a busy ED. Probes are often dropped and cords frequently run over by the wheels of the US machine. Creative solutions to prevent damage to dangling cords have been developed (Figure 3-7), but the expense of replacing transducers should be factored into any warranty and maintenance considerations. Many US manufacturers offer multiport transducer connections that allow a number of probes to be connected at once, each activated by the push of a button. Limiting the connecting and disconnecting of transducers decreases the chance of damaging fragile connector pins.

Transducers are the link between the US machine and the patient. The essential component of the transducer for generating an image is the piezoelectric element. The piezoelectric principle states that when an electrical voltage is exerted on certain materials, a mechanical pressure or vibration will be produced. A transducer's piezoelectric elements, or crystals, when vibrated by an electrical voltage will generate a mechanical sound wave. The returning sound wave in turn vibrates the elements creating an electrical voltage that carries image data back to the US machine.

Automatic Scanning

Transducers are constructed of numerous elements arranged along the width of the probe. Unlike a flashlight that emits a single continuous beam of light, the US beam is a composite of pulsed firings of the elements. Automatic scanning is the electronic activation of the elements or arrays of crystals to generate a beam for a cross-sectional image. Two types of automatic scanning currently employed by transducers are sequenced array and phased array scanning.

Sequenced Array

Sequenced array scanning involves the sequential firing of groups of elements across the transducer assembly (Figure 3-8). One scan line after another is generated along the width of the probe. Linear transducers are flat-topped and produce a rectangular beam made up of parallel scan lines. Curved transducers transmit scan lines similarly, but following the curve of the probe and create a sector or pie-shaped image. The elements are fired rapidly and multiple frames of cross-sectional images are produced per second.

Phased Array

Phased array transducers generally have a narrow flat-topped footprint. The tightly packed elements are electrically activated as a single unit, but with a slight time lag between each element resulting in an angling of the pulse direction. The electronic activation and angling of each subsequent pulse is slightly changed, and the resultant composite beam is a sector or pie-shaped (Figure 3-8).

Transducer Types

The typical arsenal of transducers for emergency US includes a low-frequency curvilinear or phased array probe, a high-frequency linear probe, and an endocavitary probe (Table 3-2). Numerous other types of transducers are manufactured. It is worthwhile evaluating the options available before purchasing a transducer.

Convex Transducers

Convex or curved transducers are low-frequency probes that produce a sector-shaped image by sequenced array scanning. These are commonly used in EM, as they are useful for most US examinations of the torso. The resolution of these probes is inferior to that of linear transducers, but the greater penetration allows for imaging of relatively deep structures such as the aorta, gallbladder, liver, kidneys, and heart. Microconvex probes have a similar but smaller footprint with a tight curvature that allows for easier use between adjacent ribs.

Phased Array Transducers

Phased array transducers are low-frequency probes made for echocardiography and general imaging of the torso. These probes have a small flat footprint ideal for maneuvering between ribs and imaging the heart.

Linear Transducers

Linear transducers are high-frequency probes that are ideal for imaging superficial structures. They are generally wide and flat-topped, and produce a rectangular image by sequenced array scanning. These transducers are useful in the ED for imaging of blood vessels and guided line placement, skin abscesses, musculoskeletal pathology, pneumothoraces, ocular and testicular pathology, and a host of other superficial parts.

Endocavitary Transducers

Endocavitary transducers are high-frequency probes that have elements arranged in a tight curve at the end of a long handle. They are designed primarily for vaginal insertion and high-resolution imaging of the female reproductive organs. These probes can be used for US-guided vascular access if no other high-frequency probe is available. The small footprint and room afforded by the handle may even prove superior to a linear probe in some instances. The probe can be used in a patient's mouth for the evaluation of oral and pharyngeal pathology, such as a peritonsillar abscess.

Frequency and Tissue Harmonic Imaging

Transducers may differ in inherent frequency. The frequency of each probe can be adjusted within a narrow range of generally a few megahertz. If greater depth is required to view pertinent structures, lowering the frequency may be beneficial. Conversely, if finer resolution is desired more than deep penetration, increasing the frequency may improve visualization.

Tissue harmonic imaging is another means of enhancing US scanning. US echoes return from tissues in multiples of the transmitted frequency. The machine can filter out the transmitted frequency and focus on receiving the second harmonic frequency (twice the transmitted frequency). The returning harmonic beam is narrower than the beam of the fundamental frequency, and, hence, lateral resolution is improved. Artifacts and distortions are also reduced with harmonic imaging.

Coupling Medium

Acoustic gel is the coupling medium used in diagnostic US. The gel obviates any air between the probe and the patient to improve US beam transmission. Apply the gel across the footprint of the probe or onto the patient's skin. A good rule of thumb is that too much gel is preferable to too little.

An US machine is made up of numerous components (Figure 3-9). The imaging system hardware consists of a beam former, signal processor, and image processor. The console allows the user to interface with and manage the imaging. The monitor screen provides for black and white as well as color viewing of the imaging. Transducers, single or multiple, have connecting ports onto the machine. Archiving tools, such as a printer or digital storage media, are available standard or as options. The following sections review the key functions that an EP should know when performing US examinations.

The Console

The US console generally consists of a keyboard for data entry and numerous knobs, buttons, dials, and toggle switches for manipulating the images. Carefully read the user's manual or undergo a detailed operations briefing by the manufacturer's application specialist after purchasing a new machine. While there are many functions that are universal to US machines, each machine has its proprietary functions.

Gain

Gain is the US analog to the volume control on a radio. It alters not the transmitted beam, but the returning echo to amplify intensity. Gain is measured in decibels and relates to the brightness of the image. Machines generally have a knob that can be dialed to adjust the gain, increasing or decreasing the overall brightness of the image (Figure 3-10).

Time gain compensation (TGC) allows for adjustments of gain at specific depths. Most TGC controls consist of a collection of sliding knobs arranged in a column (Figure 3-11). The knobs at the top of the column correspond to the near field and those at the bottom correspond to the far field. As an US beam penetrates to greater depths, the intensity of the beam is attenuated or dampened. TGC allows compensation for attenuation. For general abdominal imaging, arrange the sliding TGC knobs with a slope of increasing gain at greater depths. Use TGC to increase or decrease brightness at discreet depths of the image. To compensate for the artifactual brightness seen deep to a fluid-filled structure, decreasing the gain at that depth can improve visualization.

Depth

The depth of the field of view can be adjusted by the turn of a dial or toggle of a switch. Depth is conventionally measured in centimeters. Hash marks along the side of the image denote units of distance. Depth adjustments alter the penetration of the imaging beam and allow the user to appropriately “magnify” the organ or region of interest (Figure 3-12). If the depth is set too shallow (“overmagnification”), the organ of interest will appear too large and its deepest portion may be cut off the screen. If the depth is set too deep (“undermagnification”), the organ of interest will appear too small in the near field with wasted space in the deeper regions.

Patient Data Entry

All machines allow the entry of demographic information about the patient and the US examination. The patient data entry screen presents numerous fields that can be filled by typing on the keyboard. These fields commonly include patient name, medical record number, date of birth, and a comments section to name a few. Functions also often available on this screen are exam and transducer selection.

Freeze and Cine

Most machines allow both still images and video loops to be saved. Hitting the “freeze” button stops real-time continuous scanning. The most recent image is displayed on the screen with a few seconds worth of images (known as cine) available for review. Use the rollerball or trackpad to rewind through the cine memory of frames. This allows the viewing and subsequent saving of the most desired still image. A number of machines can allow the viewing of a loop of images as a video.

Miscellaneous Functions

There are numerous additional functions on the console. Magnify images with the “zoom” function. The “measure” button activates calipers that are moved with the trackpad or rollerball. This allows for precise measurements to the millimeter. Adjust the dynamic range to produce effects similar to modifying the contrast of a photograph. Other functions include gray-scale mapping, edge, and persistence for image alteration. A dual screen function displays two side-by-side imaging fields. Function keys can be set for easy access to commonly performed tasks.

Dimensionality

Imaging with US is akin to shining a flashlight in the dark. While a flashlight's beam is conical, the US beam is flat, and, hence, the image displayed on the screen is a two-dimensional slice. Sound transmits through most tissues to allow for visualization of deep structures in the cross-sectional slice. Scanning the beam back and forth stacks multiple two-dimensional slices in a movie format and provides a sense of spatial orientation. Rotating the probe 90° and scanning through the perpendicular plane helps to gain a three-dimensional comprehension of structures.

Orientation

Orientation is entirely dependent on how the transducer is placed on the patient. Each transducer has a marker (usually a bump, ridge, or indentation) that correlates with an indicator on the screen to establish orientation. If the transducer is held with the marker aimed cephalad, the screen indicator is located on the left side of the screen (Figure 3-13). This results in the left side of the image representing the cephalad aspect and the right side the caudal aspect (Figure 3-13). For conventional radiology imaging, the screen indicator is located on the left side. Most emergency US examinations require views with the transducer marker aimed to the patient's head and the patient's right. The image is displayed with the near field (closest to the probe) at the top of the screen and the far field (farthest from the probe) at the bottom of the screen.

Echogenicity

Fluid is anechoic, meaning US transmits through fluid without any reflected echoes. The echogenic silence is processed to generate black pixels on the screen. Highly reflective structures, such as the diaphragm and pericardium, are termed echogenic or hyperechoic. These structures will produce white imaging. Two structures of the same echo-texture are isoechoic. Less reflective tissue is hypoechoic and appears darker on the screen when comparing two tissues (Figure 3-14).

Artifacts

Imaging artifacts are frequently encountered in ultrasonography. They are erroneous representations of the anatomy in the reflected echo. It is important to understand and expect certain artifacts to prevent misinterpretation of the images. Artifacts may also help to appreciate certain structures that would otherwise be less obvious.

Shadowing

A highly reflective object allows very little of an US beam to transmit through it. Most of the beam is reflected back to the transducer and the structure is represented as hyperechoic or white on the screen. The US machine interprets the lack of returning echoes deep to a structure that is hyperechoic as the lack of reflection, similar to that seen with fluid. This is seen as a black shadow on the screen (Figure 3-15). While shadowing from ribs may obscure deep anatomy, the shadows of gallstones aid in their identification.

Reverberation

Highly reflective interfaces may result in multiple reflections. Bouncing of the sound beam between the reflector and the transducer can create false echoes known as reverberation (Figure 3-16). It can be seen when the sound beam encounters two closely spaced interfaces, such as the two walls of a needle or the visceral and parietal pleura. The echo reflects back and forth between the interfaces, but with some transmission of the beam back to the probe each time. These returning echoes generate artifactual reflections on the screen termed reverberation or comet tails (Figure 3-17).

Mirror Image

Highly reflective tissues can cause mirror-image artifacts. Structures seen on the near side of a bright reflector are displayed on the other side of the reflector as well. This is common with the diaphragm. Triangulation of the beam path delays the return time of the echo to mirror a second reflector (Figure 3-18). Mirror-image artifact can help to exclude a hemothorax by visualizing the liver or spleen cephalad to the diaphragm (Figure 3-19).

Enhancement

Attenuation does not occur when a sound beam passes through a fluid-filled structure. The beam's high intensity is maintained through the fluid. A strong echo is generated off the posterior wall of the fluid-filled structure. The artifact seen on the screen is a very hyperechoic region deep to the fluid-filled structure that may obscure the actual anatomy (Figure 3-20).

Miscellaneous Artifacts

A number of additional artifacts can interfere with imaging. Side lobes are weak beams emitted lateral to the central axis of the scanning beam that may produce false echoes. Section thickness artifact is a result of interfering information from the outside of the flat slice of the scanning beam. Refraction of the beam tangentially off an interface may produce a double image of a structure lateral to the original. Shadowing in the narrow angle of refraction may be seen deep to the interface.

Brightness Mode

Brightness mode is commonly referred to as B-Mode. This is the basic scanning mode for US. It displays the standard two-dimensional gray-scale image.

Motion Mode

Motion mode or M-Mode is used to assess moving structures. Activating M-Mode produces a vertical line on the image (Figure 3-21). The line can be moved left or right with the rollerball or trackpad. The US beam penetrating the tissue along that single line is displayed in a continuous graphical manner on the bottom of the screen (Figure 3-21). The x-axis of the display is depth and the y-axis is time. If the line is set on immobile tissue, there will be no variation in the M-Mode display. Movement along the line, such as a beating heart, allows measurements and rate determinations (Figure 3-21).

Doppler Mode

Doppler scanning allows the assessment of the presence, direction, speed, and character of blood flow and tissue movement. Doppler takes full advantage of the real-time dynamic nature of US. If an object is moving toward or away from the transducer, the frequency of the reflecting echo will be higher or lower, respectively, than the transmitted frequency. The Doppler shift is the difference between the transmitted and reflected frequencies. The US machine can calculate velocity using the Doppler shift. Turbulence, or the variance in velocity of multiple objects in flow, can be assessed. The complexities of Doppler physics and operations are beyond the scope of this chapter.

Emergency Physicians can utilize Doppler in the assessment of deep venous thromboses, testicular or ovarian torsion, compromised vascular flow in the extremities, inflamed or hypervascular tissues, to differentiate vessels from nonvascular structures for diagnostic and procedural purposes, and for advanced echocardiography.

Color Doppler Mode

A “C” button on the machine's console usually signifies color Doppler mode. Activating color Doppler produces a box, or region of interest (ROI), overlying the image. The ROI can be maneuvered over the desired area. The size of the box can be manipulated to be larger or smaller depending on the study being performed. Within the ROI, flow will be displayed in a color schematic (Figure 3-22). Flow toward the probe is typically red. Flow directed away from the probe is typically blue. Gradients of red and blue signify the speed of the flow.

Pulsed Wave Doppler Mode

Pulsed wave Doppler provides a quantitative assessment of flow velocities. Activating pulsed wave Doppler splits the screen, typically placing the image at the top of the screen and a dynamic graph of flow velocities at the bottom (Figure 3-23). A small “gate” can be positioned over a specific vascular area to measure flow. The size of the gate can be adjusted, allowing very narrow regions to be accurately assessed. The dynamic Doppler graph displays time on the x-axis and velocity on the y-axis. Movement toward the probe is depicted as a positive velocity deflection. Movement away from the probe is depicted as a negative deflection.

Power Doppler Mode

Power Doppler is another method of displaying blood flow or tissue motion (Figure 3-24). It utilizes the same ROI box as color Doppler, but with only one shade of color. Power Doppler is very sensitive in identifying the presence of flow but does not reveal direction. It is advantageous in identifying low flow states, subtle tissue motion, and imaging small or deep vessels.

There are numerous ways to archive US images. Still images can be sent to printers for hard copy archiving. Thermal printers are relatively small and can be attached to the US cart. The gray-scale printouts are fine reproductions of the image, but are at risk for degradation over time.

Digital archiving allows users to save both still images and videos with a far reduced need for physical storage space. Examinations can be saved to hard drives built into the US machine or to peripheral devices. Machines today offer many options for peripheral storage including compact discs, digital video discs, external drives with universal serial bus connections, and magneto-optical disks.

Specific formats for digital archiving of medical imaging have been developed, in particular the digital information and communication in medicine (DICOM) format. Most US machines today have the ability to save images in the DICOM file format. Picture archiving and communication system (PACS) has been developed for viewing medical imaging in DICOM and other formats. These systems are employed by radiology departments for viewing US images, as well as computed tomography, magnetic resonance imaging, plain radiographs, and other imaging modalities. Advantages to saving DICOM images on PACS include improved organization, reliability, and the ability to transmit remotely.

Ultrasound is fast becoming a commonly used modality in EDs. All EPs should become familiar with US as it will soon become the standard of care for patient evaluation, patient management, and procedural guidance. Take the time to investigate the various options available for machines, probes, and accessories. A thorough understanding of the functioning and features of an US machine is essential before making clinical decisions. Training is readily available through US fellowships, CME courses, and ultrasound manufacturers.