A brief summary of relevant terminology is presented in ( Box e3.1 ). Readers seeking more in-depth reviews of US physics, machine controls, US modes, and instrumentation are encouraged to visit the Sonoguide website ( https://www.acep.org/sonoguide/basic/ultrasound-physics-and-technical-facts-for-the-beginner/ ). Other relevant lectures on this topic can be accessed through the Academy of Emergency Ultrasound (AEUS) Vimeo Channel ( https://vimeo.com/aeus ) or website ( https://www.saem.org/about-saem/academies-interest-groups-affiliates2/aeus/education/aeus-narrated-lecture-series ), as well as various sites noted at the end of this chapter.

In general, image resolution is inversely proportional to penetration and the emergency clinician should choose the transducer with the highest resolution for the depth needed to obtain appropriate images. There are 4 basic transducer types, and selection criteria for each is listed in ( Table e3.1 ).

US biosafety includes use of the ALARA ( a s l ow a s r easonably a chievable) principle, appropriate Doppler usage, and appropriate microbiologic disinfection of the US transducer and system. Following the ALARA principle, emergency clinicians should perform EUS only when indicated and limit the time of sonographic investigation. Doppler modes should be minimized over sensitive tissue, including early gestation and germinal, mucosal, ocular, and neural tissues. All transducers should be cleaned according to the various types of examinations and the indicated disinfection type, either low-level or high-level. Surface transducers should be cleaned at the bedside with mechanical removal of gel and debris, followed by low-level disinfection with an appropriate spray or wipe. Endocavitary transducers require high-level disinfection, a more prolonged and substantial cleaning method, because they come into contact with mucous membranes. Safety also includes the use of appropriate barriers over transducers, nonsterile and sterile, as well as sterile gel or clean water.

The FAST examination technique uses a low-frequency broadband transducer (2–6 MHz) to evaluate dependent peritoneal spaces, pleural spaces, and the pericardium for free fluid, which in the trauma patient is presumed to be blood. There are four main components of the basic FAST examination: (1) the right upper quadrant (RUQ) view, (2) the left upper quadrant (LUQ) view, (3) the pelvic view and (4) the cardiac view ( Fig. e3.1 ). The E-FAST includes anterior chest views to evaluate for pneumothorax. The RUQ view evaluates for fluid in the thorax (above the diaphragm) ( Video e3.1 ), hepatorenal space (Morison pouch) and the paracolic gutter (inferior edge of the liver and right kidney) ( Video e3.2 ), moving cephalad to caudad. The LUQ view, found slightly more superior and posterior than the RUQ, should mimic the RUQ views, but also include the subdiaphragmatic space, because free intraperitoneal fluid tends to accumulate here initially. The pelvis should be evaluated in the transverse and longitudinal planes, where fluid may be detected deep to the uterus (in females) or in the retrovesical space (in males) ( Video e3.3 ). The cardiac evaluation can be performed in either the subcostal (or subxiphoid) or parasternal window ( Video e3.4 ). Evaluation for pneumothorax uses a low or high-frequency transducer at a shallow depth, placed along the anterior chest wall and will be discussed in more detail in subsequent sections.

Negative FAST images. (A) Subxiphoid view with no pericardial effusion. (B) Negative RUQ view showing the thorax and hepatorenal space without the paracolic gutter visualized. There is mirroring and loss of the spine, indicating a negative thorax. (C) Negative paracolic gutter area by the liver tip in the RUQ view. (D) LUQ view showing negative thorax, as in B, and splenorenal space. (E) Negative transverse bladder view. (F) Negative sagittal bladder view.

Typically, free fluid is anechoic, but it can have echogenicity if active extravasation, a blood clot, or bowel contents are present within the fluid. Compared with other fluid-filled structures in the abdomen and pelvis, peritoneal free fluid generally has sharp pointed edges and an irregular shape, whereas most visceral or vascular structures have intrinsically smooth oval or round contours. The volume of fluid required for a positive US study depends on the site of injury, sonographic window, and experience of the operator, but 250 mL or more is generally visible, and nearly 600 mL of fluid is required for a positive upper quadrant window. With pericardial fluid, once a certain volume is reached, the pressure in the pericardial space increases dramatically, resulting in cardiac tamponade. Generally, at least 50 mL of fluid is required to cause hemodynamic compromise in a patient without prior pericardial inflammation ( Fig. e3.2 ).

Positive FAST images. (A) Positive RUQ view showing a thin stripe of fluid in the hepatorenal space. The thorax is negative and paracolic gutter not visualized. (B) Positive LUQ view showing a hemothorax and splenorenal space. There is loss of mirroring and continuation of the spine. (C) Subxiphoid view showing a pericardial effusion circumferentially. (D) Positive transverse bladder view with free fluid noted posterior to the bladder on the right of the image.

In obstetric patients, abruption and fetal viability may necessitate an earlier operative course. FAST is unreliable for the detection of hemoperitoneum in patients with pelvic fractures. The detection of free fluid in an unstable patient with a pelvic fracture may be due to uroperitoneum from bladder injury rather than hemoperitoneum from vascular injury, clouding the decision for laparotomy versus pelvic embolization. In addition, retroperitoneal injuries to the genitourinary tract are not reliably assessed with the FAST examination. The FAST is further discussed in the “Pediatric Emergency Ultrasound” section.

The examination is performed with a low-frequency curved linear array or phased array transducer. Subcostal and intercostal windows will facilitate visualization of the gallbladder (GB), which should be evaluated in two orthogonal (perpendicular) planes ( Fig. e3.3 ). Visualization and measurement of the common bile duct (CBD) remain part of the examination and should be performed.

Normal sagittal view of the gallbladder

The diagnosis of cholelithiasis is made by identification of echogenic foci within the gallbladder lumen with associated shadowing. Other image patterns include stones with indistinct shadow, sludge, and the wall-echo-shadow (WES) sign seen in a gallbladder full of gallstones ( Video e3.5 ). Although many sonographic findings can be seen with AC, including gallstones, dilated gallbladder, increased gallbladder wall thickness (>3 mm), sonographic Murphy sign, pericholecystic fluid, and CBD dilatation, gallstones are present in 95% to 99% of AC cases ( Fig. e3.4 ). A nonmobile stone in the gallbladder neck, confirmed in the left lateral decubitus position, is highly suggestive of eventual cholecystitis. A CBD larger than 6 mm in people younger than 60 years and larger than 10 mm in older patients may indicate choledocholithiasis.

Sagittal gallbladder with signs of acute cholecystitis including stones with posterior shadowing, thickening of the wall anteriorly, and pericholecystic fluid seen within the wall of the gallbladder.

Renal US includes orthogonal views of the kidneys, with an emphasis on visualization of the renal calyces/pelvis. The sonographic windows for the two kidneys are similar to those used in the trauma upper quadrant views. The bladder view is performed from the suprapubic window in transverse and sagittal planes. Ureteral jets can be assessed by placing color Doppler over the trigone of the bladder in the transverse view. Bladder volume calculations ( Fig. e3.5 ) may be performed with on-machine calculators or by using the formula:

Bladder volume measurements.

Hydronephrosis is characterized by dilation and anechoic fluid accumulation within the renal pelvis and calyces, ranging from mild to severe ( Fig. e3.6 ). Renal and/or ureteral calculi may be identified as echogenic foci with associated shadowing and are usually located within the kidney (nonobstructive) or in the renal pelvis, proximal ureter, or uretero-vesicular junction. Color Doppler placed over the kidney can help differentiate mild hydronephrosis from the renal vasculature, as well as possibly accentuate any renal stones by producing the renal twinkle artifact.

Hydronephrosis with dilation of the calyces and renal pelvis of the kidney showing thinning of the cortex, signifying severe hydronephrosis.

Using a low-frequency transducer (curvilinear or phased array) and significant pressure to displace the overlying bowel gas, the aorta should be visualized from the subxiphoid region to the umbilicus (bifurcation) in both transverse and longitudinal planes. The transverse view should be obtained first to avoid the cylinder tangent error, which could falsely underestimate the size of the aorta ( Fig. e3.7 and Video e3.6 ). Cross-sectional measurements should be taken of the aorta, outer wall to outer wall, including any mural thrombus that might be present. Aortic dissection may be detected by a combination of abdominal and cardiac scanning, with the addition of a suprasternal notch view to the traditional cardiac windows.

Normal transverse view of the aorta.

An aortic diameter greater than 3 cm constitutes an abdominal aortic aneurysm, but risk of rupture increases with size and is rare with aneurysms smaller than 4.5 cm ( Fig. e3.8 and Video e3.7 ). A linear echogenic flap, anywhere across the lumen of the aorta, is suggestive of aortic dissection and may be associated with a different Doppler flow pattern on either side of the flap ( Fig. e3.9 and Video e3.8 ). The cardiac US examination may demonstrate an unexplained pericardial effusion, a dilated aortic root (>4 cm), aortic insufficiency, and/or a linear echogenic flap in the descending aorta.

Transverse view of a AAA with mural thrombus.

Aortic dissection flap visualized on longitudinal view of the aorta.

Cardiac US is performed through the transthoracic and transabdominal windows with the use of small curvilinear or phased array transducers. Typical views include the subcostal four-chamber view (subxiphoid), parasternal long-axis view ( Fig. e3.10 ; see Video e3.4 ), parasternal short-axis view, and apical four-chamber view. The subcostal four-chamber view, as in the FAST, is ideal for assessment of pericardial effusion and useful during cardiac arrest because it does not interfere with chest compressions. The long-axis subcostal view highlights the inferior vena cava (IVC) and can indicate volume status. The parasternal views are excellent windows for LV assessment. The apical four-chamber view is ideal for comparison of RV and LV sizes and function. Several US protocols have been developed to evaluate undifferentiated hypotension and can be used to narrow the differential diagnosis.

Normal parasternal long-axis view of the heart.

Pericardial fluid is typically anechoic, although it can contain internal echoes in cases of pericardial hemorrhage or infection. Large pericardial effusions are usually circumferential but can be loculated. As a result, assessment for pericardial effusion should include multiple views, when feasible, to confirm diagnosis and to avoid mistaking the epicardial fat pad for a pericardial effusion. Although cardiac tamponade is a clinical diagnosis, there are several suggestive echocardiographic features, including diastolic collapse of the RV ( Video e3.9 ), loss of respiratory variation of the IVC ( Fig. e3.11 ), and transvalvular flow velocity paradoxus.

Plethoric IVC showing where to take a measurement.

Assessment for global LV systolic function can be performed with visual estimation ( Video e3.10 ) and/or assessment of E-point septal separation (EPSS). EPSS is the distance between the anterior mitral valve leaflet and the ventricular septum measured using M-mode. A distance greater than 7 mm is abnormal, with larger measurements correlating to worsening systolic function. Emergency clinicians should recognize that accurate visual estimation of global LV systolic function requires experience and may prefer to categorize systolic function dichotomously as depressed or normal.

RV assessment is another useful tool for the emergency clinician when pulmonary embolism (PE) is high on the differential. With increasing right heart pressure, the RV dilates, squeezes poorly, and ultimately develops flattening of the interventricular septum, creating the “D” sign ( Figs. e3.12, e3.13 , and Video e3.11 ). The sparing of the RV apex is called the McConnell sign and is highly suggestive of PE. ^,

Apical 4-chamber view of the heart with an enlarged RV.

Parasternal short axis view with “D” sign signifying increased RV pressure.

Thoracic US is often performed with a low-frequency curvilinear array or phased array transducer, although visualization of lung sliding may be enhanced, if necessary, by the use of a high-frequency linear array transducer. The original BLUE (Bedside Lung Ultrasound in Emergency) protocol evaluated 4 areas on each hemithorax, but subsequent studies have looked at a number of protocols and additional areas of the anterior, lateral, and posterior thorax. Lung sliding, a normal finding, is identified as the visceral and parietal pleura gliding against each other during normal respiration. A lines, horizontal equally spaced echogenic artifacts deep to the pleural line, are also a normal finding ( Fig. e3.14 ).

A lines signifying normal lung.

The visualization of lung sliding excludes the presence of a pneumothorax at that location on the patient’s chest wall. Although M-mode and color Doppler techniques have been described as adjuncts to the evaluation of patients with suspected pneumothorax, neither is a necessary component of the examination. Absent lung sliding can result from a variety of causes in addition to pneumothorax, including pleural adhesions or consolidations, blebs, pleurodesis, partial or complete pneumonectomy, and contralateral mainstem bronchus intubation ( Fig. e3.15 ). A lung point sign is identified at the border of the pneumothorax, where the image shows absent lung sliding until the lung moves into the interspace with respiration ( Video e3.12 ).

M mode images of the lung. (A) Normal lung with “seashore” sign. (B) Pneumothorax with “stratosphere” or “barcode” sign.

Once the pleural line has been evaluated for sliding, then the determination of intraparenchymal fluid should be made. A lines indicate a dry lung, whereas B lines indicate the presence of fluid within the lung. B lines are vertical hyperechoic reverberation artifacts that arise from the pleura, move with respiration, extend off the screen without fading, and erase the normal A line pattern ( Fig. e3.16 ). Normally found in small numbers in the dependent areas of the lung (atelectasis), the widespread distribution of B lines, typically 3 or more in one lung window, indicates increased interstitial and/or alveolar thickening due to fluid accumulation (edema) or scarring (fibrosis). ^, As the lung accumulates fluid with consolidation, such as a lobar pneumonia, it can appear echogenic, so-called liver-like (hepatization). When diagnosing pneumonia with lung US, the consolidation needs to be in contact with the pleura to be visible within an intercostal window. Other signs can be seen with pneumonia, but dynamic air bronchograms, hyperechoic areas within bronchi that move with respiration, usually within the consolidated lung, are highly specific for alveolar consolidation. As seen in the E-FAST examination, pleural fluid appears as an anechoic collection above the diaphragm, although internal echoes may be present in cases of chronic, infected, or loculated effusions.

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