Abdominal Bedside Ultrasonography 

As compared with comprehensive imaging performed in the radiology department, emergency ultrasonography is a bedside diagnostic modality that answers time-sensitive specific questions about a patient’s condition. Emergency ultrasonography is diagnostic, can be used to assess clinical response in ongoing resuscitations, and can help guide invasive procedures. It directly influences patient management decisions and interventions in multiple ways. In terms of diagnosis, emergency ultrasonography can answer time-sensitive life-saving questions such as the following: Does this 70-year-old man with low back pain have an abdominal aortic aneurysm?Does this 19-year-old assault victim have intraperitoneal bleeding?

Bedside ultrasonography is indicated in the evaluation of symptoms such as the following:

• Abdominal pain
• Flank pain
• Low back pain
• Blunt abdominal trauma
• Hematuria
• Unexplained hypotension

It is also indicated in the workup of specific abdominal disease entities such as the following:

• Abdominal aortic aneurysm
• Acute cholecystitis
• Biliary colic
• Acute appendicitis
• Nephrolithiasis
• Urinary retention

Newer applications include the use of abdominal ultrasonography as a procedural aid (eg, in paracentesis or abscess drainage), for evaluation and monitoring of central venous filling as a surrogate for central venous pressure, and for assessment of abdominal wall pathology (eg, abscess or hernia).

When emergent treatments such as intravenous fluids or transfusion of blood are indicated, performance of abdominal sonography should not delay the initiation of these treatments.

Although ongoing resuscitation and extremis are not contraindications, abdominal sonography can be challenging to perform in such situations.

Anesthesia is generally not necessary for abdominal sonography; however, pain management should not be delayed, and patients may experience some discomfort due to probe pressure. For improved patient comfort, consider using warmed ultrasound-conducting gel, if available.

Equipment for abdominal bedside ultrasonography includes the following:

• Ultrasound machine with color flow Doppler and power Doppler
• Low-frequency (2-5 MHz) curvilinear transducer
• Acoustic coupling gel
• Gloves
• Appropriate materials to drape the patient

Patients should be evaluated in the supine position but can be moved to the lateral decubitus position for improved visualization of particular structures (eg, gallbladder).

Male patients should have the entire abdomen exposed for the examination. Take care with female patients to minimize the exposure of sensitive areas.

FAST Examination

Clinical questions, probe selection, elements of the focused assessment with sonography in trauma (FAST) examination, patient positioning, and limitations of the FAST examination are discussed below. The discussion is organized by indication.

Blunt or Penetrating Abdominal Trauma

FAST examination is nearly 100% sensitive for detecting clinically significant hemoperitoneum^[1] and, as such, has largely replaced diagnostic peritoneal lavage (DPL) in assessing for intraperitoneal free fluid. It is important to remember that the ultrasound is screening for the presence of free fluid and is not a reliable tool for visualizing the actual injury (such as liver or spleen laceration, especially in adults).

CT remains the diagnostic test of choice in hemodynamically stable patients with suspected intraabdominal injury, though serial abdominal examinations and serial FAST examinations have been used as an alternative means of ruling out significant injury in low risk patients, especially with the rising concern about radiation exposure from CT.

Clinical questions

• Is free intraperitoneal fluid present?
• Is a pericardial effusion present?

Probe selection

• Low frequency (2.0 - 5.0 MHz) curvilinear probe or small curved array or phased array probe (1.0-4.0 MHz) can be used for adequate tissue penetration.
• A small curved array or phased array probe offers the benefit of a small footprint and easier imaging between rib spaces.

Elements of the FAST examination

• Four standard views are obtained, looking for free fluid (black, or anechoic, on ultrasound).Image of the Morison pouch (arrows) demonstrating a small layer of dark free intraperitoneal fluid (F).
• Right upper quadrant view - Demonstrates fluid in the Morison pouch (hepatorenal recess), the most sensitive area for free intraperitoneal fluid. Right upper quadrant (RUQ) view of the Morison pouch (potential space between the liver and right kidney) where intraperitoneal fluid accumulates. This RUQ image depicts the liver and kidney (Kid). The center of the normal kidney (renal sinus) contains dense echoes. A marked amount of free intraperitoneal fluid in the Morison pouch, visible as a broad anechoic (black) signal between the liver edge and the anterior right kidney.
• Left upper quadrant view - Demonstrates fluid in the splenorenal recess, the subdiaphragmatic space, and along the inferior tip of the spleen and the inferior pole of the left kidney Positive left upper quadrant view on FAST examination, demonstrating perisplenic free fluid in the subdiaphragmatic space.
• Subxiphoid view - Demonstrates pericardial fluid accumulations (A parasternal view may be obtained through the chest wall at the junction of the ribs and sternum in patients with poor subxiphoid views or in patients who cannot tolerate this view because of abdominal tenderness.) Subxiphoid view of the heart showing a moderate amount of pericardial fluid extending almost entirely around the cardiac silhouette.
• Suprapubic view - Demonstrates free fluid in the pelvis (Small amounts of free fluid can be physiologic in women of childbearing age.)

Patient positioning

• The patient should be supine to allow fluid to collect in dependent areas.
• Placing the patient in the Trendelenburg position can increase the sensitivity of the right upper quadrant view for free fluid.^[2]

Limitations of the FAST examination

• Inability to distinguish between blood, urine, bile, ascites, or other fluids
• Poor sensitivity for bowel, diaphragmatic, or other solid organ injury
• Difficult for the novice or untrained clinician to identify retroperitoneal bleeding
• May be limited in patients with large body habitus
• Is not reliable for the identification of bleeding from a pelvic fracture source

Abdominal Aortic Aneurysm (AAA)

Symptomatic AAA with hemodynamic instability is a true vascular emergency and requires prompt identification in the ED, where resuscitative efforts can be initiated.

Physical examination is insufficiently sensitive to rule out AAA in the absence of a pulsatile abdominal mass, whereas the sensitivity of bedside ED ultrasonography for detection of AAA ranges from 94-100%.^{[3, 4, 5]}

While aneurysm rupture cannot be reliably detected on ultrasound, the presence of an AAA in the hemodynamically compromised patient requires emergent operative evaluation and should not be delayed in order obtain more definitive imaging. The presence of the aneurysm alone is sufficient to obtain emergent consultation.

Clinical questions

• Is an AAA present?
• Is an iliac artery aneurysm present?

Probe selection

• A standard 2.0-5.0 MHz curvilinear abdominal probe may be used to achieve adequate tissue penetration in most patients. A larger footprint probe (large curved) is often helpful to push bowel gas aside.
• Decreasing the probe frequency may be useful in patients with larger body habitus.

Elements of the examination

• The entire course of the abdominal aorta should be visualized in the transverse plane, from the xiphoid process down to the iliac bifurcation (usually at or near the umbilicus). The transverse plane gives a more reliable size estimation of the aorta than the longitudinal plane (because of the potential for tangential measuring error in the longitudinal plane).
• The common iliac arteries, which usually arise around the level of the umbilicus, should also be evaluated for aneurysm after the aorta has been evaluated.
• Longitudinal views should also be obtained. Take care to scan laterally through the barrel of the vessel and identify its widest diameter, as a "parasagittal" view may give a false negative measurement.
• AAA is typically defined as an aortic diameter >3.0 cm when measured anteriorly to posteriorly from outer wall to outer wall.
• Be sure to measure the entire aneurysm (including clot), not just the false lumen often created by the presence of echogenic thrombi adherent to the vessel walls. Measurement of this false lumen will underestimate the size of the aneurysm.
• Iliac artery aneurysm is defined as vessel diameter >1.5 cm when measured from outer wall to outer wall at its widest point.Short-axis view of an abdominal aortic aneurysm roughly 9 cm in transverse diameter. The hyperechoic vertebra is visible posteriorly and serves as a landmark in identifying the aorta. Though ultrasonography is an insensitive modality for evaluation of dissection and rupture, this image contains evidence of a false lumen, suggesting likely dissection.

  Color cine loop depicting abdominal aortic aneurysm (AAA). Video courtesy of Meghan Kelly Herbst, MD. Also courtesy of Yale School of Medicine, Emergency Medicine.

  Cine loop of abdominal aortic aneurysm (AAA). Video courtesy of Meghan Kelly Herbst, MD. Also courtesy of Yale School of Medicine, Emergency Medicine.

Patient positioning and imaging tips

• If bowel gas prevents adequate visualization using a midline approach, left lateral decubitus positioning and imaging through the liver may provide better visualization of the aorta.
• Asking the patient to inspire deeply may assist in visualizing the proximal abdominal aorta, as diaphragmatic excursion pushes the liver inferiorly.
• Holding firm pressure with a large footprint probe may help to displace bowel gas and improve visualization of the aorta.
• In older patients, the aorta may take a tortuous path. Once the aorta is identified, follow it even if it goes off of midline.
• At times, the aorta can be seen from the coronal plane (midaxillary view), using the liver or spleen as the sonographic window. Increasing the depth is necessary compared to the FAST views. This is more useful as a "rule in" tool rather than a "rule out" tool, as the aorta may not be visible in its full entirety; additional imaging modalities will be needed.
• Saccular aneurysms (small aneurysm pouches) are easier to miss compared to fusiform aneurysms. It is critical that the entire aorta be visualized in the assessment of AAA.
• Tricks for identifying the aorta include the following:

  • Increase the image depth and identify the vertebral bodies by their characteristic hyperechoic appearance with posterior acoustic shadowing; the aorta should be immediately anterior and slightly to the left of the echogenic vertebrae.
  • Do not mistake the spinal canal and the brightly walled superior mesenteric artery for the aorta.
  • Ask the patient to sniff; the inferior vena cava (IVC) often collapses during this maneuver, while the aorta typically does not.
  • You may also use color to help differentiate between the IVC and aorta. When aiming the transversely oriented probe up toward the head from the umbilicus, aortic blood is flowing toward the feet and, therefore, toward the probe. In this orientation, aortic flow will be red and IVC flow will be blue. These colors will reverse if the probe is pointed down toward the feet, so take care in using this technique.

Limitations of ultrasonography in AAA evaluation

• Ultrasonography has poor sensitivity for detecting aneurysmal leak or rupture.
• If the aneurysm ruptures, bleeding is often retroperitoneal (rather than intraperitoneal) and thus cannot be well evaluated with ultrasonography; do not look for a positive FAST examination result in these patients. The presence of the aneurysm alone is sufficient to mobilize resources in the appropriate clinical setting.
• Visualization of the aorta may be limited because of bowel gas and in patients with large body habitus.

Biliary Tract

Right upper quadrant or epigastric discomfort or pain or tenderness are the primary indications for bedside biliary ultrasonography.

Patients with jaundice may benefit from bedside biliary ultrasonography, however, the ability of novice or untrained sonographers to interpret findings is more limited. Further studies may be indicated.

Clinical questions

• Are gallstones present?
• Is evidence of cholecystitis present?
• Is the common bile duct (CBD) dilated?

Probe selection

• A small or large curved array probe of low or intermediate frequency (2.0-5.0 MHz) will suffice.
• Probes with smaller footprints allow for easier imaging through rib spaces. Probes with larger footprints offer better abdominal views. Either type is acceptable.

Elements of the examination

• The normal gallbladder appears as a pear-shaped, hypoechoic structure on ultrasound.
• Find the gallbladder under the costal margin and scan through it in both the transverse and longitudinal axes. Often, the best view is obtained between the ribs, using the liver as an acoustic window
• Interrogate the gallbladder looking for gallstones, which are hyperechoic with associated posterior acoustic shadowing. Be sure to visualize the neck of the gallbladder, as symptomatic gallstones can be found impacted in the neck. Longitudinal view of the gallbladder (gb) revealing a large gallstone (st) with associated acoustic shadowing (sh). This gallstone is located near the neck of the gallbladder. If impacted, it would need to be surgically removed. Longitudinal view of the gallbladder with 2 large stones visible in the fundus, with associated posterior acoustic shadowing. Notably, the anterior wall does not appear to be thickened and no evidence of pericholecystic fluid is present.
• Measure the anterior wall of the gallbladder (often easiest on a longitudinal view). A wall thickness greater than 3 mm is considered abnormal and is called thickened. Avoid measuring the posterior wall of the gallbladder, as posterior acoustic enhancement makes this challenging and can lead to incorrect values. Right upper quadrant view of the gallbladder in longitudinal section reveals hyperechoic material in the fundus. Note the posterior shadowing consistent with cholelithiasis. In addition, the anterior gallbladder wall appears thickened, and the presence of a narrow stripe of hypoechoic fluid along the anterior wall suggests inflammation consistent with cholecystitis.
• Assess for a sonographic Murphy sign. Reproduction of the patient’s abdominal pain by pressing with the ultrasound probe over the gallbladder constitutes a positive finding.
• Look for a hypoechoic rim of pericholecystic fluid (PCF) adjacent to the gallbladder. Pericholecystic fluid can be evidence of inflammation due to gallbladder disease but can also be secondary to hepatitis or congestive heart failure.
• While individual findings on biliary ultrasound have insufficient sensitivity and specificity, the combination of multiple findings offers improved predictive value.

  • A normal emergency physician – performed bedside ultrasound has a negative predictive value (NPV) of 90% for acute cholecystitis.^[6]
  • The combination of gallstones and anterior gallbladder wall thickening has a positive predictive value (PPV) >95% for acute cholecystitis.^[7]
  • The combination of gallstones and sonographic Murphy sign has a PPV of 92% for acute cholecystitis.^[7]
• Identify the common bile duct (CBD). A hypoechoic, tubular structure anterior to the portal vein, it can be visualized near the neck of the gallbladder in a longitudinal view or in a transverse view from the epigastrium.

  • Measure the CBD from inner wall to inner wall.
  • CBD diameter >5 mm is considered abnormal, though this cutoff increases with advancing age (a good rule of thumb is for every decade over 50 y, an additional 1 mm is allowed; eg, 6 mm at 60 y, 7 mm at 70 y). Patients often have a nonpathological dilated CBD (up to 1 cm) after cholecystectomy.
  • CBD dilatation is evidence of extrahepatic biliary obstruction, either intraductal (eg, stones) or secondary to extrinsic compression (eg, tumor), and may warrant additional imaging (CT or magnetic resonance cholangiopancreatography [MRCP]) or GI evaluation and management via endoscopic retrograde cholangiopancreatography (ERCP).

Patient positioning and imaging tips

• Supine positioning is the typical starting point, but the gallbladder is quite variable in its position and may be difficult to visualize in this position. Improved views of the gallbladder may be obtained with the patient in either a sitting position or in a left lateral decubitus position. Both of these positions bring the gallbladder out from beneath the costal margin. Asking the patient to take and hold a deep breath may bring the gallbladder farther down and into view.
• To find the gallbladder, use the following 2 landmarks: (1) the portal triad (portal vein, common bile duct, and hepatic artery) and (2) the main lobar fissure. The gallbladder lies within the main lobar fissure (hyperechoic line), is the functional division between the right half and the left half of the liver, and is present and seen in most patients.
• Imaging through the liver from the midanterior axillary line to locate the gallbladder and then moving the probe over it can also be helpful.
• If the gallbladder proves difficult to visualize despite patient positioning and inspiration, consider repeating the exam after keeping the patient NPO for a period of time.
• If uncertain whether gallbladder contents represent stones (eg, posterior acoustic shadowing is not seen), try repositioning the patient; stones generally shift to dependent areas, whereas polyps or masses remain stationary. Gallbladder sludge appears hyperechoic but does not result in acoustic shadowing and does layer or move dependently with patient positioning.
• A gallbladder full of stones may result in the wall-echo-shadow (WES) sign, where the anterior gallbladder wall is seen with a hyperechoic stripe just behind and broad, dark shadowing posteriorly. The WES sign can easily be confused with the acoustic scatter caused by adjacent small bowel.

Limitations of bedside biliary ultrasonography

• Biliary ultrasonography can be challenging (in particular, assessment of the CBD) and, thus, can be operator-dependent.
• The gallbladder can be difficult to visualize in nonfasting patients.
• The finding of gallstones does not by itself explain the patient's symptoms. While gallstones place patients at increased risk for biliary disease, they can be and are often asymptomatic.

Renal

Indications for renal ultrasonography include assessment for obstructive uropathy, as evidenced by hydronephrosis or bladder distension in the following settings:

• Unilateral flank pain, low back pain, or lower abdominal pain
• Hematuria
• Anuria or decreased urinary output

An important cautionary note is that AAAs frequently present in a manner similar to renal colic, so bedside ultrasonography to assess for and rule out an AAA is indicated in at-risk populations. Hydronephrosis from an AAA compressing the ureter can even occur, making it even more important to screen for both in at-risk groups.

Clinical questions

• Is hydronephrosis present?
• If hydronephrosis is present, is it unilateral or bilateral?
• Is the bladder distended? Is urinary retention present?

Probe selection

• A standard curvilinear abdominal probe (2.0-5.0 MHz) is typically the best choice.

Elements of the examination

• Both kidneys should be interrogated in the transverse and longitudinal planes. The asymptomatic side should be scanned first to establish a baseline for comparison.
• Normal ultrasound appearance of renal parenchyma, from exterior to interior (as follows):

  • Hyperechoic renal capsule
  • Hypoechoic renal cortex
  • Hypoechoic medullary pyramids
  • Hyperechoic renal pelvis
• Assess for hydronephrosis.

  • In hydronephrosis, the renal pelvis is dilated and filled with dark anechoic fluid. Hydronephrosis is generally classified as mild, moderate, and severe. With increasing severity of hydronephrosis, the standard architecture of the kidneys becomes more abnormal, with compression of the cortex. Image demonstrating moderate hydronephrosis (Hy) and the bear claw sign. Dilatation of the renal collecting system with extension into and splaying of the renal calyces is seen. Renal parenchyma (p). Moderate hydronephrosis, with dilatation of the collecting system and blunting of calyceal fornices. The thickness of renal parenchyma remains relatively intact.
    Cine loop depicting renal hydronephrosis. Video courtesy of Meghan Kelly Herbst, MD. Also courtesy of Yale School of Medicine, Emergency Medicine.
  • Be sure to look for hydronephrosis in both kidneys, as bilateral hydronephrosis is indicative of more distal obstruction or dysfunction.
  • Mild hydronephrosis may be present in the absence of obstruction in patients who are pregnant or overhydrated. In these instances, the finding should be present bilaterally. A very full bladder would also suggest this
• Visualize the bladder in both the transverse and sagittal planes.

  • Bladder volume can be estimated by measuring its diameter in 3 perpendicular planes and applying the formula 3/4 x (L x W x H).
  • Controversy exists over the utility of assessing for ureteral jets in obstructive uropathy.^[8] Assessing for ureteral jets can be somewhat time-consuming in a busy ED setting.

Patient positioning and imaging tips

• Supine positioning is the typical starting point.
• Asking the patient to take and hold a breath may be helpful, especially when trying to visualize the left kidney, which is more superior and posterior in the abdomen than the right kidney.
• Right lateral decubitus positioning may further assist in visualizing the left kidney.

Limitations of renal and urinary ultrasonography

• Renal stones are rarely directly seen (unless large and positioned in the proximal or distal ureter). Hydronephrosis is the surrogate marker for obstructing stones.
• Hydroureter can be challenging to visualize on ultrasound unless significant hydroureter is present.
• Hydronephrosis is a nonspecific finding that can be secondary to many causes both intrinsic (eg, nephrolithiasis) or extrinsic (eg, pelvic mass).

Appendicitis

Indications include the presence of any of the typical signs and symptoms of acute appendicitis (periumbilical or right lower quadrant abdominal pain or tenderness, abdominal rebound or guarding, fever, nausea and vomiting, or anorexia).

Clinical questions

• Is the appendix dilated?
• Is the appendix compressible?
• Is there a lack of peristalsis in the appendix?
• Is there a fecalith in the appendix?

Probe selection

• A linear array probe of intermediate to high frequency (5.0-10.0 MHz), depending on the body habitus of the patient.

Elements of the examination

• Begin scanning over the point of maximal pain, as indicated by the patient. This improves the accuracy of the examination.^[9]
• Search for the appendix by slowly and firmly compressing the abdomen with the linear probe at the point of maximal tenderness; slowly applied firm pressure may help displace overlying loops of bowel.
• Move progressively through all quadrants until the appendix is located or until a thorough abdominal survey has been completed.
• It is often helpful to use the psoas muscle and iliac vessels as a guide to locating the appendix.
• Once the appendix is found, complete the following steps:

  • Visualize the appendix along its entire length in both the short and long axes.
  • Measure the diameter of the appendix from external wall to external wall; a diameter >6 mm is considered abnormal.
  • Assess the compressibility of the appendix. An inflamed and swollen appendix does not compress under gradual compression.
  • Look for evidence of continued peristalsis. Normal peristalsis is impaired in the inflamed appendix. This may be better assessed with a long-axis view.
  • Examine the cross-sectional appearance of the appendix, as it takes on a multilaminar targetoid appearance in the setting of acute inflammation.

Patient positioning and imaging tips

• Supine positioning is standard.

Limitations of the examination

• The limited sensitivity of the examination renders it insufficient for ruling out acute appendicitis.
• This is a technically challenging and time-consuming examination with significant inter-operator variability.
• The appendix is extremely difficult to visualize in the setting of perforation and in patients with large body habitus.
• Of all the techniques discussed in this article, this is the most difficult. If suspicion for acute appendicitis remains high, further imaging, surgical consultation or both are indicated.

Paracentesis

Indications for bedside ultrasonography in paracentesis include the following:

• Bedside ultrasonography can give immediate information regarding the presence or absence of ascites because of its high sensitivity for free fluid. It is much more reliable than assessing for a fluid wave or shifting dullness on examination.
• Bedside ultrasonography has high utility in patients with suspected subacute bacterial peritonitis (SBP). It is also helpful in patients who need a tap for therapeutic indications (eg, to decrease abdominal pain from distention or to help reduce dyspnea due to abdominal distension).
• Ultrasound-guided paracentesis increases patient safety, especially as many of these patients also have a coagulopathy.

Clinical questions

• Is ascites present?
• Where are the largest pockets of fluid located?
• Where is the safest location to place the needle that also clears vital organs (such as liver, spleen, bladder, and bowel)?

Probe selection

• Use a 2.5-5.0 MHz probe similar to those used in the FAST and aorta applications above.

Elements of the examination

• Run the probe over the abdomen, evaluating for the most accessible large fluid pocket that also clears vital organs.Abdominal ultrasound showing ascites fluid (dark) surrounding loops of bowel, with a significant fluid pocket extending to a depth of around 9 cm.
• There are 2 approaches to ultrasound-guided paracentesis: static and dynamic.

  • The static approach: The site is marked and then the paracentesis procedure is performed in standard fashion.
  • If this approach is used, the patient must not move in between the marking and the procedure. Special note should also be made of the depth of the fluid collection to estimate the depth of needle placement. In addition, the angle of the probe at the time of marking should be mirrored by the angle of the needle to recreate the conditions appropriately.
  • The dynamic approach: This approach includes real-time ultrasonographic guidance.
  • The site may be marked, but after prepping and draping the patient in the usual fashion, the probe is also placed in a sterile sheath (with gel between the probe and the sheath and sterile gel on the outside of the sheath).
  • The fluid pocket is re-evaluated in real time as the needle is placed under direct ultrasonographic visualization. Care should be taken to visualize the tip of the needle.

Repositioning the patient or the probe is often the key to capturing otherwise difficult-to-obtain images. Some examples are as follows:

• Seated or left lateral decubitus positioning for biliary ultrasound
• Breath-holding in inspiration for visualizing the left kidney or gallbladder
• Imaging through the liver to visualize the gallbladder from the coronal approach when it is not easily seen anteriorly
• Mild Trendelenburg positioning for improving the sensitivity of the right upper quadrant view in the FAST examination

Be familiar with relevant anatomy and know how to distinguish sonographically similar structures (eg, have the patient sniff to distinguish the inferior vena cava (IVC) from the aorta, or use vertebral bodies to locate the abdominal aorta).

Bedside emergency department ultrasonography is preferable to radiology department imaging in an unstable patient. However, in a stable patient, more definitive imaging (eg, CT) may be indicated.

Ultrasound imaging does not use ionizing radiation and has not been shown to be associated with genetic damage or oncogenesis.

Tissue damage secondary to heat production when using Doppler imaging is a theoretical risk, so high-energy modes are typically avoided with fetal ultrasonography.

As ultrasound technology and the training to use such technology both improve, new clinical applications and indications for use in the ED will continue to grow. New and investigational applications of abdominal ultrasound include the following:

• Identification of abdominal wall pathology, including hernias, abscesses, solid masses, and wound evaluation^{[10, 11]}
• Assessment of central venous pressure using ultrasonographic evaluation of inferior vena cava size and respiratory collapse as a surrogate^{[12, 13]}
• Use of sonographic contrast agents to improve assessment for solid organ injuries in trauma patients^[14]