- Understand the normal anatomy of the contents of the abdomen
- Know which organs and pathologies are visible with each of the ultrasound views used in abdominal ultrasonography
- Understand the utility of ultrasound as an abdominal imaging modality and know when and how it should be used
- Be able to recognize several abdominal pathologies with ultrasound
Introduction to Abdominal Ultrasound
Due to the overwhelming complexity of the important structures within the abdomen, the focus of this chapter will be on the organs that are most commonly visualized with ultrasound (US) owing to their association with common pathologies:
- Common bile duct and portal vein
Normal Anatomy of The Abdomen
Please take a moment to review the locations, relationships, and sizes of the abdominal contents shown in the figures. Pay particular attention to the organs that we will focus on throughout this chapter (liver, gallbladder, common bile duct, portal vein, spleen, and appendix). The pictures are 2-D representations of 3-D tissues, and creating a mental picture of each organ and its surrounding structures will allow you to better interpret the US images.
Indications for Abdominal Ultrasonography
- Abdominal pain
- Acute abdomen
- Screening asymptomatic patients
- Suspected liver disease
- Surveillance of patients with liver disease
- Abdominal trauma (either penetrating or non-penetrating)
- To avoid exposure toionizing radiation, particularly in pediatric and pregnant patients
Technique of Abdominal Ultrasound
Ultrasonography should always be used as a compliment to patient history and physical exam findings. If diagnostic imaging is indicated, it is best to complete a full abdominal physical exam prior to performing the US exam in order to narrow down the type of US scan performed.
Right Upper Quadrant- Liver
Ask the patient to lie down in a supine position with the knees comfortably bent and the right arm raised above the head. Select a phased array or curvilinear transducer and set the US machine to the “abdominal preset.” Place the transducer along the patient’s right mid-axillary line at about the 9th or 10th intercostal space. With the indicator pointed up toward the patient’s head, obtain a longitudinal image. Notice the bright, hyperechoic diaphragm sliding across the screen as the patient inhales and exhales. The diaphragm creates a mirror image artifact that gives the false appearance of hepatic tissue above the diaphragm. Mirror image artifact is a normal finding and indicates that there is air above the diaphragm. The right lung is located superior to the diaphragm and the liver is just below it. Slide the probe inferiorly to locate the right kidney which is the rounded structure seen below the liver. Fan the transducer from superior to inferior to completely scan through the tip of the liver and identify the hepatorenal recess or “Morison’s pouch” (potential space between liver and right kidney). During this scan, note the echogenicity of the liver tissue, measure the size of the liver, and the search for the presence of any anechoic space in Morison’s pouch indicative of peritoneal free fluid.
The right upper quadrant view allows for excellent imaging of the right kidney. Fan the probe from superior to inferior to view the entire kidney to inspect the renal parenchyma. It is important to visualize the inferior pole of the right kidney when evaluating for free abdominal fluid as this is where fluid will first accumulate.
Biliary Tract and Portal Vein
Ideally, the gallbladder is best visualized with US after the patient has fasted for 8 hours because this distends the gallbladder and reduces bowel gas. Ask the patient to lie down in a supine position with the knees comfortably bent and the right arm raised above the head. The exam can include repositioning of the patient to the left lateral decubitus position to bring the gallbladder away from the ribcage and can additionally demonstrate mobility of the gallstones or biliary sludge. Set the US machine to abdominal presets. Place the curvilinear (2-5 MHz) transducer in a subcostal oblique plane (indicator pointed at the patient’s left shoulder) just below the patient’s right ribs, and angle the probe superiorly to avoid the colon. The approach can be modified to locate the gallbladder in either a sagittal or intercostal view. The gallbladder is a pear shaped organ found at the medial border of the right hepatic lobe at the liver hilum. Assess the gallbladder’s size, wall thickness and contents. The normal, fasting gallbladder has thin walls (< 3 mm) with a dark lumen.
To view the cystic duct, begin at the infundibulum of the gallbladder and ask the patient to hold a deep inhalation. Trace the cystic duct until it becomes the common bile duct (CBD). The CBD appears as a tube behind the hepatic artery and portal vein. Assess the CBD’s size (< 6 mm), wall thickness and contents. A dilated CBD > 6 mm can indicate a stone obstructing the CBD. Normally, biliary ducts within the liver are not visible unless they are pathologically dilated.
Ultrasound Examination of the Liver and RUQ
Left Upper Quadrant View- Spleen
Ask the patient to lie down in a supine position with the knees comfortably bent and the left arm raised above the head. Select a phased array or curvilinear transducer and set the US machine to the “abdominal preset.” Place the transducer along the patient’s left posterior-axillary line at about the 7th to 8th intercostal space with the indicator pointed up toward the patient’s head. The key to obtaining this image is to place your “knuckles to the bed.” This view is more posterior and superior than most think. Locate the hyperechoic diaphragm that will slide across the screen as the patient inhales and exhales. The left lung is seen above the diaphragm and the spleen is seen below it. You will notice that the US image of the spleen looks very similar to the liver due to their similar densities.
The left upper quadrant view allows for inspection of fluid in the pleural space by angling or sliding the transducer superiorly to analyze the space above the diaphragm. The mirror artifact should be present above the diaphragm.
Ultrasound Examination of the LUQ and Spleen
Abnormal Liver Parenchyma
In order to assess the normal appearance of a patient’s liver parenchyma, compare the echogenicity of the liver tissue to that of the the patient’s renal cortex. The brightness, or echogenicity, of the liver should be equal to or greater than that of the renal cortex.
Increased echogenicity of the liver tissue indicates fatty infiltration of the liver or scarring of the liver such as cirrhosis. If the liver is significantly brighter than both the spleen and the right renal cortex, there is likely fatty infiltration of the liver parenchyma or cirrhosis (if there is concurrent nodularity with small liver size).
Hepatic Cysts and Masses
Hepatic cysts may appear as well defined, round, heterogeneous masses inside the liver. If large enough these cysts may demonstrate posterior acoustic enhancement. Cysts will not contain vascular structures, B-mode can be used to confirm their absence.
Cholelithiasis, the presence of gallstones in the gallbladder, is a very common pathology that can result in biliary colic pain, inflammation, infection, abscess and peritonitis. The following signs and symptoms may indicate the presence of cholelithiasis; however, 70% of patients with gallstones are asymptomatic:
- Pain in the right upper quadrant or epigastric region
- Pain aggravated by food intake
- Pain worse at night
- Pain radiates to the back or right shoulder
- Nausea and vomiting
- Right upper quadrant tenderness and guarding
- Positive Murphy’s sign
- Elevated WBC, CRP in lab tests
US is highly sensitive and specific for the detection of gallstones and biliary obstruction, and it is preferred over CT. It is a very sensitive test, as it has been shown to detect cholelithiasis in around 98% of affected patients. The following are US findings suggestive of cholelithiasis:
- Gallstones: Bright, hyperechoic, mobile structures with posterior acoustic shadowing located within the gallbladder, cystic duct, or CBD.
- Wall Thickening: The presence of either global or focal gallbladder wall thickening is determined by measuring the wall at its thickest point. A thick wall indicates inflammation and may even suggest cancer or adenomyomatosis (a benign condition) of the gallbladder.
- Pericholecystic fluid: The presence of dark, anechoic space around the gallbladder suggests the presence of fluid. When gallbladder disease is suspected, this pericholecystic fluid might be from a rupture or perforation of the gallbladder.
Free Fluid Assessment
Ultrasound is a very sensitive, specific, and quick test to determine if there is free fluid present in the abdominal cavity. The assessment of free abdominal fluid is an essential part of the EFAST protocol which is commonly used in the assessment of trauma victims.
Right Upper Quadrant
The right upper quadrant view has the highest sensitivity for the detection of free intra-abdominal fluid. Examine the hepatorenal recess (Morison’s pouch) between the liver and the right kidney. When present, free fluid will appear as dark space in Morison’s pouch. Depending on the positioning of the patient, this view can detect the presence of a minimum of 100 mL (in right lateral decubitus position) to about 620 mL (in supine position) within the abdominal cavity. Beware that perinephric fat can be mistaken for free fluid in Morison’s pouch. It too appears on US as an anechoic space between the liver and the kidney. Additionally, free fluid can accumulate toward the inferior tip of the liver, for this reason the liver tip and inferior pole of the right kidney must be visualized when assessing for free abdominal fluid.
Left Upper Quadrant
Using the left upper quadrant view, examine the perisplenic space above the spleen but below the diaphragm for the presence of free. With the patient in the supine position, this view can detect the presence of a minimum of 500 mL within the abdominal cavity. Remember that in the RUQ fluid collects between the liver and kidney while in the LUQ fluid collects between the spleen and diaphragm.