Ultrasonography: Understanding the Principles and Its Uses in Abdominal and Pelvic Imaging

How it Works

• Ultrasonography is performed by placing the transducer on a body surface while sweeping back and forth as real-time images are displayed on a monitor. Adequate contact between the transducer and the body surface is essential to obtain a good sonographic image. Therefore, a coupling gel is applied to produce the best contact between the transducer and the body surface by eliminating air gaps ( Video 18.1 ).

• The body surface that is scanned may be external, such as the skin surface in transabdominal sonography, or internal, such as in transvaginal, transrectal, transesophageal, and endovascular sonography.

Important Points

• The creation of a sonographic image (sonogram) depends on three major components:

  • Production of high-frequency sound waves

  • Reception of a reflected wave or echo

  • Conversion of that echo into the actual image

• The sound wave is produced by the probe or transducer that sends out extremely short bursts of acoustical energy at a given frequency.

• As with all sound waves, the pulses produced by the transducer travel at different speeds depending on the density of the medium through which they are traveling.

• Where the wave strikes an interface between tissues of differing densities, some of the sound wave will be transmitted forward while some will be reflected back to the transducer ( Fig. 18.1 ).

  

• How much sound is transmitted versus how much is reflected depends on the property of the tissues that make up the interface and is called acoustical impedance. Small differences in acoustical impedance will result in greater sound transmission; large differences in acoustical impedance will result in greater sound reflection.

• For example, if the pulse encounters a gallstone, the acoustical impedance is high and most of the acoustical energy is reflected back. If the tissue that is encountered is extremely dense, such as bone, so much of the energy is reflected that there are not enough transmitted sound waves to define tissues deeper to the bone (see Fig. 18.1A ).

• If a pulse traveling through soft tissue encounters fluid, the acoustical impedance (difference in densities) is relatively low and most of the acoustical energy is transmitted (see Fig. 18.1C ).

• As the sound waves pass through tissue, their intensity decreases as they are absorbed by the tissue and converted into heat energy.

• When the reflected echo arrives back at the transducer (a matter of microseconds), it is converted from sound into electrical pulses that are then sent to the scanner itself.

• Using an on-board computer, the scanner determines the length of time it took for the echo to be received, the frequency of the reflected echo, and the magnitude (amplitude) of the signal.

• With this information, a sonographic image of the scanned body part can be generated by the computer and displayed on the screen. The images may then be captured as single, static images or recorded as a cine loop that is composed of a set of images that can be played as a video. Both this chapter and the next feature cine loops of US studies that can be viewed online ( Video 18.1 , Video 18.2 , Video 18.3 , Video 18.4 , Video 18.5 , Video 18.6 , Video 18.7 , Video 18.8 , Video 19.1 , Video 19.2 , Video 19.3 , Video 19.4 , Video 19.5 , Video 19.6 , Video 19.7 ).

Echogenicity

Important Points

• The echogenicity (brightness or darkness) of a tissue on US is determined by the amount of sound waves the tissue transmits or reflects.

• A tissue that reflects more echoes is depicted brighter or whiter on the sonogram and is called echogenic (hyperechoic).

• A tissue that transmits more echoes (resulting in less echoes reflected back to the transducer) is depicted darker on the image and is called sonolucent (hypoechoic).

• A tissue that essentially transmits all the echoes is depicted as black on the sonogram and is called anechoic.

• When a very dense structure such as a gallstone or bone reflects so many echoes that virtually no sound waves are transmitted, the tissue deeper to this structure is displayed as hypoechoic and this phenomenon is called posterior acoustic shadowing .

• When a structure transmits more echoes than the surrounding tissues, such as a cyst in the liver might, the sound waves deeper to this structure compared to surrounding structures are depicted as hyperechoic and this phenomenon is called posterior acoustic enhancement .

• The typical appearances of different types of commonly encountered tissues and structures are described in Table 18.1 .

  TABLE 18.1

  Appearance of Commonly Encountered Tissues on Ultrasound

  Tissue	Appearance	Examples
  Fluid	Hypoechoic or anechoic, depending whether the fluid is simple or complex (containing debris, pus, or blood); may have posterior acoustic enhancement	Cysts, abscesses, gallbladder, urinary bladder, spinal fluid, blood in vessels
  Calcium	Hyperechoic; may have posterior acoustic shadowing	Gallstones, renal stones, bones, calcifications in soft tissue
  Air	Hyperechoic foci; may cause posterior acoustic shadowing	Gas-forming infections (abscesses, Fournier’s gangrene, endometritis), intraperitoneal air, necrotizing enterocolitis, portal venous gas, pneumobilia

Frequency and Resolution

• The frequency of the soundwaves utilized in any given US study plays an integral role in the resolution of the US images.

• Higher frequencies result in higher resolution images and more detail. However, the higher the frequency, the shorter the penetration distance. Therefore, while higher frequencies will provide more detail, they cannot display deeper tissues because of their shorter penetration.

• US transducers produce inherently different frequencies of sound waves, usually labeled on the probe. Choosing the proper transducer for the type of study is important in order to provide as much detail as possible while also providing enough penetration to reach the desired tissue.

• In general, higher frequency transducers are appropriate for superficial structures or those that require fine detail, and lower frequency transducers are appropriate for deep structures.

Types of Ultrasound

• Several types of US are used in medical imaging. They are described in Table 18.2 .

  TABLE 18.2

  Types of Ultrasound

  A-Mode	Simplest; spikes along a line represent the signal amplitude at a certain depth; used mainly in ophthalmology.
  B-Mode	Mode most often used in diagnostic imaging; each echo is depicted as a dot and the sonogram is made up of thousands of these dots; can depict real-time motion.
  M-Mode	Used to show moving structures, such as blood flow or motion of the heart valves.
  Doppler	Uses the Doppler effect to assess blood flow; used for vascular US. Pulsed Doppler devices emit short bursts of energy that allow for an accurate localization of the echo source.
  Duplex ultrasonography	Used in vascular studies; refers to the simultaneous use of grayscale (or color Doppler) to visualize the structure of, and flow within, a vessel and spectral (waveform) Doppler to quantitate flow (see Video 18.1 ).

Doppler Ultrasonography

• You are probably familiar with the common examples of the Doppler effect illustrated by a passing train whistle or police siren . The effect generally states that sound changes in frequency as the object producing the sound either approaches or recedes from your ear ( Video 18.2 ).

• Sonography makes use of the Doppler effect to determine whether an object, usually blood, is moving toward or away from the transducer and at what velocity it is moving. Here is how it works:

  • The transducer sends out a signal of known frequency; the frequency of the echo returned is compared to the frequency of the original signal.

  • If the returning echo has a lower frequency than the original, then the object is moving away from the transducer. If the returning echo has a higher frequency than the original, then the object is moving toward the transducer.

Important Points

• The direction of flow is represented by the colors red and blue. By convention, red indicates flow toward and blue indicates flow away from the transducer.

Adverse Effects or Safety Issues

• US procedures are well-tolerated. Scans can be obtained relatively quickly, done at the bedside if necessary, and, for the most part, require no patient preparation other than abstinence from food before abdominal studies ( Table 18.3 ).

  TABLE 18.3

  Advantages and Disadvantages of Ultrasonography

  Advantages	Disadvantages
  No ionizing radiation	Difficulty penetrating through bone
  No known long-term side effects	Gas-filled structures reduce its utility
  “Real-time” images	Obese patients may be difficult to penetrate
  Produces little or no patient discomfort	Dependent on the skills of the operator scanning

• US has the short-term potential of causing minor elevation of heat in the area being scanned, though not at levels used in diagnostic imaging.

• No known long-term side effects have been scientifically demonstrated from the use of medical US in humans. Nevertheless, like all medical procedures, it should be utilized only when medically necessary. The United States Food and Drug Administration warns against the use of US during pregnancy to produce “keepsake photos or videos.”

• The advantages of US over CT and conventional angiography include the absence of the use of ionizing radiation, lack of the need for intravenous iodinated contrast, and the portability of US.

Medical Uses of Ultrasonography

• We will look at several common abnormalities in which US plays a primary imaging role.

Biliary System

• Ultrasound is the study of first choice for abnormalities of the biliary system. Patients who present with the relatively common complaint of right upper quadrant abdominal pain usually undergo an US examination as their first imaging study. CT may be helpful in cases with difficult or unusual anatomy, for detecting masses, or in determining the extent of disease already diagnosed, but CT is less sensitive than US in detecting gallstones.

Normal Gallbladder Anatomy: Ultrasound

• The gallbladder is an elliptical sac that lies between the right and left lobes of the liver in the interlobar fissure. Although different layers of its wall have different echogenic properties, the gallbladder overall consists of a fluid-filled sonolucent lumen surrounded by an echogenic wall. In the fasting patient, the gallbladder is about 4 x 10 cm in size and the wall is normally no thicker than 3 mm ( Fig. 18.3 ).

  

Gallstones and Acute Cholecystitis

• Cholelithiasis is estimated to affect more than 20 million in the United States. In almost all cases, acute cholecystitis starts with a gallstone impacted in the neck of the gallbladder or cystic duct. The presence of gallstones does not, by itself, mean that the source of a patient’s pain is the gallbladder since asymptomatic gallstones are common. Cholecystitis can also occur, though less commonly, in the absence of stones (acalculous cholecystitis).

• Because of gravity, gallstones usually fall to the most dependent part of the gallbladder, which will be influenced by the patient’s position at the time of the scan. This helps to differentiate gallstones from polyps or tumors that may be attached to a nondependent surface. Gallstones are characteristically echogenic and produce posterior acoustic shadowing ( Fig. 18.4 ).

  

• Biliary sludge can be found in the lumen of the gallbladder and is an aggregation that may contain cholesterol crystals, bilirubin, and glycoproteins. It is often associated with biliary stasis. Although it may be echogenic, sludge does not produce acoustic shadowing as gallstones do ( Fig. 18.5 ). Sludge layers along the dependent gallbladder wall or may appear as a rounded, mobile “mass” that is called tumefactive sludge.

  

Important Points

• Recognizing acute cholecystitis on US:

  • Thickening of the gallbladder wall (>3 mm) ( Fig. 18.6A )

    

  • Pericholecystic fluid (fluid around the gallbladder) ( Fig. 18.6B )

  • A positive sonographic Murphy sign (i.e., pain that is elicited by compression of the gallbladder with the US probe).

• If gallstones are present (see Fig. 18.4 ) in addition to these other US features, the diagnosis of acute calculous cholecystitis can be made . In the presence of gallstones and gallbladder wall thickening, US has a positive predictive value for acute cholecystitis as high as 94%.

• If these features exist in the absence of gallstones, the patient may have acalculous cholecystitis , which typically tends to occur in critically ill patients.

• Radionuclide scans (hepatoiminodiacetic acid [HIDA] scans) are also used in the diagnosis of acute cholecystitis.

  • Hepatoiminodiacetic acid (HIDA), which is physiologically absorbed by the liver and excreted into the biliary system, is tagged with a radioactive tracer (technetium-99m) and injected intravenously. The tagged HIDA can then be imaged with a special camera to demonstrate its normal physiologic uptake by the liver and subsequent excretion into the bile ducts, gallbladder, and small intestine.

  • In patients with obstruction of the cystic duct, the tracer will not appear in the gallbladder; in patients with obstruction of the common bile duct, the tracer will not appear in the small intestine. Either finding is typically caused by an obstructing stone ( Fig. 18.7 ).

    

Normal Bile Duct Anatomy: Ultrasound

• Ultrasound plays a key role in evaluation of the intrahepatic and extrahepatic bile ducts and the pancreatic duct. The intrahepatic biliary radicals drain into the left and right hepatic ducts, which join to form the common hepatic duct (CHD). The common bile duct (CBD) begins where the cystic duct from the gallbladder joins with the CHD. It drains into the second portion of the duodenum either within or adjacent to the head of the pancreas via the ampulla of Vater.

• The CBD lies anterior to the portal vein and lateral to the hepatic artery in the porta hepatis ( Fig. 18.8A ).

  

• The CHD and proximal CBD can be visualized on virtually all US studies of the right upper quadrant. The CHD measures no more than 4 mm (inner wall to inner wall) in diameter, and the CBD measures no more than 6 mm in diameter ( Fig. 18.8B ). The pancreatic duct measures less than 2 mm.

• Normally, the intrahepatic bile ducts are not visible on US. However, they may become dilated over time when there is prolonged CBD obstruction. Dilated intrahepatic ducts appear as additional, tubular sonolucent structures adjacent to the portal veins, which results in a tram-track appearance ( Fig. 18.9 ).

  

• Causes of bile duct obstruction include gallstones, pancreatic carcinoma, strictures, sclerosing cholangitis, cholangiocarcinoma, and metastatic disease.