Ultrasonography

Chapter 19 Ultrasonography

Understanding the Principles and Recognizing Normal and Abnormal Findings

Ultrasound (US) occupies an acoustical frequency on the electromagnetic spectrum that is hundreds of times greater than humans can hear. Ultrasonography is the medical imaging modality that utilizes that acoustical energy to localize and characterize human tissues.

How it Works

The creation of a sonographic image (sonogram) depends on three major components: the production of a high frequency sound wave, the reception of a reflected wave or echo, and the conversion of that echo into the actual image.

The sound wave is produced by a probe that contains one of more transducers, which send out extremely short bursts of acoustical energy at a given frequency.

• For most imaging, the probe is placed externally on the skin surface and moved by the sonographer who sweeps the probe back and forth over the area to be scanned while viewing the images obtained in real time on a monitor. In order to produce the best contact between the probe and the skin, a coupling gel is applied to the skin surface first.

• On occasion, more detailed images can be obtained by inserting the probe into the body, such as is done with transvaginal, transrectal, and transesophageal sonography.

Like 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 will be transmitted forward while some will be reflected back to the transducer.

How much sound is transmitted versus how much is reflected is a property of the tissues that make up the interface and is called the acoustical impedance. Large differences in acoustical impedance will result in greater sound reflection; small differences will result in greater transmission.

If the pulse encounters fluid, most of the acoustical energy is transmitted. If the pulse encounters gas or bone, so much of the acoustical energy is reflected back that it is usually not possible to define deeper structures.

When the 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 or amplitude of the signal. With this information, a sonographic image of the scanned body part can be generated by the computer and recorded digitally, on film, or sometimes on videotape.

A tissue that reflects many echoes is said to be echogenic (hyperechoic) and is usually depicted as bright or white on the sonogram; a tissue that has few or no echoes is said to be sonolucent (hypoechoic or anechoic) and is usually depicted as being dark or black.

Images can be produced in any plane by adjusting the direction of the probe. By convention, two common imaging planes utilized are those along the long axis of the body or body part being scanned, called the sagittal or longitudinal plane, or perpendicular to the long axis of the body or body part being scanned, called the transverse plane.

Also by convention, sonographic images are viewed with the patient’s head to your left and the patient’s feet toward your right; anterior is up and posterior is down.

There are several types of ultrasound used in medical imaging. They are described in Table 19-1.

TABLE 19-1 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 ultrasound. Pulsed Doppler devices emit short bursts of energy that allow for an accurate localization of the echo source and has replaced continuous wave Doppler.
Duplex ultrasonography	Utilized in vascular studies; refers to the simultaneous use of both gray-scale or color Doppler to visualize the structure of and flow within a vessel and spectral waveform Doppler to quantitate flow.

Doppler Ultrasonography

You are probably familiar with the common examples of a passing train whistle or police siren as illustrations of the Doppler effect, which generally states that sound changes in frequency as the object producing the sound approaches or recedes from your ear.

Sonography makes use of the Doppler effect to determine if an object, usually blood, is moving toward or away from the transducer and at what velocity it is moving.

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

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

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

Adverse Effects and Safety Issues

The procedure is well tolerated. Scans can be obtained quickly, done at the bedside if necessary, and, for the most part, require no patient preparation other than eating no food before abdominal studies (Table 19-2).

Ultrasound has the short-term potential to cause minor elevation of heat in the area being scanned, though not at levels used in diagnosis.

TABLE 19-2 ADVANTAGES AND DISADVANTAGES OF ULTRASONOGRAPHY

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

There are no known long–term side effects that have been scientifically demonstrated from the use of medical ultrasound in humans. Nevertheless, like all medical procedures, it should be utilized only when medically necessary. The U.S. Food and Drug Administration warns against the use of ultrasound during pregnancy to produce “keepsake photos or videos.”

Medical Uses of Ultrasonography

Ultrasound is widely used in medical imaging. It is usually the study of first choice in imaging the female pelvis and in pediatric patients, in differentiating cystic versus solid lesions in all patients, in noninvasive vascular imaging, imaging of the fetus and placenta during pregnancy, and in real-time, image-guided fluid aspiration and biopsies.

Other common uses are evaluation of cystic versus solid breast masses, thyroid nodules, tendons, and in assessing the brain, hips, and spine in newborns. Ultrasound is used everywhere from intraoperative scanning in the surgical suite to the medical unit in the battlefield and in locations as remote as the South Pole.

We will look at several common abnormalities in which ultrasound 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 pain usually undergo an ultrasound examination first. 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 ultrasound in detecting gallstones.

Normal Ultrasound Anatomy

The gallbladder is an elliptical sac that lies inferior and medial to the right lobe of the liver. 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 × 10 cm in size and the wall is normally no thicker than 3 mm (Fig. 19-1).

Figure 19-1 Normal gallbladder, sagittal view.

The gallbladder (GB) is normally filled with bile and is sonolucent. The wall of the gallbladder is less than 3 mm in size and slightly echogenic (solid white arrow). By convention on a sagittal view, the patient’s head is to your left (H), feet to your right (F), anterior is up (Ant), and posterior is down (Post).

Gallstones and Acute Cholecystitis

Cholelithiasis is estimated to affect more than 20 million Americans. 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 patient’s pain is from the gallbladder because asymptomatic gallstones are common. Cholecystitis can also occur less commonly in the absence of stones (acalculous cholecystitis).

Gallstones usually fall to the most dependent part of gallbladder, which will depend on the patient’s position at the time of the scan. This helps to differentiate gallstones from polyps or tumors, which may be attached to a nondependent surface. Gallstones are characteristically echogenic and produce acoustical shadowing because they reflect most of the signal (Fig. 19-2).

Figure 19-2 Cholelithiasis, sagittal view.

There are numerous echogenic stones (dotted white arrows) in the gallbladder (GB). The stones cast acoustical shadows as they reflect most of the sound waves (solid white arrows).

Acoustical shadowing describes a band of reduced echoes behind an echo-dense object (e.g., a gallstone) that reflects most, but not all, of the sound waves. While acoustical shadowing reduces the diagnostic effectiveness of ultrasound through such tissues as bone and bowel gas, its presence can have diagnostic value in identifying the presence of calculi, such as in the gallbladder and kidney (Fig. 19-3).

Figure 19-3 Acoustical shadowing.

There is a band of reduced echoes (solid white arrow) behind echogenic gallstones (dotted white arrows) that reflect most, but not all, of the sound waves. The presence of acoustical shadowing can have diagnostic value in identifying the presence of calculi in the gallbladder (GB).

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. While it may be echogenic, sludge does not produce acoustical shadowing like gallstones (Fig. 19-4).

Signs of acute cholecystitis on ultrasound:

• The presence of gallstones, possibly impacted in the neck of the gallbladder or cystic duct.

• Thickening of the gallbladder wall (>3 mm) (Fig. 19-5A).

• Pericholecystic fluid (fluid around the gallbladder) (Fig. 19-5B).

• A positive Murphy sign (pain that is elicited by compression of the gallbladder with the ultrasound probe).

In the presence of gallstones and gallbladder wall thickening, ultrasound has a positive predictive value for acute cholecystitis as high as 94%.

Radionuclide scans (HIDA scans) are also used in the diagnosis of acute cholecystitis.

• Hepatoiminodiacetic acid (the “HIDA” in HIDA scan) is tagged with a radioactive tracer (Technetium Tc 99m), injected intravenously and imaged with a special camera after it has been excreted by the liver into the bile and emptied into the 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 usually is caused by an obstructing gallstone (Fig. 19-6).

Figure 19-4 Sludge in the gallbladder.

Sludge (solid white arrow) in the gallbladder (GB) is associated with biliary stasis. While it may be echogenic, sludge does not produce acoustical shadowing like gallstones (the absence of shadowing is shown by solid black arrow).

Figure 19-5 Acute cholecystitis, sagittal views, two patients.

A, Thickening of gallbladder (GB) wall. The wall should be 3 mm or less. This wall is markedly thickened at 6 mm (solid white arrows). B, There is an echo-free crescent (dotted white arrow) surrounding a thickened gallbladder (GB) wall representing pericholecystic fluid. The patient had a positive sonographic Murphy sign.

Figure 19-6 HIDA scan in cystic duct obstruction.

HIDA, a radioactive labeled isotope, concentrates in the liver (L) and is then excreted into the biliary ducts. On this delayed image, the common bile duct (solid white arrow) and small bowel (dotted white arrow) fill normally because the common bile duct is patent. The cystic duct and gallbladder do not fill because the cystic is obstructed, by a stone in this case. There is a tracer-free (photopenic) area in the gallbladder fossa because the nuclide cannot fill the gallbladder lumen (solid black arrow).

Bile Ducts

Ultrasound plays a key role in evaluation of the intrahepatic and extrahepatic bile ducts and pancreatic duct. The intrahepatic biliary radicals drain into the left and right hepatic ducts, which join to form the common hepatic duct (CHD). Where the cystic duct from the gallbladder joins the CHD is the origin of the common bile duct (CBD), which drains either within or adjacent to the head of the pancreas via the ampulla of Vater into the second portion of the duodenum. The CBD lies anterior to the portal vein and lateral to the hepatic artery in the porta hepatis (Fig. 19-7).

Figure 19-7 Normal common bile duct, portal vein, and hepatic artery, sagittal view.

The common bile duct (CBD) measures 3 mm (normal <6 mm). The solid white arrow points to the hepatic artery, seen on end. The portal vein (PV) is posterior to the common duct, and the inferior vena cava (IVC) is seen posterior to the portal vein. The pancreas (P) is anterior to the CBD.

The CHD and proximal CBD can be visualized normally on virtually all ultrasound studies of the right upper quadrant. The CHD measures no more than 4 mm (inner wall to inner wall) and the CBD measures no more than 6 mm in diameter. The pancreatic duct measures <2 mm.

Normal intrahepatic bile ducts are not visible. When the CBD is obstructed, the extrahepatic ducts dilate before the intrahepatic ducts. Over time, both the intrahepatic and extrahepatic ducts will be dilated (Fig. 19-8).

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

Figure 19-8 Dilated intrahepatic and extrahepatic ducts in two different patients, sagittal images.

A, The intrahepatic (L = liver) biliary ducts are normally not visible by ultrasound. In this case, they are dilated (solid white arrow) from obstruction by a pancreatic carcinoma (not shown). B, The CBD is dilated at 15 mm (dotted white arrows) and the gallbladder (GB) is distended from an obstructing stone (not shown).

Urinary Tract

Normal Ultrasound Anatomy

The kidneys normally measure 9-12 cm in length, 4-5 cm in width, and are 3-4 cm thick. The renal sinus is home to the renal pelvis and the major branches of the renal artery and vein. Because the renal sinus contains fat, it normally appears brightly echogenic. The calyces are normally not visible. The medullary pyramids are hypoechoic. The renal parenchyma has uniformly low echogenicity, which is usually less than that of the adjacent liver and spleen (Fig. 19-9).

Renal masses are discussed in Chapter 18.

Figure 19-9 Normal right kidney, sagittal view.

The renal sinus (S) is home to the renal pelvis and the major branches of the renal artery and vein. Because the renal sinus contains fat, it normally appears brightly echogenic. The normal renal pelvis is not visible in the renal sinus. The renal parenchyma (solid white arrow) has uniformly low echogenicity and is usually less echogenic than the adjacent liver (L) or spleen.

Hydronephrosis

Hydronephrosis is defined as dilatation of the renal pelvis and calyces.

In patients experiencing renal colic, ultrasound is used primarily to evaluate for the presence of hydronephrosis since the ureters are difficult to visualize with US. The stone search itself is almost always carried out by CT scanning (see Chapter 16).

The typical appearance of obstructive uropathy is a dilated calyceal system. The echogenic renal sinus contains a dilated, fluid-filled, and therefore anechoic renal pelvis. The ureter may be dilated to the level of the obstructing stone. Severe hydronephrosis may distort the appearance of the kidney (Fig. 19-10).

Figure 19-10 Hydronephrosis, sagittal view, right kidney.

The renal sinus now contains a markedly dilated, fluid-filled and anechoic renal pelvis (P) in this patient with hydronephrosis. The renal parenchyma remains normal in size and echogenicity (dotted white arrows). The patient had an obstructing stone at the ureteropelvic junction.

Medical Renal Disease

Medical renal disease refers to a host of diseases that primarily affect the renal parenchyma. They include such diseases as glomerulonephritis, diseases that cause nephrotic syndrome, and renal involvement in collagen vascular diseases.

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Mar 2, 2016 | Posted by admin in GENERAL RADIOLOGY | Comments Off

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