9.2 Introduction

As with other areas of the body, abdominal ultrasound is done on different levels for various structures within the abdomen. This chapter will be organized by ultrasound principles. It will provide information on the nuances that different organs and different investigations dictate. Ultrasounds in the abdominal area can be performed to look for abnormal structural growths, gallstones, or problems in other places like the liver, kidneys, pancreas, or spleen. This chapter will primarily focus on renal-, gallbladder-, and liver-related discussions.

9.3 Renal Ultrasound

Kidneys are interesting because understanding renal function and disease involves several disciplines, including chemistry, biology, and physics. In clinical medicine, we often analyze renal function using standard lab tests such as urinalysis, serum creatinine, and serum blood urea nitrogen (BUN). These tests give us clues about both normal function and etiologies of pathology. Examples of how the three tests help us begin abound: too much protein in the urine in the shape of a glomerulus (called “casts”) may indicate immune-mediated glomerulonephritis, too-high creatinine alone may indicate diffuse renal failure and intravascular fluid overload, and an altered creatinine-BUN ratio may indicate intravascular fluid depletion.

In a typical ultrasound fashion, we will look at the end anatomic result of renal function and formulate a theory of a patient’s condition. Figure 9-1 shows an ultrasonography scan of the longitudinal view of the usual left kidney.

Although it might be challenging to view a nephron or its functional parts on ultrasound, we can observe the results of renal malfunction with our gross anatomic view and make some rapid observations that may help a patient. The fascia is the outer fibrous covering of many organs, and Gerota’s fascia is the one that surrounds the kidneys and adrenal glands. This fascia is particularly dense and hyperechoic, most often producing a bright reflection (white) back onto the screen in the B-mode. From this distinct outline, we can determine the size, shape, location, and consistency of the surrounding structures near the kidney. The kidney measures approximately 11–14 cm in length, 6 cm in width, and 4 cm in thickness. You may measure these at first, but you may soon only estimate the size visually. Other hyperechoic structures typically surround the kidney. The liver is located superior to the right kidney, and the spleen is located superior to the left kidney. The kidneys are retroperitoneal, or behind the peritoneal cavity. Difficulties in visualizing a kidney are due to the presence of air-filled lungs superior to it as well as the ribs. Air, of course, disperses the ultrasound waves so that reflection is complex. Ribs cause shadowing, which may completely prohibit your initial viewing attempts. Because the liver does not fully extend to the left side, the left kidney is often partially covered by the thoracic cavity and more challenging to visualize.

Other than size and shape, a general clinician ultrasound exam may include lobules, evaluation of cortex thickness, evaluation of the renal pelvis, a survey to evaluate hyperechoic renal calculi (kidney stones), and observation of the vessels entering and leaving the renal pelvis (renal arteries, renal veins, and ureters). If atrophy is noted or a patient has marked hypertension, renal artery blood flow velocity is calculated via Doppler technology to determine if there is renal artery stenosis. This latter exam is usually outside the realm of general clinical ultrasound. It may be best for the general ultrasound clinician to refer this exam to those who do the exam often.

Large kidneys, or hydronephrosis, may be due to congenital variation, but this most often is a condition due to distal obstruction. Figure 9-2 shows an ultrasound image of end-stage hydronephrosis. Most often, there is only unilateral obstruction of the ureter. As the kidney continues to make urine in the presence of ureteral obstruction, there is backflow pressure and renal swelling. This is often seen in patient care with ureteral calculi obstructing a ureter. Hematuria (blood in the urine), unilateral pain, and unilateral hydronephrosis are diagnostic of ureteral calculi, even if a calculus is too small to be seen (usually less than 3 mm). Treatment is begun based on this clinical presumption.

Choices to diagnose kidney and ureteral stones include intravenous pyelogram (IVP), plain radiographs, CT scans (which allow the synthesis of a 3D picture from multiple radiologic views), and ultrasound. Plain radiography, or shooting an X-ray through the abdomen, is the oldest evaluation method but is still used. Often hydronephrosis and occasionally an actual stone may be visualized. This method is often used to follow a visible stone over several days. IVP production involves injecting dye into a patient’s vein and taking serial plain X-rays to observe the flow of dye through the kidney and ureter. This method has lost great popularity due to the dye load on the kidney occasionally causing renal malfunction and less accuracy in diagnosis. CT technology is fast, does not require dye in this particular study, and is most accurate. The clarity of this technology is evident to the most inexperienced patient. Even a very small stone with a typical size of 1 mm may be measured more accurately. The stone may be more easily seen even if it does not contain calcium to reflect ultrasound waves. CT is used most often, but ultrasound is becoming more popular due to cost and the lack of ionizing radiation. Figure 9-3 shows an ultrasound image of a renal stone located at the pyeloureteral junction.

Over 70 million CT scans are performed in the United States every year.^[1] The malignant potential of CT scans was most famously brought to the forefront in 2007 by David Brenner and Eric Hall in the New England Journal of Medicine.^[2] Determining the medical cost of a CT scan is also a complex issue. There is a wide range of costs for CT imaging, typically running from $900 to $3,000. It is conceded by most that clinician-generated ultrasound avoids both of these menacing issues.

Clinically, significantly small bilateral kidneys with a thin cortex may indicate chronic renal disease from a diffuse process such as glomerulonephritis or chronic urinary tract infections causing scarring. This condition is distinct from a single small kidney and indicates a localized problem, such as decreased blood flow to only one kidney, known as renal artery stenosis.

9.4 Gallbladder Ultrasound

Gallbladder ultrasounds are standard diagnostic investigations that can be done by primary care and emergency providers. The capsule of the gallbladder, with its fluid-filled contents, often makes a very clearly defined reflective surface. Figure 9-4 shows an ultrasound image of a gallbladder stone. Calculi, often called “gallstones,” may be seen within the gallbladder because of the high density and reflectivity of the discrete objects sitting in often clear fluid. Another feature called “shadowing” is helpful in diagnosis. Shadowing refers to the sharply demarcated darkness that is under the gallstone. Not all gallstones cause disease or need to be addressed. Other features that can be noted on ultrasound and can indicate pathology or a diseased state in a patient include the location of the calculi, the size of the gallbladder, inflammation of the gallbladder, and acute cholecystitis.

9.4.1 Location of the Calculi

Gallstones close to the gallbladder neck are speculated to have more potential to eventually migrate through the gallbladder neck, into the cystic duct, and even into the common bile duct. Gallstones are often more challenging to visualize in the cystic duct or common bile duct due to their small size and coexisting bowel gas reflecting the ultrasound beam before it can reach the gallbladder. Other modalities are often needed to assist in diagnosis.

9.4.2 The Size of the Gallbladder

The gallbladder contracts with cholecystokinin hormone, stimulating the vagus nerve in the parasympathetic nervous system. A small gallbladder may indicate chronic inflammation and scarring. A large gallbladder may also be pathological, even indicating rare gallbladder cancer. The gallbladder size is recorded by manipulating the measurement function on the machine.

9.4.3 Inflammation of the Gallbladder

The wall of the gallbladder can be thickened from inflammation. Greater than 0.3 cm wall thickness is often defined as being pathologically thickened.

9.4.4 Acute Cholecystitis

Surrounding a thickened wall can also be inflammatory fluid, indicating an acute inflammatory response or “acute cholecystitis.” This inflammatory fluid will be seen as a dark area outside the (usually thickened) gallbladder wall, as shown in Figure 9-5.

9.5 Liver Ultrasound

Figure 9-6 is an abdominal ultrasound showing the right lobe of the liver and right kidney. Compared to the gallbladder, kidney, intestine, or bladder, the liver’s thin covering makes it less distinctive. Distinctive features include the following:

• Vascular analysis—Cirrhosis is a pathologic liver condition arising from inflammation and subsequent scarring. This scarring impacts the ease with which blood from the digestive tract and other places flows into the liver. The portal venous backflow causes portal dilation and portal hypertension. This is evaluated by measuring the portal venous velocity using the Doppler and color Doppler functions. There are ultrasound-guided procedures that are used to relieve vascular congestion.
• The duct system in the liver (intrahepatic duct)—Because of the higher lipid contact (and therefore the higher sonar beam reflectivity) of the biliary duct walls, they can be more easily visualized, making diameter determination of the ductal system more convenient. A dilated intrahepatic duct system can arise from a ductal obstruction. Frequently, obstructions may also be calculi from the gallbladder or cancers of organs farther “downstream” in the hepatic system, such as cancer of the pancreas.
• The fluids and reflective surface—The liver is a reflective surface for ultrasound beams. Pathologic fluid in the abdomen may be chronic, such as the fluid from chronic liver disease (known as ascites). It may also be acute, such as an acute intra-abdominal bleed from a motor vehicle accident. The capsular surfaces between the liver and the kidney are usually touching in a healthy individual. As discussed in the earlier chapters, the pressure of abnormal fluid forces the hepatic and renal surfaces apart, creating a new dark space referred to as Morrison’s pouch. In a patient with unlikely chronic ascites due to acute abdominal trauma, an emergency surgery may be performed to stop active bleeding into the abdomen. The emergency ultrasound exam of a patient who has received abdominal trauma is referred to as eFAST, indicating an Extended Focused Assessment with Sonography in Trauma. The original basis of this exam is the attempt to find abnormal (dark) fluid between two highly reflective surfaces that are generally close or even touching. Other than Morrison’s pouch, other areas with potential spaces are between the pericardial and myocardial heart surface, the left kidney and the spleen, and the bladder and uterus in females. In females who have had a hysterectomy and in typical male anatomy, the area under the dense capsule of the urinary bladder can show a dark region of collected fluid. FAST and eFAST exams will be discussed in a little more detail in Chapter 11.
• Abnormal liver size—The liver can be pathologically large from several diseases, including acute hepatitis and liver cancer. The size of the liver is not usually the first clue to either diagnosis but a noted pathology when the disease is already suspected.