Ultrasound appearances

There are three classic acoustic properties associated with stones in the gallbladder; they are highly reflective, mobile and cast a distal acoustic shadow. In the majority of cases, all these properties are demonstrated (Figs 3.1–3.3).

Fig. 3.1 • (A) LS and (B) TS through a gallbladder demonstrating stones with posterior acoustic shadowing.

(C) Tiny stones lie together to form a block of acoustic shadow behind the gallbladder.

Fig. 3.2 • Gallstone mobility:

(A) Shadowing from a stone in the neck of the gallbladder.

(B) With the patient erect, the stone drops to the fundus of the gallbladder.

Fig. 3.3 • (A) Floating gallstones cast a shadow from the anterior gallbladder (arrow).

(B) Floating stones in the gallbladder lumen, mimicking gas-filled bowel.

Shadowing

The ability to display a shadow posterior to a stone depends on several factors:

1 The reflection and absorption of sound by the stone. This is fairly consistent, regardless of the composition of the stone.

2 The size of the stone in relation to the beamwidth. A shadow occurs when the stone fills the width of the beam (Fig. 3.4). This will happen easily with large stones, but a small stone may occupy less space than the beam, allowing sound to continue behind it, so a shadow is not seen. Small stones must therefore be within the focal zone (narrowest point) of the beam and in the centre of the beam to shadow (Fig. 3.5). Higher frequency transducers have better resolution and are therefore more likely to display fine shadows than lower frequencies.

3 The machine settings must be compatible with demonstrating narrow bands of shadowing. The fluid-filled gallbladder often displays posterior enhancement, or increased through-transmission. If the echoes posterior to the gallbladder are ‘saturated’ this will mask fine shadows. Turn the overall gain down to display this better (Fig. 3.6). Some image processing options may reduce the contrast between the shadow and the surrounding tissue, so make sure a suitable dynamic range and image programme are used.

4 Bowel posterior to the gallbladder may cast its own shadows from gas and other contents, making the gallstone shadow difficult to demonstrate (Fig. 3.7). This is a particular problem with stones in the CBD. Try turning the patient to move the gallbladder away from the bowel. The shadow cast by gas in the duodenum, which contains reverberation, should usually be distinguishable from that cast by a gallstone, which is sharp and clean.

Fig. 3.4 • (A) This small stone is wider than the beamwidth, and so casts an acoustic shadow (arrow).

(B) When the beam focusing is moved anterior to the gallbladder, the shadow is no longer evident as the beam is now wider than the stone.

Fig. 3.5 • (A) A layer of tiny stones does not shadow when the focal zone is placed incorrectly (arrow), but the shadow is easily demonstrated (B) when the focal zone is placed at the posterior wall of the gallbladder, narrowing the beam at this point.

Fig. 3.6 • The shadow from the stone in Figure 3.4 is obscured by overamplification of the echoes behind the gallbladder.

Fig. 3.7 • (A) Shadowing from the stone in the neck of the gallbladder (arrow) is less obvious due to the structures lying behind the gallbladder.

(B) Shadowing from duodenum posterior to the gallbladder can obscure the shadowing from tiny stones within the gallbladder lumen. Changing patient position and angle of scanning moves the gas away from the gallbladder.

(C) Shadowing from stones in the contracted gallbladder lumen (arrow) is stronger and better defined than the ‘dirty’ shadowing from adjacent bowel (arrowhead).

(D) A contracted gallbladder full of stones casts a strong acoustic shadow from the gallbladder fossa. Note the echoes from the anterior gallbladder wall are distinct from the echoes from the stones.

Mobility

Most stones are gravity dependent, and this may be demonstrated by scanning the patient in an erect position (Fig. 3.2), when a mobile calculus will drop from the neck or body of the gallbladder to lie in the fundus. Some stones will float, however, forming a reflective layer just beneath the anterior gallbladder wall with shadowing that obscures the rest of the lumen (Fig. 3.3B). When the gallbladder lumen is contracted, either due to physiological or pathological reasons, any stones present are unable to move and this is also the case in a gallbladder packed with stones (Fig. 3.7D).

Occasionally a stone may become impacted in the neck, and movement of the patient is unable to dislodge it. Stones lodged in the gallbladder neck or cystic duct may result in a permanently contracted gallbladder, a gallbladder full of fine echoes due to inspissated (thickened) bile (Fig. 3.8) or a distended gallbladder due to a mucocoele (see below).

Fig. 3.8 • (A) A stone lodged in the gallbladder neck with thickened, inspissated bile filling the gallbladder lumen.

(B) The gallbladder neck is curled round and a stone is lodged within it (arrow).

Biliary reflux and gallstone pancreatitis

A stone may become lodged in the distal CBD near the ampulla. If the main pancreatic duct joins the CBD proximal to this, bile and pancreatic fluid may reflux up the pancreatic duct, causing inflammation and severe pain. Reflux up the CBD may also result in ascending cholangitis, particularly if the obstruction is prolonged or repetitive. Cholangitis may result in dilated bile ducts with mural irregularity on ultrasound, but magnetic resonance cholangiopancreatography (MRCP) is usually superior in demonstrating intrahepatic ductal changes of this nature.

Bile reflux is also associated with anomalous cystic duct insertion (Fig. 3.12A, B) which is more readily recognized on MRCP than ultrasound.

Fig. 3.12 • (A) Anomalous insertion of the cystic duct (arrows) into the lower end of the CBD.

(B) Appearances of case in (A) are confirmed on ERCP. A stone is also present in the duct.

(C) MRCP showing stones in the CBD.

(D) ERCP can be used for therapeutic purposes, such as stone removal or stent insertion.

Further management of gallstones

MRCP and endoscopic retrograde cholangiopancreatography (ERCP) demonstrate stones in the duct with greater accuracy than ultrasound, particularly at the lower end of the CBD, which may be obscured by duodenal gas on ultrasound⁵ (Fig. 3.12C, D). ERCP is invasive, carrying a small risk of morbidity or, rarely, mortality due to perforation, infection or pancreatitis, but has the advantage of providing the therapeutic option of sphincterotomy and stone removal. This is the modality of choice when stones are known to exist in the duct, (for example following MRCP) and has supplanted surgical removal in many cases.⁶

Laparoscopic cholecystectomy is the preferred method of treatment for symptomatic gallbladder disease in an elective setting. Acute cholecystitis is also increasingly managed by early laparoscopic surgery, with a slightly higher rate of conversion to open surgery than elective cases.⁷ Laparoscopic ultrasound may be used as a suitable alternative to operative cholangiography to examine the common duct for residual stones during surgery.⁸ It compares well to cholangiography, with a sensitivity and specificity of 96% and 100%, and avoids any radiation dose, but has been slow to be adopted in the UK, as it requires specialized equipment and training.⁹ Both ultrasound and cholescintigraphy are used in monitoring post-operative biliary leaks or haematoma (Fig. 3.13).

Fig. 3.13 • (A) Post-operative bile collection in the gallbladder bed.

(B) Hyperechoic, irregular ‘mass’ in the gallbladder bed, which represents a resolving haematoma after laparoscopic cholecystectomy.

Other, less common options include dissolution therapy and extracorporeal shock wave lithotripsy (ESWL). However, these treatments are often only partially successful, require careful patient selection, and also run a significant risk of stone recurrence.¹⁰

Mucocoele of the gallbladder

If the cystic duct is obstructed – usually by a stone which has failed to pass through to the CBD – the normal flow of bile from the gallbladder is interrupted. Chronic cystic duct obstruction causes the bile to be replaced by mucus secreted by the lining of the gallbladder, resulting in a mucocoele. The biliary ducts remain normal in calibre.

If the gallbladder is dilated in the absence of duct dilatation, make a careful search for an obstructing lesion at the neck; a stone in the cystic duct is more difficult to identify on ultrasound as it is not surrounded by echo-free bile (Fig. 3.8).

Mirizzi’s syndrome

Mirizzi’s syndrome is a rare cause of biliary obstruction in which compression of the biliary tree is caused by a stone in the adjacent cystic duct. This usually happens in combination with a surrounding inflammatory process which compresses and obstructs the adjacent common hepatic duct, causing distal biliary duct dilatation. This is associated with a low insertion of the cystic duct into the common hepatic duct. Occasionally a fistula forms between the hepatic duct and the gallbladder due to erosion of the duct wall by the stone. Ultimately this may lead to gallstone ileus – small bowel obstruction resulting from migration of a large stone through the cholecystoenteric fistula. If the condition is not promptly diagnosed, recurring cholangitis leading to secondary biliary cirrhosis may result.

On ultrasound the gallbladder is typically contracted and contains debris. A stone impacted at the neck may be demonstrated together with dilatation of the intrahepatic ducts with a normal-calibre lower common duct (Fig. 3.14). The diagnosis is a difficult one, as it is frequently not possible to rule out carcinoma. CT or MRI may assist in this distinction, and ERCP is still considered the ‘gold standard’ especially as it can offer therapeutic stone removal and/or stent placement.¹² Endoscopic or intraductal ultrasound, if available, have improved the diagnostic accuracy of suspected cases.¹³ Although rare, it is an important diagnosis as cholecystectomy in these cases has a higher rate of operative and post-operative complications.¹⁴

Postprandial

The most likely cause of a contracted gallbladder is physiological, following a meal. (This may still occur despite instructions to fast and it is always worth enquiring when and what the patient has last eaten or drunk.) The normal gallbladder wall is thickened when contracted, and this must not be confused with a pathological process (Fig. 3.15).

Fig. 3.15 • Normal, postprandial gallbladder with consequently thickened wall.

Pathological causes of a small gallbladder

Most pathologically contracted gallbladders contain stones. When the gallbladder cannot be identified, try scanning transversely through the gallbladder fossa, just caudal to the porta hepatis. Strong shadowing alerts the sonographer to the possibility of a contracted gallbladder full of stones.

The reflective surface of the stones and distal shadowing are apparent, and the anterior gallbladder wall can be demonstrated with correct focusing and good technique (Figs 3.7D, 3.16). Do not confuse the appearances of a previous cholecystectomy, when bowel in the gallbladder fossa casts a shadow, for a contracted stone-filled gallbladder.

Fig. 3.16 • (A) Shadowing from the gallbladder fossa indicates a contracted gallbladder packed with stones.

(B) A small layer of bile is visible between the stones and the anterior gallbladder wall.

A less common cause of a small gallbladder is the microgallbladder associated with cystic fibrosis (Fig. 3.17). The gallbladder itself is abnormally small, rather than just contracted. Cystic fibrosis also carries an increased incidence of gallstones because of the altered composition of the bile and bile stasis and the wall might be thickened and fibrosed from cholecystitis.

Fig. 3.17 • Microgallbladder in cystic fibrosis.

Porcelain gallbladder

When the gallbladder wall becomes calcified the resulting appearance is of a solid reflective structure causing a distal shadow in the gallbladder fossa (Fig. 3.18). (This can be distinguished from a gallbladder full of stones where the wall can usually be seen anterior to the shadowing [Fig. 3.7D]).

Fig. 3.18 • Porcelain gallbladder demonstrating a calcified inner layer of wall.

A porcelain gallbladder probably results from a gallbladder mucocoele – a long-standing obstruction of the cystic duct, usually from a stone. The bile inside the non-functioning gallbladder is gradually replaced by watery fluid, the wall becomes fibrotic and thickened and ultimately calcifies.

There is an association between porcelain gallbladder and gallbladder carcinoma, so a prophylactic cholecystectomy is usually performed to pre-empt malignant development.¹⁵

Shadowing from the calcified anterior gallbladder wall can obscure the gallbladder contents, and can mimic bowel in the gallbladder fossa. A plain X-ray also clearly demonstrates the porcelain gallbladder.