CHOLELITHIASIS ULTRASOUND IMAGING
Category: Diagnostic Radiology
Abstract : CHOLELITHIASIS The most commonly and reliably identified gallbladder pathology is that of gallstones. More than 10% of the population of the UK have gallstones. Many of these are asymptomatic, which is an important point to remember. When scanning a patient with abdominal pain it should not automatically be assumed that, when gallstones are present, they are responsible for the pain. It is not
CHOLELITHIASIS
The most commonly and reliably identified gallbladder pathology is that of gallstones. More than 10% of the population of the UK have gallstones. Many of these are asymptomatic, which is an important point to remember. When scanning a patient with abdominal pain it should not automatically be assumed that, when gallstones are present, they are responsible for the pain.
It is not uncommon to find further pathology in the presence of gallstones and a comprehensive upperabdominal survey should always be carried out. Gallstones are associated with a number of conditions. They occur when the normal ratio of components making up the bile is altered, most commonly when there is increased secretion of cholesterol in the bile. Conditions which are associated with increased cholesterol secretion, and therefore the formation of cholesterol stones, include obesity, diabetes, pregnancy and oestrogen therapy. The incidence of stones also rises with age, probably because the bile flow slows down. An increased secretion of bilirubin in the bile, as in patients with cirrhosis for example, is associated with pigment (black or brown) stones.
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.
Shadowing
The ability to display a shadow posterior to a stone depends upon several factors:
● The reflection and absorption of sound by the stone. This is fairly consistent, regardless of the composition of the stone.
● The size of the stone in relation to the beam width. A shadow will occur when the stone fills the width of the beam. 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. Higher-frequency transducers have better resolution and are therefore more likely to display fine shadows than lower frequencies.
● The machine settings must be compatible with demonstrating narrow bands of shadowing. The fluid-filled gallbladder often displays posterior enhancement, or increased throughtransmission. If the echoes posterior to the gallbladder are saturated this will mask fine shadows. Turn the overall gain down to display this better. 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.
● Bowel posterior to the gallbladder may cast its own shadows from gas and other contents, which makes the gallstone shadow difficult to demonstrate. This is a particular problem with stones in the common bile duct (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.
Reflectivity
The reflective nature of the stone is enhanced by its being surrounded by echo-free bile. In a contracted gallbladder the reflectivity of the stone is often not appreciated because the hyperechoic gallbladder wall is collapsed over it. Some stones are only poorly reflective, but should still cause a distal acoustic shadow.
Mobility
Most stones are gravity-dependent and this may be demonstrated by scanning the patient in an erect position, 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. 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. 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 or a distended gallbladder due to a mucocoele.
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