GALLBLADDER ULTRASOUND SCANNING

THE GALLBLADDER ULTRASOUND
The normal gallbladder is best visualized after fasting, to distend it. It should have a hyperechoic, thin wall and contain anechoic bile. Measure the wall thickness in a longitudinal section of the gallbladder, with the calipers perpendicular to the wall itself. (A transverse section may not be perpendicular to the wall, and can overestimate the thickness.) After fasting for around six hours, it should be distended with bile into an elongated pear-shaped sac. The size is too variable to allow direct measurements to be of any use, but a tense, rounded shape can indicate pathological, rather than physiological dilatation. Because the size, shape and position of the gallbladder are infinitely variable, so are the techniques required to scan it.

There are, however, a number of useful pointers to maximize visualization of the gallbladder:
● Use the highest frequency possible: 5.0 MHz or higher is especially useful for anterior gallbladders.
● Use a high line density to pick up tiny stones or polyps (reduce the sector angle and the frame rate if possible). Make sure the focal zone is set over the back wall of the gallbladder to maximize the chances of identifying small stones.
● Alter the time gain compensation (TGC) to eliminate or minimize anterior artefacts and reverberation echoes inside the gallbladder, particularly in the near field.
● Use tissue harmonic imaging to reduce artifact within the gallbladder and sharpen the image of the wall (particularly in a large abdomen).
● Always scan the gallbladder in at least two planes (find the gallbladders long axis, incorporating the neck and fundus; sweep from side to side, then transversely from neck to fundus) and two patient positions. You will almost certainly miss pathology if you do not.
● The gallbladder may be folded (the so-called Phrygian cap). To interrogate its contents fully, unfold it by turning the patient decubitus (right side raised), almost prone or erect.
● Bowel gas over the fundus can also be moved by various patient positions.

Normal variants of the gallbladder :
The mesenteric attachment of the gallbladder to the inferior surface of the liver is variable in length. This gives rise to large variations in position; at one end of the spectrum the gallbladder, attached only at the neck, may be fairly remote from the liver, even lying in the pelvis; at the other the gallbladder fossa deeply invaginates the liver and the gallbladder appears to lie intrahepatically enclosed on all sides by liver tissue. The presence of a true septum in the gallbladder is rare. A folded gallbladder frequently gives the impression of a septum but this can be distinguished by positioning the patient to unfold the gallbladder. Occasionally a gallbladder septum completely divides the lumen into two parts. True gallbladder duplication is a rare entity and it is important not to mistake this for a gallbladder with a pericholecystic collection in a symptomatic patient. Occasionally the gallbladder is absent altogether.

Pitfalls in scanning the gallbladder If the gallbladder cannot be found
● Check for previous surgery; a cholecystectomy scar is usually obvious, but evidence of laparoscopic surgery may be difficult to see in the darkened scanning room.
● Check the patient has fasted.
● Look for an ectopic gallbladder, for example positioned low in the pelvis.
● Check that a near-field artefact has not obscured an anterior gallbladder, a particular problem in very thin patients.
● Ensure the scanner frequency and settings are optimized, find the porta hepatis and scan just below it in transverse section. This is the area of the gallbladder fossa and you should see at least the anterior gallbladder wall if the gallbladder is present.
● A contracted, stone-filled gallbladder, producing heavy shadowing, can be difficult to identify due to the lack of any contrasting fluid in the lumen. Duodenum mimicking gallbladder pathology
● The close proximity of the duodenum to the posterior gallbladder wall often causes it to invaginate the gallbladder. Maximize your machine settings to visualize the posterior gallbladder wall separate from the duodenum and turn the patient to cause the duodenal contents to move.
● Other segments of fluid-containing gastrointestinal tract can also cause confusion. Stones that dont shadow
● Ensure they are stones and not polyps by standing the patient erect and watching them move with gravity. (Beware polyps on long stalks also move around.)
● The stones may be smaller than the beam width, making the shadow difficult to display. Make sure the focal zone is set at the back of the gallbladder.
● Increase the line density, if possible, by reducing the field of view.
● Scan with the highest possible frequency to ensure the narrowest beam width.
● Reduce the TGC and/or power to make sure you have not saturated the echoes distal to the gallbladder. Beware the folded gallbladder
● You may miss pathology if the gallbladder is folded and the fundus lies underneath bowel. Always try to unfold it by positioning the patient.
● A fold in the gallbladder may mimic a septum. Septa are comparatively rare and have been over-reported in the past due to the presence of folding.

Pathology or artefact?
Sometimes the gallbladder may contain some echoes of doubtful significance, or be insufficiently distended to evaluate accurately. A rescan, after a meal followed by further fasting, can be useful.This can flush out sludge, redistending the gallbladder with clear bile. It may also help to clarify any confusing appearances of adjacent bowel loops.