Technique & Views.

PROBE

The curved low-frequency probe is of choice as it provides enough depth to visualise deeper structures. It should be used in the standard abdominal setting.

PATIENT’S POSITION

The supine position is the most practical, and both deep inspiration and raising the patient’s right arm will improve visualization. If it is still difficult to obtain a proper image, positioning the patient in a left lateral decubitus, displacing the duodenum, is likely to facilitate the views.

Consider that the gallbladder empties its content after meals and therefore is harder to view if the patient has recently eaten. Also, the walls may appear thicker because of physiological GB contraction. More than one window, scanning plane and patient position are required to view the GB adequately.

THE VIEWS

Place the probe under the xiphoid process with the marker to the patient's head, and slide it across the costal margin towards the mid-axillary line. The fasted gallbladder appears as a well-demarcated, fluid-filled structure located at the medial-inferior edge of the liver. The gallbladder narrows down towards the neck, where the portal vein lies. Confirm that you are actually scanning the GB by identifying the portal triad and the “exclamation sign”.

LONGITUDINAL VIEW

Align the probe with the GB long axis to reveal the longitudinal view and scan through it while altering the probe angle to assess the fundus, body and neck properly. Depending on the patient’s habitus and anatomy, you will also visualise the IVC and Abdominal Aorta. Take a second to identify the structures accurately.

TRANSVERSE VIEW

A 90º degree rotation to the right will show the gallbladder in transverse view. Fan the probe to assess from fundus to neck, and be sure to scan as much of the GB as possible to avoid missing localized pathology. Be aware that a content-filled duodenum can resemble a large stone occupying the gallbladder; hence, it is essential to be able to differentiate between both.

Cholelithiasis.

Classically, GB calculi are described as intraluminal hyperechoic images that cast posterior acoustic shadows. If not impacted, they should also be mobile and have gravitational dependence. Consequently, scanning the patient in more than one position may elucidate an impacted gallstone, as it will not move with gravity. Neither a very small stone (<5 mm diameter) nor sludge will cast a posterior shadow, although multiple small calculi (microlithiasis) may present poorly defined shadowing.

WALL ECHO SHADOW

If the gallbladder is completely filled with stones, the normal fluid-filled structure with posterior enhancement is no longer seen. Instead, the wall echo shadow (WES) sign can be found as two parallel bright lines with a large posterior shadowing. The bright line closest to the probe is the GB wall and the line beneath represents the echogenic calculi. WES is important because it may be misinterpreted as a bowel loop, when in fact represents cholelithiasis.

Be mindful that not every echogenic image inside the GB lumen is a stone. Neither gallbladder polyps nor sludge will cast acoustic shadowing, the latter, however, has gravitational dependence, while polyps do not move when repositioning the patient.

Cholecystitis.

The diagnosis of acute cholecystitis involves clinical features, inflammatory markers, and characteristic image findings. The classic ultrasound findings are wall thickening > 3 mm, sonographic Murphy’s sign, pericholecystic fluid, and/or impacted gallstone with a distended GB. Yet, as discussed earlier, accurate identification of these signs can be difficult and involves a complex learning curve. Particularly for the novice, confidently ruling out cholecystitis by ultrasound is likely to require the experience of a radiologist.

The normal adult wall thickness is 3mm or less, but normal GB walls will also be thickened due to contraction, which is important to remember in the non-fasted ED population. The GB wall should be measured perpendicular to the long axis and at the anterior wall only, as the liquid inside the GB generates enough acoustic enhancement to make the posterior wall appear falsely thickened. A thickened wall may be seen as two echogenic lines with a hypoechoic region between them (double line sign). Wall thickening suggests acute inflammation and oedema, but can be found in other disease states such as congestive heart failure, sepsis, ascites, renal failure, HIV or and multiple myeloma.

The sonographic Murphy’s sign is assessed by applying pressure with the transducer over the GB. If this reproduces the patient’s pain, it is considered positive and strongly suggest acute GB disease. Pericholecystic free fluid can be found in acute cholecystitis and speaks of a more advanced inflammation stage. However, it may also be found in other disease states, namely pancreatitis, cirrhosis and ascites. An impacted gallstone will not move if the patient is repositioned, and will usually be accompanied by GB distension. Remember that proper interpretation of the images can only be achieved if the GB is visualised in its entire length, including fundus, body and neck. When the image is incomplete (WES) or non-definitive, a formal study or another imaging modality is needed.