Ultrasound Mickey Mouse Sign learning notes

Ultrasound Guide for Emergency Physicians, An Introduction

http://www.sonoguide.com/biliary.html

J. Christian Fox, M.D., RDMS, William P. Scruggs, M.D., RDMS

I.  Introduction and Indications


Self-resolving biliary pain generally lasts 15-minutes to 5-hours, but may persist up to 24 hours ...

Ultrasound does not expose the patient to radiation and is much more accurate than plain film radiographs or computed tomography.  Furthermore, the modality is faster and more generally more readily available than cholescintigraphy or MRI. ...

II.  Anatomy

The gallbladder is an elongated, pear shaped organ that functions to concentrate and store bile.  Along with the liver, it is an embryologic derivative of the foregut.  It normally lies on the visceral surface of the liver between the quadrate and right hepatic lobes covered by a continuation of the hepatic peritoneum.  

The gallbladder consists of the fundus, body, and neck.  The fundus is the most caudal portion of the gallbladder and often will protrude from under the inferior margin of the liver and contact the anterior wall of the abdomen.  The body of the gallbladder tapers cranially, eventually becoming the neck of the gallbladder, which continues to narrow to the junction with the cystic duct.  At its neck, the gallbladder makes a sharp turn away from the parenchyma of the liver into the peritoneal cavity where it becomes the cystic duct. 

The cystic duct carries bile to and from the gallbladder and common bile duct into the duodenum.  It is up to 5 cm long, and it joins the common hepatic duct to create the common bile duct (CBD).  The CBD courses along the free border of the lesser omentum along with the hepatic artery and the portal vein.  It passes behind the duodenum and into the head of the pancreas where it meets the ampulla of Vater at the duodenal papilla. ...


III.  Scanning Technique and Normal Findings

Gallbladder: Curvilinear abdominal probes with a frequency ranging from 2-5 MHz are ideal for examination of the gallbladder.  The lower end of this range may be necessary for sufficient penetration in larger patients.  However, sonographers should increase the frequency whenever possible as the evaluation of wall thickness, pericholecystic fluid, and gallstones is significantly improved with better resolution.

The gallbladder is identified with three basic approaches: The “subscostal sweep” , the “X minus 7” and the "flattening the probe" approach.  The subcostal sweep is generally the most effective window and is usually attempted first.  Start the scan with the probe in longitudinal orientation and the probe-indicator oriented toward the patient’s head and instruct the patient to take a deep breath.  Sweep the probe inferiorly and laterally along the subcostal margin (Video 1).

Video clip 1:  Subcostal sweep video (includes audio).

The X-Minus 7 approach is an intercostal window.  Find the xiphoid process and move laterally to the right approximately 7 centimeter.  Place the probe perpendicular to the skin between the ribs.  In most cases, the gallbladder will be found posterior to the liver parenchyma immediately beneath the probe.  In the few instances where the gallbladder is not identified, move the probe laterally, sweeping through the liver (Video 2).

Video clip 2:  Video clip showing the “X-minus 7” approach (includes audio).

In many young patients in a supine position, the gallbladder will be very anterior and cranial.  In these cases, it is often helpful to point the indicator toward the patient’s right and flatten the probe against the abdomen while aiming the beam toward the right shoulder.  Fan the beam anterior to posterior to identify the gallbladder (Video 3).

Video clip 3:  Video clip showing the “Flattening the probe” technique (includes audio).

With any of the three views, once the gallbladder is identified, stop moving the probe and make small adjustments to create the best long-axis view.  In the long-axis, the gallbladder will usually appear as a pear-shaped, hypoechoic structure with a hyperechoic wall.

Video clip 4:  Gallbladder long axis: Note the main hepatic fissure extending from the tapered end of the gallbladder to the cross-section of the portal vein.  The entire complex resembles an exclamation point.

Because the gallbladder sits in the fossa created by the right and left main lobes of the liver, the main hepatic fissure appears as an echogenic line that extends from the neck of the gallbladder to the portal vein and serves as a landmark.  The complex of the gallbladder, main hepatic fissure, and portal vein (in the short-axis) has the appearance of an exclamation point (Video 4).  Fan the ultrasound beam through the entirety of the gallbladder to identify any pathology.

As with any organ of the body, the gallbladder should be viewed in two planes.  After the long-axis is thoroughly examined, pivot the probe 90 degrees and demonstrate the short-axis (Video 5).  In the short-axis, the gallbladder will appear spherical.  Again, scan through the entire organ in the short axis (Video 6).

Video clip 5:  Rotating the probe (includes audio).

Video clip 6:  Gallbladder short axis:  The video demonstrates fanning through the short axis of the gallbladder from the neck to the fundus.

The gold standard to evaluate the gallbladder wall thickness is evaluated from the short-axis.  Measure the anterior gallbladder wall at its most narrow point.  It is important to obtain a view while the probe and ultrasound beam are perpendicular to the gallbladder wall.  If an oblique section of the wall is measured, the reading will be falsely elevated (Video 7).

Video clip 7:  Measuring the gallbladder wall:  To properly measure the gallbladder wall, view the organ in the short-axis and freeze the image on the narrowest portion of the wall.

Importantly, the normal gallbladder may be contracted.  The lumen of the gallbladder will appear very narrow in these cases and the wall will be thickened with a characteristic three layer appearance.  The inner and outer walls are echogenic while the middle layer is relatively hypoechoic (Video 8).

Video clip 8:  Contracted gallbladder wall:  This image demonstrated the classic appearance of the contracted gallbladder wall.  Note the three distinct layers of the wall.  The wall is thickened, but pathologic thickening will not demonstrate the three layers of the wall.

Common Bile Duct (CBD): The CBD is most easily identified through its association with the portal vein and the portal vein is most easily identified in the long axis of the gallbladder.  It is the ‘point’ of the exclamation point that is created with the gallbladder in the long-axis.  Follow the main lobar fissure from the neck of the gallbladder to the porta hepatis.  The portal vein will appear as a large, hypoechoic circle with echogenic walls.  The CBD and hepatic artery will appear as two smaller circles anterior to the portal vein.  Often times, it gives the appearance of a face with two ears – also called a ‘Mickey Mouse’ sign (Video 9).

Video clip 9:  Mickey Mouse sign:  The video shows an enlarged view of the portal triad in the short-axis.  The CBD and hepatic artery form the ears of the Mickey Mouse sign.  When power flow Doppler is added, the portal vein (large circle) and the hepatic artery (Mickey’s left ear) demonstrate flow.

With the indicator directed toward the patient’s right, the right ear will be the common bile duct and the left ear the hepatic artery.

The best way to evaluate the CBD is in the long axis as gallstones, strictures, or external compression may occur at any point in the tubular structure.  Maintain the Mickey Mouse sign in the center of the screen and rotate the probe 90 degrees without changing the location of the probe on the patient’s skin.  In the long axis, three echogenic lines will stretch across the screen.  The line closest to the probe will be the anterior wall of the common bile duct.  The second line is the shared anterior wall of the CBD and portal vein.  Finally, the third line is the posterior wall of the portal vein.  If there is confusion differentiating between the CBD and portal vein, color flow or power Doppler can be used.  The portal vein will demonstrate flow and the CBD will not (Video 10).

Video clip 10:  Measuring the CBD:  This video demonstrates the porta hepatis in the long-axis with power flow Doppler.  The CBD lies in the long-axis just anterior to the portal vein.  The Doppler is not absolutely necessary, but it is helpful as the hepatic artery is sometimes visualized instead.  This CBD is not measured in the video, but is of normal caliber.

Video clip 11:  This video shows a normal CBD. The clip has three sections: first, three important structures are demonstrated in regular B-Mode: The long, black structure closest to the bottom of the image is the inferior vena cava (IVC) coursing beneath the liver.  Above that is the portal vein. The portal vein is tortuous in this video and the black circular structure at the end of the portal vein to the left is the continuance of the portal vein.  Finally, above the portal vein is the narrow CBD.  In the second part flow is demonstrated using power Doppler. Note the flow through the IVC and several sections of the portal vein. In the third part of the clip, magnification (Zoom function) is applied in B-Mode and the CBD is measured.

Follow the CBD as far distally as possible by moving the probe medial in relation to the patient.  

It is very difficult to follow the CBD, as it passes posterior to the second portion of the duodenum.  It is often identified within the head of the pancreas medially.  Keep in mind that pathology other than choledocholithiasis can result in a dilated CBD and remain suspicious for tumors in the head of the pancreas.

The CBD should be measured at its largest diameter in the long-axis.  Calipers should be used to measure from the interior margin of the anterior wall to the interior margin of the posterior wall.

IV. Pathology

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