Ultrasound Imaging of the Biliary Tract 393 make the diagnosis of acute cholecystitis. The com- the gallbladder lumen that cast a posterior shadow bination of US findings that is most predictive of (Fig. 1). Although ultrasound has been demon- acute cholecystitis is the presence of a positive sone rated to have an accuracy (>95%)for the identifica Secondary signs on US examination of acute chole- <1 mm to cast a posterior shadow soft stones lacking graphic Murphy sign plus the presence of gallstones. tion of gallstones, stones that are too small, (usually cystitis include gallbladder wall thickening (>3 strong internal echoes 1 or gallstones impacted in Bs h ), a distended or hydropic gallbladder(loss of the gallbladder neck or in the cystic duct that may the normal tapered neck and development of an el- not be as readily detectable on US examination as liptic or rounded shape), and pericholecystic fluid. they silhouette with the surrounding echogenic bowel gas or intraperitoneal fat(see Fig. 1)15.If The sonographic Murphy,'s sign the gallbladder is focally tender but no gallstones are appreciated the patient should be examined The sonographic Murphy sign is defined as repro- from multiple positions and scanning planes, ducible point tenderness specifically over the gall- cluding prone, upright and decubitus positions bladder upon application of pressure by the and intercostal scanning, to facilitate complete visu transducer Ralls and colleagues [4] wrote a classic alization of the neck of the gallbladder [3, 61 article that reported a sonographic Murphy sign Harmonic imaging significantly improves visual was 87% specific for the diagnosis of acute ization of small gallstones. This type of ultrasound cholecystitis, in a patient population which only in- transmits the insonating us beam at a fundamental cluded patients who had right upper quadrant pain, frequency, such as 2.5 or 3 MHz, and receives the re- fever and an elevated white blood cell count. Laing turning echoes not only at the fundamental fre and colleagues [5| reported that the presence of quency but also at the second harmonic frequency a positive sonographic Murphys sign in combina- that is twice the fundamental frequency creating tion with the presence of gallstones has a positive the image with the higher harmonic frequency predictive value of 92%for the diagnosis of acute 17-9 By eliminating the fundamental frequency, cholecystitis. In order to avoid false positive exami- this technique significantly reduces degradation of lations, one must be careful to elicit pain directly the image by noise, since lower frequencies easily over the gallbladder, not diffusely in epigastrium, can be filtered out. In addition, scattering of the or over the liver edge. False negative examinations US beam from fat in the anterior abdominal wall may occur in patients who have received pain med- is diminished because the harmonic frequencies icine, patients who are taking steroids, para or are generated after the beam enters the body. The quadriplegics, or any patient who is not able to narrower harmonic beam also has fewer side lobes, give a reliable history or pain response. In addition, and therefore, improved lateral resolution and sig. the sonographic Murphy s sign may be absent in de- nal to noise ratio. Harmonic imaging increases nervated gallbladders, for example, in patients who the echogenicity of gallstones and strengthens have diabetes or gangrenous cholecystitis. A sono. their posterior shadows, permitting visualization diminished if the gallbaldder ruptures because ultrasound(see Fig. 1). Another technique that im this will relieve the obstruction. Therefore, careful proves visualization of stones is spatial compound attention to the patient's clinical status is important ing. Multiple images are acquired slightly off axis when assessing for a sonographic Murphy's sig from one another, which increases the signal from the persistent echoes that comprise the image and Gallstones blurs out some of the random noise. the disadvan- Gallstones are diagnosed on US by the presence of tage of compounding is that posterior shadowing is diminished, which may be a better visual cue to gravity-dependent, mobile, echogenic foci within dete llstones than the actual echoes Fig 1. Gallstones. (A)(Left) Gallstone in the gallbladder neck (arrow) casts no significant shadow and is nearly invisible. Gas in the duodenum(arrowhead)obscures the fundus of the gallbladder and casts a strong sharp shadow(asterisk).(Right) With patient in sitting position, stone (arrow)moves out of the neck and casts a clear shadow (asterisk). Adjacent duodenum(arrowheads) is now separate from the gallbladder but still casts a strong shadow, equivalent to the gallstone. B)(Left)Multiple gallstones(arrowheads), some of which cast shadow (arrows), whereas others do not. (Right) Normal caliber common duct( 6 mm at the porta)with stones(arrows) in the same patient. Choledocholithiasis may be difficult to detect, especially in the distal duct, if the stones do not shadow or are not outlined by fluid (o(Left) Longitudinal ultrasound shows a normal gallbladder. ( Right) Harmonic imaging reveals multiple small stones(arrows).(From Rubens D Hepatobiliary imaging and its pitfalls Radiol Clin North Am 2004: 42: 257-78: with permission.make the diagnosis of acute cholecystitis. The com￾bination of US findings that is most predictive of acute cholecystitis is the presence of a positive sono￾graphic Murphy’ sign plus the presence of gallstones. Secondary signs on US examination of acute chole￾cystitis include gallbladder wall thickening (>3 mm), a distended or hydropic gallbladder (loss of the normal tapered neck and development of an el￾liptic or rounded shape), and pericholecystic fluid. The sonographic Murphy’s sign The sonographic Murphy sign is defined as repro￾ducible point tenderness specifically over the gall￾bladder upon application of pressure by the transducer. Ralls and colleagues [4] wrote a classic article that reported a sonographic Murphy sign was 87% specific for the diagnosis of acute cholecystitis, in a patient population which only in￾cluded patients who had right upper quadrant pain, fever and an elevated white blood cell count. Laing and colleagues [5] reported that the presence of a positive sonographic Murphy’s sign in combina￾tion with the presence of gallstones has a positive predictive value of 92% for the diagnosis of acute cholecystitis. In order to avoid false positive exami￾nations, one must be careful to elicit pain directly over the gallbladder, not diffusely in epigastrium, or over the liver edge. False negative examinations may occur in patients who have received pain med￾icine, patients who are taking steroids, para or quadriplegics, or any patient who is not able to give a reliable history or pain response. In addition, the sonographic Murphy’s sign may be absent in de￾nervated gallbladders, for example, in patients who have diabetes or gangrenous cholecystitis. A sono￾graphic Murphy’s sign also may be significantly diminished if the gallbaldder ruptures because this will relieve the obstruction. Therefore, careful attention to the patient’s clinical status is important when assessing for a sonographic Murphy’s sign. Gallstones Gallstones are diagnosed on US by the presence of gravity-dependent, mobile, echogenic foci within the gallbladder lumen that cast a posterior shadow (Fig. 1). Although ultrasound has been demon￾strated to have an accuracy (>95%) for the identifica￾tion of gallstones, stones that are too small, (usually <1 mm to cast a posterior shadow soft stones lacking strong internal echoes [1], or gallstones impacted in the gallbladder neck or in the cystic duct that may not be as readily detectable on US examination as they silhouette with the surrounding echogenic bowel gas or intraperitoneal fat (see Fig. 1) [5]. If the gallbladder is focally tender but no gallstones are appreciated, the patient should be examined from multiple positions and scanning planes, in￾cluding prone, upright and decubitus positions and intercostal scanning, to facilitate complete visu￾alization of the neck of the gallbladder [3,6]. Harmonic imaging significantly improves visual￾ization of small gallstones. This type of ultrasound transmits the insonating US beam at a fundamental frequency, such as 2.5 or 3 MHz, and receives the re￾turning echoes not only at the fundamental fre￾quency but also at the second harmonic frequency that is twice the fundamental frequency creating the image with the higher harmonic frequency [7–9]. By eliminating the fundamental frequency, this technique significantly reduces degradation of the image by noise, since lower frequencies easily can be filtered out. In addition, scattering of the US beam from fat in the anterior abdominal wall is diminished because the harmonic frequencies are generated after the beam enters the body. The narrower harmonic beam also has fewer side lobes, and therefore, improved lateral resolution and sig￾nal to noise ratio. Harmonic imaging increases the echogenicity of gallstones and strengthens their posterior shadows, permitting visualization of stones not seen with conventional grayscale ultrasound (see Fig. 1). Another technique that im￾proves visualization of stones is spatial compound￾ing. Multiple images are acquired slightly off axis from one another, which increases the signal from the persistent echoes that comprise the image and blurs out some of the random noise. The disadvan￾tage of compounding is that posterior shadowing is diminished, which may be a better visual cue to detect typical gallstones than the actual echoes Fig. 1. Gallstones. (A) (Left) Gallstone in the gallbladder neck (arrow) casts no significant shadow and is nearly invisible. Gas in the duodenum (arrowhead) obscures the fundus of the gallbladder and casts a strong sharp shadow (asterisk). (Right) With patient in sitting position, stone (arrow) moves out of the neck and casts a clear shadow (asterisk). Adjacent duodenum (arrowheads) is now separate from the gallbladder but still casts a strong shadow, equivalent to the gallstone. (B) (Left) Multiple gallstones (arrowheads), some of which cast shadows (arrows), whereas others do not. (Right) Normal caliber common duct (6 mm at the porta) with stones (arrows) in the same patient. Choledocholithiasis may be difficult to detect, especially in the distal duct, if the stones do not shadow or are not outlined by fluid. (C) (Left) Longitudinal ultrasound shows a normal gallbladder. (Right) Harmonic imaging reveals multiple small stones (arrows). (From Rubens D. Hepatobiliary imaging and its pitfalls. Radiol Clin North Am 2004;42:257–78; with permission.) : Ultrasound Imaging of the Biliary Tract 393