|
Magnetic resonance cholangiopancreatography (MRCP) is increasingly
being used to evaluate pancreatobiliary disease, providing a noninvasive
alternative to endoscopic retrograde cholangiopancreatography (ERCP).
MRCP technique uses heavily T2-weighted MR imaging sequences to
visualize static or slow-moving fluid, such as bile and pancreatic
secretions, displaying them as high signal intensity. Current T2-weighted
imaging techniques, including HASTE or SSFSE, can acquire images
in less than one second allowing for elimination of respiratory
artifacts and minimize susceptibility effects from bowel or biliary
gas, surgical clips, and biliary stents. Two types of complementary
images are acquired during MRCP: coronal or coronal-oblique single
thick section (30-80mm) images and multiple thin-section (2-5mm)
source images, from which maximum intensity projection (MIP) images
are reconstructed.
MRCP is highly accurate for the assessment of common bile duct
calculi with a reported range of sensitivity of 86-100%, specificity
of 93-100%, a positive predictive value of 92-100%, and a negative
predictive value of 95-100%. MRI images typically demonstrate ductal
biliary stones as having a rounded or oval-shaped configuration
with a meniscus of fluid level above their proximal edge. On thin-slice
source images, stones appear as signal void lesions surrounded by
high signal intensity bile. Stones as small as 2 mm can be detected
in dilated and non-dilated ducts. On thick-slab images, large or
medium sized stones in normal-caliber ducts are easily detectable.
However, small stones that are completely surrounded by fluid may
be obscured and difficult to detect because of volume-averaging
effects.
There are several physiologic and anatomical mimickers of stones
with MRCP that can lead to a misdiagnosis of choledocholithiasis.
These include:
• Intraductal air bubbles (pneumobilia): air bubbles can
be differentiated from stones by observing that air filling defects
lie on the non-dependent portion of the bile duct against the
wall on axial images or by recognition of an air-fluid level.
• Blood clots: May appear indistinguishable from bile duct
stones.
• Tortuosity of the bile duct running in and out of the
imaging plane.
• Merging of the cystic duct into the CBD when observed
en face on coronal images which may result in a round hypointense
focus.
• Metallic clips.
• Extraductal compression from the right hepatic or the
gastroduodenal artery, which may result in a signal void focus
(as demonstrated by this case).
The hepatic arteries and gastroduodenal artery are closely related
anatomically to the bile duct. The most common sites for of non-pathologic
narrowing are the common hepatic duct, the left hepatic duct,
and the mid-portion of the common bile duct. The right hepatic
artery may compress the common hepatic duct or left hepatic duct
at the posterior aspect, whereas the gastroduodenal artery may
compress the mid-portion of the common bile duct at the anterolateral
aspect.
• Transient forceful contraction of the Vaterian sphincter
muscle complex that surrounds the distal common bile duct causing
a “pseudocalculus sign” which appears as a
smooth or irregular convex upward termination of the distal bile
duct that mimics a distal bile duct calculus.
Careful attention to the exact location of these defects and interpretation
of MRCP MIP reconstruction images in conjunction with thin-section
sources images most often allows for exclusion of the above listed
entities as representing stones.
References:
- Semelka RC. Abdominal-Pelvic MRI. New York: Wiley-Liss Inc,
c2002. pp. 319-345.
- Motohara T, Semelka RC, and Bader TR. MR Cholangiopancreatography.
Radiology Clinics of North America. 2003; 41: 89-96.
- Irie H, Honda H, Kuroiwa T, et al. Pitfalls in MR Cholangiopancreatographic
Interpretation. Radiographics. 2001; 21: 23-37.
|
;)
;)
;)