The advancements in CT and MR imaging techniques provide radiologists with additional options for biliary imaging, along with the use of sonography. With newer techniques, it is possible to achieve better characterization and earlier detection of common and rare biliary diseases. The authors review recent advances in CT biliary imaging and MR cholangiopancreatography.
is a Resident and
is an Assistant Professor, Department of Radiology, The Mount
Sinai Medical Center, New York, NY.
In recent years, there have been several improvements in biliary
imaging. While sonography is the predominant modality for the
evaluation of the biliary tree, the advent of and improvements in
computed tomographic (CT) and magnetic resonance (MR) imaging
techniques have resulted in better characterization and earlier
detection of both common and rare biliary diseases. This article
will briefly discuss recent advances in CT biliary imaging and MR
The currently available techniques for biliary evaluation have
several limitations. Although ultrasound remains the standard
initial test for assessment of the biliary tree, its limitations
include inconsistent visualization of the distal common bile duct,
operator dependence, and the difficulty in providing a reproducible
Endoscopic retrograde cholangiopancreatography (ERCP) can not only
be used to diagnose disease but can also be used for treatment.
This technique is limited by a significant failure rate of 1% to
10%, as well as associated mortality and morbidity rates of 1% and
Therefore, ERCP should be avoided when acceptable noninvasive
Percutaneous transhepatic cholangiography (PTC) also has the
potential to both diagnose and treat biliary disease; however, it,
too, suffers from significant morbidity and mortality--up to 2.5%
An additional common limitation of both invasive techniques is poor
visualization of obstructed ducts that often cannot be directly
Standard MRCP imaging technique
Two unique properties of bile that are exploited to aid
visualization by nearly all MRCP sequences are its relatively high
water content and its stasis compared with that of the adjacent
blood vessels in the portal tracts. Therefore, most MRCP sequences
are heavily T2-weighted and rely on an acquisition that is usually
slower than moving blood, producing high signal in the biliary tree
and signal voids in blood vessels that lie in close proximity.
The current workhorse of MRCP is the long echo-train T2-weighted
image, which permits rapid and effective imaging of the biliary
system. By varying the echo time (TE), the T2-weighting of the
sequence can be modulated to emphasize or localize bile by improved
visualization of adjacent soft tissue structures. This sequence is
usually acquired in the axial and coronal planes (true coronal or
oblique coronals) with a slice thickness of 4 to 5 mm for standard
2-dimensional (2D) acquisitions. By supplementing this sequence
with fat saturation and increasing the slice thickness to 40 to 60
mm, "thick-slab" MRCP images can be obtained. To allow for adequate
signal from bile, images cannot be acquired in rapid succession,
such as in a single breath-hold. The repetition time (TR) of MRCP
sequences must be sufficiently long (generally >4000 msec) to
permit recovery of the water content so that the next pulse will be
effective (Figure 1). However, this same factor (the relatively
long TR time) that enables good visualization of bile also limits
A recent addition to MR biliary imaging has been sequences that
allow for free-breathing during the acquisition of high-resolution
3-dimensional (3D) images. When compared with their 2D
counterparts, these sequences allow for a higher signal-to-noise
per voxel and, therefore, a higher spatial resolution. In addition,
it is possible to utilize zero-fill interpolation between acquired
slices. Recent studies have shown comparable diagnostic accuracy of
these sequences to the more traditional techniques.
The outcome of 3D sequences is often quite dramatic, even in a
patient who cannot tolerate short breath-holds. In the authors'
experience, the only significantly degraded studies have been
obtained when patients shift their position during the relatively
long (3 to 7 minutes) free-breathing acquisition.
This technique can be performed with a traditional respiratory
monitor (bellows) or with a navigator sequence (Figure 2). The
results can be treated like any high-contrast 3D acquisition with
further manipulation on a workstation-a technique usually reserved
for angiographic procedures. While maximum-intensity projection
(MIP) images of this data are quite similar to the thick-slab
images, postprocessing enables the removal of artifact from
sources, including fluid in the duodenum and stomach (Figure
However, it has been the authors' clinical experience that there
is some blurring of small areas of signal voids, which often
represent calculi or, less likely, air (Figure 4). This likely
represents the motion-induced edge distortion of the
higher-signal-intensity bile and, thus, obscuration of adjacent
areas of signal void during normal respiration. Further studies are
necessary to validate this observation.
While robust noncontrast MR techniques can answer almost all
diagnostic questions, there are occasions in which the additional
information yielded from a contrast-enhanced examination can be
useful. These include the evaluation of potential liver donors
and the functional assessment of the biliary anastomosis subsequent
The drawback of this approach is the requirement of intact liver
function for the uptake and excretion of contrast; therefore,
contrast-enhanced studies have decreased utility in patients with
an elevated serum bilirubin level.
There are currently 2 drugs on the market that are approved by
the Food and Drug Administration (FDA) that are excreted by the
biliary system. The older of the 2 agents is mangafodipir trisodium
(TESLASCAN, GE Healthcare, Chalfont St. Giles, UK). While this
agent is adequate in biliary imaging, it is no longer available in
the U.S. market (Figure 5). The newer agent, gadobenate dimeglumine
(MultiHance, Bracco Diagnostics, Inc., Princeton, NJ), has been
FDA-approved for central nervous system applications, and its use
for biliary imaging is off-label. It has an approximate 4% biliary
excretion and can be used as an adjunct to the normal examination,
with the addition of delayed imaging approximately 1 hour after the
initial injection. This technique may be particularly useful in a
patient whose biliary anatomy has been surgically altered so that
it is not readily accessible by ERCP. Figure 6 is an example of
such a study in a patient with a Roux-en-Y biliary anastomosis and
recurrent nonspecific abdominal pain. There was concern of possible
obstruction involving either of the 2 biliary anastomoses versus
another source of the intermittent pain. The routine MRCP
examination revealed only mild, nonspecific biliary dilatation,
which may be attributed to the anastomosis. With additional delayed
biliary imaging, a significant biliary stricture was excluded as
the source of the patient's symptoms. However, moderate stenosis of
the proper hepatic artery and complete occlusion of a right hepatic
branch were noted (Figure 6).
Although CT generally has a higher spatial resolution when
compared with MR imaging, relatively, it lacks contrast resolution,
which limits its ability to evaluate the biliary system. Therefore,
while CT is very specific for biliary pathology, its sensitivity is
CT cholangiography can be performed using iodipamide meglumine
(Cholografin, Bracco). This contrast was originally used for
imaging of the biliary system with conventional fluoroscopy. When
performing CT cholangiography at our institution, this agent is
diluted in 100 mL of normal saline and infused over 30 minutes.
Subsequent imaging occurs 45 minutes after injection, a technique
that has been described by previous authors.
This results in excellent anatomical assessment of patients with
intact liver function and is ideal for patients who are undergoing
evaluation for liver donation, as well as for the occasional
patient who cannot undergo MRCP.
While no studies are available that have compared
contrast-enhanced MRCP with CT cholangiography, in the authors'
experience with both techniques, one can obtain improved spatial
and contrast resolution with CT cholangiography (Figure 7).
Hepatic arterial assessment
Hepatic arterial compromise is frequently recognized as a source
of biliary pathology in patients who have undergone liver
transplant (Figure 8). This is because of the relatively exclusive
supply of the biliary tree by the hepatic artery, as opposed to the
remainder of the hepatic parenchyma, which has a dual blood supply.
Although significantly less common, hepatic artery compromise can
also result in biliary tree damage in the nontransplant population.
Such instances are seen in motor-vehicle trauma, surgical
complications, and hepatic artery chemo-embolization. Damage to the
right hepatic artery is often more serious, as it is often the sole
supply to the confluence of the right and left hepatic biliary
ducts. The imaging findings often mimic that of sclerosing
cholangitis with a predominant feature of biliary strictures.
Patients may present with bilomas later in the course of the
Cholangiocarcinoma and primary sclerosing
In the Western Hemisphere, there has been an increase in the
incidence of peripheral cholangiocarcinoma during the latter half
of the 20th century, which cannot be simply accounted for by a
higher detection rate.
This is in contradiction to the decrease in incidence of central
cholangiocarcinoma, which can be partially attributed to a higher
rate of cholecystectomies. Improvement in both initial detection
and correct preoperative grading of the extent of disease is
required in biliary tract malignancy imaging.
While CT is highly sensitive for intrahepatic disease detection
>1 cm, it has limited ability for correctly assessing
resectability and under-stages disease in up to 60% of patients.
MR imaging is more accurate in assessing biliary carcinomas but
still fails to correctly stage patients in 20% of cases. In both
cases, this is most likely because of microscopic perineural
Currently, MRCP is the best preoperative method for assessing the
extent of biliary invasion, even when compared with ERCP.
MR cholangiopancreatography also continues to gain acceptance as
an initial modality by which patients with suspected primary
sclerosing cholangitis (PSC) are assessed (Figure 9). This poorly
understood disease has been increasingly diagnosed largely because
of better cholangiographic techniques. Primary sclerosing
cholangitis is believed to be a premalignant condition. Most
patients ultimately succumb to cholangiocarcinoma or a colonic
malignancy related to an associated autoimmune disorder, such as
ulcerative colitis. There is an estimated annual incidence of 1% to
1.5% and prevalence of 10% of cholangiocarcinoma in patients with
Even though MR imaging, including MRCP, is believed to be one of
the best modalities for surveillance, it still has a poor
sensitivity for the de-tection of cholangiocarcinoma in this
high-risk patient population
(Figures 10 and 11).
The long-term follow-up of patients after liver transplantation
has shown a rate of recurrence of this disease in the transplanted
organ that ranges from 10% to 20%.
This diagnosis, however, is entertained only in patients who are
>90 days posttransplant and who have corresponding elevations in
appropriate liver enzymes. In all cases, hepatic artery thrombosis
must also be excluded.
Even though the tools available for the evaluation of the
biliary tree continue to grow in sophistication and clinical
application, areas of deficiency persist in biliary cancer
detection and staging. Additional tools, such as MR spectros-copy
and diffusion imaging, may hold potential for improved tumor
detection. Further investigations of noninvasive biliary imaging
modalities are warranted in order to continue its pace of
development in the years to come.