AJR 2001; 176:913-918
© American Roentgen Ray Society
Ability of MR Cholangiography to Reveal Stent Position and Luminal Diameter in Patients with Biliary Endoprostheses
In Vitro Measurements and In Vivo Results in 30 Patients
Elmar M. Merkle1,
Daniel T. Boll1,
Hans Weidenbach2,
Hans-Jürgen Brambs1 and
Andreas Gabelmann1
1
Department of Radiology, University Hospitals of Ulm, Robert Koch Str. 8,
89081 Ulm, Germany.
2
Department of Internal Medicine I, University Hospitals of Ulm, 89081 Ulm,
Germany.
Received June 26, 2000;
accepted after revision September 28, 2000.
Address correspondence to E. M. Merkle.
Abstract
OBJECTIVE. Our goal was to evaluate the ability of MR
cholangiography to show stent position and luminal diameter in patients with
biliary endoprostheses.
MATERIALS AND METHODS. Susceptibility artifacts were evaluated in
vitro in three different stent systems (cobalt alloy—based,
nitinol-based, and polyethylene) using two breath-hold sequences (rapid
acquisition with relaxation enhancement, half-Fourier acquisition single-shot
turbo spin echo) on a 1.5-T MR imaging system. The size of the stent-related
artifact was measured, and the relative stent lumen was calculated. In vivo
stent position and patency were determined in 30 patients (10 cobalt
alloy—based stents, five nitinol-based stents, and 15 polyethylene
stents).
RESULTS. In vitro, the susceptibility artifact of the cobalt stent
caused complete obliteration of the stent lumen. The relative stent lumens of
the nitinol-based and polyethylene stents were 38-50% and 67-100%,
respectively. In vivo, all stents were patent at the time of imaging. The
position of the cobalt alloy—based stent could be determined in nine of
10 patients, but stent patency could not be evaluated. Stent position of
nitinol stents could not be adequately evaluated in any of the five patients,
and internal stent diameter could be visualized in only one patient. In nine
of 15 patients, the fluid column within the implanted polyethylene stent was
seen on MR cholangiography.
CONCLUSION. The internal stent lumen could be visualized in most
patients with an indwelling polyethylene stent, but not in patients with
cobalt alloy— or nitinol-based stents.
Introduction
During the past 10 years, MR cholangiography together with percutaneous
sonography has gained general acceptance as the imaging method of choice for
evaluation of diseases of the biliary system. This technique has replaced
endoscopic retrograde cholangiography, particularly in those patients in whom
an interventional approach seems unlikely at the outset
[1]. The main applications of
endoscopic retrograde cholangiography tend to be more therapeutic than
diagnostic, including extracting bile duct calculi, obtaining biopsy material
and, increasingly, for implanting temporary or permanent stents
[2,3,4].
Literature reports concerning the role of MR cholangiography in monitoring
patient progress after stent implantation are primarily case reports
[5]. We sought to determine the
potential of MR cholangiography for evaluating the biliary tracts of patients
with biliary endoprostheses by measuring luminal stent diameter and stent
position.
Materials and Methods
All MR imaging studies were performed using a closed 1.5-T high-field MR
imaging system (Vision; Siemens, Erlangen, Germany).
Our first step consisted of determining in vitro the artifact behavior of
the three biliary endoprostheses most commonly used in our hospital: Easy
Wallstent (cobalt alloy-based; diameter, 8 mm) (Schneider, Buelach,
Switzerland); Smart Stent (nitinol wire; diameter, 8 mm) (Cordis, Miami, FL);
and the 11.5-French Cotton-Huibregtse biliary stent (polyethylene; diameter,
2.7 mm) (Cook, Moenchengladbach, Germany). Stents were first filled with water
through a tube system and then immersed in a copper sulfate solution that
exhibits an MR signal intensity equivalent to fat. For imaging, the plastic
tubes containing the stents were positioned in the center of the bore parallel
to the main magnetic field B0.
The in vitro imaging protocol consisted of the following sequences:
multislice HASTE (half-Fourier acquisition single-shot turbo spin echo)
(TR/TE, 1.9/95; acquisition time, 9 sec; in-plane resolution, 1 mm; slice
thickness, 3 mm; scan orientation, perpendicular to the long stent axis);
single-slice RARE (rapid acquisition with relaxation enhancement) (2800/1000;
acquisition time, 7 sec; in-plane resolution, 1 mm; slice thickness, 50 mm;
scan orientation, parallel to the long stent axis with frequency-encoding axis
parallel to the stent axis); multislice HASTE (1.9/95; acquisition time, 9
sec; in-plane resolution, 1 mm; slice thickness, 3 and 5 mm; scan orientation,
parallel to the long stent axis with frequency-encoding axis parallel to the
stent axis); and multislice HASTE (1.9/95; acquisition time, 9 sec; in-plane
resolution, 1 mm; slice thickness, 3 and 5 mm; scan orientation, parallel to
the long stent axis with frequency-encoding axis perpendicular to the stent
axis).
Whole-body transmit coils incorporated into the scanner were used for
radiofrequency transmission, and a circular polarized body array coil was used
for signal reception. Fat saturation was performed in all sequences using a
chemical shift technique. Measurements of the diameter of the stent lumen were
performed by a single observer and were based on signal-intensity plots drawn
orthogonally to the long axis of the stent. Patent lumina were measured along
histograms at half-maximum width
[6]. Measurements were rounded
to the nearest millimeter. The size of the stent-related artifact (signal
void) was determined relative to the known diameter.
During a 9-month period, MR cholangiography was performed on 30 patients
(average age ± SD, 63 ± 14 years) who had undergone implantation
of a biliary endoprosthesis. After informed consent in accordance with
institutional guidelines was obtained, all patients received oral
administration of 600 mL of an iron-containing contrast medium (Lumirem;
Guerbet, Sulzbach, Germany) to reduce interfering signals from the stomach and
duodenum. Before MR cholangiography, a T2-weighted axial turbo spin-echo
sequence (TR/TE, 8.2/66; acquisition time, 13 sec; slice thickness, 6 mm) was
performed to localize the biliary system. The MR cholangiographic protocol
consisted of the following sequences: single-slice RARE (2800/1100;
acquisition time, 7 sec; in-plane resolution, 1.5 mm; slice thickness, 50 mm;
chemical shift fat saturation; scan orientation, coronal and semicoronal
[angulation of 20° from the coronal toward the sagittal plane clockwise
and counter-clockwise]); and multislice HASTE (1.9/95; acquisition time, 11
sec; in-plane resolution, 1.5 mm; slice thickness, 3 mm; chemical shift fat
saturation; scan orientation, coronal and semicoronal [angulation of 20°
from the coronal toward the sagittal plane clockwise and
counterclockwise]).
Underlying diseases included chronic pancreatitis (n = 6),
pancreatic carcinoma (n = 5), carcinoma of the biliary tract
(n = 7), hepatocellular carcinoma (n = 1), liver metastases
(n = 9), and enlarged lymph nodes in the liver hilum (n =
2). Fifteen patients underwent endoscopic placement of polyethylene stents
(11.5-French [n = 11], 7-French [n = 4]), whereas another 15
patients underwent percutaneous placement of a metallic stent (Easy Wallstent
[n = 10]; Smart Stent [n = 5]). All stents were patent at
the time of examination. Percutaneous transhepatic cholangiography or
endoscopic retrograde cholangiography was performed within 48 hr of MR
cholangiography and served as the gold standard.
MR cholangiographic findings regarding the diameter of the stent lumen and
position were evaluated independently by two examiners. By definition, the
stent lumen was considered adequately evaluated in those cases in which a
"water column" could be visualized throughout the length of the
stent. Stent position was evaluated as "good" (demarcated along
its entire length with clear localization of the stent ends),
"moderate" (stent not clearly demarcated in all its segments), or
"missing." In cases in which the examiners returned divergent
opinions, a consensus was reached. All other coincidental abnormal findings
revealed on MR cholangiography were documented.
Results
In vitro, the stent lumen was best visualized in the polyethylene stent,
which showed no appreciable susceptibility artifact. Internal stent lumen was
2-3 mm, whereas the actual diameter was 2.7 mm. In the Smart Stent, which is
composed of nitinol, in vitro measurements revealed a visualized, internal
stent lumen diameter of 3-4 mm, although the actual diameter was 8 mm. In the
case of the cobalt alloy—based Easy Wallstent, the entire internal stent
lumen was overlain by artifacts (Fig.
1A,1B,1C).
Changes in phase and frequency-encoding direction had no significant effect on
the extent of the susceptibility artifacts.
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Fig. 1A. —In vitro artifact behavior of three biliary endoprostheses.
One asterisk = Smart Stent (Cordis, Miami, FL), two asterisks = polyethylene
stent (Cook, Moenchengladbach, Germany), three asterisks = Easy Wallstent
(Schneider, Buelach, Switzerland). Half-Fourier acquisition single-shot turbo
spin-echo source image (TR/TE, 1.9/95; in-plane resolution, 1 mm; slice
thickness, 3 mm; scan orientation, parallel to long stent axis).
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Fig. 1B. —In vitro artifact behavior of three biliary endoprostheses.
One asterisk = Smart Stent (Cordis, Miami, FL), two asterisks = polyethylene
stent (Cook, Moenchengladbach, Germany), three asterisks = Easy Wallstent
(Schneider, Buelach, Switzerland). Half-Fourier acquisition single-shot turbo
spin-echo source image (1.9/95; in-plane resolution, 1 mm; slice thickness, 3
mm; scan orientation, perpendicular to long stent axis).
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Fig. 1C. —In vitro artifact behavior of three biliary endoprostheses.
One asterisk = Smart Stent (Cordis, Miami, FL), two asterisks = polyethylene
stent (Cook, Moenchengladbach, Germany), three asterisks = Easy Wallstent
(Schneider, Buelach, Switzerland). Rapid acquisition with relaxation
enhancement image (2800/1000; in-plane resolution, 1 mm; slice thickness, 50
mm; scan orientation, parallel to long stent axis). Lumen of cobalt
alloy—based Easy Wallstent is completely overlain with artifact
formation. Relative interior lumen of the nitinol-based Smart Stent is
visualized at about 50%, and relative interior lumen of polyethylene stent
stands at 100%.
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In vivo examinations of the 10 patients with Easy Wallstents failed to
reveal the stent lumen in all patients. The stents themselves, however,
because of their significant susceptibility artifacts, were more amenable to
visualization and were rated as good in seven patients and as moderate in two
others (Fig.
2A,2B,2C,2D).
In one case, neither of the two sequences revealed the patient's implanted
Wallstent due to motion and respiratory artifacts.
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Fig. 2B. —46-year-old woman with breast and gastric carcinoma.
Percutaneous transhepatic cholangiogram shows cobalt alloy—based Easy
Wallstent (10-mm diameter, 70-mm length) (Schneider, Buelach, Switzerland)
implanted in common bile duct.
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Fig. 2C. —46-year-old woman with breast and gastric carcinoma.
Percutaneous transhepatic cholangiogram reveals that stent shown in B
is located in transpapillary position in which distal stent end is in contact
with opposite wall of duodenum and appears to perforate it
(arrow).
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Fig. 2D. —46-year-old woman with breast and gastric carcinoma. MR
cholangiogram (half-Fourier acquisition single-shot turbo spin-echo source
image [TR/TE, 1.9/95; in-plane resolution, 1.5 mm; slice thickness, 3 mm])
reveals stent position, including intraduodenal position of stent end
(arrow), whereas stent lumen cannot be evaluated because of artifact
formation. Patient's postinterventional course was uneventful.
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In the five patients who received nitinol stents, MR cholangiography failed
to reveal the stent lumen in four patients (Figs.
3A,3B
and
4A,4B,4C).
Because of the low degree of the associated susceptibility artifacts, even the
stent position proved difficult to visualize. MR cholangiography localized
stents with a rating of moderate in only four of five patients. In one
patient, neither of the two sequences revealed the patient's implanted Smart
Stent.
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Fig. 3B. —62-year-old man with gastric carcinoma. MR cholangiogram
(rapid acquisition with relaxation enhancement; TR/TE, 2800/1000; in-plane
resolution, 1.5 mm; slice thickness, 50 mm) shows that lumen of Smart Stent is
totally overlain by artifact formation, whereas stent itself, because of weak
associated susceptibility artifact, is inadequately visualized in cranial
section of image.
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Fig. 4C. —61-year-old man with gastric carcinoma. MR cholangiogram
(rapid acquisition with relaxation enhancement image; TR/TE, 2800/1000;
in-plane resolution, 1.5 mm; slice thickness, 50 mm) shows adequate
visualization of inner lumen, whereas stent itself, because of weak
susceptibility artifact, is not demarcated.
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In the 15 patients with polyethylene stents, the lumen was clearly
visualized in nine patients, although the stent itself, because of the lack of
susceptibility artifacts, could not be demarcated (Fig.
5A,5B).
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Fig. 5A. —65-year-old man with chronic pancreatitis. MR cholangiogram
(rapid acquisition with relaxation enhancement image; TR/TE, 2800/1000;
in-plane resolution, 1.5 mm; slice thickness, 50 mm) inadequately shows patent
polyethylene stent (arrows).
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Fig. 5B. —65-year-old man with chronic pancreatitis. Half-Fourier
acquisition single-shot turbo spin-echo source image (1.9/95; in-plane
resolution, 1.5 mm; slice thickness, 3 mm) permits unequivocal evaluation of
interior lumen of stent (arrows).
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In 12 of the 30 patients, MR cholangiography yielded important coincidental
findings (i.e., dilatation of Wirsung's duct, pancreatic pseudocyst,
cholecystolithiasis, cavernous transformation of the portal vein, pancreatic
tumor, and liver tumor), which, although already known in some cases from
patient histories, were not visualized on percutaneous transhepatic
cholangiography or endoscopic retrograde cholangiography performed as the gold
standard in the respective patients.
Discussion
At many imaging centers, MR cholangiography has essentially replaced both
IV cholangiography and endoscopic retrograde cholangiography as the imaging
method of choice for visualization of the biliary tract. The preference is
clear, based on the fact that MR cholangiography is noninvasive, does not
subject the patient to radiation exposure, and does not require IV contrast
medium. Although IV cholangiography has thus become obsolete, endoscopic
retrograde cholangiography remains indicated primarily in the interventional
field in which, aside from stone extraction and harvesting biopsy material for
cytology and chemical analysis, its use for endoscopic stent implantation has
increased in importance. This includes temporary polyethylene stents placed in
the common bile duct and permanent metallic stents placed endoscopically in
the bile duct system. These endoprostheses require regular follow-up because,
in cases of malignancy-related stenoses, they may become occluded by tumor
growth, whereas their lumina are subject to sludge incrustation that threatens
their patency regardless of the patient's diagnosis.
A further problem is that of stent dislocation
[7]. Although percutaneous
sonography offers a simple, cost-effective, and noninvasive method for
evaluating the intrahepatic biliary tract, the ability of this method to show
extrahepatic biliary structures is significantly less than that of MR
cholangiography [8]. The
capabilities of sonography are further diminished because of its difficulty in
penetrating the stent cage—a problem that complicates assessment of the
vascular lumen and the vessel wall—and the quality of the examination is
operator-dependent [6,
8,
9]. In such cases, endoscopic
retrograde cholangiography remains the method of choice because this modality
also offers the option of performing interventional procedures during the same
session. However, because patients with biliary tract abnormalities are often
multimorbid, a noninvasive imaging technique free of potential complications
would be desirable in light of the other concomitant risk factors.
There are many reasons why MR cholangiography is not used more frequently
in patients with biliary stents. The image quality of MR cholangiography is
often severely compromised by susceptibility artifacts associated with the
stent. In addition, aside from the composition of the stent alloy, other
factors, such as the magnetic field strength B0, the orientation of
the stent in relation to the magnetic field, and the individual sequence
parameters of sequence type and echo time, must be taken into consideration
[10,11,12,13].
In addition, the internal stent mesh structure may cause signal deflection
from the interior lumen (Melzer et al., presented at the International Meeting
of the Society of Magnetic Resonance in Medicine, April 2000). Furthermore,
particularly in patients with indwelling metallic stents, there are certain
safety issues to consider, such as the potential movement or dislodging of the
stent by magnetic field interactions and the heating of the endoprosthesis by
radiofrequency power deposition
[14]. However, because stents
are increasingly manufactured with MR-compatible materials, even MR
imaging—guided percutaneous or endoscopic stent implantation is
increasingly possible. Wacker et al.
[15] performed percutaneous
transhepatic cholangiography without complication in an animal model using a
0.2-T low-field MR imaging system for guidance; Wacker et al. then confirmed
these positive findings in three patients (presented at the third
international meeting of the Society for Magnetic Resonance in Medicine, April
2000). Also, MR-compatible, custom-made endoscopes have recently become
available [16].
Our in vitro results show that even with the current state-of-the-art
breath-hold MR cholangiographic sequences (RARE and HASTE), only with the
polyethylene stent is it possible to routinely visualize the inner lumen to an
adequate degree for evaluation (Fig.
1A,1B,1C).
Only with this stent type could MR cholangiography consistently correctly show
the inner lumen of the stent independent of the sequence type and the
direction of the frequency-encoding axis.
In imaging the nitinol-based Smart Stent, MR cholangiography revealed the
inner stent lumina to be in the range of 3-4 mm, which represents a relative
inner lumen of 38-50% of the actual known stent lumen. Because the difference
of 1 mm corresponds to one pixel at this imaging level, the difference is
probably most accurately described as a measurement inaccuracy. The inner
lumen of the cobalt alloy—based Easy Wallstent could not be evaluated
during breath-hold MR cholangiography sequences; the relative interior stent
lumen was 0% at all measurements.
To our knowledge, there are no published reports in the literature
describing the in vitro artifact behavior of biliary stents in routine MR
cholangiographic sequences. This subject is, however, drawing increased
attention in the field of MR angiography as the development of fast
three-dimensional gradient-echo sequences, the availability of specific
surface coils, and the bolus-triggered application of contrast medium have
combined to permit a satisfactory visualization of both the arterial and
venous vascular systems. Baum et al.
[17] found that using a
three-dimensional fast imaging with steady-state precession (FISP) sequence
with an effective slice thickness of 1.3 mm (anterior-to-posterior, 80 mm, 60
partitions, scan orientation parallel to the long stent axis), the size of the
relative stent lumen can vary from 0% to 100% depending on the composition of
the stent alloy and its design. These findings were confirmed by Buecker et
al. (ISMRM meeting, April 2000) who, using a similar sequence, reported
relative stent lumina of 0-55%. In addition, the same group was able to show
that the extent of susceptibility artifacts depends on the orientation of the
stent in relation to the magnetic field B0, which had previously
been shown only for MR imaging—compatible biopsy needles
[10]. This point may, however,
be primarily of theoretic interest, because in vivo stent position is
determined by the course of the vessel, whereas the patient's positioning in a
closed, high-field system permits only minimal adaptation.
Using a three-dimensional gradient-recalled echo sequence, Hilfiker et al.
[6] were able to see the
interior lumen of the nitinol-based Cragg (Mintec, Freeport, Bahamas) stent
with a relative stent lumen of approximately 80%. One reason for the
difference between our results and theirs may be differences in the design of
the nitinol-based Smart Stent, although the more probable explanation may be
the short TE of the three-dimensional gradient-recalled echo sequence
(approximately 2 msec), because the extent of the susceptibility artifact
increases with TE [11,
13]. Corresponding to our
data, the relative interior lumen of the cobalt alloy—based Easy
Wallstent stood at 0% in the MR angiographic measurements reported by Hilfiker
et al. As expected, in vivo localization was most reliable with the Easy
Wallstent (Fig.
2A,2B,2C,2D),
for which the associated susceptibility artifacts are sufficiently large. The
artifact formation is, however, not large enough to compromise the evaluation
of the stent surroundings (i.e., the portal region and the pancreatic head).
This confirms the findings of Girard et al.
[18] who performed MR imaging
using mid-field systems in eight patients with indwelling biliary Wallstents
and reported that visualization of adjacent structures was not influenced by
any significant artifact overlay when spin-echo sequences were used. The
signal of the interior lumen of the Easy Wallstent is, however, totally
obliterated; hence, no evaluation with regard to sludge incrustation or tumor
infiltration is possible using this stent system.
Because of the lesser extent of the associated susceptibility artifact,
evaluation of the interior lumen of the nitinol-based Smart Stent should
theoretically be possible. However, this was not confirmed in four of the five
patients we examined; demarcation of the stent itself was only partially
possible because the extent of artifact was not sufficiently large (Figs.
3A,3B
and
4A,4B,4C).
Our best results were obtained in patients with indwelling polyethylene
stents. The interior stent lumen was adequately visualized in nine of 15
patients (Fig.
5A,5B).
The stent itself, because of the general lack of susceptibility artifact, was
predictably not delineated.
Whether using MR cholangiography in patients with biliary endoprostheses
may offer superior results using mid- or low-field MR imaging scanners,
because of the lesser extent of associated susceptibility artifact formation,
remains to be addressed in future studies
[19,
20]. A totally new concept is
being studied by Melzer et al. (ISMRM meeting, April 2000), who have developed
a new type of stent with integrated capacity and conductivity adjusted and
tuned as a resonance circuit to the frequency of the MR imaging system.
In conclusion, the internal stent lumen may be visualized only in those
patients who have an indwelling polyethylene stent. Although MR
cholangiography can reveal at least the position of a cobalt alloy—based
stent, our findings suggest that MR cholangiography cannot show either the
exact position or the internal lumen diameter of nitinol stents. However, MR
cholangiography in patients with biliary stents is a safe and effective
imaging technique that can provide valuable information. When there are
positive findings, the use of MR cholangiography may often spare a severely
ill patient from having to undergo an invasive procedure such as endoscopic
retrograde cholangiography. We must emphasize that only a limited number of
biliary stents have been evaluated in our study, and thus, the results are not
applicable to all types of biliary stents.
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Abstract
Introduction
