Upper and lower endoscopy, which enable direct visualization and
biopsy of diseased GI tissue, represent the gold standard for the
evaluation of Crohn's disease. Radiology plays a significant role
as well, one that traditionally has centered on the use of barium
studies, including the barium enema and upper GI study with small
bowel follow-through. It is important to realize, however, that
barium studies show only the GI lumen and not the diseased
Helical CT holds the predominant role in cross-sectional imaging
of Crohn's disease and the evaluation of its complications.
Magnetic resonance imaging (MRI), however, has many advantages for
the evaluation of the GI tract in general, and Crohn's disease in
We first described the double-contrast technique--combining
intraluminal contrast agents and intravenous gadolinium--for MR
evaluation of gastrointestinal disease in 1997.
This MR technique takes advantage of the high contrast resolution
of MR imaging to depict enhancement of the inflamed bowel wall. We
subsequently performed a study comparing helical CT and MRI in
Crohn's disease, using double-contrast techniques.
Among our conclusions were that MRI is superior for depicting both
normal and inflamed bowel wall; shows more marked enhancement of
inflamed areas; is superior to CT for the depiction of subtle bowel
disease; and is equivalent for depicting such complications as
fistula, abscess, or phlegmon.
Some of the challenges that arise in MRI of the GI tract include
peristaltic motion, respiratory motion, and variability in the
position of the small intestine and colon. In addition, it is
necessary to distend the bowel during imaging, and, until recently,
inexpensive intraluminal contrast agents have not been widely
available. It is also necessary to distinguish subtle changes in
the bowel wall from artifact that may be related to the MR
acquisition or from collapsed bowel. Faster pulse sequences and
improved hardware have overcome many of these challenges, enabling
MR to move to the forefront of GI imaging.
We use a double-contrast protocol for MR of the GI tract (Table
First, the patient drinks two to three bottles of ReadiCat 2
(E-Z-Em, Westbury, NY) or an equal amount of water to provide
intraluminal contrast. Water is effective if scanning is
accomplished very quickly. The disadvantage, however, is that it is
reabsorbed by the colon. If the patient drinks slowly, water may
not be effective in distending the bowel.
Next, we administer 500 to 1000 mL of rectal water through a
balloon-tipped enema. We fill the balloon with water to decrease
susceptibility to artifact from the balloon.
We use single-shot spin-echo sequences to obtain very rapid,
heavily T2-weighted images in both the axial and coronal planes. It
takes approximately 19 seconds on our scanner to acquire a set of
12 images. Multiple breathholds are performed to image the abdomen
and pelvis in the axial and coronal planes.
One mg of glucagon is injected intravenously to decrease
peristalsis. This is followed immediately with a double dose of
gadolinium contrast media (0.2 mmol/kg). We then acquire
two-dimensional images using a breathhold fast spoiled
gradient-echo (SGE) sequence with fat suppression. We typically
image at high resolution, using a matrix of 512 * 192. The
bandwidth is about +20 kHz, and we use a three-quarter field of
view to shorten the time of acquisition. It takes 20 to 24 seconds
to acquire 12 images. We set up two passes in the axial plane, each
of which requires 4 breathholds, and then acquire 1 set of coronal
images. The study is performed very rapidly and takes about 15
minutes from start to finish.
Another technique we often apply is MR enteroclysis. Using the
same bowel preparation and single-shot spin-echo sequence, we
acquire a 10-cm thick section using a very long TE (600 msec),
similar to that used in magnetic resonance cholangiopancreatography
(MRCP). This acquisition takes only 2 sec to obtain, and the
resulting image looks very much like a barium study. This technique
may be used to perform dynamic MR imaging of the GI tract by
obtaining multiple sequential 2-second images following the
administration of oral contrast material. By placing these images
in a cine loop, one can review gastric and small intestinal
Among the advantages of the double-contrast approach are the low
cost and ready availability of intraluminal agents, and the
effectiveness of these agents in distending and separating the
bowel. In addition, very rapid breathhold imaging sequences reduce
motion artifact, particularly when glucagon is administered to
Perhaps most important, the double-contrast technique creates a
set of biphasic images that facilitate depiction of both the bowel
lumen and the bowel wall (Figure 1). On the T2-weighted single shot
fast spin echo images, for example, the water or ReadiCat 2 is
bright, thereby functioning as a positive agent that shows the
lumen of the bowel and intestines. This is useful for demonstrating
changes in the caliber of the lumen and in looking for stricture.
In the same patient, the same intraluminal contrast functions as a
negative agent on T1-weighted fat-suppressed gradient echo imaging.
This enables visualization of the wall of the bowel, which normally
appears as multiple thin lines or rings.
In interpreting MR images, it is important to look at the
thickness of the bowel wall and at its enhancement. The normal
bowel wall should measure 3 mm or less; anything more is considered
thickened. For defining mural enhancement, we use a non-fatty liver
as our standard. If mural enhancement is the same or less than that
of the liver, we consider it normal. Enhancement that exceeds that
of the liver is considered mildly abnormal, and enhancement equal
to that of the intravascular gadolinium is considered markedly
MR versus Helical CT
Most radiologists use helical CT to evaluate patients with
Crohn's disease. Nonetheless, the ability to image the bowel wall
is essential for defining pathology in Crohn's disease and other
intestinal disorders, and represents a critical advantage of MRI
over CT or barium studies.
Figure 2 offers a good example. In this patient, helical CT
demonstrates some mural thickening in the wall of the ileum. With
the MRI double-contrast technique, however, the lumen is distended
nicely, and the thickened wall of the ileum shows intense
enhancement. Superior conspicuity of enhancement in the inflamed
wall is a consistent advantage of MRI, one that accounts for its
having become the exam of choice for the evaluation of Crohn's
disease at our center.
Clear differences in the sensitivity of MRI and helical CT
become even more evident in subtle cases of Crohn's disease, as
depicted in Figure 3. Much of the bowel appears normal on CT. On
MRI, however, the entire bowel wall is distinctly abnormal, a
conclusion that was confirmed by endoscopic findings of pancolitis
and ileitis. We consistently find that the distribution of disease
is much better visualized on MRI than on helical CT.
Correlations with Endoscopy
Because of its close correlation with endoscopy, MRI is able to
guide clinical decision-making. We have discovered that the
activity of Crohn's disease correlates with the degree of
gadolinium enhancement, so that a thickened bowel wall that doesn't
enhance signifies inactive disease. This is an important finding,
as a patient who has chronic inactive disease will receive
different treatment than a patient with acute, actively inflamed
Figure 4 demonstrates the ability of MRI to distinguish active
from inactive Crohn's disease. Barium enema shows a stricture, but
it is impossible to determine whether the Crohn's disease is acute
or chronic. On MRI, the degree of enhancement is minimal and the
T2-weighted image is very dark. In our experience, this pattern
correlates with inactive disease. Endoscopy confirmed this
impression, demonstrating a perfectly smooth mucosa. Biopsy found
some chronic inflammation but no active disease.
By comparison, in patients with active disease, marked
thickening and enhancement of the bowel wall correlate well with
endoscopic findings of erythema, ulceration, pseudopolyps, and a
cobblestone-like mucosa, an appearance that signifies very severe
Crohn's disease (Figure 5). Similarly, MRI is a valuable tool for
the assessment of complications of Crohn's disease. Figure 6 shows
a patient with an enterovesicle fistula. On MRI, the inflammatory
mass in the terminal ileum is well visualized, as is an eccentric
thickening of the wall of the bladder near the fistula.
MRI is able to detect very focal disease as well, even though it
can be subtle. An example of this would be a patient who returns
with abdominal pain after surgical resection. In such a case,
focal, localized recurrent disease at the site of bowel anastomosis
may be seen on gadolinium-enhanced MR images with good bowel
Beyond Crohn's Disease
The same double-contrast MRI techniques that work in Crohn's
disease are equally well suited to other types of GI disease. We
use this approach not only in inflammatory bowel disease, but also
to evaluate patients with infectious enteritis and colitis,
mesenteric ischemia, and cancer. We use MRI fairly regularly to
stage colon cancer, for example, looking at the depth of
penetration of the tumor through the wall. It works well for
evaluating gastric and small bowel malignancies, and serosal
metastases from ovarian cancer and other primary tumors.
Figure 7 serves as another example of the range of applications
of double-contrast MRI. This patient had a 1-month history of
diarrhea. The helical CT was unremarkable. MRI showed that the
terminal ileum demonstrated marked thickening and enhancement,
which was not depicted on the helical CT scan. On endoscopy, there
were ulcerations at the ileocecal valve and in the distal ileum, as
well as ulcerated plaques in the right colon. This case of probable
infectious colitis again demonstrates the superior depiction of
relatively subtle changes by MRI when compared with helical CT.
Superior conspicuity of enhancement gives double-contrast MRI a
clear advantage over helical CT in the imaging of patients with
Crohn's disease. This approach is effective for imaging not only
inflammatory bowel disease, but other forms of GI disease,
including such conditions as infectious colitis, mesenteric
ischemia, and cancer. *
Thank you very much, Dr. Low. We have some time for discussion of
this very interesting topic.
I have a couple of questions. The first one relates to early and
later enhancement. I guess the early enhancement I like to think of
as perfusional, and later as sort of interstitial space. Do you
find that it's very important to keep those two data sets separate?
How do you treat them when you evaluate the bowel?
Particularly, when you are trying to distinguish or determine the
degree of activity, you need to look at the perfusional information
from the first pass. We look at that and then try to distinguish if
it is enhancing a lot or a little. The second set, everything tends
to enhance on those. We do not use those in terms of determining
the activity. That's a good point.
The other question is if you have also used that to distinguish
between Crohn's disease and ulcerative colitis? I noticed in your
cases the same thing we've found in our studies, and that is that
with Crohn's you get transmural enhancement, whereas in ulcerative
colitis, we've seen submucosal sparing consistently.
Right. I think because of the incidence of the disease, our
experience with ulcerative colitis is clearly less. But I think our
experience tends to be less intramural thickening. Although at
end-stage ulcerative colitis, we do tend to see more significant
Russell, I noticed in some of your 2D gradient-echo
gadolinium-enhanced images, that you were getting some dropout
where surgical clips were evident. Have you tried the 3D to
We played around with the 3D pulse sequence. But as Richard said,
we tend to like the appearance of the 2D better. There are
certainly some advantages of the 3D in terms of the thickness of
the sections. At this point, we haven't used that significantly.
The single-shot image is probably in that saline one, you have a
lot of bowel gas or something, and with artifact, tend to work
pretty well too.
Russell, you said you used double dose for these studies, why? Did
you try single dose and it didn't work?
It would work with single dose. It's based on our experience
looking at a lot of extrahepatic disease, particularly peritoneal
tumor. We did a lot of comparisons of single versus high dose for
peritoneal tumor. We found that for particularly things outside the
liver, they enhance more, anything that you are waiting for awhile
to enhance. So it's a logical next step to assume that it would
also work in Crohn's disease. I think clearly that does show us
more. Any time you are looking at subtle enhancement, with the
degree of enhancement of something relatively thin, it's important
and more is logically better.