AJR 2004; 183:1309-1315
© American Roentgen Ray Society
MRI of Perianal Crohn's Disease
Karin Horsthuis1 and
Jaap Stoker
1 Both authors: Department of Radiology, Academic Medical Center Amsterdam,
Meibergdreef 9, Amsterdam 1105 AZ, The Netherlands.
Received April 13, 2004;
accepted after revision May 20, 2004.
Address correspondence to K. Horsthuis.
Introduction
MRI is an effective imaging technique for the evaluation of patients
with perianal Crohn's disease. Perianal fistulas are reported to occur in up
to 38% of these patients [1].
MRI is an important tool because it can accurately reveal the location and
extent of disease, including a clinically undetected fistula or abscess
[2,
3], and can guide surgery
[4]. We present the spectrum of
MRI findings of perianal fistulating disease in patients with Crohn's disease
and an imaging-based approach of interpreting these examinations.
Anatomy
The inner layer of the anorectal canal consists of squamous and columnar
epithelium with the transitional zone at the dentate line. At this level, the
anal glands that reach to the intersphincteric space empty into the anorectal
crypts. The cause of Crohn's perianal fistulas may be an inflammation or
infection of these anal glands or penetration of fissures or ulcers in the
anorectal canal [1,
5,
6]. The muscular component of
the anal sphincter consists of an inner layer of circular smooth muscle
(internal anal sphincter), extending downward from the rectum, and an outer
striated muscular layer extending downward from the levator ani muscle,
comprising the puborectalis muscle superiorly and the external anal sphincter
inferiorly (Fig. 1A). The
horizontal part of the levator plate is constituted by pubococcygeus
(anterior) and iliococcygeus (posterior) muscles. (To prevent confusion, we
will refer to these muscles as the levator ani muscle in the figure legends.)
Between these layers is the fat-containing intersphincteric space, including
the continuation of smooth-muscle fibers of the longitudinal muscle of the
rectal wall (Fig. 1B). Outside
the anal sphincter is the fat-containing ischioanal space.
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Fig. 1A. —Healthy 35-year-old woman. C = endoanal coil. Coronal
T2-weighted endoanal image shows normal sphincter anatomy with relatively
hyperintense internal sphincter (I) and hypointense external sphincter (E).
Puborectalis muscle (P) joins levator plate (L) superiorly. Shape of
puborectalis muscle shown is physiologic variation. Often puborectalis muscle
is more closely fused with external sphincter
(Fig. 6B). Fat-containing
ischioanal space (IAS) is hyperintense.
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Fig. 1B. —Healthy 35-year-old woman. C = endoanal coil. Axial
T2-weighted endoanal image shows normal anatomy of sphincter complex; external
(E) and internal (I) sphincters are distinguishable from each other because of
substantial contrast difference. Fat-containing intersphincteric space (ISS)
is bright on this T2-weighted image (short arrow), whereas
longitudinal muscle layer (LML) situated in intersphincteric space is
hypointense (long arrow).
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Fig. 6B. —33-year-old man with Crohn's disease who exhibited distinct
perianal fistulas and abscesses despite infliximab treatment. G = gluteus
muscle. Coronal T2-weighted image shows two of three fistulas
(arrows). This sequence shows intersphincteric course of
transsphincteric fistulas. IO = internal obturator muscle, L = levator ani
muscle.
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Imaging Sequences and Planes
We propose an MRI protocol (Table
1) composed primarily of T2-weighted fast spin-echo sequences
because these sequences produce adequate contrast among the sphincter
components, fistula, and scar tissue. On T2-weighted sequences, the internal
sphincter has a homogeneous structure and is hyperintense compared with the
external sphincter. The intersphincteric space displays a signal intensity
equal to that of the surrounding fat adjacent to the hypointense longitudinal
muscle fibers, although this thin layer may not be discernible on external
phased array MRI. The striated external anal sphincter and the puborectal and
levator ani muscles are relatively hypointense. On T1-weighted fast spin-echo
sequences, relatively little difference in contrast is seen between the
sphincter muscles, except for avid enhancement of the internal sphincter after
administration of IV contrast medium.
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TABLE 1 MRI Protocol for Evaluating Patients for Perianal Fistulas on 1.5-T
Scanner with External Phased-Array Coils
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Preferably a sagittal T2-weighted sequence is performed first. This
nonangulated sequence can be used to orient axial and coronal sequences at the
anal canal, which provides the advantage of imaging in surgically relevant
planes. An axial fat-saturated T2-weighted sequence is recommended for optimal
conspicuity of fluid and inflammatory changes (Figs.
2A,
2B). A contrast-enhanced
(fat-saturated) T1-weighted sequence is helpful for differentiating between
abscess and inflammatory changes because pus does not enhance after
administration of IV contrast material, whereas inflammatory tissue does
enhance (Figs. 3A,
3B and
4A,
4B,
4C). We recommend performing a
similar (native) sequence before contrast administration to determine whether
an area of intermediate or high signal intensity on the enhanced series is in
fact hemorrhagic or proteinaceous fluid (which is also hyperintense on
unenhanced T1-weighted images) or enhancing inflammatory tissue (which is
hypointense on unenhanced T1-weighted images and shows enhancement after
contrast administration).
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Fig. 2A. —14-year-old boy with suspected Crohn's disease and perianal
fistula. G = gluteus muscle. Axial T2-weighted image shows fistula caudally
from anal sphincter coursing from perineum to scrotum. Tract (arrows)
is hardly visible because of equal signal intensities of fistula and
surrounding fat.
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Fig. 2B. —14-year-old boy with suspected Crohn's disease and perianal
fistula. G = gluteus muscle. On axial fat-saturated T2-weighted image obtained
at same level as A, diagnosis is much easier to make because brightness
of fluid-filled fistula (arrows) stands out against suppressed
fat.
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Fig. 3A. —29-year-old man with long-standing Crohn's disease. G =
gluteus muscle, IO = internal obturator muscle. Axial fat-saturated
T2-weighted image shows large horseshoe-shaped structure (arrows).
Typical horseshoe abscess extends on both sides of anal midline and has one
internal opening, but this abscess extends both superiorly and inferiorly
relative to levator ani muscle. Abscess shown has infralevatoric location
(abscess shown in Figs. 10A
and 10B has supralevatoric
location).
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Fig. 3B. —29-year-old man with long-standing Crohn's disease. G =
gluteus muscle, IO = internal obturator muscle. Axial fat-saturated
T1-weighted image after administration of IV contrast medium shows structure,
with larger part of it fully enhanced (long arrow) indicating
presence of inflammatory tissue. Right leg of structure however only shows
partial enhancement of rim (short arrow), indicating presence of
fluid in center with rim of inflammatory tissue.
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Fig. 4A. —43-year-old man with extensive Crohn's disease who underwent
proctocolectomy with creation of ileostomy. G = gluteus muscle, B = bladder.
Axial T2-weighted image shows hyperintense collection (arrow).
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Fig. 4B. —43-year-old man with extensive Crohn's disease who underwent
proctocolectomy with creation of ileostomy. G = gluteus muscle, B = bladder.
Axial fat-saturated T2-weighted image shows hyperintense structure
(arrow) indicating this could either be fluid-filled lesion (i.e.,
abscess) or granulation tissue.
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Fig. 4C. —43-year-old man with extensive Crohn's disease who underwent
proctocolectomy with creation of ileostomy. G = gluteus muscle, B = bladder.
Axial fat-saturated T1-weighted image obtained after administration of IV
contrast medium shows strong enhancement of rim of lesion, whereas core does
not enhance, indicating presence of fluid in center with rim of inflammatory
tissue (arrow).
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Fig. 10A. —29-year old man with Crohn's disease (same patient shown in
Figs. 3A,
3B). G = gluteus muscle. Axial
T2-weighted image shows supralevatoric location of horseshoe abscess
(arrows). IO = internal obturator muscle.
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Fig. 10B. —29-year old man with Crohn's disease (same patient shown in
Figs. 3A,
3B). G = gluteus muscle. Axial
fat-saturated T2-weighted image shows abscess (arrows), but exact
size and location are more conspicuous because of suppression of surrounding
fat. IO = internal obturator muscle.
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MR images were obtained on a 1.5-T scanner. External phased-array coils are
preferable to use in patients with Crohn's disease because tracts and
abscesses may be present or extend outside the field of view of an endoluminal
coil. For ano- or rectovaginal fistulas, the higher spatial resolution of
endoluminal MRI may be advantageous. These tracts often are short,
thin-walled, collapsed, and surrounded by the hyperintense veins of the ano-
or rectovaginal septum, rendering these fistulas more difficult to identify
(Figs. 5A,
5B,
5C).
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Fig. 5A. —20-year-old woman with long-standing Crohn's disease who had
undergone seton placement for anovaginal fistula. E = external sphincter, C =
endoanal coil. Axial T2-weighted endoanal image shows anovaginal fistula
(long arrow) coursing into anal canal (short arrow). Seton
can be seen as hypointense structure within hyperintense tract. I = internal
sphincter.
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Fig. 5B. —20-year-old woman with long-standing Crohn's disease who had
undergone seton placement for anovaginal fistula. E = external sphincter, C =
endoanal coil. Axial T2-weighted endoanal image obtained inferior to A
shows two branches (arrows) of anovaginal fistula. I = internal
sphincter.
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Fig. 5C. —20-year-old woman with long-standing Crohn's disease who had
undergone seton placement for anovaginal fistula. E = external sphincter, C =
endoanal coil. Sagittal T2-weighted image clearly shows path of anovaginal
fistula (arrows). E = external sphincter, P = puborectalis muscle, L
= levator ani muscle.
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Systematic Approach
We propose an approach in which the presence of fistulas is first
determined and their extension is first assessed on the axial sequences. These
sequences are then supplemented with the sequences in the longitudinal plane
(Figs. 6A,
6B). For reporting location of
fistula tracts and openings, we suggest using nomenclature commonly used by
surgeons—a clock-face orientation referring to the patient in the
classic lithotomy position. The use of regions (e.g., right anterolateral,
left posterior, and so on) is a valuable alternative.
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Fig. 6A. —33-year-old man with Crohn's disease who exhibited distinct
perianal fistulas and abscesses despite infliximab treatment. G = gluteus
muscle. Axial T2-weighted image shows three fistulas. Fistulas on right at
7-o'clock position (short arrow) and on left at 3-o'clock position
(curved arrow) are classified as transsphincteric. They both track
through external sphincter (E) into intersphincteric space. Fistula located
dorsally in midline (12-o'clock position, long arrow) at this point
courses outside sphincter complex and extends through external sphincter at
more superior level (not shown).
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First, the tract should be identified and followed throughout its entire
course. Identification of the tract is most easily performed on fat-saturated
sequences, whereas the T2-weighted sequences without fat suppression give
detailed information on the relationship of the tract and surrounding anatomic
structures (Figs. 2A,
2B and
4A,
4B,
4C). In the case of multiple
tracts, verification of communication among the tracts is relevant. Tracts are
described in accordance with the terminology described by Parks, et al.
[7] (Figs.
7 and
8).
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Fig. 7. —Diagram of fistula in ano. Extrasphincteric fistula (Es)
tracks outside of external anal sphincter and penetrates levator ani muscle
into rectum. Transsphincteric fistula (Ts) tracks from intersphincteric space
through external anal sphincter. Superficial fistula (Sf) tracks below both
internal and external anal sphincter. Intersphincteric fistula (Is) tracks
between internal and external anal sphincters in intersphincteric space.
Suprasphincteric fistula (Ss) leaves intersphincteric space over top of
puborectalis muscle and penetrates levator ani muscle before tracking to
skin.
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Fig. 8. —21-year-old man with complex fistulating Crohn's disease.
Coronal T2-weighted image shows "high" transsphincteric fistula
(solid arrow), meaning that internal sphincter is penetrated at level
higher than dentate line. Internal opening into rectum (R) is not clearly
visible but most probably is at level of levator ani muscle (L). Inferior part
of tract is composed of scar tissue (open arrow). B = bladder, G =
gluteus muscle, IO = internal obturator muscle.
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Second, the presence of abscesses and blind tracts should be studied. These
findings also are most prominent on fat-suppressed sequences, with T2-weighted
sequences used to determine the relationship between the abnormality and the
surrounding structures (Figs.
9A,
9B,
9C,
10A, and
10B).
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Fig. 9A. —33-year-old man who underwent proctocolectomy and pouch
reconstruction for Crohn's disease with symptoms indicating presence of
abscess despite earlier incision and drainage. Axial T2-weighted image shows
large abscess (A) displacing sphincter complex to left and extending into
right gluteus muscle (open arrow). In left buttock, another abscess
can be seen (solid arrow) in ischioanal fat adjoining gluteus muscle
(G).
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Fig. 9B. —33-year-old man who underwent proctocolectomy and pouch
reconstruction for Crohn's disease with symptoms indicating presence of
abscess despite earlier incision and drainage. Axial fat-saturated T2-weighted
image shows abscess (A) more clearly because bright signal of fat, in which
abscess is located, is suppressed. Abscess on left (arrow) is also
more prominently seen than on A. G = gluteus muscle.
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Fig. 9C. —33-year-old man who underwent proctocolectomy and pouch
reconstruction for Crohn's disease with symptoms indicating presence of
abscess despite earlier incision and drainage. Axial fat-saturated T1-weighted
image after administration of IV contrast medium clearly shows rim enhancement
of lesions on left (arrow) and right (A), indicating presence of
large amount of pus. G= gluteus muscle.
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As a supplement to the first two steps, the presence and extension of
fistulous disease in the levator ani muscle or the supralevator and perirectal
space should be checked; this evaluation should be performed primarily on
longitudinal sequences, especially on the coronal sequence. This step is
important to ensure detection of the clinically important extension above the
levator ani muscle (Figs. 10 C
and 10D). Abscesses and
fistulas located above the levator plate are much harder to detect clinically
and are difficult to reach preoperatively, requiring a surgical approach.
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Fig. 10C. —29-year old man with Crohn's disease (same patient shown in
Figs. 3A,
3B). G = gluteus muscle.
Coronal T2-weighted image obtained 9 months after B to determine
whether improvement had occurred after placement of setons for drainage shows
fistula (arrow) with internal opening in rectum (R) superior to
levator ani muscle (L).
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Fig. 10D. —29-year old man with Crohn's disease (same patient shown in
Figs. 3A,
3B). G = gluteus muscle. Image
from coronal T-2 weighted sequence obtained more anterior than C shows
the course of the fistula (arrow) from the recrtum (R) to the left
levator ani muscle (L).
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The fourth step is the identification of the internal openings.
Theoretically, the internal opening can be expected at the level of the
dentate line (i.e., approximately 2 cm superior to the lower edge of the anal
sphincter), this being the level at which the intersphincteric anal glands
empty into the crypts [3]. In
practice, internal openings are also identified at other levels (Figs.
11 and
12).
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Fig. 11. —40-year-old woman with Crohn's disease who underwent multiple
surgical procedures several years earlier for perianal fistulas and
experienced relapse of disease. Coronal T2-weighted endoanal image shows broad
transsphincteric fistula (long solid arrows) with internal opening
(open arrow) higher than dentate line (which cannot be visualized on
MRI). Inferiorly, fibrous tissue can be seen (short solid arrow). L =
levator ani muscle, P = puborectalis muscle, E = external sphincter.
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Fig. 12. —19-year-old woman with Crohn's disease. Coronal T2-weighted
endoanal image shows small intersphincteric abscess on left with slight
extension supralevatorically (curved arrow). From this abscess,
gracile intersphincteric tract (large solid arrow) courses caudally
into anus with subtle internal opening (open arrow) at approximate
level of dentate line. On right, second tract can be seen (small solid
arrow) following intersphincteric path just as fistula on left does. C =
coil, L = levator ani muscle, IAS = ischioanal space.
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The fifth and final step includes the evaluation for ancillary findings,
such as bone marrow edema or cancer (Figs.
13 and
14A,
14B).
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Fig. 13. —39-year-old man with complex fistulating Crohn's disease who
barely responded to infliximab treatment. Pain was localized at left ischial
tuberosity. Axial fat-saturated T2-weighted image shows fistula (short
arrow) in right gluteus muscle as well as fistula dorsal relative to
ischial tuberosity on left (long arrow). Ischial tuberosity shows
bone marrow edema (curved arrow) contiguous with fistula. Pain that
patient perceived was probably caused by reactive edema or osteomyelitis of
ischial tuberosity.
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Fig. 14A. —56-year-old woman with permanent ileostomy caused by Crohn's
disease and discharge of pus and mucus rectally that was ascribed to her vast
and complex perianal fistulating disease. Digital rectal examination was
impossible to perform because of profound perianal pain. During surgery for
excision of fistulas, large rectal tumor was revealed. Axial T2-weighted image
shows large lesion isointense compared with surrounding fat (solid
arrows). Among features differentiating abscess from mucous tumor are
presence of stalk (open arrow) and streakiness of structure seen on
the fat-saturated T2-weighted images (not shown). G = gluteus muscle.
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Fig. 14B. —56-year-old woman with permanent ileostomy caused by Crohn's
disease and discharge of pus and mucus rectally that was ascribed to her vast
and complex perianal fistulating disease. Digital rectal examination was
impossible to perform because of profound perianal pain. During surgery for
excision of fistulas, large rectal tumor was revealed. Photomicrograph of
microscopic histopathologic specimen of tumor after resection that was found
to be moderately differentiated mucinous adenocarcinoma. (H and E,
x200)
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Pitfalls
Veins can be mistaken for fistulas, but in contrast to fistulas, veins
usually are thin-walled, tortuous, symmetric structures. A pilonidal sinus may
resemble a fistula, but several findings, such as extension to the
intersphincteric space in fistulas, help one to discriminate between the two
[8] (Figs.
15A,
15B).
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Fig. 15A. —17-year-old woman with suspected perianal Crohn's disease and
clinically proven pilonidal sinus. G= gluteus muscle. Axial fat-saturated
T2-weighted image shows perianal fistula (arrow) coursing from anal
cleft to dorsal side of anal sphincter (S, arrowhead).
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Fig. 15B. —17-year-old woman with suspected perianal Crohn's disease and
clinically proven pilonidal sinus. G= gluteus muscle. Axial fat-saturated
T2-weighted image obtained more cranially than A shows hyperintense
structure (arrow) with no relation to sphincter complex ending
blindly in soft tissue of buttock. This structure is most likely pilonidal
sinus because no related intersphincteric sepsis can be seen, and lesion is
located in midline of anal cleft. Clinical report in which presence of
pilonidal sinus in area is mentioned aids in establishing correct
diagnosis.
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Hemorrhoids and anal tags may resemble small submucosal fluid collections
but are easily diagnosed at clinical examination. Furthermore, hemorrhoids can
be traced until their origin—a vein—can be seen. Clinical findings
and results of other examinations (e.g., colonoscopy) can aid in establishing
the correct diagnosis.
References
- American Gastroenterological Association. AGA technical review on
perianal Crohn's disease. Gastroenterology2003; 125:1508
-1530[Medline]
- Morris J, Spencer JA, Ambrose NS. MR imaging classification of
perianal fistulas and its implications for patient management.
RadioGraphics2000; 20:623
-635[Abstract/Free Full Text]
- Halligan S, Buchanan G. MR imaging of fistula in-ano.
Eur J Radiol2003; 47:98
-107[Medline]
- Beets-Tan RGH, Beets GL, Gerritsen van der Hoop A, et al.
Preoperative MR imaging of anal fistulas: does it really help the surgeon?
Radiol ogy2001; 218:75
-84
- Schwartz DA, Pemberton JH, Sandborn WJ. Diagnosis and treatment of
perianal fistulas in Crohn disease. Ann Intern Med2001; 135:906
-918[Abstract/Free Full Text]
- Present DH. Crohn's fistula: current concepts in man agement.
Gastroenterology2003; 124:1629
-1635[Medline]
- Parks AG, Gordon PH, Hardcastle JD. A classification of
fistula-in-ano. Br J Surg1976; 63:1
-12[Medline]
- Taylor SA, Halligan S, Bartram CI. Pilonidal sinus disease: MR
imaging distinction from fistula in ano. Radiology2003; 226:662
-667[Abstract/Free Full Text]
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Introduction
Anatomy
