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Section V • Endoanal Ultrasonography in the Evaluation of Perianal Sepsis and Fistula-in-ano 179
a b
c
Case V.22. A 28-year-old woman referred for recurrent peri-
anal fistulas. Physical examination reveals two external openings at the 6 and 9 o’clock positions, respectively. Axial
endoanal ultrasonography (EAUS) scan shows an intersphincteric hypoechoic area (acute intersphincteric abscess)
at the 6 o’clock position (arrow), with a primary tract (PT)
extending posteriorly from this area through the puborectal
muscle (suprasphincteric fistula) and a secondary tract (ST)
extending in the right side and traversing the puborectal muscle at the 9 o’clock position (a, b). After injection of peroxide
from the external opening at the 6 o’clock position, the EAUS
scan reveals an immediate communication with the anal
lumen (IO internal opening) (c). Drawing in the axial plane
d
shows extent of the abscess and fistulas (d)
180 Benign Anorectal Diseases
a b
c d
e f
Section V • Endoanal Ultrasonography in the Evaluation of Perianal Sepsis and Fistula-in-ano 181
g h
Case V.23. A 26-year-old man with recurrent perianal Crohn’s
disease. Axial endosonographic image of mid anal canal level
reveals a hypoechoic area into the right ischiorectal fossa
(acute abscess) with a primary tract extending from this area
and traversing the external and internal sphincter at the 6
o’clock position (transsphincteric fistula) (a). Three-dimen-
sional reconstruction on the coronal plane shows upward
extension of the ischiorectal abscess (b).After peroxide injection through a cutaneous orifice at the 9 o’clock position
(right lateral side) endoanal ultrasonography scans with volume render mode confirm the presence of an acute ischiorec-
i
tal abscess (c-f) with a transsphincteric fistula (g-i)
182 Benign Anorectal Diseases
a b
c d
Case V.24. A 38-year-old man complaining of fever and
perirectal pain. Digital rectal examination reveals a posterior
tender mass at the puborectalis level. Axial endosonographic
images of upper anal canal level reveal a large horseshoe
hypoechoic area in the posterior intersphincteric zone (a, b).
Three-dimensional endoanal ultrasonography scans with volume render mode confirm the presence of an acute horseshoe
intersphincteric abscess (c) and demonstrate the internal ori-
fice (d) (white arrow: internal opening; black arrows: horse-
shoe abscess). The patient underwent immediate drainage
through the intersphincteric space,as confirmed by postoper-
e
ative ultrasonography (e)
V.3.
Imaging Perianal Sepsis:
Anal Endosonography or MR Imaging?
S. Halligan
A variety of imaging techniques have been used to
evaluate fistula-in-ano and perianal sepsis over
the years. Some these are more useful than others,
and practitioners experienced in this field tend to
agree that fistulography and computed tomography (CT) scanning, even in the era of multidetector-row scanning, are not very helpful [1]. This is
because in order be of any significant clinical use,
an imaging test has to bring something to the surgeon’s table that is additional to the information
provided at his or her examination under anes-
thetic (EUA). The surgeon is primarily concerned
with the relationship of the fistula tract to the
sphincter complex – i.e., the fistula classification
[2]. This allows the surgical approach to be
planned carefully and appropriately. Fistula-inano has a tendency to recur despite seemingly
adequate surgery, and recurrence is usually due to
infection that has escaped surgical detection at
EUA and thus gone untreated. Identification of
sepsis that would otherwise have been missed is
unquestionably the prime role for preoperative
imaging in these patients, and there is now little
doubt, if any, that magnetic resonance imaging
(MRI) is best-suited to this task. However, there
are other aspects of assessment that sometimes
cause the surgeon considerable trouble.
Identifying the radial site and level of the internal
opening is one example, and is a well-recognized
problem at EUA in some patients – patients in
whom an internal opening is not detected have a
high chance of relapse. Imaging may be used to
direct the surgeon to the enteric communication
when this is not immediately apparent at EUA,
and anal endosonography (AES) is particularly
adept in this setting. It should also be borne in
mind that although radiologists (and surgeons)
are constantly searching for the “one best test,”
most investigations provide complementary and
additive information, and there are no disadvantages to performing both MRI and AES in the
same patient where local circumstances,availability,and economics allow.
Anal Endosonography
AES has already been discussed in the context of
perianal sepsis in Chapter 2 of this Section. When
considering the comparison between AES and
MRI, it is worthwhile focussing on exactly what it
is that sonography does well. It is well-established
that ultrasound generally best images structures
that are close to the transducer surface, and the
higher the frequency of the transducer, the more
this condition will apply. AES is most used for
assessment of anal sphincter injury in the context
of fecal incontinence [3] because the anal sphincters, lying close to the transducer, are imaged with
high spatial resolution. It follows then, that in the
context of fistula-in-ano, those aspects of the fistula that lie close to or within the sphincter complex will be imaged with most precision.In particular, using modern 10 -MHz transducers [4],
endosonography is particularly well suited to
identification of the internal opening because it
usually lies right at the probe surface. When considering identification of the internal opening, it
is important to realize that a tract extending right
up to the anal mucosal surface is rarely seen.
Although a breach in the subepithelial layer of the
anal canal is occasionally present, it is more com-
184 Benign Anorectal Diseases
Fig. V.40. Anal endosonography reveals a focus of inter-
sphincteric sepsis posteriorly at 6 o’clock at dentate line level
(arrow). This was the site of the internal opening – note that
the tract cannot be traced right to the anal canal lumen
mon for the position of the internal opening to be
revealed by a hypoechoic focus in the intersphinc-
teric space that abuts the internal sphincter
(Fig. V.40), often with a small corresponding
defect in the internal sphincter. Because inter-
sphincteric fistulas never stray beyond the inter-
sphincteric space, they are usually well visualized
by AES (Fig.V.41).
It also follows that isolated intersphincteric
abscesses are also well seen using AES. In the
Fig.V.41. Anal endosonography shows an intersphincteric fis-
tula (arrow). Note that the fistula is medial to the external
sphincter
Fig. V.42. Anal endosonography shows a collection in the
right posterior quadrant of the ischioanal fossa (arrows).Loss
of peripheral signal makes it difficult to trace this in its entirety (see Fig. V.49)
author’s practice, the technique has considerable
utility in patients known to have had a fistula previously and who have new symptoms suggesting
that they are developing further sepsis but in
whom digital anal examination is normal. AES
facilitates quick and easy diagnosis of intersphincteric abscesses in this situation, many of
which are small and impalpable but which are
well seen on ultrasound because of the high spa-
tial resolution of the technique.
Transsphincteric fistulas are revealed by tracts
that cross the external sphincter to reach the
ischioanal (ischiorectal) fossa. As would be
expected, extensions are revealed as hypoechoic
fluid collections, but it is here that AES starts to
lag behind MRI. The further the extensions are
from the anal canal, the less well they are visualized by AES (Fig. V.42). This is because the depth
of penetration of the ultrasound beam is limited,
especially at higher frequencies. It should also be
noted that endoanal MRI suffers from similar limitations [5]. Also, AES cannot reliably distinguish
infection from fibrosis since both appear hypoechoic [6]. This causes particular difficulties in
patients with recurrent disease since infected
tracts and fibrotic scars are frequently combined.
Attempts have been made to clarify the course of
tracts by injecting hydrogen peroxide or ultrasound contrast agents into the external opening
during examination [7, 8]. However, gas formed
within the tract as a result may cause acoustic
Section V • Endoanal Ultrasonography in the Evaluation of Perianal Sepsis and Fistula-in-ano 185
Fig. V.43. Anal endosonography showing an obvious large
extension (arrows). The surrounding anatomy is distorted,
and it is difficult to be sure whether the extension is infralevator, supralevator,or includes elements of both
shadowing that mimics an extension. Indeed, this
phenomenon potentially occurs with any tract
that contains air.For example,intersphincteric fistulas may be inadvertently classified as
transsphincteric as a result.
The levator plate (pelvic floor) is a crucial
landmark for surgeons since the treatment
options for patients with supralevator and infralevator sepsis may differ significantly. Notably, the
levator plate forms a relative barrier to surgical
drainage, and supralevator sepsis usually means
extensive surgical incision if adequate drainage is
to be achieved.AES has great difficulty visualizing
the precise position of the levator plates because
they lie in the same plane as the ultrasound beam
and, moreover, they are poorly differentiated from
surrounding structures (unlike their appearance
on MRI). The result is that it can be very difficult
to determine if a collection is supra- or infralevator (Fig. V.43). Some workers have attempted to
overcome this disadvantage by employing threedimensional (3-D) acquisition [9] (Fig. V.44), but
this remains largely experimental.
It was initially hoped that AES would revolutionize preoperative fistula classification, a view
supported by early studies [10]. However, subsequent work has been inconclusive. For example,
some investigators have found the technique useful [11, 12] while others have found it no better
than digital examination [6].
MR Imaging
In recent years, MRI has emerged as the leading
contender for preoperative classification of fistula-in-ano. It not only classifies tracts accurately,
but also identifies diseases that would otherwise
have been missed – an area where it excels more
than any other technique,including EUA. MRI has
had a palpable effect on surgical treatment and,
ultimately, patient outcome.
Coils
Fig. V.44. Three-dimensional anal endosonography
The technique for MRI is extremely simple and
can be achieved with most equipment. For example, field strength does not appear to be a critical
factor for good results [13]. Initial reports of MRI
necessarily used the body coil [14–18], and the
introduction of external phased array surface
coils further increased signal-to-noise ratio (SNR)
and spatial resolution [19, 20]. The best spatial
resolution is achieved by using dedicated endoluminal anal coils [21]. It should be noted that these
endoluminal coils are not the same as rectal coils,
being smaller and designed for location in the
anus. Their availability remains relatively restricted. Endoluminal coils are susceptible to motion
artefact, but this can be reduced by careful patient
preparation. For example, patients should be
asked to try to relax the sphincter and pelvic floor
as much as possible, and due attention should be
186 Benign Anorectal Diseases
paid to comfort,including supporting the coil and
patient with pads [22]. Spasmolytics may help to
reduce motion-induced artefacts – either 20 mg
hyoscine butylbromide (Buscopan) or 1 mg
glucagon intramuscularly.
The exact choice of coil depends on personal
preference, availability, the patient group studied,
and the clinical question in each particular
patient.A study of ten patients with cryptoglandular fistulas found an endoluminal coil superior to a
surface coil [23] whereas a subsequent study of 30
patients found a body coil superior overall because
the limited field of view inevitable with endoluminal imaging meant that distant extensions were
missed [5]. A third study compared endoluminal
and phased array coils in 20 patients and found
that while the endoluminal coil was superior for
classification of the primary track, extensions were
better imaged using the superior field-of-view of
the external coil [24]. These results suggest clearly
that a large field-of-view is necessary whenever
extensions are suspected, for example, in patients
with recurrent fistula or Crohn’s disease.The high
spatial resolution of endoluminal coils makes
them ideal for demonstrating precisely the location and height of the internal opening, rather like
AES, and they may have a special role for demonstrating ano- or rectovaginal fistulas, which are
notoriously difficult to image [25]. They are also
valuable when simultaneous information on the
degree of any sphincter disruption is needed,
which may be the case in patients who have had
previous surgery. Endoluminal coils are sometimes difficult to place due to anal stenosis or local
pain as a result of extensive infection. The author
found that an endoluminal coil could not be sited
in 17% of his patients [5], but Stoker and colleagues
failed in only 3% [22], possibly reflecting differing
patient populations.
Where circumstances allow, it is likely that
optimal examination will be achieved using a
combination of both external and endoluminal
coils. However, it should be borne in mind that
accuracy with external coils alone remains high
[5, 16–20],and lack of an endoluminal coil alone is
insufficient reason to avoid preoperative MRI of
fistula-in-ano.
Sequences
Various investigators have adopted different
strategies with respect to the sequences used to
image fistula-in-ano, but all agree that anatomic
precision is needed in combination with some
method by which infection is highlighted. Many
investigators employ the rapid and convenient
fast-spin-echo, T2-weighted sequence, which provides good contrast between hyperintense fluid
within the tract and its hypointense fibrous wall
while simultaneously enabling good discrimination between the several layers of the anal sphincter. Others have used T1-weighted sequences,
which must be combined with intravenous contrast for the fistula to be highlighted. Fat suppres-
sion techniques are also widely used, and the
author favors short T1 inversion recovery (STIR)
imaging, which combines fat suppression with
high conspicuity of active tracts.
Imaging Planes
It is vital that imaging planes are aligned with
respect to the anal canal. Because the anal canal is
tilted forward from the vertical by approximately
45°, straight axial and coronal images are unsatisfactory. Instead, oblique axial and coronal planes
orientated orthogonal and parallel to the anal
sphincter are required and are planned easily
from a midline sagittal image. It is important that
the imaged volume extends several centimeters
above the levators and also includes the whole
presacral space, both of which are common sites
for extensions. The entire perineum should also
be included. Occasionally, tracts may extend for
several centimeters, and any tract visible on the
standard volume must be followed to its termination. The imaged volume should encompass the
whole sensitive region of the coil when using
endoanal receivers.
Interpretation
The success of MRI for preoperative classification
of fistula-in-ano is a direct result of its sensitivity
for tracts and abscesses combined with high
anatomic precision and ability to image in surgically relevant planes.Accurate preoperative classification is achieved by correctly relating the imaged
fistula to the anal sphincter. Active tracks are filled
with pus and granulation tissue and thus appear as
hyperintense longitudinal structures on T2weighted or STIR sequences.Unlike ultrasound, the
lateral border of the external anal sphincter is
Section V • Endoanal Ultrasonography in the Evaluation of Perianal Sepsis and Fistula-in-ano 187
Fig. V.45. Endoanal magnetic resonance image of an exten-
sive intersphincteric fistula (arrows). Note that there in no
sepsis in the ischioanal fossa (compare with Fig.V.46)
clearly visualized in most patients using MRI. This
makes it relatively easy to determine whether a fistula is contained by the external sphincter or has
extended beyond it. If a fistula remains contained
by the external sphincter throughout its course,
then it is highly likely to be intersphincteric (Fig.
V.45). In contrast, any evidence of a tract in the
ischioanal fossa effectively excludes an intersphincteric fistula. However, transsphincteric, suprasphincteric, and extrasphincteric fistulas all share
the common feature of a tract lying beyond the
confines of the external sphincter.
Fig.V.46. Endoanal magnetic resonance image of a transsphinc-
teric fistula. There is a primary track in the left posterior aspect of
the ischioanal fossa. The internal opening is well demonstrated
(arrow)
While a transsphincteric fistula will be the
commonest cause of a tract in the ischioanal fossa
(Fig. V.46), differentiation between these three fistulas is only possible by locating the internal opening and determining the course between this and
the primary tract. The exact location of the internal opening can be difficult to define. Two questions need to be answered: what is the radial site of
the internal opening and what is its level? The vast
majority of anal fistulas open into the anal canal at
the level of the dentate line commensurate with the
cryptoglandular hypothesis of fistula pathogene-
sis. Furthermore, most fistulas enter posteriorly at
6 o’clock.Unfortunately, the dentate line cannot be
identified as a discrete anatomical entity either on
AES or MRI, even when using endoanal receiver
coils,but its general location can be estimated with
sufficient precision for imaging to be worthwhile.
The dentate line lies at approximately midanal
canal level, which is midway between the superior
border of the puborectalis muscle and the most
caudal extent of the subcutaneous external sphincter. These landmarks define the “surgical” anal
canal (as distinct from the “anatomical”anal canal,
which is shorter and defined as the canal caudal to
the anal valves). Dentate level is probably appreci-
ated best using coronal views,but with experience,
its location can be estimated from axial views with
reasonable precision. Any tract that penetrates the
pelvic floor above the level of the puborectalis
muscle is potentially a suprasphincteric or extrasphincteric fistula. The level of the internal opening
distinguishes between these, being anal in the former and rectal in the latter (Fig.V.47).
The radial site of the internal opening is simple to identify if the fistula can be traced into the
anal lumen. However, like endosonography, it is
frequently impossible to trace a tract right up to
the anal mucosa. In such cases, an intelligent
deduction must be made as to where the internal
opening is likely to be,which is best done by looking to where there is maximal intersphincteric
sepsis. The intersphincteric space and longitudinal layer is often seen as a low-intensity ring lying
between the internal and external sphincter. The
internal sphincter is hyperintense on both T2weighted fast-spin-echo and STIR sequences.
The major advantage of MRI over AES is the
facility with which it can image extensions. Like fistula tracts, extensions are revealed as hyperintense
regions on T2-weighted and STIR imaging. The
commonest type of extension is one that arises
from the apex of a transsphincteric tract and
188 Benign Anorectal Diseases
Fig. V.47. Coronal image from a body-coil magnetic reso-
nance examination showing a right-sided ischioanal tract
with an enteric communication (arrow) above the puborectalis, diagnosing an extrasphincteric fistula
Fig.V.49. Same patient as Fig.V.42.The distant extension into
the buttock (white arrows) is much better appreciated on
magnetic resonance imaging than on anal endosonography.
Note that the extension is well away from the primary tract
(black arrow)
extends into the roof of the ischioanal fossa
(Fig. V.48). The major benefit of MRI is that it can
alert the surgeon to extensions that would other-
wise be missed since they can travel several centimeters from the primary tract (Fig. V.49), which
makes them difficult to detect by clinical examination or EUA. It is especially important to search for
supralevator extensions (Fig. V.50) since these are
not only difficult to detect but pose specific difficulties with treatment. Complex extensions are
especially common in patients with recurrent fistula-in-ano or those who have Crohn’s disease [19,
20].
Fig. V.48. Surface coil magnetic resonance imaging clearly
demonstrates an extension (arrow) into the roof of the right
ischioanal fossa
Fig. V.50. Coronal image from a body-coil magnetic reso-
nance examination showing a supralevator abscess (arrow)
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