MRI of perianal fistula

1. ‫عل‬ ‫السالم‬ ‫و‬ ‫الصالة‬ ‫و‬ ‫الرحيم‬ ‫الرحمن‬ ‫هللا‬ ‫بسم‬‫ي‬
‫وااله‬ ‫من‬ ‫و‬ ‫صحبه‬ ‫و‬ ‫اله‬ ‫علي‬ ‫و‬ ‫هللا‬ ‫رسول‬ ‫محمد‬.
Welcome ladies and gentlemen in my lecture entitled:

2. MRI OF PERIANAL
FISTULA (FISTULA IN
ANO)
AM Aboelsouad, MD
Lecturer of Diagnostic Radiology, South Egypt Cancer Institute-Assiut University

4. INTENDED LEARNING OBJECTIVES(ILO)
BY THE END OF T HE LECTURE YOU WILL BE ABLE TO:-
Identify the Perianal region imaging
anatomy
Pathogenesis
How to make MRI perianal fistula
Fistula classification
How to report

5. IMAGING ANATOMY
• The imaging and clinical landmark
between rectum and anal canal i.e.
palpated during PR, is anorectal ring.
• The anorectal ring is made of the
puborectalis muscle.
• The total length of the anal canal is
about 5 cm.

6. IMAGING ANATOMY
• The anal sphincter is comprised of three
layers so it’s called anal sphincter complex:
• Internal sphincter: involuntary smooth
muscle, continuous with the circular
smooth muscle of the rectum.
• Intersphincteric space: fat.
• External sphincter: voluntary striated
muscle, continuous cranially with the
puborectal muscle and levator ani

7. IMAGING ANATOMY
• The anal sphincter on MRI appears as:
• The internal sphincter is normally more hyperintense on
T2-weighted and STIR images than the external
sphincter and shows enhancement on post-gadolinium
T1 sequences.
• The intersphincteric space is a thin T2 hyperintense &
STIR hypointense ring between the internal and external
sphincters.
• The external anal sphincter is relatively hypointense on
T2 and STIR sequences, with its lateral border
contrasting against fat in the ischioanal fossa.

9. Anal canal Internal sphincter
External sphincter

10. Anal canal mucosa
Internal sphincter
External sphincter

11. Internal sphincter
Intersphincteric space

17. PATHOGENESIS
• Anal mucous gland are lubricating glands
present at the intersphincteric space so
the intersphincteric type is the most
common fistula type.
• They open at the level of dentate line (the
histologic transitional line between the
columnar epithelium of rectum and
stratified squamous epithelium of anal
canal).

18. PATHOGENESIS
Perianal fistula is communication
between anal/rectal mucosa and
perianal skin and it’s due to:
Primary Secondary
Postop
IBD
Malignancy
Infection
Fistula
Abscess
formation
InfectionStasis
Anal mucous
gland
obstruction

19. PATHOGENESIS
Cryptoglandular hypothesis suggested
(according to Eisenhammer S 1956)
that
• Infection occurs in intersphincteric
gland  abscess formation which
discharge in variety of direction 
fistula
• Hence the anal mucous gland more
numerous in men so perianal fistula
is more in men

20. Easy downward
spread through ISS
More aggressive
infection traversing Ex
sphincter
Extremely rare as
spread through
unusual path that is
likely iatrogenic
Not due to perianal sepsis i.e.
no infected gland but likely
due to CD, Malignancy or
diverticulosis

21. HOW TO MAKE MRI
PERIANAL FISTULA?

22. HOW TO MAKE MRI
PERIANAL FISTULA?
• Technical Requirements for MRI
• A 1.5 T (or 3 T) MRI scanner
• External surface coils.
• Endoanal coils are not recommended; although they provide very high-
resolution imaging with excellent delineation of the sphincters, the
limited field of view may not detect the extensions and abscesses that
influence the surgical approach, and they are uncomfortable and invasive
to patients

23. HOW TO MAKE MRI
PERIANAL FISTULA?
• Orientation, Field of View and
Imaging Plane:
• Orientation of the scanning
planes should be either parallel
or perpendicular to the anal
canal since the anal canal is
approximately 45° from vertical,
so straight axial and coronal
images fail to achieve correct
alignment and distort the
sphincter complex.

24. HOW TO MAKE MRI
PERIANAL FISTULA?
• Orientation, Field of View and Imaging
Plane:
• An adequate field of view should be
selected to cover the whole perineum
and extend above the levator muscles to
include in inferior aspect of the presacral
space for coverage of long fistula
extensions, supralevator collections or
osteomyelitis. However, this requires
balance: too large a field of view will
diminish resolution.

25. HOW TO MAKE MRI
PERIANAL FISTULA?
• Technical Requirements for MRI
• Oblique axial plane determines the relationship of the fistula to the
sphincters. It also best assesses horseshoe extensions and the radial
location of the internal opening.
• Oblique coronal plane determines the craniocaudal length of the fistula
and fistula relationship to the levator ani muscles.
• Sagittal images determines anterior extension where anovaginal /
anoscrotal fistula is suspected or for posterior extension where sacral or
coccygeal sepsis is suspected.
• Slice thickness of 3–5 mm with no interslice gape should be used. Slices
either thicker than 5 mm or smaller than 3 mm should be avoided to
minimize volume averaging and impaired SNR respectively.

26. HOW TO MAKE MRI
PERIANAL FISTULA??
• Sequence Selection and Rationale
• Sequence choice depends to a degree on personal preference.
• So the suggested protocol is:
• Localizer in 3 directions
• T2 - 5 mm axial and sagittal (overview pelvis and orientation)
• T2 STIR – 3 or 4 mm oblique axial and oblique coronal plus sagittal
• +T1 SPIR pre & post-gadolinium – 3 or 4 mm oblique axial and coronal
plus sagittal to differentiate between abscess and granulation tissue and
to detect activity of the activity of the fistula (in case of using
immunosuppressant drugs e.eg Fleximap in IBD)*.

27. FISTULA CLASSIFICATION 1. Park classification
Easy downward
spread through ISS
More aggressive
infection traversing Ex
sphincter
Extremely rare as
spread through
unusual path that is
likely iatrogenic
Not due to perianal sepsis i.e.
no infected gland but likely
due to CD, Malignancy or
diverticulosis

28. FISTULA CLASSIFICATION
1. Park classification
• Intersphincteric 70%
• Transsphincteric 20%
• Suprasphincteric
• Extrasphincterisc (no anal
canal open but rectal open)
The inter and trans sphincteric are the most
common and both in addition to
Suprasphincteric are low perianal fistula
So

29. FISTULA CLASSIFICATION
2. St. James university hospital grading
• Grade 1 :
Simple intersphincteric fistula
• Grade 2 :
Complicated intersphincteric fistula with abscess or 2ry track
• Grade 3 :
Simple transsphincteric fistula
• Grade 4:
Complicated transsphinteric fistula with abscess or 2ry track
• Grade 5 :
supra and extra sphincteric fistula

30. FISTULA CLASSIFICATION

31. AMERICAN SOCIETY OF
GASTROENTEROLOGY CLASSIFICATION
• Superficial fistula is a fistula
that has no relation to the
sphincter or the perianal
glands and is not part of the
Parks classification
• Complex fistula starts as two
tracts that unite to form single
tract and branch again.

32. HOW TO REPORT
1) Position of the internal
mucosal opening on axial
images using anal clock
2) Craniocaudal length of the
fistula on coronal images
The surgeon’s view of the perianal region when
the patient is in the supine lithotomy position ,
corresponds to the orientation of axial MRI of the
perianal region

33. HOW TO REPORT
3) 2ry (branching) fistulas
because missed 2ry fistula is
the most common cause of
fistula relapse
4) Associated abscess
formation
5) Fistula classification
because wrong classification
increased risk of:
• inadvertent sphincter injury.
• Fistula relapse.
The surgeon’s view of the perianal region when
the patient is in the supine lithotomy position ,
corresponds to the orientation of axial MRI of the
perianal region

34. EXAMPLE FOR A REPORT
There is a perianal trans sphincteric fistula
with external opening at 4 o’clock and internal
opening at anal canal at 6 o’clock with
craniocaudal length about 6 cm and an
associated intersphincteric supralevator
extension from the apex of the track that
reaches a 2 cm abscess just above the left
levator plate.

36. CASES

37. GRADE 1/PARKS 1=SIMPLE NON
BRANCHING INTERSPHINCTERIC FISTULA

38. GRADE II/PARKS 1=INTERSPHINCTERIC
FISTULA WITH ABSCESS

39. GRADE III/PARKS 2=SIMPLE
TRANSSPHINCTERIC FISTULA

40. GRADE III/PARKS 2=SIMPLE
TRANSSPHINCTERIC FISTULA

41. GRADE IV/PARKS 2=TRANSSPHINTERIC
FISTULA BRANCHING OR WITH ABSCESS

42. GRADE V/PARKS 3&4= EXTRA& SUPRA
SPHINCTERIC

43. DDX
1. Pilonidal sinus:
superior intergluteal (natal)
cleft and result from skin
hair follicle infection. No
intersphincteric sepsis or
enteric communication.

44. DDx
2. Veins: thin walled and tortuous
seen as high signal on STIR adjacent to
rectum and within ischeorectal fossa

45. TREATMENT
• Surgery aims at
eradicating all sepsis and
maintain continence.
• Setons: wire that is cutting slowly through the
tissue to preserve continence.
• Fistulotomy: the track is incised and the
granulation tissue is removed by curettage.
• Fistulectomy: removal of fistula en-bloc and
healing of tissue by 2ry intension.
• Fibrin glue
• Ileostomy: temporary or permanent.
• Proctectomy.

46. THANK YOU
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