Transcript MRI imaging of Perianal fistula

Dr. Ahmed Refaey , FRCR
Consultant Radiologist
Riyadh Military Hospital
Format of the lecture
 Anatomy
 Pathogenesis
 Imaging techniques
 Scanning protocoles
 Classification
 Examples
ANATOMY
Anatomy
 Anatomical canal :
- extends from perineal skin
to dentate line
 Surgical canal :
- extends from perineal skin
to anorectal ring ( 1-1.5cm
abov e dentate line )
- total length 4-5 cm
Anal sphincter
comprised of 3 layers
 Internal sphincter
- continuance of circular smooth
muscle of rectum, involuntary,
contracts at rest & relaxes at
defecation
 Intersphincteric space
 External sphincter
- voluntary striated muscle ,
continuous cranially with
puborectal muscle &levator ani
 Puborectal muscle has its origin on both sides of the symphysis
pubis, forming a sling around the anorectum
 The puborectal
muscle is contracted
at rest and accounts
for the 8O0
angulation of the
anorectal junction . It
relaxes during
defecation
 On axial and
coronal MR
images , the
different
layers of anal
sphincter and
the
surrounding
structures can
be displayed
perfectly
Coronal
Axial
PATHOGENESIS
Perianal fistula
 Abnormal connection between the
epithilialised surface of the anal canal and
the skin.
Causes
 1ry
- obstruction of anal gland which leads to stasis &
infection with abscess & fistula formation ( most
common cause )
 2ry
- iatrogenic ( post hemorrhoiedal surgury )
- inflammatory bowel dis. ( crohn’s disease )
- infections ( viral , fungal or TB )
- malignancy
IMAGING TECHNIQUES
Imaging techniques
 Fistulography
 Endosonography
 CT
 MRI
Perianal fistulography
Anal endosonography
CT
MRI protocol
 T1W &T2W fse axial and coronal
 T2W with fat sat
 T1W + CM
 FOV 200
 T2W
----- anatomy
 T2W with fat sat ---- fistula
The anal clock
P: anterior
perineum
n: natal cleft
The anal
clock
 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
Reporting
 Position of the
mucosal opening
on axial images
using anal clock
 Distance of
mucosal defect to
perianal skin on
coronal images
 2ry fistulas or
abscess
CLASSIFICATION
Classification
 Parks classification
1- intersphincteric
2- transsphincteric
3- extrasphincterisc
4-suprasphincteric
Intersphincteric & transsphincteric are the most common
Intersphincteric --> 70 %
Transsphincteric -->20%
St. James university hospital
classification
MR imaging Grading of
perianal fistulas
MRI Grading of perianal fistulas
 Grade 1 :
simple linear intersphincteric fistula
 Grade 2 :
intersphincteric fistula with abscess or 2ry track
 Grade 3 :
transsphincteric fistula
 Grade 4:
transsphinteric fistula with abscess or2ry track
within ischeorectal fossa
 Grade 5 :
supralevator & translevator fistula
Grade 1 :
simple linear intersphincteric fistula
 Intersphincteric fistula
 Axial T2W with and without fat
saturation
 The intersphincteric fistula
located at 6 o’clock
Intersphincteric fistula
Perianal fistula with an abscess
Grade 2 :
intersphincteric fistula with
abscess or 2ry track
Grade 3 :
transspincteric fistula
Transsphincteric fistula
 The defect through internal & external
sphincter at 6 o’clock is clearly visible
 Transsphincteric fistula at 11 o’clock
Grade 4:
transsphinteric fistula with
abscess or2ry track within
ischeorectal fossa
Grade 5 :
supralevator & translevator
fistula
Suprasphincteric fistula
 Two tracts in
ischeorectal
region
 The right sided
tract runs over
the puborectal
muscle (asterisc)
& the mucosal
opening lies at
the level of
dentate line
(black arrow)
Extrasphincteric fistula
 A small abscess in
left ischeoanal
fossa , the fistula
runs through
levator ani , it is
therefore above the
sphincter complex
and
extrasphincteric
Complex fistula
 2 tracts in left buttock




form single tract
The fistula breaks through
the external sphincter
In intersphincteric space it
divides again into 2 tracts
One ends blindly in the
intersphincteric space
The other breaks through
the internal sphincter with
mucosal defect at 1 o’clock
Differential diagnosis
Pielonidal sinus
 Small abscess just above the nates
 No relation with sphincter complex
Proctitis
 No fistula was seen
 Diffuse thickening of rectal mucosa due to proctitis
Ischiorectal space abscess
 An abscess in ischiorectal space with no
connection to the sphincter complex
REFERENCES
 Goodsall DH, Miles WE. Diseases of the anus and rectum.
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