The document provides an overview of perianal fistulas and abscesses, including the relevant anatomy, classification systems, pathogenesis, and imaging techniques for evaluation using CT and MRI. It discusses the Parks and St. James's University Hospital classification systems and how to identify and characterize perianal fistulas and abscesses on imaging based on their relationship to the anal sphincter complex. The goal is to help radiologists accurately diagnose and describe perianal diseases for appropriate medical or surgical management.

1. Perianal Fistula and Abscess:
A Simple Guide for Beginners
Authors:
Natalia Ramírez Pedraza, MD; Aarón Horeb Pérez Segovia, MD;
Juan Alberto Garay Mora, MD; Kurt Techawatanaset, MD;
Andrew W. Bowman, MD, PhD; Miguel Angel Cruz, MD; Mónica
Chapa-Ibargüengoitia, MD; Sofía Arizaga Ramirez, MD; María
Rebeca Arizaga Ramírez, MD

2. From the Department of Radiology, Instituto Nacional de Ciencias Médicas y Nutrición
Salvador Zubirán, Vasco de Quiroga 15, Belisario Domínguez Secc 16, Tlalpan, Mexico
City, Mexico 14080 (N.R.P., A.H.P.S., J.A.G.M., M.C.I., S.A.R., M.R.A.R.); Department of
Radiology, Medical College of Wisconsin, Milwaukee, Wis (K.T.); Department of
Radiology, Mayo Clinic, Jacksonville, Fla (A.W.B.); and Department of Radiology,
Instituto Nacional de Cardiología Ignacio Chávez, Mexico City, Mexico (M.A.C.).
Address correspondence to: N.R.P. (e-mail: natalia_ramirezp@live.com.mx).
Recipient of a Certificate of Merit award for an education exhibit at the 2020 RSNA
Annual Meeting.
All authors have disclosed no relevant relationships.

3. Introduction:
Perianal fistulas and abscesses are relatively uncommon entities; however, they are associated with
significant morbidity. Prompt diagnosis with an accurate, complete and thorough report is crucial for
the appropriate medical or surgical management of perianal disease. This presentation seeks to
provide radiologists with the knowledge required to identify and classify the variants of perianal
disease and to effectively communicate the findings. This will be achieved by reviewing the relevant
normal anatomy of the perianal region, as well as the various presentations of perianal fistulas and
abscesses as seen at CT and MRI.

4. Table of Contents:
1. Anatomy of the Rectum and Anal Region
2. Cause and Pathogenesis of Perianal Fistula and Abscess
3. Perianal Fistula Classification Systems
4. Techniques for Imaging Perianal Fistula and Abscess at CT and MRI
5. Reporting
6. Pearls and Pitfalls
7. Summary

5. • #1 Review anatomy of the anal region.
• #2 Understand the classification systems for perianal fistulas.
• #3 Review CT and MRI evaluation of perianal fistula and abscess.
• #4 Discuss key points to include in the radiology report and their implications for
patient management.
• #5 Review pearls and common pitfalls.

6. Anatomy
RECTUM
• Arterial Supply: Middle rectal artery
• Venous Drainage: Portal system via
superior rectal/Inferior Mesenteric Vein
• Lymphatic Drainage: Inferior Mesenteric
Nodes
• Nerve Supply: Inferior Hypogastric
Plexus

7. Anatomy
ANAL CANAL
Terminal structure of the GI tract:
• Arterial Supply: Inferior rectal artery
• Venous Drainage: Pudendal Veins
Internal Iliac Veins
• Lymphatic Drainage: Superficial Inguinal
Nodes
• Nerve Supply: Pudendal Nerves and
Inferior Rectal Nerves

8. Anatomy

9. Anal Canal Anatomy
Dentate Line
• Landmark for the histologic junction
(rectal columnar epithelium - anal
squamous epithelium).
• Transition point for blood supply and
innervation.
• Cannot be seen at MRI, and surgeons
cannot palpate it.
Anal Verge
Penetrates the internal
sphincter into the
intersphincteric groove,
but not the external
sphincter.
Anal Glands
Junction (anal – skin)

10. Anal Canal Anatomy
• Three muscular fiber bundles:
• D: Surrounds the upper third of the anal canal.
• S: Largest of the three.
• Sc: “J” Shape extends below the internal anal
sphincter (IAS).
Levator Ani (LA)
External Anal Sphincter (EAS)
• Outermost circular layer of the anal canal
• Skeletal muscle, responsible for voluntary
continence
• Thickness ~ 4 mm
• 15% of anal sphincter tone
• Cranially extends up to puborectalis (PR)
muscle
Divides the perineum from the pelvic cavity

11. “Tube within a tube’’ configuration of muscular anal canal. The inner tube is the internal anal
sphincter (IAS), the outer tube is external anal sphincter (EAS).
Anal Canal Anatomy
• Space between the CLM and EAS
• Smooth muscle (continuous with
the rectum)
• Responsible for resting tone (85%
of anal sphincter tone)
• Thickness, about 3.5 mm

12. T2-weighted oblique coronal image. Internal anal sphincter (I), external anal sphincter (EAS) and its three components: deep
(D), superficial (S), and subcutaneous (Sc). Levator ani muscles (LA) composed of illiococcygeus (IL) and puborectalis (PR)
muscles. Intersphincteric space represented by *. Circular Longitudinal muscle represented by arrowhead.
Anatomy of the Anal Canal at MRI
IAF = ischioanal fossa

13. Anatomy of the Anal Canal at MRI
Internal anal sphincter (I), external anal sphincter (EAS) and
its subcutaneous (Sc) superficial (S), and deep component
(D), levator ani (LA), intersphincteric space (*/IS), circular
longitudinal muscle (CLM) represented by arrowhead.

14. Anatomy of the Anal Canal at MRI
Internal anal sphincter (IAS), external anal sphincter (EAS)
and its subcutaneous (Sc) superficial (S) and deep
component (D), levator ani (LA), intersphincteric space (IS)
represented by *, circular longitudinal muscle (CLM)
represented by arrowhead.
• External anal sphincter
(EAS), only the fibers of
subcutaneous (Sc)
portion
• Inverted “U-shaped”
area (outer contour
marked in blue)
• “Tube within a tube’’
configuration
• Levator Ani: “U-shaped” area
• Anterior: Pelvic cavity
(purple area)
• Posterior: Perineum
(white area)

15. Anatomy at CT
For CT evaluation, use of the multiplanar reformation (MPR) tool to align the images to the true coronal plane of the anal
canal can help to clearly identify structures and allow better assesment of anatomy.
In aligment
C
O
R
O
N
A
L
P
L
A
N
E
EAS = external anal sphincter, IAF= ischioanal fossa, IAS = internal anal sphincter, IRF = ischiorectal
fossa, IS = intersphincteric space, LA = levator ani muscles, PC = pelvic cavity.
Not in alignment

16. Anatomy at CT
For CT evaluation, use of the MPR tool to align the images to the true axial plane of the anal canal can help to clearly identify
structures and allow better assesment of anatomy.
Not in alignment In aligment
A
X
I
A
L
P
L
A
N
E
EAS = external anal sphincter, IAF= ischioanal fossa, IAS = internal anal sphincter, IRF = ischiorectal
fossa, IS = intersphincteric space, LA = levator ani muscles, PC = pelvic cavity.

17. Obstruction of the anal
gland.
Glandular secretions
accumulate.
Infection and
abscess formation.
Infection spreads to the
fatty tissue of the
intersphincteric space (IS), the
“path of least resistance”,
leading to formation of
fistulous or sinus tracts.
Cause
Primary cause: Cryptoglandular Theory
90% of fistulous tracts

18. Secondary causes:
• Crohn disease
• Tuberculosis
• Infection
• Diverticulitis
Inflammatory causes:
Infectious GI tract agents
in immunocompetent
patients
Opportunistic Infectious
agents in
Immunosuppressed
patients
• Escherichia coli
• Enterococcus
• Bacteroides fragilis
• Herpes
• Cytomegalovirus
• Neisseria gonorrhoeae
• Chlamydia trachomatis
• Trauma
• Malignancy
• Radiotherapy
• Surgery
Axial contrast-enhanced CT (CECT) image in an immunosuppressed
man with HIV with severe proctitis that developed abscess in internal
anal sphincter (arrow).
Cause

19. Cause
Perianal
Fistula
• Abnormal communication between the anal canal and
the perianal or perineal skin
• Chronic condition
Infected
Fistula
• Fluid or air-filled soft-tissue tract surrounded by
inflammation
Abscess
• Well-defined round to oval-shaped fluid or air
collection
• Central necrotic material
• Acute manifestation
T2-hyperintense
tissue (white
arrow)
Fibrotic T2-
hypointense tissue
(pink arrows)
Fistula:
Abscess:
Fibrotic tissue:
Hypointense tissue
(pink arrow)
Cor T2 T2
T2

20. Classification System
◦ Classification according to the relationship of primary tract to the anal sphincter.
◦ The landmark is the external anal sphincter.
◦ Four different types:
◦ Intersphincteric: 45%
◦ Transsphincteric: 30%
◦ Suprasphincteric: 20%
◦ Extrasphincteric: 5%
◦ All may be complicated with abscesses and secondary tracts.
Parks Classification System:
Fig 1- Intersphincteric (arrow), transsphincteric (*),
suprasphincteric (arrowhead), extrasphincteric (◁).
*
◁

21. Parks Classification System
Axial T2, short-tau inversion recovery (STIR) and T1 fat-saturated (FS) postcontrast images show two intersphinteric fistulas located at 5
and 7 o’clock (arrows). On the coronal diffusion-weighted imaging (DWI) and STIR images, the arrows show hyperintense tracts located
in the intersphincteric space without communication to the external anal sphincter.
Type 1 - Intersphincteric Fig 1.1 - Left intersphincteric (arrow).
Axial
T2
Axial F1 FS CE Coronal DWI
Axial STIR Coronal STIR

22. Parks Classification System
Type 2 – Transsphincteric
50-year-old man with perianal fistula. Axial (Ax) T2 fat-saturated (FS), sagittal (Sag) T1 FS, and diffusion-weighted imaging (DWI) images
demonstrate a transsphincteric fistula (TSF) at 6 o’clock position with a path that courses through the left ischioanal fossa (IAF) and a
sinus into the subcutaneous cellular tissue of the left gluteal region. There is no communication to skin.
Fig 1.2 - Transsphincteric (*).
*
Sag DWI
Sag T1 FS
Ax T2 FS

23. Parks Classification System
Type 3 - Suprasphinteric
Coronal (Cor) contrast-enhanced
(CE) T2- and T1-weighted fat-
saturated MR images show a
fistula that crosses the internal
anal sphincter (yellow arrow), the
tract courses cranially along the
intersphincteric space (white
arrow), passes over the top of the
puborectalis muscle (pink arrow),
then descends into the
ischiorectal fossa (red arrows).
Fig 1.3 - Right suprasphincteric
(arrowhead).
Cor T2 Cor T2 Cor T2
Cor T1 FS CE Cor T1 FS CE Cor T1 FS CE

24. Parks Classification System
Type 4 - Extrasphinteric
Coronal (Cor) STIR- weighted image shows a right
extrasphinteric (pelvic) abscess (yellow arrow)
with a translevator fistula traversing the
ischiorectal fossa (pink arrows). White arrow
represents the levator ani muscle, and the rectum
is denoted by the white asterisk.
Fig 1.4 - Right extrasphincteric (◁).
◁
Cor STIR
*

25. Classification System
St James’s University Hospital:
o Axial and coronal anatomic landmarks
o Delineates the course of the primary fistulous tract, site of secondary
extentions and abscess
o Five grades:
• G1: Simple linear intersphincteric fistula.
• G2: Intersphincteric fistula with abscess or secondary tract.
• G3: Transsphincteric fistula.
• G4: Transsphincteric fistula with abscess or secondary tract within
the ischiorectal or ischioanal fossa.
• G5: Supralevator and translevator disease.
Fig 2 - Grades
1
2
3
4
5

26. St James’s University Hospital Classification System
Grade 1: Simple Linear Intersphincteric Fistula
Coronal (Cor) and axial (Ax) T2-weighted images show an
intersphincteric simple linear fistula tract at the 4 o’clock position
(arrows) without complications. FS = fat-saturated.
Fig 2.1 - Left G1 fistula.
a = coronal, b = axial.
a
b
Ax T2 Ax T2 FS
Cor T2

27. St James’s University Hospital Classification System
Grade 2: Intersphincteric Fistula with Abscess or Secondary Tract
Coronal (Cor) and axial T2-
weighted images show an
intersphincteric fistula at 3 and 5
o’clock positions (pink arrows)
confined by the external
sphincter. The abscess is marked
with yellow arrows and tract
with pink arrows.
Fig 2.2- Left G2 fistula.
a) Coronal. b) axial
a
b
Axial T2
CorT2

28. St James’s University Hospital Classification System
Grade 3: Transsphincteric Fistula Fig 2.3 - Right G3 fistula.
a = coronal, b = axial.
a
b
Cor T2
Coronal (Cor) T2-weighted images shows a right transsphincteric fistula. The white arrows represent
the internal opening of the fistula. The tract pierces both layers of the sphincter complex extending
into the ischiorectal fossa (pink arrows). CE = contrast-enhanced, FS = fat-saturated.
Cor T2
Cor T1 FS CE

29. St James’s University Hospital Classification System
Grade 4: Transsphincteric Fistula with Abscess or Secondary
Tract within the Ischiorectal or Ischioanal Fossa
Cor T2
Axial (Ax) fat-saturated (FS) T1-weighted contrast-enhanced (CE) images as well
as axial and coronal (Cor) T2- weighted images show a left transsphincteric
fistula (pink arrows) with multiple associated abscesses in the left ischioanal
fossa (red arrows).
Ax T2
Ax T1 CE
Fig 2.4 - Left G4 fistula.
a = coronal, b = axial.
a
b
CorT2
Axial T2 FS

30. St James’s University Hospital Classification System
Grade 5: Supralevator and Translevator Disease
Sagittal (Sag) T2-weighted image and axial
(Ax) T2 video show a supralevator (pelvic)
abscess (red arrow) with a translevator
fistula traversing the right ischiorectal and
ischioanal fossa (IRF) (yellow arrows). PC =
pelvic cavity.
Sag T2
Fig 2.5 - Right G5 fistula Coronal.
Ax T2

31. Techniques for Imaging
CT:
HIV patient with abscesses (asterisks) that extend
above the levator ani muscle (arrows), affecting the
pelvic cavity.
* *
 Quick to identify lesions requiring urgent surgical treatment
 Good for evaluation of emergency room (ER) patients with
suspicion for infected fistulous tracts or perianal abscess
 Lower cost
 Poor tissue contrast resolution
 May not adequately visualize subtle fistulas
 Does not identify fissures
Cor CECT
* *

32. CT Protocol:
Techniques for Imaging
Concentration: 370 mg
100 mL or 1.5 – 2 mL/kg
2 mL/sec
70 sec delay
0.6 mm section thickness
Sagittal and coronal images
IV Iodinated Contrast:
Contrast Dose:
Rate:
Acquisition Time:
Axial Images:
Reformats:

33. Techniques for Imaging
MRI:
T2-weighted fat-saturated (FS) axial images
show a right transsphincteric fistula with a
right ischiorectal fossa abscess (red arrows).
Reference Standard
 Better definition of anatomy
 Demonstrates hidden areas of infection or additional
extension of disease
 Best imaging to depict fistulous tracts
 Identify associated complications
 Depict fissures
T2 FS
T2

34. Techniques for Imaging
MRI:
T2-weighted fat-saturated (FS) axial images
show a right transsphincteric fistula with a
right ischiorectal fossa abscess (arrows).
T2 FS
T2
FS
DWI
T2
Diffusion-weighted imaging (DWI) when used
in combination with T2-weighted imaging
allows for better delineation of perianal
fistula. It is a fast sequence that can be
added to the entire study.
T2-weighted images without fat saturation
better display the anatomy.
Fat-saturated images better depict fistulas.
T2

35. Techniques for Imaging
MRI Protocol:
Sagittal T2- weighted images. (A) Oblique axial and (B) oblique coronal imaging planes
according to the anal canal axis. CE = contrast-enhanced, FS = fat-saturated, STIR = short-
tau inversion recovery.
⦿ 3T preferred
⦿ Three Planes
⦿ Section thickness: 3 mm
⦿ Axial, coronal and sagittal: T2
⦿ Axial and coronal: STIR / T2 FS
⦿ Axial and coronal after gadolinium-
based contrast agent: T1 CE FS
A B

36. Techniques for Imaging
MRI Protocol:
Flip angle (degree) TR (msec) TE (msec) Slice thickness (mm) FOV (mm2)
Ax obl T2 155 4000 82 3 220 x 220
Cor obl T2 137 4630 104 3 220 x 220
T2 FS/STIR 120 4920 56 3 220 x 220
T1 FS CE 9 3.29 1.29 3 240 x 201
T1 FS CE Hi-res 13 4.36 2.23 2.5 280 x 280
Ax = axial, CE = contrast-enhanced, Cor = coronal, FOV = field-of-view, FS = fat-saturated, Hi-res
= high resolution, obl = oblique, STIR = short-tau inversion recovery.

37. • Regardless of the classification used, mention the precise location of the anatomical structures involved as well as
the site of fistulas and abscesses. Any of the following methods can be used:
Anal clock (surgeon’s view of the perianal region).
Patient is oriented in the supine position (lithotomy position) with axial slices
used for localization.
QUADRANT LOCATION ANAL CLOCK
Reporting
AxT2

38. Radiological reports must have the following three points:
Reporting
Axial T2
Internal
Opening
• The internal orifice at the lumen of the anal canal.
• Most are located at the level of the dentate line and at the
posterior midline.
Course
External
Opening

39. Radiological reports must have the following hree3 points:
Reporting
Axial T2
Internal
Opening
• The internal orifice at the lumen of the anal canal.
• Most are located at the level of the dentate line and at the
posterior midline.
Course
• Course of the primary fistulous tract and its relationship with
the internal anal sphincter (IAS) and external anal sphincter
(EAS).
• Secondary associated tracts.
• Describe any extension into the ischioanal, perineal, gluteal
or supralevator regions.
• Remember: Chronic fistulas have more complications and
associated extensions.
External
Opening

40. Radiological reports must have the following three points:
Reporting
Axial T2
Internal
Opening
• The internal orifice at the lumen of the anal canal.
• Most are located at the level of the dentate line and at the
posterior midline.
Course
• Course of the primary fistulous tract and its relationship
with the internal anal sphincter (IAS) and external anal
sphincter (EAS).
• Secondary associated tracts.
• Describe any extension into the ischioanal, perineal, gluteal
or supralevator regions.
• Remember: Chronic fistulas have more complications and
associated extensions.
External
Opening
• May be more than one.
• Chronicity can make it difficult to identify the external
orifice.
• Sinus Tract: refers to a primary tract that terminates
blindly in subcutaneous fat tissue (image, arrow).

41. Reporting
Intersphincteric fistula with internal origin at 1 o'clock
position in the middle third of the internal anal
sphincter. Through T2-weighted sequence, the
hyperintense fistulous path is observed, which is
correlated to activity and patency of the fistula, with the
intergluteal fold opening to the skin.
Example 1
AxT2

42. Reporting
Example 2
Left posterolateral transsphincteric fistula with
internal origin at the 4 o'clock position in the
upper third of the internal anal sphincter and
perforation of the external anal sphincter. A
fistulous course is observed through the
ischioanal fossa with a hypointense seton within.
AxT2

43. Reporting
43-year-old man diagnosed with
HIV/AIDS and complex perianal
fistula.
Axial (Ax) and coronal (Cor) T2-
weighted images show a
transsphincteric anal fistula at 6
o’clock position that
communicates with a sinus in
the left ischioanal fossa and on
the right there is a
suprasphincteric path that
involves the levator ani muscle.
Intersphincteric accessory
pathways coexist and several
skin communication sites are
observed in the gluteal and
perineal folds.
Example 3 – Complex Fistula
Ax T2 Cor T2

44. Reporting
56-year-old man with perianal fistula. Axial (Ax) T2 (a), T2 fat-saturated (FS) (b), axial high-resolution T1 FS contrast-
enhanced (CE) (c), and sagittal (Sag) T1 FS CE image (d) show a transsphincteric anal fistula (white arrow) at the 6 o’clock
position communicating with an abscess (pink arrow) in the left ischioanal fossa, which communicates with the skin in
the left gluteal fold (not shown).
Example 4 – Pre-treatment
AxT1 FS CE
c
Sag T1 FS CE
AxT2
d
a
AxT2 FS
b

45. Reporting
Example 4 – Post-treatment
Same patient mentioned in previous slide. Follow-up exam 1 month later following seton
placement (e-h) demonstrates decreased inflammation and slightly decreased size of the
abscess (pink arrow). (h) Shows the course of the seton (oval ring, red arrow). Fistula
represented by white arrows.
AxT2 AxT2 FS AxT1 FS CE Sag T2 FS
e f g
h

46. • Always look for the levator ani muscle to define the supralevator space/pelvic cavity. It is important to
evaluate this structure because when it is involved, it is a surgical emergency.
Pearls:
Pearls and Pitfalls
Levator ani muscle (arrows); supralevator space/pelvic cavity (purple color). Ax = axial.
Cor

47. • Extension into ischiorectal, ischioanal, perineal or gluteal spaces often denotes a transsphincteric fistula.
Pearls:
Pearls and Pitfalls
CorT2
Fig 3 – Coronal diagram representing right ischiorectal and left ischioanal
abscess.
Coronal (Cor) T2-weighted image show a left transsphincteric
fistula (blue arrow) with associated abscesses in the left
ischiorectal (purple arrow) and ischioanal fossa (red arrow).

48. • Fistulous tracts that only involve muscle fibers of the subcutaneous (SC) bundle of the external anal
sphincter can still be considered an intersphinteric fistula because very few fibers are affected and
management does not change.
Pearls:
Pearls and Pitfalls
Cor T2
Cor NECT
Fig 4 – Coronal (Cor) diagram. Coronal non-contrast enhanced CT (NECT) and coronal T2 images representing left intersphinteric fistula involving SC bundle (yellow arrows).

49. • The ischioanal fossa is the most common site of extension.
Pearls:
Pearls and Pitfalls
Ax T2
Fig 5 – Coronal diagram indicating the ischional fossa. Axial (Ax) T2-weighted image representing
abscess in right ischioanal fossa (yellow
arrow).

50. • High-resolution T1-weighted fat-saturated (FS) contrast-enhanced (CE) images may aid in diagnosing
complex fistula disease (only on that sequence can a second fistula be seen [red arrow]). Hi res = high
resolution, STIR = short-tau inversion recovery.
Pearls:
Pearls and Pitfalls
Ax T2 STIR Ax T1 FS CE Ax T1 FS CE hi res

51. • Common mimics of perianal fistulas:
• Proctitis
• Necrosis
• Pilonidal Disease
• Hidradenitis Suppurativa
• Anovaginal Fistula
• Anal Fissure
Pitfalls:
Anovaginal fistula (white arrows). A = anal canal, V = vagina. Ax = axial. Hidradenitis Suppurativa. Sag = sagittal.
Pearls and Pitfalls
T2 Sag T2 Ax
V
V
A
A
T2 Sag

52. • CT can be used in the emergency department (ED) setting, especially to rule out pathologic conditions
requiring urgent surgical management.
• For CT evaluation, use the MPR tool to align your images to the true axial and coronal planes of the anal
canal.
• MRI is the reference standard modality, it should be performed for a complete assessment, even with a
normal CT scan in a symptomatic patient.
• An adequate radiological report must be clear and concise, with a comprehensive characterization and
localization of the anatomic structures involved by the fistulas and abscesses.
Summary

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