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MRI of Perianal Crohn's Disease

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

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 
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

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 
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.

View larger version (127K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (135K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (186K): [in this window] [in a new window] [as a PowerPoint slide]   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.

Imaging Sequences and Planes

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 
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.

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).

View larger version (136K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (161K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (173K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (156K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (117K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (149K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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).

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (147K): [in this window] [in a new window] [as a PowerPoint slide]   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.

Systematic Approach

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 
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.

View larger version (174K): [in this window] [in a new window] [as a PowerPoint slide]   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).

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).

View larger version (97K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (183K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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).

View larger version (177K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (188K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (145K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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.

View larger version (170K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (169K): [in this window] [in a new window] [as a PowerPoint slide]   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).

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).

View larger version (178K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (120K): [in this window] [in a new window] [as a PowerPoint slide]   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.

The fifth and final step includes the evaluation for ancillary findings, such as bone marrow edema or cancer (Figs. 13 and 14A, 14B).

View larger version (152K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (159K): [in this window] [in a new window] [as a PowerPoint slide]   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.

View larger version (198K): [in this window] [in a new window] [as a PowerPoint slide]   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)

Pitfalls

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 
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).

View larger version (119K): [in this window] [in a new window] [as a PowerPoint slide]   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).

View larger version (154K): [in this window] [in a new window] [as a PowerPoint slide]   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.

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

Top Introduction Anatomy Imaging Sequences and Planes Systematic Approach Pitfalls References

 

1. American Gastroenterological Association. AGA technical review on perianal Crohn's disease. Gastroenterology2003; 125:1508 -1530[Medline]
2. 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]
3. Halligan S, Buchanan G. MR imaging of fistula in-ano. Eur J Radiol2003; 47:98 -107[Medline]
4. 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
5. 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]
6. Present DH. Crohn's fistula: current concepts in man agement. Gastroenterology2003; 124:1629 -1635[Medline]
7. Parks AG, Gordon PH, Hardcastle JD. A classification of fistula-in-ano. Br J Surg1976; 63:1 -12[Medline]
8. 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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This Article Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Horsthuis, K. Articles by Stoker, J. Search for Related Content PubMed PubMed Citation Articles by Horsthuis, K. Articles by Stoker, J. Social Bookmarking                      What's this? Hotlight (NEW!) What's Hotlight?