Imaging in anal fistulas
To diagnose anal fistulas (Fistula disease), two main radiological methods are used that complement the clinical picture and help the surgeon plan the intervention:
- Magnetic resonance imaging (MRI): This is a crucial stage of preoperative preparation. It has been proven that a high-quality preoperative MRI reliably reduces the rate of recurrence, especially when dealing with complex fistulas with a branching structure.
- Endosonography: Endoscopic ultrasound is indispensable for assessing the structures of the anal canal. The method excellently visualizes the internal opening of the fistula and allows tracking fistula tracks that pass directly inside the muscle mass of the sphincter.
CT colonography in the diagnostics of colon pathologies
According to large-scale studies (e.g., SIGGAR), CT colonography (or virtual colonoscopy) serves as a reliable and effective alternative to traditional optical colonoscopy and barium enema. This method is actively used in examining patients with clinical symptoms of colorectal cancer or suspected large polyps.
CT colonography is also used as a screening tool for completely asymptomatic patients, including those with a burdened family history. To improve compliance (patients' willingness to undergo the procedure), the efficacy of noncathartic CT colonography—bowel cleansing without aggressive and poorly tolerated laxatives—is being actively studied. The use of contrast coating significantly improves the visualization and detection of flat polyps. However, the physician must always keep in mind typical diagnostic pitfalls and the risk of post-imaging colorectal cancer. All studies are regulated by strict ESGAR consensus guidelines.
MRI in rectal cancer
In malignant neoplasms of the rectum (Rectal Cancer), pelvic MRI is recognized as the absolute gold standard for preoperative staging, which is especially important for lower third rectal tumors.
Fundamental MERCURY studies have proven that accurate assessment of the circumferential resection margin - CRM using magnetic resonance imaging serves as the most reliable predictor of disease-free survival. Based on the CRM status, the risk of local recurrence can be predicted. In addition, MRI allows for an objective assessment of the response of a locally advanced tumor to neoadjuvant therapy, which directly correlates with the patient's ultimate survival rates.
Radiological evaluation in Crohn's disease and IBD
Inflammatory bowel diseases, particularly Crohn's disease, are often accompanied by the formation of fistulas and require a comprehensive radiological evaluation. The methods of choice here are CT enterography, MR enterography, and specialized intestinal ultrasound.
These methods allow for precisely determining the extent of the lesion, assessing the degree of inflammatory activity of the intestinal wall, and identifying various abdominal complications. The entire diagnostic algorithm is built on the basis of joint consensus guidelines from the European societies ECCO and ESGAR. The results of the large METRIC study confirm the exceptionally high diagnostic accuracy of MR enterography and small bowel ultrasound, which is relevant both for newly diagnosed Crohn's disease and for diagnosing its flares and recurrences.
Briefly: answers to topical questions
According to which classification do radiologists evaluate anal fistulas on MRI?
Radiologists evaluate anal fistulas on MRI using the Parks classification. It is based on the relationship of the fistula tract to the anal sphincter muscles and includes the following types:
- Trans-sphincteric (trans-sphincteric) — passing through part of the sphincter.
- Intersphincteric (intersphincteric) — located between the sphincters.
- Suprasphincteric (suprasphincteric) — looping over the top of the sphincter apparatus.
- Extrasphincteric (extrasphincteric) — passing outside the sphincter complex, for example, in the ischioanal fossa.
What parameters are evaluated in MRI staging of rectal cancer?
In MRI staging of rectal cancer, local tumor spread, lymph node status, and treatment response are evaluated. The main parameters include:
- T-stage (T-stage) — tumor size and depth of invasion through the muscular wall.
- N-stage (N-stage) — regional lymph node involvement (assessed by morphological criteria).
- Circumferential resection margin (circumferential resection margin, CRM) — distance from the tumor to the resection margin.
- Extramural venous invasion (extramural venous invasion, EMVI) — direct tumor invasion into a blood vessel.
- Mesorectal fascia involvement (mesorectal fascia involvement) — infiltration of the fascia by aggressive tumors.
- Intersphincteric plane involvement (intersphincteric plane) — tumor extension into the intersphincteric space.
- Tumor regression grade (MRI Tumour Regression Grade, MR-TRG) — assessment of response to therapy.
What abdominal complications of Crohn's disease are detected using CT and MR enterography?
Using CT and MR enterography in Crohn's disease, extramural manifestations and complications of the inflammatory process are detected. These include:
- Abscesses (abscesses).
- Fistulas and sinuses (fistulae and sinuses) — hyperintense on fat-suppressed T2-weighted images and showing intense contrast enhancement.
- Phlegmon (phlegmon) — an ill-defined inflammatory mass without clear abscess formation in the adjacent omentum or mesentery.
- Narrowing and strictures (narrowing and strictures) with potential pre-stenotic dilatation.
What typical false-positive findings and pitfalls are encountered in CT colonography?
The main false-positive finding in CT colonography is a high-attenuation diverticulum, which can mimic a polyp.
- Diverticulum with fecalith (diverticulum) — appears as a polypoid lesion on 3D endoluminal rendering. For differential diagnosis, 2D views are used, which show it to be a diverticulum filled with feces and often containing small gas bubbles.
Another typical diagnostic pitfall is missing flat lesions, which are extremely difficult to detect as they may manifest only as slight wall thickening.
Class and exam questions
Why is MRI performed for complex anal fistulae?
Preoperative MRI allows for precise mapping of fistula tracts relative to the sphincter muscles, which reliably reduces the risk of surgical errors and postoperative recurrences.
What is the advantage of endosonography?
It allows for a detailed examination of the anal canal wall from the inside, accurately localizing the internal opening of the fistula and the tracts passing through the sphincter.
What is non-cathartic bowel preparation for CT colonography?
This is a method of bowel preparation for the procedure without the use of aggressive laxatives (noncathartic CT colonography), making the procedure more comfortable for the patient.
What is the role of MRI in rectal cancer according to the MERCURY study?
MRI is the gold standard for assessing the circumferential resection margin (CRM), which predicts the risk of local recurrence and disease-free survival.
What else to study on this topic
- Specifics of performing endoanal ultrasound (technique and modes).
- CT colonography protocols and the use of contrast coating for flat polyps.
- Assessment of tumor response to neoadjuvant therapy in locally advanced cancer.
- MR enterography: assessment of inflammation activity in Crohn's disease.
- ESGAR criteria in the standardization of large bowel imaging.
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