Traditional contrast examinations
Despite the rapid development of tomography, intraluminal contrast-enhanced fluoroscopy is still used to solve localized surgical problems.
- Water-soluble contrast enema. Iodine-based agents are used (Gastrografin, Urografin). The method is optimal for checking the integrity of intestinal anastomoses after surgery, visualizing fistulous tracts, and ruling out mechanical bowel obstruction. It is important to remember complications: such contrast media are hypertonic. If the patient has a stricture, the contrast causes fluid to shift into the lumen proximal to the narrowing, leading to overdistension and the risk of perforation. Therefore, modern practice attempts to use isotonic solutions.
- Double-contrast barium enema (Double-contrast barium enema). An examination using a barium suspension and air insufflation for full bowel distension without blind spots. In modern clinical practice, this method is considered obsolete for detecting polyps or colorectal cancer—it has been completely replaced by cross-sectional imaging.
Computed tomography and CT colonography
Computed tomography has become the primary tool for emergency and elective diagnostics of intestinal pathologies.
- Basic abdominal CT. Includes intravenous contrast enhancement, sometimes supplemented by oral administration of barium or water-soluble contrast. Targeted rectal administration is used when anastomotic leaks are suspected. Normally, the mucosa shows uniform enhancement, and the pericholonic/perirectal adipose tissue appears homogeneous.
- CT colonography (CT colonography, CTC). A highly informative visualization method of the bowel distended with carbon dioxide (using an automated insufflator). To improve distension, an antispasmodic (Buscopan) is administered intravenously. A key preparation step is fecal tagging: the patient takes a contrast agent so that fecal masses acquire high X-ray attenuation and do not mimic polyps. Scanning is performed in two positions (prone and supine), after which the physician correlates 2D axial slices with 3D endoluminal reconstructions.
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CT contrast enhancement patterns
The pattern of contrast enhancement of the bowel wall (mural enhancement pattern) is a crucial diagnostic criterion in computed tomography.
| Enhancement type | Potential pathological processes |
|---|---|
| Pronounced (Avid) | Acute infection, IBD flare-up, ischemia, malignant tumors |
| Homogeneous | Lymphoma, chronic ischemia, chronic IBD |
| Heterogeneous | Tumors, lymphoma (especially post-chemotherapy), endometriosis infiltration |
| Layered / target sign | Vasculitis, IBD, radiation colitis, graft-versus-host disease |
| Decreased | Ischemia (compromised arterial or venous blood flow) |
Magnetic resonance imaging and ultrasound
Unlike CT, magnetic resonance imaging involves no ionizing radiation, making it indispensable for chronic conditions.
- Pelvic MRI. It is the gold standard for assessing local staging of rectal tumors (including involvement of the mesorectal fascia), as well as for detecting perirectal fistulas. As an alternative to endoscopy in IBD, MR colonography (MR-colonography) is used, performed with luminal distension using water or gas.
- Defecography (Evacuation proctography). A dynamic examination of the act of defecation performed via fluoroscopy (using thick barium paste) or MRI (using ultrasound gel). It includes rest, evacuation, and post-evacuation phases. In MR defecography, the degree of pelvic organ prolapse is measured relative to the pubococcygeal line (pubococcygeal line).
- Ultrasound. Transabdominal ultrasound is performed using graded compression, but visualization is limited by intraluminal gas. Endosonography has significantly higher accuracy. High-frequency probes allow the differentiation of 4 layers in the anal canal (subepithelium, internal sphincter, longitudinal muscle, external sphincter) and 5 layers in the rectal wall.
Briefly: answers to topical questions
What structures and parameters are evaluated on pelvic MRI for rectal cancer staging?
During rectal cancer staging on a pelvic MRI, the status of lymph nodes, the mesorectal fascia, the presence of extramural venous invasion, and the tumor's response to treatment are evaluated.
- Lymph nodes — morphological features are assessed (irregular contour, heterogeneous internal signal).
- Mesorectal fascia (mesorectal fascia) — involvement of the fascia by the tumor is determined, which is critical for assessing resection margins.
- Extramural venous invasion (extramural venous invasion) — dilatation and tortuosity of the draining vein are identified, with its lumen filled with abnormal soft tissue.
- Tumor regression grade (MRI Tumour Regression Grade) — the ratio of dense fibrosis to residual viable tumor after therapy is assessed.
Class and exam questions
Why is barium enema no longer used for polyp screening?
Double-contrast barium enema has blind spots and is inferior in accuracy to tomographic methods. It has now been completely replaced by CT colonography (CTC).
What is the danger of a water-soluble enema in bowel narrowing?
Iodine-based agents are hypertonic. Penetrating above a stricture, they cause a massive influx of fluid into the lumen, wall overdistension, and a risk of bowel perforation.
What is the role of PET in the diagnostics of intestinal diseases?
18F-FDG PET is used to precisely detect distant metastases and extramural spread of colorectal cancer prior to surgical treatment.
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What else to study on this topic
- Technique and agents for fecal tagging in CT colonography
- Differential diagnostic imaging of inflammatory bowel diseases (IBD)
- MR defecography: biomechanics of the rest, evacuation, and post-evacuation phases
- Endosonographic anatomy of the rectal sphincter apparatus
- Assessment of the mesorectal fascia and pelvic tumor staging on MRI
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Other topics in the section "Diagnostic Radiology of the Abdominal Organs"
Anatomy of the colonColonic polypsColorectal cancerRectal cancerAnal canal cancerColonic diverticular diseaseColitidesParasitic Infections of the ColonToxic MegacolonColonic StricturesVolvulusRetrorectal MassesColonic motility disordersRectal fistulasX-ray examination of the intestineCT enterography / colonography with contrast
Entire section "Diagnostic Radiology of the Abdominal Organs"Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.