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Computed tomography (CT) is a non-invasive diagnostic imaging modality that uses electromagnetic radiation (X-rays) to create detailed images of the body structure.
CT entails the use of a CT machine that produces multiple X-ray beams from different directions focused on the patient who is positioned inside the gantry on the patient couch. The X-rays are later processed to create images of the examined part of the body based on the differences in tissue density of the body parts. For example, some tissues (e.g., air, fluid) allow a majority of X-rays to pass through, while others (e.g., bone) allow only a few X-rays to penetrate. Therefore, when an X-ray beam is focused on the body, tissues with different densities are delineated clearly and the images are reconstructed accordingly.
Although the exposure to electromagnetic radiation is greater in a CT scan than in a conventional plain X-ray radiograph, the images obtained are more precise and meticulous.
CT scan of the abdomen and pelvis facilitates the diagnosis of acute abdomen, diseases of the gastrointestinal tract and other internal abdominal organs. Ideally, an abdominal CT scan is performed unenhanced (without contrast),n followed by contrast if indicated. Contrast medium enhances the visibility of the tissue structures by enhancing the contrast among various tissues.
The use of plain chest and abdominal radiographs has many advantages, for example, low radiation exposure, short duration, cost-effectiveness, and greater convenience. However, plain radiographs occasionally fail to accurately delineate the location of the abnormality. CT scan is preferred in such cases and is often the modality of choice.
Oral Contrast and Abdominal CT
Oral contrast is beneficial for the diagnosis of specific etiologies of an acute abdomen. Small-sized abnormalities of the upper gastrointestinal tract, such as ulcers, obstructions or space-occupying lesions, are better visualized with contrast-enhanced CT, especially oral contrast enhancement.
Its use is limited in certain conditions, such as:
- Patients with ileus who cannot tolerate oral contrast
- Patients with illnesses who cannot tolerate oral contrast and require an intragastric tube route
- The time between the administration of contrast and the CT scan. Patients typically need to wait approximately 90 minutes before a CT scan, which might be unethical in some scenarios or impractical in an emergency setting.
Intravenous Contrast and Abdominal CT
Intravenous contrast is often used in patients with suspected gastrointestinal inflammatory pathology, arteriovenous malformations, or space-occupying lesions.
The rationale for using intravenous contrast is that inflammatory lesions are more likely to absorb a specific contrast due to endothelial cell disruption compared with healthy tissues.
The use of intravenous contrast is limited under certain conditions, for example, allergic reaction to intravenous contrast and renal impairment.
Intravenous contrast dose adjustments are necessary for patients with renal impairment. Patients with acute abdomen are prone to acute kidney failure, which is a contraindication for the use of intravenous contrast.
Rectal Contrast and Abdominal Computed Tomography
Rectal contrast facilitates visualization of colonic pathologies, such as:
- Inflammatory changes
The risks associated with rectal contrast are limited to contrast extravasation and procedural delays.
Emergency Abdominal CT
In an emergency setting, abdominal CT is often preferred for the timely diagnosis and management of hemodynamically stable patients. It is impractical to follow the usual complete protocol. Therefore, contrast use should be individualized.
For example, contrast use is not required in case of suspected intra-abdominal hemorrhage, while contrast is required in suspected parenchymal pathologies. Patient with suspected bowel obstruction might need oral and rectal contrast.
Impact of Emergency Abdominal CT on the Management Plan
CT scan of the abdomen reveals a significant pathology in up to two-thirds of the patients presenting with an acute abdomen. An emergency abdominal CT scan is used to detect the most common pathologies including inflammatory, infectious, and obstructive diseases.
After a CT scan of the abdomen, no intervention is needed in one- half of the patients, while surgical intervention is needed n one-fourth of the patients. Therefore, emergency CT scans of the abdomen facilitate the management of patients and are indicated in hemodynamically stable patients presenting with an acute abdomen.
Multiple Detector CT
Multiple detector computed tomography (MD-CT) scans of the abdomen are beneficial in an emergency setting. MD-CT is faster than conventional CT and produces images of better quality.
The diagnostic quality of MD-CT in pathologies of the small bowel and colon is excellent and comparable to magnetic resonance imaging (MRI). The terminal ileum is usually better visualized compared with the rest of the small bowel. In one study, approximately 95% of the patients with confirmed colonic pathology had an abnormal MD-CT scan of the colon. MD-CD also facilitates accurate detection of fistulas, extraintestinal abscesses, and enlarged lymph nodes.