A Computed tomography (CT) scan is a non-invasive diagnostic imaging procedure that uses electromagnetic radiations (X-rays) to create detailed images of the body structures.
It involves the use of a machine that produces multiple X-ray beams. The patient is positioned inside a CT scan machine and multiple pulses of X-rays are generated from different directions that are later processed to create images of the examined part of the body.
These images are created using a basic principle that different body structures have different tissue densities, for instance, some tissues (e.g. air, fluid) allow a majority of X-rays to pass through them, while others (e.g. bone) allow only a few X-rays to cross; therefore, when an X-ray beam is allowed to cross, the tissues with different densities are delineated clearly and the images are accordingly reconstructed.
A CT scan of the abdomen and pelvis helps in the diagnosis of an acute abdomen, diseases of the gastrointestinal tract and other internal abdominal organs. In an optimum setting, an abdominal CT scan should be performed unenhanced (without a contrast), with an intravenous contrast, an oral contrast, and a rectal contrast simultaneously. A contrast medium helps in better visibility of the tissue structures by enhancing the contrast among various tissues.
The use of plain chest and abdominal radiographs has many advantages; for instance, low radiation exposure, short procedure duration, less expensive, and more convenient, but plain radiographs sometimes fail to accurately delineate the location of the abnormality; hence, a CT scan of preferred is preferred in these cases and is often an investigation of choice.
Oral Contrast and Abdominal Computed Tomography
Oral contrast use is beneficial in certain diagnoses of the acute abdomen. The small-sized abnormalities of the upper gastrointestinal tract, for instance, ulcers, obstructions or space-occupying lesions, are better visualized with CT scans enhanced by an oral contrast.
Its use is limited in certain circumstances, such as:
- The patients with ileus are intolerant to the oral contrast.
- The patient is too sick to take the oral contrast. In that case, the use of an intragastric tube route might be indicated.
- The time between the administration of the contrast and the CT scan. Patients typically need to wait for approximately 90 minutes before they perform a CT scan, which might be unethical in some scenarios or unfeasible in an emergency setting.
Intravenous Contrast and Abdominal Computed Tomography
Intravenous contrast is often used in suspected gastrointestinal inflammatory pathologies, arteriovenous malformations, or space-occupying lesions.
The rationale behind using an intravenous contrast is that inflammatory lesions are more likely to take a certain contrast due to endothelial cell disruption compared to healthy tissues.
The use of intravenous contrast is limited in certain conditions, for instance, an allergic reaction to intravenous contrasts, and renal impairment.
Rectal Contrast and Abdominal Computed Tomography
Rectal contrast helps in better visualization of colonic pathologies, such as:
- Inflammatory changes
Emergency Abdominal Computed Tomography
In an emergency setting, an abdominal CT scan is often a preferred investigation in hemodynamically stable patients for an urgent timely diagnosis and management. It is unfeasible to follow the usual complete protocol; therefore, contrast use should be individualized.
For instance, contrast use is not required in suspected intra-abdominal hemorrhage, while it is required in suspected parenchymal pathologies. A patient with suspected bowel obstruction might need both oral and rectal contrast.
Impact of Emergency Abdominal Computed Tomography on the Management Plan
A CT scan of the abdomen reveals a significant pathology in up to two-thirds of the cases presenting with an acute abdomen. The most commonly identified pathology on an emergency abdominal CT scan is inflammatory, followed by infectious and obstructive pathologies.
It is estimated that after a CT scan of the abdomen, no intervention is needed in half of the cases, while surgical intervention is needed only in one-fourth of the cases; therefore, the emergency CT scans of the abdomen help in the management plan of the patient and are indicated in hemodynamically stable patients presenting with an acute abdomen in the emergency.
Multiple Detector Computed Tomography
Multiple detector computed tomography (MD-CT) scans of the abdomen are beneficial in the emergency setting. This technique is faster than conventional CT scans and produces images of better quality.
The diagnostic quality of MD-CT in the small bowel and colonic pathologies is excellent and is comparable to magnetic resonance imaging (MRI). The terminal ileum is usually better visualized compared to the rest of the small bowel. In one study, approximately 95% of the patients with confirmed positive colonic pathology had an abnormal MD-CT scan of the colon. It also accurately detects fistulas, extraintestinal abscesses, and enlarged lymph nodes.