Computed tomography (CT) is an imaging modality that utilizes x-ray photons for image production, with digital reconstruction. The CT scanner essentially consists of an x-ray tube and detectors. The x-ray tube produces an x-ray beam that passes through the patient. This beam is captured by the detectors and reconstructed to create a two or three dimensional image.
The analog data captured by the scanner is digitally converted by various algorithms into reconstructed images, which represent a cross-sectional slice through the patient at that level. Each image is acquired at a slightly different angle and results from a different reconstruction algorithm. The individual volume elements that make up the image are each displayed as a two-dimensional pixel, each of which carries a designation of density or attenuation, represented by a Hounsfield unit (HU).
Contrast media may be employed during a CT study. It helps to distinguish structures of similar density in the body. The most commonly used in CT studies are intravenous and oral contrast agents. Many abnormalities, such as bleeding, extravasation or neoplasms become more evident through contrast perfusion. Oral contrast may also be given to opacify the cavities of the gastrointestinal organs, allowing for greater differentiation of these spaces.
Common clinical applications
- Brain (with or without contrast and perfusion study)/Cranial/Head/Neck CT
- CT myelography
- Chest/mediastinum/high resolution CT
- Abdominal CT
- Pelvic CT
- CT urography
- CT Colonography
- Cardiac CT
- CT angiography
- Quantitative Computed Tomography, QCT Densitometry
- As with plain radiography, CT utilizes x-ray radiation to produce images, however the radiation doses from CT are higher due to multiple exposures.
- Pregnancy status must be established prior to exam. X-ray procedures should be delayed until after pregnancy where possible or non radiation imaging such as ultrasound or MRI utilized.
- As children are more radiosensitive, x-ray procedures should be performed with caution.
- Procedural complications due to intervention such as haemorrhage, infection, shock, rarely death as a result of the procedure.
- Hypersensitivity reaction to contrast agents may range in severity from flushing to anaphylaxis. Risk factors for immediate hypersensitivity reactions to IV contrast include previous allergic reaction to IV contrast, history of asthma or food allergies.
- Patients with renal insufficiency/failure, multiple myeloma, severe cardiac disease such as congestive heart failure, aortic stenosis, should not receive IV contrast. Other examples include severe dehydration, diabetics, sickle cell disease, etc.