Computed Tomography (CT)

This is a technology that uses computer-processed x-rays to produce tomographic images (virtual 'slices') of specific areas of the body, allowing a view to what is inside it without cutting it open. In Medical imaging CT cross-sectional images are used for diagnostic and therapeutic purposes in various medical disciplines.

There are several advantages that CT demonstrates over traditional X-Ray. First, CT completely eliminates the superimposition of images of structures outside the area of interest. Second, because of the inherent high-contrast resolution of CT, differences between tissues that differ in physical density by less than 1% can be distinguished. Finally, data from a single CT imaging procedure consisting of either multiple contiguous or one helical scan can be viewed as images in the axial, coronal, or sagittal planes, depending on the diagnostic task.

CT is regarded as a moderate- to high-radiation diagnostic technique. The improved resolution of CT has permitted the development of new investigations, which may have advantages; compared to conventional radiography, for example, CT angiography avoids the invasive insertion of a catheter. CT colonography (also known as virtual colonoscopy or VC for short) may be as useful as a barium enema for detection of tumours, but may use a lower radiation dose. CT VC is increasingly being used as a diagnostic test for bowel cancer and can eliminate the need for a colonoscopy.

The radiation dose for a particular diagnosis depends on multiple factors: volume scanned, patient build, number and type of scan sequences, and desired resolution and image quality. CT scan has been shown to be more accurate than radiographs in evaluating anterior inter-body fusion.

Usage of CT has increased dramatically over the last two decades. CT has become an important tool in medical imaging to supplement x-rays and medical ultrasonography. It is also used for preventive medicine or screening for disease, for example CT colonography for patients with a high risk of colon cancer, or full-motion heart scans for patients with high risk of heart disease. The main uses of CT in medical diagnostics are to examine:

• Head: CT scanning of the head is typically used to detect infarction, tumours, calcifications, haemorrhage and bone trauma. Tumours can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. CT scanners are very useful to investigate and diagnose cases involving stroke or head trauma.


• Lungs: CT is used for detecting both acute and chronic changes in the internals of the lungs where conventional X-rays do not show such defects. A variety of techniques are used, depending on the suspected abnormality. For evaluation of chronic cases such as emphysema, fibrosis, etc., thin sections with high spatial frequency reconstructions are used; often scans are performed both in inspiration and expiration. This special technique is called high resolution CT. Therefore, it produces a sampling of the lung and not continuous images.


• Pulmonary Angiogram: CT pulmonary angiogram (CTPA) is a diagnostic test used to diagnose pulmonary embolism (PE). It employs computed tomography and an iodine based contrast agent to obtain an image of the pulmonary arteries.


• Cardiac: with the advent of faster and multi-slice CT scanners (more than 64 slices), high resolution and high speed can be obtained at the same time, allowing excellent imaging of the coronary arteries (cardiac CT angiography).


• Abdominal and Pelvic: CT is a sensitive method for diagnosis of abdominal diseases. It is used frequently to determine stages of cancer and to follow progress. It is also a useful test to investigate acute abdominal pain.


• Extremities: CT is often used to image complex fractures, especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries and dislocations can easily be recognised with a 0.2 mm resolution.