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Keywords
Computed Tomography Angiography (CTA) «»
Computed Tomography Myelography (CT Myelography)
Introduction
Procedure
Technical Details
Risks
Computed tomography (CT) is a combination of focused x-ray beams and the computerized production of an image that provides physicians with views of internal body structures. This modality is particularly useful when quick imaging of the brain and/or spine is needed (such as in a trauma), or as a first-line imaging exam in patients with suspected problems of the brain. A brain CT image is depicted below.
Computed tomography angiography (CTA) is a variation on the standard CT which is used to visualize blood vessels. After injecting some contrast material into the blood, a CT scan is performed after which a computer highlights the structures that contain contrast (i.e. blood vessels) and a three-dimensional picture of the blood vessels is created. This technique is typically used in combination with a standard CT in order to obtain images that show abnormalities such as blockage, stenosis (abnormal narrowing) or aneurysms (abnormal dilation) of arteries and veins in the brain and neck, as well as certain other blood vessels in the body. A brain CTA is depicted below.
Computed tomography myelography (CT Myelography) is an imaging modality that combines myelography with CT scanning to provide better visualization of certain types of tissues within the spinal canal. This procedure involves the injection of a contrast material into the cerebrospinal fluid through a lumbar puncture followed by a CT scan of the area of interest. The contrast material will highlight the cerebrospinal fluid space; therefore, any tissues that exist within this space will become more conspicuous. CT myelography is typically used for imaging of the spine in those patients who can not obtain an MRI for a variety of reasons. A lumbar spine CT myelography image is depicted below.
The imaging process typically takes place on a CT scanner similar to the one depicted below.
The patient lies back on a narrow cushioned table which slides into a doughnut-shaped tube that houses the CT scanner. Once the area of the body to be imaged has been properly positioned, a very thin image through the area is obtained after which the table moves down a fraction of an inch and the next image is obtained. Each image acquisition takes from less than one to several seconds and the entire exam will last anywhere from less than one to several minutes, depending on the area of the body being imaged. During this time the patient is not allowed to move since this can cause distortions in the image.
In some cases, a chemical agent can be injected into the blood stream or the cerebrospinal fluid in order to improve the contrast between healthy and diseased tissues.
The CT scanning process is typically quick and comfortable. In many cases, the entire examination will be performed by a CT technician who is not a doctor (with the exception of CT myelography where injection of the contrast agent into the cerebrospinal fluid will typically be performed by a physician). However, the supervising radiologist should be available to consult as necessary during the exam, and will view and interpret the results.
During a CT scan, multiple x-rays are produced by an x-ray source in the form of a beam that rotates around the patient. These x-rays pass through the body in various angles and some of the rays are absorbed by different body tissues. The x-rays that are not absorbed exit the body and are detected by the machine, and used to create images on a television screen. Since various tissues absorb x-rays in different ways, the amount of x-ray that reaches the detector can be used to determine the type of tissue being examined.
CT scanners use ionizing radiations (x-rays) which while relatively harmless at low doses, can potentially increase the risk of certain types of cancers in chronic or repeated exposure. Furthermore, exposure to x-rays during pregnancy can increase the risk of birth defects. Therefore, all individuals, especially woman who are, or may be, pregnant, should carefully discuss this issue with their physician and radiologist before undergoing a scan in order to weigh the benefits of the CT scan against its potential risks.
Also, contrast agents that contain iodine may sometimes be injected into the blood stream or into the cerebrospinal fluid in order to improve the image quality. In rare instances, some patients may be allergic to these chemicals or may have a medical condition or be taking a medication which makes it dangerous to receive these agents. Therefore, if these contrasts materials are are to be used, patients should discuss their medical history, medications, and any other concerns with their physician and radiologist prior to receiving the injection.
