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CT Scan Questions and answers
What is a CT scan?
CT is an abbreviation for computed tomography or computed axial tomography (CAT) scan. This type of scan uses x-rays and a computer to create detailed cross-sectional images of the body. These images provide more detailed information than a normal x-ray. CT scans usually produce two-dimensional images of a “slice” or section of the body, although the data can also be reconstructed to build three-dimensional images.
What can be detected by a CT scan / When is CT used?
Although a CT scan has many applications, it is mainly used for a rapid examination of patients who may have internal injuries and/or bleeding from accidents or other types of trauma. A CT scan can visualize nearly all parts of the body and is used to diagnose diseases or injuries as well as to plan medical, surgical or radiation treatment. It can diagnose muscle and bone disorders, bone fractures and tumours. It can detect and monitor conditions such as cancer, heart disease, lung nodules and liver masses. A CT scan is also used to pinpoint the exact location of a tumour, infection or blood clot. In addition, it can help to guide procedures such as biopsies, surgeries or radiation therapy.
The results of a CT scan can be used to help a doctor diagnose a condition and to assess the effectiveness of previous treatment as well as to plan future treatment.
What happens during a CT scan?
A CT scanner is a large, doughnut-shaped machine. It has a sliding table in the centre so that a patient may be placed effortlessly into the machine. He or she remains horizontal, at rest during the entire scan procedure. The table slowly moves through the scanner while the x-rays rotate around the body and the machine emits thin beams of x-rays. x-ray detectors pick up the strength of this beam. The more dense the tissue, the fewer x-rays pass through. This information is fed into a computer where different densities show up different types of tissue and create a picture.
It is important to keep as still as possible during the CT scan, to achieve the highest possible quality of the images. You might also be asked to hold your breath at times. A CT scan of a single body part typically takes around 30 minutes.
CT versus MRI
A CT scan uses X-Rays, whilst an MRI scan uses magnets and radio waves.
An MRI scan shows tendons and ligaments, whilst a CT scan does not.
An MRI scan is preferable for examining the spinal cord.
A CT scan is better suited for cancer, pneumonia, abnormal chest x-rays and bleeding on the brain, especially after an injury.
However, a brain tumour is more clearly visible on a MRI scan.
A CT scan shows organ tears and organ injury more rapidly and thus is more suitable for trauma cases.
Broken bones and vertebrae are more clearly visible on a CT scan.
CT scans provide a better image of the lungs and organs in the chest cavity between the lungs.
How does a CT scan work?
Unlike traditional x-rays, which use a stationary tube, a CT scanner uses a mechanical x-ray source that rotates around the circular opening of a donut-shaped structure called a giant gantry. During a CT scan, the patient lies on a slowly moving bed on the cylinder bridge as the x-ray tube rotates around the patient, projecting narrow beams of x-rays through the body. Instead of film, CT scanners use special digital x-ray detectors, which are located directly in front of the x-ray source. As the x-rays pass through a body part, they are collected by detectors and transmitted to a computer.
Each time the x-ray source completes a full cycle, the CT scan computer uses sophisticated mathematical techniques to create a two-dimensional image slice. The tissue thickness shown on each image slide may vary depending on the CT scan machine used, but is generally between 1 and 10 milimetres. When each “slice” is complete, the image is stored and the mechanical bench progressively advances along the gantry. The x-ray scanning process is repeated to produce another image slice. This process continues until slides are collected, covering the required area.
What is a contrast agent and when is it used?
With a CT scan, soft tissues do not show up as clearly as dense substances like bones. Therefore, a dye – a contrast agent – may sometimes be administered to help internal structures appear more clearly on the x-ray images. The contrast agent “blocks” the x-rays and appears white on the scan, highlighting blood vessels, organs or other structures. Contrast materials are usually made of iodine or barium sulphate. They may be administered in one of three ways: orally, by injection or by enema.
Safety of a CT scan
A CT scan is a quick and painless procedure and invariably safe. There is however a small risk from exposure to x-ray radiation or of an allergic reaction to the contrast dye. The amount of radiation during a CT scan varies depending on how much of the body is being scanned. CT scanners are designed to ensure that levels of radiation are kept to the minimum necessary for the related procedure.
What are the risks from a CT Scan?
Exposure to ionizing radiation, more than during a plain x-ray, occurs during a CT scan. However, the low doses of radiation used in CT scans have not been shown to cause long-term harm. At much higher doses, the potential risk of cancer increases slightly. However, this small potential risk is outweighed by a CT scan’s many benefits. To obtain the required medical information, the lowest possible doses of radiation are always used. Although some patients are allergic to the contrast materials, most of the time, any reaction is mild and only leads to minor itchiness or a rash.
The amount of radiation used in a CT scan is minimal and so the risk of exposure is low. Radiation exposure during pregnancy can lead to birth defects and s
o patients knowing or suspecting they are pregnant should tell the doctor. Using a contrast agent may cause an allergic reaction. Patients planning a CT scan who are allergic or sensitive to medication, contrast agents, iodine or shellfish should inform their doctor. In some cases, the contrast agent can harm the kidneys, especially if the person is taking glucophage, a medicine for diabetes. Patients with a history of kidney problems should tell their doctor. Certain factors or conditions can affect the accuracy of a chest CT scan. These factors include metal objects in the chest, such as surgical clips or a pacemaker, body piercings on the chest and/or barium in the esophagus from a recent examination.