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L.Srikumar Pai
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What is CT Scan
( CT : Computer Tomography )

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MRI Scan | CT Scan


The CT (Computed Tomography) scan or the CAT (Computer-Aided Tomography) scan is a diagnostic procedure that takes a detailed cross-sectional x-ray picture of a "slice" of the body.

To get a CT scan, the patient lies very still on a table, which is slowly moved horizontally while the x-ray machine rotates around the patient and takes pictures from many angles. A computer then combines the pictures into a very detailed cross-sectional image. The length of the procedure depends on the areas to be x-rayed. For the chest area, the procedure takes about 20 seconds.

CT scans can show the shape, size and the exact location of organs and tissues in any "slice" of the body more clearly than other diagnostic tools. This technique can help find enlarged lymph nodes, which might contain cancer that has spread from the lung. CT scans are more sensitive than routine chest x-rays in finding early lung cancers. CT scans are also used in detecting masses in the liver, adrenal glands, brain and other internal organs that may be affected by the spread of lung cancer.

Who is a good candidate for a CT scan?

CT scans are designed to help identify medical conditions, diseases or traumas. CT scans are useful, then, for patients who have signs or symptoms of a particular condition, or for patients who have undergone injuries and are suspected of having internal damage or trauma. With a CT scan, a doctor can determine if a disease, trauma or other medical condition is present, and can take the next appropriate measure. People with symptoms have a higher probability of having a disease or condition than the general population. As well, symptoms usually indicate a more advanced form of a particular condition, meaning that the condition will likely be readily and easily detectable on CT images.

On a related note, CT scans can also help rule out certain medical conditions, diseases or traumas. A patient may be exhibiting signs or symptoms of a particular condition, but a CT scan may reveal that the condition does not exist. The doctor can thus eliminate that disease as a potential root of a problem. For example, a doctor may suspect a brain tumour in a patient who reports persistent dizziness and/or headache. A CT scan might reveal that a tumour is not present. Thus, the patient’s doctor can eliminate “brain tumour” as a possible root cause for the dizziness and headache, and can take measures to explore other possible conditions.

Finally, CT can be useful in situations where a medical condition is known and where further medical action is required. Such situations include:

  • Planning a surgery
  • Guiding a needle to obtain a biopsy (tissue sample)

In individuals where there is a known medical condition, a CT scan is an excellent way in determining:

  • the extent of the problem
  • the location of the problem
  • the effect any treatment is having on the problem
Is a CT scan painful?

No. Computed Tomography is a painless procedure. However, a CT scan often requires the patient to lie still in one position for a long period of time. It also often requires the patient to hold his or her breath (though with fast spiral CT scanners, the scan is very quick). These aspects of the CT scan can be uncomfortable. Some patients may require a sedative.

What are some recent advances in CT scan technology?

A spiral CT scan, also called a helical CT scan, is a newer type of CT scan. During a spiral CT scan, the x-ray machine moves continuously around the body in a spiral path, creating cross-sectional images of the body.

Spiral CT has the following benefits over a conventional CT:

  • Can create 3–dimensional pictures of areas inside the body
  • Can detect small abnormal areas with greater precision
  • Faster, so the procedure takes less time overall (Spiral CT scans can obtain images of the abdomen or lungs in 20-30 seconds)

Though spiral CT is a newer type of CT scan, technological advances mean that a large amount of CT scanners today are capable of spiral CT scans.

Spiral CT detectors allow for faster, higher-quality image acquisition and less radiation exposure for the patient. The current spiral CT scans are called multidetector CT and are most commonly 4- or 16-slice systems. 16-slice scanner systems allow the radiologist to acquire 32 image slices per second. These scanners allow for the scanning of the chest or in 10 seconds or less. Such speed is good news for all patients, but especially to those who may have problems lying still for long periods of time: the elderly, children, or critically ill patients. As well, many CT scan systems have the capability to scan and create images of multiple slices simultaneously. These advances mean that large sections of the body can be imaged in a relatively short period of time.

In conventional CT scans, small lesions are sometimes missed if a patient breathes differently on consecutive scans. Because the latest spiral CT scanners allow for a scan to take place in a single breath, the likelihood of finding small lesions is much higher.

The electron beam CT, also known as the EBCT, is another recent advancement in CT technology. Like conventional CT scanners, the EBCT scanner creates snapshots of “slices” of the body, but it does not need any moving parts to create the individual images. Thus, image acquisition is much faster with EBCT scanners than with conventional CT scanners.

What is the difference between conventional X-rays and CT scans?

X-rays are rays of electromagnetic radiation used to diagnose and treat trauma and disease. When x-ray beams pass through the body, 2-dimensional images are created based on shadows made by body structures in the area being photographed. The image depends on the body structure’s absorption of the x-rays.

CT scans, meanwhile, produce 3-dimensional cross-section images of a particular body part. These images, too, are measures of x-ray absorption; however, many slices of the body join together to form an image. Typically, bone turns up white, air turns up black, and tissues and mucous turn up in shades of gray.

CT is more powerful and more detailed than conventional x-rays.

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