Computerized tomography how does it work

While CT scans can be vital tools for assessing health, there are risks associated with the scan. X-rays are a source of ionizing radiation, which can damage sensitive tissues such as lymphoid organs and blood.

CT scans around the abdomen are not advised for pregnant women because of a chance the fetus would be exposed to harmful radiation. More time in the CT scanner may lead to higher-quality images but also a higher radiation dose, which is often unnecessary, said Dr. A CT scan of just the chest area exposes the patient to about 70 times the amount of radiation as a traditional chest X-ray, according to Harvard Health Publishing.

Duong said it's important to balance CT scan image quality with the amount of radiation exposure — a practice doctors call ALARA, or as low as reasonably achievable. There are a few ways to reduce radiation exposure, Duong said. For example, image only when necessary and only the body part needed, and use lower-energy radiation and newer technology, such as more sensitive X-ray detectors.

Occasionally, patients might experience allergic reactions to the contrast materials, but major reactions are rare. If allergies are known ahead of time, medications may be given to reduce the effects of the contrast material, according to the Radiological Society of North America. A CT scan has many uses, but it's particularly well-suited to quickly examine people who may have internal injuries from car accidents or other types of trauma.

During a CT scan, you're briefly exposed to ionizing radiation. The amount of radiation is greater than you would get during a plain X-ray because the CT scan gathers more-detailed information. The low doses of radiation used in CT scans have not been shown to cause long-term harm, although at much higher doses, there may be a small increase in your potential risk of cancer.

CT scans have many benefits that outweigh any small potential risk. Doctors use the lowest dose of radiation possible to obtain the needed medical information. Also, newer, faster machines and techniques require less radiation than was previously used.

Talk with your doctor about the benefits and risks of your CT scan. Tell your doctor if you're pregnant. Although the radiation from a CT scan is unlikely to injure your baby, your doctor may recommend another type of exam, such as ultrasound or MRI, to avoid exposing your baby to radiation.

At the low doses of radiation used in CT imaging, no negative effects have been observed in humans. In certain cases, your doctor may recommend that you receive a special dye called contrast material. This can be something that you are asked to drink before your CT scan, or something that is given through a vein in your arm or inserted into your rectum. Although rare, the contrast material can cause medical problems or allergic reactions. Most reactions are mild and result in a rash or itchiness.

In rare instances, an allergic reaction can be serious, even life-threatening. During a CT scan, the patient lies on a bed that slowly moves through the gantry while the x-ray tube rotates around the patient, shooting narrow beams of x-rays through the body. Instead of film, CT scanners use special digital x-ray detectors, which are located directly opposite the x-ray source.

As the x-rays leave the patient, they are picked up by the detectors and transmitted to a computer. Each time the x-ray source completes one full rotation, the CT computer uses sophisticated mathematical techniques to construct a 2D image slice of the patient. The thickness of the tissue represented in each image slice can vary depending on the CT machine used, but usually ranges from millimeters.

When a full slice is completed, the image is stored and the motorized bed is moved forward incrementally into the gantry. The x-ray scanning process is then repeated to produce another image slice.

This process continues until the desired number of slices is collected. Image slices can either be displayed individually or stacked together by the computer to generate a 3D image of the patient that shows the skeleton, organs, and tissues as well as any abnormalities the physician is trying to identify.

This method has many advantages including the ability to rotate the 3D image in space or to view slices in succession, making it easier to find the exact place where a problem may be located.

CT scans can be used to identify disease or injury within various regions of the body. For example, CT has become a useful screening tool for detecting possible tumors or lesions within the abdomen.

A CT scan of the heart may be ordered when various types of heart disease or abnormalities are suspected. CT can also be used to image the head in order to locate injuries, tumors, clots leading to stroke, hemorrhage, and other conditions. It can image the lungs in order to reveal the presence of tumors, pulmonary embolisms blood clots , excess fluid, and other conditions such as emphysema or pneumonia.

A CT scan is particularly useful when imaging complex bone fractures, severely eroded joints, or bone tumors since it usually produces more detail than would be possible with a conventional x-ray. As with all x-rays, dense structures within the body—such as bone—are easily imaged, whereas soft tissues vary in their ability to stop x-rays and, thus, may be faint or difficult to see.

The technologist begins by positioning you on the CT exam table, usually lying flat on your back. They may use straps and pillows to help you maintain the correct position and remain still during the exam. Many scanners are fast enough to scan children without sedation. In special cases, children who cannot hold still may need sedation.

Motion may cause blurring of the images and degrade image quality the same way that it affects photographs. The exam may use contrast material, depending on the type of exam. If so, it will be swallowed, injected through an intravenous line IV or, rarely, administered by enema.

Next, the table will move quickly through the scanner to determine the correct starting position for the scans. Then, the table will move slowly through the machine for the actual CT scan. Depending on the type of CT scan, the machine may make several passes. The technologist may ask you to hold your breath during the scanning. Any motion, including breathing and body movements, can lead to artifacts on the images.

This loss of image quality can resemble the blurring seen on a photograph taken of a moving object. When the exam is complete, the technologist will ask you to wait until they verify that the images are of high enough quality for accurate interpretation by the radiologist.

The CT examination is usually completed within 30 minutes. The portion requiring intravenous contrast injection usually lasts only 10 to 30 seconds. CT exams are generally painless, fast, and easy.

Multidetector CT reduces the amount of time that the patient needs to lie still. Though the scan is painless, you may have some discomfort from remaining still for several minutes or from placement of an IV. If you have a hard time staying still, are very nervous, anxious, or in pain, you may find a CT exam stressful. The technologist or nurse, under the direction of a doctor, may offer you some medication to help you tolerate the CT exam.

If the exam uses iodinated contrast material, your doctor will screen you for chronic or acute kidney disease. The doctor may administer contrast material intravenously by vein , so you will feel a pin prick when the nurse inserts the needle into your vein. You may feel warm or flushed as the contrast is injected.

You also may have a metallic taste in your mouth. This will pass. You may feel a need to urinate. However, these are only side effects of the contrast injection, and they subside quickly.

If you swallow oral contrast material, you may find the taste mildly unpleasant. However, most patients can easily tolerate it. If you receive an enema, you can expect to experience a sense of abdominal fullness. You may also feel an increasing need to expel the liquid. If so, be patient; the mild discomfort will not last long.

When you enter the CT scanner, you may see special light lines projected onto your body. These lines help ensure that you are in the correct position on the exam table. With modern CT scanners, you may hear slight buzzing, clicking and whirring sounds. These occur as the CT scanner's internal parts, not usually visible to you, revolve around you during the imaging process. You will be alone in the exam room during the CT scan, unless there are special circumstances. For example, sometimes a parent wearing a lead shield may stay in the room with their child.

However, the technologist will always be able to see, hear and speak with you through a built-in intercom system. With pediatric patients, a parent may be allowed in the room but may need to wear a lead apron to minimize radiation exposure. After a CT exam, the technologist will remove your intravenous line. They will cover the tiny hole made by the needle with a small dressing. You can return to your normal activities immediately.