Computerized Tomography: CT Scan

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Computerized Tomography: CT Scan

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How Does a CT Scan Work?

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The entire series of pictures produced in CT is like a loaf of sliced bread—you can look at each slice individually 2-dimensional pictures , or you can look at the whole loaf a 3-dimensional picture. Computer programs are used to create both types of pictures. Modern CT machines take continuous pictures in a helical or spiral fashion rather than taking a series of pictures of individual slices of the body, as the original CT machines did. Helical CT also called spiral CT has several advantages over older CT techniques: it is faster, produces better quality 3-D pictures of areas inside the body, and may detect small abnormalities better.

In addition to its use in cancer, CT is widely used to help diagnose circulatory blood system diseases and conditions, such as coronary artery disease atherosclerosis , blood vessel aneurysms, and blood clots ; spinal conditions; kidney and bladder stones; abscesses; inflammatory diseases, such as ulcerative colitis and sinusitis; and injuries to the head, skeletal system, and internal organs. During a CT procedure, the person lies very still on a table, and the table passes slowly through the center of a large donut-shaped x-ray machine.

With some types of CT scanners, the table stays still and the machine moves around the person. The person might hear whirring sounds during the procedure. At times during a CT procedure, the person may be asked to hold their breath to prevent blurring of the images. Sometimes, CT involves the use of a contrast imaging agent, or dye. The dye may be given by mouth, injected into a vein, given by enema , or given in all three ways before the procedure.

The contrast dye highlights specific areas inside the body, resulting in clearer pictures. Iodine and barium are two dyes commonly used in CT. In very rare cases, the contrast agents used in CT can cause allergic reactions. Some people experience mild itching or hives small bumps on the skin. Symptoms of a more serious allergic reaction include shortness of breath and swelling of the throat or other parts of the body. People should tell the technologist immediately if they experience any of these symptoms, so they can be treated promptly. Very rarely, the contrast agents used in CT can cause kidney problems for certain patients, such as those with impaired kidney function.

Patient kidney function can be checked with a simple blood test before the contrast agent is injected. CT is a noninvasive procedure and does not cause any pain. However, lying in one position during the procedure may be slightly uncomfortable. The length of a CT procedure depends on the size of the area being scanned, but it usually lasts only a few minutes to half an hour.

For most people, the CT is performed on an outpatient basis at a hospital or a radiology center, without an overnight hospital stay. Some people are concerned about experiencing claustrophobia during a CT procedure. However, most CT scanners surround only portions of the body, not the whole body. Therefore, people are not enclosed in a machine and are unlikely to feel claustrophobic. Women should let their health care provider and the technologist know if there is any possibility that they are pregnant.

Depending on the part of the body to be scanned, the provider may reduce the radiation dose or use an alternative imaging method. However, the level of radiation exposure in a CT scan is believed to be too low to harm a growing fetus. Patient undergoing CT of the abdomen. Drawing shows the patient on a table that slides through the CT machine, which takes x-ray pictures of the inside of the body.

Studies have shown that CT can be effective in both colorectal cancer screening including screening for large polyps and lung cancer screening. CT colonography also known as virtual colonoscopy can be used to screen for both large colorectal polyps and colorectal tumors. CT colonography uses the same dose of radiation that is used in standard CT of the abdomen and pelvis, which is about 10 millisieverts mSv 1. By comparison, the estimated average annual dose received from natural sources of radiation is about 3 mSv.

As with standard colonoscopy, a thorough cleansing of the colon is performed before this test. During the examination, air or carbon dioxide is pumped into the colon to expand it for better viewing. CT colonography is less invasive than standard colonoscopy and has a lower risk of complications. However, if polyps or other abnormal growths are found on CT colonography, a standard colonoscopy is usually performed to remove them.

Whether CT colonography can help reduce the death rate from colorectal cancer is not yet known, and most insurance companies and Medicare do not currently reimburse the costs of this procedure. Also, because CT colonography can produce images of organs and tissues outside the colon, it is possible that noncolorectal abnormalities may be found. Some of these "extracolonic" findings will be serious, but many will not be, leading to unnecessary additional tests and surgeries. Previous studies had shown that screening with standard chest x-rays does not reduce the death rate from lung cancer. The estimated amount of radiation in a low-dose helical CT procedure is 1.

Those who have never smoked tobacco products are considered to be at too low a risk of lung cancer to benefit from lung cancer screening. Despite the effectiveness of low-dose helical CT for lung cancer screening in heavy smokers, the NLST identified risks as well as benefits. For example, people screened with low-dose helical CT had a higher overall rate of false-positive results that is, findings that appeared to be abnormal even though no cancer was present than those screened with standard x-rays.

The benefits of helical CT in screening for lung cancer may vary, depending on how similar someone is to the people who participated in the NLST. The benefits may also be greater for those with a higher lung cancer risk, and the harms may be more pronounced for those who have more medical problems like heart or other lung disease , which could increase problems arising from biopsies and other surgery. However, because low-dose lung CT can produce images of organs and tissues outside the lung, it is possible that non pulmonary abnormalities such as renal or thyroid masses may be found. As with extracolonic findings from CT colonography, some such findings will be serious, but many will not. Under the Affordable Care Act , all Marketplace health plans, and many other health plans, must cover lung cancer screening with low-dose CT as a preventive care benefit for adults ages 55 to 80 years who have a 30 pack-year smoking history e.

People who think they might qualify for screening with low-dose helical CT should discuss the appropriateness and the benefits and risks of lung cancer screening with their doctors. They should also check with their health insurance company to determine whether this screening is covered by their insurance plan. Medicare covers the costs of annual lung cancer screening with low-dose CT in beneficiaries considered to be at increased risk based on their smoking history.

Annual screening is covered for Medicare beneficiaries who are between 55 and 77 years old; have no signs or symptoms of lung cancer; are either a current smoker or have quit smoking within the last 15 years; have smoked an average of one pack a day for 30 years i. Medicare coverage includes a prescreening counseling visit with the health professional who wrote the order to review the benefits and risks of lung cancer screening. Total, or whole-body, CT creates pictures of nearly every area of the body—from the chin to below the hips. This procedure, which is used routinely in patients who already have cancer, can also be used in people who do not have any symptoms of disease.

However, whole-body CT has not been shown to be an effective screening method for healthy people. Most abnormal findings from this procedure do not indicate a serious health problem, but the tests that must be done to follow up and rule out a problem can be expensive, inconvenient, and uncomfortable, and they may expose the patient to extra risks, such as from an invasive procedure like a biopsy that may be needed to evaluate the findings. In addition, whole-body CT can expose people to relatively large amounts of ionizing radiation —about 12 mSv, or four times the estimated average annual dose received from natural sources of radiation. Most doctors recommend against whole-body CT for people without any signs or symptoms of disease. CT is done first to create anatomic pictures of the organs and structures in the body, and then PET is done to create pictures that provide functional data about the metabolic pathways chemical reactions that take place in a cell to create and use energy that are active in tissues or cells.

Cancer cells often use different metabolic pathways than normal cells. Numerous radiopharmaceuticals have been developed. Because cancerous tumors usually metabolize glucose more rapidly than normal tissues, they take up more FDG and appear different from other tissues on a PET scan. Other PET imaging agents can provide information about the level of oxygen in a particular tissue, the formation of new blood vessels, the presence of bone growth, whether tumor cells are actively dividing and growing, and whether cancer may have spread. The combined procedure may improve the ability to diagnose cancer, to determine how far a tumor has spread, to plan treatment, and to monitor response to treatment. Some people may be concerned about the amount of radiation they receive during CT.

CT imaging involves the use of x-rays , which are a form of ionizing radiation. Exposure to ionizing radiation is known to increase the risk of cancer. Standard x-ray procedures, such as routine chest x-rays and mammography , use relatively low levels of ionizing radiation. The radiation exposure from CT is higher than that from standard x-ray procedures, but the increase in cancer risk from one CT scan is still small. Not having the procedure can be much more risky than having it, especially if CT is being used to diagnose cancer or another serious condition in someone who has signs or symptoms of disease. It is also important to note that everyone is exposed to some background level of naturally occurring ionizing radiation every day.

The average person in the United States receives an estimated effective dose of about 3 millisieverts mSv per year from naturally occurring radioactive materials, such as radon and radiation from outer space 1. By comparison, the radiation exposure from one low-dose CT scan of the chest 1. The widespread use of CT and other procedures that use ionizing radiation to create images of the body has raised concerns that even small increases in cancer risk could lead to large numbers of future cancers 2 , 3. Before a CT urogram, tell your health care team if you:. You may be asked to drink water before a CT urogram and not to urinate until after the procedure.

This expands your bladder. But, depending on your condition, guidelines about what to eat and drink before your CT urogram may vary. Before your CT urogram, a member of your health care team may:. For a typical CT urogram, you lie on your back on an exam table, though you may be asked to lie on your side or stomach. Straps and pillows may be used to help you maintain the correct position and keep still during the exam. You may be asked to change positions during the CT urogram. An IV line is placed into a vein in your hand or arm through which the X-ray dye is injected. You may feel a warm, flushed sensation when the dye is injected and notice a metallic taste in your mouth for a minute or two.

The contrast material may briefly make you feel like you have to urinate. Before the exam begins, the table moves quickly through the scanner to determine the correct starting position for your scans. For the actual CT urogram, the table moves slowly through the machine while the images are taken. If needed, the machine may make several passes. The machine makes slight buzzing and clicking sounds while it takes pictures. To keep the images from blurring, the technician may ask you to hold your breath for a few seconds during the scanning. After the CT urogram is complete, the technician checks the images for quality while you wait. When the test is done, the IV line is removed from your arm and the IV entry point is covered with a dressing.

Most people return to their usual activities after the test. CT is used to gain additional detailed information after myelograms, discograms, and after selected types of arthrograms. CT is also used to guide procedures such as biopsies, aspirations of fluid collections, to guide facet joint injections and nerve root blocks. The CT scan is ordered by your referring physician and is interpreted by a radiologist. A radiologist is a physician with dedicated training in the safe utilization of imaging equipment and cross-sectional image interpretation. The radiologist will protocol prescribe the specific examination parameters. A specially trained technologist will operate the CT scanner according to the prescribed protocol.

The radiologist will supervise and confirm that the examination is performed accurately, interpret the study, and provide a written report to your physician. HSS uses the most advanced CT scanning technologies available. In addition, our radiologists who analyze and interpret CT scans are uniquely qualified for sub-specialization of diagnosing orthopedic and musculoskeletal conditions. An X-ray tube and a series of detectors are contained in a doughnut-shaped machine. You will lie down on a platform that will move you slowly through this doughnut.

Some CT scan examinations will require an injection of an iodinated contrast agent dye into your vein. If contrast is needed, an intravenous IV line will be placed for the injection. If your CT scan will be performed as part of another procedure surgery, biopsy, etc. If the CT scan is to include an injection of a contrast agent, it is advisable that you do not eat or drink for a several hours before the procedure. If you have any allergies, especially allergies to medications, contrast agent, local anesthesia, Betadine soap, or latex, be sure to inform your physician at the time of scheduling of the procedure, and also inform the CT technologist and the radiologist before the start of the procedure. If you are pregnant or think you may be pregnant, be sure to inform your physician, the technologist, and the radiologist prior to the procedure.

CT scans expose patients to radiation, but they are within the limits determined to be acceptable safe by the National Radiation Safety Commission. If intravenous IV contrast dye is required, there is a risk of allergic reaction, as well as minor bleeding or soreness at the contrast injection site. Although many X-ray image slices may be obtained on a portion of your body, each is done so with a very thin X-ray beam in order to minimize exposure and scatter the radiation. The X-ray exposure you may receive will be within the safety limits put forth by the National Radiation Commission. HSS radiology equipment is monitored daily by highly trained technologists and a radiation physicist.

When appropriate, radiation-sensitive of the body will be shielded during the scan. Possible alternatives may include routine X-rays, ultrasound images or MRIs.

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