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CT Scan

Computer Aided Tomography (CAT) is causing a revolution in the medical fields such as radiology, neurology, and nuclear medicine. Called computed tomography or CT scanning, the procedure uses multiple X-rays to provide a virtual look inside the body, allowing physicians to better assess disease risk and, in some cases, find evidence of disease that might otherwise have gone undetected.

By combining "ordinary" X-ray technology with sophisticated computer signal processing, it is possible to generate an image of the tissues of the body which is unobscured by other organs. An ordinary X-ray system takes pictures by passing X-rays through the body and recording the interference patterns onto a photograph. The different tissues in the body absorb the X-ray beams to varying degrees, and the film responds to the intensity of the X-rays received. The resulting photograph displays the accumulated absorption patterns to the tissue.CAT is a procedure in which cross-sectional images (x-rays taken from many different angles) of structures of the body are created and processed through a computer forming a three dimensional image called a tomogram. It is the three dimensional imaging that makes CAT scans more effective than x-rays.

In medical nuclear imaging, CATscans are used to view organs containing a type of radiation known as gamma-emitting radionuclides. Original imaging methods called scintillation cameras are analogous to conventional X-ray pictures. However, the usefulness of scintillation cameras is limtied because:

• Only organs with a high radionuclide concentration can be separated from their environment
• The resolution is limited, such that improving resolution reduces the detector efficiency
• The radioisotopes widely used have low-energy gamma radiation, which is subject to much absorption in the body causing the detector efficiency to vary significantly with depth, and to be affected by the presence of bone overlying the organ of interest.

Recent developments use tomographic reconstruction to provide a high-contrast image of organs and structures containing positron-emitting radioisotopes.

Johnson Lightfoote, M.D., associate professor of clinical radiology and medical director of radiology at USC University Hospital, says that CT imaging is particularly useful because it provides high-resolution images of soft tissue, blood vessels, lungs and bones. "CT scanning does not replace tests such as mammograms, Pap smears, prostate-specific antigen screening or colonoscopy, but it is useful for identifying coronary artery calcification that can signify hidden disease, as well as osteoporosis, and tumors in the lungs, liver, colon and other organs," Lightfoote says.

The full-body CT scan may detect changes in organs that often appear before anatomical changes or noticeable symptoms, as well as tumors or other abnormalities. The scan is used preventively, to detect disease in its earliest and most treatable stage, and diagnostically, to help determine the course of therapy for those already being treated, he says.

A CT scanner is a large machine with a hole through its center. During a scan, patients lie on their backs on a table that slides into the opening as a rotating X-ray device takes multiple images from various angles. A computer then uses the images to create a composite representation of the area scanned.

The procedure is painless and usually takes only a few minutes. Depending on the type of scan being performed, patients may receive contrast dyes-by mouth, injection or enema-to enhance the images.

According to the Food and Drug Administration, the amount of radiation received during a CT scan is about the same as the amount of natural background radiation a person would receive in eight months to three years, depending on the size of the area scanned. Compared to the dosage of radiation a person receives from a standard X-ray, which is small, a person receives about 100 times as much from a head CT scan and 500 times as much from a CT scan of the abdomen.

Lightfoote says that level equates to an additional instance of cancer for one out of 1 million people undergoing the procedure -and that for people under 40, the slight risk probably does not outweigh the potential benefit. Generally, patients undergoing CT scans are age 40 or older and have a personal history, such as smoking or industrial exposure, or a family history that puts them at higher risk for heart disease or cancer.

A CT scan's strength is in helping determine who is at the highest and lowest risk for cardiovascular and lung diseases. Even so, a clear scan does not necessarily mean a patient is disease-free-and a scan that contains an abnormality does not necessarily demonstrate the presence of disease. Lightfoote stresses that because of that, patients should always have their scans interpreted by a board-certified and experienced radiologist.

Advances in technology are making those kinds of discriminations easier. Lightfoote says that electron-beam CT, one of the first machines available, traded off resolution for speed to solve the problem of capturing images of a beating heart. Now, more advanced CT scanners offer unprecedented speed and high resolution-scans from the shoulder to pelvis now can take just a few seconds, he says.

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Cardiac CT for Calcium Scoring

Cardiac CT for calcium scoring is a non-invasive way of obtaining information about the location and extent of calcified plaque in the coronary arteries—the vessels that supply oxygen-containing blood to the heart wall. Plaque is a build-up of fat and other substances, including calcium, which in time can narrow the arteries or even close off blood flow to the heart. The result may be painful angina in the chest or a heart attack. Calcium is a marker of coronary artery disease. The findings on cardiac CT, expressed as a calcium score, may help decide what measures can be taken to avoid these events. Another name for this test is coronary artery calcium scoring.

The goal of cardiac CT for calcium scoring is to detect coronary artery disease (CAD) at an early stage when there are no symptoms and to determine its severity. It is a screening study that may be recommended by a physician if you have risk factors for CAD but no clinical symptoms yet. The procedure is most often suggested for men aged 45 years or older and for women who are aged 55 and above or are postmenopausal. Some patients choose to have the test on their own even if their doctors have not recommended it, in order to discover early-stage CAD if present, or reassure themselves that they probably do not have advanced CAD.

The major risk factors for CAD, other than age, are:

• Abnormally high blood cholesterol levels
• A family history of heart disease
• Diabetes
• High blood pressure
• Cigarette smoking
• Being overweight or obese
• Being physically inactive

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CT Angiography

CT (computed tomography) angiography (CTA) is an examination that uses x-rays to visualize blood flow in arterial and venous vessels throughout the body, from arteries serving the brain to those bringing blood to the lungs, kidneys, and arms and legs. CT combines the use of x-rays with computerized analysis of the images. Beams of x-rays are passed from a rotating device through the area of interest in the patient's body from several different angles to create cross-sectional images, which then are assembled by computer into a three-dimensional picture of the area being studied. Compared to catheter angiography, which involves placing a sizable catheter and injecting contrast material into a large artery or vein, CTA is a much less invasive and more patient-friendly procedure—contrast material is injected into a small peripheral vein by using a small needle or catheter. This type of exam has been used to screen large numbers of individuals for arterial disease. Most patients undergo CT angiography without being admitted to a hospital.

CTA is commonly used to:

• Examine the pulmonary arteries in the lungs to rule out pulmonary embolism, a serious but treatable condition.

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CT of the Abdomen

CT imaging is particularly useful because it can show several types of tissue with great clarity, including organs such as the liver, spleen, pancreas and kidneys. Using specialized equipment and expertise to create and interpret CT scans of the lower gastrointestinal (GI) tract, the colon, and the rectum, an experienced radiologist can accurately diagnose many causes of abdominal pain, such as an abscess in the abdomen, an inflamed colon or colon cancer, diverticulitis and appendicitis. Often, no additional diagnostic work-up is necessary and treatment planning can begin immediately.

Because it is a non-invasive procedure that provides detailed, cross-sectional views of all types of tissue, CT is becoming the preferred method for diagnosing many diseases of the bowel and colon, including diverticulitis and appendicitis, and for visualizing the liver, spleen, pancreas and kidneys. In cases of acute abdominal distress, CT can quickly identify the source of pain. Especially when pain is caused by infection and inflammation, the speed, ease and accuracy of a CT examination can reduce the risk of serious complications caused by a burst appendix or ruptured diverticulum and the subsequent spread of infection.

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CT of the Chest

CT of the chest is used to take a closer look at findings detected on conventional chest x-rays or may be used to investigate and try to explain clinical signs or symptoms of disease of the chest. The CT examination may provide more specific information regarding the nature and extent of the findings or, in some cases, determine that the chest is normal.

CT may be used to detect and evaluate the extent of tumors that arise in the lung and mediastinum, or tumors that have spread there from other parts of the body. CT is routinely used to assess whether tumors are responding to treatment.

You may have heard that, in recent years, some people have chosen to have a chest CT scan to screen for lung cancer. This makes the most sense for those who are former or current cigarette smokers, as they are at much greater risk of cancer than are nonsmokers. The best hope of curing lung cancer is to find it as early as possible, making it easier to treat. CT is able to detect even very small abnormalities that could be early lung cancer, which would not be visible on a conventional chest x-ray. A special low-dose CT technique is used for lung cancer screening. CT of the chest is not able to detect every cancer.

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CT of the Head

CT scanning provides more detailed information on head injuries, stroke, brain tumors, and other brain diseases than do regular radiographs (plain x-ray films). It also can show bone, soft tissues and blood vessels in the same images. CT of the head and brain is a patient-friendly exam that involves radiation exposure. This exam is used for:

• Detection of bleeding, brain damage and skull fractures in patients with head injuries.
• Detecting a blood clot or bleeding within the brain shortly after a patient exhibits symptoms of a stroke.

• Detection of stroke, especially with a new technique called Perfusion CT.
• Evaluation of the extent of bone and soft tissue damage in patients with facial trauma, and planning surgical reconstruction.
• Detection of bleeding in a patient with a sudden severe headache who may have a ruptured or leaking aneurysm.
• Detection of most brain tumors.
• Diagnosing diseases of the temporal bone on the side of the skull, which may cause hearing problems.
• Detection of enlarged brain cavities (ventricles) in patients with hydrocephalus.
• Determining whether inflammation or other changes are present in the paranasal sinuses.
• Planning radiation therapy for cancer of the brain or other tissues.
• Guiding the passage of a needle used to obtain a tissue sample (biopsy) from the brain.
• Non-invasive assessment of aneurysms or arteriovenous malformations through a technique called CT angiography.
• Detecting diseases or malformations of the skull.
• Three-dimensional imaging of the skull and brain structures.

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Pediatric CT Scan

In children as well as adults, CT scanning may be preferred to the customary x-ray exam. It may be performed in newborns and infants as well as older children and adolescents. CT scans may be done with or without contrast materials that are taken by mouth or injected into a vein. These contrast-enhanced exams often depict a particular organ, tissue, or blood vessel more clearly.

A CT scan may be recommended in order to detect a wide range of abnormalities or diseases in any part of your child's body. Among the more common reasons for requesting a CT scan are infectious or inflammatory disorders, abdominal pain, headaches, and injury-related changes. CT also may be used to demonstrate birth defects, detect cancer, and to periodically check to see if previously treated disease has recurred.

CT can evaluate the blood vessels serving the brain, face or neck. In children suffering head injuries the exam can display or rule out serious complications such as bleeding within the brain or other forms of brain damage. In addition, CT can display the spinal cord and the bones making up the spinal column.

Except for the chest x-ray, CT is the most commonly used imaging procedure for evaluating the chest. Some of the more common conditions that may be imaged by chest CT are:

• complications from infections such as pneumonia;
• a tumor that arises in the lung or has spread there from a distant site;
• airway disease such as inflammation of the bronchi (breathing passages);
• birth defects.

CT can demonstrate injured blood vessels or lung damage in children. Using a newer type of CT called multidetector CT it is possible to obtain very detailed pictures of the heart and large blood vessels of the chest in children, even newborn infants.

CT is well-suited for visualizing diseases or injury of important organs in the abdomen including the liver, kidney and spleen. A common condition that may be diagnosed by CT is appendicitis. A CT scan is especially helpful in a child with appendicitis who does not have typical symptoms or who has symptoms but an inconclusive ultrasound study. CT has proved helpful for evaluating adolescents who have inflammatory disorders of the bowel such as colitis and for detecting abdominal tumors or birth defects.

CT scans of the pelvic region may help to find cysts or tumors of the ovary, abnormalities of the bladder, stones in the urinary tract, and disease of the pelvic bones.

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CT of the Sinuses

The paranasal sinuses are hollow, air-filled spaces located within the bones of the face and surrounding the nasal cavity. The nasal cavity is a system of air channels connecting the nose with the back of the throat. There are four pairs of sinuses, each connected to the nasal cavity by small openings. The paranasal sinuses are lined with a special tissue called a nasal mucous membrane that secretes mucus to keep the nose and sinuses moist and thus to humidify the air passing toward the lungs. Normally, air passes in and out of the sinuses and mucous drains from the sinuses into the nose.

Computed tomography (CT) can provide important information about sinus and nasal structure. CT is able to detect sinuses that are filled with fluid or sinus membranes that are thickened. It can assist your physician in diagnosing sinusitis, which can be due to either infection or inflammation in one or more of the sinuses. This condition may be acute (sudden and short), chronic (ongoing and long-term) or recurring (repeated). Among the causes of sinusitis are allergies, upper respiratory infection and an obstruction in the nose. When the nasal mucous membrane swells, the opening from the sinuses to the nasal cavity can become blocked. Fluids that would normally drain into the nose become trapped in the sinuses and infection can set in. Pressure, pain, headache, fever and tenderness in the sinus area can result.

CT may also be used to define the anatomy before surgery or to give further information about certain tumors of the nasal cavity and sinuses.

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CT of the Spine

Computed tomography (better known as CT or CAT scan) of the spine, is a type of x-ray examination that uses a machine called a scanner to obtain multiple images of the spinal column, as well as three-dimensional images if needed. Modern CT scanners employ a method called spiral (or helical) CT, which produces images of the spine and, with the aid of a computer, processes the images to create cross-sectional "slices" of the area of interest. These images may be examined on a computer monitor or printed out and viewed like conventional x-rays. CT images are far more detailed than those obtained by a conventional x-ray unit. In addition, CT is a very useful diagnostic method because it can display and distinguish many different types of tissue in the same region, including bone, muscle, soft tissue and blood vessels.

Compared to regular x-rays, CT scanning uses a relatively low dose of radiation. It is not an invasive procedure, although contrast material sometimes is injected into a vein before scanning to show fine structural details and highlight any abnormalities. The bony structure of the spinal vertebrae is clearly and accurately shown by CT scanning, as are the intervertebral disks and, to some degree, the spinal cord.

Perhaps the most frequent use of spinal CT is to detect—or rule out—spinal damage in patients who have been injured. Other uses:

• CT is a very helpful means of evaluating the spine before and after surgery.
• CT scanning is able to detect various types of tumor in the vertebral column, including those that have spread there from another area of the body. Some tumors that arise elsewhere are first identified by finding deposits of malignant cells (metastases) in the vertebrae; prostate cancer is an example.
• In patients with narrowing of the spinal canal, vertebral fracture, infection, or degenerative disease such as arthritis, CT of the spine may provide important information when carried out by itself or in addition to magnetic resonance imaging (MRI). One of the most common causes of spinal pain that may be diagnosed by CT is a herniated intervertebral disk.
• When a patient is at risk of osteoporosis, CT can accurately measure bone density in the spine and predict whether vertebral fractures are likely to occur.
• CT is a valuable means of guiding certain diagnostic procedures such as the biopsy of a suspicious area to detect cancer, or the removal of fluid from a localized infection (abscess).

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