Locations & Hours | Radiology Services & Procedures | Insurance Info | Policies & Payments Computerized Tomography (CT) by Stuart H. Caplan, M.D. Why did my doctor order a CT scan? Computerized Tomography (CT, CT scan, CAT scan) is a test that your physician can use to look for a wide variety of diseases and injuries throughout your body. CT combines the use of x-rays and advanced computer technology to produce detailed two-dimensional images of the area of your body about which your physician is currently concerned. By examining these images, a radiologist can evaluate the nature, extent, and location of many infections, injuries, benign tumors, malignant tumors (cancer) and other abnormalities such as strokes, gallstones, kidney stones, and an intestinal obstruction. Your physician may order a number of different CT tests. CT scans of the head, brain, sinuses, face, and orbits looks for causes of headaches, other pain, seizures, confusion, dizziness, weakness and changes in vision and hearing. CT scans of the chest look for causes of chest pain, shortness of breath, persistent cough, and coughing up blood; evaluate the heart and lungs; and provide more information about findings on chest x-ray films. CT scans of the spine look for fractures and for causes of pain, numbness, tingling and weakness. CT scans of the abdomen and pelvis look for causes of belly or stomach pain, lack of appetite, nausea, vomiting, weight loss and bleeding. They also evaluate organs such as the liver, spleen, pancreas, and kidneys. CT scans of other parts of the body such as the neck, arms, and legs can also help diagnose a variety of injuries and diseases. When necessary, a radiologist can also use CT to biopsy a mass or to drain a collection of fluid in your body, avoiding the need for surgery. How does CT work? A CT scanner consists of a donut like gantry and a moveable bed called a "table". The gantry contains both an x-ray generator, which will project x-rays through your body and a detector or detectors, which will receive the x-rays after they pass through your body. When x-ray is passed through your body, they are blocked or absorbed (weakened or attenuated) to different degrees by different tissues or materials in your body. High-density tissues such as bone and high-density material such as metal attenuate x-rays most while low-density tissue such as lungs and low-density material such as air and gas attenuate x-rays least. Once the detector or detectors in the gantry receive the attenuated x-rays that pass through your body, they send information to the scanner's computer about the strength and location of those x-rays. The computer uses the density and location information from the detector to paint pictures of the inside of your body. The densest tissues usually appear white; the least dense tissues, black. Most tissues fall somewhere in between in appears in various shades of gray. What will I experience during a CT scan? When you arrive for your CT scan, a radiologic technologist (RT) may ask you questions to make sure you have the best and safest CT study possible. If you wish, you may also ask questions of your own. It is very important that you tell the technologist if you're pregnant; if you think you might be pregnant; if you are diabetic; if you have problems with your kidneys or your heart; if you take glucophage, glucovance or metformin; if you have asthma; and if you have any allergies, especially allergies to seafood, iodine, and x-ray contrast. The RT will assist you onto the CT table, which extends outward from the doughnut-like gantry. The technologist will then control your examination from a computer workstation in a room adjacent to the CT scanner. He or she will watch you closely during the examination. You will be able to talk to the technologist by intercom during the study. The gantry may tilt slightly to allow the scanner to obtain the best possible pictures. You should be able to see out both sides of the gantry during the examination. The table may move slowly up and down, forward and backward before the exam to place the part of your body being studied inside the gantry. It will move slowly forward or backwards during that exam to make sure that the scanner takes pictures of the entire part of your body being studied. During the study, you may hear a whirring noise as the detector ring rotates in the gantry. CT is generally painless although some people find lying on the table for long periods uncomfortable. Fortunately, advances in technology have made CT studies much shorter than they once were. Most CT scans now last only ten to 45 minutes. If you receive intravenous contrast (IV contrast, x-ray dye) for your CT, then you may experience momentary pain during the needle stick and either a metallic taste in your mouth or flushing after the contrast is injected. The technologist may ask you to hold your breath at one or more points during the examination. Otherwise, it is extremely important that you remain as still as possible during your study to prevent blurring of images. Most people find that the easiest way to remain still is to relax until the study is over. How do I prepare for a CT scan? You should dress in loose comfortable clothing for the study; if possible without any metal part such as zippers, snaps, buttons, hooks, or buckles. If you are unable to choose clothing without metal, you may be asked to change into a gown for the exam. You should leave any jewelry at home. You may be asked to remove your hairpins, hearing aids, eyeglasses, dentures or bra before your study. Intravenous contrast (IV contrast, x-ray dye) is used during many CT examinations. Because of this you may be asked to remain N.P.O. (to eat and drink nothing) after midnight or for four or more hours before your study. You may be asked instead to have only clear liquids (such as water, clear fruit juices or drinks or Jell-O) before your CT. You should not take glucophage, glucovance, or metformin before your CT. Otherwise, unless you are specifically told not to do so, you should take any medication your physician has prescribed for you at the time you would usually take it. Oral contrast is used during many CT examinations of the abdomen and pelvis both to better demonstrate the stomach, small bowel, and colon and to help separate the gastrointestinal tract from other normal and abnormal tissues on CT images. Oral contrast contains a metal such as barium or iodine that blocks x-rays and appears white on most x-ray images. If your examination calls for it, you should receive instructions on when to drink oral contrast, but you will probably be asked to drink three to four twelve ounce cups beginning over one hour before your CT scan. Oral contrast is not absorbed by your body in significant amounts, but can lead to constipation in some cases. What if I am pregnant? Because CT uses x-rays, it should be avoided if you are pregnant, especially if you are in your first trimester (first three months of pregnancy). If you are pregnant or think you might be pregnant, tell the CT technologist before your study. In many cases, another examination that does not use x-rays can be substituted for CT. In other cases, the CT can be rescheduled until after delivery or at least after the first trimester. In those cases where a CT must be performed and can't be delayed, it may be possible to shield your uterus from some x-rays. What if I am nursing? If you are nursing, you can still have a CT scan. If your CT scan requires the use of I.V. contrast, then you may wish to pump enough milk before the study to last 24 hours after the study. After 24 hours, it is safe to resume breast-feeding your infant. Does CT have risks? CT is a very safe examination performed thousands of times each day. If you are pregnant or think you might be pregnant, then tell the CT technologist before your study as the x-ray used for CT scans can harm unborn babies. If your study requires use of I.V. contrast, then a risk of allergic reaction (anaphylaxis) exists. This risk can be higher if you have asthma or if you are allergic to seafood, iodine, or I.V. contrast (x-ray dye). Allergic reactions to I.V. contrast include itching, hives, swelling of the face and throat, and difficulty breathing. In very rare cases, death has occurred due to allergic reactions to contrast. The intravenous contrast (I.V. contrast, x-ray dye) used for some types of CT scans can affect the way the kidneys work. If your kidneys are not working as well as they should be, the type and amount of I.V. contrast used for the study may need to be changed, the CT may need to be performed without I.V. contrast, or another test may need to be substituted for the CT study. Because of this, before your CT scan, your physician may arrange for you to have blood drawn for laboratory tests (sometimes called B.U.N and creatinine) that will show how well your kidneys are working. Several drugs used to treat diabetes (as well as obesity and polycystic ovarian disease) may increase the risk of renal failure. If you take glucophage, glucovance, or any other medications containing metformin, discuss this with your physician. Generally, these drugs should not be taken for 24 hours before the use of I.V. contrast and should not be taken for 48 hours after the use of I.V. contrast. After 48 hours, your physician should check your kidney function before you begin to take any of these drugs again.