How does CT work?

Imagine that you had a loaf of bread with several very small pieces of glass baked inside. You are hungry but not hungry enough to eat pieces of glass. So how do you find and remove them from that delicious loaf? You cannot see them on the outside of the loaf, so you must look inside. You could x-ray the whole loaf in several different projections, and if the glass fragments are dense enough, you may be able to locate some of them. You'd be able to find even more if you could slice the entire loaf and examine each slice individually. Basically, that is what the CT scanner does to the human body: "slices it up."

Keep in mind that CT is a computerized imaging test. When we take a conventional x-ray, we rely on the interaction between the radiation and the various structures in the body. For x-rays, that information is displayed in high-resolution analogue form (increasingly, digital forms are being used). The problem is that all soft tissue looks about the same, because all soft tissue interacts in basically the same way with x-radiation. So the liver, gallbladder, and the important bile ducts, for example, are all hidden in the same large soft tissue shadow. The story is a bit different with CT. The physics allow for greater discrimination between the various structures that make up that soft tissue glob.

The typical CT scanner is composed of three main sections. First, there is the table that resembles a bed or stretcher, which slides into a gantry. The gantry is the second main section, and it contains the detector array, among other goodies. The array sweeps around your body sending and receiving a fine beam of x-rays. These x-rays pass through your body and into a detector at the other side, at many different points on the circle, so each individual detector will see the beam coming through an individual part of the body. This is a lot of information, and the detectors need to convene and come to a consensus about what they have seen. That is the purpose of the third main section, the computer. The computer takes all the information from the detectors and makes a picture of the particular slice. Remember, a typical CT study may use anywhere from 12 to 40 slices. The table slides you in or out of the scanner (depending on what part of the body is being scanned), the array spins around your body getting information about that slice and then moves on to the next slice where the same process is repeated. So it is, take a picture, stop, slide, stop, and take another picture, stop, and slide.

Not only is it a jerky motion, but the procedure takes time. And as time passes, things inside your body move. Bowel gas moves around, your heart beats, and every time you take a breath, your lungs, chest, and entire upper abdomen shift around. If you cough or happen to take a breath between two of the slices, the whole sequence is thrown off. The radiologist can end up with more than one slice of one part of the body and no slices of another part. If the radiologist is trying to find a small nodule in your lungs, for example, and you cough or breathe, the nodule could easily be overlooked.

The arrival of the spiral, or helical, CT has greatly improved the CT by sliding and taking pictures continuously. The test takes much less time and slices can be obtained more quickly. The cough and breath-holding problem of the conventional CT scanner doesn't exist with helical CT, since you scan an entire chest in one short breath hold. There is much less chance of overlooking an entire section. You can get numerous slices in a very short period of time. But there are still some problems. To do helical CT, the gantry had to be redesigned because the detector array has to travel in a continuous circle. The computer also required upgrading to accommodate translating a third dimension into a two-dimensional display. For those with geometric skills, we have added a "Z" plane variable to simple "X, Y" slice. When the conventional scanner got its slice, it did not have to worry about the Z plane. Now that the patient is sliding into or out of the detector, Z becomes a variable that must be considered. The CT produces so many slices that it becomes somewhat difficult to review all of them separately. So what can you do with all these images? What are the best ways to look at them?

First, you could sit down in front of a computer monitor and fly through your images. Say you wanted to see if there was a stone in the lower ureter (the tube that connects the kidney to the bladder). You could start at the top of that tube, focus on it, and fly through the rest of it, almost like being in it. And you could go back and forth. You could focus on all sorts of other structures: the liver, the pancreas, the bile ducts, the aorta, to mention a few. Of course, if you ran into anything that is strange, you could go back to the rough data, the individual slices. You could reformat the images. The slices that the computer routinely gives you are cross-sections of whatever is sliced, head, chest, abdomen, etc. You may want to see a different perspective of things. The good news is that with a rapid spiral sequence, there is plenty of data to get the computer to set up slices in a different plane. Or you could ask the computer to show you only the structures that have x-ray dye in them or, even better, create a virtual reality where you "travel" through the body observing everything.

IV Contrast or No IV Contrast

There's a popular misconception that using intravenous dye for CT scans makes everything better and easier to see. But that is not always true. Sometimes dye doesn't help one iota, and sometimes it can make things even worse. Though rare, some patients suffer allergic reactions. And finally, the newer contrast agents are quite expensive. But there are times when dye is definitely needed, such as for detecting the spread of malignancy in the brain or liver. In situations where patients are allergic to contrast, MRI can be an excellent alternative. The contrast agents used in MRI are unlike the CT dyes, and the likelihood of a reaction is rare.

If your doctor and/or radiologist decides that you need a contrast scan, make sure that you have mentioned an allergic history (especially to previous dye injections). Also ask if you really need the dye, and if you do, request the newer, milder dye.

When is a CT needed?

The CT scanner has become the key player in the imaging arsenal. The places where CT is not used include breast diagnosis (generally), obstetrics, and in looking at the various soft tissue structures that make up our joints. Other than those few exceptions, CT is most often the procedure of choice. Head CT has become almost a standard part of any work-up for conditions such as trauma; severe headache; prolonged, persistent headache; and changes in mental status. In other places in the body, radiologists do fine needle aspirations (biopsies) under CT guidance, as well as under ultrasound guidance. In some institutions, the abdominal CT scan has become the screening test for severe abdominal pain when renal stones, appendicitis, or diverticulitis is suspected.

The CT study of the pulmonary arteries has become an exceptionally important tool in the diagnosis of life-threatening pulmonary clots (emboli). The list of applications for CT seems to be growing daily.

Risks and Potential Complications

The only risk of a CT scan is radiation, the same risk as all other x-ray tests. If intravenous contrast is used, there is that risk to be considered as well. Overall, the risks of CT are minimal.

What actually happens?

Prior to the study, a technologist or nurse will probably interview you. If you do not receive intravenous contrast, the test is very easy to take. You will be asked to lie (usually on your back) on the CT table, which is often padded concave to receive your body. Unless you have severe back pain while lying supine, or you are severely claustrophobic, this should not be an unpleasant experience. Of course, you will be apprehensive and nervous, but the test should be over very quickly. Depending on the nature of your study, you may be asked to drink variable amounts of oral contrast before the test. Most of these preparations, though not delicious, are tolerable. Unlike many MRI gantries, the CT gantry is relatively open and only the most claustrophobic patients have a problem.

What is it like afterward?

If you have the oral dye, and especially if it's barium, make sure that you drink lots of liquids. The oral dyes used for CT are much less dense than those used in the upper GI series, and the potential of impaction is much less. Those who have had the IV dye should have no postprocedural problems. Delayed reactions are exceedingly rare.

Aside from a minimum of annoyances, you will walk away from the CT scan unscathed. But when will you be able to find out the results? That will depend on many factors, including the urgency of the study, the seriousness of the problem, and the staffing in the x-ray department. For urgent studies the results should be known shortly. If you have the test early in the morning, you should know the results by late afternoon or the following day at the latest. Can you ask the radiologist about his or her impressions right after the test? Often the radiologist is elsewhere and may not be available for an initial impression. Also, CT scans are very involved studies that require numerous images, may require reformatting, and may also require review of previous studies. There is a long way to go from raw data to diagnosis with CT scans, all of which may take a bit of time.