Computerized Tomography

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Overview

The CT scan has become the bread-and-butter test of medical imaging; though there are those MRI scan advocates who would suggest that magnetic resonance imaging is more useful. Notwithstanding, it is correct to say that CT scans are very good in some cases and MRI scans are good in other cases. CT scan usually costs a bit less than MRI and it is generally better tolerated by patients.

How does CT work?

Cystic Tumor of Pancreatic Tail	Right Subdiaphragmatic Abcess
Click on image for larger scan.

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: "creates image slices."

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 the soft tissue.

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 this contains the detector array. The array sweeps around the 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 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 the entire chest is scanned 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 with this scanner. 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 (e.g., head, chest, abdomen). 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.

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