The Physics of CAT Scans
Welcome to Part 2 in my series of “The Physics Of” medical imaging. Today’s topic is CAT scans.
CAT stands for Computed Axial Tomography. In the medical community they simply call them CT scans, because axial, as you’ll find out, is unnecessary in the title. CT scans are generally used for studying the chest, abdomen and pelvis. They have a very good soft-tissue contrast and very high-resolution, making them particularly useful for diagnosing cancers.
So what is a CT scan? In a nutshell, CT scans are high-resolution images which use x-rays to image the body in many “slices”. (For a little background on how x-rays work, you can read my previous article in this series.)
The problem with a simple x-ray image is that it is only two-dimensional. For example, look at this chest x-ray.
You can see the ribs just fine, but how far into the body are the ribs? What is the diameter of the spine and how far back from the ribs is the spine?
You simply can’t tell because the image is only in two dimensions, length and width we will call them. We need some information on the third dimension, the depth of the image in order to answer the previous questions.
So now the question is how do we do this? How can we get 3-D information from a 2-D image? Well the answer turns out to be that we need to take many 2-D images from different angles and then “stitch” them back together. In the medical physics community, this “stitching” is called a reconstruction.
Now here’s how it is done. We get the patient to lie on their back on a gantry table. The patient is then slid back into the CT machine, which is a big donut shape and the patient goes in the hole.
Inside the donut part of the machine is an x-ray source, and directly across from the source is a detector. So the x-rays are shot through the patient and into the detector. The advantage of having the CT scan be a donut shape is that the x-ray source and the detector can then be rotate around the patient, and a series of x-rays can be taken from all angles in a very short period of time (seconds). Some CT scanners even have multiple detectors and sources in order to reduce the image acquisition time.
Generally, a couple hundred x-rays are taken from all angles around the patient in order to generate a CT image. The information is divided into many “slices”, meaning if you were to cut your body across the waist, and do that a bunch of times to you cut the body into a bunch of slices a few millimeters thick, that is what you will see on a CT image. This is called an “axial” slice.
Generating the CT image is a bit complicated, but the basic idea is this: with each x-ray we take, we get some information on how many x-rays are blocked by your body from that angle. For example if we take an x-ray from the front of you, we can see the ribs and the spine, just like the image above. Now lets take an x-ray from the side. From here, we can see how close the ribs are from the front of your body, and how far back the spine is.
When we move the detector around and take another x-ray from a different angle, we get more information on how many x-rays are blocked by your body from that new angle. When we do this a whole bunch of times, we get a very clear picture of where in your body the x-rays are getting blocked. This information is then “reconstructed” in a computer, and a CT image is created.
Because of the shape of the CT machine, all CT images are “axial” slices (slices through the middle), which is why we no longer call them CAT scans, although in the general population this term is still fairly popular.
Ok so this is all well and good, but is it safe? I mean, we are giving the patient a few hundred x-rays all at once!
Well, any radiation exposure leads to a proportionally increased risk to develop cancer. But lets put the amount of radiation absorbed by the body (the “dose”) in a CT scan into context.
The amount of dose you receive from a CT scan is the same that you would get from normal background radiation in roughly 1 – 3 years. So having one or two CT scans is not a big deal, but if you need repeated diagnostic imaging performed then the doctor may look to other methods of imaging, such as MRI which has no radiation dose associated with it. There is always a risk-benefit analysis when it comes to CT imaging, and the benefits generally far outweigh the risks.
Bottom line: CT scans are highly useful and quite safe when used properly.
Stay tuned for my next installment when I’ll tackle Nuclear Medicine.