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The Physics of CAT Scans

September 9, 2010 Leave a comment Go to comments

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.

CT image of the lungs. The white arrow indicates a lung tumour. Source: Cancernews.com

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.

Source: radiologyinfo.org

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.

CT Scanner Source: Wired.com

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.

Source: AAPM

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.

An axial slice CT image. Source: thoracic.org

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.

Chest x-ray from the side. Source: crosscanadaround.ca

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.

Source: radiologyinfo.org

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.

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  1. Jason
    January 4, 2011 at 10:00 am

    Interesting article.

    I’ve heard it said that radiation dose is cumulative. Would you explain how that is so?

    Is it like rolling a pair of dice consecutively and adding the points each time to a running total? In other words past DNA mutations get further mutated with each new scan?

    Or is each roll a new risk – so like that classical gambling odds, every CT scan gives you a 1000:1 shot, not very great for one scan but when you take those same odds many times it starts adding up.

    Which view is closer to reality for CT radiation risk?

    • January 5, 2011 at 3:38 pm

      Think of it this way:

      If you smoke 2 packs of cigarettes a day for a year, or 1 pack a day for 2 years, the damage to your lungs is roughly the same. It doesn’t matter how quickly you smoke the cigarettes, what matters is how many you smoke. In this case, the damage to your lungs due to smoking is cumulative.

      Risk of illness due to radiation is similar in this way. Whether you receive a large dose all at once, or a small dose over a longer period of time, the risk of developing ill effects would be the same. Thats what is meant by “cumulative”.

  2. jc
    January 20, 2011 at 11:45 pm

    So if I understand you correctly it means that CT scans are like flipping a coin or rolling dice or spinning a roulette wheel. Each spin, flip, or dose is independent of the last.

    If someone flips a coin 50 times and each time comes up heads, the odds that the next flip will turn up heads is still 50%.

    The same with smoking. Just one cigarette might cause cancer. Now given the odds of one cigarette starting cancer are pretty low, but each time you smoke you are “flipping the coin” so to speak and so the odds in that sense are “cumulative.” But the odds don’t change with each additional cigarette.
    AND -just like a coin toss- it doesn’t matter if you flip 50,000 coins one time, or flip one coin 50,000 times you will see the same results, same with smoking and that sounds like what you’re saying about CT radiation.

    IF a person had 10 ct scans in their life, the next ct scan has the same chance of carcinogenesis that the previous 10 scans had.

    But is it possible that when you have a CT scan it actually increases the odds of the next scan causing cancer?

    My hypothesis, correct if i’m wrong, is that each time DNA gets mutated it becomes more “unstable” or susceptible to further mutations.

    What I’m saying is this: if I’m stupid enough to play Russian Roulette with a 6-shot revolver with one round in it I’m taking a small yet serious risk each time I squeeze it (1 in 6, not really small but its all relative).
    Now what if after each missed shot I add another bullet to the gun? My odds that the next shot will kill me goes up each time.

    It might sound pedantic but it really interests me and so far I have not been able to find an answer.

    Someone might ask why does it matter? I would answer that it matters a great deal.

    For me, if I’ve already had a CT scan and my DNA carries mutations from a previous scan, then the next CT scan I consider should be weighed ever more carefully since it carries even greater risk than previous scans. It might cause a mutation on top of a mutation already present.

    On the other hand, if each scan carries an equal risk as the last scan I might be willing to take another small risk (b/c maybe, just MAYBE the cells carrying DNA mutations have since carried out apoptosis and body has since “flushed out” old mutations)

    • January 21, 2011 at 9:23 am

      Hey JC,

      Thanks for your question. The short and sweet answer is the more CT scans you get, the higher your risk of developing cancer. Radiation dose is cumulative, so each CT scan you get, your risk gets higher. Each scan is not independent of the previous ones, so your “coin-flipping” analogy isn’t quite accurate for this case.

      It is always a bit murky dealing with probabilities. It makes it easier to look at the larger picture. For example, smoking. How much you smoke is related to your risk of developing lung cancer. However, you can’t really say that each cigarette has a certain chance of giving you cancer AND that risk is cumulative. If that were the case, eventually your risk of getting cancer should be 100%, which just isn’t the case. Each cigarette will increase your risk of cancer, but the chance of you getting cancer will never reach 100%.

      Similarly with CT scans, or any form of radiation. The more you get, the higher your risk.

      I hope that answers your question. Unfortunately there are no definite answers when it comes to this subject, only probabilities and statistics. But remember, the dose from a CT scan is the same as what you would get from background radiation in about 1-3 years. The risks are not large, but your doctor will always take into account your medical history, including x-rays and CT scans, when determining the best imaging test for you. If you have had a few CT scans in the past, he may opt for an Ultrasound or MRI instead.

  3. jc
    January 21, 2011 at 1:33 pm

    Thank you for replying. Hopefully if I ever need a scan again the scanners they use in the future will use much less radiation than they do today… or maybe MRI will be cheaper by then. If that happens it will be one less thing for people to worry about.

  4. Monica
    February 17, 2011 at 10:48 pm

    Although dose is cumulative, cellular repair may occur over time as long as the stem cells are not eradicated. Besides medical x-rays, we receive natural background radiation. It is a low dose of radiation over your entire lifetime. So you can’t view as every x-ray exposure as cumulative and damaging, but rather how is the dose delivered? Whole body or localized? One large dose at once: think atomic bomb victims, or irregular doses with intervals of recovery period, or a small dose over a long time (natural background radiation). Also about risk never reaching 100%, it is also never 0% as cancer is a spontaneous event.

  5. jc
    February 28, 2011 at 11:39 am

    I’ve been researching this intensely for a few weeks now. I went to my school library and went through some oncology books and medical journals and looked at some epidemiological studies and reports on in-vitro cell studies on the effects of radiation on future cancer 20 years down the road.

    And from what I have found it may be a bit premature to go with the “dose is cumulative and that’s all that matters” picture.

    ALl the reports I’ve read have all said the same thing: that it’s much too early to say that DOSE RATE and dose concentration aren’t relevant to carcinogenesis. There is conflicting evidence and much dispute over mathematical models of dose-rate/effect relationship.

    It is quite possible that the amount of radiation from just 1 ct scan, since it is delivered in such a short amount of time, can initiate cancer by creating Reactive Oxidative Species and that this risk is much greater than the kind of prolonged or fractionated radiation that natural background radiation produces.

    Each electron that is ionized within a cell by an x-ray photon has enough kinetic energy to bounce around inside like a pinball machine, creating dangerous free radicals in multiple locations and wreaking havoc on your DNA.It can create double-strand DNA breaks and shatter chromosomes; and the damage can be so severe as to be beyond repair. The upside is that it takes upwards of 20 years for solid cancer to develop. But for a person in their 30s or younger, 20 years can come up on you faster than you imagine.

    An abdominal CT is equal to about 800 chest x-rays, so unless someone under 40 already has cancer it might be wise for them to request a safer imaging technique like MRI or ultrasound.

  6. vickie beamish
    March 14, 2011 at 12:11 am

    1. Should a patient’s thyroid and eyes be protected when the head is scanned?

    2. When a patient is left in the machine after a CT scan whilst a discussion takes place and then the patient is scanned a second time (immediately after discussion between staff) has the patient had any radiation between the two scans?

    2. A CT scan of the chest is equivalent to how many chest x-rays?

    • March 14, 2011 at 2:23 pm

      1. Usually you would wear a lead vest which should cover your thyroid if you head is being scanned. It is difficult to shield the eyes, however, so they are not shielded.

      2. No. The actual CT scan only takes a couple of seconds, so if there was any pause between scans you have not received any extra radiation exposure.

      3. The dose of a CT scan of the chest can vary quite a bit, but usually it is equivalent to between 10 and 100 chest x-rays.

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