Posts Tagged ‘x-ray’

Relative Radiation

March 21, 2011 Leave a comment

With the current crisis in Japan with the nuclear power plant, the media has latched onto the public’s fears of radiation.

And y’know what? I understand that fear. Radiation is invisible. It could be anywhere and you won’t know it until it’s too late.

But I am not afraid. Why?

The thing is that I know how much radiation we get on a daily basis, and how it compares with certain medical procedures and working near radiation sources, like a nuclear power plant. Most people do not, and the media plays on those fears to drive up ratings.

Thankfully, Randall Munroe of xkcd fame has created another wonderful (and timely) poster illustrating the relative doses associated with doing certain tasks or living near certain places.

Click on the image see the large version.

Some of the most interesting comparisons:

  • You get more than 3 times the radiation dose living within 50 miles of a coal power plant than you do living within 50 miles of a nuclear power plant
  • Flying round-trip from New York to Los Angeles would give you the equivalent dose of living within 10 miles of the Three Mile Island accident.
  • Living in a stone, brick, or concrete building for 15 years gives you a larger radiation dose than anyone got from the Three Mile Island accident.
  • Using a CRT monitor for a year gives you a larger radiation dose than living next to a nuclear power plant for a year (but then again, who uses CRT monitors anymore?)

So while the Japan nuclear crisis is indeed serious, it is no reason to stop using nuclear power in general.

Oh, and the radiation dose from cell phones is zero, because phones don’t generate ionizing radiation and they don’t cause cancer. Relax, people.

Do Cell Phones Cause Cancer? See What a Real Expert Says

November 24, 2010 6 comments

I’ve written about the issue of cell phones and Wi-Fi and the supposed health risks associated with their use. The resounding scientific evidence shows they are perfectly safe, but the media continues to stoke fear in the public with unbalanced coverage.

When reading a news story about this sort of thing, I always wonder why they don’t ask an actual expert. The answer is that an expert would say how implausible the story is, and that they are silly for reporting it.

That doesn’t sell newspapers.

Instead, they find the one person on the fringe who maintains that there is a magical mechanism by which non-ionizing radiation can harm us. But thankfully, Phil Plait posted on his blog today something I have been looking for a long time.

I’m not a researcher (anymore). I don’t have a Ph.D, so I can tell you what I think, and I can tell you I know what I’m talking about, but I will never have as much credibility as a real university professor in physics or electrical engineering.

Enter this talk by Professor Christopher Davis from the University of Maryland’s Department of Electrical and Computer Engineering. It was given at a National Capitol Area Skeptics meeting, and posted on YouTube. It’s in 5 parts and each part is about 13 minutes long.

It is a fascinating talk and not too technical, so you don’t need a science background to understand the main points. He even touches on backscatter x-ray scanners which have been in the news quite a bit lately. Enjoy!

The Physics of CAT Scans

September 9, 2010 9 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:

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.

CT Scanner Source:

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:

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:

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.

X-Rated X-Rays

June 22, 2010 4 comments

Just when you think you’ve seen everything.

This came across my attention whilst reading one of my favourite blogs, Bad Astronomy. It seems a company which makes displays for medical devices called EIZO, got the memo that “Sex Sells”, and jumped on the bandwagon. They hired an advertising agency called Butter, based in Germany, and Butter came up with an x-ray pin-up calendar. Awesome.

I dunno. I realize its an x-ray, and you really can’t see anything. But I think these are kinda sexy.

It’s the lingerie prinicple I suppose. Keeping something hidden is sometimes even sexier than being totally revealed.

So this is a science blog after all, and if you are interested at all in how X-rays work, here is a brief and simple description (and heres a link to another explanation on How Stuff Works).

X- ray images are made in a similar fashion as to a regular photograph on film. Only in the case of x-ray images, it is photons in the x-ray range of the electromagnetic spectrum (about 0.1 to 10 nanometers) rather than visible light photons (wavelengths in the 400-750 nanometer range) which make the image.

When x-ray photons pass through the body, they will interact with the various tissues. Bones are very good at absorbing x-ray photons, thus not many x-rays make it through to expose the film, making bones appear light on the resulting image. Tissues with a lower density, such as muscle and fat, will allow the x-rays to pass through, and appear dark on the image.

Thats it in a nutshell. But at the end of the day, x-rays are just freakin’ cool.

[Update: If you would like more information on how x-rays work, I have written a new post about it here. Check it out!]