Posts Tagged ‘X-rays’

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.


February 17, 2011 1 comment

Photo Courtesy of NASA

Happy Valentine’s Day, indeed.

The largest solar flare in 4 years erupted on Monday night. It is what as known as an X-class flare. As the name implies, it is a big-time event.

Solar flares are classified based upon the intensity of x-rays emitted between the wavelengths of 1 and 8 Angstroms. (An angstrom is 1 x 10-10 meters).

In order to be an X-class flare, the intensity of the flare must be greater than 10-4 Watts/m2. This flare is an X2 flare, meaning it has an intensity of about 2 x 10-4 Watts/m2.

The flare is sending waves of energetic particles at the Earth, which have already caused radio disruptions in China.

The main event will be coming Today, however. The bulk of the particles will be hitting the Earth’s magnetic field sometime today, possibly causing some intense Aurora’s.

So if you are outside at all this evening, take a quick look up and see if anything is happening. Regions closer to the equator which don’t normally see Aurora’s may just get lucky tonight.

(Bonus points to those who got the ‘Airplane!’ reference in the title of this post)

Update: Phil Plait uploaded a cool video of the flare. Check it out:

Physics Christmas Carols

December 24, 2010 Leave a comment

Continuing with the Christmas theme of recent posts, I stumbled upon this awesome little collection of Christmas Carols re-written with physics-related lyrics. Pure gold!

My favourite: Phrosty the Photon (sung to Frosty the Snowman)

Phrosty the Photon was quite a quantum sight,

with a zero mass and an endless life,

and a speed approaching light.

There must have been some magic in a physics lab one year,

for when they studied X-ray beams

ole Phrosty did appear, Ohhhhhh,

Phrosty the Photon says he knows he’s not that large,

but he said one day if he comes this way,

he’ll give us all a charge.

Thumpity thump thump, thumpity thump thump, moving fast as light.

Thumpity thump thump thumpity thump thump, Phrosty’s out of sight!!

Google Celebrates X-Rays!

November 8, 2010 Leave a comment

It is the 115th anniversary of the discovery of x-rays by Wilhelm Conrad Roentgen.

To celebrate, Google has put up a ‘Google Doodle’ featuring x-ray radiography.

This is awesome! It gives worldwide attention to one of the greatest medical discoveries in history. Hell, it’s one of the greatest scientific discoveries in history!

I’ve written posts about the history of x-rays, as well as their use in radiography and CAT Scans. Check’em out if you want to learn a little more about this awesome form of radiation.

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.

The Physics Of X-Ray Imaging

August 31, 2010 3 comments

So here is Part One of my series of the “Physics Of” medical imaging. First up is the most recognizable: X-ray Radiography.

Radiography (which uses x-rays, but the images are generally called “X-Rays”) are the most common form of medical imaging, and are incredibly useful. Thousands of images are performed everyday and medicine was revolutionized when this non-invasive means to study the body was discovered.

But how exactly do we get x-rays and use them for imaging?

Lets start with a bit of history. The first X-ray image was created by a guy named Wilhelm Rontgen in 1895.

Wilhelm Rontgen

Rontgen called them “X” rays because they were an “unknown” type of radiation, and the name kind of stuck.

The first image was of Rontgen’s wife’s hand, and is pretty cool because you can actually make out her wedding ring.

First image using X-rays of Wilhelm Rontgen's wife's hand

I actually find this a bit funny. I just picture a crazy looking physicist saying “Honey! C’mere! Stick your hand in front of this radiation for a second!”

Luckily for Mrs. Rontgen, x-rays, in small doses, are not very dangerous. So what exactly are x-rays?

X-rays are electromagnetic waves just like visible light, radio waves and microwaves. They have a wavelength range of roughly 0.01 to 10 nanometers (1 nanometer = 1 billionth of a meter).

When talking about x-ray imaging, however, its easier to think of x-rays in terms of photons. Photons are like tiny wave “packets” and electromagnetic waves can be described as a big collection of photons.

X-rays are generated in an x-ray tube (unsurprisingly). Basically, a bunch of electrons are shot at a piece of metal (usually tungsten, the same metal used in old school incandescent light bulbs). Now what happens next is a little complicated, but really cool…

So the electron travels at a certain speed toward the piece of tungsten; it has kinetic energy, which is the energy of motion. But as it gets close to the Tungsten it will run into an electric field produced by the metal, and will actually slow down.

X-ray Tube

Now, in physics there is principle called the conservation of energy. Basically this just says that energy can never be created or destroyed, it can only change form. So when the kinetic energy (energy of movement) of the electron drops (when it slows down) that lost energy has to go somewhere. Where it goes, in fact, is in the generation of an x-ray. The electron will actually emit an x-ray when it gets slowed down by the tungsten. Pretty sweet eh?

Schematic of X-ray tube. Electrons come in from the bottom, strike the tungsten target (the anode) and emits x-rays

This is actually a type of radiation called Bremsstrahlung, which is German for “braking radiation”.

Schematic Diagram of Bremsstrahlung

Ok, so now we got x-rays, how do we make an image?

Well, if we fire x-rays at, oh lets say, YOU! the x-rays will interact with your body. How you ask?

Well when an x-ray passes through the body, it may get absorbed or scattered by the body. An x-ray gets absorbed when the x-ray hits an electron in our body, and the electron “jumps” out of the atom. This is called the photoelectric effect.

The Photoelectric Effect

The x-ray may also get scattered. This just means that the x-ray will get close to the nucleus of an atom and get kind of turned in another direction due to the electric field of the nucleus. This is known as Compton Scattering.

Compton Scattering Effect

In spots of our body that very dense like bones, the x-rays have a much higher chance of getting absorbed or scattered than if they pass through muscle or fat, which are less dense. So if we were to stick a piece of film which is sensitive to x-rays behind someone getting a radiograph, you would get lots of x-rays hitting the film when they pass through muscle or fat, but very few pass through bones (or metal, if you’re really unlucky).

So on the radiograph muscles and fat show up dark, and bones show up white. BAM! Radiograph!

Chest Radiograph

See, now that wasn’t so bad was it? Pretty interesting if you ask me.

The next installment of my “Physics Of” medical stuff  series will be something that takes x-rays to the next level: Computed Axial Tomography, commonly called “CAT” scans.

Woo Hoo! Isotopes Rule!

August 27, 2010 Leave a comment

Gotta love the Isotopes!

After a 15 month hiatus, the Chalk River nuclear reactor in Ontario, Canada, is starting to once again produce isotopes used for medical  imaging.

The Chalk River reactor produced one third of medical isotopes used for imaging procedures all over the world. Namely, it produced Molybdenum-99, which is created as a fission product in the nuclear reaction.

The Molybdenum-99 isotope is unstable, and will decay into Technetium-99m. The Technetium can then be injected into a patient to perform medical scans.

Molybdenum-99 is kept in specialized containers while it decays into Technetium-99m

Now, I did my Masters thesis on Magnetic Resonance Imaging, so these medical procedures bring back some good memories for me. As such, I am going to start a series of posts describing the “Physics Of…” various medical techniques.

These will include things like X-Rays, PET scans, CAT scans, etc. So look forward to that, its a subject I hold very dear to my heart.