Ask a Scientician

I’d like to add to what Dave wrote about the differences between CAT scans, MRIs, ultrasound, X-rays, Pete Wentz, and Justin Timberlake

Starting with X-rays.  X-rays are a type of invisible radiation, just like infrared, ultraviolet, and radio waves.  X-rays have a very short wavelength and this allows them to pass through objects that are not very dense such as skin and muscle.  Dense objects, such as bone, will reflect X-rays.  By scanning an object with X-rays, one can use the pattern of where X-rays pass unhindered and where they are reflected to create a picture.  The reflected portions are denser material, although an X-ray cannot tell you what that material is.  When imaging a human, it has to be bone because there is nothing denser.  Also, if you take a person apart the X-ray pattern matches the human skeletal pattern.  I don’t advise doing this.

An X-ray produces a two dimensional picture.  A three-dimensional picture, however, provides more diagnostic information.  This 3D picture is a CAT or CT scan.  We can use a computer to combine many 2-dimensional X-rays pictures taken from different angles into a full 3D picture.  Essentially, CAT (computed axial tomography) simply means “computer generated picture (“computed”) created using an X-ray device (“tomography”) rotated around an object (“axial”)”.   It’s simply a three-dimensional X-ray.

An ultrasound is very similar to an x-ray in that it reflects energy waves off of objects and uses the pattern of reflection to create a picture.  Ultrasound waves are much lower in frequency than X-rays.  Because they have a low frequency, they do not penetrate very far into objects.  Thus, they are perfect for imaging things near the skin surface.  Because they are low frequency, they do much less harm to living tissue than X-rays.  The low frequency means they have a low energy content and will interact with tissue in a less destructive manner than X-rays.  This is a major reason why ultrasound is preferred for imaging fetuses.  Like Dave said, this is essentially the same technique used by bats and other critters to echo-locate.  By sending out a sound and listening to the reflection, we (and the bats) can build up a picture of the immediate environment.

Now we get to MRIs.  The true term is NMRI or nuclear magnetic resonance imaging.  The word nuclear was removed due to concerns that the public would be unnecessarily fearful.  This scan relies on the fact that the water molecules that make up the human body can be manipulated with a magnetic field.  No- you cannot be controlled against you will in this manner.  The effect is extremely small, but large enough to use for imaging.  An MRI uses a very powerful magnetic field to line up all of the hydrogen atoms contained in the water molecules the field.  A second magnetic field is then used to push some of the hydrogens out of alignment.  As the pushed hydrogen atoms relax and return to their alignment with the original magnetic field, they release energy in the form of radio waves.  (Radio waves are a very low energy, very long wavelength form of radiation).  The energy pattern given off by the relaxing hydrogen atoms is used to produce a picture (just like the way a picture is formed from reflecting radiation during an X-ray).  The usefulness of an MRI comes from the fact that the pushed hydrogen atoms relax at rates in different body tissues (muscle, fat, cartilage, etc).  This means that different body structures (the brain, joints, kidneys, etc) can be imaged in good detail.

A quick note: Dave mentioned that X-rays are ionizing radiation.  This means that the rays are strong enough to knock electrons off of atoms and molecules to create charged particles (ions).  This is not a great thing since ions react with living tissue, often in very destructive ways.  The occasional dental or medical X-ray is nothing to worry about because it’s a short, one time dose and is not much more than a few weeks of exposure to normal background radiation.  Whenever possible, the least invasive type of scan is used to minimize ionization and other interactions with healthy live tissue.

Pete Wentz would look like a leprechaun if you dressed him in some Irish greenery.  Aside from the cool album title of their last record, I’m not familiar.

Justin Timberlake makes catchy but not very exciting tunes, awesome commercials, weak movies, and great SNL appearances.  I’d totally hang with him in a bar.