It's Not Just for Babies

Yesterday, my feet were a bit swollen and painful, so my oncology doctor advised me to go to the local hospital to make sure it wasn't a blood clot.  After sitting there for 5 hours through blood tests and an ultrasound, they decided it wasn't a clot, but weren't sure what it was.  Things seem much better this morning, but just so the trip wasn't a total loss, I figured I'd talk a bit about how ultrasound works, since it was a pretty cool part of the experience.

As the name implies, ultrasound uses high-frequency sound waves to look inside your body.  The waves reflect off the various internal structures and return to the sensor.  By measuring the strength of the echo compared to the original wave, we can estimate how dense the reflecting structure is.  By timing the echo, we can also determine how deep the structure is.

One of the advantages of ultrasound over other imaging techniques, like CT scans or MRIs, is that it can directly measure the movement of blood (hence its usefulness in finding clots).  This is done by measuring the Doppler shift of the reflected wave.  Since we know what frequency we're emitting, we can measure the echo's frequency for comparison.  If it's higher, we know the wave reflected off something moving toward the sensor, while if the frequency is lower, we're detecting something moving away.  While scanning my legs, the technician would occasionally ask me to flex my foot, increasing the blood flow.  This would cause a spike in the signal from my blood vessels, and served as evidence that I was clot-free.

Here's a picture of a typical ultrasound image (from Wikipedia):

The red indicates a red-shift (movement away), while the shading indicates density.

I mentioned to my technician that I write a physics blog, and this would probably be a post, and he responded that 90% of ultrasound techs (including him) forget all the physics they had to learn immediately after taking their test.  Clearly it's not vital knowledge, since he did a fine job without it, but if I were using a device like that on a day-to-day basis, I would want to know exactly how it worked.  It's just not very satisfying to use a magical black box...