Baby Thermometers

Babies have very limited methods of communicating what they want or what is wrong with them. Crying can mean any number of different things. At this point, I am fairly good at figuring out when my daughter is tired or hungry or upset about strangers or just bored. However, sometimes she just cries and gets very upset for no clear reason and it is difficult to figure out what is wrong.

A couple of weeks ago, my daughter was very fussy and needier than usual and just refused to nap. After a while, I decided that there was definitely something wrong and that she seemed sick. One of the main tools for determining if our child is sick is our thermometer. Being a scientist (or a geek, if you prefer), I am of course interested in how different thermometers work and how they measure my daughter’s temperature.

While it seems that there are a wide variety of baby thermometers – ear thermometers, rectal thermometers, under arm thermometers and forehead thermometers – the most common types fall into just two categories of operation. The rectal and under arm thermometers use contact between the skin and a metal component on the thermometer to read temperature while the ear and forehead thermometers use the radiation emitted by your baby’s body to calculate temperature. These thermometers use very different techniques to measure temperature than the bulb thermometer many of us grew up with.

Contact Sensors

The metal contact sensors are most commonly electrical sensors. There is a thermistor in the metal detector portion of the thermometer. This is a resistor that changes its resistance depending on temperature. To understand how this works, we need to think about some simple electronics.

Consider a very simple electronic circuit like the one below:

A battery provides a voltage – 9 Volts in this picture. A constant current travels around the circuit because there is just a single path through the loop. An ammeter is a device that is used to measure the current – this is a measurement of how much charge passes through that point in a given amount of time.

Ohm’s Law tells us that voltage, V, is related to current, I, and resistance, R, as follows:

So what happens when my resistor is sensitive to temperature? When you put the resistor in contact with your body, it heats up and its resistance decreases. The battery voltage stays constant, so as the resistance decreases, the current through the system must increase. This makes sense, right? If there is less resistance to the flow of charge, more charges will pass the ammeter in a given amount of time.

By measuring this current, and knowing the voltage of the battery, you can easily calculate the resistance of your temperature sensitive resistor using the equation above. The resistance vs. temperature is something that is very well known by the manufacturer, so once you know the resistance, you know the temperature of your baby who is in contact with the thermometer.

The circuit in your thermometer has a few more components, but this basically how it works. And it does all the calculating for you and just displays a temperature.

Infrared Sensors

If you bring your child to the doctor’s office, they will likely take his or her temperature using an infrared sensor. This is what my nurse uses. She takes a device with a flat, round end and lightly moves it across my daughter’s forehead and reads out the temperature. This is very fast and is great for a kid (like mine) who hates to be touched by strangers.

How does this work? If you remember, a few weeks ago, we talked about Blackbody Radiation. Our bodies emit light, but it is in the infrared where we cannot see it with our eyes. The peak wavelength emitted from our bodies is about 9 microns (Check out the PhET website to see for yourself). The amount of light and the peak wavelength of the light both depend on the temperature of our bodies.

The thermometer uses a thermopile to measure the radiation coming from our bodies (How Stuff Works). Heat from our bodies heats two pieces of metal that make up the thermopile. A voltage is created between the pieces of metal that depends on the heat of our bodies. (Wikipedia)

How do we measure this voltage and turn it into a temperature? We could use the same circuit that we used above. This time, have a constant resistor and use the thermopile as our voltage source. We can measure the current to determine the voltage created by the heat from our bodies and determine the temperature from that voltage.

The infrared sensors are very fast, which is definitely a plus with a screaming, squirmy baby. Both types of thermometers work best when the temperature is taken inside the body – a rectal thermometer or ear thermometer. However, we were discouraged by the hospital from using a rectal thermometer on a newborn because they said it is to easy to cause them harm because they are so tiny. And ear thermometers can be a little tricky because they need to be aimed at the baby’s eardrum to get an accurate reading.

We have one of the forehead thermometers at home so I definitely wanted to make sure I understand how they worked. It turns out my daughter did not have a fever when she was sick a few weeks ago. I did take her to the doctor, though, and she did have an ear infection. The poor kid was sick for a while since the first set of antibiotics did not work on her, but is feeling better. And is now fussy for some other completely unknown reason…

I hope my daughter continues to be healthy and not have any fevers, but I do think it is interesting to know how current thermometers make use of technology to help me figure out what is going on with my little girl.

And, of course, I am looking forward to the time when she can tell me what is wrong and there is a lot less guesswork in trying to figure out if she is sick.


CT Scans

Someone I know had a CT scan last week. These scans are very commonplace these days, but I was wondering how many people know they work or even what CT stands for. I learned about this when was in graduate school and studying a method of imaging that is very similar to CT scans, but using laser light instead of x-rays.

These scans are usually called either CT (Computed Tomography) or CAT (Computed Axial Tomography) scans. The two terms seem to be used interchangeably. The ‘computed’ part is easy to understand – a computer is used to construct the final image. ‘Tomography’ refers to the method of taking multiple pictures of an object from different points of view in order to build up a two-dimensional image from one-dimensional pictures. ‘Axial’ means that these pictures are taken by rotating a camera in a circle around a center axis.

So how does this work in practice? While the method does not specify what type of light is used, doctors usually use x-rays for these scans. What are x-rays? They are part of the electromagnetic spectrum, which most of us just refer to as ‘light.’ We know all about the many colors of visible light because we can see them – from violet to red and everything in between. But as the wavelength of light gets shorter (and the frequency gets higher), the energy of the light increases. You can think of this as a wave on a string that oscillates faster and faster. As this happens, the wavelength (distance between peaks) gets smaller and smaller, like in the simple graphic below:

X-rays have shorter wavelengths & higher frequencies than the visible light we see. Much, much shorter. Here’s a graph of the different wavelengths of light and how they compare to visible light:

Light generally can not be used to measure things that are much smaller than the wavelength of that light (there are a few interesting exceptions). So x-rays can measure things that are much smaller, or with much better resolution, than microwaves or radio waves or even visible light. Of course, x-rays have wavelengths on the order of a nanometer – that is 100,000 times smaller than the width of a human hair! So that resolution probably is not needed for typical imaging in the body.

How do these x-rays interact with your body? A beam of x-rays is sent through the body, and some parts of your body absorb more of the light, and some absorb less. What you see on the camera on the far side is a shadow showing what parts of your body absorbed the light and which did not. Visible light is not good for this because it stops at your skin – you can see a shadow of your outside shape, but no features on the inside.

The fact that x-rays get absorbed throughout our bodies is one of the reasons that they can be so dangerous and why we should not be exposed to them too often. In addition to helping to diagnose cancer with CT scans, excessive exposure can cause cancer. Those high energy waves are absorbed by our cells and that energy can be used to alter and damage the cells. Short exposures of low doses are considered safe, but long term exposure can cause serious problems. Sadly, many of the early scientists who discovered and studied x-rays died of cancers caused by this research because they did not yet know of the dangers or how to protect themselves.

Now that we know what x-rays are, let’s get back to these multiple views. Why do we need to look at the body from different angles? This is the tomography part. If you have ever had a CT scan, you know that you need to lie still while an x-ray source and detector spin around you to get different views of your body. We know now that the pictures are basically shadows of your insides, so let’s look at some shadow pictures of some household objects to see how important it is to get multiple views.

Below are two pictures of two different objects in my house. The pictures were taken by shining a bright light on the object and then photographing the shadow created on the wall.

These two objects look to be almost exactly the same, and there is not a lot of information available on what the objects even are.

I then rotated the objects 90°, and these are the new shadows on the wall:

Now we can see that the object on the left is a paring knife and the object on the right is a serrated knife. Multiples views of the object give us more information about its shape and size in different dimensions.

Below are a few more fun shadow pictures from my house. As you look at the pictures, think about the following questions:

  • Can you tell what the objects are?
  • Do you need all the views to understand what you are looking at?
  • Are some views more useful than others?
  • How does having the multiple views help you better understand what you are looking at?

This last object is partially transmissive – light travels through the wings, but not the edges. This is similar to a picture of your body where the x-rays are not completely absorbed by some parts of your body. This gives us more information about the internal structure of the object being viewed.

So CT scans use x-rays, which are short wavelength light, and operate by taking multiple shadow pictures of our bodies at many different angles to reconstruct a full image. The picture below shows how this is done.

A computer adds up all the data on all those different lines, giving us a full circle cross section picture of the body. I will skip the details of the computer programming for now and just look at the results. I do not have any pictures myself of CT scans, but here is a really cool set of cross sectional scans of a human brain from Wikipedia’s page on Computed Tomography.

It’s really amazing that we can get so much information about how our body works from what are essentially shadow pictures. I frequently have students in my introductory Physics courses who are interested in being doctors. I think I should add a day on CT scans to the unit on light to see if I can convince them that Physics really is something useful for them to learn. If anyone out there has some more medical applications you would like me to explore in the future, I am all ears – there are tons of fun ones!