Hi, I'm Libby Bucholz, and I'm an assistant professor of the practice
here at Duke University in the Department of Biomedical Engineering.
Have you ever wondered how CT images are made?
Well today we're going to take a peek inside the hood, and open it up so that you guys can get a better look.
So CT stands for computed tomography. It's also known as CAT scan
which stands for computed axial tomography.
The two people who invented CT, sir Godfrey Hounsfield and Alan Cormac
both earned very well deserved Nobel prizes in 1979, but it is a feat of modern engineering that we have these
amazing systems creating images of our body, every single day.
So to start with, we use x rays to create CT images.
Why do we use x rays, who decided that anyway?
we know that x rays are harmful to our health.
Well, x rays it turns out meet two criteria to chief criteria for creating medical images.
The first is that their wavelength has to be small enough that we can resolve small tissues.
And so our resolution can never be greater than our wavelength
So we need to use only wavelength of light that will allow us to achieve that resolution.
The second criteria is that it needs to be able to penetrate around 25 centimeters of tissue
or needs to be able to get through our guts.
And so those two chief requirements, basically eliminate everything except x rays and gamma rays
and gamma rays are used in other imaging technologies that is not CT.
So to create CT images, we need an X ray tube
and physicists have been building X ray tubes, since the late 1800s so this really isn't a problem
but unlike traditional X ray, where we just take a single picture CT is going to create hundreds of pictures
hundreds of different X ray images together and then we're going to reconstruct them all together to create
he high resolution images that you see CT images creating right now.
And so to do that we need to mount our X ray tube in our CT scanner, and that X ray tube is going to rotate
around our patient about three times a second.
The X ray tube is about the size of a VW engine, so this really is no joke.
And we need to call in some mechanical engineers who are going to help us make sure that this is
working, and then it's calibrated correctly and it's doing exactly what it's supposed to be doing.
Then the X rays are going to pass through our patient.
And so to figure out how the X rays are passing through our patients
we need to talk to the biologists who are going to tell us what our body is composed of
and what we can put in the body if we wanted to change that contrast
and the physicists who tell us how those x rays are going to interact with matter to begin with.
For CT, it's really important that our x rays both pass through our patient and are absorbed by our patient.
And that they're done so differentially, meaning that in different parts of the body,
they're going to be absorbed more or less depending on what part of the body they're in.
So for example, if we had 100% transmission of our x rays that really wouldn't be helpful, because then
we would get no contrast there would be no difference in the absorbance of X rays through bone or liver.
And so when we went to reconstruct our image. Yes, we would be able to see through the body but we
wouldn't be able to see that this was liver or bone. We wouldn't be able to differentiate any of the tissues.
So in order for us to get that those images that we need where we can see the different parts of the body
are what they are. We need contrast, and to do that we really need our x rays, some of them to be absorbed and
and some of them to be transmitted depending on what tissue that they're in.
I like to call that the Goldilocks phenomenon where we need it to be just right
We don't want it to be too much transmission, or too much absorption
we want it to be right in the middle so that we get that excellent contrast.
Okay, so now we've had our x rays pass through our patient and hopefully some of them have been absorbed
and some of them have been transmitted and now we have to stop those.
and it is extremely important that we stop every single X ray that made it through our patient
because otherwise what were we doing in the first place?
So, we need to stop those x rays, and we need to convert that X ray energy into
some kind of electrical signal. And so the first step is creating these special crystals.
They're called scintillation crystals and Einstein was the first person who discovered these phosphors
in the mid 1900s
But now we use pretty nice scintillation crystals such as cesium iodide or sodium iodide
that will allow a lot of light photons to be created.
So every time an X ray is stopped by one of these scintillation crystals
about 1000 light photons will be created
and to create these special materials, we really need material scientists to help us out.
And so these material scientists are going to build better smaller crystals that are able to stop more x rays
and create more light photons because if each x ray produces 1000 light photons
That means that we're going to have a drastic increase in our SNR, which means that we're going
to get a lot more signal and we're going to get a lot better images.
Now we need to convert our photons
our light signal these 1000 light photons that we just created with every x ray,
we need to convert it into electrical signal.
To do that, we really need electrical engineers to come in and for every single crystal those material scientists
we need an electrical engineer to create a photo detector that will read that raw signal
so that we can have that raw signal and start to reconstruct our data.
Now, we have that raw data, and in the 1970s when CT was first being made that raw data was enough to spend
it could it could take days to reconstruct your data.
you could go in and get scanned for 30 minutes and then
they literally would not have the images four days later,
Even today with amazing computing power that we have reconstruction still takes
orders of magnitude longer than the actual image acquisition.
So in order to get that raw data and store that massive amount of raw data and process that raw data,
we need computer scientists who are going to help us with that.
So we have the computer scientists who are helping us store this raw data and we need to process
it into an image, and the process that we're going to use to reconstruct it is known as filtered back projection.
And this was largely developed by mathematicians who were sitting many many years ago,
thinking about theoretical approaches to this and that, and who knew it had a new life
in CT and reconstructing our raw data.
So this is an example of raw CT data.
The raw CT data is known as a sinogram and it is just the set of all x ray projections
and you can see that it kind of wobbles through the picture as you change
the X ray, the direction that the X rays are coming.
And then we reconstruct our data using a process known as filtered back projection.
And what we do is we actually smear the raw data along the direction that was acquired
and then add it up with the other smeared data. And we smear all the projections,
until we have our reconstructed image.
So you can see the image as we get more and more smears
and we add them together we get an image that is closer and closer to what we want it to be,
to those high resolutions fantastic images that we that we are so used to seeing in CT.
So to create our CT image, we needed a physicist to help us build our X ray tube
we needed mechanical engineers to put it into the scanner,
we need biologists and physicists to tell us how those x rays are going to interact with matter.
We need a material scientist to build a special scintillation crystals.
We needed electrical engineers to create photo detectors.
We needed a computer scientists and mathematicians to help us with the reconstruction,
and all of that together is biomedical engineering. Thank you for listening.