How do you distinguish between a CT scan and an MRI scan?
In my anatomy exam, we are given pictures and we are asked to distinguish whether this is an
MRI or a CT scan. What are some features that can help me distinguish between the two?
CT does use x-rays and the way x-rays interact with tissue is simple as far as imaging is
concerned. x-rays pass through tissue and get attenuated as x-ray photons are scattered by
tissue atoms. The denser the tissue along the x-ray path, the more photons are scattered. So,
we send the x-ray through the sample and measure what comes out on the other side. The
attenuation measured at the receiver is the total attenuation along the entire line of the x-ray
beam.
Let’s say you’re getting a regular x-ray, and not a CT, and you’re standing up straight, with your
body facing the x-ray source. Let’s call the x-ray beam axis the z axis. The image we obtain will
be a value for each point on the x-y plane opposite your body, and this value is directly
proportional to the sum of tissue density along z. We say that’s a projection of your body
density map onto the x-y plane where the z-axis gets compressed/summed.
So, what makes a CT different from an x-ray? Imagine keeping that x-ray source at a constant
distance from the center of your body and rotating it around your body slightly to obtain
another x-ray image from another angle or another perspective. Imagine we keep doing this
around 180 degrees. The result is many x-ray images. How do we combine them to obtain a CT
scan? We then take these projections, smear them along their respective z axis (axis
perpendicular to the projection plane), and then add the smeared images.
It looks like this:
https://www.youtube.com/watch?v=MTBhqcVjQ8Q#action=share
Here’s a little bit more of an explanation, via another YouTube video:
https://www.youtube.com/watch?v=8V2QBD8nh_s#action=share
MRI on the other hand works on a different principle: the basic idea is that we image hydrogen
in the body, which is amply present in water, and also in all the organic molecules (fats,
proteins, etc.). The hydrogen nucleus (a proton), aligns on average with the magnetic field
(which we supply – the MRI scanner consists of a huge magnet). Now, because there are so
many protons in the volume we measure, the ensemble average behaves classically. So, we
don’t need to consider quantum mechanics for most imaging applications. Classically speaking,
we tip the proton magnetic moment away from its alignment using radiofrequency energy, and
the proton magnetic moment then precesses at its resonance frequency, which is directly
proportional to the field strength, emitting a signal that we measure. In doing so, it loses
energy, interacts with its environment and gives off a radiofrequency signal. How the proton
magnetic moments precess, the spread in their precession frequency, and how long it takes
them to go back to equilibrium, all these are affected by various tissue properties.
We only measure from one antenna, and so the signal from the whole section being scanned is
added together into one measured signal; but how do we resolve which part of the signal
comes from which spatial location? Recall that we said that the resonance frequency of
precession of magnetic moments is proportional to the magnetic field we apply. So, we use
gradients (linear variations) in the field to vary the resonance frequency across the sample. That
way, we can resolve where each frequency is coming from. It’s a bit more involved than this,
but this is the simple explanation.
This introductory free online book/tutorial by Prof. Joseph Hornak is a great intro for someone
who wishes to get better acquainted with MRI: The Basics of MRI .
So, from this, you should be able to note one big difference between MRI and CT. CT only gives
you one parameter: density. MRI tells you a lot more. It’s looking at hydrogen in different
contexts and different tissue environments, and we have multiple ways to probe tissue
