I am often asked about the difference between the "CAT scan" (CT) of the head and the MRI of the head; why get one over the other. Or the questions comes in many different forms such as:
1. They keep giving me (or gave me recently) a CT scan in the ER, Dr. Kirk. Why do you need an MRI now?
2. Does the MRI give me more radiation?
3. Why didn't my primary care physician just do an MRI already, or in the ER, if that is more likely to tell us the answer you are looking for?
4. Why is that MRI so expensive?
I think it is important for you, as the patient getting these tests, to understand the basics, and you really can understand them. I won't get too elaborate here, so just be patient with me.
Let's talk about CTs.
This is what a CT (Computed Tomography) machine looks like. For our purposes below, I am addressing CT's of the head and neck/spine, since that is what I order 99% of the time.
As you can see, very little of your body is actually inside the machine and the actual imaging part of the machine itself is not very deep, more like sweeping into the hole of a donut rather than deep into a coffin.
If we actually take the functional part of the CT machine out, it looks like this:
Really, it's just a spinning, better version of this below. This is the old x-ray machine everyone grew up with and many of us made us of as kids when we broke this or that bone in our arm or leg.
This is the type of X-ray machine that takes the familiar pictures such as these:
A CT machine essentially works like a complex, 3-Dimentional version of a basic X-ray machine.
HOW does it work? It uses radiation.
Now this is the important part that differentiates it from an MRI so read closely.
Everything in the human body has a different degree of density. Bone is more dense than blood for instance. So depending on how dense a certain part of your body is, it is able to slow down x-rays with variable ability, and the detector senses this difference and sends the detailed information to a computer to crunch the numbers and create a quality picture that helps pick up differences in say... the density of healthy brain from brain tissue with blood in the wrong place since blood has a different density than brain tissue.
Bone is quite dense since it serves the purpose of protecting our brain from impacts.
VERSUS
Blood, which is mostly water and less dense since it needs to gently squeeze through blood vessels and let nutrients and oxygen absorb through its walls.
Again, in a CT, a motor turns the ring so that the X-ray tube and the X-ray detectors revolve around the body. Each full revolution scans a narrow, horizontal "slice" of the body. The control system moves the platform farther into the donut hole so the tube and detectors can scan the next slice.
In this way, the machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. It's not usually necessary to scan the entire body, of course, only a small section of interest.
Since we examine the body slice by slice, from all angles, CT scans are much more comprehensive than conventional X-rays. They are an invaluable tool in modern medicine.
This is a typical "slice" image of the human brain below, using the CT. Later I'll put an image side-by-side with an MRI. In this picture, you can see the skull around the brain in white, fluid around the brain and inside the brain in special compartments called ventricles as black, and the brain tissue itself as grey. The front of the head is at the top and back is at the bottom.
Naturally-occurring "background" radiation exposure.
I have been asked a lot about this more recently in light of raised concerns in the world about nuclear energy and radiation exposure. So I thought I would address it here.
We are exposed to radiation from natural sources all the time. The average person in the U.S. receives an effective dose of about 3 mSv (millisievert) per year from naturally occurring radioactive materials and cosmic radiation from outer space. These natural "background" doses vary throughout the country.
People living in the plateaus of Colorado or New Mexico receive about 1.5 mSv more per year than those living near sea level. The added dose from cosmic rays during a coast-to-coast round trip flight in a commercial airplane is about 0.03 mSv. Altitude plays a big role, but the largest source of background radiation comes from radon gas in our homes (about 2 mSv per year). Like other sources of background radiation, exposure to radon varies widely from one part of the country to another.
To explain it in straight forward, relative terms, we can compare the radiation exposure from one chest x-ray as equivalent to the amount of radiation exposure one experiences from our natural surroundings in 10 days.
Following are comparisons of effective radiation dose with background radiation exposure for several radiological procedures. I included a chart with many types of studies.... but I boxed in red the CT of the head with and without contrast (which needs an additional "sweep" of the head after contrast is added), and a CT of the spine:
To put it into perspective... if you get a CT of the head, it's as thought you were exposed to extra 8 months of living (in background radiation time), than someone else who didn't.
...Or.... people in Colorado are essentially almost receiving an extra regular CT of the head per year by just living, than people living in, say, Houston.
Okay, now let's talk about MRIs.
I'll compare the two a bit after this section.
This our MRI here at the office. It is currently housed in a traveling mobile unit that comes by every Wednesday to do our MRIs.
This is what the MRI (Magnetic Resonance Imaging) machine looks like. As you can see, if you go back up to the top, it's "deeper" than the CT machine, more coffin-like than donut-like. And this lack of openness is what bothers some people.
When patients slide into an MRI machine, they take with them the billions of atoms that make up the human body. For the purposes of an MRI scan, we're only concerned with the hydrogen atom, which is abundant since the body is mostly made up of water.
And water, if you remember, is just a bunch of H2O: 2 Hydrogen atoms and 1 Oxygen atom.
All of the hydrogen atoms are going in various directions, but when placed in a magnetic field, the hydrogen atoms line up in the direction of the field.
So you can imagine that that has to ONE POWERFUL MAGNET. And it is.... It has to line up hydrogen at the atomic level. AND this is why we stress about certain metals in your body. IF it's a metal affected by a magnet (not all are), then it will either rip it right out, move it inside your body, or heat it up. We don't want any of that to happen since most of it was placed with the intention to stay put, or we want it not to wiggle anyway if you got that metal in you on accident.
So, unlike a CT which depends on density, the MRI depends on the movement on water (hydrogen) essentially in a much more detailed way and more finely differentiate different tissues.
Example: A tumor in the brain may be just as dense as the brain tissue around it (so it can hide from a CT), BUT since it's not working like normal brain tissue, it often utilizes water differently, and an MRI can pick this up.
I'm too claustrophobic... now what?
MRI machines are evolving so that they're more patient-friendly. Many people simply can't stand the cramped confines, and the bore/hole may not accommodate obese people.
-There are more open scanners, which allow for greater space, but these machines have weaker magnetic fields, meaning it may be easier to miss abnormal tissue.
-Very small scanners for imaging specific body parts are also being developed, but this doesn't help me as a Neurologist usually since I always seem to need the brain or spine which puts the claustrophobic part of people inside.
There other ways to try to reduce the feeling of being closed in:
-IV and by-mouth medication to calm you/help you sleep through. Putting a person completely out is tricky, less safe, and usually not recommended since if your heart stops or you stop breathing inside an MRI machine, it's hard to know, hard to get you out quickly, and hard to get a doctor who can really do something about you dying in a position to help you... so we try to avoid that unless absolutely necessary, because it involves essentially hiring an Anesthesiologist physician to supervise the test. You can't just do controlled poison to knock someone out without a specialist doing it.
-The special coils on many MRI scanners have mirrors so you can see outside, thereby easing the claustrophobic sensation.
- Some MRI machines now have glasses you can put on which allow you to watch a show with earphones in to "escape" the fact you are in a small space with a noisy machine around you.
I like this image. It shows that there are different magnet fields going different directions to help produce the detailed images of an MRI but sweeping a different field across the body different ways. It's also part of the reason the MRI machine makes different noises during the study, as it changes directions, juking and jiving around you, encased its sterile-looking plastic shell.
And this is an example "slice" of MRI image of the brain. In this image, different layers of the skull and its musculature are different shades of gray around the outside. The grey matter of the human brain (where all the thinking cells are) is a darker grey, and the the white-ish part is the so-names white matter, which is the railroad tracks by which the thinking cells sent their message down through the brain to the spinal cord. The black in the middle is the water inside the ventricles. The sort slight-grey areas on both sides in the middle are parts of the basal ganglia which functions to smooth out your movements.
Side-by-side, the MRI (first) and CT (second) look like this:
CT versus MRI
Why not always get an MRI then?
CT:
-Quicker/less testing time (Makes it better in an emergency and gives more people access to testing)
-Patients can be less stable and get a CT since many machines that keep critical people alive are affected by MRIs
-Less of a weight and obesity limitation
-No metal-in-body limitation
-More quiet than an MRI
-Open like a large donut (decreased claustrophobia concern)
-Much more affordable (usually about a third of the price)
-You often don't need an MRI when a CT is sufficient to see a large number of pathologies
-Is great for looking for blood and bone damage/abnormalities
MRI:
-NO RADIATION
-Better for soft tissue delineation
-For me as a neurologist, is better for aging an acute stroke or recent strokes and helping in such diseases as epilepsy by looking at very subtle changes in the soft tissue of the brain... and many other subtle brain changes
-Noisy, narrow, expensive, has limitations of use such as weight & body size & certain metals in the body
So there you go. That is the basics of what a CT and MRI are about as important imaging modalities in modern medicine, and little of the differences between them. I hope this helps you understand the difference a bit better. Email me to tell me if something is unclear here/I need to elaborate more.
