Phew. I know, right?!
I’ve been saying I’ll write a post about the physics of MRI for months. Never got around to it. Mostly because I knew that since magnetic resonance imaging (MRI) was my field of research, that I would want to go into a lot of detail. I loved the time I spent doing MR research, and now I’ll finally share with all of you how it works.
Let’s start at the beginning.
We are all made of atoms. Atoms consist of a nucleus (made of neutrons and protons) orbited by electrons. The most common atom in our bodies is Hydrogen, which is a single proton orbited by a single electron. It is the simplest atom in nature, and it is quite fitting that it makes up the majority of our bodies.
We are mostly made of hydrogen because we are mostly water. Water has 2 hydrogen atoms and 1 oxygen atom (H20). When we do an MRI, what we are actually taking an image of is the hydrogen in our bodies.
How is that done? Well protons have charge, a positive charge. Any particle that has charge also has what is called a magnetic moment. A magnetic moment is essentially a measure of the strength and direction of the magnetic field of particle. For a proton, the magnetic moment looks like this:
Usually, all the magnetic moments in your body are jumbled about in all directions. That’s why you are not magnetic. When this is the case, we say that your net magnetic moment is zero.
But if we apply a magnetic field from the outside, we can get the moments in your body to line up with the field, like so:
In order to do that, we need BIG MAGNETS. That’s why MRI’s are so strong; you need that strong magnetic field to get the moments in your body to line up. (The diagram above is an exaggeration; in reality, only 1 in a million of the magnetic moments will line up with the field, on average).
How strong is an MRI magnet? Magnetic fields are measured in units of Tesla (in honour of Nikola Tesla). A typical MR scanner has a field of 1.5 Tesla. For comparison, this is about 30 000 times stronger than the Earth’s magnetic field, the field that makes a compass needle point north.
Now your body has what is called a net magnetization. This means that the magnetic moments have lined up and you are a little bit magnetic.
But that’s only half the battle. Next, we have to get some kind of signal from your body to make an image. How do we do that?
Well, have you ever swiped a credit card in a machine? What would happen if you just held it still? Would it still work?
The answer is no. Your credit card only works because the black strip in the back is magnetic. When you swipe it through the machine, it creates an electric current. This is due to Faraday’s Law, which says that a changing magnetic field in a loop of wire will create an electric current in that wire.
We can apply this concept to your body too now that its been magnetized by the MRI machine. You see, MRIs actually have a radio transmitter/receiver inside of them as well. How does THAT help?
Unlike a credit card, we can’t swipe YOU through the MRI machine. That’s not feasible. But we can manipulate the fact that you are now magnetized. This can be done using a radio-frequency pulse. Basically this is just a short, intense burst of radio waves at your body. This will actually make your net magnetic moment spin.
If your net magnetic moment is now spinning, it creates the exact same effect as if we were to swipe you through the MRI machine: it generates an electric current in a receiver in the MRI. Cool eh?
Ok so now we have a signal from your body by magnetizing it, and then making that net magnetization vector spin. How does that make an image?
So we made your net magnetization spin by using a radio-frequency pulse. How fast it spins depends on how strong the magnetic field is. So what MR scanners do is they make the magnetic field a bit different at each point in your body. That way, the magnetization from each part of your body spins at a different rate (or frequency) and we can then determine what part of your body is giving the signal.
Confused? Think of it this way:
Let’s say you had a piano. You hit a key on the left side of the piano and what happens? It makes a deep, low-frequency sound. Now you hit a key on the other end of the piano and you get a high-pitched, high frequency sound.
Now pretend you weren’t looking at the piano at all, and someone else hit a key. If you hear a low-frequency sound, you know they hit a key on the left side of the piano. If you hear a high frequency sound, you know it came from the right side. You were able to determine the position of the key by its frequency.
Its the exact same idea in MRI. Using the frequency of the signal we get, we can determine where in your body that signal came from. We then put all those signals from all parts of your body together, and we get an MRI image.
I know, there’s a lot of steps to MRI, so I’ll recap:
- We magnetize your body by using a really big, strong magnet.
- We make your net magnetization spin using a radio-frequency pulse.
- This spinning magnetization generates an electric current in the radio receiver in the MRI machine.
- We can tell where in your body this signal came from by its frequency.
- Putting all the signals from all over your body together, we can make an image.
So there you go. Simple right?
If you want some more information you can check out the MRI article on HowStuffWorks.
Or if you want something a little more technical and goes a bit deeper into MR theory (e.g. relaxation, pulse sequences), check out this online book by J.P. Hornak, or the Wikipedia page on the physics of MRI.
A couple of days ago, a woman in Germany gave birth at the Berlin’s Charité Hospital while inside an MRI scanner
so scientists could study the birthing process in more detail.
A hospital spokesperson said the entire procedure went well, and both mother and baby are doing well.
Researchers designed a special “open” MRI machine in order to accommodate the experiment. MRI’s are quite loud though, so the mother still had to wear earmuffs, and the procedure was stopped after the amniotic sac broke, in order to protect the baby’s hearing.
MRIs use large coils of wire to generate a strong magnetic field to image the body. Generally, the bigger the magnet means a better picture, so the opening in which the patients lie is as small as possible. In this case, it was more advantageous to have a more open design. A photo of the actual MRI machine used in this experiment was not given, but it would look something like this.
So what was the point of all this? Scientists want to study the birth process better in order to understand what causes complications, and prevent them.
Experiments like this are going to continue, as 5 more mothers have volunteered for the procedure.
Man, how many times have I said I would write a post about the “Physics of MRI”? Quite a few…its coming I promise, cause I have a couple more cool MRI studies to share… stay tuned.
And I get proud of myself when I run 3 or 4 miles. This sure is humbling…
But running 2,800 miles (4,500 km) in 2 months is exactly what a group of a few dozen “Ultramarathoners” do every few years in Europe. Its called the TransEurope Footrace, and I am in awe.
Last year, 44 of the 66 participants in this race allowed themselves to be medically examined through the course of the race, to find out exactly what happens to people who exert themselves this way.
The results were presented at the Radiological Society of North American meeting in Chicago this week. The study was entitled: Longitudinal Follow-up of Changes of Body Tissue Composition in Ultra-Endurance Runners during 4.500 km Trans Europe Foot Race 2009 Measured by Whole-Body MR Imaging on a Mobile MR Imaging Truck-trailer. (Yes, the same conference that had the acupuncture presentation I wrote about yesterday.)
So they followed these runners around with an MRI machine in their truck (!) and through the course of the race took 6 full body scans of the runners and measured their body fat content and muscle volume. The results?
We found muscle mass catabolism also in the exposed muscles of the leg. This occurs in every subject. Over all nearly 34% of nonvisceral body fat has been gone after the race. But there was nearly 20% of visceral fat loss, also.
So they found that 7% of muscle mass in the leg was lost through the course of the race as well.
I’m not sure whats more impressive about this study: that they ran 4,500 km, or that they followed them with a friggin’ MRI machine in a friggin’ truck!
A presentation is being given today at the 96th Scientific Assembly and Annual Meeting of the Radiological Society of North America.
A couple of media outlets have jumped on this presentation, titled “Influence of Acupuncture on Pain Modulation during Electrical Stimulation: An fMRI Study“.
The headline in the Telegraph reads: Acupuncture’s effect ‘isn’t just psychological’
In the Daily Mail it reads: Acupuncture is no placebo and does relieve pain, say scientists
The Telegraph headline is misleading, and the Daily Mail headline is just plain wrong. And, as I’ll point out, both are overstating the findings of the study, as are the scientists who performed it.
So first off, fMRI stands for Functional Magnetic Resonance Imaging. It is a type of MRI scan which can determine which parts of the brain become “activated” by measuring the amount of blood flow to each part of the brain.
It is a fascinating field of study, and in a future post I will explain the physics of MRI, but for now lets just say that fMRI is (somewhat) able to tell which parts of the brain “turn on” when you do certain tasks.
So what happened in this particular study is this: the authors got 18 healthy volunteers and shocked their ankle with an electric shock to induce pain. At the same time, they imaged their brain using fMRI.
Next, they took the same 18 people, performed acupuncture on them, and then shocked their ankle again and took another fMRI of their brain.
They compare the two images, before and after the acupuncture, to see which parts of the brain light up (or don’t) to see if they could see any differences in how the brain reacts to pain stimuli with and without acupuncture.
And wouldn’t you know it? They did see a difference. Their conclusion:
Activation of brain areas involved in pain modulation was significantly reduced or modulated under acupuncture and the majority of the detected areas were not influenced by the analyzed covariate. However, left anterior insular cortex and orbitofrontal / superior frontal gyrus activation was modulated by stimulus intensity. We hypothesize that insula activation seems to be correlated to the stimulus and pain intensity while the importance of frontal activation increases during acupuncture and may be an acupuncture specific effect.
Essentially, they found that after the acupuncture, parts of the brain which control pain were not activated as strongly. Not only that, but the affective response to pain (the frontal cortex) was changed after the acupuncture as well. Pretty convincing right?
No. It’s not . First off, a similar response has been shown by Wager et al. in 2004 that placebos induce the same effect.
Second, it has also been shown that expecting pain can alter one’s response to pain. This study had the volunteers get their ankle shocked first, then they got acupuncture and had to be shocked again. They were expecting the pain, so this may have affected the results.
Third, there was no control group. A proper study should have had a placebo type of acupuncture, such as pricking the skin with toothpicks (which has been done before) or placing the needles at non-acupuncture points. Or they should have tried some other type of pain relief, like massage or relaxation prior to the second shock, to see if there was any difference.
Fourth, the study only had 18 volunteers. To make a claim that acupuncture works based on such a small group is irresponsible.
Fifth (geez, five points?!) fMRI is not easy to gather accurate conclusions from. The workings of the brain are affected by many things and brain responses can be non-localized.
I am not refuting the results of the study, only the conclusions drawn by the authors and the media. The data itself is not surprising. In fact, it is exactly what you would expect since placebos have shown similar results. But to draw the conclusion that this is an acupuncture specific effect from this data is fallacious.