Magnetic resonance imaging, or MRI, is a non-invasive medical imaging technique. It is used primarily for imaging soft tissues such as the brain, spinal cord, and abdominal organs.

MRI works by placing your body in a strong magnetic field. As a result of this external field, the magnetic moments of Hydrogen atoms in your body (think of them as little bar magnets) tend to align with each other. This alignment produces a net magnetisation which can be measured and used to create images.

The net magnetisation is measured by first applying a radio-wave to the body. This causes the magnetisation to tip partly into a plane that is perpendicular to the external magnetic field.

Once this is complete, the magnetisation begins to precess about the external field, much like a spinning top as it begins to slow. This rotating magnetisation can be measured by placing a coil of wire next to the body, which acts as an antenna and picks up the signal.

Overview: MRI uses magnetic fields to align subatomic particles in tissue and then uses a radio frequency to perturb them. The time the particles take to relax to their original state is used in conjunction with locational information to generate a 3D image.

MRI uses three magnetic fields. The static field is constant and determined by the strength of the main electromagnet. The patient is placed inside the center of this magnet. This static field causes all the subatomic particles composing the tissue to align their spin states. This alignment gives the system a base state to measure from.

Once the particles are aligned, another magnetic field, "pulse sequence", is released from a radio antenna. The pulse sequence is set based on the resonant frequency of the substance the scientist wishes to study. The most common substances are water or fat. The pulse sequence causes the particles responsive to the resonant frequency to relax from the aligned state to their original state. A receiver coil measures the relaxation time. The intensity of the signal recorded by the coil indicates the concentration of the substance in question.

Locational information is provided by a gradient field.

The combination of locational and intensity information is used to generate the three dimensional image of the tissue.


She blamed it all on the Halloween pumpkin sticker some sadist had placed in the chamber, just at eye level. There were other little thumb-size stickers in there, but she had been unable to take her eyes off of the evil pumpkin. The little orange demon had been talking to her, with its blacker than black eyes and one white tooth.


* * * * *


The back pains had gotten to the point where she was tired of complaining about it. She'd gone to an orthopedic surgeon who recommended an MRI both on her neck and lower back. Both could be done at the same time to save time and money. She'd had an MRI done years before, and it hadn't been any big deal. Of course, at that time, she'd wanted a free Valium and had told the technician that she was claustrophobic and needed some medication. The MRI had gone OK, but she had felt like shit the rest of the day, groggy from the drugs she was no longer used to. So, this time, she had decided to just let it ride and was sure it would be fine. It wasn't.

She had to be there first thing in the morning, which was much earlier than she'd gotten used to getting up. No coffee, no cigarette, just waiting in the parking lot for the sun to come up and the office to open. By 7:30, she was in the room with the weird lady who did the MRIs. This lady had eyes that did not express any emotion. And her hair looked weird. It was some sort of strange red, but not a natural red. It certainly did not coordinate well with her blue medical outfit.

"What is your whole name?" the MRI lady asked.

"Sharon Abbot."

"Do you have any metal in your body? Do you have any partial plates? Do you have a pacemaker? Have you ever gotten metal flakes of any sort in your eyes?" And on and on about metal.

"No, no, no, no...." (What would happen if I did have a metal flake in my eye, she wondered?)

After putting on a very ugly and very large pair of blue pants, she laid down on the table in front of the chamber. The MRI lady told her that they would do her neck first, and put a sort of restraint over her head. "This will take about 20 to 25 minutes. Here are some ear plugs. It will get kinda loud at times. Here is a bulb you can squeeze if you need to talk to me." And she was gone out of the room. Sharon thought, "I can do 20 minutes in this thing. No sweat."

The table moved into the chamber. It wasn't too bad at first. She kept her eyes open and noticed the blue line right down the top of the chamber, as if it were dissecting her body into two equal halves. And she noticed the stickers. They seemed to have a Thanksgiving/Halloween theme. A cornucopia was on one side of the blue line. A half dozen others were a bit further up and further down; she never did really see what they were. But, on the left side of the blue line was a pumpkin. It seemed nice enough at first, but that would change very soon.

As the loud noises began, she tried to close her eyes and drift into a sort of sleep. This was impossible with the noises, and so she tried to count the time to see how much longer it would be. This, too, was impossible. Time didn't seem to exist any more. That's when the panic began.

Some sort of evil thoughts were creeping into her head. Thoughts which had not been considered in many, many years. "I'm getting old, aren't I? I wouldn't be in here if I was young and healthy. How is my life going to end? In pain? In misery? What will I do at the end? Will I put a gun to my head? Will I tie a scarf around my head to stop the splatter of blood, like the guys in The Deer Hunter did in that game of Russian Roulette? . . . . Wait, why am I thinking about this sort of stuff? (Well, doesn't everyone, at one time or another? WAIT . . . NO: THEY DON'T!)"

The MRI lady spoke to her on some sort of intercom. "Ms. Abbot, you moved during that sequence. We will have to do it again. Please try to be perfectly still and try not to swallow."

"How much longer will this take?" she asked in a voice that must have been filled with terror.

"This will take 6 or 7 minutes, IF you can remain still."

"Can you let me out of here for a few minutes before we continue?"

In a very cruel voice, the MRI lady said, "If I let you out, we will have to start from the beginning. Is THAT what you want?"

Sharon heaved a deep sigh. "No, go ahead."

As the noise geared back up, she tried to make her eyes look as far down toward her feet as she could, to see the outside world. This worked for a while, but it began to hurt her eyes, so she looked back at the blue line and the stickers. Closing her eyes seemed to be the worst thing for the "bad thoughts," so she was determined to keep them open. That's when it dawned on her that the rearrangement of her molecules by this cursed machine was causing her brain to do this. This machine was driving her to madness with its mad-scientist rays and beams. Just then, the noise stopped and the MRI lady said, "OK, we're done with that one."

When the table was fully out of the chamber, she sat up, threw the earplugs to the floor and angrily said, "I tell you what. I will see the doctor about this one on my neck, and we can worry about the lower back MRI later." This did not sit well with the automaton lady who was standing by the table. In a clipped voice, she said, "If that's what you want. It's your call."

However, after sitting in the big open room for a minute, Sharon decided she could go ahead and tough it out. The thought of having to come back to this place seemed somehow worse than going ahead and getting it over with. "Oh, I guess not. Let's go ahead and get it over with."

The MRI lady seemed pleased. "This will be easier; you won't have your head restrained. That's the worst part." And within seconds, she was re-ear-plugged and back in the chamber and the now-happy sadistic demon from Hell was out of the room again. The noises started again. And that's when it happened. The voices started.

She could see last week when a little boy was in this same chamber; a small boy with a brain tumor who would not live another year. She could hear his tiny voice saying, "You think it's bad for you. How do you think I felt when I was in here? What do you think was going through my head?" She could hear the voice from a middle-aged man with bone cancer who had been in here only yesterday. "I'm dying and I have two small children to support. My wife doesn't love me any more because she's already planning for when I'm gone. I won't see my children grow up. What do you think I was thinking about when I was lying RIGHT WHERE YOU ARE NOW, less than 24 hours ago?" The voices began to tell her what they were thinking about.

It was that damn pumpkin. It was talking to her. Somehow these voices had been trapped behind that white tooth staring at her, and now they were flowing freely into her head. They grew in strength and she could hear several voices at once, all of them filled with their own particular version of terror and fright. She almost blacked out.

When the MRI lady came in smiling and removed her from the chamber, she said, "OK, it's all over now."

"Yes. Yes it is," Sharon said. She got dressed in a daze and spent the rest of the day in a fog. The next day, she logged onto the internet and discovered that around 30% of people who have MRI's experience panic attacks during the process. She doubted if any of them had experienced a talking pumpkin which changed their lives forever.

MRI provides incredibly detailed images of the structure of the brain, with a resolution of under 1mm. Certain types of MRI can even give information about regional blood flow and the metabolic or biochemical state of selected brain regions.

MRI (which, for the reason bobort pointed out, older doctors will call NMR) is based on the measurement of the interaction of a magnet and a magnetic field. Consider, as an example, the earth's magnetic field and a compass magnet. At rest, the compass needle points north. If the needle is tapped, however, it will swing back and forth at a frequency directly proportional to the magnetic field strength. The needle will continue to oscillate until overtaken by friction. Since the oscillation frequency is proportional to magnetic field strength, knowledge about the spacial variation of the field could, in principle, be used to detect the location of the needle on the earth's surface (though rather crudely). In MRI, atomic nuclei (principally hydrogen) act as the compass needle, and a strong magnet plays the role of the earth's field. All the atomic nuclei are aligned by the magnetic field, then "tapped" with a brief radio frequency pulse. They then emit energy in an oscillatory fashion as they return to the alignment imposed by the field. By detecting the oscillations, extremely detailed images can be created.

The resolution of MRI depends on the strength of the magnetic field. Most clinical machines have field strengths of 1.5 - 4 Tesla, providing millimeter to quarter-millimeter resolution.

Neuroscience, Sinaur Associates (QP355.2.N487 1997)

Cool Facts About MRI Scanners

I spent a summer working for General Electric Medical Systems in Australia installing MRI machines for various hospitals and radiology practises. Here are some facts and stories that I learnt during my work experience.

  • Metal Kills:
    If you ever get a MRI scan done on you, you will be asked to remove all metallic objects from your person before entering the room. The reason for this is that the magnetic field is so strong that it will literally rip magnetic metallic items from you.

  • Cheap Husband:
    There is a story of a woman who went for a MRI scan and was asked by the radiographer to remove her earings. She assured the radiographer that they were solid 18k gold earings from her husband and therefore not magnetic. Unbeknownst to her, the earings were only gold plated and so when she entered the MRI scanner, the earings tore from her ear lobes and stuck to the scanner - that would have been one unpopular husband that night I'm guessing!

  • Metal Flakes:
    One question they will ask you is if you have metal flakes in your eyes (a common problem with metal workers). If you do and you go into an MRI machines, your eyeball could literally tear itself to shreds when the metal flakes try to leave your eye for the scanner!

  • Superconductor:
    To achieve accurate results, the magnetic field should be as parallel as possible. It is for this reason that the MRI scanner is a long tube into which patients are slid. When the patient is inside the scanner (ie inside the tube) they are surrounded by a huge superconductor.

  • Coolant:
    To keep the superconductor functioning, it is immersed in liquid nitrogen. Most of the bulk of the MRI machine is taken up with insulating and containing the liquid nitrogen.

  • Emergency Vent:
    If you look at an MRI machines, it will have a tube connected at the top of the machine and going up into the roof. The purpose of this tube is in case the cooling unit in the scanner breaks down - which means that the liquid nitrogen would start to heat up and boil - boiling liquid nitrogen rapidly expands - and this tube is the emergency vent.

  • No Radio Please!:
    Because the MRI scanner is very sensitive, all external radio waves must be eliminated. To do this, the entire room is lined with copper sheeting. To get wires and stuff into the room, pipes of a certain length are used to tunnel through the wall. These pipes are of a length which will cause radio waves to superimpose on themselves and therefore cancel themselves out. If you take a radio into a MRI room and shut the door, you will get zero radio reception!

  • Blinding Copper:
    One of the MRI machines that I helped install had to be lowered into the hospital via the roof. We couldn't work from about 11am to 2pm while the sun was overhead because the glare reflecting from the copper lined walls was blinding!

  • Physics Lesson:
    One of the most impressive demonstrations of the strength of the magnetic field in the MRI scanner was when one of the techs grabbed a piece of non-magnetic metal and placed it on edge inside the MRI scanner. He then let go. Instead of the piece of metal falling flat straight away (as you would expect), it very slowly keeled over. It occured in a slow motion kind of speed. The reason this occurred was simple physics. A conductor moving in a magnetic field will generate an electric current which in turn generates a magentic field which will try to oppose its motion.

MRI is the short name of the Manchester Royal Infirmary.

Situated on Oxford Road opposite Whitworth Park halls of residence, it is the largest hospital complex in Europe. It is also a University Hospital meaning that this is where doctors and nurses are trained. Being such a hospital, links to academia mean that much research is carried out there from psychology, neurology, techniques etc. all the way to biomedical devices, bio-informatics and GIS.

MRI machines generate an impressive amount of Tesla. High School Physics explains the rudimentary elements of it.

Many people get claustrophobia because they are in a narrow, vibrating, noisy tube, sandwiched under a 1 ton magnet and a table. I was a bit nervous looking at the mammoth machine. It doesn't hurt, but until I realized that the thing was properly bolted and wouldn't fall under the magnetism, I was fine. Plus, they gave me nice music to listen to, in what looks like a stethoscope with thick ear buds to block out the Hmmmmmm. The reason is so that there is no metal, the speakers are outside the machine, piping sound in through tubes. I got to pick the radio station.

During the 1990's, advances in technology created the Open MRI. No, not open source, open enclosure. More advanced RF antennas meant you could move the magnets further apart, and you could pick up weaker signals. Aside from clearer films, you now had a good six feet between magnets. From photos of the new enclosure, you sit on a table. There's half of a hula hoop ring going above you and joining to the sides of the table, to be the antenna. Above you is a large metal circular plate, the magnet. Beneath you is another magnet. It's much roomier, and less noisy. I'm not a radiologist, so I don't know if there's a difference in quality of the films though. Also, now they now use a rare element called Niobium in their superconducting magnets, which comes from South America.

I've had some fun with MRIs too. After I got a head scan with a closed MRI, the tech picked up a screwdriver and gave it to me. Tightly clenching it, I walked in to the machine, and felt an invisible hand begin to tug and tug on the screwdriver. Waving it in the tunnel, it almost latched onto the ceiling. Luckily the machine was turned off, and the outside had some shielding, or I would have been dragged accross the room.

There are two types of closed MRIs, permanent magnets and electromagnets. The reason closed MRIs vibrate so much and are so noisy, is the magnetism. With permanent magnets, one magnet is moved into position relative to the other magnet. There is such attraction generated that the magnet begins to shake. It's perfectly safe, and bolted and wielded into place, but creates a vibration. Electromagnets turn on and off, when it's on, you feel the vibrations. It feels rather like a massage bed set to low. Other causes for the noises and vibration is the moving internal machinery, the antennas moving into place once the magnet is switched off.

The MRI technology was developed at the State University of New York at Stony Brook, where it went by the name NMR, Nuclear Magnetic Resonance. The first machine was built with permanent magnets, dozens of them in metal brick form stacked to form the plates. The lab was situated right above the parking garage, which was unlucky for the cars below. People began to notice that all the cars parked in a certain corner wouldn't start if they were left there for a while. It turns out the machine wasn't shielded enough, and the magnetism was somehow draining all the car batteries below. The floor, as well as the walls, soon got lead or copper shielding after that.

Another interesting story there: One day, the custodian, not The Custodian somehow ignored the red "In Progress" signs and entered while using the floor buffing machine. Immediately the machine was yanked off the ground, and dragged into the tunnel, where I imagine a patient was lying. The patient was OK, just had to crawl out the other side. The custodian was fired, and the radiologists were left with the task of getting a heavy twisted hunk of metal out from in between two permanent magnets. In the end, a tow truck had to use a winch to slowly pull the tangled floor buffer out.

Aside from the clearer resolution and the Open MRI, there haven't been any new advances, except the fMRI which can help determine which parts of the brain are functioning at one time.

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