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A Diagnostic Procedure
I used the following in the following discussion; the first four are excellent references which you might be interested in reading for yourself.
- A Machine Called Indomitable, by Sonny Kleinfield (1985). This book is an excellent read, written by a journalist, "the remarkable story of a scientist's inspiration, invention, and medical breakthrough."
- Nuclear Medicine, a volume of the Encyclopedia of Health (1994), by Wendy and Jack Murphy.
- Medicine's New Vision by Howard Sochurek (1988). This book has excellent photographs, descriptions, and stories of people whose disease was diagnosed by modern imaging techniques.
- Everything You Need to Know About Medical Tests, edited by Breanndan Moore (1996).
- Various Organic Chemistry texts, as well as the Handbook for Organic Chemistry Lab (CU Boulder), written by yours truly.
The Technical Facts
Magnetic Resonance Imaging, "MRI", is a relatively new imaging technique. The first images were scanned in the 1970s, and in the 1980s its use became widespread. It really should be called "NMRI", as it is based on a technique employed by scientists to study the structure of molecules, a technique called NMR for Nuclear Magnetic Resonance. But, the public associates "nuclear" with nuclear bombs, radioactive substances, and other such nasties, therefore, they dropped the word nuclear to make it more palatable to the layman.
In the context of NMR, nuclear stands for the nucleus of the atom. Recall, the nucleus is the positively charged "center" of an atom. Some atoms have nuclei which "spin" on an axis, examples are hydrogen, carbon, nitrogen, fluorene, and phosphorus.
Why is this "spin" important to MRI? Consider hydrogen, with a nucleus of one proton. This nucleus spins around like a top. Since it has a positive charge, as it spins it generates a magnetic dipole along the spin axis, as shown in the figure to the right. In a normal, non-treated sampling of hydrogen atoms, the magnetic dipoles will orientate in random directions. However, if a magnetic field is applied, they will line up with the magnetic field:
I have shown the spinning nucleis' magnetic dipoles in two colors on purpose, because half of the dipoles will be of a higher energy "red" and half will be of a lower energy "blue". Once they are lined up by a magnet, they are zapped with energy in the form of low energy electromagnetic radiation (radiowaves), which causes the blue protons to become as energetic as the red protons:
The lower energy "blue" protons are able to accept one and only one size package (or quantum: quantum chemistry!) of energy -- energy in the form of light, or radiation -- and if that package is given to it, it jumps to a higher energy level, and becomes just like a red.
But, there is only room in the higher energy level for half of the high energy dipoles, so half of them must soon give up their energy and drop back down. When they drop back down, they each give off a tiny amount of energy:
The technicians know exactly what frequency of radiation to look for, because the nuclei are only giving up specific size packages of energy. It is the energy given off by the protons which signals the detector and leads to the MRI image given to the doctor.
What happens to the other atoms in the body, all the ones that are not hydrogen? Well, each nuclei that is able to (i.e., has a magnetic dipole) will line up with the field. But, the package of energy which each of these nuclei can accept is different than the package size the hydrogens take, so, they will not jump to a higher energy level when zapped with the radiowaves, and hence will not give off radiation.
So, you might ask, why would detecting hydrogens in the body help the diagnostician? Most of the signals in an MRI scan originate from the hydrogens of water molecules, because these hydrogens are far more abundant in tissues than are the hydrogens of organic compounds. The difference in the way water is bound in different tissues is what produces much of the signal variation among the organs as well as the signal variation between healthy and diseased tissue. With practice and experience, people -- technicians -- learn to decipher the scans and tell diseased tissue from healthy tissue.
What It's Like To Have an MRI
To have an MRI, the whole person must fit into a huge magnet and then lie very still for 45 minutes to 2 hours. You are able to speak through an intercom to the technician. Some people become quite claustrophobic. The instrument produces no harmful rays so the technician or family members can remain close by. During the scan, the magnet will effect anything metal, so you must not wear any jewelry or anything made of metal. Some people say that their fillings tingle during the scan. The magnetic is a superconducting magnet, cooled with liquid helium. As each separate image is scanned, the pulsing magnet bangs and thumps. Sometimes a contrast is injected to produce better pictures.
Advantages MRI Over X-Ray Scans
The electromagnetic radiation used to charge the hydrogen atoms is much, much lower in energy and much less damaging than X-rays. The magnet itself has not been shown to have any adverse effect on humans, even though it produces a magnetic field 10,000-30,000 times greater than that of the earth! There is no limit to the number of MRI scans a person can have.
MRI can see right through bones and other hard tissues. It gives images of brain tumors, abscesses, swelling, bleeding, nerve damage, and other disorders that increase the fluid content of tissues.
The instruments cost well over $2 million. They require about $10,000 per month for upkeep, like the liquid helium. Plus, they generally take longer to acquire than do X-ray type scans. These costs are of course passed on to the patient -- depending on the length of the scan, the MRIs cost $2,000 - $4,000 in MRI time alone, not accounting for the time of the various technicians and doctors also involved.
The Parent Facts
An adult may require mild sedation to overcome the claustrophobic effects of an MRI, but what about a child? This will depend on many things, like age, personality, and the current state of the child's disease.
- "Because Eric needed general anesth. for scans, we would always try to get a couple of procedures done at the same time, so Eric was under the general for the duration of the scan and usually an LP or BM or whatever else. He would go to the anesthesia suite (known here as PACU), be put under, be transported up to MRI and then returned to PACU for whatever the other treatment was and then recovery. The anesthesiologist stayed with him the whole time."
- "The first few times Mary had a great deal of trouble staying still. She was frightened of the noise (it sounds like a loud moterboat or jackhammer) and she didn't like it. They wanted to sedate her but she promised to keep still (anything to avoid another needle). After a while (the last couple of years) she learned to calmly ask for the earplugs and go to sleep. They used to talk to her on a loudspeaker to tell her when the next set would begin, but she finally told them to knock it off, she was asleep.
- MRI's are such a time consuming effort, I always tried to schedule several things (CT, bone scan, hearing tests) in the same day. You have to plan to take the entire day off and just be with your child. They never seemed to go off without a hitch. If we were on time for an appt. (or early) we would have to wait. If we were running late, we would get scolded for messing up their "schedule" (like they ever stuck to it). Nothing metallic could be worn into the room with the MRI. Once I had a barrette in my hair with a metal clasp and when I got close to the machine, the magnet lifted the barrette off my scalp towards the machine. Mary couldn't wear overalls with buckles, shoes with eyelets, jewelry, her Mickey Mouse watch or her hat with the barrette attached. Everything was stored in a locked cubby outside the room. She would usually be given pillows, blankets, and a foam knee-rest to make lying there as comfortable as possible. She hated changing into a hospital gown, so we made sure that plain T-shirts and stretch pants were what she wore. Most technicians were pretty good about allowing her to wear her own clothes.
- She would stick earplugs in her ears, and after a while, the drone of the machine taking its pictures would put her to sleep. Most of the time her arms would be at her side, but once she put them behind her head. The images that we saw later on film with her arms up looked pretty funny. We also had to be careful that her central line was away from her body and not in the pictures."
- "When Jake has his MRI's he doesn't have to much problems as he's older and I was able to explain to him what was going to happen. They had headphones on him so he was able to talk to the MRI people and he could see us at the end of the machine through a mirror that reflected so he wasn't to scared. His MRI's are of the head and his only take about 20-30 mins."
Interesting sites on MRI.
- Everything you wanted to know, and more, is at a site by Joseph Hornak, at the Rochester Institute of Technology. He has essentially created a Hypertext on MRI. Tons of math for the interested.
- Your mri exam at Joseph Hornak's site. If you miss the rest of the site, at least go here for a discription of an exam.