Just had an MRI. The machine went through at least five different sequences, for lack of a more technical term. By this, I mean the machine produced 5 sets of disctinct patterns of buzzing noises. Each set lasted about 5 minutes. What do these patterns mean, or I am hearing the electromagnets being pulsed in different cycles?
How does all this form an image? Is there some similarity here to how a microwave resonates the water molecules in food? Could an MRI cook you, if the machine were improperly used?
bensonGemini’s link will give you more details, but to answer some of your questions:
(1) MRI is basically the same technique which, in analytical chemistry, we call NMR (Nuclear Magnetic Resonance). NMR and MRI use magnetic fields to resonate and manipulate the magnetic field associated with protons in atomic nuclei. From this one can “map” the arrangements atomic nuclei in a molecule to determine its structure. Basic techniques will show which atoms are attached to one another. More complex NMR techniques can map structures over several bond lengths, as well as intra-molecular (3D) structures. The most common types of NMR resonate the protons of hydrogen atoms or carbon-13 atoms.
Perhaps to avoid giving patients the mistaken impression that nuclear radiation is somehow involved, the technique is called MRI for medical purposes.
(2) MRI uses magnetic fields to resonate/manipulate the nuclei of the water molecules in your cells, to produce essentially a water-density map of your body and organs. This is completely different from the microwave, which uses electromagnetic radiation to vibrate the molecules and heat them. So no, you can’t be “cooked” in an MRI.
I would guess that it is resonating the protons of the hydrogen atoms in water molecules, but I suppose it might involve the protons in the oxygen molecules (of water) – or both.
While it is true that there is no radiation used in an MRI, still if you are vibrating molecules with some form of sympathetic frequency, you are adding energy. This energy would be dissipated as heat energy. However, I WAG, the added energy is small enough not to be a concern, of course.
Could this be why you feel so very warm during an MRI? And/or, is it heat off the electromagnet(s) around you? Regarding my other question, I’ll have to check out that link (provided above), but until then (in case it is not mentioned)…what is the function of the pulses of loud noises heard during an MRI?
"The machine makes a tremendous amount of noise during a scan. The noise sounds like a continual, rapid hammering. Patients are given earplugs or stereo headphones to muffle the noise (in most MRI centers you can even bring your own cassette or CD to listen to). The noise is due to the rising electrical current in the wires of the gradient magnets being opposed by the main magnetic field. The stronger the main field, the louder the gradient noise. "
An MRI is just Nuclear Magnetic Resonance. A nucleous with an non-zero spin That just means only certain isotopes can be used.) has two equivalent spin states. When a magnetic feild is applied these spin states become non-equivalent, making one spin state more energetically favorable. (This magnetic feild is why you can’t take metal objects into an MRI.)
The energy difference between the two states will be descreet, and if an electromagnetic wave of the right frequency is absorbed the nucleous will go into the exited state. Through a round about method it is effectively the quantity of the EM absorbtion that is recorded. (It is the emmision that is actually recorded.)
I would guess MRI’s are strictly designed to analyze H1 atoms. The relative abundancies of O17 and C13 would make the process prohibitively time consuming.
That said, an MRI could analize for many different kinds of hydrogens. The MRI can distinguish between a hydrogen on a water molecule and a hydrogen on a fat molecule, because each of these protons will exist in a different electronic environment.
Jinx, I have never had an MRI myself, so I can’t speak to the feeling of warmth, but I would guess that it is, indeed, heat dissipation from the electromagnets themselves. The amount of energy involved in manipulating the spin-states of protons (as Christopher describes above) is quite small. In any case, nuclear particles don’t dissipate this excess energy primarily as “heat” (infra-red) energy: what they do is change spin states. A tiny amount of heat dissipation might accompany this process, but not nearly enough to affect the molecule’s vibrational and translational energy states-- that is, how “hot” it is.
Or to put it another way: I am not aware that we have ever thermally decomposed a molecule by doing an NMR analysis of it.