29 February 2012

MRI stands for MAGNETIC resonance imaging


The most interesting text was in a New York Times story:







  1. They let me wear my shoes into the room where the MRI was located. I took them off without a problem before getting on the platform, but when I went to put them back on, one show was sucked into the machine.

  2. To freak people out even more, I could add that MRI is the layman-friendly version of the scientific term Nuclear Magnetic Resonance, which is what an MRI does. However, it was found too many people freak out over the 'nuculer' part of NMR, so it was dropped from the name.


    What MRI/NMR does is look at the magnetic behavior of atomic nuclei in a very quickly oscillating magnetic field. Because the atoms (and hence their nuclei) are stuck in molecules and the molecules are stuck in tissues, you can figure out information about the different surroundings of those atomic nuclei. An atom nucleus surrounded by a lot of hydrogen (water) behaves differently than an atom surrounded by a lot of carbon (fat). This is in the end what gives the MRI image.

    All in all the magnetic fields have to be very large because atomic nuclei are very small.

  3. The article is still mostly correct. MRI safety is extremely important and the accidents that happen today are rare (but still very deadly). Much money is spent on practical safety (labeling, metal detectors, education) and physical safety (modern machines actually have a secondary magentic field that helps cancel the main magnetic field away from inside the bore/central machine). If it weren't for this extra field, the effective radius of ferromagnetic attraction could be several hundred feet (as in, way past the waiting room). A few of points:

    - Pacemakers implanted over the last 10 years are safe for MRI from a magnetic standpoint, however, because shifting magnetic fields in the MRI can cause electric current (remember the right hand rule in high school physics), that current can disrupt the function of the pacer. If a patient is dependent on the pacemaking abilities of the pacer rather than the debrillating/AICD needs, then it is perfectly safe (though it still needs to be reset after the scan). In all practically, this is rarely done because both the cardiac needs of the patient and the ability to reset the pacer are not known or available at the time of scan.
    - having foil (medicine patches) or piercings are generally not dangerous because of ferromagnetic attraction, but because of that induced current (which burns).
    - a shoe should not fly toward the machine unless it has metal rivets, or some metal (e.g. a nail) stuck in the sole.
    - I'm not sure about the freaking about the name part, but the fundamental difference between NMR and MRI is that MRI uses magnetic gradients which are necessary to locate that hydrogen proton in space. NMR does not have spacial differentiation capacity nor does it create an image.

    Nach (radiology MD)

    1. Thank you, Nach. I'm always pleasantly surprised by the expertise of readers here.

    2. No worries. I've been lurking for a couple years. Keep up the good work.

  4. My eyelids are permanently "tattooed" because of iron oxide particles in the eyeshadow I was wearing when I got an MRI in the 80's. The skin on my eyelids burnt, then then peeled like sunburn, and the color remained. So happy it was brown and not blue!

  5. @ Nach: Interesting comments. I did not know about the second field. I have wondered why you can approach MRIs much closer than NMRs.

    On the difference between MRI and NMR. MRI got the "I" because the (3D) imaging is a special art in itself. A name difference is also in place because NMR is a much broader technique than MRI. MRI in the end images hydrogen (/water) concentration. NMR is much broader in the sense that it can look at the actual physical (and chemical) properties of every nucleus that has non-zero spin. IN other words, MRI uses a tiny sliver of the array of options that NMR offers, and then adds the imaging component, which - if I may add - is immensely cool.

    My comment was more intended to explain the dropping of the 'nuclear' part of the name. As far as I know that was really done because the public is scared of everything that is nuclear, because it associates the term 'nuclear' with atom bombs, ignoring that every atom has a nucleus.

    1. Definitely true. NMR has a lot of great capability. MRI tries to replicate some of that ability in MR spectroscopy. For example, with MRS, I can use peaks at ppm to find lactate or N-acetyl aspertate to help differentiate my patient's tumor from abscess (NMR) and to help find the borders of such lesions (precise location using imaging/MRI). But that's only a stepping stone. I always think that medicine technology is 10 years behind everyone else's technology.

  6. OK science journalists are trying to kill me:

    "They draw most of their power from supercooled helium [...]"

    This is not even remotely true. The supercooled helium is present to provide a very low temperature that allows for superconduction in the electromagnets.

    Superconduction allows for (1) the machines to use much less energy and (2) for the coils not to turn into molten metal due to the heat that would be generated from that much current passing through a normal wire with non-negligible resistance.

    The reason that the helium must be vented is not that it provides power (it doesn't). What is important to know is that under superconducting conditions a current flowing through a loop of wire will (almost) never stop. The current going round and round the coils in the electromagnet just keeps moving and since moving current always creates a magnetic field the magnetic field stays in place.

    Only when more typical temperatures (at least more typical for earth) are restored by venting the helium does electrical resistance kick in and stop the flow of current which in turn drops the magnetic field.

    Provides power indeed.... grrr...

    1. Yes - I baulked at that and would have made much the same comments as you if I hadn't been beaten to it! This is where Wikipedia scores - you can edit such nonsense out, but I suppose the New York Times is above such things.

    2. You're right, of course. I've modified the text as best I can for quoted material.

  7. While on the subject of Helium, I should point out that we're running out of He because the US government is not making Tritium anymore for atom bombs. Wow, great segway back to the scary atom bomb.


    While sad for balloons, the He shortage is wreaking havoc in the science community where people need He to keep supermagnets running.

  8. That's why people need to listen to the techNOLOGIST who are preforming their exam, answer the screening questions fully and honestly and believe us when we tell them that something is a safety hazard. We have studied this, been trained, passed our licensing exams and really, honestly, truly know what we're doing (Well, there's always that one bad apple). It's not because we're mean people and want to make your life difficult. We actually want to help you while keeping you safe. Also, (as a personal pet peeve, sorry) having an x-ray is completely different than an MRI. If I tell you that you don't have to remove your watch or belly button ring when I'm taking x-rays of your neck, I really mean it. I'm not just some idiot who wandered in off the street when no one was looking and started pushing a button.

  9. @ Anonymous: But you're not called a doctor, and hence you're education probably stopped after Kindergarten. I recently had to explain to a friend that going to an academic hospital and being treated by professors (and gasp! residents) is really better (for his rare symptoms) than by his rural (old) PCP because the residents have been recently educated with the newest knowledge, while his PCP is relying on education from decades ago. Very difficult argument to make.

  10. I work in medical research and have had my own little oddities with the MRI machine. As soon as I'm in the room with the MRI (well, really starting a few feet outside the room), I feel unsteady, cognitively "off" and my vision begins to form moving waves. I've always double check to make sure I don't have any metal on my person, but everytime I go in there I have to make sure I'm close to a wall to hang on to. I'm not in there long; usually just as the tech is administering the contrast. I asked multiple people in the dept if they've heard of this before, but no one can give me any insight. I know that people often feel side effects after they've actually HAD an MRI, but I've never heard of anyone being affected in such a manner simply by standing in the same space as the machine. Anyone know what gives?

  11. @ Anonymous: Based on the physics, there's not much that can cause that. Unfortunately, physics generally has no explanations of anything on the mind, since that's beyond the scope of physics.

    1. I object. Every particle in the brain obeys the laws of physics.

      Certainly there is a great deal of uncertainty at our current level of knowledge about how the physical changes on particles in the brain affect our inner mental experience. But studies of brain lesions and patients with traumatic brain injuries do indicate that physical damage to areas of particular brain regions do cause particular changes in brain function and subjective experience.

      Really there is no reason to believe that there is anything spooky going on in the brain that would be outside the scope of physics.


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