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I Estimating sound pressure (in an MRI scanner)

  1. Jan 19, 2017 #1
    Hello,

    excuse me if these happen to be basic questions, but I'm a psychologist/neuroscientist in training without any particular experience in physics, and I am just confronted with a very specific problem.

    I need to measure the sound pressure experienced inside of an MRI scanner. Unfortunately, due to the strong magnetic field (3T) and my lack of an MRI-compatible decibel meter, I can only estimate it with sound pressure measurements taken from a certain distance.

    So, how could I achieve this?

    I guess that the first part of the question (ignoring the fact that it's in an MRI scanner) can be framed as "How can I estimate the sound pressure at the source of the sound from one or several measurements made at certain distances from it?".
    What I have found by now is the formula for estimating changes in sound pressure using the Inverse Square Law, as described here: http://hyperphysics.phy-astr.gsu.edu/hbase/Acoustic/isprob2.html
    However, this doesn't work for my problem, because from what I gathered I will always arrive at estimations of Infinite dB at the sound source using this formula (what with dividing by zero and all). What am I doing wrong here?

    A second issue is the fact that the sound is generated by the tubular MRI scanner, and the noise level that interests me is the one in the center of the tube. I imagine that this complicates the issue further, as the sound probably is changed by being reflected by the walls of the tube before exiting the scanner. I guess to factor this in completely one would have to know all kinds of parameters concerning the scanner tube. However, I am completely content with a "quick and dirty" solution to this - All I really need is a sensible estimation of the maximum sound pressure I could expect inside there.

    Thank you very much!

    Best,
    Lukas
     
  2. jcsd
  3. Jan 19, 2017 #2

    A.T.

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    If you are specficially interested in the noise level within the tube, you should put one end of a plastic pipe into it and measure the noise that arrives at the other end. The falloff within the pipe can be determined using some other loud sound source of similar frequency, that doesn't produce magnetic fields (so you can place your device at both ends).
     
  4. Jan 19, 2017 #3

    berkeman

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    A.T. beat me to the sound tube suggestion. :smile:

    But the noises coming from the MRI machine are from the mechanical mechanisms, right? Does having the magnetic field turned off change the noises coming from the mechanisms? I wouldn't think so. Can the machine just be run in a test mode with the magnetic field turned off for your sound measurements?
     
  5. Jan 19, 2017 #4

    A.T.

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    I think the noise is partly from the coils, and does depend on the magnetic field.
     
  6. Jan 19, 2017 #5

    Nidum

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    Use a subjective method . Ask patients how loud the noise is . Give them a chart of standard noise levels and ask them to say which level is nearest match .

    Standard chart relates perceived sound levels to dBA noise level and to sound pressure .
     
  7. Jan 19, 2017 #6

    Dale

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    So it really depends on what your purpose is. For instance, if you are trying to make safety assessments then you really need an in-bore measurement. But if you just need to regress out a confounding factor in a fMRI experiment then your distant measurement should be fine.
     
  8. Jan 21, 2017 #7
    Wow, thank you all for your answers! :)

    That sounds like a good idea! Concerning the falloff: So the sound source I use does not have to produce the same sound pressure as is experienced inside the MRI tube? And how do I calculate the falloff then? Just as the difference in dB at the two ends? And will the falloff be independent of the actual sound pressure level of the source (that is, will the falloff I find with some other sound source be equal or similar to the falloff of the sounds of the MRI scanner, even if the two sources don't necessarily have the same pressure)?

    Yes, the noise comes from the vibrations of the coils, produced by the extremely quick changes in current, and thus magnetic field. I don't think that it would make a significant difference in the noise when you turn of the main magnetic field, However, you can't just turn it off like that! The main magnetic field of MRI scanners is always on (contrary to depictions in popular culture), and you can only turn it off by quenching the scanner, that is, releasing all of the liquid helium that cools it down... which comes with huge costs, possible damage to the scanner, and also quite some safety issues. Of course, you could measure the sound levels like that before a scanner goes online, or when it is decommissioned - but that is clearly not an option for me ;) .

    In fact, it is about a study with dogs, for which we have to record the scanner noise to (a) ensure that the dogs' health/hearing is not harmed by the scans, and (b) to then train the dogs to get accustomed to the noise before the actual scans. Regarding (a), many studies all over the world have already established that, with proper hearing protection etc. the scans we want to perform are not in any way harmful to the dogs. However, the ethics commission of our university is crazily petty about this, and will only accept an evaluation done with the exact same procedure inside the exact same scanner.
     
  9. Jan 21, 2017 #8

    Dale

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    Then you really need to get a measurement in the bore. The acoustic noise level is different in different locations within the bore, and is changed significantly by the presence of a subject.

    To get a measurement in the bore, you do not need a MRI compatible decibel meter, just a MRI compatible microphone whose cables you route outside the 5 gauss line. Here is a study explaining the methodology:

    http://onlinelibrary.wiley.com/doi/10.1002/1522-2586(200102)13:2<288::AID-JMRI1041>3.0.CO;2-P/full

    That is surprising. I have seen fMRI studies done at Auburn University using dogs, and I am sure there are many others from all over the world, as you say. The ones that I have seen even used awake unsedated dogs which were trained to voluntarily put their head in the bore and lie still for the scan.

    http://ocm.auburn.edu/newsroom/news...at-would-predict-a-dogs-detection-ability.htm
     
  10. Jan 21, 2017 #9

    berkeman

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    Yeah, doggie earmuffs seem to be the solution. Or noise cancelling earphones, if you want to be high-tech... :smile:
     
  11. Jan 22, 2017 #10
    Thanks for the papers, they're very helpful! And yeah, as I said, the ethics people are needlessly pedantic about this, and won't take evidence from other studies as proof that our experiment will not be harmful. But I guess I have to work with the ethics commission that is given to me :wink:

    Yeah, we already got earmuffs :smile: . We still need to know the sound pressure level inside the scanner though, to assess if the earmuffs will reduce it enough as to avoid harm!
     
  12. Jan 22, 2017 #11
    It is your study and your equipment, so being prudent doesn't hurt.
    Can it be said that the other studies have not measured the noise level prior.
     
  13. Jan 22, 2017 #12

    berkeman

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    Oh, and I just realized why you can't use sound-cancelling headphones. Duh. o0)
     
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