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MRI excitation time vs relaxation time

  1. Jun 5, 2013 #1
    So.. if you apply a fluctuating magnetic field to an atom at its correct resonance frequency, its pole will go out of alignment of the applied field. How long does it take for the proton/electron to go 90 degrees out of the applied field vs how long it takes to return
  2. jcsd
  3. Jun 5, 2013 #2


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    It is unclear from your post how much of NMR you "get" and how much is confusion.
    1. The phrase "fluctuating magnetic field" is an odd one that I've never seen used for NMR. The applied time-varying field B1 is at radio frequency (RF), must be circularly polarized, and must be oriented at right angles to the static B0 field.
    2. "to an atom"--do you realize that only those few atoms with the correct nuclear spin can be used for NMR?
    3. "applied field"--I hope you are referring to the B0 field here.
    4. You can't do NMR on electrons.
    5. The transit time is determined by the strength of the applied B1 field. The return is governed by the T1 and T2 relaxation rates.
    I suggest that you read a basic text on MRI or NMR to get these and other concepts straight.
  4. Jun 5, 2013 #3
    I'm pretty sure I get the basics of NMR, but I'm trying to learn, so please be patient.

    From what I understand, when an individual is placed in an MRI machine, the nuclei of atoms in his or her body with an odd number of protons align with the B0 field. My understanding is that these nuclei "spin" at a speed that is related to how strong the B0 field is. If the fluctuating B1 field matches this spin speed, the magnetic poles of a small percent of these atoms spins at an increasing angle away from the axis parallel to the B0 field. I didn't know the B1 field NEEDS to be circularly polarized, but that makes sense. Once the B1 field is turned off, the nuclei of the atoms snap back and release a pulsating field relating to how they "relax." Electrons are the target particle in ESR and not NMR.. so I guess I was generalizing when I mentioned electrons.

    Do I understand it alright or am I completely off?

    My question is how fast a typical "transit time" is compared to the "returning" time. I'm not a physicist, but I'm an inventor and I only want to fill my head with the basics so sorry if I sound dumb! I'm new to this.
  5. Jun 6, 2013 #4
    And by transit time do you mean excitation time? I don't know terms.
  6. Jun 6, 2013 #5


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    The correct term is "precess," and they do it only if excited. When sitting in a static B0 field, the spins are simply aligned or anti-aligned with B0 (in the simple classical picture).
    Please use the term "oscillatory" or "rotating", because the word "fluctuations" typically refers to random variations.
    No, again spin is an intrinsic property of the nucleus. You mean that the B1 frequency matches the Larmor or precession frequency.
    Yes, only the CP component matching the precession sense will couple to the spin system.
    They don't snap, but rather relax exponentially
    not exactly, they don't release anything. They induce in the pickup coil an oscillating EMF at the Larmor frequency that is exponentially damped according to the relaxation rates.
    As I mentioned, the transit or excitation time is proportional to the strength of the applied B1 field. 1 ms is a typical value used in MRI. Relaxation times depend entirely on the material being examined. In MRI, hydrogen atoms (protons), primarily in water, are imaged. Purified water has a relaxation time of many seconds, and cerebral spinal fluid is also very long. Water in bones is surrounded by a solid matrix with strong dissipation mechanisms, so relaxation is very short. I forget actual values but you can look them up easily.
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