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Electromagnetic radiation / mri question

  1. Sep 24, 2005 #1
    In reading about magnetic resonance imaging, I was surprised to learn that the coil/antenna used to send a radiofrequency pulse into the person being scanned is not a coil/antenna at one end of the person, but a large coil actually wrapped around the person.

    I know how alternating current through a coil can generate EM radiation spreading outward from the coil, but how does a coil generate EM radiation ie. a radiofrequency pulse within the coil?

    Am I confused about some part of this?

    Thanks for your help.
  2. jcsd
  3. Sep 24, 2005 #2
    MRI has little to do with EM radiation. It works with and detects magnetic fields. Alternative magnetic field is used to exicite the internal spin transition, and a big coil is used to detect them as a change of the magnetic field in a body. So MRI does not rely on EM waves to probe the body, it relies on the detection of the magnetic field.
    BTW, there is no proplem to have EM radiation inside the coil. In fact, it exist, but if coil size is smaller then the wavelength, it can be neglected. This may be a problem for big coils, but it is rather unwanted effect.
  4. Sep 24, 2005 #3
    The receiving half of an MRI unit is esentially one very sensitive radio receiver and probably has components that would be familiar to any ham radio operator or audiophile geek.

    There are in fact several types coils used in an MRI system that transmit and receive radio waves (some coils do both). Gradient coils are responsible for generating the spatially varying magnetic fields that help to localize the RF signal coming from the patient. A large body coil, typically integrated into the bore of the magnet is used to transmit and receive RF signals and surrounds the patient. Other 'coils', such as head, extremity, shoulder, other types of body coils are detachable and usually placed on, near or around the part being imaged. The head coil (sometimes called a bird cage for its appearance) is positioned around the head. Shoulder coils are usually flexible and placed on the shoulder, rather than around.

    The coils are just that, windings of wire to form an antenna. Coils come in fairly sophisticated designs to optimize their sensitivity patterns and the way they receive signals.

    This is incorrect. MRI has everything to do with EM radiation. MRI relies on detecting radiofrequency waves being emitted from protons as they realign with the main magnetic field. The magnet is only used to align protons in one direction or another. Radio waves at the resonant frequency of the protons are used to push the proton spins out of alignment with the main magnetic field (known as flipping). The duration of the flipping pulse determines how much the proton spins are flipped (usually 90° or 80°). When the RF pulse is turned off, the proton spins gradually realign with the main magnetic field, and emit radio waves in the process. These radio waves are detected and used to form the image.
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