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Hydrogen atom spins in longitudinal direction Hydrogen atom spins in transverse direction Picture source: Lesics at t=189 and t=205.

I recently visited a friend who just had MRI scan. From his explanation, I have some question about magnetic power applied to him.

Consider a B magnetic field strength is applied to him when he was being had MRI diagnose, which the B is sum of the main magnetic field strength (Bo) and the gradient magnetic field strength (Bg) applied to a specific location his body which was being diagnosed (called slicing). B is in longitudinal direction. Bo and Bg constructively interfering. To flip the hydrogen atom from longitudinal to transverse then an RF pulse is applied so the hydrogen atom's orientation is now 90 degree (perpendicular) of its initial direction.

Then my question are:

  • What was the magnitude of the RF pulse's power (in B) applied to flip that hydrogen atom to transverse direction?
  • What kind of power from that RF pulse was involved during MRI diagnose? Because RF has magnetic field and electric field at the same time.
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  • $\begingroup$ Small nitpick - it should probably be phrased as "flip the proton/hydrogen nucleus" - MRI involves only the nucleus' spin state changing. $\endgroup$
    – AXensen
    Apr 6, 2023 at 11:13

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What was the magnitude of the RF pulse's power (in B) applied to flip that hydrogen atom to transverse direction?

That depends on the duration of the RF pulse. If we assume a $1\mathrm{\ ms}$ duration then it would be $6\mathrm{\ \mu T}$

What kind of power from that RF pulse was involved during MRI diagnose? Because RF has magnetic field and electric field at the same time.

The magnetic field produces the excitation of the patient’s spins. The electric field does exist, but it is kept as small as feasible to reduce tissue heating. Hence the antenna designs used in MRI are loop antennas rather than dipole antennas.

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  • $\begingroup$ The magnetic field produces the excitation of the patient’s spins. The electric field does exist, but it is kept as small as feasible to reduce tissue heating. Hence the antenna designs used in MRI are loop antennas rather than dipole antennas. Understand. Indeed, it is using loop rather than dipole antenna. By so, it is actually not correct to call it RF pulse as there is no electric field. it should be only magnetic pulse. $\endgroup$ Apr 6, 2023 at 12:06
  • $\begingroup$ That depends on the duration of the RF pulse. If we assume a 1 ms duration then it would be 6 μT. If I am right, from your comment, the magnetic field strength from the RF coil is not depends on the longitudinal magnetic field (B=Bo+Bg), but on the duration only. Is that really so? $\endgroup$ Apr 6, 2023 at 12:08
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    $\begingroup$ @AirCraftLover yes, the RF field strength depends only on the current in the RF coil, not the current in the main coil or the gradient coils. It is correctly called a RF coil. There is a RF E field (which produces SAR), it is just smaller than the B field, but it is present. $\endgroup$
    – Dale
    Apr 6, 2023 at 16:44
  • $\begingroup$ Thank you for your explanation. $\endgroup$ Apr 6, 2023 at 17:34
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    $\begingroup$ That works because even though it is small it is at the resonant frequency. So even that small field can gradually tip the magnetization. Energy transfer is very efficient at the resonant frequency $\endgroup$
    – Dale
    Apr 6, 2023 at 20:14

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