Correcting Misconceptions about the Frequency of RF Pulses in MRI

In summary, the magnetic moment of protons precesses at a frequency ω0=γB0 about the static magnetic field. To move the magnetisation vector into the transverse plane, an RF pulse is applied at a frequency ω0. Despite the fact that the B1 field strength of the RF pulse is much smaller than B0, this is possible due to the resonant system storing energy over many cycles. There are two important motions at play: precession, driven by B0 at a high frequency, and nutation, driven by B1 at a lower frequency. This is because B0 is in the Tesla range while B1 is in the micro Tesla range. The initial confusion was due to a misunderstanding of
  • #1
BobP
74
1
The magnetic moment of protons precesses at a frequency ω0=γB0 about the static magnetic field.

In order to move the magnetisation vector into the transverse plane the RF pulse must be applied at a frequency
ω0. However, as B1 (the field strength of the RF pulse) is << B1 how is this possible (as ω0=γB0 >> ω1=γB1)

Does this makes sense. Please can someone explain where I am going wrong
Thanks
 
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  • #2
I presume that, as with any resonant system, a small disturbance at the resonant frequency will add a little energy each cycle until the cumulative effect is large. The resonant system will store energy from the RF field over many cycles.
 
  • #3
BobP said:
The magnetic moment of protons precesses at a frequency ω0=γB0 about the static magnetic field.

In order to move the magnetisation vector into the transverse plane the RF pulse must be applied at a frequency
ω0. However, as B1 (the field strength of the RF pulse) is << B1 how is this possible (as ω0=γB0 >> ω1=γB1)

Does this makes sense. Please can someone explain where I am going wrong
Thanks
I am not sure what you are asking. There are two motions that are important here. One is the precession, that is driven by B0 and so by the Larmor equation it is typically in the 100 MHz range. The other is nutation, that is driven by B1 and is typically in the 100 Hz range. This corresponds to the fact that the B0 field is in the Tesla range while the B1 field is in the micro Tesla range.
 
  • #4
Dale said:
I am not sure what you are asking. There are two motions that are important here. One is the precession, that is driven by B0 and so by the Larmor equation it is typically in the 100 MHz range. The other is nutation, that is driven by B1 and is typically in the 100 Hz range. This corresponds to the fact that the B0 field is in the Tesla range while the B1 field is in the micro Tesla range.
My question was based on a misunderstanding of the difference between frequency of the RF pulse and Larmor frequency.
I foolishky assumed they were the same which is why I was confused. However problem is now fixed :)

Thanks for your help though :)
 

1. What is the role of RF pulses in MRI?

The role of RF pulses in MRI is to excite the protons in the body, which generates a signal that is used to create images. These pulses are emitted at a specific frequency and duration to produce the desired image contrast.

2. How is the frequency of RF pulses determined in MRI?

The frequency of RF pulses is determined by the strength of the magnetic field used in the MRI machine. This field strength is typically measured in units of Tesla (T) and can range from 0.2T to 7T, with higher field strengths resulting in higher frequency RF pulses.

3. What is the relationship between RF pulse frequency and image resolution in MRI?

The frequency of RF pulses has a direct impact on the image resolution in MRI. Higher frequency pulses can produce images with higher resolution, as they are able to excite protons more precisely and provide more detailed information about the tissues being imaged.

4. How does the frequency of RF pulses affect the image contrast in MRI?

The frequency of RF pulses can also affect the image contrast in MRI. Different tissues have different resonance frequencies, which can be manipulated by changing the frequency of the RF pulses. This allows for the creation of images with varying levels of contrast, aiding in the diagnosis of different types of tissue abnormalities.

5. Are there any safety concerns related to the frequency of RF pulses in MRI?

Yes, there are safety concerns related to the frequency of RF pulses in MRI. High frequency RF pulses can produce heat in the tissues being imaged, which can cause discomfort or even tissue damage. Therefore, MRI machines are designed to limit the amount of RF energy emitted to ensure patient safety.

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