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Anonymouse
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I'm reading about the theory behind MRI, specifically from Farr's Physics for Medical Imaging, 2nd Edition, by Allisy-Roberts and Williams.
According to this book, the spins of the free protons in a sample line up, either parallel or antiparallel, with the externally applied magnetic field. In the case of a 1T field, there will be an excess of approximately 3 in every 1000000 protons aligned in the lower energy state parallel with the field, giving rise to a net magnetism Mz (where z represents the axis of the external field).
For MRI, a signal is generated by a set of RF coils that inject an RF pulse in a direction perpendicular to z. The book explains that this has the effect that some or all of the spin-up protons pick up energy, turn spin down, affecting Mz, which may be reduced, disappear or even reverse.
In particular, the book explains that for a so-called "180° pulse", an RF pulse of a certain total energy will give to each and every dipole exactly the energy required to tip them through 180°. This temporarily reverses the net magnetic vector Mz.
My question is, as far as I understand, if an RF pulse at the resonant frequency is directed towards the sample, then the chance of flipping a proton from the low to high energy state also applies to flipping a proton from the high to low energy state. Therefore, the best that could be achieved is a 50% equilibrium between the high and low energy states, giving Mz a minimum value of 0 (i.e. it cannot be negative).
How is it that "each and every dipole" will be flipped through 180°, thus reversing Mz? I think the dipoles will be flipped at random, and that Mz will be reduced, but cannot be reversed.
Am I misunderstanding something?
According to this book, the spins of the free protons in a sample line up, either parallel or antiparallel, with the externally applied magnetic field. In the case of a 1T field, there will be an excess of approximately 3 in every 1000000 protons aligned in the lower energy state parallel with the field, giving rise to a net magnetism Mz (where z represents the axis of the external field).
For MRI, a signal is generated by a set of RF coils that inject an RF pulse in a direction perpendicular to z. The book explains that this has the effect that some or all of the spin-up protons pick up energy, turn spin down, affecting Mz, which may be reduced, disappear or even reverse.
In particular, the book explains that for a so-called "180° pulse", an RF pulse of a certain total energy will give to each and every dipole exactly the energy required to tip them through 180°. This temporarily reverses the net magnetic vector Mz.
My question is, as far as I understand, if an RF pulse at the resonant frequency is directed towards the sample, then the chance of flipping a proton from the low to high energy state also applies to flipping a proton from the high to low energy state. Therefore, the best that could be achieved is a 50% equilibrium between the high and low energy states, giving Mz a minimum value of 0 (i.e. it cannot be negative).
How is it that "each and every dipole" will be flipped through 180°, thus reversing Mz? I think the dipoles will be flipped at random, and that Mz will be reduced, but cannot be reversed.
Am I misunderstanding something?