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MRI RECIT

magnet on/off, RF on/off

TermDefinition
1. MAGNET OFF Before exposure to the main magnetic field B₀, the hydrogen protons in the body are randomly oriented.
2. MAGNET OFF Because hydrogen has a primary spin due to its odd mass number, each proton has a magnetic moment, meaning each behaves like a tiny magnet.
3. MAGNET OFF However, since these spins are randomly distributed, their magnetic moments cancel each other out, resulting in no net magnetization and no polarization.
4. MAGNET OFF At this point, M equals zero, Mz equals zero, and Mxy equals zero.
5. MAGNET OFF There is no established magnetic dipole and no MRI signal produced.
6. MAGNET OFF The patient is not a magnet because without spin alignment, there is no measurable magnetization.”
1. MAGNET ON When the patient is placed inside the MRI scanner, the external magnetic field B₀, measured in tesla, is applied.
2. MAGNET ON The hydrogen protons undergo alignment, where more protons align parallel rather than antiparallel to B₀.
3. MAGNET ON This imbalance causes polarization, establishing a longitudinal net magnetization vector, or Mz, parallel to B₀ and the z-axis.
4. MAGNET ON As alignment occurs, the magnetic moments begin precession, which is the wobbling motion of spinning protons around B₀, at a characteristic rate called the Larmor frequency.
5. MAGNET ON At equilibrium, M equals one, Mz equals one, and Mxy equals zero.
6. MAGNET ON Although there is signal present along the z-axis, it cannot be measured because it is parallel to B₀.
7. MAGNET ON At this stage, the patient becomes a magnet due to the net magnetization but only temporary.
1. RF ON When the frequency of the external RF pulse equals the Larmor frequency of the nucleus, the spin system absorbs energy, leading to resonance.
2. RF ON This RF pulse has a whip-like action that causes the protons to precess in-phase, a condition known as phase coherence, which is necessary to generate a measurable MRI signal.
3. RF ON The absorbed energy flips the longitudinal magnetization vector Mz by 90 degrees into the transverse plane, making it perpendicular to B₀.
4. RF ON This produces transverse magnetization, or Mxy, which can now be detected by the receiver coil.
5. RF ON At this point, M equals one, Mz equals zero, and Mxy equals one.
6. RF ON The protons are in an excited state, with an increased number of high-energy spin-down nuclei.”
1. RF OFF After the RF pulse is turned off, the protons undergo relaxation, which is the process of returning to equilibrium.
2. RF OFF Along the z-axis, spins recover through T1 relaxation, restoring longitudinal magnetization.
3. RF OFF Simultaneously, in the transverse plane or xy axes, spins lose phase coherence or goes through dephase/out-f-phase through T2 relaxation, causing transverse magnetization to decay.
4. RF OFF As relaxation progresses, the value of M is equals one, Mz equals one, and Mxy equals zero.
5. RF OFF During this decay, a signal known as Free Induction Decay (FID) is released and measured by the receiver RF coil.
Created by: yulyae
 

 



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