MRI basics - CBIC, Amy Chan:
Brief description of MRI basics such as magnetism, T1 relaxation, and the T2 and T2* (T2 “star”) phenomena. CBIC Web Notes provided by Amy Chan, BS Copyright 2006.
Magnetism
Magnetism is a property of matter that is a result of the orbiting electrons in atoms. The nucleus of an atom contains some number of protons and neutrons, each of which intrinsically possesses angular momentum. The orbiting electrons cause the atoms to have a magnetic moment associated with an intrinsic angular momentum of a proton or neutron is called 'spin'. Magnetic field strengths are measured in units of gauss (G) and Tesla (T). One Tesla is equal to 10,000 gauss. The earth's magnetic field is about 0.5 gauss.
Nuclear Spin and Magnetic Moment
An important physical constant in MRI is the ratio of the magnetic moment to the spin, which is called the gyromagnetic ratio, γ. The gyromagnetic ratio, which is different for each nucleus, indicated the frequency with which a nucleus will precess around an applied magnetic field produced by an MRI magnet. This means that nuclei, such as hydrogen, will spin at a known frequency under a specific magnetic field.
Table above represents gyromagnetic ratio and spin of nuclei of interest in imaging.
Resonance and RF
Resonance is vibration with high amplitude when nuclei are excited (going from a low energy state to a higher energy state) by energy at a certain radio frequency (RF) electromagnetic wave. Protons in a magnetic field act like tiny toy tops that wobble as they spin. The rate of the wobbling or precession is the resonance or Larmor frequency.
Figure above shows precession of magnetic moment of the nucleus µ around an applied magnetic field B0 with an angular frequency of precession ω0.
Relaxation
Relaxation occurs at the stop of an RF wave when the nuclei relax back to the equilibrium state and release the absorbed energy back to the environment.
T1 Relaxation
T1 (Spin-Lattice or Longitudinal) relaxation time describes the rate that nuclear spins return to equilibrium.
 
Nuclei going from the high energy state to the low energy is associated with loss of energy to the surrounding nuclei. T1 relaxation is characterized by the longitudinal return of the net magnetization to its ground state of maximum length in the direction of the main magnetic field. The rate of return is an exponential process as is shown in the right figure above.
T2 Relaxation
T2 (Spin-Spin or Transverse) Relaxation is the time constant that describes the rate of signal decay.
 
T2 relaxation occurs when spins in the high and low energy state exchange energy but do not loose energy to the surrounding lattice. As nuclei release their excess energy, the magnetic moments interact with each other causing a decrease in the transverse magnetization or decay.
T2* Relaxation
T2* is the time it takes for the transverse magnetization to decay to 37% of its original magnitude. It is characterized by B0 inhomogeneity and loss of transverse magnetization at a rate greater than T2. It is caused by magnetic field inhomogeneity and occurs in all magnets.
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