Nuclear Spin Relaxation
In NMR, a strong magnetic field is used to partially polarize the nuclear spins. Taking protons as the most common example, the excess of proton spin in the direction of the magnetic field constitutes a small net magnetization of the material. To set up the conditions for the observation of an NMR signal, strong radio frequency radiation is applied to the sample at the appropriate frequency to produce "spin flips". From the quantum point of view, the RF photons are absorbed by some of the protons to flip them from parallel to the magnetic field to anti-parallel, a higher energy state.
When the exciting RF field is switched off, the protons tend to returned to their lower energy state. This "relaxation" back to a state where their spins are parallel to the static magnetic field produces a small amount of RF radiation which is detected as the NMR signal. Two different time constants for decay are typically observed.
Approximate T1 relaxation times
of various tissues for a field
strength of 1 Tesla. |
| Tissue | T1(ms) |
| Fat | 180 |
| Liver | 270 |
| Renal cortex | 360 |
| White matter | 390 |
| Spleen | 480 |
| Gray matter | 520 |
| Muscle | 600 |
| Renal medulla | 680 |
| Blood | 800 |
| Cerebrospinal fluid | 2000 |
| Water | 2500 |
| The longer of the two time constants is usually labeled T1 and is associated with the decay of the field component that is parallel to the applied static magnetic field B0. This field direction is usually taken to define the z-axis of the system. This time constant is sometimes called the longitudinal time constant. It is also called the spin-lattice relaxation time. Since the magnetic potential energy is proportional to the projection along this axis, a change in the magnetization along this axis involves the exchange of energy. This implies that the spin has interacted with its environment.
For application to magnetic resonance imaging of the body, the interaction is a good thing because it implies the capacity to differentiate between different types of tissue.
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Approximate T2 relaxation times
of various tissues for a field
strength of 1 Tesla. |
| Tissue | T2(ms) |
| Muscle | 40 |
| Liver | 50 |
| Renal cortex | 70 |
| Spleen | 80 |
| Fat | 90 |
| White matter | 90 |
| Gray matter | 100 |
| Renal medulla | 140 |
| Blood | 180 |
| Cerebrospinal fluid | 300 |
| Water | 2500 |
Data from Bushong
| The shorter two time constants is usually labeled T2 and is associated with the decay of the field component that is perpendicular or transverse to the applied static magnetic field B0. This time constant is sometimes called the transverse time constant. It is also called the spin-spin relaxation time. As shown in the table at left, this time constant also varies with the type of tissue.
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Index
Nuclear Spectra Concepts
References
Hobbie
Ch 17
Bushong
MRI |