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Mz Recovers Via
T1 Relaxation
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The process of giving off RF energy occurs as the spins go from a high energy state to a low energy state, realigning with Bø. The RF emission is the net result of the Z component (Mz) of the magnetization recovering back to Mø. Not all of the energy given off is detectable as an RF pulse. Some of the energy goes to heating up the surrounding tissue, referred to as the lattice. In a global, or rather, universal sense, this system can be divided into the spins, and the rest of the universe, or a very large lattice. This type of spin-lattice interaction is the result of the excited system returning to thermal equilibrium. In the classical description, the Mz component begins to grow at the expense of the Mxy component.
 Spin-Lattice Relaxation: The process whereby energy absorbed by the excited protons or spins is released back into the surrounding lattice, reestablishing thermal equilibrium. |
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T1 Recovery Curve
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Mz = Mø * ( 1 - e-t/T1 )
| -t/T1 |
Mz |
| -1 |
0.632 |
| -2 |
0.865 |
| -3 |
0.950 |
| -4 |
0.982 |
| -5 |
0.993 |
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The time course whereby the system returns to thermal equilibrium, or Mz grows to Mø, is mathematically described by an exponential curve. This recovery rate is characterized by the time constant T1, which is unique to every tissue. As will be discussed in detail later, this uniqueness in Mz recovery rates is what enables MRI to differentiate between different types of tissue. At a time t=T1 after the excitation pulse, 63.2% of the magnetization has recovered alignment with Bø.
T1 Relaxation: Spin-Lattice relaxation. The exponential recovery of longitudinal (aligned with Bø) magnetization. Mz returns to Mø.
 In general, T1 values are longer at higher field strengths. |
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