Nuclear Spin Relaxation

核自旋弛豫

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.

在核磁共振（NMR）中，使用强磁场部分极化核自旋。以质子为例，磁场方向上的自旋过剩构成了材料的微小净磁化。为了观察NMR信号，需在适当频率下对样品施加强射频辐射以产生'自旋翻转'。从量子观点来看，射频光子被部分质子吸收，使其自旋从平行于磁场的方向翻转到反平行方向，进入更高能量状态。

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.

当激发的射频磁场关闭后，质子倾向于回到较低的能量状态。这种‘弛豫’回到自旋与静态磁场方向一致的状态会产生一小部分射频辐射，该辐射被检测为NMR信号。通常会观察到两个不同的衰减时间常数。

Approximate T₁ relaxation times of various tissues for a field strength of 1 Tesla. 1特斯拉场强下各种组织的近似T1弛豫时间
Tissue 组织	T₁(ms)
Fat 肥	180
Liver 肝脏	270
Renal cortex 肾皮质	360
White matter 白质	390
Spleen 脾	480
Gray matter 灰质	520
Muscle 肌肉	600
Renal medulla 肾髓质	680
Blood 血液	800
Cerebrospinal fluid 脑脊液	2000
Water 水	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.

两个时间常数中较长的那个通常标记为T1，并与平行于所施加的静态磁场B0的场分量的衰减相关联。这个方向通常用来定义系统的z轴。这个时间常数有时也被称为纵向时间常数。它也被称为自旋-晶格弛豫时间。由于磁势能与沿此轴的投影成正比，沿此轴方向的磁化变化涉及能量的交换。这表明自旋已与其环境发生了相互作用。

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.

对于应用在人体磁共振成像中，这种相互作用是好事，因为它表明具有区分不同种类组织的能力。

Approximate T₂ relaxation times of various tissues for a field strength of 1 Tesla. 1特斯拉场强下各种组织的近似T2弛豫时间
Tissue 组织	T₂(ms) T² (ms)
Muscle 肌肉	40
Liver 肝脏	50
Renal cortex 肾皮质	70
Spleen 脾	80
Fat 肥	90
White matter 白质	90
Gray matter 灰质	100
Renal medulla 肾髓质	140
Blood 血液	180
Cerebrospinal fluid 脑脊液	300
Water 水	2500

Data from Bushong

Bushong 提供的数据

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.

较短的两个时间常数通常标记为T₂，并与垂直于或横向于所施加的静态磁场B₀的场分量的衰减有关。这个时间常数有时也被称为横向时间常数。它也被称为自旋-自旋弛豫时间。如左表所示，这个时间常数也随着组织类型而变化。