神经系统影像学入门, Keith A. Johnson, M.D., 哈佛医学院

 Neuroimaging Primer, Keith A. Johnson, M.D., Harvard Medical School

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磁共振信号源 MR signal sources

可参看信号强度列表 See also signal intensity table.

• 将质子置于磁场中时，它们就会振动。
• When protons are placed in a magnetic field, they oscillate.
• 它们振荡时的频率取决于所在磁场的强度。
• The frequency at which they oscillate depends on the strength of the magnetic field.
• 如用与质子振动频率相同的电磁波激发它时，质子就能够吸收能量。之后，质子再次将能量释放以回到初始时的平衡状态。质子在返回初始状态时所重新辐射或释放的能量就是我们所观察的磁共振信号。
• Protons are capable of absorbing energy if exposed to electromagnetic energy at the frequency of oscillation. After they absorb energy, the nuclei release or reradiate this energy so that they return to their initial state of equilibrium. This reradiation or transmission of energy by the nuclei as they return to their initial state is what is observed as the MRI signal.
• 质子回到初始时的平衡状态不是瞬时发生得，而是需要持续一段时间。
• The return of the nuclei to their equilibrium state does not take place instantaneously, but rather takes place over some time.
• 质子回到初始时的平衡状态的过程由两个过程控制：
• The return of the nuclei to their initial state is governed by two physical processes:
  • 弛豫到与磁场平行的核磁化平衡状态，接着
  • the relaxation back to equilibrium of the component of the nuclear magnetization which is parallel to the magnetic field, and
  • 再弛豫到与磁场垂直的核磁化平衡状态。
  • the relaxation back to equilibrium of the component of the nuclear magnetization which is perpendicular to the magnetic field.
• 质子回到初始时的平衡状态的过程中的两次弛豫时间相当于：

• The time that it takes for these two relaxation processes to take place is roughly equal to:

  • 第一过程相当于T1时间 time T1 for the first process, and
  • 第二过程相当于T2时间 time T2 for the second process.
• 磁共振成像中信号的强弱主要取决于三个因素：

• The strength of the MRI signal depends primarily on three parameters.

  • 组织中所含质子的密度：质子密度越大，信号越强。Density of protons in a tissue: The greater the density of protons, the larger the signal will be.
  • T1 
  • T2
• 影像中比较不同的脑组织也依赖于组织中这三个因素的不同。
• In the image,contrast different brain tissue also depended on the difference of the three parameters in the tissue.
• 对于身体中大多数的“软”组织，由于质子密度非常均一，在影像上没有方法来区别这些组织的信号。
• For most "soft" tissues in the body, the proton density is very homogeneous and therefore does not contribute in a major way to signal differences seen in a image.
• 但是对于不同的软组织，T1和T2 却是不相同的。所以，在主要针对软组织进行比较时，根据T1和T2是可以分辨出不同的。
• However, T1 and T2 can be dramatically different for different soft tissues, and these parameters are resonsible for the major contrast between soft tissues.
• 组织的黏滞性和硬度对T1和T2影响很大。一般来说，组织的黏滞性和硬度越高，T1和T2的值越小。
• T1 and T2 are strongly influenced by the viscosity or rigidity of a tissue. Generally speaking, the greater the viscosity and rigidity, the smaller the value for T1 and T2.
• 通过改变支配质子的的电磁能大小的方法来调控磁共振的信号是可行得。这种调控是通过改变质子密度，T2和T2三种因素对所观察的信号的作用来实现得。因此有多种磁共振成像的技术（加权）可供选择以加强某些特性或削减其它的特性。

• It is possible to manipulate the MR signal by changing the way in which the nuclei are initially subjected to electromagnetic energy. This manipulation can change the dependence of the observed signal on the three parameters: proton density, T1 and T2. Hence, one has a number of different MR imaging techniques ("weightings") to choose from, which accentuate some properties and not others.

  撰稿： Sam Patz, Ph.D.     Contributed by Sam Patz, Ph.D.

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