（医学类）磁共振波谱的基本原理及临床应用（一）

    MRS所需的信号的激发、空间定位、探测采集等技术上均与MRI类似，但其最终的表现形式不同。MRS是将按时间域（Time domain）分布的函数转换为按频率域（Frequency domain）分布的谱线。

•Same physical principles: Excitation, refocussing, relaxation mechanisms

•Difference: In imaging, the signal of hydrogen nuclei in water molecules is measured, because of its very high natural abundance. Two protons, fully equivalent to each other, give rise to one resonance frequency. In spectro, one wants to acquire signal of protons in molecules, other than water.

•Unique resonance frequency, to be explained later on in this presentation

    w0为拉莫尔进动频率，g为磁旋比，取决于质子本身的性质，对于某一个质子来说是常说。

    ω为角频率；f为线频率。

    This FID is a mixture of the various frequencies, transmitted by the tissue under investigation. A Fourier Transform is a mathematical technique that transforms the signal out of the time domain to a signal in the frequency domain. After FT, it’s possible to recognize the various frequencies that were present in the FID.

    Nucleus surrounded by electrons, causing the proton to experience a somewhat lower magnetic field. Configuration of electrons is different for the various molecules, so is the experienced B0, meaning that the same nucleus (1H for example) will have different resonance frequency, depending on molecular environment.

    Placing a molecule in a magnetic field causes its electrons to circulate. In circulating, these electrons generate secondary induced magnetic fields. Circulation of electrons around the proton itself always induces a field that opposes to the applied field. This process diminishes the local field experienced by the proton, and the proton is said to be shielded.

    Conversely, circulation of electrons around nearby nuclei may generate fields that may either oppose or reinforce the proton's local field. If the induced field reinforces the applied field, however, the local field experienced by the proton is augmented, and the proton is said to be deshielded. 

    Consider the water molecule H2O and the ethanol, CH3-CH2-OH. The hydrogen nuclei (protons) in the water have a different chemical shift than those in the ethanol. But even the protons in the CH3- group have a different chemical shift from those in the -CH2- group and again different from those in the -OH group.

  如果不进行水抑制的话，水的共振峰会掩盖其他浓度弱得多的分子中氢原子产生的信号。