MRI相关基础内容

Functional MRI (FMRI)
功能性磁共振成像（fMRI functional magnetic resonance imaging）是一种新兴的 神经影像学方式，其原理是利用磁振造影来测量神经元 活动所引发之血液动力的改变。由于fMRI的非侵入性、 没有辐射暴露问题与其较为广泛的应用，从1990年代开 始就在脑部功能定位领域占有一席之地。目前主要是运 用在研究人及动物的脑或脊髓。 广义的fMRI包括三类： （1）脑血流测定技术，包括注射造影剂、灌注加权和 BOLD效应成像。 （2）脑代谢测定技术，包括1H和31P的化学位移成像。 （3）神经纤维示踪技术，包括扩散张量和磁化学转移 成像。从时空分辨率、无侵入性和实用性等方面考虑，目 前应用最广泛的是BOLD效应的fMRI，也即通常的fMRI。
Only a small fraction, less than 0.5%, of the body’s tissues are naturally paramagnetic. Paramagnetic agents may be added to enhance relaxation and provide a mechanism for contrast enhancement. Thus, paramagnetic contrast agents are not imaged directly in MRI. Instead, the presence of the agent influences the appearance of the image.
MOTION SUPPRESSION TECHNIQUES
Various schemes have been proposed for suppressing motion artifacts（伪影） in MRI. Table 24-1 lists several of these techniques. Methods of motion suppression may be divided into three groups: techniques that suppress motion in the patient, those that make novel use of scan data to remove the effects of motion, and the use of faster pulsing schemes
BOLD效应：fMRI是基于神经元功能活动对局部氧耗量和 脑血流影响程度不匹配所导致的局部磁场性质变化的原理。 血红蛋白包括含氧血红蛋白和去氧血红蛋白，两种血红蛋 白以磁场有完全不同的影响。含氧合血红蛋白是抗磁性物 质，对质子弛豫没有影响。去氧血红蛋白属顺磁物质，可 产生横向磁化弛豫时间（T2）缩短效应（perferential T2 proton relaxation effect,PT2PRE）。因此，当去氧血红 蛋白含量增加时，T2加权像信号减低。当神经兴奋时， 电活动引起脑血流量显著增加，同时氧的消耗量也增加， 但增加幅度较低，其综合效应是局部血液氧含量的增加， 去氧血红蛋白的含量减低，削弱了PT2PRE， T2加权像 信号增强。总之，神经元兴奋能引起局部T2加权像信号 增强，反过来就是T2加权像信号能反映局部神经元的活 动。这即BOLD效应（blood oxygenation level dependent）。早期的fMRI是单纯利用神经元活动的血流 增强效应，利用注射顺磁造影剂的方法来实现的，后来随 成像技术的发展，才形成的BOLD。
Suppression of Motion of Patient抑制病人运动
Methods such as patient restraint, sedation（镇静）, acquisition during breath holding, and gated acquisition are obvious extensions of techniques that have had some success in other imaging applications. Ultimately, breath holding is practical for most patients only if scan times are reduced to the order of seconds.
Gd-DTPA
ferromagnetic
Ferromagnetic materials develop magnetic polarization (i.e., become magnetic) in the presence of an applied magnetic field. Ferromagnetism is a phenomenon in which hundreds to thousands of atoms having unpaired electrons group together into “domains.” If many domains are aligned, then the material is said to be magnetized. If the alignment persists in the absence of an applied magnetic field, then the material is a permanent magnet. Because a large grouping of atoms is required for ferromagnetism, ferromagnetic contrast agents do not exist as solutions of metal ions. They are given in the form of compounds such as iron oxides (Fe2O3, Fe3O4). These agents selectively decrease T2 and cause a decrease in MR signal in conventional pulse sequences. Thus ferromagnetic MR contrast agents provide “negative” contrast.
Fast Scanning Techniques快速扫描技术
gradient echo
Half-Fourier Technique
There is symmetry in the data produced by repeated application of the phase-encoding gradient (the phaseencoding “steps” of signal acquisition). In theory, it is necessary to acquire only half of the steps, say those produced with a negative gradient, with the other half, the positive gradient steps, reproduced mathematically from those acquired.
Paramagnetic Paramagnetic contrast agents contain unpaired electrons. They develop their own magnetic fields by tumbling in solution at some fraction of the Larmor frequency. The tumbling creates alternating magnetic fields that affect precessing nuclei. An interaction between the unpaired electrons and the precessing nuclei increases the rate of relaxation of neighboring protons, and the presence of variations in the magnetic field introduced by the paramagnetic agent increases dephasing. Thus, both T1 and T2 are shortened by the presence of a paramagnetic agent.
A series of functional magnetic resonance (FMRI) images taken during a study of learning ability and its relation to short term memory activity. In the first image, the subject is given a task that involves motor skills and the visual system (to press a key in response to the appearance of an object on a view screen). In the second, the subject is told to remember a sequence as different objects are shown on a screen. The third image is obtained by subtracting the previous two under the hypothesis that it represents what was common to both tasks. Although not definitive in itself, this study is one of a number of novel approaches to new areas of study in understanding brain function that has been made possible with FMRI.