New Imaging Technique Reveals Structural Changes In Tourette's

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Magnetization Transfer Imaging（磁化传递成像）是与扩散张量成像等价的技术，用于成像白质。介绍一下两个技术的原理

DTI和megnetization transfer技术的原理是什么？

弥散张量成像（diffusiontensorimaging，DTI）是反映水分子随机运动的一种成像方法，可提供关于组织微观结构和排列状态的信息，主要定量指标包括平均弥散率（mean diffusity，MD）反映组织弥散速度和部分各向异性（fractional anisotropy，FA）反映组织内弥散方向。在中枢神经系统,水分子弥散受细胞膜、细胞器和髓鞘等组织微观构筑的影响，凡是能影响组织完整性和排列方向的病变，均可引起MD和FA值的变化。MS的主要病理特点是炎性反应，髓鞘脱失，轴突变性和胶质增生，其中髓鞘脱失和轴突变性均可以严重影响脑组织的完整性和排列方向，从而引起MD和FA值的变化。

磁化传递成像（magnetization transfer imaging，MTI）是基于大分子结合水质子与自由水质子之间存在相互作用，在距水的中心频率1.5Hz附近施加一个饱和脉冲，可以选择性饱和大分子结合的水质子，这种饱和效应被传递到自由水质子，从而降低了MR的信号强度。磁化传递率（MT ratio，MTR）是衡量磁化传递效应的指标，通过在打开和关闭饱和脉冲时分别采集数据，根据公式MTR=(M0-Ms)/M0X100%，计算出 MTR值，M0和Ms分别为关闭和打开饱和脉冲时的信号强度，再通过后处理计算出脑内每个体素的MTR，并绘制出磁化传递率直方图（MTR histogram），常用参数有平均MTR峰高及峰位置等。中枢神经系统中自由水质子对应于组织中的水质子，而结合水质子对应于髓鞘和其他细胞膜大分子中的质子，信号丢失的程度取决于给定组织中大分子的密度。因此，MTR降低提示组织内大分子含量的减少，反映了髓鞘的丢失或轴索密度的减少。MS病灶 MTR的演变具有规律性，新鲜病灶MTR明显下降，随后的1-6个月逐渐部分或完全恢复正常，其原因在于急性期轴索保存较好，脱髓鞘和水肿引起新鲜病灶 MTR下降，而髓鞘再生和水肿消退是MTR随后恢复的病理基础。


结论：因为MTI能够降低MRI信号强度，因此理论上可以推测对于MTI技术来说，较小的场强也能够达到DTI的显示效果。所以该技术可以为医院配备的较小场强的（0.5T）MRI机器提供脑白质损伤以及连接情况的分析。


ScienceDaily (May 11, 2009) — Magnetization Transfer Imaging, MTI, has been used to visualize previously unknown alterations in the cerebral architecture of patients with Tourette's syndrome. The researchers also found a correlation between the extent of some of the structural changes and symptom severity.

Kirsten Müller-Vahl, from Hanover Medical School, led a team of researchers who used normal MRI scanning and the new MTI technique to investigate the brains of 19 Tourette's patients and 20 controls. They identified alterations in the frontal lobe of the Tourette's group that they suggest may be responsible for the pathology of the syndrome. Müller-Vahl said, "Our in vivo findings, using two sensitive and unbiased techniques, support the hypothesis that alterations in frontostriatal circuitries underlie Tourette's pathology".

The MTI technique used by the researchers has never before been applied to the study of Tourette's. It is a refinement of the nuclear magnetic resonance technique and allows for the detection of changes invisible to conventional MRI scanners. Tissue alterations in comparison to controls were detected in brain areas involved in the selection, programming, initiation, and control of movement. The authors conclude, "We suggest that Tourette's is primarily caused by a dysfunction in prefrontal cortex areas rather than the basal ganglia, as has been previously thought".

Tourette's syndrome is estimated to affect between 1-10 children per 1000 and, although the severity of a person's tics tends to decline with age, as many as 1% of the adult population may have some form of tic disorder. Symptoms include various facial, phonic and other motor tics - the well-known propensity for 'un-voluntary' swearing is in fact relatively uncommon, only affecting about 10% of Tourette's patients.

Journal reference:

1. Kirsten R Muller-Vahl, Jorn Kaufmann, Julian Grosskreutz, Reinhard Dengler, Hinderk M Emrich and Thomas Peschel. Prefrontal and anterior cingulate cortex abnormalities in Tourette Syndrome: evidence from voxel-based morphometry and magnetization transfer imaging. BMC Neuroscience, (in press)

Adapted from materials provided by BMC Neuroscience, via EurekAlert!, a service of AAAS.