The Brain 105
Diffusion Tensor Image
changed to the one we use today: Magnetic Resonance Imaging, ‘MRI. The system works by applying a strong magnetic field to your body to excite the hydrogen atoms. Since we are mostly H2O there are plenty of these.
Three magnetic fields are used. First, an extremely strong field is applied to the whole body. This causes all the hydrogen atoms in the water and fat to spin in line with the field of the machine. Next a gradient field is applied to the top of your head so it is slightly more magnetized than the bottom of your feet and, finally, a pulse of magnetism is applied to the top of your head. The spinning hydrogen atoms line up a little more when this pulse is applied and then randomize again when it is switched off. As they randomize, they give off energy. The clever part is the gradient field which causes the atoms to give off energy at slightly different times — the top of your head first, your neck a fraction of a
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