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In a Nutshell

During an MRI examination the patient is placed in an external homogeneous magnetic field of high intensity ($\bar{B}_0$). The magnetic moments of hydrogen nuclei become partly arranged in this field, the patient becomes “magnetized” (will have an own resulting magnetization). (This magnetization is very low.) With radio waves, the frequency of which is appropriate, this magnetization can be excited, i.e. it can be rotated from the direction of the field $\bar{B}_{0}$ which generates it. The rotated magnetization will precess in the external field, thus generating a changing magnetic field. This changing field induces a voltage in the coils that are placed around the patient, which thus can be measured.
By making the magnitude of the homogeneous field $\bar{B}_{0}$ position-dependent with gradient coils, the frequency of the precession and the phase of the moments will also be position-dependent, the result of which is that the volume elements of the sample can be separated.
The behaviour of the magnetization is different in the different tissues. The most frequently examined parameters are the spin-grid relaxation time ($T_{1}$ ), which connects the spin system to the thermal motion of the environment, and the time constant that is characteristic of the interaction of the spins with each other ($T_{2}$).

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