NIUMAG
How to Uniform the Homogeneity of the Magnetic Field
The homogeneity of the magnetic field which generates only by the magnet sometimes is not very perfect. However, if we want to get a perfect image, we will need a uniform magnetic field. In our NMR instrument, we can use the gradient amplifier unit to improve the homogeneity of the magnetic field.
（1）Find the correct central frequency.
（2）Switch on the power supply of the Gradient Amplifier Unit.
Fig 1 switch on the power supply of GAU
After the gradient unit start, a gradient field will be obtained by the gradient unit through the gradient coil, so the uniformity of the original magnetic field in the magnet is being destroyed. Therefore the first thing we need to do is electronic shimming after we start the gradient unit, so that the magnetic field uniformed again. The detail about how to electronic shimming is explained below:
（3）Increase O1 with the increment of 10 KHz.
（4）Click Acquisition (the 4th icon in the toolbar) to change the parameter panel (shown at the left side of the imaging software window) to Acquisition Page.
（5）Click “Par_Fixed” to choose “Par_Fixed” sub-page. Then set GxOffset to zero, GyOffset to zero and GzOffset to zero.
（6）Click GS (the 7th icon in the toolbar) to start sampling.
Fig 2: FID signal when the magnetic field is non uniform the sampling signal of oil with GxOffset=0 Hz, GyOffset=0 Hz, GzOffset=0 Hz
（7）Adjust the homogeneity of the magnetic field in X-direction by GxOffset.
（8）Increase GxOffset from zero with the increment of 100. If the relaxation become slower and slower, find the GxOffset which is corresponding to the slowest relaxation. Otherwise, decrease GxOffset from zero with the increment of 100. And find the GxOffset which is corresponding to the slowest relaxation.
（9）Write down the GxOffset value.
（10）Adjust the homogeneity of the magnetic field in Y-direction by GyOffset. (refer to GxOffset setting).
（11）Adjust the homogeneity of the magnetic field in Z-direction by GzOffset (refer to GzOffset setting).
（12）Click Stop (the 8th icon in the toolbar) to stop sampling.
Fig 3 FID signal when the magnetic field is uniform
Attention:
Fig 2 shows the situation when the field is no uniform, the curve of the amplitude is not in the form of exponential attenuation. If we meet this situation, we can manually adjust by icons Gxoffset、Gyoffset、Gzoffset. Fig 2 shows the situation when the field is uniform,
FID signal attenuate in the form of exponential, through manual shimming reaches to
Maximum, or not, we can judge whether the field is uniform.
