GRADIENT COIL SIMULATION

Model

In this example the Z-coil used to generate the low frequency gradient field in an MRI system is simulated. Due to the static nature of the problem an accurate and fast solution is possible using FEM based low frequency solvers.

The complexity of the geometry can be greatly reduced since it is not necessary to model the metal loops in coil. Rather these can be represented by current loops excitation for the simulation. Furthermore, these current loops need not be discretized by the grid, but are simply used to calculate the field distributions in the coil.

Simulation

The phase of the current source excitations needs to be set correctly for opposite and adjacent loops.

Two different solvers are applied:

1) Empty coil: the simulation is run using the magneto-static (MS) low frequency solver. The H-field inside and around the coil is calculated.

2) Coil with patient: the lossy dielectric tissue of the patient requires that the simulation is run using the magneto-quasi-static solver. The H-field inside and around the coil is calculated and the induced current distribution (J) is calculated in the patient. NOTE: the induced current is purely imaginary.

Results

The gradient field shows the gradient region of the coil: this can be optimized to obtain steeper, more constant gradients in the region of interest. Furthermore, the active current cancellation can be optimized to reduce the field outside the coil, which is important for worker safety standards.

Induced currents in the patient can also be studied for safety purposes.