Bloch Simulator: Advanced uses

The following simulations made with the Bloch Simulator are representative of the programs capabilities to explore more complicated matters. They were recorded by choosing appropriate initial conditions and pulse sequence elements (RF pulses, spoilers,...) via the graphical user interface.

  • Stimulated echo. A more complicated sequence is shown in this movie, where the x-axis is used to indicate the position along the direction of a field gradient. Hence, a row of spin isochomates precessing at different frequencies are shown. After a 90 degree excitation pulse, a phase roll accumulates due to the variation in Larmor precession frequency. The wavelength of the phase roll decreases until a second 90 degree pulse is employed. This rotates the disc of spins and the transversal component of the magnetization dephases. After spoiling (i.e. projection of the magnetization on to the z-axis) it becomes clear that the residual longitudinal magnetization forms a sinusoidal pattern. A third 90 degree RF pulse rotates this sinusoidal pattern into the transversal plane. It is partially refocused by the gradient after a waiting period equal to the first dephasing period.
  • Spin ensemble. The net magnetization (shown in green) is made up from many small contributions pointing in different directions. This is a result of the imperfect alignment caused by the relatively weak fields used for MRI (the field is strong compared to the earth field, for example, but the thermal energies by far exceeds the energy differences associated with nuclear orientation). The near isotropic spin-distribution is rotated as a fixed structure by RF. Hence it is sufficient to consider only the dynamics of the net magnetization for regions where the field variations are insignificant.
    Note
    : The spins are not only pointing up or down, as often said. This and other MRI myths are discussed here.