The present embodiments relate to a method for determining a set of control parameters of a control sequence for a magnetic resonance device.
The imaging method of magnetic resonance is already known in the prior art. An object to be examined is introduced into a relatively high basic magnetic field (e.g., the B0 field). In order to be able to acquire magnetic resonance data (e.g., in a layer), the spins of the layer are excited, and the decay, for example, in this excitation is observed as a signal. Gradient errors may be generated by a gradient coil arrangement, while high frequency excitation signals (e.g., high frequency pulses) are emitted via a high frequency transmission coil. The high frequency pulses generate a high frequency field (e.g., the B1 field), and the spins of resonantly excited nuclei, spatially resolved by the gradients, are tilted by a flip angle with respect to the magnetic field lines of the basic magnetic field. If the spins of the nuclei relax again, high frequency signals are emitted. The emitted high frequency signals are picked up by suitable receiving antennae and processed further in order to be able to thus reconstruct magnetic resonance image data.
Conventional high frequency transmitting coils are operated in a “homogeneous mode,” (e.g., in a “CP-Modus”). A single high frequency pulse with a defined fixed phase and amplitude is given on all components of the transmission coil (e.g., all transmission rods of a birdcage antenna). To increase the flexibility and create new degrees of freedom to improve imaging, parallel transmission, in which a plurality of transmission channels are each loaded with single pulses that may differ from each other, may also be provided. All of these single pulses, which may be described, for example, by the phase and amplitude parameters, are defined as a whole in a control sequence that is described by a corresponding set of parameters. A multi-channel pulse, which is composed of single pulses for the different transmission channels, may be a “pTX pulse” (for “parallel transmission”).
Calculation methods (e.g., optimization methods) are known for determining a set of control parameters of a control sequence for a transmitter of a magnetic resonance device including a plurality of transmission channels. A target magnetization (e.g., a magnetic resonance excitation quality specification) may be specified. For example, a desired spatially resolved flip angle distribution that corresponds to a target magnetization may be given. A target function may then be defined. A suitable control sequence (e.g., the single pulses for the channels) is then determined by the optimization method (e.g., a target function optimizer). Reference is made purely by way of example for such a method for determining control sequences for parallel excitation methods to the article by W. Grissom et al., “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation”, Mag. Res. Med. 56, 620-629, 2006.
Together with additional control specifications (e.g., the associated gradient pulses), the control sequence forms the measuring protocol that allows automatic control of the magnetic resonance device for a measurement.
Parallel transmission therefore permits the excitation to be spatially modulated. A class of optimizations has the aim of optimally homogeneous excitation within a certain volume or imaging region. All nuclei excited may have the same flip angle. The single pulses used, described by the set of control parameters (e.g., calculated from an optimization method), are taken as a basis for the data about the B1 fields to also achieve such an homogeneous flip angle distribution or homogeneous excitation. This uses a patient- and measuring volume specific adjustment measurement that forms the basis of pulse calculation. B1 maps of the individual coil elements or transmission channels may be measured for this purpose. In the case of the specific object and the specific imaging region, an analytical or numerical optimization algorithm calculates, from a basically infinite, multi-dimensional solution space for the homogeneous excitation, optimum parameters for the different degrees of freedom of the high frequency excitation (e.g., the high frequency amplitude and the high frequency phase) for each time step and each transmission channel and, optionally, additional gradients for each time step.