VOP Compression Workflow | XFdtd

Post-process averaged SAR results.

XF generates the specific absorption rate (SAR) matrices required by virtual observation point (VOP) compression algorithms by utilizing the workflow outlined below. Running a single, finite-difference time-domain (FDTD) simulation computes field values for each transmit element of an MRI coil, and the fields are then combined with varying weights and provided to the SAR averaging routines during post-processing.

Users can support the required data generation by integrating the following specifications with the project setup workflow:

The averaged SAR results and the voxel information are then available for use with an external VOP compression algorithm.

XF supports additional variations of this workflow:

Voltage Source

A circuit component is used to excite each transmit coil and the corresponding feed definition is specified as a voltage source with a 50 ohm internal resistance and 0 degree phase shift. Because a definition is reusable, a project requires just one definition that is applied to each circuit component. The various phase shifts necessary for generating the SAR matrices are applied as operating modes during post-processing.

SAR Sensor

The SAR averaging sensor is defined with the desired settings. Although SAR averaging is not performed during the FDTD simulation, these settings define how SAR averaging is computed during post-processing.

FDTD Simulation

In order to combine fields during post-processing, they must first be computed for each circuit component excited individually. This is accomplished by enabling S-parameters and selecting all circuit components.

Also in the S-parameters tab, the SAR averaging option must be set to compute during post-processing only. When selected, the calculation engine saves electric field data for each circuit component excited individually, pre-processes the volume-finding step of SAR averaging, and writes valid voxel data to disk. During post-processing, the electric field data is combined using the principle of superposition, and those combined fields, along with both volume and valid voxel information, are then passed to the SAR averaging routines.

Additionally, one or more steady-state frequencies must be specified.

Valid Voxel Data

VOP compression algorithms require knowledge of voxels used in the SAR averaging routines. XF follows the definitions for valid, used, and unused voxels provided in the IEC/IEEE 62704-1 standard. Only valid voxels have an associated averaging volume that remains the same regardless of field combination settings. Used and unused voxels each have averaging volumes that vary with different excitations.

The valid voxel output files are saved during the FDTD simulation in the *.xf/Simulations/######/Run0001/output directory, and utilize the following names:

Users can read each of these binary files as follows:

The following voxel record identifies the X, Y, Z location of each valid voxel. Read N voxel records. Users should note that the voxels are listed in lexicographic order within the file.

Schematic

An FDTD block must be added to the schematic and a voltage source must be connected to each of its terminals. Each voltage source has an associated amplitude, phase, and active status. Each desired field combination must be set as an operating mode. For example, an 8-element MR coil and 64 field combinations produces 64 operating modes.

Users can reduce the number of setup steps by minimizing the the delta between the default values and the values for each operating mode in the workspace's default schematic. For example, setting each voltage source as active with a 0 volt amplitude and 0 degree phase allows the operating modes to turn on two sources simultaneously by changing the amplitude to 1 volt.

Voltage source settings impact both feed loss computations and input power. An inactive voltage source is treated as its equivalent resistance and contributes to feed loss computations. The power measured across an active source with an amplitude of zero contributes to input power.

Post-Process Averaged SAR

Create a simulation that combines a circuit schematic with FDTD results, and select the set-up option allowing SAR averaging for each operating mode. The available hardware determines whether SAR averaging is run serially on the local machine or queued on a compute cluster.

Averaged SAR Data

Averaged SAR results are available in the results browser once the simulation is complete. Results are exported to either Matlab or comma-separated value (*.csv) files through the right-click menu. Users should note that batch export is not available through the results browser and that reading the SAR data directly off disk might be preferable.

The averaged SAR output files are located in the post-processed simulation *.xf\Simulations\######\Run0001\{operating-mode-name}\output\SteadyStateOutput\f0 directory. Users can find additional information about binary file format on the volumetric SAR file format page of Remcom's support site.

Template Schematic

In order to reuse a common matrix of voltage source amplitude, phase, and active status, users can save a schematic to a library and then add it with its operating modes to a subsequent project, simulation, or design. This avoids re-entering the matrix values for more efficient circuit creation.

Projects with the same port-ordering convention require only that users update the FDTD block with the desired FDTD simulation and request SAR averaging when applying a circuit schematic during post-processing.