Calibration method for an MPI(=Magnetic particle imaging) apparatus
A calibration method for an MPI (=magnetic particle imaging) apparatus for conducting an MPI experiment, wherein the calibration method comprises m calibration MPI measurements with a calibration test piece and uses these measurements to create an image reconstruction matrix with which the signal contributions of N voxels within an investigation volume of the MPI apparatus are determined, wherein compressed sensing steps are applied in the calibration method with a transformation matrix that sparsifies the image construction matrix, and wherein only a number M<N of calibration MPI measurements for M voxels are carried out, from which the image reconstruction matrix is created and stored. This specifies an efficient method for determination of the system matrix for the MPI imaging method, which does not require much time to determine an MPI system function and nevertheless achieves a high degree of precision.
1. A method for calibrating an MPI apparatus for performing an MPI experiment of a volume under investigation, the method comprising the steps of:
a) determining compressed sensing steps using a transformation matrix that sparsifies an image reconstruction matrix, the transformation matrix comprising analytic transformations, a modelled MPI-system matrix, a simulated or a measured system matrix;
b) carrying out M calibration MPI measurements for M voxels of a calibration sample, the M voxels being selected in a random or pseudo-random fashion; and
c) using the transformation matrix of step a) and the measurements of step b) to create and store an image reconstruction matrix with which signal contributions of N voxels within the volume under investigation are determined, wherein M<N.
2. The method of claim 1 , wherein the calibration sample is shifted within the volume under investigation to locations of the M voxels and one calibration MPI measurement is performed for each voxel.
3. The method of claim 1 , wherein the calibration sample remains stationary during the calibration MPI measurements and magnetic fields of the MPI apparatus are varied in such a fashion that static magnetic fields that prevail at locations of the M voxels are reconstructed, one after another.
4. The method of claim 1 , wherein the calibration sample remains stationary during the calibration MPI measurements and magnetic fields of a system calibration unit that is independent of the MPI apparatus are varied in such a fashion that static magnetic fields that prevail at locations of the M voxels are reproduced, one after another.
5. The calibration method of claim 1 , wherein a volume of the calibration sample is larger than a volume of a voxel.
6. The calibration method of claim 1 , wherein the calibration sample has a non-homogeneous inner structure.
7. The calibration method of claim 1 , wherein the transformation matrix includes Discrete Fourier Transformation (DFT), Discrete Cosine Transformation (DCT), Discrete Wavelet Transformation (DWT) or discrete Chebyshev Transformation (DTT), wherein, in case of multi-dimensional imaging, the multi-dimensional variants of those transformations are utilized.
8. The calibration method of claim 1 , wherein the following applies: 100<N<10 12 and M/N<0.5, M/N<0.1 or M/N<10 −3 .
9. The calibration method of claim 1 , wherein a convolution kernel, which can be spatially dependent, is determined from the created and stored image reconstruction matrix and used for improving a spatial resolution in subsequent performance of an X space reconstruction.
10. The method of claim 1 , further comprising the steps of:
d) carrying out an MPI experiment; and
e) constructing an MPI image following step d) using the image reconstruction matrix that is created and stored in step c).
11. An MPI apparatus suitable for performing a calibration method, the MPI apparatus having an electronic control unit, wherein the electronic control unit comprises:
means for determining compressed sensing steps using a transformation matrix that sparsifies an image reconstruction matrix, the transformation matrix comprising analytic transformations, a modelled MPI-system matrix, a simulated or a measured system matrix;
means for carrying out M calibration MPI measurements for M voxels of a calibration sample, the M voxels being selected in a random or pseudo-random fashion; and
means for using said transformation matrix and said M calibration MPI measurements in order to create and store an image reconstruction matrix with which signal contributions of N voxels within the volume under investigation are determined, wherein M<N.
12. The MPI apparatus of claim 11 , further comprising:
means for carrying out an MPI experiment; and
means for constructing an MPI image using said image reconstruction matrix.