The trajectory binary file specification#
The k-space trajectories are transformed to a binary file which is processed by a given scanner through arbitrary gradient sequences. At Neurospin, we have specifically tested MR systems from Siemens-Healthineers vendor. This file mainly specifies an arbitrary gradient profile which is played on scanner at gradient raster time rate (10e-6 seconds).
The binary file format is specified as follows:
Name |
Type |
Size |
Unit |
Description |
|---|---|---|---|---|
Version |
FLOAT |
1 |
n.a. |
file version this new version would be “5.0” |
Dimension |
FLOAT |
1 |
n.a. |
2 -> 2D , 3 -> 3D |
FOV |
FLOAT |
D |
m |
FOV size (x,y,z) : z absent if 2D dimension |
Minimum OSF |
FLOAT |
1 |
n.a. |
Minimum OS for the trajectory |
Gamma |
FLOAT |
1 |
Hz/T |
For Na / MRSI imaging |
Spokes |
FLOAT |
1 |
n.a. |
Number of spokes |
Samples |
FLOAT |
1 |
n.a. |
Number of samples per spoke |
K-space center |
FLOAT |
1 |
n.a. |
Relative value in the range [0-1] to define center of spokes |
MaxGrad |
FLOAT |
1 |
mT/m |
Maximum absolute gradient in all 3 (or 2) directions |
recon_tag |
FLOAT |
1 |
n.a. |
Reconstruction tag |
timestamp |
FLOAT |
1 |
n.a. |
Time stamp when the binary is created |
Empty places |
FLOAT |
9 |
n.a. |
Yet unused : Default initialized with 0 |
kStarts |
FLOAT |
D*Nc |
1/m |
K-space location start |
kEnds |
FLOAT |
D*Nc |
1/m |
K-space location end points. Only in version>5.0 |
Gradient array |
FLOAT |
D*Nc*Ns |
unitary |
Gradient trajectory expressed in the range [-1; 1] relative to MaxGrad |
mrinufft.trajectories.io module helps convert a trajectory as numpy array to a binary file and vice versa.
All the trajectory FLOAT’s are specified with float32 always.
Note that different versions of the binary file format may have different fields and mri-nufft supports IO for version >= 4.1`