Determination of the coordinate transformation between an optical motion tracking system and a magnetic resonance imaging scanner
Improved cross-calibration between magnetic resonance imaging (MRI) coordinates and optical tracking coordinates is provided. Initial calibration is performed with a calibration tool that includes wireless active markers that can be tracked using the MRI scanner, and an optical marker that can be tracked using the optical tracking system. Data from one or more poses of this tool are used to provide an initial cross-calibration. In use, this initial calibration is corrected to account for differences between actual camera position and the reference location. Here the reference location is the camera location at which the initial calibration was performed.
1. A method of cross-calibrating magnetic resonance imaging (MRI) coordinates with coordinates of an optical tracking system in an MRI system, the method comprising:
a) providing a calibration tool, wherein the calibration tool includes optical features recognizable by the optical tracking system, wherein the calibration tool includes wireless active MRI markers recognizable by the MRI system, wherein the optical features and the wireless active MRI markers have fixed relative positions on the calibration tool;
b) performing an initial calibration by measuring MRI and optical coordinates of the calibration tool while a first camera of the optical tracking system is disposed at a reference location, wherein the first camera is attached to a head coil of the MRI system;
wherein the initial calibration comprises taking MRI and optical data for one or more distinct poses of the calibration tool and determining an initial cross-calibration coordinate transformation relating MRI coordinates to optical coordinates;
c) before or during imaging of an MRI subject, i) determining a position of the first camera relative to the reference location, ii) determining a corrected calibration using the camera position and the initial cross-calibration coordinate transformation by updating a homogenous transformation matrix of the initial cross-calibration with a position change of the first camera, and
iii) using the corrected calibration to relate MRI coordinates to optical coordinates for the subject being imaged;
wherein the initial cross-calibration can be represented by the homogenous transformation matrix
H
=
[
R
v
(
0
,
0
,
0
)
1
]
,
wherein R is a 3 by 3 rotation matrix of the initial cross-calibration, wherein v is a 3 by 1 translation vector of the initial cross-calibration, and wherein the homogenous transformation matrix of the corrected calibration is given by,
H
′
=
[
R
v
′
(
0
,
0
,
0
)
1
]
,
wherein v 1 is a 3 by 1 translation vector relating to the position change of the first camera.
2. The method of claim 1 , wherein each wireless active MRI marker includes an MR-visible sample coupled to a resonant circuit, and wherein positions of the wireless active MRI markers can be determined via inductive coupling of the resonant circuits of the wireless active MRI markers to one or more receive coils of a magnetic resonance system.
3. The method of claim 1 , wherein the optical features are individually distinguishable, and wherein the optical features are arranged in a predetermined pattern having known relative positions, whereby partial observation of the pattern suffices to completely determine a position of the optical marker.
4. The method of claim 1 , wherein an MRI-visible camera marker is affixed to the first camera, and wherein the determining a position of the first camera relative to the reference location comprises determining a position of the MRI-visible camera marker using the MRI system.
5. The method of claim 1 , wherein a bore optical marker is affixed to a bore of the MRI system at a known position relative to the reference location, and wherein the determining a position of the first camera relative to the reference location comprises observing the bore optical marker with the optical tracking system.
6. The method of claim 1 , wherein a second camera is affixed to a bore of the MRI system at a known position relative to the reference location, and wherein the determining a position of the first camera relative to the reference location comprises observing an optical marker affixed to the first camera with the second camera.
7. The method of claim 1 , wherein the fixed relative positions of the optical features and the wireless active markers on the calibration tool are known inputs for the initial calibration.
8. The method of claim 1 , wherein a z-direction is along a bore of the MRI system, and wherein the determining a position of the first camera relative to the reference location comprises determining a z-offset of the camera.
9. The method of claim 8 , wherein the z-offset is determined from a position encoding of a patient table of the MRI system.
10. The method of claim 1 , wherein the initial calibration comprises taking MRI and optical data for three or more distinct poses of the calibration tool, thereby providing two or more distinct motions that are used for calibration.
11. The method of claim 10 , wherein the initial calibration is computed without a priori knowledge of the fixed relative positions of the optical features and the wireless active markers on the calibration tool, and wherein the fixed relative positions of the optical features and the wireless active markers on the calibration tool are determined from the three or more distinct poses of the calibration tool.