Device and method for calibrating tracking systems in imaging systems

Abstract

A device and a method for calibrating the coordinate system of imaging systems having a tracking system prior or during image data acquisition, e.g. by way of magnetic resonance tomography.

Claims

1. A method for calibrating tracking systems in imaging systems and for prospective motion correction of a moving object being imaged or of a moving object being imaged in MRT, in IMRT or in CT, the method comprising the steps of: a) providing a tracking system having a tracking coordinate system; b) providing at least one imaging system having an imaging coordinate system; c) providing at least one first marker that is stationary relative to the imaging system as a reference marker having a position and orientation; d) cross calibrating the tracking coordinate system and the imaging coordinate system; e) measuring a position and orientation of the first marker in the tracking coordinate system; f) calibrating the position and orientation of the first marker in the imaging coordinate system; g) monitoring a position and orientation of the first marker in the tracking coordinate system; h) correcting the cross calibration of step d) using the results of step g), wherein the correction in the cross calibration is carried out using quaternion algebra; i) disposing at least one second marker on the moving object; j) detecting a position and orientation of the second marker in the tracking system coordinate system during imaging; k) converting, following step h), the position and orientation of the second marker detected in step j) into the coordinate system of the imaging system using quaternion algebra; and l) performing, following steps g) through k), a prospective motion correction such that an imaging volume remains stationary with respect to the moving object during imaging.

2. The method of claim 1, wherein said tracking system is arranged within said imaging system.

3. The method of claim 1, wherein said tracking system is arranged outside of said imaging system.

4. The method of claim 1, wherein said first marker is arranged within said imaging system.

5. The method of claim 1, wherein said first marker is arranged outside of said Imaging system.

6. The method of claim 1, wherein each of a plurality of markers is arranged in imaging systems such that positions and orientations thereof in a coordinate system of a respective imaging system are calibrated, wherein a position and orientation of the second marker arranged on the movable object is converted from a coordinate system of one imaging system into coordinate systems of other imaging systems.

7. The method of claim 1, wherein calibration is performed once as a cross calibration of the coordinate system of the tracking system with coordinates c.sub.0 and C, wherein c.sub.0 is the translation quaternion of the tracking system and C is a rotation quaternion of the tracking system and a position and orientation (r.sub.0, R) of the first marker in the coordinate system of the tracking system are stored and a current position and orientation (x.sub.0, X) of the reference point in the coordinate system of the tracking system is used for calibration in order to calculate a current cross calibration (s.sub.0, S) as follows:
S=X*RC, wherein X* is a conjugated rotation quaternion of a quaternion X and
s.sub.0=c.sub.0+C*r.sub.0C−S*x.sub.0S, wherein C* and S* are conjugates of rotation quaternions C and S.

8. A method for calibrating tracking systems in an imaging apparatus and for prospective or for retrospective motion correction of a moving object being imaged by the imaging apparatus or of a moving object being imaged in MRT, in IMRT or in CT, the method comprising the steps of: a) providing a tracking system having a tracking coordinate system; b) providing at least one imaging system having an imaging coordinate system; c) providing at least one first marker that Is stationary relative to the imaging system as a reference marker having a position and orientation; d) cross calibrating the tracking coordinate system and the imaging coordinate system; e) measuring a position and orientation of the first marker in the tracking coordinate system; f) calibrating the position and orientation of the first marker in the imaging coordinate system; g) removing the tracking system from the imaging apparatus; h) reinstalling the tracking system in the imaging apparatus; i) measuring a position and orientation of the first marker in the tracking coordinate system following step h); j) correcting the cross calibration of step d) using the results of step i), wherein the correction in the cross calibration is carried out using quaternion algebra; k) disposing at least one second marker on the moving object; l) detecting a position and orientation of the second marker in the tracking system coordinate system during imaging; m) converting, following step j), the position and orientation of the second marker detected in step l) into the coordinate system of the imaging system using quaternion algebra; and n) performing, following steps i) through m), a prospective or a retrospective motion correction such that an imaging volume remains stationary with respect to the moving object.

9. The method of claim 8, wherein said tracking system is arranged within said imaging system.

10. The method of claim 8, wherein said tracking system is arranged outside of said imaging system.

11. The method of claim 8, wherein said first marker is arranged within said imaging system.

12. The method of claim 8, wherein said first marker is arranged outside of said imaging system.

13. The device of claim 8, wherein each of a plurality of markers is arranged in imaging systems such that positions and orientations thereof are calibrated in a coordinate system of a respective imaging system, wherein a position and orientation of a marker arranged on a movable object is transferred from a coordinate system of an imaging system to coordinate systems of other imaging systems through measurement of position and orientation using said tracking system.

14. The method of claim 8, wherein calibration Is performed once as a cross calibration of the coordinate system of the tracking system with coordinates c.sub.0 and C, wherein c.sub.0 is the translation quaternion of the tracking system and C is a rotation quaternion of the tracking system and a position and orientation (r.sub.0, R) of the first marker in the coordinate system of the tracking system are stored and a current position and orientation (x.sub.0, X) of the reference point in the coordinate system of the tracking system is used for calibration in order to calculate a current cross calibration (s.sub.0, S) as follows:
S=X*RC, wherein X* is a conjugated rotation quaternion of a quaternion X and
s.sub.0=c.sub.0+C*r.sub.0C−S*x.sub.0S, wherein C* and S* are conjugates of rotation quaternions C and S.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 schematically shows an arrangement of a device 1 for calibrating a tracking system in an imaging system; and

(2) FIGS. 2a, 2b and 2c schematically show an arrangement of a plurality of imaging systems with associated coordinate systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(3) The reference numerals of corresponding elements in the figures are identical.

(4) In accordance with FIG. 1, the schematically illustrated device 1 comprises a tracking system 10 which is arranged in an imaging system 20 e.g. for MRT. The tracking system 10 has a coordinate system 50. The device moreover has a first marker 70 which is stationary relative to the imaging system 20 as a reference marker and the position and orientation of which are calibrated in a coordinate system 60 of the imaging system 20.

(5) The tracking system 10 and the first marker 70 are arranged within the imaging system 20.

(6) A second marker 30 is provided on a movable object 40 such that the position and orientation of the marker 30 can be detected in the coordinate system 50 of the tracking system 10 during imaging and can be transferred to the coordinate system 60 of the imaging system 20.

(7) In principle, the device 1 for calibrating tracking systems 10 in imaging systems 20a, 20b, 20c, e.g. for MRT or IMRT or CT, may comprise at least the following components: a tracking system 10 with a coordinate system 50, at least one imaging system 20a, 20b, 20c, and at least one first marker 70a, 70b, 70c that is arranged stationarily relative to the imaging system 20a, 20b, 20c as a reference marker, and the position and orientation of which are calibrated in a coordinate system 60a, 60b, 60c of the imaging system 20a, 20b, 20c as is also illustrated below in FIGS. 2a-2c.

(8) FIGS. 2a, 2b and 2c schematically show an arrangement of several imaging systems with associated coordinate systems analogous to FIG. 1, wherein in FIGS. 2a, 2b and 2c, the same object 40 is examined in each case with a first marker 70a, 70b, 70c in different imaging or therapy modalities 20a, 20b, 20c.

(9) In accordance therewith, each of a plurality of markers 70a, 70b, 70c is arranged in imaging systems 20a e.g. for MRT, 20b e.g. for CT, and 20c e.g. for IMRT but also PET such that their positions and orientations in the coordinate systems 60a, 60b, 60c of the respective imaging system 20a, 20b, 20c have been calibrated, wherein the position and orientation of the object marker 30 can be transferred from the coordinate system 60a of the imaging system 20a to the coordinate systems 60b and/or 60c of the imaging systems 20b and/or 20c.

(10) The second marker 30 is advantageously mounted for arrangement on the movable object 40 such that the position and orientation of the marker 30 in the coordinate system 50 of the tracking system 10 can be detected during imaging and can be transferred to or be converted into the coordinate system 60a, 60b, 60c of the imaging system 20a, 20b, 20c.

(11) The first marker 70a, 70b, 70c thereby cooperates with the tracking system 10 in such a fashion that a changing position of the tracking system 10 during imaging can be detected via the tracking system 10 by means of the first marker 70a, 70b, 70c and the tracking system 10 can be re-calibrated.

(12) The tracking system 10 can thereby be arranged within or outside of the imaging system 20a, 20b, 20c.

(13) The first marker 70a, 70b, 70c can also be arranged within or outside of the imaging system 20a, 20b, 20c.

(14) Each of a plurality of markers 70a, 70b, 70c are arranged in respective imaging systems 20a, 20b, 20c such that their positions and orientations in the coordinate system 60a, 60b, 60c of the respective imaging system 20a, 20b, 20c are calibrated, wherein the position and orientation of the marker 30 arranged on the movable object 40 can be converted from the coordinate system 60a of the imaging system 20a into the coordinate systems 60b, 60c of the imaging systems 20b, 20c.

(15) In each case, the imaging modality is naturally a different one, e.g. MRT, CT, IMRT, PET, SPECT and the like, wherein the reference marker is a different one in each case, since it is permanently connected to the imaging device. The tracking system may also be a different one or the same and be mobile or be stationary within or outside of the imaging system.

LITERATURE

(16) 1) Thesen S, Heid O, Mueller E, Schad L R. Prospective acquisition correction for head motion with image-based tracking for real-time fMRI. Magn Reson Med 2000; 44:457-465. 2) van der Kouwe A J, Benner T, Dale A M. Real-time rigid body motion correction and shimming using cloverleaf navigators. Magn Reson Med 2006; 56:1019-1032. 3) Ooi M B, Krueger S, Thomas W J, Swaminathan S V, Brown T R. Prospective real-time correction for arbitrary head motion using active markers. Magn Reson Med 2009; 62:943-954. 4) Dold C, Zaitsev M, Speck O, Firle E A, Hennig J, Sakas G. Prospective head motion compensation for MRI by updating the gradients and radio frequency during data acquisition. Med Image ComputAssist Interv 2005; 8 (Pt 1):482-489. 5) Zaitsev M, Dold C, Sakas G, Hennig J, Speck O. Magnetic resonance imaging of freely moving objects: prospective real-time motion correction using an external optical motion tracking system. Neuroimage 2006; 31:1038-1050. 6) Quin L, van Gelderen P, Derbyshire J A, Jin F, Lee J, de Zwart J A, Tao Y, Duyn J H. Prospective head-movement correction for highresolution MRI using an in-bore optical tracking system. Magn Reson Med 2009; 62:924-934. 7) B. C. Andrews-Shigaki, B. S. R. Armstrong, M. Zaitsev, et al., “Prospective Motion Correction for Magnetic Resonance Spectroscopy Using Single Camera Retro-Grate Reflector Optical Tracking”, Journal of Magnetic Resonance Imaging, vol. 33, pp. 498-504, 2011 8) Aksoy M, Forman C, Straka M, Skare S, Holdsworth S, Hornegger J, Bammer R. Real time optical motion correction for diffusion tensor imaging. Magn Reson Med 2010; 66:366-378. 9) Forman C, Aksoy M, Hornegger J, Bammer R. Self-encoded marker for optical prospective head motion correction in MRI. Med Image Comput Comput Assist Interv 2010; 13 (Pt 1):259-266. 10) Aksoy M, Forman C, Straka M, Cukur T, Hornegger J, Bammer R.; Hybrid prospective and retrospective head motion correction to mitigate cross-calibration errors. Magn Reson Med. 2012 May; 67(5):1237-51. doi:10.1002/mm.23101. Epub 2011 Aug. 8. 11) Boegle R, Maclaren J, Zaitsev M. Combining prospective motion correction and distortion correction for EPI: towards a comprehensive correction of motion and susceptibility-induced artifacts. MAGMA 2010; 23:263-273. 12) I. Kadashevich, D. Appu, O. Speck: “Automatic motion selection in one step cross-calibration for prospective MR motion correction”. In: Proceedings of 28th Annual Scientific meeting of ESMRMB 2011. DOI:10.1007/s10334-011-0269 13) Hamilton, William Rowan, “On quaternions, or on a new system of imaginaries in algebra”, Philosophical Magazine, vol. 25, pp. 489-495, 1844.