1. Declaration of Relevant Financial Interests or Relationships David Atkinson: I have no relevant financial interest or relationship to disclose with regard to the subject matter of this presentation.

2. Motion Compensation Strategies David Atkinson D.Atkinson@ucl.ac.ukD.Atkinson@ucl.ac.uk Centre for Medical Imaging Division of Medicine University College London

3. Talk Outline Effect of motion during acquisition. Motion compensation using pulse programming. Source material and Further Reading: Handbook of MRI Pulse Sequences. Bernstein et al.

4. Motion At Any Time Can Corrupt Entire Image k-space acquired in time image s(x) Fourier Transform The sum over all k in the Fourier Transform means that motion at any time can affect every pixel.

5. Motion and Timings TR TE TR Inter-shot time often much longer than shot duration “Shot”: k-space points acquired in a short time

6. Motion Compensation During shots –Flow compensation: gradient modifications Between shots –Prospective compensation: gradient and RF –Retrospective correction see Friday’s Sunrise Course. –Motion estimate required: navigator measures

7. Correction for motion during gradients: Flow Compensation 0 th moment (gradient area) Non-zero for PE. u is time integration variable 1 st moment (area of gradient x time) Zero first moment compensates at one time point for constant velocity

8. Not velocity compensated Binomial 121 Velocity compensated. G 0 th 1 st

9. Slice Select and Readout Flow Compensation Slice encoding schematic Readout schematic Echo centre Isodelay time of RF pulse Binomial 121 (flow comp)

10. Phase Encode Flow Comp Net gradient area non zero. Time origin is at echo, after gradients Same gradients, note origin important when area non-zero

11. Corrections for Motion Between Shots Effect of motion on k-space. Measure motion. Prospective scan adjustments –gradients and RF (e.g. slice/volume following).

12. K-Space Corrections for Affine Motion Image Motion Translation (rigid shift) Rotation Expansion General affine K-Space Effect Phase ramp Rotation (same angle) Contraction Affine transform

13. Estimating Motion (pulse sequences) 1D navigators. FID navigators. 2D and 3D navigators. Self-navigated sequences.

14. 1D Navigators: FID Navigators & k y =0 echo FID navk y =0 echo RF slice phase readout acq

15. 1D Navigators (k y =0) Readout with no phase encoding. 1D FT gives projection through object. “Classic” navigator + recent resurgence in balanced sequences. With localised receive coils, some extra information, e.g. respiratory and cardiac phases. kxkx kyky

17. Pencil Beam Navigators Cylindrical excitation. Typically used to measure diaphragm superior- inferior motion. van der Meer R W et al. Radiology 2007;245:251-257

18. Pencil Beam Excitation RF GxGx GyGy GzGz Spiral trajectory in excitation k-space.

19. Pencil Beam Excitation: How does it work? [Bernstein et al, from Glaser] kyky kxkx B1B1 B1B1 GxGx GyGy GzGz Spatial excitation

20. 2D and 3D Navigation Fast low spatial resolution image (2D). Orbital path through k-space (2D). Spherical shell in k-space (3D). [Welch MRM 47:32 (2002)]

21. Self-Navigated Sequences Frequent sampling of k-space central region. –Radial –Spiral –PROPELLOR etc.

22. Prospective modifications: Volume Following Affine correction to k-space coordinates can compensate for affine motion. K-space coordinates are gradient integral. Affine gradient correction can compensate affine motion.

23. [Nehkre et al. MRM 54:1130 (2005)] More general than inter-shot correction

24. Additional Strategies for Reducing Motion Sensitivity Averaging. Gating and Triggering. Short TE sequences. Saturating moving tissue out of ROI. Phase Encode Re-Ordering.

25. Summary Optimal motion compensation depends on sequence timings / expected motion. Multiple techniques. Tools include: –Motion compensated sequences. –Acquisition Strategies. –Navigation and Field Probes. –Prospective correction. –Retrospective correction – especially for non-affine motion.