2D-CSI in a fraction of the time using multiple receiver coils

1. 2D-CSI in a fraction of the time using multiple receiver coils Simon J. Doran1, Adam J. Schwarz2 and Martin O. Leach2 1Department of Physics, University of Surrey, Guildford, Surrey, UK 2Institute of Cancer Research & Royal Marsden NHS Trust, Sutton, Surrey, U.K.

2. Acknowledgements Funding from Cancer Research Campaign [CRC] National Institutes of Health (NIH) NHS (South Thames)

3. Background • Use of phased array receiver coils in surface coil studies • - improves SNR • - improves coverage (e.g. ~volumetric coverage) • Spatial receive profiles of the coil array elements already employed in imaging schemes to reduce the total acquisition time • Aim of this study is to develop such techniques for application to chemical shift imaging (CSI) spectroscopy acquisitions, primarily for application to extra-cranial tumours.

4. Reduced imaging time with phased arrays: Ra-Rim method • Advantages: • Works for arbitrary array sensitivity profiles. (Orthogonal coil basis functions [as in SMASH] not required). • Don’t require pure coil sensitivity maps. Any sequence can be used for the reference.

5. 1 Ra and Rim (1993) MRM 30 142-145 Reduced imaging time with phased arrays: Ra-Rim method1 For C coils, acquire: MxN reference image: R ( MxNxC ) Mx(N/L) folded image: I ( MxN/LxC ) These are related to each other via the array sensitivity function, which is the same for both.

6. Folded images acquired in fraction of time N N / L E.g. L = 2

7. Folded image Reference image Unknown to be found Folding parameter Pixel number in fold direction Coil number Reconstruction scheme To find relationship x jl between desired (unfolded) and reference image, solve the matrix equation I jc = R jc lx jlfor x jl, for each j (i.e. N/L times). Desired image obtained as Icfinal =Rcx

8. Application to CSI • Additional complex, chemical shift dimension in undersampled data set. • Use imaging (rapidly acquired gradient echo) reference data for coil sensitivity maps. • 16 x 16 x 1024 x 4 2D-CSI [for comparison] • 8x 16 x 1024 x 4 undersampled 2D-CSI • 16 x 16 x 1 x 4 reference images • (in practice rebinned from clinical 256 x 256 images) • Three compartment phantom • Siemens Vision, body-phased-array (C=4) • (No k-space apodisation)

9. Extension to CSI Spectral dimension 1024 Repeat the basic Ra-Rim 1024 times, once on each plane. Spatial dimensions 16 x 16

10. Phantom & coil arrangement Transverse section Side view 20mM Cho oil 50mM Cho Coil elements

11. Reconstruction of undersampled images Reference Reconstructed T1w Folded T1w

12. Reference images from individual elements Acquired at 256x256 k-space truncated to 16x16 for registration with CSI

13. Reconstructed Full CSI CSI metabolite images : oil Folded CSI (oil CH2) Rebinned ref. image

14. CSI metabolite images : water Folded CSI (water) Reconstructed Rebinned ref. image Full CSI

15. Folded spectra from within oil and choline balls

16. CH2 CH3 (water) Unfolded spectrum from within oil and choline balls Unfolded Reference from full 16x16 CSI residual lipid choline

17. Conclusions • Ra-Rim method has been extended to reduce acquisition time in CSI spectroscopic studies in (pseudo-) abdominal sites, using product coil and rapidly acquired image data as coil sensitivity reference. • ‘Unfolding’ process moves aliased signal to its correct location. • Further work is need both to quantify the minor differences seen between restored (unfolded) and reference CSI signals and to improve the method.

18. Acknowledgements Funding from Cancer Research Campaign [CRC] National Institutes of Health (NIH) NHS (South Thames)