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

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

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.

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.

Reduced imaging time with phased arrays: Ra-Rim method 1 1 Ra and Rim (1993) MRM For C coils, acquire: M x N reference image: R ( M x N x C ) M x ( N / L ) folded image: I ( M x N / L x C ) These are related to each other via the array sensitivity function, which is the same for both.

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

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

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] 8 x 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)

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

Phantom & coil arrangement Transverse sectionSide view 20mM Cho 50mM Cho oil Coil elements

Reconstruction of undersampled images Folded T1w Reference Reconstructed T1w

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

CSI metabolite images : oil Folded CSI (oil CH 2 ) Reconstructed Full CSI Rebinned ref. image

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

Folded spectra from within oil and choline balls

UnfoldedReference from full 16x16 CSI Unfolded spectrum from within oil and choline balls CH 2 CH 3 (water) choline residual lipid

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.

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