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, UK2Institute 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 method1

1 Ra and Rim (1993) MRM 30 142-145

For C coils, acquire:

MxN reference image: R ( M x N x C )

Mx(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 jl between desired (unfolded) and

reference image, solve the matrix equation

I jc = R j

c l x jl for x j

l, for each j

(i.e. N/L times).

Desired image obtained as Icfinal = Rc x

Foldedimage

Referenceimage

Unknown to be found

Pixel numberin fold direction

Coil number Foldingparameter

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 dimension1024

Repeat the basic Ra-Rim1024 times, once on eachplane.

Phantom & coil arrangement

Transverse section Side view

20mMCho

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 CH2) 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

Unfolded Reference from full 16x16 CSI

Unfolded spectrum from within oil and choline balls

CH2

CH3

(water)

choline residuallipid

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