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