title: Coil combination for receive array spectroscopy: Are data-driven methods superior to methods using computed field maps?
abstract: OBJECTIVE
  Combining spectra from receive arrays, particularly X-nuclear spectra with low signal-to-noise ratios (SNRs), is challenging. We test whether data-driven combination methods are better than using computed coil sensitivities.
  
  
  BACKGROUND
  Several combination algorithms are recast into the notation of Roemer's classic formula, showing that they differ primarily in their estimation of coil receive sensitivities. This viewpoint reveals two extensions of the whitened singular-value decomposition (WSVD) algorithm, using temporal or temporal + spatial apodization to improve the coil sensitivities, and thus the combined spectral SNR.
  
  
  METHODS
  Radiofrequency fields from an array were simulated and used to make synthetic spectra. These were combined with 10 algorithms. The combined spectra were then assessed in terms of their SNR. Validation used phantoms and cardiac (31) P spectra from five subjects at 3T.
  
  
  RESULTS
  Combined spectral SNRs from simulations, phantoms, and humans showed the same trends. In phantoms, the combined SNR using computed coil sensitivities was lower than with WSVD combination whenever the WSVD SNR was >14 (or >11 with temporal apodization, or >9 with temporal + spatial apodization). These new apodized WSVD methods gave higher SNRs than other data-driven methods.
  
  
  CONCLUSIONS
  In the human torso, at frequencies ≥49 MHz, data-driven combination is preferable to using computed coil sensitivities. Magn Reson, 2015. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Magn Reson Med 75:473-487, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine.
authors.affiliation: Oxford Centre for Clinical Magnetic Resonance Research, University of Oxford, John Radcliffe Hospital, Oxford, United Kingdom.
authors.forename: Christopher T
authors.initials: CT
authors.lastname: Rodgers
authors.affiliation: Oxford Centre for Clinical Magnetic Resonance Research, University of Oxford, John Radcliffe Hospital, Oxford, United Kingdom.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Robson
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D005260:Female; D006321:Heart / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D009682:Magnetic Resonance Spectroscopy / Q000295:instrumentation / Q000379:methods*; D008297:Male; D019047:Phantoms, Imaging; D012680:Sensitivity and Specificity; D059629:Signal-To-Noise Ratio; D055815:Young Adult
keywords: MR spectroscopy; WSVD; WSVD+Apod; WSVD+Apod+Blur; adaptive combination theory; array; coil combination
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