Abstract
PurposeCombining 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.TheorySeveral 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.MethodsRadiofrequency 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 31P spectra from five subjects at 3T.ResultsCombined 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.ConclusionIn 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.
Highlights
Magnetic resonance spectroscopy (MRS) opens a window on biochemical processes in healthy and diseased subjects [1]
We tested two approaches to obtain these: 1) using the Biot-Savart law (BS BÀ1 ) and 2) using the Biot-Savart law and multiplying by a phantom-calibrated phase/gain coefficient per element (BS BÀ1 phased). Both approaches were worse than the data-driven whitened singular value decomposition (WSVD), WSVDþApod, and WSVDþApodþBlur combination methods, except at signal-to-noise ratios (SNRs) so low that the combined data was unsuitable for further analysis anyway
For receive array spectra acquired from torso-sized objects at frequencies ! 49.9 MHz, the data-driven methods considered here all outperform, for any reasonable SNR, combination using coil sensitivities computed by the Biot-Savart law in Roemer’s formula
Summary
Magnetic resonance spectroscopy (MRS) opens a window on biochemical processes in healthy and diseased subjects [1]. Most metabolites are present in small concentrations, so MRS methods and applications benefit from an increased signal-to-noise ratio (SNR). Non-proton spectroscopy, such as 31P-MRS or 13C-MRS, has even lower intrinsic SNR than 1H-MRS because of the smaller gyromagnetic ratios of these nuclei. These methods are hindered even more acutely by their low SNR [2,3]. Combining MR spectra is difficult because the singleelement spectra in each voxel contain important phase and frequency information that is not present in images [9]. Combining X-nuclear spectra is especially challenging because these spectra usually have low SNR and lack a strong reference signal such as the 1H water peak
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
