Abinand C Rejimon1, Diana Y Lee2, Christopher M Bergeron3, You Zhuo4, Wenshu Qian5, Richard G Spencer6, Mustapha Bouhrara7. 1. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: abinand.rejimon@nih.gov. 2. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: diana.lee5@nih.gov. 3. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: christopher.bergeron@nih.gov. 4. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: joy.zhou@nih.gov. 5. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: qian.wenshu@nih.gov. 6. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: spencerri@mail.nih.gov. 7. Laboratory of Clinical Investigation, National Institute on Aging, NIH, 251 Bayview Boulevard, Baltimore, MD 21224, USA. Electronic address: bouhraram@mail.nih.gov.
Abstract
PURPOSE: To extend the null signal method (NSM) for B1 mapping to 3 T magnetic resonance imaging (MRI). BACKGROUND: The NSM operates in the steady state regime and exploits the linearity of the spoiled gradient recalled echo (SPGR) signal around the 180° flip angle (FA). Using linear regression, B1 maps are derived from three SPGR images acquired at different FAs with a short repetition time. While the conventional NSM allows accurate mapping of B1 for moderate B1 variation, we observed that this method fails for the larger B1 variations typical of high-field MRI. METHODS: We analyzed the effect of the FA range of the acquired SPGR images on B1 determination using the NSM for 3 T MRI through extensive numerical and in vivo analyses. B1 maps derived from the extended angle-range NSM (EA-NSM) were calculated and compared to those derived from the conventional, more restricted angle range, NSM, and to those derived from the reference, but much more time-consuming, double angle method (DAM). Furthermore, we investigated the compatibility of EA-NSM B1 mapping and the half-scan and SENSE reconstruction methods for accelerating acquisition time. RESULTS: Our results show that the use of the conventional FA range leads to substantial inaccuracies in B1 determination. Both numerical and in vivo analyses demonstrate that expanding the FA range of the acquired SPGR images substantially improves the accuracy of B1 maps. Furthermore, B1 maps derived from EA-NSM were demonstrated to be quantitatively comparable to those derived from the lengthy DAM protocol. We also found that B1 maps derived from SPGR images using the EA-NSM and imaging acceleration methods were comparable to those derived from images acquired without acceleration. Finally, the use of half scanning combined with SENSE reconstruction permits whole-brain B1 mapping in ~1 min. CONCLUSIONS: The EA-NSM permits accurate, fast, and practical B1 mapping in a 3 T clinical setting. Published by Elsevier Inc.
PURPOSE: To extend the null signal method (NSM) for B1 mapping to 3 T magnetic resonance imaging (MRI). BACKGROUND: The NSM operates in the steady state regime and exploits the linearity of the spoiled gradient recalled echo (SPGR) signal around the 180° flip angle (FA). Using linear regression, B1 maps are derived from three SPGR images acquired at different FAs with a short repetition time. While the conventional NSM allows accurate mapping of B1 for moderate B1 variation, we observed that this method fails for the larger B1 variations typical of high-field MRI. METHODS: We analyzed the effect of the FA range of the acquired SPGR images on B1 determination using the NSM for 3 T MRI through extensive numerical and in vivo analyses. B1 maps derived from the extended angle-range NSM (EA-NSM) were calculated and compared to those derived from the conventional, more restricted angle range, NSM, and to those derived from the reference, but much more time-consuming, double angle method (DAM). Furthermore, we investigated the compatibility of EA-NSM B1 mapping and the half-scan and SENSE reconstruction methods for accelerating acquisition time. RESULTS: Our results show that the use of the conventional FA range leads to substantial inaccuracies in B1 determination. Both numerical and in vivo analyses demonstrate that expanding the FA range of the acquired SPGR images substantially improves the accuracy of B1 maps. Furthermore, B1 maps derived from EA-NSM were demonstrated to be quantitatively comparable to those derived from the lengthy DAM protocol. We also found that B1 maps derived from SPGR images using the EA-NSM and imaging acceleration methods were comparable to those derived from images acquired without acceleration. Finally, the use of half scanning combined with SENSE reconstruction permits whole-brain B1 mapping in ~1 min. CONCLUSIONS: The EA-NSM permits accurate, fast, and practical B1 mapping in a 3 T clinical setting. Published by Elsevier Inc.
Entities:
Keywords:
B(1) mapping; Null signal method; Spoiled gradient recalled echo
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