Sensitivity regularization of the Cramér‐Rao lower bound to minimize B1 nonuniformity effects in quantitative magnetization transfer imaging. Issue 6 (7th May 2018)

• Record Type: Journal Article
• Authors: Boudreau, Mathieu; Pike, G. Bruce
• Is Part Of: Magnetic resonance in medicine. Volume 80:Issue 6(2018)
• Page Start: 2560
• Page End: 2572
• Publication Date: 2018-05-07
• Subjects: Nuclear magnetic resonance -- Periodicals; Electron paramagnetic resonance -- Periodicals; 616.07548
• DOI: 10.1002/mrm.27337
• Languages: English
• ISSNs: 0740-3194

Abstract

Abstract : Purpose: To develop and validate a regularization approach of optimizing B1 insensitivity of the quantitative magnetization transfer (qMT) pool‐size ratio ( F ). Methods: An expression describing the impact of B1 inaccuracies on qMT fitting parameters was derived using a sensitivity analysis. To simultaneously optimize for robustness against noise and B1 inaccuracies, the optimization condition was defined as the Cramér‐Rao lower bound (CRLB) regularized by the B1 ‐sensitivity expression for the parameter of interest ( F ). The qMT protocols were iteratively optimized from an initial search space, with and without B1 regularization. Three 10‐point qMT protocols (Uniform, CRLB, CRLB+B1 regularization) were compared using Monte Carlo simulations for a wide range of conditions (e.g., SNR, B1 inaccuracies, tissues). Results: The B1 ‐regularized CRLB optimization protocol resulted in the best robustness of F against B1 errors, for a wide range of SNR and for both white matter and gray matter tissues. For SNR = 100, this protocol resulted in errors of less than 1% in mean F values for B1 errors ranging between −10 and 20%, the range of B1 values typically observed in vivo in the human head at field strengths of 3 T and less. Both CRLB‐optimized protocols resulted in the lowest σF values for all SNRs and did not increase in the presence of B1 inaccuracies. Conclusion: This work demonstrates a regularized optimization approach for improving the robustness of auxiliary measurements (e.g., B1 ) sensitivity of qMT parameters, particularly the pool‐size ratio ( F ). Predicting substantially less B1 sensitivity using protocols optimized with this method, B1 mapping could even be omitted for qMT studies primarily interested in F .