Bloch-Siegert B1+- mapping for human cardiac 31P-MRS at 7 Tesla

Journal article

Purpose: Phosphorus MR spectroscopy (³¹P-MRS) is a powerful tool for investigating tissue energetics in vivo. Cardiac ³¹P-MRS is typically performed using surface coils that create an inhomogeneous excitation field across the myocardium. Accurate measurements of B₁⁺ (and hence flip angle) are necessary for quantitative analysis of ³¹P-MR spectra. We demonstrate a Bloch-Siegert B₁⁺-mapping method for this purpose. Theory and Methods: We compare acquisition strategies for Bloch-Siegert B₁⁺-mapping when there are several spectral peaks. We optimize a Bloch-Siegert sensitizing (Fermi) pulse for cardiac ³¹P-MRS at 7T and apply it in a 3D chemical shift imaging sequence. We validate this in phantoms and skeletal muscle (against a dual-TR method) and present the first cardiac ³¹P B₁⁺ -maps at 7T. Results: The Bloch-Siegert method correlates strongly (Pearson’s r = 0.90 and 0.84) and has bias <25Hz compared to a multi-TR method in phantoms and dual-TR method in muscle. Cardiac 3D B₁⁺ -maps were measured in five normal volunteers. B₁ maps based on phosphocreatine and alpha-adenosinetriphosphate correlated strongly (r=0.62), confirming that the method is T₁ insensitive. Conclusion: 3D ³¹P Bloch-Siegert B₁⁺ -mapping is consistent with reference methods in phantoms and skeletal muscle. It is the first method appropriate for ³¹P B₁⁺-mapping in the human heart at 7T.

Authors: Clarke, W; Robson, M; Rodgers, C

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Magnetic Resonance in Medicine
• Volume: 76
• Issue: 4
• Pages: 1047–1058
• Publication date: 2015-10-28
• DOI: 10.1002/mrm.26005
• ISSN: 0740-3194
• Keywords: Bloch-Siegert, B1+-mapping; cardiac; 31P-MRS; phosphorus; heart; spectroscopy; 7T