Improvement of the repeatability of parallel transmission at 7T using interleaved acquisition in the calibration scan.

Background: Respiration-induced phase shift affects B ₀ /B ₁ ⁺ mapping repeatability in parallel transmission (pTx) calibration for 7T brain MRI, but is improved by breath-holding (BH). However, BH cannot be applied during long scans.
Purpose: To examine whether interleaved acquisition during calibration scanning could improve pTx repeatability and image homogeneity.
Study Type: Prospective.
Subjects: Nine healthy subjects.
Field Strength/sequence: 7T MRI with a two-channel RF transmission system was used.
Assessment: Calibration scanning for B ₀ /B ₁ ⁺ mapping was performed under sequential acquisition/free-breathing (Seq-FB), Seq-BH, and interleaved acquisition/FB (Int-FB) conditions. The B ₀ map was calculated with two echo times, and the B ₁ ⁺ map was obtained using the Bloch-Siegert method. Actual flip-angle imaging (AFI) and gradient echo (GRE) imaging were performed using pTx and quadrature-Tx (qTx). All scans were acquired in five sessions. Repeatability was evaluated using intersession standard deviation (SD) or coefficient of variance (CV), and in-plane homogeneity was evaluated using in-plane CV.
Statistical Tests: A paired t-test with Bonferroni correction for multiple comparisons was used.
Results: The intersession CV/SDs for the B ₀ /B ₁ ⁺ maps were significantly smaller in Int-FB than in Seq-FB (Bonferroni-corrected P < 0.05 for all). The intersession CVs for the AFI and GRE images were also significantly smaller in Int-FB, Seq-BH, and qTx than in Seq-FB (Bonferroni-corrected P < 0.05 for all). The in-plane CVs for the AFI and GRE images in Seq-FB, Int-FB, and Seq-BH were significantly smaller than in qTx (Bonferroni-corrected P < 0.01 for all).
Data Conclusion: Using interleaved acquisition during calibration scans of pTx for 7T brain MRI improved the repeatability of B ₀ /B ₁ ⁺ mapping, AFI, and GRE images, without BH.
Level of Evidence: 1 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.
(© 2017 International Society for Magnetic Resonance in Medicine.)