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

Respiration-induced phase shift affects B₀ /B₁⁺ 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.

To examine whether interleaved acquisition during calibration scanning could improve pTx repeatability and image homogeneity.

Prospective.

Nine healthy subjects.

7T MRI with a two-channel RF transmission system was used.

Calibration scanning for B₀ /B₁⁺ mapping was performed under sequential acquisition/free-breathing (Seq-FB), Seq-BH, and interleaved acquisition/FB (Int-FB) conditions. The B₀ map was calculated with two echo times, and the B₁⁺ 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.

A paired t-test with Bonferroni correction for multiple comparisons was used.

The intersession CV/SDs for the B₀ /B₁⁺ 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).

Using interleaved acquisition during calibration scans of pTx for 7T brain MRI improved the repeatability of B₀ /B₁⁺ mapping, AFI, and GRE images, without BH.

1 Technical Efficacy Stage 1 J. Magn. Reson. Imaging 2017.