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

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Abstract

Background

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.

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 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.

Statistical Tests

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

Results

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).

Data Conclusion

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.

Data Conclusion

Level of Evidence: 1

Data Conclusion

Technical Efficacy Stage 1

Data Conclusion

J. Magn. Reson. Imaging 2017.

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