Improving FLAIR SAR efficiency at 7T by adaptive tailoring of adiabatic pulse power through deep learning B1+ estimation

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Improving FLAIR SAR efficiency at 7T by adaptive tailoring of adiabatic pulse power through deep learning urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0002 estimation

Shahrokh Abbasi‐Rad, Kieran O’Brien, Samuel Kelly, Viktor Vegh, Anders Rodell, Yasvir Tesiram, Jin Jin, Markus Barth, Steffen Bollmann

Abstract

Purpose

The purpose of this study is to demonstrate a method for specific absorption rate (SAR) reduction for 2D T2‐FLAIR MRI sequences at 7 T by predicting the required adiabatic radiofrequency (RF) pulse power and scaling the RF amplitude in a slice‐wise fashion.

Methods

We used a time‐resampled frequency‐offset corrected inversion (TR‐FOCI) adiabatic pulse for spin inversion in a T2‐FLAIR sequence to improve urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0004 homogeneity and calculated the pulse power required for adiabaticity slice‐by‐slice to minimize the SAR. Drawing on the implicit urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0005 inhomogeneity in a standard localizer scan, we acquired 3D AutoAlign localizers and SA2RAGE urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0006 maps in 28 volunteers. Then, we trained a convolutional neural network (CNN) to estimate the urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0007 profile from the localizers and calculated pulse scale factors for each slice. We assessed the predicted urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0008 profiles and the effect of scaled pulse amplitudes on the FLAIR inversion efficiency in oblique transverse, sagittal, and coronal orientations.

Results

The predicted urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0009 amplitude maps matched the measured ones with a mean difference of 9.5% across all slices and participants. The slice‐by‐slice scaling of the TR‐FOCI inversion pulse was most effective in oblique transverse orientation and resulted in a 1 min and 30 s reduction in SAR induced delay time while delivering identical image quality.

Conclusion

We propose a SAR reduction technique based on the estimation of urn:x-wiley:07403194:media:mrm28590:mrm28590-math-0010 profiles from standard localizer scans using a CNN and show that scaling the inversion pulse power slice‐by‐slice for FLAIR sequences at 7T reduces SAR and scan time without compromising image quality.

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