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Variable Flip Angle Optimization for Fetal Brain Imaging With Reduced Specific Absorption Rate.

Dominique Franson^1,2,3, Camilo Calixto^2,3, Hongli Fan^1,2,3

¹Siemens Medical Solutions USA Inc, Boston, Massachusetts, USA.

NMR in Biomedicine

|June 17, 2025

Summary

This summary is machine-generated.

Optimized variable flip angle (VFA) fetal neuroimaging reduces specific absorption rate (SAR) and scan time compared to constant flip angle (CFA) HASTE scans. This VFA approach maintains diagnostic image quality while improving efficiency and patient comfort.

Keywords:

HASTE fetal MRI fetal brain single‐shot turbo spin echo specific absorption rate variable flip angle

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Area of Science:

Magnetic Resonance Imaging (MRI)
Medical Physics
Radiology

Background:

Half-Fourier Acquisition with Single-Shot Turbo Spin Echo (HASTE) is standard for fetal brain imaging.
HASTE scans face challenges with high specific absorption rate (SAR) and inefficiency due to SAR limits.

Purpose of the Study:

To design and evaluate an optimized variable flip angle (VFA) pattern for fetal HASTE neuroimaging.
The goal was to reduce SAR and repetition time (TR) while preserving image quality compared to constant flip angle (CFA) HASTE.

Main Methods:

VFA pattern optimized to minimize signal difference and constrain SAR to 65% of CFA, reducing TR by 29%.
Extended phase graph formalism used for signal calculation; simulations predicted performance.
VFA and CFA scans tested in phantoms and fetuses at 3T; SAR, acquisition times, and image quality (by radiologists) were assessed.

Main Results:

Phantom studies showed VFA scans had similar SNR and CNR, with slightly lower signal and contrast than CFA.
In vivo, VFA scans demonstrated significantly reduced SAR, measurement times, and total scan times.
Radiologist ratings found no significant difference in overall image quality between VFA and CFA scans.

Conclusions:

Optimized VFA HASTE scans achieve diagnostic equivalence to CFA scans, offering 65% of SAR and 71% of acquisition time.
Lower SAR reduces heating and SAR pauses, enabling faster scans by reducing TR.
Time savings enhance patient comfort, allow for scan repetitions, or facilitate advanced imaging sequences.