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Lowering the B1 threshold for improved BEAR B1 mapping.

Kalina V Jordanova1, Dwight G Nishimura1, Adam B Kerr1

  • 1Magnetic Resonance Systems Research Laboratory, Department of Electrical Engineering, Stanford University, Stanford, California, USA.

Magnetic Resonance in Medicine
|April 8, 2015
PubMed
Summary
This summary is machine-generated.

This study enhances the B1 estimation using adiabatic refocusing (BEAR) method by using flattened hyperbolic secant pulses. This modification extends the range for accurate radiofrequency field excitation amplitude (B1) mapping, especially at lower magnitudes.

Keywords:
B1 mappingadiabatic pulsesflattened hyperbolic secant pulses

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Area of Science:

  • Magnetic Resonance Imaging (MRI)
  • Radiofrequency (RF) Field Measurement
  • Quantitative MRI

Background:

  • Accurate measurement of the transmit radiofrequency (RF) field excitation amplitude (B1) is crucial for quantitative magnetic resonance imaging (MRI) applications.
  • Current B1 mapping methods, such as the B1 estimation using adiabatic refocusing (BEAR) method, have limitations in measuring lower B1 magnitudes due to the adiabatic threshold of hyperbolic secant pulses.

Purpose of the Study:

  • To modify the BEAR method to extend its capability for measuring lower B1 magnitudes.
  • To improve the accuracy and range of B1 mapping in MRI.

Main Methods:

  • The study redesigned the BEAR method by replacing standard hyperbolic secant pulses with flattened hyperbolic secant pulses.
  • Flattened hyperbolic secant pulses possess lower adiabatic thresholds, extending the measurable B1 range.
  • Optimization of flattened hyperbolic secant pulse parameters was performed to minimize phase sensitivity to frequency variations, ensuring accurate phase-to-B1 mapping.

Main Results:

  • Simulations and in vivo validation at 3 Tesla (3T) demonstrated the performance of the modified BEAR method.
  • Accurate B1 maps were acquired using reduced nominal peak B1 values for n ≤ 8.
  • The nominal B1 was decreased by 52%, and sensitivity to B1 increased by a factor of 3.8 at 3T.

Conclusions:

  • The modified BEAR method, utilizing flattened hyperbolic secant pulses, successfully reduces the adiabatic threshold.
  • This enhancement allows for accurate phase-to-B1 mapping over a wider frequency range and enables measurement of lower B1 values.
  • The improved method increases the applicability of B1 mapping for low B1 magnitude measurements in MRI.