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Related Experiment Videos

A simple correction for B1 field errors in magnetization transfer ratio measurements.

Rebecca S Samson1, Claudia A M Wheeler-Kingshott, Mark R Symms

  • 1NMR Research Unit, Department of Neuroinflammation, Institute of Neurology, UCL, WC1N 3BG London, UK.

Magnetic Resonance Imaging
|March 28, 2006
PubMed
Summary

B1 errors significantly impact magnetization transfer ratio (MTR) measurements. A new correction method using B1 field mapping accurately adjusts MTR values, improving quantitative accuracy in MRI scans.

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Area of Science:

  • Magnetic Resonance Imaging
  • Quantitative MRI Techniques
  • Biophysical Modeling

Background:

  • Magnetization transfer ratio (MTR) measurements are susceptible to B1 errors, stemming from radiofrequency (RF) nonuniformity and transmitter calibration issues.
  • These B1 errors can lead to significant inaccuracies (5-10%) in quantitative MRI, potentially broadening MTR histograms and affecting tissue characterization.

Purpose of the Study:

  • To investigate the impact of B1 errors on MTR measurements across various brain tissue types.
  • To develop and validate a systematic correction method for B1-induced MTR inaccuracies.
  • To implement and assess a B1 field mapping technique for routine MTR data correction.

Main Methods:

  • Modeling the dependence of MTR on B1 using binary spin bath theory with a continuous wave (CW) approximation.

Related Experiment Videos

  • Performing MTR measurements on 18 tissue types in five controls on a 1.5-T scanner, systematically reducing MT pulse amplitude by up to 20%.
  • Implementing a B1 field mapping technique based on the double angle method (DAM) with fast spin-echo (FSE) readout.

    • Main Results:

    • A single linear correction equation, mtr(normalized)=0.812b(1normalized)+0.193, was derived, effectively correcting MTR for B1 reductions up to 20% across different tissues.
    • Post-correction, MTR values were within 1.5 percent units (pu) of their true values (RMS error=0.7 pu), a significant improvement from the uncorrected mean error of 7.0 pu.
    • The FSE DAM B1 field mapping technique proved robust, with whole-brain B1 histograms showing full-width half-maximums (FWHMs) between 6.8% and 11.5% of the nominal B1 value.

    Conclusions:

    • A systematic, tissue-independent correction method for B1 errors in MTR measurements is feasible and effective.
    • B1 field mapping using the FSE DAM technique provides accurate quantification of B1 errors, enabling reliable MTR map and histogram correction.
    • While the correction scheme is sequence-dependent, the developed methodology offers a robust approach to enhance the accuracy of quantitative MTR imaging.