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A variable echo-number method for estimating R2 in MRI-based polymer gel dosimetry.

Yoichi Watanabe1, Hitoshi Kubo

  • 1Department of Therapeutic Radiology, University of Minnesota, Minneapolis, Minnesota 55455, USA. watan016@umn.edu

Medical Physics
|April 2, 2011
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Summary
This summary is machine-generated.

A new variable echo-number (VAREC) method accurately estimates spin-spin relaxation rate R2 for magnetic resonance imaging (MRI) polymer gel dosimetry. This method avoids anomalous R2 values caused by long echo times, improving dose quantification accuracy.

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Area of Science:

  • Medical Physics
  • Radiotherapy Physics
  • Magnetic Resonance Imaging

Background:

  • Spin-spin relaxation rate R2 is crucial for magnetic resonance imaging (MRI)-based polymer gel dosimetry to quantify absorbed dose.
  • Traditional R2 estimation methods can yield anomalous values with long echo times due to signal decay to background levels.

Purpose of the Study:

  • To propose and evaluate a novel variable echo-number (VAREC) method for accurate R2 estimation in polymer gel dosimetry.
  • To address the issue of anomalous R2 values arising from long echo trains in MRI.

Main Methods:

  • Implemented nonlinear least-squares (NLLS), VAREC, and maximum likelihood estimator (MLE_R) algorithms for R2 estimation.
  • Tested methods on phantoms and BANG3-type polymer gels irradiated to doses from 0 to 50 Gy using a 32-echo CPMG sequence on 3 T MRI scanners.
  • Defined the VAREC threshold as a multiplier (alpha) of the standard deviation of Gaussian noise.

Main Results:

  • The NLLS method produced incorrect R2 values at doses above 10 Gy.
  • VAREC method with alpha=2 showed agreement with MLE_R within measurement uncertainty.
  • Optimal R2 value uncertainties were achieved with alpha=2 or 3.

Conclusions:

  • The VAREC algorithm offers a simple, fast, and robust solution for R2 estimation in MRI dosimetry.
  • Recommends VAREC with alpha=2 or 3 for R2 estimation in multispin echo MRI protocols.
  • Enhances the accuracy and reliability of absorbed dose quantification using polymer gel dosimetry.