On the validity of 3D polymer gel dosimetry: III. MRI-related error sources

Abstract

In MRI (PAGAT) polymer gel dosimetry, there exists some controversy on the validity of 3D dose verifications of clinical treatments. The relative contribution of important sources of uncertainty in MR scanning to the overall accuracy and precision of 3D MRI polymer gel dosimetry is quantified in this study. The performance in terms of signal-to-noise and imaging artefacts was evaluated on three different MR scanners (two 1.5 T and a 3 T scanner). These include: (1) B₀-field inhomogeneity, (2) B₁-field inhomogeneity, (3) dielectric effects (losses and standing waves) and (4) temperature inhomogeneity during scanning. B₀-field inhomogeneities that amount to maximum 5 ppm result in dose deviations of up to 4.3% and deformations of up to 5 pixels. Compensation methods are proposed. B₁-field inhomogeneities were found to induce R₂ variations in large anthropomorphic phantoms both at 1.5 and 3 T. At 1.5 T these effects are mainly caused by the coil geometry resulting in dose deviations of up to 25%. After the correction of the R₂ maps using a heuristic flip angle-R₂ relation, these dose deviations are reduced to 2.4%. At 3 T, the dielectric properties of the gel phantoms are shown to strongly influence B₁-field homogeneity, hence R₂ homogeneity, especially of large anthropomorphic phantoms. The low electrical conductivity of polymer gel dosimeters induces standing wave patterns resulting in dose deviations up to 50%. Increasing the conductivity of the gel by adding NaCl reduces the dose deviation to 25% after which the post-processing is successful in reducing the remaining inhomogeneities caused by the coil geometry to within 2.4%. The measurements are supported by computational modelling of the B₁-field. Finally, temperature fluctuations of 1 °C frequently encountered in clinical MRI scanners result in dose deviations up to 15%. It is illustrated that with adequate temperature stabilization, the dose uncertainty is reduced to within 2.58%.

Evaluation of accuracy and precision in polymer gel dosimetry

“…The poor accuracy of these results was largely attributed to temperature fluctuations between the calibration and test phantoms. A further study by Vandecasteele and De Deene repeated this experiment and reduced the mean dose discrepancy to 2.58% when temperature stabilization was held within 0.2°C. These improved results are comparable to those found in this work, and although the increased dose sensitivity of the MRI readout allows for dose measurements at a lower dose level, temperature sensitivity requires careful environmental control to avoid large dose errors.…”

“…Overall, when NIPAM consistency is considered, the interbatch reproducibility is such that an average, or “generic,” calibration is possible and the potential of using a generic calibration curve is an important factor going forward for clinical applications of this gel dosimeter. Previous studies of NIPAM‐based gel dosimeters have also looked at the reproducibility of gel dose response across a small sample of gel batches for both X‐ray CT and optical CT readout. In neither case was an attempt to translate this toward generic calibration made.…”

Investigation of Temperature Dependence of Polymer Gels for Use with Scanning Magnetic Resonance Imaging

“…MGS Research, Inc., Guilford, CT), and PAGAT (Polyacrylamide Gel and THPC) gels [14] were prepared. The BANG-3 type polymer gels were prepared using a BANG kit.…”

On the Need for Comprehensive Validation of Deformable Image Registration, Investigated With a Novel 3-Dimensional Deformable Dosimeter

“…Both Yeo et al (14, 15) and Niu et al (13) present 3D methodologies utilizing deformable polymer-based gel dosimeters, read out in the former case by optical computed tomography (optical-CT) and in the latter by magnetic resonance imaging (MRI). There are disadvantages to MRI not only in terms of the inherent accuracy of the methodology, but also in terms of cost and accessibility (16, 17). Optical-CT is capable of high accuracy at low cost, but in the case of polymer gels, scatter effects limit the size of the dose distribution that may be evaluated without sophisticated corrections (18).…”