An automated fast MR-imaging method for localized measurements of dose distributions using NMR-Fricke gel dosimetry. Evaluation of influences on the measurement accuracy.

Strahlentherapie und Onkologie : Organ der Deutschen Rontgengesellschaft ... [et al]

PubMedID: 8677503

Reichl B, Matthaei D, Richter J, Haase A. An automated fast MR-imaging method for localized measurements of dose distributions using NMR-Fricke gel dosimetry. Evaluation of influences on the measurement accuracy. Strahlenther Onkol. 1996;172(6):312-9.
PURPOSE
The calculation of radiation absorbed dose using a chemical dosimeter and NMR-imaging techniques provides non-invasive 3D dosimetry. The main problems with this new method are measurement accuracy and diffusion effects. These involve influences on chemical preparations, correct data acquisition (evaluation of the T1 values) and a lack of methods for reducing diffusion effects. The aim of this study was to develop a computer assisted localized dosimetry method using fast and quantitative NMR-imaging techniques in order to reduce diffusion effects and to determine the accuracy of this new approach. We show successful localized dosimetry experiments and present a new method for the calculation of the diffusion coefficient.

MATERIAL AND METHODS
The dosimeter gel contained a modified Fricke solution and was stabilized with gelatin. Several compositions of dosimeter gels and different conditions were investigated in order to optimize the treatment. IR-Snapshot-Flash has been used to determine the T1-values. The diffusion coefficient of the bound ferric ions was estimated by taking advantage of the short acquisition times of this fast NMR-imaging technique.

RESULTS
Dependencies of the dosimeter system were investigated. The calculated error limits for evaluation of the measurement accuracy are in the range of 10 to 20%. The accurate localized determination of the T1-values and the diffusion has considerable influence on the measurement. Fast and quantitative NMR-imaging methods for accurate T1-calculations like IR-Snapshot-Flash reduce this effect.

CONCLUSION
Our results show the advantage of the use of fast MR-imaging techniques for NMR Fricke dosimetry. The diffusion effects are reduced effectively, so even details can be imaged. We also developed a new method for the calculation of the diffusion coefficient, which can be used for paramagnetic ions in any aqueous solution.