Magnetic resonance in polymer gel dosimetry: techniques and optimization

1. Introduction Magnetic resonance imaging (MRI) can non-invasively and non-destructively probe the magnetization of hydrogen atoms on water molecules in large objects with a high spatial resolution, and in three dimensions. This unique combination of properties is exploited in polymer gel dosimetry since original gel dosimeters, in which viscosity changes were measured and related to the absorbed dose [1], were adapted to obtain a three dimensional dose distribution [2]. The original publication for a 3D dosimeter using MRI dates back to 1990 [3] where a well-known Fricke dosimeter [4] was combined with an aqueous agarose matrix. This followed the observation that irradiation of a ferrous sulfate solution leads to measurable changes in NMR relaxation times [5]. In polymer gel dosimeters, a free radical-induced polymerization reaction leads to the formation of polymer clusters or chains in a gelatin matrix [6-8]. The contrast mechanisms by which MRI can, directly or indirectly, detect the presence of this polymer have already been reviewed [9] and will not be detailed here. Rather, this review will summarize the different MRI techniques that have been applied to date to probe polymer gel dosimeters. The ultimate goal of this, and other, imaging technique is to provide the most precise and accurate dose determination, which implies an optimization in terms of signal-to-noise (SNR) and the absence of imaging artefacts or distortions. All researchers and medical physicists using polymer gel dosimeters do not have access to the same scanners and/or cannot modify the pulse sequences packaged by the manufacturer. This review will thus specify the type of scanner, the field strength, the version, and whether the sequence was available or whether it was modified. Finally, the type of sequence (i.e., multi-slice or 3D) will be reported. Note that some reports were not covered in this review, as they did not provide the uncertainties on the measurements and thus could not be compared adequately. The paper will follow the chronological development of MRI techniques, and their optimization, applied to polymer gel dosimeters. It is not the scope to trace these techniques back to their original authors, since this aspect is covered in a number of excellent reviews and textbooks [e.g., [10,11]].