Improved polymer gels for radiation dosimetry by MRI

这些研究的目的是开发可用于准确测量三维辐射剂量分布的改进的聚合物凝胶。聚合物凝胶剂量计是我们实验室首次开发的用于磁共振成像的仪器，它记录了单体混合物在水凝胶中照射产生的水质子的核磁共振弛豫时间的变化。这种凝胶是组织等效的，以高空间分辨率提供独特而准确的三维综合剂量分布测量。尽管如此，目前可用的聚合物凝胶在实践中的使用仍然存在重大问题，限制了其更广泛的使用。这些困难包括凝胶的制备和储存困难，凝胶必须在使用前保持低氧状态，因此不能倒在塑料或任意容器中；它们的可变敏感性，主要是由于微量氧抑制其反应的影响；以及有限剂量反应，其基础是测量T2值的变化，而T2值容易出现误差。拟议中的研究将导致更敏感、更稳定的凝胶的显著改进，医院物理学家可以在正常气氛和任何容器中制造这种凝胶，而无需特殊程序。在初步研究中，我们已经在室内空气中生产出了比目前的凝胶更敏感的凝胶。它们使用了一种基于铜离子和抗坏血酸的新型引发剂体系，当凝胶受到辐射时，该体系与分子氧结合并需要分子氧才能开始聚合。我们将通过探索该系统的不同浓度和组件的影响来优化这些剂量计的性能。我们还表明，辐照凝胶中的弛豫是通过自由水和与聚合物中的半固态池接触的不稳定质子之间的磁化转移(MT)发生的。我们将设计和验证MT敏感序列的性能，避免T2测量的缺陷，用于成像凝胶以提取准确的剂量信息。我们还假设，聚合物凝胶的剂量响应简单地反映了这个半固态质子池的大小随着剂量的增加而线性增加，我们将进行新的核磁共振和其他物理测量来验证这一模型。总体而言，我们的目标是为实际应用生产更多改进的凝胶，并更好、更定量地了解它们对辐射的响应。

The studies proposed aim to develop improved polymer gels that may be used for accurate measurements of radiation dose distributions in three dimensions. Polymer gel dosimeters were first developed in our laboratory for use with MRI, which records the changes in NMR relaxation times of water protons produced by irradiation of mixtures of monomers in an aqueous gel. Such gels are tissue equivalent and provide unique and accurate measurements of integrated dose distributions in three dimensions with high spatial resolution. Nonetheless, there remain significant problems in the use of currently available polymer gels in practice that have limited their wider use. These include difficulties of preparation and storage of gels, which must be made and kept hypoxic until used and which therefore cannot be poured in plastic or arbitrary containers: their variable sensitivity, which largely arises from the influence of trace amounts of oxygen which inhibit their response: and the limited dose response, which is based on measuring changes in T2 values that are subject to errors. The studies proposed would lead to significantly improved gels that are more sensitive, more stable, and which can be made in a normal atmosphere and any container by a hospital physicist without special procedures. In preliminary studies we have produced gels in room air that are much more sensitive than current gels. These use a novel initiator system based on copper ion and ascorbic acid which combines with and requires molecular oxygen to start polymerization when the gel is irradiated. We will optimize the performance of these dosimeters by exploring the effects of different concentrations and components of this system. We have also shown that relaxation in irradiated gels occurs via magnetization transfer (MT) between free water and labile protons in contact with a semi-solid pool in the polymer. We will design and verify the performance of MT-sensitive sequences that avoid the pitfalls of T2 measurements for imaging gels to extract accurate dose information. We also postulate that the dose- response of polymer gels simply reflects a linear increase in the size of this semi-solid proton pool with increasing dose and we will perform novel NMR and other physical measurements to verify this model. Overall we aim to produce much improved gels for practical applications and obtain a better and more quantitative understanding of how they respond to radiation.