Magnetic resonance imaging approaches to gel dosimetry for validation of conformal radiotherapy treatment plans

Gel dosimeters are suitable for the purpose of 3-D dose verification using imaging methods such as magnetic resonance (MR) imaging, which can provide a spatial resolution of ∼1 mm or better. The two main kinds of gels investigated to date (Fricke and polymer-based gel dosimeters) face various problems which have impeded their full clinical use. The Fricke-based gel dosimeters suffer the problem of spatial information loss with time due to diffusion of radiation-induced ferric ions. Polymer-based gels are quite toxic (carcinogens, teratogens, etc.) and can't be prepared in normoxic conditions. Possible solutions to these problems are the use of fast imaging methods to limit diffusion for Fricke gels and development of less or non toxic gels for the case of polymer gels.; In this thesis, the ability of the MR imaging Look-Locker method to act as a dose verification tool for Fricke gel dosimeters due to its speed is studied. This is done through the modification of the conventional Look-Locker method to accommodate larger tip angles thereby improving on its precision and accuracy. The optimum method of acquiring the image data in Fourier or k-space so as to limit the effect of image blurring due to a time point spread function inherent of this imaging method is proposed.; Also studied is the prospect of using two new polymer-based gel dosimeters with reduced toxicity as a dose verification tool. This is demonstrated by verifying a treatment planned dose distribution using these gel dosimeters and MR imaging. A multiple spin-echo MR imaging sequence is used as the imaging tool, with possible problems which can affect the accuracy of the measured dose distribution such as B1 field inhomogeneity and geometrical distortions due to non-linear gradient magnetic fields mapped out and corrected for.