Magnetic resonance imaging of Fricke-doped agarose gels for the visualization of radiotherapy dose distributions in a lung phantom

Abstract

When calculating a treatment plan for a radiation beam that passes through an inhomogeneity such as lung, the dose at each point needs to be modified to correct for the effect of the inhomogeneities. Many algorithms exist for making such corrections. Algorithms must be verified by measurements in phantoms. Measurements with an ionization chamber in a solid phantom can only be performed at a limited number of points, giving poor spatial resolution. Film dosimetry gives better spatial resolution, but the introduction of non-tissue equivalent photographic film represents a considerable perturbation of a tissue equivalent phantom. Shultz et al (1990) and Olsson et al (1990) have recently described techniques for the measurement of absorbed dose using magnetic resonance imaging (MRI) of agarose gels doped with Fricke (iron (II) sulphate) solution. After Fe²⁺ has been oxidized by irradiation to Fe³⁺, the gel fixes the position of the ions, so that the distribution of Fe³⁺ in the gel corresponds to the dose distribution. The gel is then imaged to produce a map of the longitudinal proton relaxation time (T¹). As a result of the paramagnetic properties of Fe³⁺, T¹ is inversely proportional to Fe³⁺ concentration and hence to absorbed dose of radiation. Thus, the technique provides a method of determining a continuous distribution of radiation dose within a phantom. This paper describes preliminary work using the technique on a phantom containing a volume of low density material designed to simulate lung.