Exploration of a virtual MVCT for use in radiation therapy planning

Radiation therapy planning systems utilize CT imaging to determine the electron density of patient anatomy and measurement phantoms. The properties of the kilovoltage photons from a CT scanner are different to the megavoltage photons used for treatment on a linear accelerator, meaning that a CT Number to electron density calibration is required. However not all materials have the same CT Number to electron density conversion as it is a complicated relationship, depending on the atomic number, physical density and electron density of the material. The aim of this work was to develop a technique to identify such materials using the megavoltage X-ray of a linear accelerator and electronic portal imaging device (EPID). The technique uses the pixel values of a single planar image to identify an unknown material that is contained within a known substance, such as a metal prosthesis within tissue. Another use is the accurate determination of the electron density of a phantom material for use in dosimetry measurements. Simple, homogeneous phantoms were measured to within 3 % of their reported electron density. The identification of an inner portion of a multi-part phantom was also successful, with reported values matching the measured results within experimental uncertainty.