Determination of the modulation transfer function of digitally reconstructed radiographs in radiotherapy treatment planning using a point phantom.

Digitally reconstructed radiographs (DRRs) have become an important tool in radiotherapy treatment planning and treatment verification but their use can be limited by poor image quality. A point phantom comprised of a 1 mm diameter tungsten carbide ball bearing in a wax block has been manufactured as a means of determining a quantitative measure of DRR spatial resolution by calculating the modulation transfer function (MTF) of the image. This method is an alternative to the more often used bar pattern method. The effect of varying CT slice thickness upon DRR spatial resolution has been assessed and a power law relationship between the two variables has been established.