Temperature dependence of HU values for various water equivalent phantom materials.

The temperature dependence of water equivalent phantom materials used in radiotherapy and diagnostic imaging has been investigated. Samples of phantom materials based on epoxy resin, polyethylene, a polystyrene-polypropylene mixture and commercially available phantom materials (Solid Water, Gammex RMI and Plastic Water, Nuclear Associates) were scanned at temperatures from 15 to 40 degrees C and HU values determined. At a reference temperature of 20 degrees C materials optimized for CT applications give HU values close to zero while the commercial materials show an offset of 119.77 HU (Plastic Water) and 27.69 HU (Solid Water). Temperature dependence was lowest for epoxy-based materials (EPX-W: -0.23 HU degrees C(-1); Solid Water: -0.25 HU degrees C(-1)) and highest for a polyethylene-based material (X0: -0.72 HU degrees C(-1)). A material based on a mixture of polystyrene and polypropylene (PSPPI: -0.27 HU degrees C(-1)) is comparable to epoxy-based materials and water (-0.29 HU degrees C(-1)).