Effect of fat content on single- and dual-energy CT measurements of bone mineral: determination using a new system of tissue-mimicking phantom materials

The effects of fat content on single- and dual-energy CT measurements of bone mineral were quantified using a set of tissue-mimicking phantoms which more accurately represents the in-vivo situation than previous phantoms. The key to performing these measurements in CT is to have a mixture of tissue types within each image voxel, a condition which is not satisfied with standard phantoms. The phantoms used in these studies were solid materials which mimicked 17 different homogeneous mixtures of bone, muscle, and fat. The concept of creating phantoms to mimic different mixtures of these tissues is new. The materials are epoxy-resin based and have different mixtures of phenolic microspheres, polyethylene, and calcium carbonate suspended in them. Single- and dual-energy CT were used to image the phantom materials, and the effects of fat content on bone-mineral measurements were determined. The single-energy CT measurements show how fat content causes an underestimation of the amount of bone mineral present in a specimen, with the underestimation increasing as a function of fat content. With 25% and 50% fat by volume, the single-energy measurements underestimated bone volume percentage by 2.7% and 3.6% respectively. With dual-energy CT, fat content has no effect on the measurement of bone mineral. These results are not surprising. In fact, the effects of fat content on single- and dual-energy CT measurements have been studied many times previously. However, a system of accurately measuring these effects using a set of phantom measurements with physiologically accurate tissue-mimicking materials has not been developed previously. Using these phantoms, dual-energy CT measurements can be accurately calibrated for measurements of bone mineral while the errors possible while measuring bone mineral with single-energy CT can be quantified for any given imaging parameters.