Prospects and challenges of rendering tissue density in Hounsfield units for cone beam computed tomography

Objectives. Limitations in rendering of tissue density in Hounsfield units (HUs) for cone beam computed tomography (CBCT) are described and illustrated using a phantom and two CBCT systems to demonstrate grayscale measurement variability. Materials and methods. The basis of the HU scale, its correlation with measured computed tomography (CT) numbers, and the limitations in the accuracy of such correlation due to artifacts are discussed. Rendering of tissue densities based on HU values of two CBCT systems [NewTom VGi and Hyperion X9, respectively large and small field of view (FOV)] are measured using a phantom. Results. Data produced from small FOV CBCT acquisition are generally less affected by artifacts compared with large FOV CBCT. Conclusions. Artifacts challenge the accurate conversion of density values into HUs. Care should be taken when interpreting quantitative density measurements obtained with CBCT. With more advanced software and methods, it may be possible to improve the consistency and accuracy of density measurements.