An assessment of photoacoustic and photon counting multispectral x-ray imaging techniques for imaging gold nanorods in vivo as part of predicting dose enhancing effects

Radiotherapy, whilst useful in the treatment of many cancers, suffers from the drawback of unavoidably delivering radiation doses to healthy tissue. Gold nanoparticles (GNPs) have been the subject of much interest as they are able to generate a local dose enhancing effect (DEE), allowing lower intensity beams to achieve a given effect, sparing healthy tissue. It is important that GNP distributions can be ascertained before radiotherapy begins so that accurate treatment plans can be made. Two imaging modalities that show great promise in being able to identify these GNPs in vivo without delivering excessive DEEs are photoacoustic imaging (PAI) and photon counting multi-spectral x-ray imaging (x-CSI). This work forms part of a larger project to assess the feasibility of using x-CSI and PAI to predict DEEs during radiotherapy planning. This work assessed the degradation of gold nanorods (GNRs) in a photoacoustic system sold by iThera, specifically comparing the protective effects against degradation afforded by a silica coating as opposed to GNR PEGylation. This coating is also hoped to prevent spectral broadening when the GNRs are endocytosed, allowing PAI to confirm uptake of GNRs by cells. With regards x-CSI, this work uses a combination of Monte Carlo and Finite Element modelling techniques to realistically model cylindrical phantoms of high-Z materials in order to demonstrate the potential improvements in imaging contrast that x-CSI can produce over traditional energy integrated x-ray imaging techniques. Results on the assessment of both techniques are discussed.