Effect of a Radiotherapeutic Megavoltage Beam on Ultrasound Contrast Agents

A Preliminary Investigation of Radiation-Sensitive Ultrasound Contrast Agents for Photon Dosimetry

Radiotherapy treatment plans have become highly conformal, posing additional constraints on the accuracy of treatment delivery. Here, we explore the use of radiation-sensitive ultrasound contrast agents (superheated phase-change nanodroplets) as dosimetric radiation sensors. In a series of experiments, we irradiated perfluorobutane nanodroplets dispersed in gel phantoms at various temperatures and assessed the radiation-induced nanodroplet vaporization events using offline or online ultrasound imaging. At 25 °C and 37 °C, the nanodroplet response was only present at higher photon energies (≥10 MV) and limited to <2 vaporization events per cm2 per Gy. A strong response (~2000 vaporizations per cm2 per Gy) was observed at 65 °C, suggesting radiation-induced nucleation of the droplet core at a sufficiently high degree of superheat. These results emphasize the need for alternative nanodroplet formulations, with a more volatile perfluorocarbon core, to enable in vivo photon dosimetry. The current nanodroplet formulation carries potential as an innovative gel dosimeter if an appropriate gel matrix can be found to ensure reproducibility. Eventually, the proposed technology might unlock unprecedented temporal and spatial resolution in image-based dosimetry, thanks to the combination of high-frame-rate ultrasound imaging and the detection of individual vaporization events, thereby addressing some of the burning challenges of new radiotherapy innovations.

Activation of Phase Change Contrast Agents Using Ionizing Radiation

The feasibility of activating phase change contrast agents (PCCA) made from Definity (Lantheus Medical Imaging) using X-rays was investigated. A 10 mL of Definity PCCA (0.1 mL PCCA/mL) were injected into gelatin phantoms and irradiated using doses of 0, 30, 50, and 100 Gy. Size distribution and PCCA activation were measured after irradiation. Definity PCCAs were activated at a threshold of 50 Gy. Changes were visible with microscopy, visual inspection of T-flasks, and ultrasound imaging of gelatin phantoms. Moreover, increasing the radiation dose above 50 Gy appeared to further activate PCCA. These results indicate Definity PCCAs may be useful for ultrasound-based radiation dosimetry.

Next-Generation Colloidal Materials for Ultrasound Imaging Applications

Ultrasound has important applications, predominantly in the field of diagnostic imaging. Presently, colloidal systems such as microbubbles, phase-change emulsion droplets and particle systems with acoustic properties and multiresponsiveness are being developed to address typical issues faced when using commercial ultrasound contrast agents, and to extend the utility of such systems to targeted drug delivery and multimodal imaging. Current technologies and increasing research data on the chemistry, physics and materials science of new colloidal systems are also leading to the development of more complex, novel and application-specific colloidal assemblies with ultrasound contrast enhancement and other properties, which could be beneficial for multiple biomedical applications, especially imaging-guided treatments. In this article, we review recent developments in new colloids with applications that use ultrasound contrast enhancement. This work also highlights the emergence of colloidal materials fabricated from or modified with biologically derived and bio-inspired materials, particularly in the form of biopolymers and biomembranes. Challenges, limitations, potential developments and future directions of these next-generation colloidal systems are also presented and discussed.

Deep learning for dose assessment in radiotherapy by the super-localization of vaporized nanodroplets in high frame rate ultrasound imaging

Objective. External beam radiotherapy is aimed to precisely deliver a high radiation dose to malignancies, while optimally sparing surrounding healthy tissues. With the advent of increasingly complex treatment plans, the delivery should preferably be verified by quality assurance methods. Recently, online ultrasound imaging of vaporized radiosensitive nanodroplets was proposed as a promising tool for in vivo dosimetry in radiotherapy. Previously, the detection of sparse vaporization events was achieved by applying differential ultrasound (US) imaging followed by intensity thresholding using subjective parameter tuning, which is sensitive to image artifacts. Approach. A generalized deep learning solution (i.e. BubbleNet) is proposed to localize vaporized nanodroplets on differential US frames, while overcoming the aforementioned limitation. A 5-fold cross-validation was performed on a diversely composed 5747-frame training/validation dataset by manual segmentation. BubbleNet was then applied on a test dataset of 1536 differential US frames to evaluate dosimetric features. The intra-observer variability was determined by scoring the Dice similarity coefficient (DSC) on 150 frames segmented twice. Additionally, the BubbleNet generalization capability was tested on an external test dataset of 432 frames acquired by a phased array transducer at a much lower ultrasound frequency and reconstructed with unconventional pixel dimensions with respect to the training dataset. Main results. The median DSC in the 5-fold cross validation was equal to ∼0.88, which was in line with the intra-observer variability (=0.86). Next, BubbleNet was employed to detect vaporizations in differential US frames obtained during the irradiation of phantoms with a 154 MeV proton beam or a 6 MV photon beam. BubbleNet improved the bubble-count statistics by ∼30% compared to the earlier established intensity-weighted thresholding. The proton range was verified with a −0.8 mm accuracy. Significance. BubbleNet is a flexible tool to localize individual vaporized nanodroplets on experimentally acquired US images, which improves the sensitivity compared to former thresholding-weighted methods.