(G-272) Vaporization of Endoskeletal Droplets by Indirect Ionizing Radiation

Introduction:: X-ray acoustic computed tomography (XACT) is a novel imaging technique with the potential for radiation dosimetry. By using the acoustic waves generated by radiation absorption, XACT can create real-time images during radiation therapy. Since the intensity of these images is linked to the radiation dose, they can also be used for relative dosimetry. Recent studies have demonstrated XACT’s ability to collect dose distributions during treatment while overlaying them with anatomical images [1]. Despite these advancements, x-ray acoustics suffer from a low signal-noise ratio (SNR). We hypothesize that we can increase the SNR for XACT by introducing vaporizable endoskeletal droplets as ultrasound contrast agents.

Nanodroplets have been shown to vaporize under radiation. When seen through ultrasound, these vaporization profiles have been shown to coordinate radiation delivery with sub-millimetric (0.5 ± mm) accuracy, while enhancing contrast [2]. Vaporizable endoskeletal droplets (VEDs) offer the same capabilities as nanodroplets but with better stability and tunability of vaporization temperature. VEDs consist of a liquid perfluorocarbon droplet with a solid hydrocarbon core. When exposed to heat, the hydrocarbon core melts and mixes with the liquid perfluorocarbon, weakening its cohesion, and causing vaporization. This mechanism allows for better tunability of vaporization through a selection of the hydrocarbon core [3]. The aim of this research is to test the feasibility of VED vaporization by radiation.


Materials and Methods:: VEDs were made from liquid perfluoropentane (n-C₅F₁₂) and solid nonadecane (n-C₁₉H₄₀). Within the hydrocarbon core, zinc 2,11,20,29-tetra-tert-butyl-2.3-napthalocyanine dye was incorporated (2% by weight of hydrocarbon), improving x-ray absorption due to the presence of the high-Z material. The VEDs were measured to have a mean diameter of 1.532 ± 1.17 μm. The VEDs were split into 3 samples (10⁸ drops/mL per sample) and homogeneously distributed into a tissue-mimicking gel through manual stirring (Carbopol 940, 0.2% wt/vol) The gel spatially fixed the droplets and resulting microbubbles, allowing for easy detection of new vaporization events. Each sample was placed in a water tank directly underneath a Varian TrueBeam Linear Accelerator (Figure 1). The surface of the water was placed 100 cm away from the radiation source while the samples were placed 1.4 cm underneath the surface correlating to d_max (depth of maximum dose) for our radiation beam.

Each set of samples was irradiated with 50 Gy of dose by a 6FFF (Flattening-Filter Free) MV photon beam with a 5x5 field size. The radiation field was off centered by 5 cm to the right so only half of the gel was exposed providing the left side as background. During irradiation, our ULA-OP system with a LA322 transducer (center frequency = 9.75 MHz, pulse repetition frequency 8 kHz, and peak-negative Pressure = 1.08 MPa) was used to capture the vaporization events in B-mode.


Results, Conclusions, and Discussions:: For the three samples that were irradiated, Figure 2 represents the sample with the highest number of vaporization events recorded. The right side of our images (Figure 2b) presents more vaporization events than the left side (Figure 2a). Of the vaporization events recorded, 8 new vaporization events occurred in the presence of radiation compared to 0 on the left side.

Acknowledgements (Optional): :

References (Optional): : [1] W. Zhang, I. Oraiqat, H. Lei, P. L. Carson, I. EI Naqa, and X. Wang, “Dual-Modality X-Ray-Induced Radiation Acoustic and Ultrasound Imaging for Real-Time Monitoring of Radiotherapy,” BME Front., vol. 2020, May 2020, doi: 10.34133/2020/9853609.

[2] G. Collado-Lara et al., “Spatiotemporal Distribution of Nanodroplet Vaporization in a Proton Beam Using Real-Time Ultrasound Imaging for Range Verification,” Ultrasound Med. Biol., vol. 48, no. 1, pp. 149–156, Jan. 2022, doi: 10.1016/j.ultrasmedbio.2021.09.009.

[3]G. Shakya, S. E. Hoff, S. Wang, H. Heinz, X. Ding, and M. A. Borden, “Vaporizable endoskeletal droplets via tunable interfacial melting transitions,” Sci. Adv., vol. 6, no. 14, p. eaaz7188, Apr. 2020, doi: 10.1126/sciadv.aaz7188.