Abstract
The characteristic depth dose deposition of ion beams, with a maximum at the end of their range (Bragg peak) allows for local treatment delivery, resulting in better sparing of the adjacent healthy tissues compared to other forms of external beam radiotherapy treatments. However, the optimal clinical exploitation of the favorable ion beam ballistic is hampered by uncertainties in the in vivo Bragg peak position. Ionoacoustics is based on the detection of thermoacoustic pressure waves induced by a properly pulsed ion beam (e.g., produced by modern compact accelerators) to image the irradiated volume. Co-registration between ionoacoustics and ultrasound imaging offers a promising opportunity to monitor the ion beam and patient anatomy during the treatment. Nevertheless, the detection of the ionoacoustic waves is challenging due to very low pressure amplitudes and frequencies (mPa/kHz) observed in clinical applications. We investigate contrast agents to enhance the acoustic emission. Ultrasound microbubbles are used to increase the ionoacoustic frequency around the microbubble resonance frequency. Moreover, India ink is investigated as a possible mean to enhance the signal amplitude by taking advantage of additional optical photon absorption along the ion beam and subsequent photoacoustic effect. We report amplitude increase of up to 200% of the ionoacoustic signal emission in the MHz frequency range by combining microbubbles and India ink contrast agents.
Highlights
The characteristic depth dose deposition of ion beams, with a maximum at the end of their range (Bragg peak) allows for local treatment delivery, resulting in better sparing of the adjacent healthy tissues compared to other forms of external beam radiotherapy treatments
The contrast agents (CA) phantom was consecutively filled with three different CA: SonoVue microbubbles diluted in deionized water (MB), a mixture of SonoVue microbubbles and India ink diluted in deionized water (MBink) and India ink diluted in deionized water (India ink)
We experimentally showed that combining India ink to microbubbles increases the amplitude of the ionoacoustic signals produced when a proton beam enters the targeted volume (CA phantom)
Summary
The characteristic depth dose deposition of ion beams, with a maximum at the end of their range (Bragg peak) allows for local treatment delivery, resulting in better sparing of the adjacent healthy tissues compared to other forms of external beam radiotherapy treatments. Ionoacoustics is based on the detection of the direct acoustic signal resulting from the energy deposition which is enhanced in the Bragg peak region. It is worth noting that the dose dependence of the ionoacoustic signals implies change of the pressure wave frequency and amplitude within the treatment field as the dose distribution could be distorted due to heterogeneous tissue composition or variation in the proton beam energy (adapted during the treatment to reach a specific penetration depth). The low pressure amplitudes and the low frequency range, in the order of some mPa in the 10 to 50 kHz range for clinically relevant proton beam energies[10], make the detection of ionoacoustic signals challenging and compromise the co-integration with ultrasound imaging systems which operate in the MHz range. In this work we investigate the ionoacoustic response obtained from these two types of CA (microbubbles and India ink-based) depending on the time structure of the pulsed beam (temporal excitation) and the dose distribution (spatial excitation) for low-energy mono-energetic proton beams
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