Ionoacoustics

The field of ionoacoustics deals with the online verification of a treatment plan during proton and ion beam therapy. Potentially, by measuring ionoacoustic signals during irradiation and their travel time through the body, the theoretically calculated irradiation field can be matched with the actual irradiation field in real time.

One of the major advantages of proton and ion beam therapy is its characteristic dose distribution, where the majority of the dose is delivered at the so-called Bragg peak at the end of its range and ideally in the tumor. The implementation of this irradiation target is prone to error because the exact position of the Bragg peak depends on many factors, such as patient positioning or minor changes in anatomy. Therefore, when creating a treatment plan for tumor therapy, it is common to intentionally enlarge the tumor region to be irradiated by large margins to ensure that the tumor region is irradiated with sufficient dose. As a result, normal tissue close to the tumor is irradiated as well, resulting in increased side effects. Ionoacoustics can help to reduce these margins and measure the exact position of the Bragg peak in real time during irradiation.

The energy deposition of the proton or ion beam at the Bragg peak causes a local temperature increase and thus a local pressure increase. After propagation of this pressure through the body, it can be detected as an ultrasound wave using suitable sensors. The time of flight of the ultrasound wave from the Bragg peak to the sensor can be extracted from the measured signals and is used to calculate the distance between Bragg peak and sensor. In combination with a live ultrasound image of the irradiated region, the Bragg peak position can be marked directly in the ultrasound image relative to surrounding organs.

A major challenge in the detection of ionoacoustic signals is the low signal-to-noise ratio (SNR). Current research is mainly concerned with optimizing the beam parameters (e.g., pulse duration, pulse shape, and beam current) to achieve the highest possible SNR at a constant dose. In addition, the signals are denoised in a post-processing procedure to further increase the SNR. Thus, it is possible to achieve range verifications in the submillimeter range in homogeneous phantoms at clinically relevant doses.

Future tasks will be the transfer of this methodology to human-like phantoms with heterogeneities and thus demonstrate the clinical applicability of ionoacoustics.