Many of these current approaches, however, involve complex instrumentation and bulky detectors; what's needed is a compact and cost-effective alternative. Speaking at the ICTR-PHE meeting, held last week in Geneva, Switzerland, Katia Parodi described novel investigations on a technique that may just meet these requirements: "ionoacoustic" imaging.

Parodi, Chair for Medical Physics at Ludwig Maximilian University (LMU) in Munich, Germany, explained that ionoacoustic imaging is very similar to optoacoustic imaging, in which light pulses absorbed by tissue cause localized thermal expansion and generate acoustic waves. The difference is that in this case, the therapeutic ion beam is used to induce the heating effect. "The idea is that most of the energy deposition will be concentrated in the area of the Bragg peak," she said. "This results in thermal expansion and generation of an acoustic wave, which we can then detect with an acoustic transducer."

For eventual clinical application, the researchers envision an ionoacoustic imaging system with a penetration depth of several centimetres and sub-millimetre resolution. Ultimately, they propose to combine anatomic ultrasound imaging prior (or even in parallel) to treatment with ionoacoustic detection of the Bragg peak during ion beam therapy.

System tests

To test the potential of this approach, the research team - a collaboration between Ludwig-Maximilians-Universität München (Katia Parodi and colleagues); Helmholtz Zentrum München and Technische Universität München (Vasilis Ntziachristos and colleagues); and Universität der Bundeswehr München (Günther Dollinger and colleagues)- performed proton beam experiments at the Maier-Leibnitz-Laboratory's Tandem accelerator. They used 20 MeV protons, chosen to provide a sharp Bragg peak, to irradiate a water phantom with pulsed beams of varying pulse length (1.5 ns – 4 µs). The ensuing acoustic signals were detected using a 3.5 MHz cylindrically focused ultrasound transducer and a spherically focused 10 MHz transducer.

For beam intensities above 105 protons/pulse, a clear acoustic signal could be acquired. The signal showed three spikes, attributed to the Bragg peak, the entrance window and a reflection. The distance between the first and second of these spikes is proportional to the ion beam penetration depth and can be used to assess beam range. The measured range resolution agreed with that seen in simulations. "Sub-millimetre resolution of the Bragg peak position seems to be possible, quantification is ongoing," said Parodi.

The water phantom set-up was also examined with a 0.52 mm thick aluminium absorber inserted to reduce the range by 1 mm. This range difference could be clearly resolved in the ionoacoustic measurements.

Next, the researchers moved onto 2D Bragg peak imaging, achieved by xy scanning the 10 MHz transducer. In parallel, they imaged the Bragg peak using Gafchromic film and performed Geant4 Monte Carlo simulations. The ionoacoustic images of the Bragg peak agreed reasonably well with both the film and the simulated images, both with and without the aluminium absorber.

Introducing tomography

Very recently, the researchers have begun to investigate ionoacoustic tomography, using a 64-channel ultrasound transducer array originally developed by Ntziachristos et al for Multispectral Optoacoustic Tomography (MSOT)1,2, which enabled recording of the first three-dimensional images.

Parodi concluded that ionoacoustic detection offers a direct way to observe the energy deposition of an ion beam in tissue, with the potential for 2D and 3D imaging with sub-millimetre range resolution. The minimum detectable signal was so far found to be 104 protons/pulse, representing a 1012 eV energy threshold.

The next steps in this project will involve testing the ionoacoustic effect using more realistic targets and higher-energy ion beams from conventional and novel high-dose-rate sources, as well as simulating the entire imaging process and exploring the technique's applicability to various clinical treatment sites.

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