X-ray acoustic imaging for external beam radiation therapy dosimetry using a commercial ultrasound scanner

Photoacoustic (PA) imaging has been used for numerous applications in clinical medicine and preclinical studies. Usually, nanosecond laser pulses are used to generate PA signals. In laser-based PA imaging, the contrast is based on optical absorption. The generated PA signals can be detected at greater depths, given that the limits imposed by photon scattering of purely optical based techniques are overcome. However, less effort has been directed towards the use of X-ray photons, which have more penetration depth than photons in the visible NIR region. Modern linear accelerators can provide polychromatic X-ray with sufficient power density to produce microseconds X-ray pulses capable of inducing ultrasonic waves in the material. Based on this concept, the X-ray acoustic computer tomography (XACT) was proposed to generate images by combining X-ray excitation and ultrasonic detection. In XACT an ultrasonic single element transducer with central frequency of 500 kHz was moved 360° around the sample. The present study proposes the use of X-ray photons to generate x-ray acoustic (XA) imaging using a commercial ultrasound system, where a linear ultrasound probe was used to acquire XA signals during external beam radiation therapy (RT). To validate our system, lead samples were positioned inside a water tank and then were irradiated to generate XA signals for 6 MV and 15 MV energies. The XA signals were captured by the ultrasound transducer operating in a frequency range between 5 MHz and 14 MHz, using the delay and sum beamforming to generate the images. We obtained XA images of lead samples consistent to XACT and the signals analysis showed XA signals with amplitude increased for higher dose-rates. These results demonstrate the feasibility of generating XA images, which provided dosimetric information during RT using a linear accelerator and a commercial ultrasound system.