Purpose: The goal of radiotherapy is to deliver dose to the tumor while sparing organs at risk (OAR). Some cases can become challenging due to organ motion or breathing. X-ray induced acoustic tomography (XACT) offers an alternative for image guided radiotherapy (IGRT) without using additional imaging dose. This study compared XACT images from different medium.
Methods: A low frequency single element transducer was used to acquire XACT signals on water and oil from a 4 cm square photon field. The transducer was fixed in front of the radiation field at an appropriate distance and depth of measurement. Beam delivery was performed using a Varian TrueBeam with 10X-FFF energy and 4 microsecond linac pulse to trigger acquisition. The XA signals passed through a shielded preamplifier, then collected in an oscilloscope with high averaging. Data collection was performed at every 6° collimator angle, for a total of 60 signals. MATLAB was used for signal processing and image reconstruction using simple backprojection. 2D images were obtained for the field in both water and oil medium. The frequency spectrum was compared for both medium and background signals.
Results: The reconstructed square field was clearly visible in oil, while the image from water was considerably weaker presenting artifacts. Each signal contains rarefactions where the middle corresponds to the boundaries of the field. The signal intensity reached 1mVpp in water, while in oil it was 15mVpp. The frequency spectrum in oil presented larger response at lower frequencies, having at 50kHz a ratio of 11 to the water medium.
Conclusion: This XACT system was able to reconstruct the image of the photon field. Signal strength is highly dependent on the acoustic properties and radiation absorption of the medium. Further analysis of frequency response could give insight on using dual mode ultrasound to improve image quality.