Experimental evaluation of x‐ray acoustic computed tomography for radiotherapy dosimetry applications

Abstract

Purpose: The aim of this work was to experimentally demonstrate the feasibility of x-ray acoustic computed tomography (XACT) as a dosimetry tool in a clinical radiotherapy environment. Methods: The acoustic waves induced following a single pulse of linear accelerator irradiation in a water tank were detected with an immersion ultrasound transducer. By rotating the collimator and keeping the transducer stationary, acoustic signals at varying angles surrounding the field were detected and reconstructed to form an XACT image. Simulated XACT images were obtained using a previously developed simulation workflow. Profiles extracted from experimental and simulated XACT images were compared to profiles measured with an ion chamber. A variety of radiation field sizes and shapes were investigated. Results: XACT images resembling the geometry of the delivered radiation field were obtained for fields ranging from simple squares to more complex shapes. When comparing profiles extracted from simulated and experimental XACT images of a 4 cm 9 4 cm field, 97% of points were found to pass a 3%/3 mm gamma test. Agreement between simulated and experimental XACT images worsened when comparing fields with fine details. Profiles extracted from experimental XACT images were compared to profiles obtained through clinical ion chamber measurements, confirming that the intensity of XACT images is related to deposited radiation dose. Seventy-seven percent of the points in a profile extracted from an experimental XACT image of a 4 cm 9 4 cm field passed a 7%/4 mm gamma test when compared to an ion chamber measured profile. In a complicated puzzle-piece shaped field, 86% of the points in an XACT extracted profile passed a 7%/4 mm gamma test. Conclusions: XACT images with intensity related to the spatial distribution of deposited dose in a water tank were formed for a variety of field sizes and shapes. XACT has the potential to be a useful tool for absolute, relative and in vivo dosimetry.

Simulation of x‐ray‐induced acoustic imaging for absolute dosimetry: Accuracy of image reconstruction methods

“…While XACT using BP has been studied extensively due to its potential for relative dosimetry, it has not been studied for its potential for absolute dosimetry. By comparing results from both BP and ITR imaging algorithms for their accuracy in the dose reconstruction, we investigated the feasibility of using XACT imaging for absolute dosimetry.…”

“…Also, the effects of acoustic noise on the accuracy of dose reconstruction are not investigated in this work. While noise is an important problem in XA imaging, it can be suppressed by filtering techniques or averaging the signal from multiple measurement …”

“…In RT, XA is generated by medical linear accelerators (LINACs) . The generated XA signal has been shown to be proportional to the absorbed dose . X‐ray pulses from a LINAC are typically around 4–6 µs in duration.…”

“…Various groups have performed simulations of the XA signals generated from LINAC irradiation using a combination of Monte Carlo dose deposition simulations and 2D acoustic wave transport simulations. To the best of author's knowledge, all currently published works regarding XACT imaging modality use back projection (BP) techniques for 2D dose reconstruction which show potential for relative dosimetry . Here, we explored the potential of XA tomographic techniques for 3D absolute dosimetry.…”

Ionizing radiation‐induced acoustics for radiotherapy and diagnostic radiology applications

“…Later studies investigated using XACT to image dose distributions of various shapes and sizes in a homogeneous water tank . Experimental XACT images were obtained by keeping an immersion ultrasound transducer stationary while the linac collimator was rotated.…”