Towards quantitative in vivo dosimetry using x‐ray acoustic computed tomography

Abstract

BackgroundRadiation dosimetry is essential for radiation therapy (RT) to ensure that radiation dose is accurately delivered to the tumor. Despite its wide use in clinical intervention, the delivered radiation dose can only be planned and verified via simulation. This makes precision radiotherapy challenging while in‐line verification of the delivered dose is still absent in the clinic. X‐ray‐induced acoustic computed tomography (XACT) has recently been proposed as an imaging tool for in vivo dosimetry.PurposeMost of the XACT studies focus on localizing the radiation beam. However, it has not been studied for its potential for quantitative dosimetry. The aim of this study was to investigate the feasibility of using XACT for quantitative in vivo dose reconstruction during radiotherapy.MethodsVarian Eclipse system was used to generate simulated uniform and wedged 3D radiation field with a size of 4 cm 4 cm. In order to use XACT for quantitative dosimetry measurements, we have deconvoluted the effects of both the x‐ray pulse shape and the finite frequency response of the ultrasound detector. We developed a model‐based image reconstruction algorithm to quantify radiation dose in vivo using XACT imaging, and universal back‐projection (UBP) reconstruction is used as comparison. The reconstructed dose was calibrated before comparing it to the percent depth dose (PDD) profile. Structural similarity index matrix (SSIM) and root mean squared error (RMSE) are used for numeric evaluation. Experimental signals were acquired from 4 cm 4 cm radiation field created by Linear Accelerator (LINAC) at depths of 6, 8, and 10 cm beneath the water surface. The acquired signals were processed before reconstruction to achieve accurate results.ResultsApplying model‐based reconstruction algorithm with non‐negative constraints successfully reconstructed accurate radiation dose in 3D simulation study. The reconstructed dose matches well with the PDD profile after calibration in experiments. The SSIMs between the model‐based reconstructions and initial doses are over 85%, and the RMSEs of model‐based reconstructions are eight times lower than the UBP reconstructions. We have also shown that XACT images can be displayed as pseudo‐color maps of acoustic intensity, which correspond to different radiation doses in the clinic.ConclusionOur results show that the XACT imaging by model‐based reconstruction algorithm is considerably more accurate than the dose reconstructed by UBP algorithm. With proper calibration, XACT is potentially applicable to the clinic for quantitative in vivo dosimetry across a wide range of radiation modalities. In addition, XACT's capability of real‐time, volumetric dose imaging seems well‐suited for the emerging field of ultrahigh dose rate “FLASH” radiotherapy.

X-ray-induced acoustic computed tomography and its applications in biomedicine

“…Reconstruction algorithms such as the conventional back projection 28 and model-based 30,31,44 in PAI have been adapted to XACT and further improved XACT's reconstruction for getting quantitative information. 33,45 The application of XACT imaging within the realm of biomedicine holds tremendous potential. The inherent connection between the XA signal and absorber density opens a captivating pathway-the ability to quantify human body density within images.…”

“…This map acts as a valuable tool for cross-referencing with the precise location of the target organ, ensuring meticulous beam alignment and accurate dose administration. 15,33,36,20,46 This serves as the foundation for XACT imaging's potential in image-guided radiation therapy. A noteworthy advantage lies in XACT's non-interference with the radiation beam; transducers positioned outside the beam path eliminate the need for correction factors prevalent in other dosimetry techniques.…”

“…The research in PAI also motivates and contributes to the development of XACT. Reconstruction algorithms such as the conventional back projection 28 and model-based 30 , 31 , 44 in PAI have been adapted to XACT and further improved XACT’s reconstruction for getting quantitative information 33 , 45 . The application of XACT imaging within the realm of biomedicine holds tremendous potential.…”

“…29,30 While they may not completely resolve the issue of limited views, these algorithms furnish more quantitative data [Fig. 3(c)], 33 which holds significance, particularly for radiation dose monitoring. Subsequently, the model-based XACT algorithm has been extended to three dimensions.…”

“…5 This promising trajectory has spurred the development of novel imaging instrumentation and advanced image algorithms aimed at extracting precise quantitative dose information during radiotherapy. By harnessing the capabilities of XACT, 33 clinicians and researchers are poised to usher in a new era of radiation therapy that not only visualizes but quantifies the therapeutic impact, culminating in more informed and optimized treatment strategies.…”