An article authored by Varian-based researchers about the company’s Acuros® CTS, a new software tool for scatter correction in Varian’s Iterative cone-beam CT (CBCT) capability, was among four of the highest quality articles selected by the editors of Medical Physics to appear in the May 2018 Editor’s Choice column. The two-part article, “Acuros CTS: A Fast, Linear Boltzmann Transport Equation Solver for Computed Tomography Scatter--Part I: Core Algorithms and Validation,” 1 and “Acuros CTS: A Fast, Linear Boltzmann Transport Equation Solver for Computed Tomography Scatter--Part II: System Modeling, Scatter Correction, and Optimization,” 2 was selected for this honor because of its potential scientific impact and reader interest. It was also selected to appear on the cover of the May 2018 edition.
“Congratulations to the engineering and applied research teams for this outstanding achievement in Iterative CBCT imaging,” remarks Kolleen Kennedy, president, Varian Oncology Systems and corporate executive vice president. “What a unique honor for the work done by the Varian team that will certainly drive global market interest in Iterative CBCT capabilities and improve patient care through high quality imaging.”
Scatter Challenges CBCT Image Quality
CBCT is an invaluable clinical tool. With a wide-area detector such as an amorphous silicon flat-panel imager, CBCT makes it possible to acquire a large amount of volumetric data in a single rotation, which is advantageous in the areas of radiotherapy, interventional procedures, and dentistry. However, scatter correction remains one of the main challenges for CBCT image quality. The wide-area beam produces a large amount of scatter in the projection images, making it difficult to acquire high-quality CBCT images. Without scatter correction, the quality of reconstructed images can be affected by shading, cupping, quantification inaccuracies, and inhomogeneities.
Varian’s team began their research with the goal of finding an improved scatter correction for CBCT that could be executed in clinically acceptable times without additional cumbersome hardware. In the first part of their published research, Varian’s team describes the use of Acuros CTS for rapidly and accurately estimating scatter in x-ray projection images that deterministically solves the linear Boltzmann transport equation (LBTE). In the second part, they apply Acuros CTS to scatter correction of CBCT scans on Varian’s TrueBeam® system, exploring the trade-offs between scatter estimation accuracy and run times.
Part I: Core Algorithms and Validation
In part I, Varian’s team describes the first complete implementation and validation of a deterministic linear Boltzmann Transport Equation (LBTE) solver (Acuros CTS) for estimating scatter in kilovolt (kV) projection images. The LBTE describes the behavior of particles as they interact with an object across spatial, energy, and directional domains. Acuros CTS deterministically solves the LBTE by modeling photon transport associated with an x-ray projection.
Acuros CTS utilizes efficient numerical methods that preserve the precision of the solution despite high discretization in the spatial, energy, and directional domains. Varian’s team demonstrates the ability of Acuros CTS to produce scatter estimates equivalent to Monte Carlo simulations while benchmarking vastly reduced computation times. Consequently, Varian’s new software is a fast and precise tool capable of modeling the behavior of photon transport throughout the imaging chain.
Part II: System Modeling, Scatter Correction, and Optimization
The second part of the article describes how Acuros CTS can be used to correct for scatter in projection images from phantom and clinical data acquired on a clinical CBCT system, Varian’s TrueBeam® radiotherapy machine.
The team achieved high correction precision with computation times compatible with the clinical workflow by carefully modeling the imaging chain and optimizing several parameters. Their phantom and clinical scans demonstrate that Acuros CTS can be used to improve the precision of scatter correction, enhancing CBCT image quality. Furthermore, the increased image uniformity is expected to improve the visualization of soft-tissue structures and further enhance patient positioning and offline review.
The improvements in scatter correction provided by Acuros CTS are available as part of Varian’s Iterative CBCT capability. By improving image quality and enabling better soft-tissue visualization, Iterative CBCT seeks to deliver image quality necessary for adaptive radiotherapy, which requires images with high CT number precision. Acuros CTS also has the potential to positively impact scatter correction in other CBCT applications, such as image-guided interventions, dentistry, and industrial imaging, and may lead to improved dose reporting for cone-beam CT scans and diagnostic CT scans.
1Acuros CTS: A fast, linear Boltzmann transport equation solver for computed tomography scatter - Part I: Core algorithms and validation. Maslowski A, Wang A, Sun M, Wareing T, Davis I, Star-Lack J. Med Phys. 2018 May;45(5):1899-1913. doi: 10.1002/mp.12850. Epub 2018 Apr 6. https://rdcu.be/VQfG
2 Acuros CTS: A fast, linear Boltzmann transport equation solver for computed tomography scatter - Part II: System modeling, scatter correction, and optimization. Wang A, Maslowski A, Messmer P, Lehmann M, Strzelecki A, Yu E, Paysan P, Brehm M, Munro P, Star-Lack J, Seghers D. Med Phys. 2018 May;45(5):1914-1925. doi: 10.1002/mp.12849. Epub 2018 Mar 23. https://rdcu.be/VQKd