A novel method for quantification of beam's‐eye‐view tumor tracking performance

Abstract

Purpose In-treatment imaging using an electronic portal imaging device (EPID) can be used to confirm patient and tumor positioning. Real-time tumor tracking performance using current digital megavolt (MV) imagers is hindered by poor image quality. Novel EPID designs may help to improve quantum noise response, while also preserving the high spatial resolution of the current clinical detector. Recently investigated EPID design improvements include but are not limited to multi-layer imager (MLI) architecture, thick crystalline and amorphous scintillators, and phosphor pixilation and focusing. The goal of the present study was to provide a method of quantifying improvement in tracking performance as well as to reveal the physical underpinnings of detector design that impact tracking quality. The study employs a generalizable ideal observer methodology for the quantification of tumor tracking performance. The analysis is applied to study both the effect of increasing scintillator thickness on a standard, single-layer imager (SLI) design as well as the effect of MLI architecture on tracking performance. Methods The present study uses the ideal observer signal-to-noise ratio (d′) as a surrogate for tracking performance. We employ functions which model clinically relevant tasks and generalized frequency-domain imaging metrics to connect image quality with tumor tracking. A detection task for relevant Cartesian shapes (i.e. spheres and cylinders) was used to quantify trackability of cases employing fiducial markers. Automated lung tumor tracking algorithms often leverage the differences in benign and malignant lung tissue textures. These types of algorithms (e.g. soft tissue localization – STiL) were simulated by designing a discrimination task, which quantifies the differentiation of tissue textures, measured experimentally and fit as a power-law in trend (with exponent β) using a cohort of MV images of patient lungs. The modeled MTF and NPS were used to investigate the effect of scintillator thickness and MLI architecture on tumor tracking performance. Results Quantification of MV images of lung tissue as an inverse power-law with respect to frequency yields exponent values of β = 3.11 and 3.29 for benign and malignant tissues, respectively. Tracking performance with and without fiducials was found to be generally limited by quantum noise, a factor dominated by quantum detective efficiency (QDE). For generic SLI construction, increasing the scintillator thickness (gadolinium oxysulfide – GOS) from a standard 290 μm to 1720 μm reduces noise to about 10%. However, 81% of this reduction is appreciated between 290 and 1000 μm. In comparing MLI and SLI detectors of equivalent individual GOS layer thickness, the improvement in noise is equal to the number of layers in the detector (i.e. 4) with almost no difference in MTF. Further, improvement in tracking performance was slightly less than the square-root of the reduction in noise, approximately 84–90%. In comparing an MLI detector with an SLI with a GOS scintillator of...

A new registration algorithm of electronic portal imaging devices images based on the automatic detection of bone edges during radiotherapy

“…The anticipated benefit of IGRT is realized basing on online confirmation of the registration at the time of radiotherapy. Megavoltage (MV) electronic portal imaging device (EPID) has been still widely used as an online verification tool for the treatment field and dosimetry in radiation therapy, although the emergence of kilovoltage (kV) imager mounted on a linear accelerator [1][2][3][4][5][6][7] .…”

Low‐dose megavoltage cone‐beam computed tomography using a novel multi‐layer imager (MLI)

“…The stacked format of the MLI increases x-ray detection efficiency without the loss of resolution associated with thick scintillator detectors. 2 In this work, we address the potential for low-dose, high-image-quality MVCBCT acquisition with the MLI. Based on the MLI construction, we expected to achieve improved image quality performance compared to AS1200 EPID at low dose.…”

“…1 This novel design can benefit task-specific imaging in radiotherapy such as portal imaging, motion-tracking, and MV cone-beam computed tomography (CBCT) by improving image contrast and noise characteristics. [2][3][4] Previously published work with the MLI included quantitative image metric evaluation on planar images, 1,5 theoretical design optimization studies for MVCBCT, [2][3][4] and feasibility and optimization of MV spectral imaging. 6,7 Low-dose MVCBCT has been a goal for image-guided radiotherapy.…”

“…The multi‐layer imager (MLI) was developed for megavoltage (MV) imaging using layered stacks of detectors to achieve improved performance in detective quantum efficiency (DQE), and noise power spectrum (NPS) . This novel design can benefit task‐specific imaging in radiotherapy such as portal imaging, motion‐tracking, and MV cone‐beam computed tomography (CBCT) by improving image contrast and noise characteristics . Previously published work with the MLI included quantitative image metric evaluation on planar images, theoretical design optimization studies for MVCBCT, and feasibility and optimization of MV spectral imaging …”

Characterizing a novel scintillating glass for application to megavoltage cone‐beam computed tomography

“…Recently, a great deal of research effort has been devoted to the development of high detective quantum efficiency (DQE) EPIDs with the goal of improving image quality at MV energies. Adoption of multilayer architecture, structured and pixilated scintillators, novel scintillation materials, and novel direct conversion detector design have all been investigated and found to improve detector DQE. Presently, we introduce a new, inexpensive scintillating glass called LKH‐5 (Collimated Holes Inc., Campbell, CA, USA) and characterize the fundamental, planar imaging performance.…”