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

Abstract

Purpose The purpose of this study was to evaluate the performance of a prototype electric portal imaging device (EPID) with a high detective quantum efficiency (DQE) scintillator, LKH‐5. Specifically, image quality in context of both planar and megavoltage (MV) cone‐beam computed tomography (CBCT) is analyzed. Methods Planar image quality in terms of modulation transfer function (MTF), noise power spectrum (NPS), and DQE are measured and compared to an existing EPID (AS‐1200) using the 6 MV beamline for a Varian TrueBeam linac. Imager performance is contextualized for three‐dimensional (3D), MV‐CBCT performance by measuring imager lag and analyzing the expected degradation of the DQE as a function of dose. Finally, comparisons between reconstructed images of the Catphan phantom in terms of qualitative quality and signal‐difference‐to‐noise ratio (SDNR) are made for 6 MV images using both conventional and LKH‐5 EPIDs as well as for the kilovoltage (kV) on‐board imager (OBI). Results Analysis of the NPS reveals linearity at all measured doses using the prototype LKH‐5 detector. While the first zero of the MTF is much lower for the LKH‐5 detector than the conventional EPID (0.6 cycles/mm vs 1.6 cycles/mm), the normalized NPS (NNPS) multiplied by total quanta (qNNPS) of the LKH‐5 detector is roughly a factor of seven to eight times lower, yielding a DQE(0) of approximately 8%. First, second, and third frame lag were measured at approximately 23%, 5%, and 1%, respectively, although no noticeable image artifacts were apparent in reconstructed volumes. Analysis of low‐dose performance reveals that DQE(0) remains at 80% of its maximum value at a dose as low as 7.5 × 10−6 MU. For a 400 projection technique, this represents a total scan dose of 0.0030 MU, suggesting that if imaging doses are increased to a value typical of kV‐CBCT scans (~2.7 cGy), the LKH‐5 detector will retain quantum noise limited performance. Finally, comparing Catphan scans, the prototype detector exhibits much lower image noise than the conventional EPID, resulting in improved small object representation. Furthermore, SDNR of H2O and polystyrene cylinders improved from −1.95 and 2.94 to −15 and 18.7, respectively. Conclusions Imaging performance of the prototype LKH‐5 detector was measured and analyzed for both planar and 3D contexts. Improving noise transfer of the detector results in concurrent improvement of DQE(0). For 3D imaging, temporal characteristics were adequate for artifact‐free performance and at relevant doses, the detector retained quantum noise limited performance. Although quantitative MTF measurements suggest poorer resolution, small object representation of the prototype imager is qualitatively improved over the conventional detector due to the measured reduction in noise.

Deep learning enables MV-based real-time image guided radiation therapy for prostate cancer patients

“…This improved the robustness and transferrability of the model between institutions. Furthermore, recent research has been dedicated towards the improvement of detective quantum efficiency and contrast-tonoise ratio of the MV imager, which include the multi-layer imager (MLI) design (Myronakis et al 2017, Hu et al 2018, Harris et al 2020, 2021, pixelated scintillation (Star-Lack et al 2015) and advanced scintillation materials (Hu et al 2019). Harris et al (2021) demonstrated a 41.7% increase in marker tracking efficiency (number of frames that successfully track markers compared to maximum number of trackable frames) when using a novel multilayer MV imager.…”

Frequency-dependent optimal weighting approach for megavoltage multilayer imagers

“…In this study, we consider layer configurations that combine complementary detectors: one layer with high sensitivity and one with better spatial resolution. These particularly scintillators have been characterized in previous studies (Rottmann et al 2016, Hu et al 2019, and fabricated into development EPID panels, enabling future experimental validation. Furthermore, employing this weighting method may enable use of novel scintillator materials, which a priori are not suitable for a general-purpose planar imaging device by themselves, but potentially provide improved performance when combined with other layers with different responses.…”

“…Previous studies had validated this standard EPID model using Geant4 applications (Blake et al 2013, Myronakis et al 2016, Shi et al 2018. The bottom layer in configuration 1 (figure 1(a)) features a continuous slab of a 3.0mm thick LKH-5 scintillating glass (ρ = 3.85 g cm −1 ) while in configuration two (figure 1(b)) the scintillation glass matrix was based on the detector described by Hu et al (2019), which is composed of 12 mm thick individual blocks of LKH-5 with planar dimensions of 1.512 × 1.512 mm 2 with 20 μm separation (including glue and 88% reflective double-sided aluminum deposition Mylar). LKH-5 is a terbium doped glass with the following composition: oxygen (30%), barium (40%), silicon (20%) and terbium (10%) and it was modeled using the information provided by the manufacturer.…”

“…Optical properties of the different materials such as refractive index n and absorption length l were set according to previous studies (Myronakis et al 2016, Shi et al 2018, Hu et al 2019. Furthermore, the scintillation yield is sampled from a normal distribution with expectation value equal to 600 and 400 optical photons/MeV for the GOS phosphor and the LKH-5, respectively.…”

Clinical translation of a new flat-panel detector for beam’s-eye-view imaging

“…Considerable research has been devoted to improving EPIDs, in order to support advanced clinical applications. Investigators have explored different scintillation materials (Hu et al 2019), pixelated scintillators (Star-Lack et al 2015), and structured phosphors (Zhao et al 2004), for example. A cost-effective and easily implementable approach is to stack multiple layers of currently available components to improve photon collection without increasing Swank noise (Swank 1973).…”