Physics considerations in MV-CBCT multi-layer imager design

Abstract

Megavoltage (MV) cone-beam computed tomography (CBCT) using an electronic portal imaging (EPID) offers advantageous features, including 3D mapping, treatment beam registration, high-z artifact suppression, and direct radiation dose calculation. Adoption has been slowed by image quality limitations and concerns about imaging dose. Developments in imager design, including pixelated scintillators, structured phosphors, inexpensive scintillation materials, and multi-layer imager (MLI) architecture have been explored to improve EPID image quality and reduce imaging dose. The present study employs a hybrid Monte Carlo and linear systems model to determine the effect of detector design elements, such as multi-layer architecture and scintillation materials. We follow metrics of image quality including modulation transfer function (MTF) and noise power spectrum (NPS) from projection images to 3D reconstructions to in-plane slices and apply a task based figure-of-merit, the ideal observer signal-to-noise ratio (d') to determine the effect of detector design on object detectability. Generally, detectability was limited by detector noise performance. Deploying an MLI imager with a single scintillation material for all layers yields improvement in noise performance and d' linear with the number of layers. In general, improving x-ray absorption using thicker scintillators results in improved DQE(0). However, if light yield is low, performance will be affected by electronic noise at relatively high doses, resulting in rapid image quality degradation. Maximizing image quality in a heterogenous MLI detector (i.e. multiple different scintillation materials) is most affected by limiting total noise. However, while a second-order effect, maximizing total spatial resolution of the MLI detector is a balance between the intensity contribution of each layer against its individual MTF. So, while a thinner scintillator may yield a maximal individual-layer MTF, its quantum efficiency will be relatively low in comparison to a thicker scintillator and thus, intensity contribution may be insufficient to noticeably improve the total detector MTF.

XperCT Sharpening Reconstruction for Cone-Beam Computed Tomography Guided Lung and Bone Interventions

“…The primary limitation to the adoption of CBCT in diagnostic and procedural arenas is image quality. 3 While flat-panel detectors offer comparable spatial resolution to standard CT, characteristic artifacts and noise can limit detail in reconstructions. 1 Methods to produce detailed imaging while minimizing radiation or contrast exposure in both pediatric and adult interventional procedures would be highly beneficial.…”

“…1,2 However, the adoption of CBCT as an adjunctive imaging modality in some arenas has been slowed by image quality limits and concerns over radiation dosing. 3 Particularly in artifact-prone areas such as lung and bone, limits in image detail may encourage the operator to prefer standard computed tomography (CT), ultrasound, magnetic resonance imaging (MRI), or fluoroscopic guidance. Early experience, however, suggests that CBCT provides effective image guidance for bone 4 and lung interventions.…”

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

“…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.…”

Development of a novel high quantum efficiency MV x‐ray detector for image‐guided radiotherapy: A feasibility study

“…Various approaches have been explored to improve the DQE while maintaining the spatial resolution of the MV xray detectors, such as using pixelated scintillator arrays [4][5][6][7][8][9][10][11][12][13][14][15] and building multilayer imagers. 1,[16][17][18][19] A variety of segmented scintillators composed of noncrystalline materials such as Gd 2 O 2 S:Tb powder 13 and crystalline materials such as thallium-doped cesium iodide, 4,5,10 bismuth germanate 11,12 , and cadmium tungstate 9,14,15 have been investigated to optimize the tradeoff between DQE and spatial resolution. However, the cost of crystalline scintillator is high and it is complex to fabricate pixelated crystalline arrays without defects.…”

“…13 Another approach involves the use of a stack of four conventional MV x-ray detection layers. 1,[16][17][18][19] By combining signals from all four layers, a zero-frequency DQE of about 6.7% was observed and the SNR was increased by a factor of 1.7. 1 A limitation of this approach might be the degradation in modulation transfer function (MTF) if more layers are used to further increase the DQE, due to the defocusing caused by beam divergence.…”