Chang-Woo Seo

Design and image-quality performance of high resolution CMOS-based X-ray imaging detectors for digital mammography

In digital X-ray imaging systems, X-ray imaging detectors based on scintillating screens with electronic devices such as charge-coupled devices (CCDs), thin-film transistors (TFT), complementary metal oxide semiconductor (CMOS) flat panel imagers have been introduced for general radiography, dental, mammography and non-destructive testing (NDT) applications. Re- cently, a large-area CMOS active-pixel sensor (APS) in combination with scintillation films has been widely used in a variety of digital X-ray imaging applications. We employed a scintillator- based CMOS APS image sensor for high-resolution mammography. In this work, both powder-type Gd2O2S:Tb and a columnar structured CsI:Tl scintillation screens with various thicknesses were fabricated and used as materials to convert X-ray into visible light. These scintillating screens were directly coupled to a CMOS flat panel imager with a 25 50 mm 2 active area and a 48 mm pixel pitch for high spatial resolution acquisition. We used a W/Al mammographic X-ray source with a 30 kVp energy condition. The imaging characterization of the X-ray detector was measured and analyzed in terms of linearity in incident X-ray dose, modulation transfer function (MTF), noise-power spectrum (NPS) and detective quantum efficiency (DQE).

Quantitative lung nodule detectability and dose reduction in low-dose chest tomosynthesis

Quantitative imaging analysis has become a focus of medical imaging fields in recent days. In this study, Fourier-based imaging metrics for task-based quantitative assessment of lung nodules were applied in low-dose chest tomosynthesis. Compared to the conventional filtered back-projection (FBP), a compressed-sensing (CS) image reconstruction has been proposed for dose and artifact reduction. We implemented the CS-based low-dose reconstruction scheme to a sparsely sampled projection dataset and compared the lung nodule detectability index (d’) between the FBP and CS methods. We used the non-prewhitening (NPW) model observer to estimate the in-plane slice detectability in tomosynthesis and theoretically calculated d’ using the weighted amounts of local noise, spatial resolution, and task function in Fourier domain. We considered spatially varying noise and spatial resolution properties because the iterative reconstruction showed non-stationary characteristics. For analysis of task function, we adopted a simple binary hypothesis-testing model which discriminates outer and inner region of the encapsulated shape of lung nodule. The results indicated that the local noise power spectrum showed smaller intensities with increasing the number of projections, whereas the local transfer function provided similar appearances between the FBP and CS schemes. The resulted task functions for the same size of lung nodules showed the same pattern with different intensity, whereas the task function for different size of lung nodules presented different shapes due to different object functions. The theoretically calculated d’ values showed that the CS schemes provided higher values than the FBP method by factors of 2.64-3.47 and 2.50-3.10 for two different lung nodules among all projection views. This could demonstrate that the low-dose CS algorithm provide a comparable lung nodule images in comparison to FBP from 37.9% up to 28.8% reduced dose in the same projection views. Moreover, we observed that the CS method implemented with small number of projections provided similar or somewhat higher d’ values compared to the FBP method with large number of projections. In conclusion, the CS scheme may present a potential dose reduction for lung nodule detection in the chest tomosynthesis by showing higher d’ in comparison to the conventional FBP method.

Fast low-dose compressed-sensing (CS) image reconstruction in four-dimensional digital tomosynthesis using on-board imager (OBI)

Patient respiration induces motion artifacts during cone-beam CT (CBCT) imaging in LINAC-based radiotherapy guidance. This could be relieved by retrospectively sorting the projection images, which could be called as a respiratorycorrelated CBCT or a four-dimensional (4D) CBCT imaging. However, the slowness of the LINAC head gantry rotation limits a rapid scan time so that 4D-CBCT usually involves large amounts of radiation dose. Digital tomosynthesis (DTS) which utilizes limited angular range would present a faster 4D imaging with much lower radiation dose. One drawback of 4D-DTS is strong streak artifacts represented in the tomosynthesis reconstructed images due to sparsely sampled projections in each phase bin. The authors suggest a fast low-dose 4D-DTS image reconstruction method in order to effectively reduce the artifacts in a sparse imaging tomosynthesis condition. We used a flat-panel detector to acquire tomosynthesis projections of respiratory moving phantom in anterior-posterior (AP) and lateral views. We entered a sinusoidal periodic respiratory motion for an input signal to the phantom. An external monitor of Varian real-time position management (RPM) system was used to estimate the input respiratory motion, thereby four respiratory gating phases were determined to retrospectively arrange the projections. For streak line reduction, we introduced a simple iterative scheme suggested by McKinnon and Bates (MKB) and regarded it as a prior input image of the proposed lowdose compressed sensing (CS) method. Three different 4D-DTS image reconstruction schemes of conventional Feldkamp (FDK), MKB, and MKB-CS were implemented to phase-wise projections of both AP and lateral views. All reconstructions were accelerated by using a GPU card to reduce the computation times. For assessment of algorithmic performance, we compared a streak reduction ratio (SRR) and a contrast-to-noise-ratio (CNR) among the outcome images. The results showed that SRRs for MKB and MKB-CS schemes were 0.24 and 0.69, respectively, which indicates that the proposed MKB-CS method provided smaller streaking artifacts than conventional one by factor of 2.88. The resulted CNRs of coronal tomosynthesis images at peak-inhale phase were 3.24, 6.36, and 10.56 for FDK, MKB, and MKB-CS, respectively. This shows that the proposed method provides better image quality compared to others. The reconstruction time for MKB-CS was 196.07 sec, showing that our GPU-accelerated programming would enhance the algorithmic performance to match clinically feasible times (~3 min). In conclusion, the proposed low-dose 4D-DTS reconstruction scheme may provide better outcomes than the conventional methods with fast speed, and could thus it could be applied in practical 4D imaging for radiotherapy.

Use and imaging performance of CMOS flat panel imager with LiF/ZnS(Ag) and Gadox scintillation screens for neutron radiography

In digital neutron radiography system, a thermal neutron imaging detector based on neutron-sensitive scintillating screens with CMOS(complementary metal oxide semiconductor) flat panel imager is introduced for non-destructive testing (NDT) application. Recently, large area CMOS APS (active-pixel sensor) in conjunction with scintillation films has been widely used in many digital X-ray imaging applications. Instead of typical imaging detectors such as image plates, cooled-CCD cameras and amorphous silicon flat panel detectors in combination with scintillation screens, we tried to apply a scintillator-based CMOS APS to neutron imaging detection systems for high resolution neutron radiography. In this work, two major Gd2O2S:Tb and 6LiF/ZnS:Ag scintillation screens with various thickness were fabricated by a screen printing method. These neutron converter screens consist of a dispersion of Gd2O2S:Tb and 6LiF/ZnS:Ag scintillating particles in acrylic binder. These scintillating screens coupled-CMOS flat panel imager with 25x50mm2 active area and 48μm pixel pitch was used for neutron radiography. Thermal neutron flux with 6x106n/cm2/s was utilized at the NRF facility of HANARO in KAERI. The neutron imaging characterization of the used detector was investigated in terms of relative light output, linearity and spatial resolution in detail. The experimental results of scintillating screen-based CMOS flat panel detectors demonstrate possibility of high sensitive and high spatial resolution imaging in neutron radiography system.