Soohwa Kam

Planar cone-beam computed tomography with a flat-panel detector

For a dedicated x-ray inspection of printed-circuit boards (PCBs), a bench-top planar cone-beam computed tomography (pCT) system with a flat-panel detector has been built in the laboratory. The system adopts the tomosynthesis technique that can produce cross-sectional images parallel to the axis of rotation for a limited angular range. For the optimal operation of the system and further improvement in the next design, we have evaluated imaging performances, such as modulation-transfer function, noise-power spectrum, and noise-equivalent number of quanta. The performances are comparatively evaluated with the coventional cone-beam CT (CBCT) acquisition for various scanning angular ranges, applied tube voltages, and geometrical magnification factors. The pCT scan shows a poorer noise performance than the conventional CBCT scan because of less number of projection views used for reconstruction. However, the pCT shows a better spatial-resolution performance than the CBCT. Because the image noise can be compensated by an elevated exposure level during scanning, the pCT can be a useful modality for the PCB inspection that requires higher spatial-resolution performance.

Effect of the phosphor screen optics on the Swank noise performance in indirect-conversion x-ray imaging detectors

The optics between the scintillators and photodiode arrays of indirect-conversion x-ray imaging systems requires careful design because it can be a cause of secondary quantum sink, which reduces the detective quantum efficiency at high spatial frequencies. The aim of this study was the investigation of the effect of the optical properties of granular phosphor screens — including optical coupling materials and passivation layers in photodiode arrays — on the imaging performance of indirect-conversion x-ray imaging detectors using the Monte Carlo technique. In the Monte Carlo simulations, various design parameters were considered, such as the refractive index of the optical coupler and the passivation layer, the reflection coefficient at the screen backing, and the thickness of the optical coupler. We developed a model that describes the optical pulse-height distributions based on the depth-dependent collection efficiency obtained from the simulations. We used the model to calculate the optical Swank noise. A loss in the number of collected optical photons was inevitable owing to the introduction of intermediate optics and mismatches in the optical design parameters. However, the collection efficiency marginally affected the optical Swank factor performance. The results and methodology of this study will facilitate better designs and optimization of indirect-conversion x-ray detectors.

Signal and noise analysis of flat-panel sandwich detectors for single-shot dual-energy x-ray imaging

We have developed a novel sandwich-style single-shot (single-kV) detector by stacking two indirect-conversion flat-panel detectors for preclinical mouse imaging. In the sandwich detector structure, extra noise due to the direct x-ray absorption in photodiode arrays is inevitable. We develop a simple cascaded linear-systems model to describe signal and noise propagation in the flat-panel sandwich detector considering direct x-ray interactions. The noise-power spectrum (NPS) and detective quantum efficiency (DQE) obtained from the front and rear detectors are analyzed by using the cascaded-systems model. The NPS induced by the absorption of direct x-ray photons that are unattenuated within the photodiode layers is white in the spatial-frequency domain like the additive readout noise characteristic; hence that is harmful to the DQE at higher spatial frequencies at which the number of secondary quanta lessens. The model developed in this study will be useful for determining the optimal imaging techniques with sandwich detectors and their optimal design.

Effects of the energy-separation filter on the performance of each detector layer in the sandwich detector for single-shot dual-energy imaging

A novel sandwich-style single-shot detector has been built by stacking two indirect-conversion flat-panel detectors for preclinical dual-energy mouse imaging. Although this single-shot method is more immune to motion artifacts compared with the conventional dual-shot method (i.e., fast kVp switching), it may suffer from reduced image quality because of poor spectral separation between the two detectors. Spectral separation can be improved by using an intermediate filter between the two detector layers. Adversely, the filter reduces the number of x-ray photons reaching the rear detector, hence probably increasing image noise. For a better design and practical use of the sandwich detector for single-shot dual-energy imaging, imaging performances of each detector layer in the sandwich detector are investigated for various spectral-separation extents and applied tube voltages. The imaging performances include the modulation-transfer function, the Wiener noise-power spectrum, and the detective quantum efficiency. According to the experimental results, impacts of the intermediate filter on the imaging performances of each detector layer are marginal. The detailed experimental results are shown in this study.

Physics-based modeling of computed tomography systems

We present a theoretical framework describing projections obtained from computed tomography systems considering physics of each component consisting of the systems. The projection model mainly consists of the attenuation of x-ray photons through objects including x-ray scatter and the detection of attenuated/scattered x-ray photons at pixel detector arrays. X-ray photons are attenuated by the Beers-Lambert law and scattered by using the Klein-Nishina formula. The cascaded signal-transfer model for the detector includes x-ray photon detection and light photon conversion/spreading in scintillators, light photon detection in photodiodes, and the addition of electronic noise quanta. On the other hand, image noise is considered by re-distributing the pixel signals in pixel-by-pixel ways at each image formation stage using the proper distribution functions. Instead of iterating the ray tracing over each energy bin in the x-ray spectrum, we first perform the ray tracing for an object only considering the thickness of each component. Then, we assign energy-dependent linear attenuation coefficients to each component in the projected images. This approach reduces the computation time by a factor of the number of energy bins in the x-ray spectrum divided by the number of components in the object compared with the conventional ray-tracing method. All the methods developed in this study are validated in comparisons with the measurements or the Monte Carlo simulations.

A framework of modeling detector systems for computed tomography simulations

Ultimate development in computed tomography (CT) technology may be a system that can provide images with excellent lesion conspicuity with the patient dose as low as possible. Imaging simulation tools have been cost-effectively used for these developments and will continue. For a more accurate and realistic imaging simulation, the signal and noise propagation through a CT detector system has been modeled in this study using the cascaded linear-systems theory. The simulation results are validated in comparisons with the measured results using a laboratory flat-panel micro-CT system. Although the image noise obtained from the simulations at higher exposures is slightly smaller than that obtained from the measurements, the difference between them is reasonably acceptable. According to the simulation results for various exposure levels and additive electronic noise levels, x-ray quantum noise is more dominant than the additive electronic noise. The framework of modeling a CT detector system suggested in this study will be helpful for the development of an accurate and realistic projection simulation model.

Characterization of Screen-Printed Mercuric Iodide Photoconductors for Mammography

We describe energy-dependent incomplete signal generation in semiconductor detectors with a simple planar geometry using deep charge trapping and depth-of-interaction models. Based on this formalism, we have characterized mercuric iodide ( HgI2) photoconductors by extracting performance parameters from x-ray-induced signals under mammographic imaging conditions. The HgI2 sample photoconductors were prepared using a simple screen-printing method. For an x-ray tube output from tungsten target and 30-kV setting, the quantum absorption efficiency of samples with a thickness of approximately 0.1 mm was measured to be ~ 70%. X-ray sensitivity was measured to be 0.4 nC cm - 2 mR - 1. Mobility-lifetime products for electrons and holes were measured as ~ 2 ×10 - 6 cm2 V - 1 and ~ 0.9 ×10 - 6 cm2 V - 1, respectively. Considering incomplete charge collection, the W-value (or w) was estimated to be ~ 27 eV. Since the characterization method described in this study is simple and does not require sophisticated equipment, it can be useful for the characterization of photoconductor materials.

An experimental study on the shift-variant MTF of CT systems using a simple cylindrical phantom

The modulation transfer function (MTF) is a typical parameter to measure the spatial resolution, which is an essential factor for evaluating the performance of computed tomography (CT) systems. It is known that the CT system does not follow the shift-invariant manner because of the cone-beam geometry and the transformation from the cylindrical coordinates to the axial coordinates when the image reconstruction is employed. Several studies reported that if the position of impulse receded from the center of a region of interest (ROI), the MTF degraded continuously. In this study, the trend of shift-variant characteristics of CT systems was measured and analyzed using a novel multi-cylindrical phantom. This study used to determine a point spread function (PSF) and MTF of a CT system using a simple cylindrical phantom. First of all, the optimal diameter of cylinder phantoms was experimentally determined as 70 mm to obtain reliable PSFs. Two kinds of field of views (FOVs), 40 cm and 60 cm, were used to vary reconstructed pixel sizes. The shift-variant MTF curves were acquired at five off-center positions per FOV. For the effective analysis of MTF shiftvariance, the integrated MTF values were calculated and used. In the result, the MTF slightly decreased as diameter increased from CT center in the central region within the distance of 10 cm. Moreover, a considerable MTF decrease suddenly occurred around the distance of 15 cm in the actual FOVs. The decreasing trend of the off-center spatial resolution of CT cannot be neglected in recent radiologic and radio-therapeutic fields requiring high degree of image precision, especially in sub-mm images. It is recommended that the ROI is laid on the CT center as close as possible. A novel cylindrical phantom was finally suggested to effectively measure PSFs with optimal diameters for clinical FOVs in this study. This phantom is cheap and convenient to use because it was only made of acryl with simple geometry. It is expected that the spatial resolution of CT can be easily monitored using our methodology in clinical CT sites.

Optical crosstalk in CT detectors and its effects on CT images

Detectors for computed tomography (CT) typically consist of scintillator and photodiode arrays which are coupled using optical glue. Therefore, the leakage of optical photons generated in a scintillator block to neighboring pixel photodiodes through the optical glue layer is inevitable. Passivation layers to protect the silicon photodiode as well as the silicon layer itself, which is inactive to the optical photons, are another causes for the leakage. This optical crosstalk reduces image sharpness, and eventually will blur CT images. We have quantitatively investigated the optical crosstalk in CT detectors using the Monte Carlo technique. We performed the optical Monte Carlo simulations for various thicknesses of optical components in a 129 × 129 CT detector array. We obtained the coordinates of optical photons hitting the user-defined detection plane. From the coordinate information, we calculated the collection efficiency at the detection plane and the collection efficiency at the single pixel located just below the scintillator in which the optical photons were generated. Difference between the two quantities provided the optical crosstalk. In addition, using the coordinate information, we calculated point-spread functions as well as modulation-transfer functions from which we estimated the effective aperture due to the optical photon spreading. The optical crosstalk was most severely affected by the thickness of photodiode passivation layer. The effective aperture due to the optical crosstalk was about 110% of the detector pixel aperture for a 0.1 mm-thick passivation layer, and this signal blur was appeared as a relative error of about 3-4% in mismatches between CT images with and without the optical crosstalk. The detailed simulation results are shown and will be very useful for the design of CT detectors.

SU‐E‐I‐17: Comparison of Two Novel Algorithms for the Modulation Transfer Function of CT Using a Simple Cylindrical Phantom

PURPOSE To compare and analyze two novel algorithms for the assessment of modulation transfer functions (MTFs) of computed tomography (CT) systems using a simple acrylic cylindrical phantom METHOD AND MATERIALS: Images of the acrylic cylindrical phantom were acquired by a GE LightSpeed 16 RT (GE Healthcare, Milwaukee, WI) using 120 kVp, 330 mA, 2.5 mm slice thickness, 10 cm field-of view (FOV), four reconstruction kernels (e.g. standard, soft, detail, bone, and lung). Two different algorithms were used to analyze images for MTF assessment. First, Richard et al. suggested a task-based MTF assessment method through an edge spread function (ESF) which described pixel intensities as a function of distance from the center. The MTF was obtained as the absolute value of Fourier transform of the differentiated ESF. Second, Ohkubo et al. devised an effective method to determine the point spread function (PSF) of CT system accompanied with verification. The line spread function (LSF), which was the one-dimensional integration of the PSF, was used to obtain the MTF. We validated the reliability of two above-mentioned methods through the comparison with a conventional method using a thin tungsten wire phantom. RESULTS The measured MTFs by two methods were mostly similar each other for standard, soft, and detail kernels. In 0.6 lp/mm, the MTF difference between two methods were 0.012(standard), 0.004(soft), and 0.037(detail). They also coincided with the MTF by the conventional method well. However, there were considerable distinctions for bone and lung kernels containing edge enhancement that might cause undershoots near the peak of the LSF. CONCLUSIONS We compared two novel methods to assess task-based MTFs for clinical CT systems especially using a simple acrylic cylindrical phantom with high-convenience and low-cost, and validated them against a conventional method. This work can provide a practical solution to users for the quality assurance of CT.