Hyun-Ju Ryu

A Monte Carlo simulation study of the effect of energy windows in computed tomography images based on an energy-resolved photon counting detector

The energy-resolved photon counting detector provides the spectral information that can be used to generate images. The novel imaging methods, including the K-edge imaging, projection-based energy weighting imaging and image-based energy weighting imaging, are based on the energy-resolved photon counting detector and can be realized by using various energy windows or energy bins. The location and width of the energy windows or energy bins are important because these techniques generate an image using the spectral information defined by the energy windows or energy bins. In this study, the reconstructed images acquired with K-edge imaging, projection-based energy weighting imaging and image-based energy weighting imaging were simulated using the Monte Carlo simulation. The effect of energy windows or energy bins was investigated with respect to the contrast, coefficient-of-variation (COV) and contrast-to-noise ratio (CNR). The three images were compared with respect to the CNR. We modeled the x-ray computed tomography system based on the CdTe energy-resolved photon counting detector and polymethylmethacrylate phantom, which have iodine, gadolinium and blood. To acquire K-edge images, the lower energy thresholds were fixed at K-edge absorption energy of iodine and gadolinium and the energy window widths were increased from 1 to 25 bins. The energy weighting factors optimized for iodine, gadolinium and blood were calculated from 5, 10, 15, 19 and 33 energy bins. We assigned the calculated energy weighting factors to the images acquired at each energy bin. In K-edge images, the contrast and COV decreased, when the energy window width was increased. The CNR increased as a function of the energy window width and decreased above the specific energy window width. When the number of energy bins was increased from 5 to 15, the contrast increased in the projection-based energy weighting images. There is a little difference in the contrast, when the number of energy bin is increased from 15 to 33. The COV of the background in the projection-based energy weighting images is only slightly changed as a function of the number of energy bins. In the image-based energy weighting images, when the number of energy bins were increased, the contrast and COV increased and decreased, respectively. The CNR increased as a function of the number of energy bins. It was concluded that the image quality is dependent on the energy window, and an appropriate choice of the energy window is important to improve the image quality.

The effects of spectral distortion on multi-energy X-ray imaging based on photon counting detector

We studied the effect of spectral distortions on X-ray imaging observed by photon-counting detectors. The photon counting-based imaging system used in this study consisted of a micro focus X-ray source and cadmium telluride (CdTe) detector.

Comparison of material decomposition methods in contrast enhancement digital mammography based on photon counting

Photon counting detector with energy discrimination has made it possible to decompose materials. In this study, we compared two material decomposition methods in CEDM using a Monte Carlo simulation. The K-edge and dual energy imaging methods were used for material decomposition. We designed the cadmium telluride (CdTe) detector based on photon counting using GEANT4 Application for Tomographic Emission (GATE) version 6.0 simulation tools. The CdTe detector was 44.8×44.8 mm2 and thickness of 1 mm. To verify the material decomposition ability, we designed the phantom with GATE. The phantom was a cylinder of breast equivalent tissue material (ICRU-44) containing iodine inserts at various thicknesses ranging from 0.3 to 1.5 mm. Thicknesses of breast equivalent tissue material was 20, 30, 40, and 50 mm. First, to verify the material decomposition ability of K-edge imaging, we obtained image from 34–44 keV energy window because the K-edge energy of iodine is 33.2 keV. Second, for the dual energy imaging methods, we acquired two images with below and above the iodine K-edge energy using one exposure with energy selective windows. Data for material decomposition algorithm were obtained for various combinations step wedge phantom of breast tissue equivalent materials and iodine. The images from the phantom were used to produce the inverse mapping functions. The ability of material decomposition was investigated in terms of a contrast-to-noise ratio (CNR). According to the results, the CNR with K-edge imaging method was higher than dual energy method. The calculated values of CNR with K-edge imaging method were approximately 1.60 times higher than with dual energy imaging method. Our studies may be used as a basis for future studies on X-ray imaging based on photon counting.

Comparison of photon counting and conventional scintillation detectors in pinhole SPECT system for small animal imaging

The photon counting detector using cadmium telluride (CdTe) or cadmium zinc telluride (CZT) is a promising imaging modality and provides many benefits compared to conventional scintillation detectors. When using the pinhole collimator with the photon counting detector, we are able to improve both spatial resolution and sensitivity. The purpose of this study was to evaluate the photon counting and conventional scintillation detectors in a pinhole single photon emission computed tomography (SPECT) system. We designed five pinhole SPECT systems of two types. One was the CdTe photon counting detector, and the other was the conventional NaI(Tl) scintillation detector. We conducted simulation studies and evaluated the imaging performance. The results showed that the spatial resolution of CdTe photon counting detector was 0.38 mm and the sensitivity in this detector was 1.40 times higher than conventional NaI(Tl) scintillation detector in the same detector thickness condition. Also, the average scatter fraction of the CdTe photon counting and the conventional NaI(Tl) scintillation detectors were 1.93% and 2.44%, respectively. In conclusion, we successfully evaluated various pinhole SPECT systems for small animal imaging.

Simulation studies of a high resolution SPECT system for a photon counting semiconductor detector

Photon counting detector using cadmium telluride (CdTe) or cadmium zinc telluride (CZT) has benefits compared to conventional scintillation detector. These materials have advantageous physical characteristics for nuclear medicine imaging. Recently, for improvement of the sensitivity and the spatial resolution, many researchers have been investigated using these materials. By using the pixelated parallel-hole collimator, we may be able to improve the sensitivity in small pixel condition and the spatial resolution. In this study, we simulated the SPECT system using the photon counting detector based on CdTe and CZT, and evaluated the performance of these systems. We performed a simulation study using the Geant4 Application for Tomographic Emission (GATE) simulation. The proposed system was equipped with a CdTe and CZT detector with very small pixels such as 0.35 × 0.35 mm2. The results showed that the proposed system could be acquired the 0.46 mm spatial resolution located at 2 cm from the collimator. In conclusion, our results demonstrated that we established the high resolution SPECT system with the pixelated parallel-hole collimator.

Improved visualization of plaque in cardiac CT angiography by optimal energy weighting with photon counting detector: A simulation study

For the detecting plaque buildup and calcification in the coronary artery, the cardiac computed tomography (CT) angiography is widely used in clinic. However, the soft and calcified plaque is hard to distinguish from contrast enhanced blood vessel with conventional CT. The photon counting detector has an advantage over the conventional detector by detecting energy information of each photon. The application of optimal energy weighting factor for each energy window based on energy information of each photon can be improve the identification of plaque. The purpose of this study is to investigate, with simulation, the ability to distinguish plaque from contrast enhanced vessel using optimal energy weighting factor. The photon counting imaging system consists of a micro focus x-ray source and CdTe sensor was simulated using the Geant4 Application for Tomographic Emission (GATE) V6. The source to phantom rotation distance and phantom rotation to detector distance were set as 600 and 400 mm, respectively. The energy spectra were calculated at 80 kVp with 1 mm aluminum filter using Spekcalc program and used as source. A phantom consisted of iodine and hydroxyapatite (HA) which simulates contrasts enhanced blood vessel and plaque, respectively. The projection based energy weighting factors at each energy window were calculated and applied. The identification of HA with respect to iodine was evaluated using signal-difference-to noise ratio (SDNR). The SDNR improvements, as compared to image from charge integrating detector were 1.07 and 1.42 for photon counting and energy weighted image at 50 mg/ml of HA, respectively. The SDNR were improved from 1.43 to 21.88 with optimal energy weighting for each concentration. The results showed that the plaque could be distinguished from contrast enhanced blood vessel with optimal energy weighting. However, the result can be depends on the concentration of background contrast media, therefore additional simulation required.

Optimization of hardware-based photon rejecter to separate electronic noise in the photon counting detector

The cadmium telluride (CdTe)-based photon-counting detector is limited in capability under a high x-ray flux. A photon rejecter composed of aluminum, for example, can reduce this limitation by modulating the incident number of photons. In addition to this function, the optimal photon rejecter can separates electronic noise. The aim of this work was to optimize the photon rejecter for high quality image acquisition by removing electronic noise from the actual pulse signal. We acquired data with various types of photon rejecter materials composed of aluminum (Al) and iodine at three different tube voltages (50, 70 and 90 kVp). A phantom composed of high atomic number materials was imaged to evaluate the efficiency of the photon rejecter. Photon rejecters composed of 1-mm Al, 10-mm Al, and a combination of 10-mm Al and iodine provided optimum capability at 50, 70 and 90 kVp, respectively. Each optimal combination of photon rejecter material and voltage effectively separated electronic noise from the actual pulse signal and gave highest contrast for materials on the image. These optimized types of photon rejecter can effectively discriminate electronic noise and improve image quality according to the different tube voltages.

Development of a pinhole collimator SPECT-CT system using a CdTe detector sample

SPECT system with a co-registered x-ray CT image allows the fusion of functional information and morphologic information. In this study, a pinhole collimator SPECT system sharing a CdTe detector with a CT was designed. Geant4 application for tomographic emission (GATE) v.6 was used for the design simulation. Sensitivity and spatial resolution were measured to evaluate the system characteristics on an ideal condition. A pinhole collimator was designed to obtain high spatial resolution and better sensitivity. The diameter of the collimator was 0.5 mm, and the distance from the center of the field of view (CFOV) was set to 25 mm. The source to detector distance was set to 69 mm. The acquisition time of each projection was 40 seconds, and 60 projections were obtained for tomographic image acquisition. The reconstruction was performed using ordered subset expectation maximization (OS-EM) algorithms. The total system sensitivity was measured in cps/kBq, and the spatial resolution was defined by the full width at half maximum (FWHM) from the point spread function (PSF) of the point source image. Modulation transfer function (MTF) was also measured from the PSF. The spatial resolution of the system calculated from the FWHM of Gaussian fitted PSF curve was 0.48 mm, and the sensitivity of the system measured 0.354 cps/kBq from a Tc-99m point source. A phantom study was performed to verify the dual modality system design. The system will be built as designed, and it can be applied for the pre-clinical imaging system.

Optimization of high resolution collimator with CdTe detector: a simulation study

Photon counting detectors using cadmium telluride (CdTe) or cadmium zinc telluride (CZT) have benefits compared to conventional scintillation detectors. Recently, many studies have been conducted using these materials to improve the sensitivity and spatial resolution of photon counting detectors. The purpose of this study was to optimize the design of a collimator to achieve excellent resolution and high sensitivity on the gamma camera system based on the CdTe detector. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe detector using the Geant4 Application for Tomographic Emission (GATE) simulation. This detector consists of small pixels (0.35 × 0.35 mm2). We designed two parallel-hole collimators with different shapes and verified their usefulness. One was the proposed pixelated parallel-hole collimator in which the hole size and pixel size are the same, and the other was the hexagonal parallel-hole collimator, which has a hole size similar to that of the pixelated parallel-hole collimator. We evaluated the sensitivity, spatial resolution, and contrast resolution to determine which parallel-hole collimator was best on the PID 350 CdTe detector. The sensitivity was 22.65% higher for the pixelated parallel-hole collimator than for the hexagonal parallel-hole collimator. Also, the pixelated parallel-hole collimator provided 10.7% better spatial resolution than the hexagonal parallel-hole collimator and contrast resolution was improved by 8.93%. In conclusion, we successfully established a high resolution gamma camera system with a pixelated parallel-hole collimator.

The effect of energy weighting on x-ray imaging based on photon counting detector: a Monte Carlo simulation

Photon counting detector based on semiconductor materials is a promising imaging modality and provides many benefits for x-ray imaging compared with conventional detectors. This detector is able to measure the x-ray photon energy deposited by each event and provide the x-ray spectrum formed by detected photon. Recently, photon counting detectors have been developed for x-ray imaging. However, there has not been done many works for developing the novel x-ray imaging techniques and evaluating the image quality in x-ray system based on photon counting detectors. In this study, we simulated computed tomography (CT) images using projection-based and image-based energy weighting techniques and evaluate the effect of energy weighting in CT images. We designed the x-ray CT system equipped with cadmium telluride (CdTe) detector operating in the photon counting mode using Geant4 Application for Tomographic Emission (GATE) simulation. A micro focus X-ray source was modeled to reduce the flux of photons and minimize the spectral distortion. The phantom had a cylindrical shape of 30 mm diameter and consisted of ploymethylmethacrylate (PMMA) which includes the blood (1.06 g/cm3), iodine, and gadolinium (50 mg/cm3). The reconstructed images of phantom were acquired with projection-based and image-based energy weighting techniques. To evaluate the image quality, the contrast-to-noise ratio (CNR) is calculated as a function of the number of energy-bins. The CNR of both images acquired with energy weighting techniques were improved compared with those of integrating and counting images and increased as a function of the number of energy-bins. When the number of energy-bins was increased, the CNR in the image-based energy weighting image is higher than the projection-based energy weighting image. The results of this study show that the energy weighting techniques based on the photon counting detector can improve the image quality and the number of energy-bins used for generating the image is important.