Sangtaek Kim

Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.

Phantom Experiments on a PSAPD-Based Compact Gamma Camera With Submillimeter Spatial Resolution for Small Animal SPECT

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8 mm X 8 mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq 99mTc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.

Ultrafast multipinhole single photon emission computed tomography iterative reconstruction using CUDA

We have developed an ultrafast statistical iterative reconstruction method for multipinhole single photon emission computed tomography using high performance graphics processing unit computing and have demonstrated a significant performance improvement in reconstruction using computer-generated and experimental sinogram data.

Depth-of-Interaction Compensation Using a Focused-Cut Scintillator for a Pinhole Gamma Camera

Preclinical SPECT can potentially be a powerful platform to study fundamental biological processes and drug interactions in small animals. Gamma cameras for such SPECT systems require high spatial resolutions in order to adequately map the uptake of radioisotopes in small animals. Pinhole collimators offer one of the best technically feasible ways to achieve a high resolution. However, pinhole geometry introduces parallax errors, particularly toward the edge of the field of view, limiting the system spatial resolution. The parallax errors arise from the variable depth of interaction (DOI) of gammaray/scintillator events, especially when gamma rays enter a scintillator at steep angles. There have been several efforts to address parallax errors in pinhole SPECT, including algorithm-based DOI modeling and correction and the use of a curved fiber bundle to collimate light from a curved scintillator [1, 2]. Another way to overcome parallax errors in a pinhole gamma camera is to use a focused-cut scintillator, which is pixellated so that the pixels are focused towards the pinhole of a collimator [3]. Thus, the path of a primary ray that has passed through the pinhole only intersects with a single pixel in the scintillator. Here, we experimentally test a pinhole gamma camera with a focused-cut (FC) scintillator. We measure the resolution across a continuous scintillator and across a straight-cut (SC) pixellated scintillator and show that the thicker FC scintillator has comparable parallax error in comparison with a thinner SC scintillator (3 mm vs. 1 mm). Thus, FC scintillators are shown to offer both the high resolution of thin pixellated scintillators and the high sensitivity of thicker scintillators.

Temperature Dependent Operation of PSAPD-Based Compact Gamma Camera for SPECT Imaging

We investigated the dependence of image quality on the temperature of a position sensitive avalanche photodiode (PSAPD)-based small animal single photon emission computed tomography (SPECT) gamma camera with a CsI:Tl scintillator. Currently, nitrogen gas cooling is preferred to operate PSAPDs in order to minimize the dark current shot noise. Being able to operate a PSAPD at a relatively high temperature (e.g., 5°C) would allow a more compact and simple cooling system for the PSAPD. In our investigation, the temperature of the PSAPD was controlled by varying the flow of cold nitrogen gas through the PSAPD module and varied from -40°C to 20°C. Three experiments were performed to demonstrate the performance variation over this temperature range. The point spread function (PSF) of the gamma camera was measured at various temperatures, showing variation of full-width-half-maximum (FWHM) of the PSF. In addition, a 99 mTc-pertechnetate (140 keV) flood source was imaged and the visibility of the scintillator segmentation (16 × 16 array, 8 mm × 8 mm area, 400 μ m pixel size) at different temperatures was evaluated. Comparison of image quality was made at -25°C and 5°C using a mouse heart phantom filled with an aqueous solution of 99 m Tc-pertechnetate and imaged using a 0.5 mm pinhole collimator made of tungsten. The reconstructed image quality of the mouse heart phantom at 5°C degraded in comparision to the reconstructed image quality at -25°C. However, the defect and structure of the mouse heart phantom were clearly observed, showing the feasibility of operating PSAPDs for SPECT imaging at 5°C, a temperature that would not need the nitrogen cooling. All PSAPD evaluations were conducted with an applied bias voltage that allowed the highest gain at a given temperature.

Dependence of self-pulsation characteristics of multisection index-coupled distributed feedback lasers on section lengths

We investigate the effect of section lengths on the self-pulsation (SP) characteristics of a multisection index-coupled distributed feedback (DFB) laser that is composed of two spectrally detuned DFB sections and a phase-tuning section between them. Compound-cavity modes in multisection DFB lasers can be obtained by the concept of Fabry-Perot cavity modes. The simple equation for a CS number, which represents the number of compound-cavity modes within the stop-band width of a DFB section, is derived. By using the CS number, we were able to predict the SP characteristics such as abrupt changes of SP frequency due to mode hopping. Stable SP characteristics without abrupt changes of SP frequency for the variation of the spectral detuning and the phase shift in a phase-tuning section were obtained in cases with small CS numbers, which can be obtained by adjustment of section lengths.

A preclinical SPECT camera with depth-of-interaction compensation using a focused-cut scintillator

Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activity in animals. SPECT cameras using pinhole collimators offer high resolution that is needed for visualizing small structures in laboratory animals. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the scintillator detector at steep angles, introducing parallax errors due to variable depth-of-interaction in the scintillator, especially towards the edges of the detector field of view. These parallax errors ultimately limit the resolution of pinhole preclinical SPECT systems, especially for higher energy isotopes that can easily penetrate through millimeters of scintillator material. A pixellated, focused-cut scintillator, with its pixels laser-cut so that they are collinear with incoming rays, can potentially compensate for these parallax errors and thus open up a new regime of sub-mm preclinical SPECT. We have built a 4-pinhole prototype gamma camera for preclinical SPECT imaging, using an EMCCD camera coupled to a 3 mm thick CsI(Tl) scintillator whose pixels are focused towards each 500 μm-diameter pinhole aperture of the four pinholes. The focused-cut scintillator was fabricated using a laser ablation process that allows for cuts with very high aspect ratios. We present preliminary results from our phantom experiments.

Bidirectional Lasing Characteristics of Rectangular Ring Laser with Three-Guide Coupler

The lasing characteristics of three-guide coupled rectangular ring laser containing active and passive sections were investigated numerically and experimentally. The rectangular laser cavity consists of four low-loss total internal reflection mirrors and an output coupler made from three passive coupled waveguides. The laser having active section lengths of 350 µm and total cavity lengths of 780 µm was fabricated. The lasing threshold current in the clockwise circulating direction is approximately 35 mA, while that in the counterclockwise circulating direction is around 40 mA under continuous wave operation. The lasing characteristics are bidirectional operation of the single mode with a side mode suppression ratio better than 27 dB.

Depth-of-interaction compensation using a focused-cut scintillator for a pinhole gamma camera

Preclinical SPECT offers a powerful means to understand the molecular pathways of drug interactions in animal models by discovering and testing new pharmaceuticals and therapies for potential clinical applications. A combination of high spatial resolution and sensitivity are required in order to map radiotracer uptake within small animals. Pinhole collimators have been investigated, as they offer high resolution by means of image magnification. One of the limitations of pinhole geometries is that increased magnification causes some rays to travel through the detection scintillator at steep angles, introducing parallax errors due to variable depth-of-interaction in scintillator material, especially towards the edges of the detector field of view. These parallax errors ultimately limit the resolution of pinhole preclinical SPECT systems, especially for higher energy isotopes that can easily penetrate through millimeters of scintillator material. A pixellated, focused-cut (FC) scintillator, with its pixels laser-cut so that they are collinear with incoming rays, can potentially compensate for these parallax errors and thus improve the system resolution. We performed the first experimental evaluation of a newly developed focused-cut scintillator. We scanned a Tc-99 m source across the field of view of pinhole gamma camera with a continuous scintillator, a conventional “straight-cut” (SC) pixellated scintillator, and a focused-cut scintillator, each coupled to an electron-multiplying charge coupled device (EMCCD) detector by a fiber-optic taper, and compared the measured full-width half-maximum (FWHM) values. We show that the FWHMs of the focused-cut scintillator projections are comparable to the FWHMs of the thinner SC scintillator, indicating the effectiveness of the focused-cut scintillator in compensating parallax errors.

Multi-material decomposition using low-current X-ray and a photon-counting CZT detector

We developed and evaluated an x-ray photon-counting imaging system using an energy-resolving cadmium zinc telluride (CZT) detector coupled with application specific integrated circuit (ASIC) readouts. This x-ray imaging system can be used to identify different materials inside the object. The CZT detector has a large active area (5×5 array of 25 CZT modules, each with 16×16 pixels, cover a total area of 200 mm × 200 mm), high stopping efficiency for x-ray photons (∼ 100 % at 60 keV and 5 mm thickness). We explored the performance of this system by applying different energy windows around the absorption edges of target materials, silver and indium, in order to distinguish one material from another. The photon-counting CZT-based x-ray imaging system was able to distinguish between the materials, demonstrating its capability as a radiation-spectroscopic decomposition system.