Kyeongjin Park

Preliminary evaluation of a brain PET insertable to MRI

There is a new trend of the medical image that diagnoses a brain disease as like Alzheimer dementia. The first qualified candidate is a PET-MRI fusion modality because MRI is a more powerful anatomic diagnosis tool than other modalities. In our study, in order to solve the high magnetic field from MRI, the development was consisted with four main items such as photo-sensor, PET scanner, MRI head-coil and attenuation correction algorithm development.

A modified a 3-Tr CMOS X-ray image sensor for low-distortion pixel output in source follower

This paper presents a low distortion source follower (LDSF) method in 3-Tr (Transistor) CMOS X-ray sensor. Most of the important parameters in source follower is a gate to source voltage (V_gs) difference. This additional circuit reduces fill factor of the photodiode from 86% to 78% in the 50 μm × 50 μm size. This circuit requires only a 1 P-mos and 1 N-mos. Simulation test represented that the LDSF method makes V_gs more constant. It means that V_gs is not easily affected by V_ds.

Development of a digitalized position decoder circuit for brain PET consisting of Silicon Photomultiplier

This paper presents a circuit of a digitized position decoder circuit for brain PET consisting of Silicon Photomultiplier (SiPM). Difference of proposed circuit and traditional circuit is the existence of analog-to-digital convertor (ADC). In this work, we conclude that the combination of ADC, field programmable gate array (FPGA) and digital data acquisition (DAQ) board can be substitute for an existing analog DAQ.

Simulation study of a plastic scintillator for an electrical personal dosimeter

Plastic scintillators show several desired characteristics in dosimetry; especially tissue-equivalence is a definite advantage as an electrical personal dosimeter (EPD). The EPD based on an array-type Silicon Photomultiplier (SiPM) coupled to the polystyrene scintillator was proposed at the last time and has been improving. In this research, for the EPD application, the characteristics of plastic scintillator were studied through MCNP5 and LightTools simulation. For the reliable MCNP simulation, Gaussian energy broadening (GEB) was applied with the computed parameters from measured spectrum. The difference between simulation and measurement were 2.2% and 3.5% in terms of energy and FWHM each. To simulate the small-size effect in the plastic scintillator, the plastic scintillators of various sizes were simulated. The collapse of counting curve was shown in the MCNP simulation result. This was due to the escape of Bremsstrahlung photons and more clearly shown in the smaller scintillator, higher energy. In the LightTools simulation, the light collection efficiency of each size was simulated. Like the prediction, the efficiency decreased when the size of scintillator increased. The simulation results showed the plastic scintillator for the EPD must be small for the light collection, but also have minimum limit of size to avoid loss of count at the high energy.

Large area X-ray CMOS digital pixel sensor based on pulse width modulation for high frame rate applications

In wafer-scale CMOS image sensors, the 3-transistor structure suffers from its low speed. To overcome this limitation, we propose a digital pixel sensor which has a digital-pixel output instead of analog-pixel output as in a conventional 3-transistor pixel. The digital pixel sensor can provide a high frame because it eliminates analog-to-digital conversion time. In addition, it removes the noise from long analog-signal paths. As a prototype, we designed a 4.5mm × 4.0mm chip having 24 × 16 pixels of 100μm × 100μm using standard 0.18μm CMOS technology. With 2MHz clock, the readout time for each pixel was 6micro seconds. When 3000pixels are in each column of a full-wafer-size CMOS image sensor, 50fps can be achieved with the digital pixels based on pulse width modulation.

Study on the fast signal transfer for large-area X-ray image sensors

A large area X-ray CMOS image sensor (LXCIS) is a well-known imaging device for high speed and resolution. In design and fabrication process, we found several problems in making LXCIS, especially in signal transferring. A 3-transistor active pixel sensor (3T APS) in LXCIS has a long signal line about 16.896 cm as a worst case. This long signal line consists of metal and it has resistance and capacitance about 21.12 kΩ and 71.87 pF each. We have optimized 3T APSs transistors, applied boosting circuit, and designed a low parasitic resistance and capacitance. From our simulation result, we obtained a high speed operation, which ranges from 13.5 frame per second (FPS) to 18.6 FPS in 1536 × 3072 pixel arrays, and a high dynamic range by increasing maximum voltage of pixel output signal.

Evaluation of dosimetry characteristics of a Silicon Photo-Multiplier with a plastic scintillator by Cs-137

A Silicon Photo Multiplier (SiPM) is states of arts radiation detector, it is alternative to PM-Tube. But, availability research of SiPM is not entirely satisfactory. In this study, we suggest the radiation detector module coupled with the SiPM and the plastic scintillator, and then, evaluate the availability of detector module for Cs-137 dosimetry. To confirm the dosimetry characteristics, we experimented count difference depends on equivalent dose rate and equivalent dose comparing with MCNP calculation, and, calculating the conversion coefficient depends on count rate and total count that finding the region keeps the linearity of calculating the dose. These experiment results means the SiPM with the plastic scintillator is able to use the dosimetry application for Cs-137. New detector module has the advantages that smaller volume than GM Tube and ion chamber and the lower price than indirect detector module using inorganic scintillator as CsI(TI). So, it will helpful to relieving the public anxiety about radiation exposure by popularizing the dosimetry device.