Hyoungtaek Kim

Characterization of silicon photomultipliers at National Nano-Fab Center for PET-MR

The silicon photomultipliers (SiPMs) were fabricated for magnetic resonance compatible positron emission tomography (PET) applications using customized CMOS processes at National NanoFab Center. Each micro-cell consists of a shallow n+/p well junction on a p-type epitaxial wafer and passive quenching circuit was applied. The size of the SiPM is 3 × 3 mm(2) and the pitch of each micro-cell is 65 μm. In this work, several thousands of SiPMs were packaged and tested to build a PET ring detector which has a 60 mm axial and 390 mm radial field of view. I-V characteristics of the SiPMs are shown good uniformity and breakdown voltage is around 20 V. The photon detection efficiency was measured via photon counting method and the maximum value was recorded as 16% at 470 nm. The gamma ray spectrum of a Ge-68 isotope showed nearly 10% energy resolution at 511 keV with a 3 × 3 × 20 mm(3) LYSO crystal.

Silicon photomultiplier modules for MRI-compatible PET

Silicon photomultiplier (SiPM) modules were developed for use in positron emission tomography-magnetic resonance imaging (PET-MRI), which is a hybrid medical imaging technology. A PET-MRI is very efficient in the early diagnosis of representative senile diseases, including cancer, Alzheimers disease, and Parkinsons disease. SiPMs comprise the core image sensor for MR-compatible PET applications since they have a low operational voltage, high gain, good timing resolution, ruggedness, insensitivity to magnetic fields, compactness, and low cost. In PET systems, SiPM microcells can be optimized by making a trade-off between photon detection efficiency (PDE) and dynamic range. The SiPM modules used in this study were fabricated at the National NanoFab Center (NNFC) of South Korea by using a customized CMOS processes. The SiPM modules were evaluated by first packaging them with a cost-effective PCB package instead of with a conventional ceramic package. Measurements on 1,400 SiPMs indicated a uniform breakdown voltage of 20.54 V with a standard deviation of 0.07 V. Moreover, the SiPM modules present a high and uniform energy resolution of 13.6% with a standard deviation of 0.5% at 511 keV with 3 × 3 × 20 mm3 cerium-doped lutetium-yttrium oxyorthosilicate (Lu2(1−x)Y2xSiO5:Ce, LYSO) crystal coupling. These results indicated that the proposed devices offer adequate performance to form the foundation of an image sensor technology for MRI-compatible PET.

Fabrication and comparison Gd 2 O 2 S(Tb) and CsI(Tl) films for X-ray imaging detector application

During the last decade, digital X-ray imaging systems have been replacing analog X-ray imaging systems of conventional X-ray film-screen combination for radiography applications. Indirect detection methods consisted of an X-ray converter (or a scintillator film) and photodiode arrays are more widely used in medical diagnoses and industrial fields. Two major scintillation materials such as terbium doped gadolinium oxysulfide (Gd 2 O 2 S:Tb, GOS) and thallium doped cesium iodide (CsI:Tl) are commonly used. In this work, GOS scintillator films were manufactured by mixing and thermal hardening of Gd 2 O 2 S:Tb powder, dispersion agent, hardening agent, and other organic additives. And CsI:Tl scintillator films with columnar structure were also fabricated by the thermal evaporation method. The scintillation properties, such as emission spectrum and light yield etc., of the GOS and CsI:Tl films were measured by X-ray luminescence and photo-luminescence (PL) methods. The maximum luminescent intensity of both scintillators was observed at 540–560nm wavelength. In order to investigate the imaging performances of both GOS and CsI:Tl films as converters of X-ray imaging detectors, both scintillator films were coupled with an CCD sensor. The light response to X-ray dose, signal-to-noise ratio (SNR), spatial resolution were measured and analyzed under the same X-ray conditions. As X-ray dose increases, the SNR curves showed linear relationship. And the spatial resolution of two scintillator films was resolved at 7∼8lp/mm.

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.

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.

Time transient reverse current behavior of a-Si:H p-i-n diode

When a-Si:H pin diodes are used for medical imaging application, the reverse biased dark current is a sensitive characteristic of diode performance, and the time-transient reverse current behavior may limit the sensitivity and stability of p-i-n diode. Because of defect states within a band gap, reverse current shows a time dependent behavior. We investigate this transient behavior introducing the time dependent electric field, which is originated from the variation of ionized dangling bond density due to trapped charge emission. We assume the components of reverse current are the thermal generation current and the injection current at p-i interface. We also discussed the thermal generation current has a time independent component and a time and bias dependent component. Reverse current transient was calculated using this analytical model and compared with the experimental results.