• 한국자기공명학회논문지
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• 제24권3호
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• pp.66-69
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• 2020

In ultra-low field magnetic resonance imaging (ULF-MRI), the strength of a static magnetic field can be comparable to that of gradient field. On that occasion, the gradient field is accompanied by concomitant gradient field, which yields distortion and blurring artifacts on MR images. Here, we focused on the distortion artifact and derived the equations capable of correcting it. Its usefulness was confirmed through the corrections in both simulated and experimental images. This solution will be effective for acquiring more accurate images in low and/or ultra-low magnetic fields.

• 한국소음진동공학회논문집
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• 제23권3호
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• pp.274-281
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• 2013

Field measurement method of heavy/soft impact sound pressure level which is regulated in JIS and ISO has been using in Korea, Japan and Canada. It is reported that heavy/soft impact sound pressure level was varied by the sound field condition of receiving room such as sound absorption power and room volume. In this study, it is checked that heavy/soft impact sound pressure level was affected by the receiving sound field condition. Rubber ball and bang machine sound pressure level was measured in the vertically connected reverberation chamber. In oder to check the effect of receiving sound field on heavy/soft impact sound pressure, sound absorption power was changed with polyester sound absorption blankets with air space and glass wool. The reverberation time at 1 kHz band was changed from 10 s to 0.2 s by sound absorption material. Rubber ball sound pressure level measured without sound absorption material was 58 dB in $L_{i,Fmax,AW}$, but the level was 46 dB with sound absorption treatment. From this result, it is confirmed that sound field correction method is needed in the heavy/soft impact sound pressure level measurement method using bang machine and rubber ball.

• Nuclear Engineering and Technology
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• 제42권6호
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• pp.670-679
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• 2010

In this paper, a new approach using a pixel-based correction method was developed to fix the non-uniform responses of flat-bed type scanners used for radiochromic film dosimetry. In order to validate the method's performance, two cases were tested: the first consisted of simple dose distributions delivered by a single port; the second was a complicated dose distribution composed of multiple beams. In the case of the simple individual dose condition, ten different doses, from 8.3 cGy to 307.1 cGy, were measured, horizontal profiles were analyzed using the pixel-based correcton method and compared with results measured by an ionization chamber and results corrected using the existing correction method. A complicated inverse pyramid dose distribution was made by piling up four different field shapes, which were measured with GAFCHROMIC$^{(R)}$EBT film and compared with the Monte Carlo calculation; as well as the dose distribution corrected using a conventional method. The results showed that a pixel-based correction method reduced dose difference from the reference measurement down to 1% in the flat dose distribution region or 2 mm in a steep dose gradient region compared to the reference data, which were ionization chamber measurement data for simple cases and the MC computed data for the complicated case, with an exception for very low doses of less than about 10 cGy in the simple case. Therefore, the pixel-based scanner correction method is expected to enhance the accuracy of GAFCHROMIC$^{(R)}$EBT film dosimetry, which is a widely used tool for two-dimensional dosimetry.

• 대한방사선기술학회지:방사선기술과학
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• 제32권2호
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• pp.205-212
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• 2009

반응보정인자(h)는 고체팬텀에서 전리함의 반응을 물에서의 값으로 변환하기 위한 인자이다. RW3 고체 팬텀의 경우에 고에너지 X-선에 대한 반응보정인자는 선질과 깊이에 의존하는 것으로 알려져 있으나 조사면 크기와 SSD(Source to surface distance), 그리고 전리함 종류에 따른 의존성은 알려진 바가 없다. 본 연구에서는 알려진 의존성을 고찰하고 알려지지 않은 인자들에 대한 의존성을 조사하였다. 본 측정에서는 파머형전리함(FC65G, IBA, Germany)과 소형전리함(CC13, IBA, Germany)이 사용되었으며 대상 선질은 6 MV와 15 MV X-선이었다. 측정 결과 반응보정인자는 6 MV의 경우에 깊이 5 cm와 10 cm에서 각각 h = 1.015, 1.021, 그리고 15 MV의 경우에 깊이 5와 10 cm에서 각각 h = 1.024, 1.029로 나타났다. 결론적으로 반응보정인자는 선질과 깊이에는 의존하였지만 조사면 크기와 SSD에 따른 변화는 적었다. 전리함에 있어서 대상의 두 전리함에 대해서는 차이가 없었으나 다른 종류의 전리함에 대해서 추가적 연구가 필요하다고 생각한다. 본 결과는 RW3에서 전리함을 이용한 측정시 측정값의 분석에 활용될 수 있다.

• 한국광학회지
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• 제30권6호
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• pp.237-242
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• 2019

가상현실 기기의 렌즈는 사용자의 편의성을 위한 적은 부피와 높은 현실감을 위한 넓은 시야각을 동시에 만족시켜야 하기 때문에 왜곡 수차가 필연적으로 발생하는데, 일반적인 렌즈보다 시야각이 넓고 왜곡 수차가 크기 때문에 측정이 어렵다. 본 논문에서는 가상현실 기기의 특성을 고려한 두 가지 왜곡 측정 방법을 제안하였다. 하나는 카메라 교정 방법 기반의 다중 이미지를 이용한 왜곡 측정법이며, 또 다른 하나는 카메라를 포함한 측정부를 직접 회전시켜 왜곡을 측정하는 다중 측정점을 이용한 왜곡 측정법이다. 제안된 방법들의 검증을 위해 시판된 가상현실 기기인 Google Cardboard 제품의 왜곡을 측정하고 설계 데이터를 통한 시뮬레이션 결과와 비교하였으며, 두 방법으로 측정된 왜곡 값은 시뮬레이션 결과와 비교했을 때 잘 부합하는 것으로 나타났다.

• 응용통계연구
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• 제16권2호
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• pp.305-319
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• 2003

본 연구에서는 바이어스 필드에 의해 왜곡된 MRI 영상에 대한 분할을 위해 확장된 EM 알고리즘을 기반으로 한 통계적 접근법을 제시한다. 영상의 명암값을 자료로 하는 분할기법들은 고주파 성분의 잡음 뿐만 아니라 영상을 불균질하게 만드는 바이어스 필드라는 저주파 성분의 왜곡에 특히 취약하다. 이 문제를 해결하기 위해 본 논문에서는 잡음을 효과적으로 제어하기 위해 마코프랜덤필드가 적용된 정규혼합모형을 고려하며, 효과적인 바이어스 필드의 보정을 위해 페널티-우도를 도입하여 추정하는 방법으로 고안되었다.

• 한국광학회지
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• 제31권1호
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• pp.20-25
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• 2020

넓은 시야를 가지는 2반사광학계를 설계하기 위하여서는 제2거울에 의한 광선의 차폐와 시야 가림이 없도록 비축시야를 사용하여야 한다. 그러나 이러한 2반사경계에서는 설계변수가 적어 비축수차를 충분하게 보정할 수 없었다. 이 연구에서는 조리개를 편심시켜 보다 성능이 개선된 비축시야 2반사광학계를 설계하였다.

• 대한화학회지
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• 제17권5호
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• pp.332-336
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• 1973

Integral Hellmann-Feynman Theorem을 degenerate case에 연장 전개하였다. 연장된 formula의 특성을 조사하고 근사식을 제안하였다. Conventional crystal field theory에 대한 고차섭동 에너지 보정의 가능성을 논의 하였다.

• Journal of the Optical Society of Korea
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• 제19권6호
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• pp.658-664
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• 2015

Lens designers generally refer to flat image fields and attempt to minimize the field curvature. Present-day CMOS image sensors for mobile phone cameras, however, are not flat, but curved. Sometimes it is necessary to generate an intentional field curvature according to the degree and direction of the CMOS image-sensor’s curvature. This paper presents the degree of curvature of a CMOS image sensor measured using an interferometer, and proposes an effective compensation method that minimizes the net field curvature through optimizing the air gap between lens elements, which is demonstrated using simulations and experiments.

• Nuclear Engineering and Technology
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• 제55권8호
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• pp.2935-2940
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• 2023

A polystyrene phantom was developed following the guidance of the International Atomic Energy Association (IAEA) for gamma knife (GK) quality assurance. Its performance was assessed by measuring the absorbed dose rate to water and dose distributions. The phantom was made of polystyrene, which has an electron density (1.0156) similar to that of water. The phantom included one outer phantom and four inner phantoms. Two inner phantoms held PTW T31010 and Exradin A16 ion chambers. One inner phantom held a film in the XY plane of the Leksell coordinate system, and another inner phantom held a film in the YZ or ZX planes. The absorbed dose rate to water and beam profiles of the machine-specific reference (msr) field, namely, the 16 mm collimator field of a GK Perfexion^TM or Icon^TM, were measured at seven GK sites. The measured results were compared to those of an IAEA-recommended solid water (SW) phantom. The radius of the polystyrene phantom was determined to be 7.88 cm by converting the electron density of the plastic, considering a water depth of 8 g/cm². The absorbed dose rates to water measured in both phantoms differed from the treatment planning program by less than 1.1%. Before msr correction, the PTW T31010 dose rates (PTW Freiberg GmbH, New York, NY, USA) in the polystyrene phantom were 0.70 (0.29)% higher on average than those in the SW phantom. The Exradin A16 (Standard Imaging, Middleton, WI, USA) dose rates were 0.76 (0.32)% higher in the polystyrene phantom. After msr correction factors were applied, there were no statistically significant differences in the A16 dose rates measured in the two phantoms; however, the T31010 dose rates were 0.72 (0.29)% higher in the polystyrene phantom. When the full widths at half maximum and penumbras of the msr field were compared, no significant differences between the two phantoms were observed, except for the penumbra in the Y-axis. However, the difference in the penumbra was smaller than variations among different sites. A polystyrene phantom developed for gamma knife dosimetry showed dosimetric performance comparable to that of a commercial SW phantom. In addition to its cost effectiveness, the polystyrene phantom removes air space around the detector. Additional simulations of the msr correction factors of the polystyrene phantom should be performed.