• Investigative Magnetic Resonance Imaging
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• v.25 no.3
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• pp.193-196
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• 2021

Congenital absence of the bilateral internal carotid arteries (ICA) is a very rare occurrence. Recognition of this rare anomaly is important, when considering intracranial endovascular interventions in the event of thromboembolic events with revascularization, transsphenoidal surgery, and the surveillance and detection of associated cerebral aneurysms. We report a case of a 25-year-old man who presented with headache since 2 years ago, and was incidentally discovered to have a congenital bilateral absence of ICAs.

• Communications for Statistical Applications and Methods
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• v.11 no.1
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• pp.181-187
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• 2004

One of objectives in epidemiologic studies is to detect the amount of change caused by a specific risk factor. Risk ratio is one of the most useful measurements in epidemiology. When we perform the inference for this measurement with rare events, the standard approach based on the normal approximation may fail, in particular when there are no disease cases observed. In this paper, we discuss and evaluate several existing methods for constructing a confidence interval of risk ratio through simulation when the disease of interest is a rare event. The results in this paper provide guidance with how to construct interval estimates for risk difference and risk ratio when there are no disease cases observed.

• Journal of the Korea Society for Simulation
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• v.8 no.3
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• pp.77-89
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• 1999

Simulating rare events, such as probability of cell loss in ATM networks, machine failure in highly reliable systems, requires huge simulation efforts due to the low chance of occurrence. Importance Sampling (IS) has been applied to accelerate the occurrence of rare events. However, it has a drawback of effective biasing scheme to make the estimator of IS unbiased. Adaptive Importance Sampling (AIS) employs an estimated sampling distribution of IS to the system of interest during the course of simulation. We propose Nonparametric Adaptive Importance Sampling (NAIS) technique which is nonparametrical version of AIS. We test NAIS to estimate a probability of rare event in M/M/1 queueing model. Comparing with classical Monte Carlo simulation, the computational efficiency and variance reductions gained via NAIS are substantial. A possible extension of NAIS regarding with random number generation is also discussed.

• Journal of Korean Neurosurgical Society
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• v.53 no.1
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• pp.46-48
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• 2013

Chondrosarcoma is a very uncommon malignant primary bone tumor, especially, it occurs extremely rare in the spine. A 52-year-old man was admitted to the emergency room with sudden paraplegia. Twelve hours prior to a paraplegic event, he visited an outpatient clinic with discomfort and tenderness around the medial border of the right scapular, and his neurologic status was absolutely intact. Magnetic resonance imaging showed a lobulated soft tissue mass from T3 to T5, which extended to the epidural space. Computed tomography scans showed soft tissue mass on the spinal posterior arch and osteolytic change of the adjacent bony structures. Emergent surgery was performed and the lesion was removed. Dark reddish blood and gel-like material were encountered around the dura and posterior arch during the operation. Multiple pulmonary nodules were found on a chest CT scan and a biopsy of one of them had been proven to be a metastasis of chondrosarcoma. The histologic examination showed dedifferentiated chondrosarcoma. The patient's neurologic deficit was improved slowly from ASIA A to ASIA D. Chondrosarcoma in the spine is extremely rare, even more with acute hemorrhage and sudden expansion into the epidural space. We named it chondrosarcoma apoplexy. We should consider the possibility of a hemorrhagic event when the patient's neurologic deficit worsens suddenly with spinal bone tumor.

• Nuclear Engineering and Technology
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• v.47 no.1
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• pp.59-65
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• 2015

Background: After the Fukushima accident, the importance of hazard analysis for extreme external events was raised. Methods: To analyze typhoon-induced hazards, which are one of the significant disasters of East Asian countries, a statistical analysis using the extreme value theory, which is a method for estimating the annual exceedance frequency of a rare event, was conducted for an estimation of the occurrence intervals or hazard levels. For the four meteorological variables, maximum wind speed, instantaneous wind speed, hourly precipitation, and daily precipitation, the parameters of the predictive extreme value theory models were estimated. Results: The 100-year return levels for each variable were predicted using the developed models and compared with previously reported values. It was also found that there exist significant long-term climate changes of wind speed and precipitation. Conclusion: A fragility analysis should be conducted to ensure the safety levels of a nuclear power plant for high levels of wind speed and precipitation, which exceed the results of a previous analysis.

• Nuclear Engineering and Technology
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• v.54 no.6
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• pp.2084-2093
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• 2022

Probabilistic safety assessment is widely used to quantify the risks of nuclear power plants and their uncertainties. When the lognormal distribution describes the uncertainties of basic events, the uncertainty of the top event in a fault tree is approximated with the sum of lognormal random variables after minimal cutsets are obtained, and rare-event approximation is applied. As handling complicated analytic expressions for the sum of lognormal random variables is challenging, several approximation methods, especially Monte Carlo simulation, are widely used in practice for uncertainty analysis. In this study, a theoretical approach for analyzing the sum of lognormal random variables using an efficient numerical integration method is proposed for uncertainty analysis in probability safety assessments. The change of variables from correlated random variables with a complicated region of integration to independent random variables with a unit hypercube region of integration is applied to obtain an efficient numerical integration. The theoretical advantages of the proposed method over other approximation methods are shown through a benchmark problem. The proposed method provides an accurate and efficient approach to calculate the uncertainty of the top event in probabilistic safety assessment when the uncertainties of basic events are described with lognormal random variables.

• Nuclear Engineering and Technology
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• v.54 no.8
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• pp.2933-2940
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• 2022

Approximations such as the delete-term approximation, rare event approximation, and minimal cutset upper bound (MCUB) need to be prudently applied for the quantification of a seismic probabilistic safety assessment (PSA) model. Important characteristics of seismic PSA models indicate that preserving the success branches in a primary seismic event tree is necessary. Based on the authors' experience in modeling and quantifying plant-level seismic PSA models, the effects of applying negate-down to the success branches in primary seismic event trees on the quantification results are summarized along with the following three insights gained: (1) there are two competing effects on the MCUB-based quantification results: one tending to increase and the other tending to decrease; (2) the binary decision diagram does not always provide exact quantification results; and (3) it is identified when the exact results will be obtained, and which combination provides more conservative results compared to the others. Complicated interactions occur in Boolean variable manipulation, approximation, and the quantification of a seismic PSA model. The insights presented herein can assist PSA analysts to better understand the important theoretical principles associated with the quantification of seismic PSA models.

• The Bulletin of The Korean Astronomical Society
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• v.46 no.2
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• pp.65.1-65.1
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• 2021

The Donggungilgi (東宮日記) is the daily records of the Siganwon (侍講院), which was a royal office in the Joseon dynasty that took charge of the education for the crown prince who dwelled in the Donggung (East Palace). This literature contains records of meteorological and astronomical observations as well as educational matters. The Sohyeon-Donggungilgi (昭顯東宮日記) includes records from 1625 to 1645, when Prince Sohyeon, the first son of King Injo (仁祖), was the crown prince. We investigate the records of solar halo observations in the Sohyeon-Donggungilgi. For consistency, we restrict our investigation to the period before the second Manchu invasion of Korea (i.e., 1625 to 1635). We extract 2,684 records and classify them into ten events according to the terms in their descriptions. The largest and smallest number of observation records are for the Hun (暈) and Geuk (戟) events (1,794 and 7 records, respectively). To verify what each event represents in modern atmospheric terms, we refer to historical documents of the Seoungwanji (書雲觀志, Treaties on the Bureau of Astronomy) and Cheonmundaeseong (天文大成, Great Achievements in Astronomy). We also calculate the solar altitude based on the observation hour and compare the descriptions to compute simulations provided by Arbeitskreis Meteore e.V.. We find that the descriptions of the Hun, Junghun (重暈), Yi (珥), and Baekhonggwanil (白虹貫日) events indicate a 22˚ halo, 22˚ and 46˚ halos, a parhelion, and a parhelic circle, respectively, Alternatively, we estimate that the Gwan (冠), Dae (戴), Bae (背), Li (履), and Gyohun (交暈) events describe arcs tangent to a 22˚ or 46˚ halo such as a upper or lower tangent arc, a circumzenithal arc, or a parry arc. We suggest that further studies are required for the Geuk event because the descriptions of this event differ from both documents referred to this study. In the sense that the number of observation records of the Geuk event is the smallest, however, this event may describe a rare phenomenon. We believe that this work will contribute to the study of historical records of solar or lunar halos.

• Journal of Chest Surgery
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• v.38 no.5 s.250
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• pp.400-402
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• 2005

Herniation of the kidney through a traumatic diaphragmatic rupture in itself is rare. However, complete avulsion of the renal pedicle implies not only a more rare event, but also a surgical emergency. We report a case of a patient with complete avulsion of renal vessels and ureter of an intrathoracic kidney herniated through a diaphragmatic rupture caused by blunt trauma. Prompt diagnosis with a computer tomographic scan and immediate surgery saved the patient's life.

• Archives of Craniofacial Surgery
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• v.20 no.6
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• pp.412-415
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• 2019

Salivary gland tumors usually appear in solitary mass in single salivary gland. The coexistence of tumors with different histological types occurring within a unilateral parotid gland is an extremely rare event. We experienced a case which two different types of malignant tumors developed at different time points in same gland; metachronous tumors. The second tumor was excised widely and reconstruction was performed by free tissue transfer. Sensory and motor nerve to the left cheek appeared to be intact, and circulation was adequate. This rare case was presented in this article.