• Investigative Magnetic Resonance Imaging
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• v.23 no.1
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• pp.1-16
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• 2019

Dynamic contrast enhanced (DCE) magnetic resonance (MR) imaging plays an important role in non-invasive detection and characterization of primary and metastatic lesions in the liver. Recently, efforts have been made to improve spatial and temporal resolution of DCE liver MRI for arterial phase imaging. Review of recent publications related to arterial phase imaging of the liver indicates that there exist primarily two approaches: breath-hold and free-breathing. For breath-hold imaging, acquiring multiple arterial phase images in a breath-hold is the preferred approach over conventional single-phase imaging. For free-breathing imaging, a combination of three-dimensional (3D) stack-of-stars golden-angle sampling and compressed sensing parallel imaging reconstruction is one of emerging techniques. Self-gating can be used to decrease respiratory motion artifact. This article introduces recent MRI technologies relevant to hepatic arterial phase imaging, including differential subsampling with Cartesian ordering (DISCO), golden-angle radial sparse parallel (GRASP), and X-D GRASP. This article also describes techniques related to dynamic 3D image reconstruction of the liver from golden-angle stack-of-stars data.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.1
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• pp.31.1-31.1
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• 2011

We present wide-field near-IR imaging polarimetry of 30 Doradus in the Large Magellanic Cloud, using the InfraRed Survey Facility (IRSF). We obtained polarimetry data in J, H, and Ks bands using the JHKs-simultaneous imaging polarimeter SIRPOL. Since many Galactic field stars along the line-of-sight to the Large Magellanic Cloud are contaminated in our data, we developed methods to identify the foreground sources using the proper motion data. We investigated polarimetric properties between the Galactic foreground stars and the stars in the LMC.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.2
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• pp.76.3-76.3
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• 2019

Intracluster stars are believed to be gravitationally bound to a galaxy cluster, however, not to individual cluster galaxies. Their presence is observed as diffuse light typically in the central region extended from the brightest cluster galaxy. The diffuse light, often referred to as intracluster light (ICL), is difficult to quantify in distant high-redshift galaxy clusters because of the significant surface brightness dimming although ICL observations in high-redshift clusters provide powerful constraints on the origin of intracluster stars. In this poster, we present ICL study of the distant galaxy cluster SPT2106-5844 at z=1.132 with Hubble Space Telescope IR imaging data. With careful control of systematics, we successfully quantify the total amount of the ICL, measure the color profile, and obtain its two-dimensional distribution. Our measurement of the high abundance of the intracluster stars in this young cluster favors the ICL formation scenario, wherein production of intracluster stars are predominantly associated with the BCG formation.

• Investigative Magnetic Resonance Imaging
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• v.18 no.2
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• pp.87-106
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• 2014

Purpose : To describe how a robust implementation of a radial 3D gradient-echo sequence with stack-of-stars sampling can be achieved, to review the imaging properties of radial acquisitions, and to share the experience from more than 5000 clinical patient scans. Materials and Methods: A radial stack-of-stars sequence was implemented and installed on 9 clinical MR systems operating at 1.5 and 3 Tesla. Protocols were designed for various applications in which motion artifacts frequently pose a problem with conventional Cartesian techniques. Radial scans were added to routine examinations without selection of specific patient cohorts. Results: Radial acquisitions show significantly lower sensitivity to motion and allow examinations during free breathing. Elimination of breath-holding reduces failure rates for non-compliant patients and enables imaging at higher resolution. Residual artifacts appear as streaks, which are easy to identify and rarely obscure diagnostic information. The improved robustness comes at the expense of longer scan durations, the requirement for fat suppression, and the nonexistence of a time-to-center value. Care needs to be taken during the configuration of receive coils. Conclusion: Routine clinical use of radial stack-of-stars sequences is feasible with current MR systems and may serve as substitute for conventional fat-suppressed T1-weighted protocols in applications where motion is likely to degrade the image quality.

• Journal of The Korean Astronomical Society
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• v.38 no.2
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• pp.319-324
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• 2005

To date, more than 150 exo-solar planets have been observed by various methods such as spectroscopic, photometric, astrometric, gravitational lensing, pulsar timing methods. However, all these are indirect methods; they do not directly image the planets. Only free-floating planets or their 'ana-log' have been directly detected so far. Thus the next milestone is the direct imaging of any kinds of planetary mass objects orbiting around normal (young) stars, which might have been associated with protoplanetary disks, the sites of planet formation. I will describe some SUBARU efforts to detect self-luminous young giant planets as companions as well as direct imaging of the protoplanetary disks of ${\~}$100 AU size. The results of near-infrared coronagraphic imaging with adaptive optics are briefly presented on AB Aur, HD 142527, T Tau, and DH Tau. Our results demonstrate the importance of high-resolution (${\~}$0.1 arcsec) direct imaging over indirect observations such as modeling based on spectral energy distributions. The SUBARU observations are a prelude to ALMA from the morphological point of view.

• Journal of The Korean Astronomical Society
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• v.29 no.2
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• pp.269-278
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• 1996

We present the color-magnitude diagrams (CMD) of more than 24,000 stars in the field of an intermediate age open cluster M11, based on wide field CCD imaging. The morphology of the CMDs varies strikingly as the distance from the center of the cluster increases. From the surface number density analysis, we confirm the mass segregation effect in this cluster: the bright, massive stars are centrally more concetrated than the faint, low mass stars. The slope of the field-corrected surface density with respect to magnitude progressively increases as the radius increases, up to r = 5'. Most of the field stars in or near the cluster main sequence band and in the bright part of the red stars in the CMDs appear to be nearly at the same distance as M11, and they are considered to be the major component of disk stars in the Sagittarius-Carina arm.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.83.2-83.2
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• 2014

Flare and rotational variabilities induced by stellar activity are important for studying the effect of magnetic fields on the evolution of red dwarf stars. The level and frequency of magnetic activity in these stars have a different aspect at every moment of the observations due to the effect of age-rotation relation. The use of both tracers is thus essential to have a relatively homogeneous set of stellar activity data for statistical studies. The archival light curves and imaging data of the open cluster M37 taken by MMT 6.5m telescope were used for this work. In order to achieve much more accurate photometric precisions and also to make the most efficient use of the data, the entire imaging database were re-analyzed with our new time-series photometry technique and carefully calibration procedures. Based on the new light curves, we study, for the first time, a variety of aspects of those two variabilities in red dwarfs and their relation to magnetic activity. In this talk, we present all observational evidences that support the idea that the strength of magnetic activity is closely connected with the rotation rate of a star and its evolutionary status (age-activity-rotation paradigm). In conclusion, we suggest future directions to improve our understanding of stellar activity in cool stars with photometric time-series data.

• Publications of The Korean Astronomical Society
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• v.30 no.2
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• pp.223-224
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• 2015

We present the results of visual double stars speckle observations from 2013 using a Zeiss Double Refractor 60 cm with visual focal length f = 1,078 cm, and CCD SBIG ST-402 MEA. A Bessel V filter with ${\lambda}=550nm$ was placed in front of the CCD camera to reduce the chromatic aberration of the objective lens. The objects selected for this observation were calibration candidates and program stars with separations ranging from 0.9-6 arc second, and were located in both the northern and southern hemispheres. Seeing at Bosscha Observatory is generally 1-2 arc second, imposing a limit on visual double star separation below which the system cannot be resolved by long exposure imaging (longer than ~50 ms). Speckle interferometry methods are used to resolve double stars with separations below the typical size of seeing effects. A series of images were captured in fast short-time exposures (~50 ms) using a CCD camera. The result of our experiment shows that our system can be used to measure separations of 0.9 arc second (for systems with small ${\Delta}m$) and ${\Delta}m{\approx}3.7$ (for wide systems).

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.44.1-44.1
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• 2016

In this paper, we present a catalogue of the spectra of bright stars observed during the sky survey using the Far-Ultraviolet Imaging Spectrograph (FIMS), which was designed primarily to observe diffuse emissions. By carefully eliminating the contamination from the diffuse background, we obtain the spectra of 70 bright stars observed for the first time with a spectral resolution of $2-3{\AA}$ over the wavelength of $1370-1710{\AA}$. The far-ultraviolet spectra of an additional 139 stars are also extracted with a better spectral resolution and/or higher reliability than those of the previous observations. The stellar spectral type of the stars presented in the catalogue spans from O9 to A3. The method of spectral extraction of the bright stars is validated by comparing the spectra of 323 stars with those of the International Ultraviolet Explorer (IUE) observations.

• Journal of Astronomy and Space Sciences
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• v.29 no.1
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• pp.85-91
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• 2012

Optical interferometry and polarimetry have separately provided new insights into stellar astronomy, especially in the fields of fundamental parameters and atmospheric models. We present: scientific justifications for "full-Stokes" optical interferometric polarimetry (OIP); updated instrument requirements; preliminary beam combiner designs; polarimeter design; end-to-end OIP data reduction; and realistic reimaged full-Stokes models of Be stars with a suitable number of telescopes plus noise sources. All of this work represents preliminary research to construct an OIP beam combiner.