Yonghyun Ha - Academia.edu (original) (raw)

Papers by Yonghyun Ha

Schematic diagrams describing the connection between OLED display and PC.</p

PLOS ONE

Since recovery time of the RF coil is long at low field MRI, the rising and the ring-down times o... more Since recovery time of the RF coil is long at low field MRI, the rising and the ring-down times of the square pulse are also long, which means the applied sinc pulse can easily be distorted from the changing amplitude. However, both the rising time and ring-down time can be calculated using Q-factor. Using this information, an RF square pulse were compensated by appending two square pulses before and after the RF pulse. The durations of these RF square pulses were calculated using the Q-factor. Since the amplitude of the sinc pulse changes continuously, a series of square pulses were applied to apply sinc pulse to the coil. The minimum number of square pulses and the amplitude of the square pulses were calculated. It was successfully demonstrated that the sinc pulse can be compensated using a series of square pulses. The more number of square pulses were used, the smoother sinc pulse was applied to the RF coil. The Q-factor was experimentally calculated from the ring-down time of a ...

(a) A schematic diagram and (b) a photo of the OLED display installed on the head coil.&... more (a) A schematic diagram and (b) a photo of the OLED display installed on the head coil.</p

The display is cantered on the Larmor frequency of the MRI system (123.25 MHz) and cover... more The display is cantered on the Larmor frequency of the MRI system (123.25 MHz) and covers a bandwidth greater than the bandwidth used in the fMRI acquisitions.</p

System set-up to measure noise with LNA board and field probe.</p

The schematic of visual angle, display width and distance.</p

Calculated visual angle of dual OLED compared to that of commercial screens mounted outs... more Calculated visual angle of dual OLED compared to that of commercial screens mounted outside the scanner bore.</p

(a) OLED with shield box, and (b) the detailed structures of the shield box. The shield ... more (a) OLED with shield box, and (b) the detailed structures of the shield box. The shield box contains the OLED itself and the video converters without additional shielding between the individual units.</p

Nature Neuroscience

Hydrocephalus, characterized by cerebral ventricular dilatation, is routinely attributed to prima... more Hydrocephalus, characterized by cerebral ventricular dilatation, is routinely attributed to primary defects in cerebrospinal fluid (CSF) homeostasis. This fosters CSF shunting as the leading reason for brain surgery in children despite considerable disease heterogeneity. In this study, by integrating human brain transcriptomics with whole-exome sequencing of 483 patients with congenital hydrocephalus (CH), we found convergence of CH risk genes in embryonic neuroepithelial stem cells. Of all CH risk genes, TRIM71/lin-41 harbors the most de novo mutations and is most specifically expressed in neuroepithelial cells. Mice harboring neuroepithelial cell-specific Trim71 deletion or CH-specific Trim71 mutation exhibit prenatal hydrocephalus. CH mutations disrupt TRIM71 binding to its RNA targets, causing premature neuroepithelial cell differentiation and reduced neurogenesis. Cortical hypoplasia leads to a hypercompliant cortex and secondary ventricular enlargement without primary defects in CSF circulation. These data highlight the importance of precisely regulated neuroepithelial cell fate for normal brain-CSF biomechanics and support a clinically relevant neuroprogenitor-based paradigm of CH.

$Research paper thumbnail of Development of novel RF coils for <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mrow></mrow><mn>23</mn></msup></mrow><annotation encoding="application/x-tex">^{23}</annotation></semantics></math>23Na imaging at ultra-high field MRI$

Sodium (23Na) magnetic resonance imaging (MRI) provides valuable cellular and metabolic informati... more Sodium (23Na) magnetic resonance imaging (MRI) provides valuable cellular and metabolic information. Since the Larmor frequency of the 23Na nucleus is different from that of protons (1H), a radiofrequency (RF) coil tuned to the 23Na Larmor frequency is required for sodium MR imaging. However, the MR signal of 23Na is much smaller in amplitude than that of 1H due to its low intrinsic MR sensitivity and lower natural abundance and 1H images are very often required in conjunction with the 23Na images. For this reason, 23Na/1H double-tuned RF coils are often used for 23Na studies. In this case the proton signal is used for localisation and B0 shimming. Double-tuned RF coils are often designed by including additional lossy components such as LC-traps or PIN-diodes. Thus, the sensitivity of the coil – which is proportional to the quality factor of the resonant circuit – is reduced at both resonance frequencies, when compared to that of a single-tuned coil. The aim of this thesis is to dev...

IEEE Transactions on Medical Imaging, 2018

We thank Mr. T. Felder for his assistance to design the coil former and Ms. C. Rick for English p... more We thank Mr. T. Felder for his assistance to design the coil former and Ms. C. Rick for English proofreading. NJS in part has received funding from the European Union Seventh Framework Program (FP7 2007-2013) under Grant Agreement No. 602621-Trimage. 1 The first authorship is shared between Ha and Choi.

Journal of Magnetic Resonance, 2018

A folded four-ring quadrature birdcage coil was designed and constructed with a double-tune confi... more A folded four-ring quadrature birdcage coil was designed and constructed with a double-tune configuration of an outer high-pass coil for 1 H (400 MHz) and inner low-pass coil for 23 Na (105.72 MHz at 9.4 T). The coil was evaluated on the bench and in the scanner, comparing its performance with that of singletuned coils and a large four-ring coil. All coils were tuned and matched and the isolation between two quadrature ports was found to be better than À13.7 dB for 1 H and À27 dB for 23 Na. Signal-to-noise ratios (SNRs) were calculated and 23 Na flip angle maps were acquired. 23 Na SNR of the folded four-ring reached $93% of that obtained with the single-tuned coil. A set of in vivo 1 H and 23 Na axial images to cover the whole rat brain were obtained. The performance of the folded four-ring coil and its benefit for 23 Na imaging experiments have been demonstrated. This proposed four-ring coil could avoid length restrictions, e.g. the shoulders, by folding the outer rings vertically. This facilitates the construction of doubletuned four-ring birdcage coils just to fit the head, leading to higher filling factors and better SNR.

IEEE Transactions on Nuclear Science, 2017

This paper focuses on the design and the characterization of a frequency-selective shield for PET... more This paper focuses on the design and the characterization of a frequency-selective shield for PET detector modules of hybrid MR-PET scanners where the shielding of the PET cassettes is located close to the observed object. The proposed shielding configuration is designed and optimized to guarantee a high shielding effectiveness of up to 60 dB for B1-fields at the Larmor frequency of 64 MHz to prevent interactions between the RF coil and PET electronics. On the other hand, the shield is transparent to the gradient fields with the consequence that eddy current artifacts in the acquired EPI images are significantly reduced with respect to the standard solid-shield configuration. The frequency-selective behavior of the shield is characterized and validated via simulation studies with CST MICROWAVE STUDIO in the MHz and kHz range. Bench measurements with a RF coil built in-house demonstrated the high shielding effectiveness at the Larmor frequency. Moreover, measurements on a 4 T human scanner confirmed the abolishment of eddy current artifact and also provided an understanding of which surfaces eddy currents flow with respect to the sequence parameters. Simulations and measurements for the proposed shielding concept were compared to a solid copper shielding configuration.

Journal of Magnetic Resonance, 2016

Non-proton MRI has recently garnered gathering interest with the increased availability of ultra ... more Non-proton MRI has recently garnered gathering interest with the increased availability of ultra highfield MRI system. Assuming the availability of a broadband RF amplifier, performing multinuclear MR experiments essentially requires additional hardware, such as an RF resonator and a T/R switch for each nucleus. A double-or triple-resonant RF probe is typically constructed using traps or PIN-diode circuits, but this approach degrades the signal-to-noise ratio (SNR) and image quality compared to a singleresonant coil and this is a limiting factor. In this work, we have designed the required hardware for multinuclear MR imaging experiments employing six single-resonant coil sets and a purpose-built animal bed; these have been implemented into a home-integrated 9.4 T preclinical MRI scanner. System capabilities are demonstrated by distinguishing concentration differences and sensitivity of X-nuclei imaging and spectroscopy without SNR penalty for any nuclei, no subject interruption and no degradation of the static shim conditions.

Journal of magnetic resonance (San Diego, Calif. : 1997), 2017

A double-tuned (1)H/(19)F coil using PIN-diode switches was developed and its performance evaluat... more A double-tuned (1)H/(19)F coil using PIN-diode switches was developed and its performance evaluated. The is a key difference from the previous developments being that this design used a PIN-diode switch in series with an additionally inserted inductor in parallel to one of the capacitors on the loop. The probe was adjusted to (19)F when the reverse bias voltage was applied (PIN-diode OFF), whilst it was switched to (1)H when forward current was flowing (PIN-diode ON). S-parameters and Q-factors of single- and double-tuned coils were examined and compared with/without a phantom on the bench. Imaging experiments were carried out on a 9.4T preclinical scanner. All coils were tuned at resonance frequencies and matched well. It is shown that the Q-ratio and SNR of double-tuned coil at (19)F frequency are nearly as good as those of a single-tuned coil. Since the operating frequency was tuned to (19)F when the PIN-diodes were turned off, losses due to PIN-diodes were substantially lower re...

2010 IEEE International Ultrasonics Symposium, 2010

Magnetic nanoparticles residing in living tissues under external magnetic field may induce tissue... more Magnetic nanoparticles residing in living tissues under external magnetic field may induce tissue displacement that can be measured by ultrasound imaging. We propose a method to take magnetic force images which is proportional to the magnetic nanoparticle density under the assumption of mechanical coupling of the nanoparticles with background tissues. After measuring the axial displacement component by ultrasound imaging, we

PLOS ONE, 2018

Functional MRI (fMRI) is a well-established method used to investigate localised brain activation... more Functional MRI (fMRI) is a well-established method used to investigate localised brain activation by virtue of the blood oxygen level dependent (BOLD) effect. It often relies on visual presentations using beam projectors, liquid crystal display (LCD) screens, and goggle systems. In this study, we designed an MR compatible, low-cost display unit based on organic light-emitting diodes (OLED) and demonstrated its performance.