Younghun Kim

All Publications

• High-spatial-resolution transcranial focused ultrasound neuromodulation using frequency-modulated pattern interference radiation force. Ultrasonics Kim, Y. H., Kang, K. C., Kim, J. N., Park, K. K., Firouzi, K., Khuri-Yakub, B. T. 2024; 140: 107298

  Abstract

  Stimulating the brain in a precise location is crucial in ultrasound neuromodulation. However, improving the resolution proves a challenge owing to the characteristics of transcranial focused ultrasound. In this paper, we present a new neuromodulation system that overcomes the existing limitations based on an acoustic radiation force with a frequency-modulated waveform and standing waves. By using the frequency-modulated pattern interference radiation force (FM-PIRF), the axial spatial resolution can be reduced to a single wavelength level and the target location can be controlled in axial direction electronically. A linear frequency-modulated chirp waveform used in the experiment was designed based on the simulation results. The displacement of the polydimethylsiloxane (PDMS) cantilever was measured at intervals of 0.1 mm to visualize the distribution of radiation force. These results and methods experimentally show that FM-PIRF has improved spatial resolution and capability of electrical movement.

  View details for DOI 10.1016/j.ultras.2024.107298

  View details for PubMedID 38531115

• Acoustic radiation force for analyzing the mechanical stress in ultrasound neuromodulation. Physics in medicine and biology Kim, Y. H., Lee, C. H., Firouzi, K., Park, B. H., Pyun, J. Y., Kim, J. N., Park, K. K., Khuri-Yakub, B. T. 2023; 68 (13)

  Abstract

  Objective. Although recent studies have shown that mechanical stress plays an important role in ultrasound neuromodulation, the magnitude and distribution of the mechanical stress generated in tissues by focused ultrasound transducers have not been adequately examined. Various acoustic radiation force (ARF) equations used in previous studies have been evaluated based on the tissue displacement results and are suitable for estimating the displacement. However, it is unclear whether mechanical stress can be accurately determined. This study evaluates the mechanical stress predicted by various AFR equations and suggests the optimal equation for estimating the mechanical stress in the brain tissue.Approach. In this paper, brain tissue responses are compared through numerical finite element simulations by applying the three most used ARF equations-Reynolds stress force ((RSF)), momentum flux density tensor force, and attenuation force. Three ARF fields obtained from the same pressure field were applied to the linear elastic model to calculate the displacement, mechanical stress, and mean pressure generated inside the tissue. Both the simple pressure field using a single transducer and the complex standing wave pressure field using two transducers were simulated.Main results. For the case using a single transducer, all three ARFs showed similar displacement. However, when comparing the mechanical stress results, only the results using the RSF showed a strong stress tensor at the focal point. For the case of using two transducers, the displacement and stress tensor field of the pattern related to the standing wave were calculated only from the results using the RSF.Significance. The model using RSF equation allows accurate analysis on stress tensor inside the tissue for ultrasound neuromodulation.

  View details for DOI 10.1088/1361-6560/acdbb5

  View details for PubMedID 37366067

• Measurement of the acoustic reflectance function based on Lamb wave using high f-number transducers. Ultrasonics Kang, K. C., Joung, D. Y., Hun Kim, Y., Kyu Park, K., Firouzi, K., Khuri-Yakub, B. T. 2023; 132: 107003

  Abstract

  Ultrasonic reflectivity using a V(z) technique is a powerful characterization method in acoustic microscopy to measure the elastic properties of materials. Conventional techniques generally use a low f-number with high frequency; however, to measure the reflectance function of the highly attenuative material, a low frequency is essential. In this study, the transducer-pair method based on Lamb waves is used to measure the reflectance function of a highly attenuative material. The results demonstrate the feasibility of the proposed method using a commercial ultrasound transducer with high f-number.

  View details for DOI 10.1016/j.ultras.2023.107003

  View details for PubMedID 37099939

• Design of Piezoelectric Acoustic Transducers for Underwater Applications SENSORS Pyun, J., Kim, Y., Park, K. 2023; 23 (4)

  Abstract

  Interest in underwater transducers has persisted since the mid-1900s. Underwater transducers are designed in various shapes using various materials depending on the purpose of use, such as to achieve high power, improve broadband, and enhance beam steering. Therefore, in this study, an analysis is conducted according to the structural shape of the transducer, exterior material, and active material. By classifying transducers by structure, the transducer design trends and possible design issues can be identified. Researchers have constantly attempted new methods to improve the performance of transducers. In addition, a methodology to overcome this problem is presented. Finally, this review covers old and new research, and will serve as a reference for designers of underwater transducer.

  View details for DOI 10.3390/s23041821

  View details for Web of Science ID 000940047400001

  View details for PubMedID 36850418

  View details for PubMedCentralID PMC9966007

• Increasing the transmission efficiency of transcranial ultrasound using a dual-mode conversion technique based on Lamb waves. The Journal of the Acoustical Society of America Kang, K. C., Kim, Y. H., Kim, J. N., Kabir, M., Zhang, Y., Ghanouni, P., Park, K. K., Firouzi, K., Khuri-Yakub, B. T. 2022; 151 (3): 2159

  Abstract

  Transcranial focused ultrasound (FUS) is a noninvasive treatment for brain tumors and neuromodulation. Based on normal incidence, conventional FUS techniques use a focused or an array of ultrasonic transducers to overcome the attenuation and absorption of ultrasound in the skull; however, this remains the main limitation of using FUS. A dual-mode conversion technique based on Lamb waves is proposed to achieve high transmission efficiency. This concept was validated using the finite element analysis (FEA) and experiments based on changes in the incident angle. Aluminum, plexiglass, and a human skull were used as materials with different attenuations. The transmission loss was calculated for each material, and the results were compared with the reflectance function of the Lamb waves. Oblique incidence based on dual-mode conversion exhibited a better transmission efficiency than that of a normal incidence for all of the specimens. The total transmission losses for the materials were 13.7, 15.46, and 3.91dB less than those associated with the normal incidence. A wedge transducer was designed and fabricated to implement the proposed method. The results demonstrated the potential applicability of the dual-mode conversion technique for the human skull.

  View details for DOI 10.1121/10.0009849

  View details for PubMedID 35364946