Hyungseok Jang

Objectives: A characteristic of chronic obstructive pulmonary disease is mucus hypersecretion. In this proof-of-concept study, we aimed to characterize airway mucus using a quantitative ultrashort echo time (UTE) magnetic resonance imaging (MRI) sequence in a controlled in vitro synthetic mucus model to establish a technical foundation for future in vivo studies. Materials and methods: UTE double-echo steady-state (UTE-DESS) was performed to acquire the signal from free induction decay-like signal and spin/stimulated echo-like signal echoes; quantitative MRI parameters (T1, T2, and diffusivity) were estimated. Synthetic mucus phantoms were prepared using mucin (0.20%, 0.50%, and 1.0%) mixed with xanthan gum (0.10%, 0.25%, and 0.50%) to replicate the viscoelastic properties of human laryngeal mucus, and samples were embedded in 1% agar for imaging. Results: The mucus phantoms with varying concentrations of mucin and xanthan exhibited visual differences in color and opacity. Estimated T1 values were 2.23 ± 0.16, 2.37 ± 0.20, and 2.09 ± 0.18 s (mean ± standard deviation) for the three tubes with relative mucus concentrations of 1, 2.5, and 5, respectively. Corresponding T2 values were 806.6 ± 105.6, 687.4 ± 78.0, and 475.0 ± 41.3 ms, while diffusivity values were 2.36 ± 0.14, 2.31 ± 0.18, and 2.33 ± 0.18 µm ² /ms for the three tubes. Only the T2 relaxation times showed a significant linear correlation with mucus concentration ( r = -0.9998, p = 0.0112). Conclusion: UTE-DESS MRI could be feasible for characterizing airway mucus. T2 relaxation time showed a strong inverse correlation with increasing mucus concentration, indicating its sensitivity to mucus content. Key Points: Question Can a novel quantitative ultrashort echo time MRI sequence noninvasively characterize airway mucus properties and overcome conventional lung imaging limitations in muco-obstructive diseases? Findings In this controlled in vitro model, UTE-DESS MRI mapping demonstrated that T2 relaxation times alone showed a strong inverse correlation with increasing mucus concentrations. Relevance statement This exploratory proof-of-concept establishes a technical foundation for radiation-free, noninvasive T2 mapping to monitor and phenotype chronic obstructive pulmonary disease and other mucoobstructive lung diseases in future in vivo studies.

To investigate the feasibility of a noncontrast MRI framework combining velocity‐selective arterial spin labeling (VSASL) and artificial neural network (ANN)–based gradient‐echo sampling of spin‐echo (GESSE) analysis for quantifying exercise‐induced changes in calf muscle perfusion and oxygen metabolism. Ten healthy volunteers (mean age: 31.1 ± 5.45 years) underwent MRI scans before and after standardized eccentric calf muscle exercise. Muscle blood flow (MBF) was assessed using VSASL with 3D GRASE readout, while muscle oxygen extraction fraction (MOEF) was estimated from GESSE data using an ANN‐based method. Skeletal muscle oxygen consumption (SMVO ₂ ) was calculated based on Fick's principle. All maps were registered to T2‐weighted images, and linear mixed‐effects models were applied to evaluate the effects of time and muscle group. Following exercise, a significant increase in MBF was observed ( Δ = 6.91 mL/100 g/min, p < 0.001), primarily in the gastrocnemius and soleus muscles. MOEF also increased significantly ( Δ = 0.014, p = 0.029), with no significant differences across muscles. SMVO ₂ showed a significant overall increase ( Δ = 25.56 μmol O ₂ /100 g/min, p < 0.001), with a significant interaction between time and muscle ( p = 0.028). The greatest SMVO ₂ increases were observed in the gastrocnemius (68.76% ± 36.1%) and soleus (51.94% ± 44.0%). This noncontrast approach using VSASL and ANN‐based GESSE demonstrates the feasibility of spatially resolved quantification of calf muscle hemodynamics and oxygen metabolism. This method may serve as a valuable tool for assessing microvascular and metabolic function in both research and clinical practice.

Myelin-weighted imaging with inversion-recovery ultrashort-echo-time (IR-UTE) is highly motion-sensitive, yet extra hardware or longer scans are impractical. We evaluated whether a 1D superior–inferior (SI) navigator with bit-reversed spoke-angles mitigates motion artifacts without extending acquisition. Dual-echo IR-UTE was implemented at 3T. After an adiabatic inversion pulse, 21 spokes were acquired per segment around the white-matter null point, and dual-echo subtraction suppressed residual long-T2 signals. Adding SI-navigators at the end of each segment allows motion detection without lengthening TR. And bit-reversal is used to pseudo-randomize the spoke-angles. Bloch simulations of a 2D synthetic brain removed 10% of spokes to mimic gating. Three volunteers were scanned: (i) sequential-ordering, no-motion; (ii) bit-reversed, no-motion; (iii) bit-reversed with deliberate head nods. The navigator rejected 1,280 of 12,000 spokes (10.7%) during nodding, and the same gating was reconstructed on motion-free data. Simulation showed coherent streaks for sequential ordering but an isotropic point-spread function for bit-reversal. In vivo, gating degraded only the sequential dataset. bit-reversal preserved subcortical and deep-white-matter detail. During intentional nodding, gating with bit-reversal enhanced myelin contrast, outperforming the image reconstructed from the ungated data. The SI-navigator plus bit-reversal enables effective motion gating without hardware or time penalty, supporting routine motion-robust myelin mapping.

Background/Objectives: Tissue conductivity reflects ionic composition (e.g., sodium), providing critical insights into various diseases. Ultrashort echo time quantitative conductivity mapping (UTE-QCM) offers a method to obtain this information, which is particularly effective for musculoskeletal (MSK) tissues with short T2 relaxation times. The aim of this study is to develop a UTE-QCM framework using ultrashort echo time double echo steady-state (UTE-DESS) and validate its feasibility in the knee. Methods: An ultrashort echo time double echo steady-state (UTE-DESS) sequence was used to acquire S+ and S− images and estimate the transmit radiofrequency field (B1⁺) phase at 3T. The B1⁺ phase was derived by canceling the phase evolution in the free induction decay using these images. This phase data was then processed using two widely used QCM reconstruction methods for comparison: parabolic fitting and an integral-based method. The proposed UTE-QCM framework was validated using a phantom containing three different concentrations of sodium chloride (0%, 0.5%, and 1%). Additionally, three healthy volunteers were recruited to validate UTE-QCM in knee imaging. Results: In both phantom and in vivo experiments, the integral-based QCM demonstrated improved robustness to noise compared to parabolic fitting. In the sodium phantom, the estimated conductivity showed high linearity with sodium concentrations. In the in vivo knee, the generated conductivity maps successfully visualized both long and short T2 tissues. Conclusions: We demonstrated the feasibility of UTE-QCM as a novel quantitative imaging tool targeting short T2 tissues in the MSK system. This technique may facilitate the diagnosis and prognosis of joint disorders.

This study aims to demonstrate the feasibility of ultrashort echo time (UTE)-based susceptibility source separation for musculoskeletal (MSK) imaging, enabling discrimination between diamagnetic and paramagnetic tissue components, with a particular focus on hemophilic arthropathy (HA). Three key techniques were integrated to achieve UTE-based susceptibility source separation: Iterative decomposition of water and fat with echo asymmetry and least-squares estimation for B0 field estimation, projection onto dipole fields for local field mapping, and χ-separation for quantitative susceptibility mapping (QSM) with source decomposition. A phantom containing varying concentrations of diamagnetic (CaCO₃) and paramagnetic (Fe₃O₄) materials was used to validate the method. In addition, in vivo UTE-QSM scans of the knees and ankles were performed on five HA patients using a 3T clinical MRI scanner. In the phantom, conventional QSM underestimated susceptibility values due to the mixed-source cancelling the effect. In contrast, source-separated maps provided distinct diamagnetic and paramagnetic susceptibility values that correlated strongly with CaCO₃ and Fe₃O₄ concentrations (r = −0.99 and 0.95, p < 0.05). In vivo, paramagnetic maps enabled improved visualization of hemosiderin deposits in joints of HA patients, which were poorly visualized or obscured in conventional QSM due to susceptibility cancellation by surrounding diamagnetic tissues such as bone. This study demonstrates, for the first time, the feasibility of UTE-based quantitative susceptibility source separation for MSK applications. The approach enhances the detection of paramagnetic substances like hemosiderin in HA and offers potential for improved assessment of bone and joint tissue composition.

Purpose: To propose a signal acquisition and modeling framework for multi‐component tissue quantification that encompasses transmit field inhomogeneity, multi‐component relaxation and magnetization transfer (MT) effects. Theory and Methods: By applying off‐resonance irradiation between excitation and acquisition within an RF‐spoiled gradient‐echo scheme, in combination with multiple echo‐time acquisitions, both Bloch‐Siegert shift and magnetization transfer effects are simultaneously induced while relaxation and spin exchange processes occur concurrently. The spin dynamics are modeled using a three‐pool framework, from which an analytical signal equation is derived and validated through numerical Bloch simulations. Monte Carlo simulations were further performed to analyze and compare the model's performance. Finally, the feasibility of this novel approach was investigated in vivo in human brain and knee tissues. Results: Simulation results showed excellent agreement with the derived analytical signal equation across a wide range of flip angles and echo times. Monte Carlo analyses further validated that the three‐pool parameter estimation pipeline performed robustly over various signal‐to‐noise ratio conditions. Multi‐parameter fitting results from in vivo brain and knee studies yielded values consistent with previously reported literature. Collectively, these findings confirm that the proposed method can reliably characterize multi‐component tissue parameters in macromolecule‐rich environments while effectively compensating for inhomogeneity. Conclusion: A signal acquisition and modeling framework for multi‐component tissue quantification that accounts for magnetization transfer effects and inhomogeneity has been developed and validated. Both simulation and experimental results confirmed the robustness of this method and its applicability to various tissue types in the brain and knee.

Quantitative susceptibility mapping (QSM) is a powerful magnetic resonance imaging (MRI) technique for assessing tissue composition in the human body. For imaging short-T2 tissues in the musculoskeletal (MSK) system, ultrashort echo time (UTE) imaging plays a key role. However, UTE-based QSM (UTE-QSM) often involves repeated acquisitions, making it vulnerable to inter-scan motion. In this study, we investigate the effects of motion on UTE-QSM and introduce strategies to reduce motion-induced artifacts. Eight healthy male volunteers underwent UTE-QSM imaging of the knee joint, while an additional seven participated in imaging of the ankle joint. UTE-QSM was conducted using multiple echo acquisitions, including both UTE and gradient-recalled echoes, and processed using the iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) and morphology-enabled dipole inversion (MEDI) algorithms. To assess the impact of motion, datasets were reconstructed both with and without motion correction. Furthermore, we evaluated a two-step UTE-QSM approach that incorporates tissue boundary information. This method applies edge detection, excludes pixels near detected edges, and performs a two-step QSM reconstruction to reduce motion-induced streaking artifacts. In participants exhibiting substantial inter-scan motion, prominent streaking artifacts were evident. Applying motion registration markedly reduced these artifacts in both knee and ankle UTE-QSM. Additionally, the two-step UTE-QSM approach, which integrates tissue boundary information, further enhanced image quality by mitigating residual streaking artifacts. These results indicate that motion-induced errors near tissue boundaries play a key role in generating streaking artifacts in UTE-QSM. Inter-scan motion poses a fundamental challenge in UTE-QSM due to the need for multiple acquisitions. However, applying motion registration along with a two-step QSM approach that excludes tissue boundaries can effectively suppress motion-induced streaking artifacts, thereby improving the accuracy of musculoskeletal tissue characterization.

Direct myelin imaging with inversion-recovery ultrashort-echo-time (IR-UTE) is highly motion-sensitive, yet extra hardware or longer scans are impractical. We evaluated whether a superior–inferior (SI) self-navigator with bit-reversed spoke-angles mitigates motion artifacts without extending acquisition. Dual-echo IR-UTE was implemented at 3T. After an adiabatic inversion pulse, 21 spokes were acquired per segment around the white-matter null point, and dual-echo subtraction suppressed residual long-T2 signals. Adding SI-navigators at the end of each segment allows motion detection without lengthening TR. And bit-reversal is used to pseudo-randomize the spoke-angles. Bloch simulations of a 2D synthetic brain removed 10% of spokes to mimic gating. Three volunteers were scanned: (i) sequential-ordering, no-motion; (ii) bit-reversed, no-motion; (iii) bit-reversed with deliberate head nods. The navigator rejected 1,280 of 12,000 spokes (10.7%) during nodding, and the same gating was reconstructed on motion-free data. Simulation showed coherent streaks for sequential ordering but an isotropic point-spread function for bit-reversal. In vivo, gating degraded only the sequential dataset. bit-reversal preserved subcortical and deep-white-matter detail. During intentional nodding, gating with bit-reversal enhanced myelin contrast, outperforming the image reconstructed from the ungated data. The SI-navigator plus bit-reversal enables effective motion gating without hardware or time penalty, supporting routine motion-robust myelin mapping.

Purpose: Introduce a unified acquisition and modeling strategy to simultaneously quantify magnetization transfer (MT), tissue susceptibility () and . Theory and Methods: Magnetization transfer is induced through the application of off‐resonance irradiation between excitation and acquisition of an RF‐spoiled gradient‐echo scheme, where free pool spin–lattice relaxation (), macromolecular proton fraction () and magnetization exchange rate () were calculated by modeling the magnitude of the MR signal using a binary spin‐bath MT model with inhomogeneity correction via Bloch‐Siegert shift. Simultaneously, a multi‐echo acquisition is incorporated into this framework to measure the time evolution of both signal magnitude and phase, which was further modeled for estimating and tissue susceptibility. In this work, we demonstrate the feasibility of this new acquisition and modeling strategy in vivo on the brain tissue. Results: In vivo brain experiments were conducted on five healthy subjects to validate our method. Utilizing an analytically derived signal model, we simultaneously obtained 3D , , , and maps of the whole brain. Our results from the brain regional analysis show good agreement with those previously reported in the literature, which used separate MT and QSM methods. Conclusion: A unified acquisition and modeling strategy based on an analytical signal model that fully leverages both the magnitude and phase of the acquired signals was demonstrated and validated for simultaneous MT, susceptibility and quantification that are free from bias.