Shigeo Wada

Voiced fricatives involve complex aerodynamic interactions between turbulent noise generation in the oral cavity and vocal fold vibration. In this study, aeroacoustic simulations combining a simplified vocal tract model of the alveolar fricative /s/ with a two-mass vocal fold model were conducted to investigate the effects of vocal fold vibration and tongue motion on fricative sound production. The compressible Navier-Stokes equations were solved for a vocal tract model with the tongue dynamically lowered from the fricative configuration to the vowel /u/ configuration. Comparisons between vibrating and non-vibrating conditions showed that vocal fold vibration reduced high-frequency fricative noise above 4 kHz by approximately 5 dB under identical subglottal pressure. Flow field revealed reduced turbulent velocity fluctuations around the teeth in the vibrating condition, indicating that part of the aerodynamic energy was transferred to sustaining vocal fold oscillation. During tongue lowering, the glottal airflow rate increased more gradually in the vibrating condition than in the non-vibrating condition. Consequently, the fricative noise decayed earlier in the voiced condition, suggesting that vocal fold vibration facilitates an earlier transition from fricative noise to vowel production, potentially contributing to smoother consonant–vowel transitions in voiced fricatives.

BACKGROUND AND OBJECTIVES: The pathophysiology of obstructive hydrocephalus caused by cerebral aqueduct obstruction or stenosis, as well as communicating hydrocephalus with oscillating cerebrospinal fluid (CSF) flow and vortices in the third ventricle (3V), remains unclear. In addition, clear criteria for selecting between ventriculoperitoneal shunting (VPS) and endoscopic third ventriculostomy (ETV) have yet to be established. This study aimed to investigate the utility of 4-dimensional flow MRI for assessing CSF dynamics to enhance our understanding of hydrocephalus pathophysiology and guide treatment selection. METHODS: Using 4-dimensional flow MRI (velocity encoding = 5 cm/s), we evaluated CSF dynamics before and after VPS and ETV in patients with obstructive and communicating hydrocephalus, focusing on velocity vectors, streamlines, and vorticity. RESULTS: In obstructive hydrocephalus due to cerebral aqueduct obstruction, we initially expected a complete absence of CSF circulation in the 3V and cerebral aqueduct before ETV. However, slow flow was observed around the foramen of Monro and even within the obstructed aqueduct. After ETV, rapid CSF flow with vortex formation was observed in the 3V. In obstructive hydrocephalus, while VPS reduced ventricular size, it did not enhance CSF movement in the 3V. In communicating hydrocephalus with third ventricular floor ballooning, rapid CSF flow and abnormal vortices in the 3V persisted after ETV; however, these abnormalities were mitigated by VPS. CONCLUSION: These findings suggest that both CSF stagnation and abnormal vortices in the 3V contribute to the pathophysiology of obstructive and communicating hydrocephalus. Therefore, treatment strategies should prioritize interventions that optimize CSF flow dynamics.

Purpose: Diffusion‐weighted imaging (DWI) and intravoxel incoherent motion (IVIM) imaging are well‐established approaches for evaluating cerebrospinal fluid (CSF) flow in subarachnoid and perivascular spaces, and have recently been applied to study ventricular CSF flow. However, DWI does not directly measure flow velocity, and the physical implications of DWI measurements are unclear. This study aimed to provide a theoretical interpretation of the DWI and IVIM imaging of CSF flow velocity fields. Theory: The general semi‐analytical form of the signal attenuations caused by fluid flow and the resultant apparent diffusion coefficient were derived from the Bloch–Torrey equation for arbitrary values. Methods: The fundamental properties of the signal attenuation in laminar flow velocity fields were investigated. A Monte Carlo simulation of the IVIM parameter estimation was performed based on these signal attenuations, taking background noise into consideration. Results: The developed theoretical framework indicates that signal attenuations in DWI detect intravoxel flow velocity standard deviations ranging from approximately 0.1 to 10 mm/s within the range of practical scan parameter settings. The lower bounds of the DWI flow profiles appeared where the flow effect was an order of magnitude lower than the molecular diffusion effects, even when increased. The IVIM fitting parameters reflected the flow effects of the signal attenuations despite an inconsistency with the original IVIM model assumptions. Conclusion: The physical implications of signal attenuation in DWI have been theoretically clarified. This framework provides a useful basis for understanding CSF flow dynamics and considering appropriate imaging settings.

Purpose: Computational simulation of phase‐contrast MRI (PC‐MRI) is an attractive way to physically interpret properties and errors in MRI‐reconstructed flow velocity fields. Recent studies have developed PC‐MRI simulators that solve the Bloch equation, with the magnetization transport being modeled using a Lagrangian approach. Because this method expresses the magnetization as spatial distribution of particles, influences of particle densities and their spatial uniformities on numerical accuracy are well known. This study developed an alternative method for PC‐MRI modeling using an Eulerian approach in which the magnetization is expressed as a spatially smooth continuous function.Methods: The magnetization motion was described using the Bloch equation with an advection term and computed on a fixed grid using a finite difference method, and k‐space sampling was implemented using the spoiled gradient echo sequence. PC‐MRI scans of a fully developed flow in straight and stenosed cylinders were acquired to provide numerical examples.Results: Reconstructed flow in a straight cylinder showed excellent agreement with input velocity profiles and mean errors were less than 0.5% of the maximum velocity. Numerical cases of flow in a stenosed cylinder successfully demonstrated the velocity profiles, with displacement artifacts being dependent on scan parameters and intravoxel dephasing due to flow disturbances. These results were in good agreement with those obtained using the Lagrangian approach with a sufficient particle density.Conclusion: The feasibility of the Eulerian approach to PC‐MRI modeling was successfully demonstrated.

Left upper lobectomy (LUL) with left superior pulmonary vein (LSPV) resection alters the left atrium (LA) physiological states and LA hemodynamics associated with thrombosis, although this underlying mechanism is poorly understood. Therefore, we investigated the effects of LSPV resection and associated LA physiological changes on LA hemodynamics using four-dimensional computed tomography image-based computational simulations. Three cases were considered: the LA before and after LUL extracted from computed tomography images and artificial LSPV resection without physiological changes. Comparisons among the three cases demonstrated that physiological changes associated with LSPV resection are the possible factors that affect the LA hemodynamics after LUL.

The brain is a fluid-saturated poroelastic material and keeps its mechanical balance between solid and fluid phases in an intracranial space. Changes in the intracranial water contents due to the loss of neurons with aging lead to brain deformation as cerebral atrophy, while this mechanical implication remains poorly understood. This study aims to develop a computational mechanical model of cerebral atrophy with considering anatomical brain shape and the volume change of brain tissues. The brain was modeled as compressible hyperelastic materials and its volume decrease with aging was modeled as plastic deformation by changing its reference state. The mechanical balance state of the brain with its volume decrease was computed using Galerkin finite element method. Obtained results showed characteristic brain deformations consistent with clinical knowledge. This finding suggests that mechanical balances would be one of the essential factors to understand the mechanism of brain deformation with aging.

How do regional brain volume ratios and cerebral blood flow (CBF, mL/min) change with aging, and are there sex differences? This study aimed to comprehensively evaluate the relationships between regional brain volume ratios and CBF in healthy brains. The study participants were healthy volunteers who underwent three-dimensional T1-weighted MRI, time-of-flight MR angiography, and four-dimensional (4D) flow MRI between 2020 and 2022. The brain was automatically segmented into 21 brain subregions from 3D T1-weighted MRI, and CBF in 16 major intracranial arteries were measured by 4D flow MRI. The relationships between segmented brain volume ratios and CBFs around the circle of Willis were comprehensively investigated in each decade and sex. This study included 129 healthy volunteers (mean age ± SD, 48.2 ± 16.8; range, 22-92 years; 43 males and 86 females). The association was strongest between the cortical gray matter volume ratio and total outflow of the intracranial major arteries distal to the circle of Willis (Pearson’s correlation coefficient, r: 0.425). In addition, the mean flow of the total inflow and outflow around the circle of Willis were significantly greater in women than men, and significant left-right differences were observed in CBFs even on the peripheral side of the circle of Willis. Moreover, the correlation was strongest between the left cortical gray matter volume ratio and the combined flows of the left anterior and posterior cerebral arteries distal to the circle of Willis (r: 0.486). There was a trend toward greater total intracranial CBF, especially among women in their 40s and younger, who had a larger cortical gray matter volume. This finding may be one of the reasons for the approximately twofold higher incidence of cerebral aneurysms and subarachnoid hemorrhage, and a threefold higher incidence of migraine headaches.

Purpose: Phase‐contrast MRI (PC‐MRI) of cerebrospinal fluid (CSF) velocity is used to evaluate the characteristics of intracranial diseases, such as normal‐pressure hydrocephalus (NPH). Nevertheless, PC‐MRI has several potential error sources, with eddy‐current‐based phase offset error being non‐negligible in CSF measurement. In this study, we assess the measurement error of CSF velocity maps obtained using 4D flow MRI and evaluate correction methods.Methods: CSF velocity maps of 10 patients with NPH were acquired using 4D flow MRI (velocity‐encoding = 5 cm/s). Distributed phase offset error was estimated for a whole 3D background field by polynomial fitting using robust regression analysis. This estimated phase offset error was then used to correct the CSF velocity maps. The estimated error profiles were compared with those obtained using an existing 2D correction approach involving local background information near the region of interest.Results: The residual standard error of the polynomial fitting against the phase offset error extracted from the measured velocities was within 0.2 cm/s. The spatial dependencies of the phase offset errors showed similar tendencies in all cases, but sufficient differences in these values were found to indicate requirement of velocity correction. Differences of the estimated errors among other correction approaches were in the order of 10⁻² cm/s, and the estimated errors were in good agreement with those obtained using existing approaches.Conclusion: Our method is capable of estimating the measurement error of CSF velocity maps obtained from 4D flow MRI and provides quantitatively reasonable characteristics for the main CSF profile in the cerebral aqueduct in patients with NPH.

Despite numerous experimental observations regarding heart failure with preserved ejection fraction (HFpEF), which is characterized mainly by left ventricular hypertrophy and a left ventricular ejection fraction over 50%, myocardial dynamics under HFpEF have not yet been fully clarified, particularly regarding the relationship between myocardial strain distribution and myocardial work. To address this issue, we numerically investigated radial distribution of myocardial strain during a cardiac cycle with fixed internal volume at the end of the systolic and diastolic phases under different mechanical conditions, such as those involving myocardial thickness and elasticity of myocardial fibers. The myocardium was a modeled as a visco-hyperelastic continuous material. This model was taken into account that active contractile stress along the myocardial fiber direction depends on membrane potential change. Our numerical results showed that both radial and circumferential strains decreased as wall thickness increased, which reflected cardiac hypertrophy, but that myocardial work became larger than that observed with thin ventricular walls. Further, the change in left ventricular diastolic internal pressure caused circumferential strain, while fiber stiffness contributed to radial strain. Since peak circumferential strain was well estimated by the maximum difference between total internal and myocardial volumes, measuring the epicardial contraction rate should be helpful in understanding patients with HFpEF.