In this work MRI-based spirometry is presented as a method for noninvasively assessing pulmonary mechanical function on a regional basis. A SPAMM tagging sequence was modified to allow continuous dynamic imaging of the lungs during respiration. A motion-tracking algorithm was developed to track material regions from time-resolved grid-tagged images. Experiments were performed to image the lungs during quiet breathing and volumetric strain was calculated from the measured displacement maps. Regional volume calculations, derived from volumetric strain, were integrated over the entire lung and compared to segmented volume calculations with good agreement. Results from this work demonstrate that MRI spirometry has the potential to become a clinically useful tool for measuring regional ventilation and assessing pulmonary diseases that regionally affect the mechanical function of the lung. The ability of the lung to expand and contract is vital to promoting respiration. Even though the lung is conventionally examined on the whole, it is generally understood that regional differences in function do exist (1). Moreover, many pathologic conditions, e.g., acute respiratory distress syndrome (2) and status asthmaticus (3), are believed to be due to airways with locally impaired function. At the present time, clinical assessment of pulmonary function is largely performed on the entire pulmonary system using conventional spirometry and thus is incapable of delivering localized information. The objective of this work was to develop a methodology for an image-based pulmonary function test using grid-tagging MRI, ultimately to deliver regional metrics for lung function.Regional ventilation measures have proven more sensitive than traditional spirometry for defining therapy and providing early detection of disease (4). As such, there has been an effort to noninvasively obtain regional ventilation or spirometric measures during respiration using MRI. However, technical limitations on temporal and spatial resolution and tracking lung deformation have inhibited dynamic studies from yielding functional regional lung measurements. For example, promising images have been generated with hyperpolarized 3 He gas exhibiting ventilated regions, yet lung motion complicates parametric mapping as a function of time (5-8).Many novel MR methods have been developed to image the lung parenchyma (9 -11). MR imaging of the lung parenchyma is inherently challenging, as efforts are hampered by low proton density and local magnetic field inhomogeneity. As such, a number of techniques have focused on imaging the easily discernible chest wall boundary for calculating tidal volume of the right and left lungs (12-15). For example, Suga et al. (12) performed dynamic studies imaging chest wall motion to understand secondary effects of ventilation defects and the relationship between pulmonary mechanics and the respiratory muscles external to the lungs. In this study, the imaging time for a single image was long-0.8 s-compared to quiet breathing and typical ...
Purpose:To study the feasibility of using the MRI technique of segmented true-fast imaging with steady-state precession arterial spin-labeling (True-FISP ASL) for the noninvasive measurement and quantification of local perfusion in cerebral deep gray matter at 3T. Materials and Methods:A flow-sensitive alternating inversion-recovery (FAIR) ASL perfusion preparation was used in which the echo-planar imaging (EPI) readout was replaced with a segmented True-FISP data acquisition strategy. The absolute perfusion for six selected regions of deep gray matter (left and right thalamus, putamen, and caudate) were calculated in 11 healthy human subjects (six male, five female; mean age ϭ 35.5 years Ϯ 9.9).Results: Preliminary measurements of the average absolute perfusion values at the six selected regions of deep gray matter are in agreement with published values for mean absolute cerebral blood flow (CBF) baselines acquired from healthy volunteers using positron emission tomography (PET). Conclusion:Segmented True-FISP ASL is a practical and quantitative technique suitable to measure local tissue perfusion in cerebral deep gray matter at a high spatial resolution without the susceptibility artifacts commonly associated with EPI-based methods of ASL.
This paper contains temporally and spatially resolved flow visualization and DPIV measurements characterizing the frequency–amplitude response and three-dimensional vortex structure of a circular cylinder mounted like an inverted pendulum. Two circular cylinders were examined in this investigation. Both were 2.54 cm in diameter and ~140 cm long with low mass ratios, m* = 0.65 and 1.90, and mass–damping ratios, m*ζ = 0.038 and 0.103, respectively. Frequency–amplitude response analysis was done with the lighter cylinder while detailed wake structure visualization and measurements were done using the slightly higher-mass-ratio cylinder. Experiments were conducted over the Reynolds number range 1900≤Re≤6800 corresponding to a reduced velocity range of 3.7 ≤ U* ≤ 9.6. Detailed examination of the upper branch of the synchronization regime permitted, for the first time, the identification of short-time deviations in cylinder oscillation and vortex-shedding frequencies that give rise to beating behaviour. That is, while long-time averages of cylinder oscillation and vortex-shedding frequencies are identical, i.e. synchronized, it is shown that there is a slight mismatch between these frequencies over much shorter periods when the cylinder exhibits quasi-periodic beating. Data are also presented which show that vortex strength is also modulated from one cylinder oscillation to the next. Physical arguments are presented to explain both the origins of beating as well as why the quasi-periodicity of the beating envelopes becomes irregular; it is believed that this result may be generalized to a broader class of fluid–structure interactions. In addition, observations of strong vertical flows associated with the Kármán vortices developing 2–3 diameters downstream of the cylinder are presented. It is hypothesized that these three-dimensionalities result from both the inverted pendulum motion as well as free-surface effects.
Background: To date hydrocephalus researchers acknowledge the need for rigorous but utilitarian fluid mechanics understanding and methodologies in studying normal and hydrocephalic intracranial dynamics. Pressure volume models and electric circuit analogs introduced pressure into volume conservation; but control volume analysis enforces independent conditions on pressure and volume. Previously, utilization of clinical measurements has been limited to understanding of the relative amplitude and timing of flow, volume and pressure waveforms; qualitative approaches without a clear framework for meaningful quantitative comparison.
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