Pulsation Motion Research Articles

Informed RESTORE: A method for robust estimation of diffusion tensor from low redundancy datasets in the presence of physiological noise artifacts

Physiological noise artifacts, especially those originating from cardiac pulsation and subject motion, are common in clinical Diffusion tensor-MRI acquisitions. Previous works show that signal perturbations produced by artifacts can be severe and neglecting to account for their contribution can result in erroneous diffusion tensor values. The Robust Estimation of Tensors by Outlier Rejection (RESTORE) method has been shown to be an effective strategy for improving tensor estimation on a voxel-by-voxel basis in the presence of artifactual data points in diffusion-weighted images. In this article, we address potential instabilities that may arise when using RESTORE and propose practical constraints to improve its usability. Moreover, we introduce a method, called informed RESTORE designed to remove physiological noise artifacts in datasets acquired with low redundancy (less than 30-40 diffusion-weighted image volumes)--a condition in which the original RESTORE algorithm may converge to an incorrect solution. This new method is based on the notion that physiological noise is more likely to result in signal dropouts than signal increases. Results from both Monte Carlo simulation and clinical diffusion data indicate that informed RESTORE performs very well in removing physiological noise artifacts for low redundancy diffusion-weighted image datasets.

Spatially variable Rician noise in magnetic resonance imaging

Magnetic resonance images tend to be influenced by various random factors usually referred to as "noise". The principal sources of noise and related artefacts can be divided into two types: arising from hardware (acquisition coil arrays, gradient coils, field inhomogeneity); and arising from the subject (physiological noise including body motion, cardiac pulsation or respiratory motion). These factors negatively affect the resolution and reproducibility of the images. Therefore, a proper noise treatment is important for improving the performance of clinical and research investigations. Noise reduction becomes especially critical for the images with a low signal-to-noise ratio, such as those typically acquired in diffusion tensor imaging at high diffusion weightings. The standard methods of signal correction usually assume a uniform distribution of the standard deviation of the noise across the image and evaluate a single correction parameter for the whole image. We pursue a more advanced approach based on the assumption of an inhomogeneous distribution of noise in space and evaluate correction factors for each voxel individually. The Rician nature of the underlying noise is considered for low and high signal-to-noise ratios. The approach developed here has been examined using numerical simulations and in vivo brain diffusion tensor imaging experiments. The efficacy and usefulness of this approach is demonstrated here and the resultant effective tool is described.

Development of wireless NIRS system with dynamic removal of motion artifacts

Along with recent advances in neuroscience, near-infrared spectroscopy (NIRS) has been widely used to measure changes in cerebral oxygenation non-invasively. An NIRS system can be constructed to be portable unlike other imaging modalities, but its signals are often distorted by artifacts generated by arterial pulsation, vasomotion and head motion. To overcome these problems, we have developed a wireless NIRS system with a real time accelerometer that allows noise reduction. Our wireless NIRS system includes a microcontroller, a Bluetooth communication, a fPCB (flexible printed circuit board), an accelerometer, batteries, LEDs (light emitting diode) and PDs (photodiode). Distorted signal caused by head motion was removed by active noise cancellation (ANC) algorithm. Two different tasks, arterial occlusion and brain hypoxia, were tested to validate the performance of our system. During arterial occlusion and breath holding, the NIRS signal showed corresponding hemodynamic changes such as increase in deoxy-hemoglobin (Hbr) and decrease in oxy-hemoglobin (HbO2) concentrations. Signal distortions generated by head motion were effectively removed. Wireless NIRS system combined with real time noise cancellation algorithm has great potential to be utilized in brain-computer interface (BCI) for physically challenged people, dynamic exercise tasks and cognitive studies for children. Furthermore we expect the system will be highly applicable in neuroscience.

Compressible pulsating convection through regular and random porous media: the thermoacoustic case

The effects of material, geometry, length and position of the porous channels on energy transfer in air-filled enclosures carrying a compressible pulsating wave are investigated. The pulsating fluid motion is created by an acoustic driver in a resonant chamber. Three different porous materials (Corning Celcor, Reticulated Vitreous Carbon (RVC), and Mylar plastic), three different geometries (square, open foam, and circular cross-section), six different lengths, “L” (varying between 1 and 6.5 cm, L = 0.01–0.068 λ, where λ is the wavelength of the fundamental acoustic mode), and eight different positions (hot end of the channel, varying between 0.5 and 8 cm) of the channels from the pressure anti-node is experimentally measured. The surface temperature distribution on the channel wall and temperature difference generated across the channel walls are measured while energy flow along the channel walls is calculated analytically. The experimental results are compared with a 1-D numerical code and found excellent agreement. The material, geometry, length, and position of the porous channel strongly affect the energy interactions between the porous channel and the working fluid. The temperature difference generated across the porous RVC channel increases as the porosity increases form 20 to 80 PPI; but decreases if the porosity increases further. Corning Celcor shows improved temperature difference generated across the channel as the length of the channel increases; but then decreases if the length is further increased. The results of this study are applicable to the design of thermoacoustic devices.

Controlled-cycle pulsed liquid–liquid chromatography. A modified version of Craig's counter-current distribution

A new liquid–liquid chromatography technique developed from a combination of controlled-cycle operation and a pulsed-mixing technique is suggested and validated. The controlled-cycle pulsed liquid–liquid chromatography (CPLC) system operates without involving a centrifuge and consists, of a series of multistage units, and a method for imparting pulsation motion to the liquids inside the units (the pulsation cycle). This chromatography technique can be considered as an improved continuous form of Craig's counter-current distribution method, or, alternatively, as a form of droplet chromatography with the cycling mode of operation. The theoretical model has been designed to account for the effects of the basic parameters influencing the CPLC operation. The theoretical model's suitability was proved by direct comparison between the experimental and model responses. The CPLC devices containing 1, 2, 4 and 5 multistage columns (each column was divided into 26 stages) have been designed, fabricated and tested; experiments were conducted to test the chromatographic behavior of organic (monocarboxylic) and mineral acids. The mass transfer rate in the stages depends on the nature of both—phase and sample systems: the highest values were achieved in experiments with acetic acid by using the octane/water biphasic system, where an equilibrium concentration distribution between stationary and mobile phases in the stages was attained. The results obtained demonstrated the potential of the new technique for preparative and industrial scale separations.

Effect of Osmolality and Selected Ions on Retraction of the Distome Body into the Cercaria Tail Chamber of Proterometra macrostoma (Trematoda: Azygiidae)

The furcocystocercous cercariae of the digenetic trematode, Proterometra macrostoma , possess a tail chamber into which their distome body withdraws prior to emergence from their snail intermediate host. The process of distome retraction and the conditions that trigger it in this species are not clear. The objectives of the present study were (1) to describe the retraction process in P. macrostoma; (2) to assess whether osmolality affects cercarial retraction; (3) to evaluate the effect of selected ions on retraction; and (4) to compare the swimming effectiveness of naturally ( = in vivo) retracted versus in vitro retracted cercariae. Retraction of the cercaria body into its tail chamber required only 2 min or less once initiated. The process began with the development of a chamber within the anterior end of the worm's tail. The chamber's lip advanced in a pulsating motion over the stationary distome. Retraction was completed with the constriction and fusion of the chamber lip once it passed over the anterior end of the distome, sealing the latter within the tail chamber. There was a significant difference in the proportions of cercariae with bodies retracted into tails, bodies not retracted, and bodies separated from tails in artificial pond water (APW) versus artificial snail water (ASW). A greater number of cercariae withdrew into their tail chambers in ASW (59/124; 47.6%) than in APW (21/124; 16.9%). In APW, more bodies separated from their tails (24/124; 19.4%) than in ASW (3/124; 2.4%). In both solutions (APW: 63.7% = 79/124; ASW: 50% = 62/124), a majority of cercariae never retracted. In APW, 76.2% of distomes retracting into their tails did so within the first 5 min compared to only 30.5% in ASW. There was no significant difference in the proportions of cercariae with bodies retracted into tails, bodies not retracted, and bodies separated from tails based on isosmotic replacement of individual ions, i.e., Na(+), K(+), Ca(++), or Mg(++), in ASW with Li(+). There was also no significant difference in the vertical swimming burst distance in cercariae whose bodies were initially retracted into their tails in vitro versus in vivo.

MULTI-SITE OBSERVATIONS OF PULSATION IN THE ACCRETING WHITE DWARF SDSS J161033.64–010223.3 (V386 Ser)

Nonradial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSS J161033.64-010223.3 (V386 Ser) using seven observatories located around the world in May 2007 over a duration of 11 days. We report the best fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible. Using time-series spectra acquired in May 2009, we determine the orbital period of SDSS J161033.64-010223.3 to be 83.8 +/- 2.9 min. Three of the observed photometric frequencies from our May 2007 campaign appear to be linear combinations of the 609s pulsation mode with the first harmonic of the orbital period at 41.5min. This is the first discovery of a linear combination between nonradial pulsation and orbital motion for a variable white dwarf.

Acute Aortic Syndromes: New Insights from Electrocardiographically Gated Computed Tomography

The development of retrospective electrocardiographic (ECG)-gating has proved to be a diagnostic and therapeutic boon for computed tomography (CT) imaging of patients with acute thoracic aortic diseases, such as aortic dissection/intramural hematoma (AD/IMH), penetrating atherosclerotic ulcer (APU), and ruptured/leaking aneurysm. The notorious pulsation motion artifacts in the ascending aorta confounding regular CT scanning can be eliminated, and involvement of the sinuses of Valsalva, the valve cusps, the aortic annulus, and the coronary arteries in aortic dissection can be clearly depicted or excluded. Motion-free images also allow reliable identification of the site of the primary intimal tear, the location, and extent of the intimomedial flap, and branch artery involvement. ECG-gated CTA also allows the detection of more subtle lesions and variants of aortic dissection, which may ultimately expand our understanding of these complex, life-threatening disorders.

Hypersonic shock wave evidence in RR Lyrae stars

Context. We carried out a new high resolution spectroscopic analysis of S Arae, one of the largest amplitude RR Lyrae star, to constrain the dynamical structure of the outermost layers of RR Lyrae stars. Aims. New high precision spectroscopic observations of S Arae were obtained using UVES at VLT–UT2 and FEROS at ESO–1.52 telescopes, during the rise of the S Arae light curve, over an entire pulsation period and covering a large wavelength range, from 376 to 1000 nm. Methods. We used the first line moment method to measure the radial velocity and to establish the radial velocity curves. The atmosphere stratification was done by the line identification method derived from the Kurucz atmosphere models. Results. For the first time, we show the existence of hypersonic shock waves in the photosphere of RR Lyrae stars. Our spectroscopic results reveal a new mechanism: the neutral line disappearance phenomenon. All neutral metallic absorption lines, over the whole spectrum, undergo a total line disappearance during the hump, while the ionized lines show line doubling connected by the two-step Schwarzschild’s mechanism. Thus, a hypersonic shock wave propagating through the photosphere around the maximum light phase results in the line disappearance. The shock energy is high enough to ionize the atoms, forming the layer crossed by the shock wave. The shock at first recedes, then becomes stationary and finally advances with a very high Mach number crossing the high atmosphere of the star. We stress the need for additional theoretical efforts to come to a better understanding of hypersonic shock wave and dynamic pulsational motion relationship.

Unsteady flow organization of compressible planar base flows

The unsteady flow features of a series of two-dimensional, planar base flows are examined, within a range of low-supersonic Mach numbers in order to gain a better understanding of the effects of compressibility on the organized global dynamics. Particle image velocimetry is used as the primary diagnostic tool in order to characterize the instantaneous near wake behavior, in combination with data processing using proper orthogonal decomposition. The results show that the mean flowfields are simplified representations of the instantaneous flow organizations. Generally, each test case can be characterized by a predominant global mode, which undergoes an evolution with compressibility, within the Mach number range considered. (The term “global mode” is defined herein as an organized global dynamical behavior of the near wake region, recognizing that the near wake dynamics may be describable in terms of several global modes.) At Mach 1.46, the predominant global mode can be characterized by a sinuous or flapping motion. With increasing compressibility, this flapping mode decreases, and the predominant global mode evolves into a pulsating motion aligned with the wake axis at Mach 2.27. These global modes play an important role in the distributed nature of the turbulence properties. The turbulent mixing processes become increasingly confined to a narrower redeveloping wake with increasing compressibility. Global maximum levels of the streamwise turbulence intensity and the kinematic Reynolds shear stress occur within the vicinity of the mean reattachment location, and show no systematic trend with compressibility. In contrast, the global maximum level of the vertical turbulence intensity moves upstream from the redeveloping wake toward the mean reattachment location. The vertical turbulence intensity decays thereafter more slowly than the other turbulence quantities. Overall, the local maximum levels of the turbulence properties decrease appreciably with increasing compressibility.

Cerebrospinal Fluid Flow in the Normal and Hydrocephalic Human Brain

Advances in magnetic resonance (MR) imaging techniques enable the accurate measurements of cerebrospinal fluid (CSF) flow in the human brain. In addition, image reconstruction tools facilitate the collection of patient-specific brain geometry data such as the exact dimensions of the ventricular and subarachnoidal spaces (SAS) as well as the computer-aided reconstruction of the CSF-filled spaces. The solution of the conservation of CSF mass and momentum balances over a finite computational mesh obtained from the MR images predict the patients' CSF flow and pressure field. Advanced image reconstruction tools used in conjunction with first principles of fluid mechanics allow an accurate verification of the CSF flow patters for individual patients. This paper presents a detailed analysis of pulsatile CSF flow and pressure dynamics in a normal and hydrocephalic patient. Experimental CSF flow measurements and computational results of flow and pressure fields in the ventricular system, the SAS and brain parenchyma are presented. The pulsating CSF motion is explored in normal and pathological conditions of communicating hydrocephalus. This paper predicts small transmantle pressure differences between lateral ventricles and SASs (approximately 10 Pa). The transmantle pressure between ventricles and SAS remains small even in the hydrocephalic patient (approximately 30 Pa), but the ICP pulsatility increases by a factor of four. The computational fluid dynamics (CFD) results of the predicted CSF flow velocities are in good agreement with Cine MRI measurements. Differences between the predicted and observed CSF flow velocities in the prepontine area point towards complex brain-CSF interactions. The paper presents the complete computational model to predict the pulsatile CSF flow in the cranial cavity.

The Effects of Time Varying Curvature on Species Transport in Coronary Arteries

Alterations in mass transport patterns of low-density lipoproteins (LDL) and oxygen are known to cause atherosclerosis in larger arteries. We hypothesise that the species transport processes in coronary arteries may be affected by their physiological motion, a factor which has not been considered widely in mass transfer studies. Hence, we numerically simulated the mass transport of LDL and oxygen in an idealized moving coronary artery model under both steady and pulsatile flow conditions. A physiological inlet velocity and a sinusoidal curvature waveform were specified as velocity and wall motion boundary conditions. The results predicted elevation of LDL flux, impaired oxygen flux and low wall shear stress (WSS) along the inner wall of curvature, a predilection site for atherosclerosis. The wall motion induced changes in the velocity and WSS patterns were only secondary to the pulsatile flow effects. The temporal variations in flow and WSS due to the flow pulsation and wall motion did not affect temporal changes in the species wall flux. However, the wall motion did alter the time-averaged oxygen and LDL flux in the order of 26% and 12% respectively. Taken together, these results suggest that the wall motion may play an important role in coronary arterial transport processes and emphasise the need for further investigation.

Multiple shock waves in the atmosphere of the Cepheid X Sagittarii?

Context. Shock waves in Cepheids have often been invoked, both from observational and theoretical points of view. However, classical shock wave signatures, such as emission or line doubling, have hardly been detected. Aims. In this paper, we suggest that our spectra of the classical Cepheid X Sgr can be interpreted by means of the passage of 2 shock waves per pulsation period. Methods. We study new, high-resolution (120 000) spectra of X Sgr that show very complicated patterns within metallic lines. Results. Spectra show up to 3 components in most of the lines of the spectra during most of the pulsation cycle. These components seem to follow a pulsation motion. In the blue wing, the appearance of a new component is observed twice per pulsation period, which can be interpreted by 2 consecutive shock waves, one being apparently related to the classical κ -mechanism at work in these stars. The origin of the second shock is still unclear. Conclusions. X Sgr is an exceptional Cepheid according to its observed multi-components behaviour. Additional observations are requested in order to establish the eventual effect of the binary orbit on the pulsation motion.

Averaged dynamics of two-phase media in a vibration field

The averaged dynamics of various two-phase systems in a high-frequency vibration field is studied theoretically. The continuum approach is applied to describe such systems as solid particle suspensions, emulsions, bubbly fluids, when the volume concentration of the disperse phase is small and gravity is insignificant. The dynamics of the disperse system is considered by means of the method of averaging, when the fast pulsation and slow averaged motion can be treated separately. Two averaged models for both nondeformable and deformable particles, when the compressibility of the disperse phase becomes important, are obtained. A criterion when the compressibility of bubbles cannot be neglected is figured out. For both cases the developed models are applied to study the averaged dynamics of the disperse media in an infinite plane layer under the action of transversal vibration.

Malignant Right Coronary Artery Anomaly Simulated by Motion Artifacts on MDCT

The aim of our study was to determine the prevalence of anomalous right coronary artery imitation due to motion artifacts in MDCT. Routine chest MDCT for reasons other than cardiac or vascular imaging is usually performed using breath-hold technique but without retrospective ECG gating and consequently yields pulsating motion artifacts. A possible artifact in front of the aortic root imitates an anomalous right coronary artery originating from the left posterior sinus. This course of the right coronary artery is considered a malignant variant and raises the question of far-reaching consequences such as a bypass operation. We performed a prospective study involving 355 patients undergoing routine chest CT examinations. To determine the prevalence of anomalous right coronary artery imitation caused by this motion artifact, all images were evaluated prospectively by an experienced radiologist. Twenty-one patients (5.9%) were suspected of having a malignant variant of the right coronary artery. However, in all patients prior chest CT or additional coronary MR angiography showed a normal origin of the right coronary artery. Routine chest MDCT without retrospective ECG gating may produce artifacts around the aorta simulating a malignant variant of the right coronary artery. Considering the low incidence of this malignant interarterial variant, the need for routine chest CT examinations combined with ECG gating and further workup can be disputed from an economic point of view. This artifact should be known to avoid unnecessary further examinations.

Viscous forces on acoustically levitated gas bubbles

When a gas bubble is subjected to an oscillating acoustic field in a gravitational environment, it seeks a location where time-averaged net force vanishes (‘acoustic levitation’). At this location, the bubble undergoes periodic, coupled volumetric pulsation and translational motion. In this study, we extend the existing inviscid theory of acoustic levitation to include viscous drag forces arising from translational motion. Equilibrium locations are computed and compared with predictions from inviscid theory. It is found that the effects are quite small, producing negligible effect on the levitation position.

Radial velocity variations in pulsating Ap stars

We have used precise stellar radial velocities to study the pulsational motion of the rapidly oscillating Ap star 10 Aql. Observations were made on three separate nights using the high resolution spectrograph of the Harlan J. Smith 2.7 m telescope at McDonald Observatory. A high radial velocity precision was achieved by using an iodine gas absorption cell. The integrated radial velocity measurements using the full wavelength region covered by iodine absorption lines (5000–6300 Å) failed to detect any coherent pulsational velocity variations to a level of 2.5–5 m s-1. An analysis over a much narrower wavelength range revealed that pulsational radial velocity variations are indeed present in 10 Aql, but only in 5 spectral lines. The amplitude of these variations ranged from about 100 m s-1 to as high as 398 m s-1 for an unidentified feature at λ5471.40 Å. Other spectral features showing pulsational radial velocity variations are tentatively identified as Sm II and Tm II.

Validation of averaged equations for thermal vibrational convection in near-critical fluids

Within an averaging approach, the governing equations and effective boundary conditions describing both the average and pulsation motion of a near-critical fluid subjected to high-frequency vibrations are obtained. Vibrations induce the non-homogeneities in average temperature. Owing to these non-homogeneities, the average flows can be generated even in isothermal cavity under weightlessness. These flows are examined for 1D and 2D configurations. The direct numerical simulations fulfilled earlier confirm the averaged model, we obtain the same flow structures by essentially smaller requirements for computational time. To cite this article: A.Vorobev et al., C. R. Mecanique 332 (2004).

Silo music and silo quake: granular flow-induced vibration

Acceleration and sound measurements during granular discharge from silos are used to show that silo music is a sound resonance produced by silo quake. In tall and narrow silos, the latter is produced by stick–slip friction between the wall and the granular material. For the discharge rates studied, the occurrence of flow pulsations is determined primarily by the surface properties of the granular material and the silo wall. The measurements show that the pulsating motion of the granular material drives the oscillatory motion of the silo.

Development of “Low-tech” Solar Thermal Water Pumps for Use in Developing Countries

Solar water pumps, based on electro-mechanical pumps powered by PV arrays, are commonly used and commercially available. However, one of the difficulties for their wider application in developing countries, where there is a high average insolation, is their relatively excessive cost. This arises mainly due to the high cost of the PV elements. Hence, this paper describes some developmental work and results of experimental tests on “low-tech” solar thermal water pumps which were built on the basis of Stirling engines with fluid pistons coupled to flat-plate solar collectors. Temperatures and pressures in the cycle are comparatively low, thus cheap design materials, such as glass and plastic, and a simple technology, available in the majority of mechanical workshops, can be used for their manufacture and consequently reduce their cost. Several design modifications of the above solar thermal water pumps have been developed and tested. The results obtained demonstrate that existing installations can be effectively applied for water pumping with a dynamic head which varies between 2-5 m. Furthermore, data from experimental tests shows that the pulsating motion of water in channels of the flat-plate solar collectors increases the collector’s efficiency by approximately 8-10%, which is a considerable advantage when a pump is used as part of a house solar heating system.