Query: Phase velocity mapping simpson

title: Velocity mapping of fast flows using a linearly ramped gradient waveform.
abstract: We report a new pure phase encoding measurement for velocity mapping. Velocity sensitization is achieved using a repeating, linearly ramped gradient waveform instead of rectangular bipolar pulsed field gradients. This approach reduces eddy current effects and results in the sample experiencing a gradient waveform that more closely matches the ideal input. Errors in k space mapping and calculated velocity values are reduced when contrasted with the previous measurement method. Velocity maps were acquired of high speed (c. 6 m/s) water flow through a pipe constriction. The application of linearly ramped gradient waveforms to non velocity encoded imaging measurements is discussed.

title: Magnetic resonance phase contrast velocity mapping for flow quantification in irregular heart rhythms using radial k space ultrashort echo time imaging.
abstract: Phase contrast velocity mapping sequences utilising ultrashort echo time (UTE) radial k space sequences have been used to reduce intravoxel dephasing at high velocities. We evaluated the accuracy of the UTE flow sequence for mitral regurgitation (MR) quantification, including patients with atrial fibrillation.  Forty patients underwent cardiac MRI for indirect MR quantification by assessment of aortic flow using a UTE phase contrast sequence (TE 0.65 ms) combined with left ventricular stroke volume. Retrospective ECG gating was used in sinus rhythm (30 patients) prospective ECG triggering in atrial fibrillation (10) MR was also quantified by a standard phase contrast sequence (TE 2.85 ms, standard flow method) and by comparing stroke volumes (volumetric method)  UTE flow derived MR measurement showed modest agreement in sinus rhythm (95% limits of agreement: 38.2 ml; 29.8% and atrial fibrillation 33.7 ml; 30.3% compared to standard flow assessment. There was little systematic bias in sinus rhythm (mean offset 4.4 ml 3.5% compared to standard flow assessment) but a slight bias towards greater regurgitation in atrial fibrillation +15.2 ml +14.0% There were wider limits of agreement between the UTE flow method and volumetric method than between the regular flow method and the volumetric method in sinus rhythm 48.4 ml; 36.4% mean offset: 12.2 ml 9.0% and similar limits of agreement in atrial fibrillation 29.6 ml; 25.8% +12.0 ml +10.3%  UTE flow imaging is inferior to conventional flow techniques for MR assessment in patients with sinus rhythm as well as atrial fibrillation. However, the number of atrial fibrillation patients in this initial study is small.

title: Phase analysis in maximal sprinting: an investigation of step to step technical changes between the initial acceleration, transition and maximal velocity phases
abstract: The aim of this study was to investigate spatiotemporal and kinematic changes between the initial acceleration, transition and maximum velocity phases of a sprint. Sagittal plane kinematics from five experienced sprinters performing 50 m maximal sprints were collected using six HD video cameras. Following manual digitising, spatiotemporal and kinematic variables at touchdown and toe off were calculated. The start and end of the transition phase were identified using the step to step changes in centre of mass height and segment angles. Mean step to step changes of spatiotemporal and kinematic variables during each phase were calculated. Firstly, the study showed that if sufficient trials are available, step to step changes in shank and trunk angles might provide an appropriate measure to detect sprint phases in applied settings. However, given that changes in centre of mass height represent a more holistic measure, this was used to sub divide the sprints into separate phases. Secondly, during the initial acceleration phase large step to step changes in touchdown kinematics were observed compared to the transition phase. At toe off, step to step kinematic changes were consistent across the initial acceleration and transition phases before plateauing during the maximal velocity phase. These results provide coaches and practitioners with valuable insights into key differences between phases in maximal sprinting.

title: Single shot velocity mapping by rewinding of velocity encoding with Echo Planar Imaging.
abstract: The ability of single shot NMR imaging methods to follow the time evolution of a velocity distribution within an object is strongly limited by the phase errors accumulated as velocity maps are acquired. In the particular case of Carr Purcell based sequences combined with Echo Planar Imaging acquisition, phase accumulates through subsequent images, hampering the possibility to acquire several velocity maps, which would be useful to determine transient behavior. In this work, we propose the use of a rewinding velocity encoding module applied after the acquisition of each image during the CPMG echo train. In this way, the first velocity module imparts a velocity dependent phase prior to the image acquisition and the second pair cancels this phase out before the next refocusing radiofrequency pulse is applied. The performance and limits of this method are studied by acquiring 100 images of a co rotating Couette cell over a period of 1.6 s as a function of the rotation speed. The method is applied to determine the kinematic viscosity of a water/alcohol mixture, which is a relevant topic in many physical, chemical and biological processes.

title: Joint Inversion of Rayleigh Wave Phase Velocity, Particle Motion, and Teleseismic Body Wave Data for Sedimentary Structures
abstract: Mapping seismic structures of sedimentary basins is of great importance to better evaluate energy resources and seismic hazards. In this study, we develop a joint Bayesian Monte Carlo nonlinear inversion method that utilizes the complementary nature of Rayleigh wave phase velocity, Rayleigh wave particle motion, and teleseismic Pwave in responding to sedimentary structures. Synthetic tests demonstrate that our joint inversion can retrieve the input sedimentary models better than inversions with surface or body wave data alone. We apply our method to waveform data of two broadband stations inside the Songliao Basin in northeast China to invert for sedimentary structures beneath the two stations. We verify our results by successfully isolating the Moho P to S conversion from sediment reverberations in surface receiver functions with a wavefield downward continuation technique. We further demonstrate that we can extend our method to single station inversions by excluding the Rayleigh wave phase velocity in the inversion.

title: Autocalibrated multiband CAIPIRINHA with through time encoding: Proof of principle and application to cardiac tissue phase mapping
abstract: In conventional multiband (MB) CAIPIRINHA, additional reference scans are acquired to allow the separation of the excited slices. In this study, an acquisition reconstruction technique that makes use of the MB data to calculate these reference data is presented. The method was integrated into a 2D time resolved phase contrast MR sequence used to assess velocities of the myocardium.

title: Measuring the phase velocity of twisted wavefronts.
abstract: Traditionally one defines the speed of a wave as a property of the medium. Recent studies in photonics have challenged this idea, indicating that spatial shaping of the optical wavefront has can alter the arrival time of single photons when compared to an entangled reference photon. However, relying only on time of flight measurements leads to ambiguity in the measurement. Unlike photonics, one can directly measure the phase and intensity of sound, permitting unambiguous measurements. We developed a bespoke 28 element ultrasonic phased array transducer to generate short pulses that carrying orbital angular momentum. Through spatial mapping, we observed pulse modulation that indicates a localised change in phase velocity by over 10 m/s. We present a geometrical argument for this effect, proposing that the phase velocity varies to compensate for the effective path length increase arising from spatial wavefront shaping. We expect that this pulse dispersion is a general effect for any spatially shaped wavefront and that it could be utilised to manipulate readings from pulse based sensor technologies such as SONAR, medical ultrasound, and underwater communication systems.

title: Activation Mapping With Integration of Vector and Velocity Information Improves the Ability to Identify the Mechanism and Location of Complex Scar Related Atrial Tachycardias
abstract: Activation mapping of scar related atrial tachycardias (ATs) can be difficult to interpret because of inaccurate time annotation of complex electrograms and passive diastolic activity. We examined whether integration of a vector map can help to describe patterns of propagation to better explain the mechanism and location of ATs.  The investigational mapping algorithm calculates vectors and applies physiological constraints of electrical excitation in human atrial tissue to determine the arrhythmia source and circuit. Phase I consisted of retrospective evaluation in 35 patients with ATs. Phase II consisted of prospective validation in 20 patients with ATs. Macroreentry was defined as a continuous propagation in a circular path >30 mm; localized reentry was defined as a circular path 30 mm; a focal source had a centrifugal spread from a point source.  In phase I, standard activation mapping identified 28 of 40 ATs (70% 25 macroreentry and 3 focal tachycardias. In the remaining 12 ATs, the mechanism and location could not be identified by activation and required entrainment or empirical ablation for termination (radiofrequency time, 17.3+ 6.6 minutes) In comparison, the investigational algorithm identified 37 of 40 (92.5% ATs, including 5 macroreentry, 3 localized reentry, and 1 focal AT not identified by standard mapping. It also predicted the successful termination site of all 37 of 40 ATs. In phase II, the investigational algorithm identified 12 macroreentry, 6 localized reentry, and 2 focal tachycardias that all terminated with limited ablation (3.2+ 1.7 minutes) It identified 3 macroreentry, 3 localized reentry, and 1 focal AT not well characterized by standard mapping. The diagnosis of localized reentry was supported by highly curved vectors, resetting with increasing curve and termination with limited ablation (22+ 6 s)  Activation mapping integrating vectors can help determine the arrhythmia mechanism and identify its critical components. It has particular value for identifying complex macroreentrant circuits and for differentiating a focal source from a localized reentry.

title: Velocity mapping of steady water flow through methane hydrate bearing samples
abstract: The productivity of methane from natural gas hydrate reservoirs is predominantly controlled by the fluid migration mechanisms in hydrate bearing sediments. Thus, the characterization of the effects of hydrate distribution on fluid migration is significant in the exploitation of natural gas hydrates. In this work, a phase contrast method was used to visualize water flow velocity fields at three cross sections under different injecting flow rates by using magnetic resonance imaging (MRI) Four experimental cases were performed to directly measure the cross sectional velocity fields in a sand packed hydrate bearing sample with hydrate saturations varying from 22.5% to 32.7% The effect of hydrate distribution on the water velocity fields was discussed. It was found that hydrate formed nonuniformly in the sand packed sample in all four cases, and the hydrate distribution predominantly affected the water effective permeability and cross sectional velocity fields when steady water flow was injected. When hydrate saturation was higher than 22.5% a two region flow pattern was observed with a clear boundary between the fast flowing region and the slow flowing region. The cross sectional flow rates calculated according to the velocity field images fitted well with the injecting flow rates.

title: Visualization of CSF Flow with Time resolved 3D MR Velocity Mapping in Aqueductal Stenosis Before and After Endoscopic Third Ventriculostomy
abstract: PurposeThe aim of this study was to evaluate timed resolved three dimensional (3D) magnetic resonance (MR) velocity mapping as a method for investigation of cerebrospinal fluid (CSF) flow changes in patients with aqueductal stenosis (AS) treated by endoscopic third ventriculostomy (ETV).MethodsThe MR velocity mapping was performed in 12 AS patients before and after ETV and in 10 healthy volunteers by using a 3 Tesla MR system. Time resolved 3D MR velocity mapping data were acquired with a standard 3D phase contrast (PC) sequence with cardiac triggering. Values of mean (vmean) and maximum (vpeak) velocity were measured at several ventricular structures using dedicated software.ResultsOf the patients 11 showed a satisfactory clinical improvement after ETV, whereas one patient needed subsequent shunt implantation. All AS patients showed significant hypomotile CSF flow dynamics in the third ventricle when compared to healthy subjects before surgery (p 0.05) In contrast, CSF flow velocity was increased within the Foramen of Monro in AS patients. After ETV, all AS patients showed a decrease of CSF flow dynamics within the third ventricle. Mean and peak CSF flow velocities through the ventriculostomy were 1.72 0.59 cm/s (vmean) and 3.53 0.79 cm/s (vpeak) respectively after ETV. The patient who needed shunt implantation after ETV had the lowest flow velocities through the ventriculostomy.ConclusionThis study demonstrates that timed resolved 3D MR velocity mapping is a useful imaging investigation for diagnostics and follow up in patients with AS. This new technique provides an insight into the physiological CSF flow changes related with AS and its treatment.