PurposeTo develop a method that achieves simultaneous brain and neck time-of-flight (ToF) magnetic resonance angiography (MRA) within feasible scan timeframes.MethodsLocalized quadratic (LQ) encoding is efficient for both signal-to-noise ratio (SNR) and in-flow enhancement. We proposed a spiral multiband LQ method to enable simultaneous intracranial and carotid ToF-MRA within a single scan. To address the venous signal contamination that becomes a challenge with multiband (MB) ToF, tilt-optimized non-saturated excitation (TONE) and partial-Fourier slice selection (PFSS) were further introduced in the LQ framework to mitigate the venous signal and improve artery contrast. A sequential spiral MB and LQ reconstruction pipeline was employed to obtain the brain-and-neck image volumes.ResultsThe proposed MB method was able to achieve simultaneous brain and neck ToF-MRA within a 2:50-min scan. The complementarily boosted SNR-efficiency by MB and LQ acquisitions allows for the increased spatial coverage without increase in scan time or noticeable compromise in SNR. The incorporation of both TONE and PFSS effectively alleviated the venous contamination with improved small vessel sensitivity. Selection of scan parameters such as the LQ factor and flip angle reflected the trade-off among SNR, blood contrast, and venous suppression.ConclusionsA novel MB spiral LQ approach was proposed to enable fast intracranial and carotid ToF-MRA with minimized venous corruption. The method has shown promise in MRA applications where large spatial coverage is necessary.

Purpose To generate efficient gradient waveforms for spiral MRI which mitigate the high-frequency attenuation inherent in gradient systems. Theory and Methods Spiral MRI has many clinical advantages, including high temporal and SNR efficiency. One of the challenges for robust spiral MRI is a high sensitivity to imperfections in the gradient system, which requires some form of correction in order to map data correctly in k-space. A previous numerical algorithm for generating spiral gradient waveforms was modified to reduce its high-frequency content with minimal increase in waveform duration. Results Examples are shown of compact frequency gradient waveforms. Software implementing the algorithm is made available. Conclusion An algorithm to produce gradient waveforms with a compact frequency spectrum is described. This algorithm results in greatly reduced overall error and better compatibility with gradient systems than the original algorithm from which it was derived.

While magnetic resonance velocimetry (MRV) has been used in technological flow studies for over 30 years, it has not yet reached the levels of usage as more traditional experimental techniques such as particle image velocimetry or laser Doppler anemometry. This work involves a relatively simple U-bend geometry with complex three-dimensional turbulent flow characteristics which was shared with four research groups, including a combined effort from the U.S. Military Academy/Stanford University, and teams from Hanyang University, the University of Rostock, and the Mayo Clinic. The geometry—including upstream flow development—was shipped between groups with nominally similar experimental conditions, and the acquired data are presented including both two- and three-dimensional comparisons. In addition, details on how each team conducted the MRV experiments are provided, with each team using a different set of procedures and hardware. The results are remarkably similar, with only a few variations at the flow regions with the highest in-plane velocity gradients showing differences outside the combined uncertainty of the results.

Particle ingestion into turbine engines is a widespread problem that can cause significant degradation in engine service life. One primary damage mechanism is deposition of particulate matter in internal cooling passages. Musgrove et al. proposed a compact particle separator that could be installed between the combustor bypass exit and turbine vane cooling passage inlet. The design had small pressure losses but provided limited particle separation, and its performance has proved difficult to replicate in subsequent experiments. Borup et al. recently developed a Magnetic Resonance Imaging (MRI) based technique for making full-field, 3D measurements of the mean particle concentration distribution in complex flows. A particle separator based on the Musgrove et al. design was fabricated out of plastic using 3D printing. The primary difference from earlier designs was the addition of a. drain from the collector, through which 3% of the total flow was extracted. The separator efficiency was measured at two Reynolds numbers, using water as the working fluid and 33 -micron titanium microspheres to represent dust particles. Particle Stokes number was shown to play the dominant role in determining efficiency across studies. MRI was used to obtain the 3D particle volume fraction and 3 -component velocity fields. The velocity data showed that flow was poorly distributed between the separator louvers, while the collector flow followed the optimal pattern for particle retention. The particle distribution data revealed that strong swirling flow in the collector centrifuged particles towards the outer wall of the collector and into a partitioned region of quiescent flow, where they proceeded to exit the collector via the drain. Future designs could be improved by re-arranging the louvers to produce a more uniform flow distribution, while maintaining the effective collector design.

Microparticle transport in the human airway is an important problem with major implications for global healthcare. However, nasal cavity flows are not well understood due to morphological variations and the difficulty of acquiring data in realistic geometries. Magnetic resonance imaging (MRI) was used to obtain the 3D, three-component mean velocity field for flow through a scale replica of the Carleton-Civic standardized nasal geometry (Liu et al. in J Appl Physiol 106(3):784–795, 2009). Velocity measurements at two flow rates (representative of steady inhalation at 15 and 30 L/min) reveal the presence of a strong swirling flow structure in the nostril, stagnant or weakly reversed flow in the lower meatus, and a separation region at the top of the nasopharynx. The measurements suggest a transitional or weakly turbulent flow regime. A recently developed MRI-based technique was used to measure the 3D mean particle concentration distribution for two particle-laden streaks introduced near the nostril opening. The particles were representative of 0.75 micron dust-like particles inhaled at 30 L/min. The streak injected near the nostril base remained confined to the lower portion of the nasal cavity, while the streak injected near the tip was dispersed across most of the passage cross-section past the nasal valve. Mixing occurred most rapidly in the nostril and in the nasopharynx. The results suggest that some level of drug targeting in the nasal cavity may be achievable by choosing the introduction site at the nostril opening.

Diffuser holes are used extensively for film cooling because they dramatically improve film-cooling effectiveness relative to bound holes, but there is concern that the drawbacks of a compound angle (CA) may reduce the beneficial effects. This work Magnetic Resonance Velocimetry (MRV) to obtain the 3D, component mean velocity field for a shaped hole with a pitch angle of 30 degrees, blowing ratio of unity, and compound angles of 10 and 20 degrees. The data are compared to a previous MRV measurement of an identical hole with zero skew angle. in the 0 and 20 degree cases, a separation bubble is observed in the downstream wall of the diffuser. Streamtubes emanating,from the diffuser exit show the asymmetry of the flow as the jet is accelerated to align with the mainstream flow. Streamtube analysis shows evidence of competing effects of CA on film cooling performance: a wider streamtube footprint may increase courage while a decrease in streamtube thickness may make courage more susceptible to turbulent mixing. Analysis of the jet trajectory, defined as the streamtube centroid, shows that the realignment of the jet fluid with the freestream direction occurs in pie region of the diffuser exit. Although significant qualitative changes in film cooling performance are not expected, some ingestion of mainstream flow may occur due to vortices present in the diffuser below the plane of the blade surface.

Transverse decay rate (R2∗) mapping is an established method for measuring iron overload in various biological tissues. Recently, R2∗ mapping was used to measure the mean 3D concentration distribution of micron-size particles dispersed in turbulent flows. However, some discrepancy was observed between the measured R2∗ and the expected decay based on existing theory. The present paper examines three flow-related mechanisms that could be responsible for this discrepancy. Computational simulations were used to study the effects of relative particle-fluid motion and preferential concentration by turbulence, while the effect of enhanced proton dispersion due to turbulence was examined via the existing MRI relaxation theory. Each flow phenomenon was shown to produce a different effect on the signal-time curve, as well as the extracted R2∗. Comparison to experimental data in a square channel flow showed that relative motion between the particles and fluid was the most likely cause of the discrepancy in the previous experiments; however, all three effects may be present in both medical and non-medical flows, and their differing effects on the MRI signal may eventually allow for their identification from MRI data.

Discrete hole film cooling is widely employed to protect turbine blades and vanes from hot combustion gases entering the high-pressure turbine stage. Accurate prediction of the heat transfer near film cooling holes is critical, and high-fidelity experimental data sets are needed for validation of new computational models. Relatively few studies have examined the effects of periodic main flow unsteadiness resulting from the interaction of turbine blades and vanes, with a particular lack of data for shaped hole configurations. Periodic unsteadiness was generated in the main flow over a laidback, fan-shaped cooling hole at a Strouhal number (St = fD/U) of 0.014 by an airfoil oscillating in pitch. Magnetic resonance imaging (MRI) with water as the working fluid was used to obtain full-field, phase-resolved velocity and scalar concentration data. Operating conditions consisted of a hole Reynolds number of 2900, channel Reynolds number of 25, 000, and blowing ratio of unity. Both mean and phase-resolved data are compared to the previous measurements for the same hole geometry with steady main flow. Under unsteady freestream conditions, the flow separation pattern inside the hole was observed to change from an asymmetric separation bubble to two symmetric bubbles. The periodic unsteadiness was characterized by alternating periods of slow main flow, which allowed the coolant to penetrate into the freestream along the centerplane, and fast, hole-impinging main flow, which deflected coolant toward the laidback wall and caused ejection of coolant from the hole away from the centerplane. Mean adiabatic surface effectiveness was reduced up to 23% inside the hole, while mean laterally averaged effectiveness outside the hole fell 28–36% over the entire measurement domain. A brief comparison to a round jet with and without unsteadiness is included; for the round jet, no disturbance was observed inside the hole, and some fluctuations directed coolant toward the wall, which increased mean film cooling effectiveness. The combined velocity and concentration data for both cases are suitable for quantitative validation of computational fluid dynamics predictions for film cooling flows with periodic freestream unsteadiness.

Magnetic resonance imaging (MRI) techniques were used to investigate a discrete, \(30^{\circ }\)-inclined round jet in crossflow subjected to periodic freestream unsteadiness. The freestream perturbations were generated by an oscillating airfoil upstream of the jet. The experiment operated at a Strouhal number of 0.014, channel Reynolds number of 25,000, hole Reynolds number of 2900, and jet blowing ratio of unity. 3D phase locked velocity measurements were obtained over the entire channel using magnetic resonance velocimetry (MRV). 3D time-averaged temperature measurements were acquired using magnetic resonance thermometry (MRT), along with phase-locked temperature measurements in the 2D centerplane of the channel and jet. The freestream flow just upstream of the jet was characterized by streamwise velocities ranging from \(0.88 U_\text {bulk}\) to \(1.23 U_\text {bulk}\) and wall-normal velocities from \(-0.11 U_\text {bulk}\) to \(0.02 U_\text {bulk}\). Flow inside the hole was observed to be insensitive to the freestream fluctuations, as velocities and temperatures in the hole remained largely unchanged throughout the cycle. Outside the hole, changes to the streamwise velocity produced an oscillating jet blowing ratio that led to the lengthening and shortening of the counter-rotating vortex pair (CVP) as well as a varying degree of coolant separation from the film cooled wall. During one portion of the cycle, downwashing freestream flow (i.e., flow with negative wall-normal velocities) promoted strong re-attachment and lateral spreading of the jet. Mean, spanwise-averaged film cooling effectiveness values were compared to those of an earlier experiment with a steady freestream and identical geometry, Reynolds number, and blowing ratio. Film cooling performance in the near-hole region was higher with steady freestream flow. However, at downstream locations, the downward transport of coolant by the periodic downwashing flow led to a higher mean surface effectiveness than in the steady case.

A novel method, denoted MRP (short for Magnetic Resonance Particle concentration), was developed to obtain 3D volume fraction measurements for a dispersed particulate phase in turbulent water flows using Magnetic Resonance Imaging (MRI). MRI images taken near a single stainless steel particle suspended in agarose gel showed good agreement with the analytical solution for the disturbance to a uniform magnetic field induced by an immersed sphere. For a random distribution of particles, a linear relationship between the MRI signal decay rate (R-2(*)) and particle volume fraction (phi(v)) has previously been predicted in the MRI literature. This relationship was investigated for various types of particles suspended in agarose gel vials. Good agreement with theory was observed for particles with a high magnetic susceptibility difference from water. R-2(*) was also measured in a square channel flow containing a uniform distribution of titanium particles at two fully turbulent Reynolds numbers. Experimental results again agreed well with theory in the majority of the channel for both Reynolds numbers studied. Data from this flow were used to examine the expected SNR and dynamic range for MRP in future experiments. Some discrepancy was observed near the entry region of the channel, with possible explanations including inflowing fluid and large-scale flow structure effects behind the channel's mixing pin array. Finally, the new method was used to measure the 3D concentration distribution for a streak of titanium particles injected into a turbulent square channel flow with angled ribs. The transport of the streak was analyzed quantitatively, and a minor asymmetry in the channel geometry was shown to have important implications for the mean transport of the particle streak.

Quantitative, fully three-dimensional particle concentration data were obtained for a streak of ∼30 micron titanium micro-spheres injected isokinetically into the center of flow at Re = 20,000 through a ribbed serpentine passage. The rib height to channel height ratio was 10% and the pitch was 60% channel height. The ribs were oriented at 45° to the flow and staggered on opposite walls. Each passage was 10 ribs long. Concentration measurements were made using a newly developed Magnetic Resonance Imaging (MRI) based technique with an uncertainty of ±0.018% particles by volume. Three-component mean velocity data were also obtained using an existing MRI technique. The concentration data are in good agreement with the expected large-scale particle transport based on the mean flow field. Quantitative analysis is performed in the high-concentration portion of the streak where turbulent dispersion plays a significant role. Particles also tend to concentrate just upstream of the angled ribs, forming rivulets that persist as the flow returns to the channel center. These results suggest the potential for improved design of dust-resistant cooling systems based on experimental data. It is also concluded that the data set comprises a suitable validation case for Computational Fluid Dynamics (CFD) simulations.