A rapid magnetization preparation for MRI measurements of sprays.

The process of spray atomization, i.e., the transformation of a continuous liquid jet into µm-sub-mm sized droplets, is ubiquitous in industry yet quite complex to analyze theoretically and study experimentally. One of the main strengths of MRI is its ability to sensitize spatially-resolved NMR signal to a wide variety of physical and chemical parameters. However, standard preparation schemes are difficult to employ for studies of sprays due to sprays' fast speeds (>10-20m/s). In addition, sprays are usually low-density systems, leading to a poor SNR and a need for massive signal averaging and long acquisition time. In this paper, we reduced the interval between the preparation and the readout stages by performing SPI encoding on the rising gradients. This also enabled the use of 90-degree flip angles to maximize the spray signal and saturate the stationary water signal while avoiding unwanted slice-selection. The use of gradients during preparation stage was eliminated due to their time-consuming rise and stabilization times limiting possible preparation schemes to a combination of RF pulses and delays. The two preparation schemes presented here are Time-of-Flight (TOF) and T1ρ-weighting schemes. The total duration of the sequence (without TR) was 240-1100µs for the TOF and 410µs for T1ρ. The T1ρ prepared images of the near-atomization region (11 spin-locking frequencies, 0-15kHz) showed a strong signal attenuation at higher frequencies. In series of TOF images the clearly noticeable displacement of the liquid parcel can be utilized to measure spray speeds.

MR relaxometry of micro-bubbles in the vertical bubbly flow at a low magnetic field (0.2T).

Measurements of the vertical bubbly flow were performed at a low magnetic field of 0.2T. The void fraction data were acquired. The susceptibility-induced changes in T2 relaxation time were analyzed using the previously introduced approaches by Sukstanskii et al. and Ziener et al., originally developed for the Magnetic Resonance analysis of randomly distributed and isolated spherical inclusions, and a simple model of a spherical particle, respectively. The CPMG signal decay due to the presence of spherical inclusions was approximated as linear vs. CPMG inter-echo times to extract the average inclusion's size information. Two equations were derived for a simplified analysis of gas-liquid systems with basic T2 measurements, and without prior knowledge on the gas-liquid susceptibility or a need for the magnetic gradient setup. They can provide estimates for the void fraction and the average inclusion size, provided the CPMG inter-echo time requirements are met. For the control samples, there was a good agreement with the theory. For the bubbly flows, a good agreement was observed between the Magnetic Resonance and optics-based estimates for the slowest airflow rate. The deviation, however, increased for higher airflow rates. The introduced approach lends itself to the characterization of multi-phase systems such as cavitating media and well-separated bubbly flows.

Magnetic susceptibility based magnetic resonance estimation of micro-bubble size for the vertically upward bubbly flow.

The approach originally developed for the Nuclear Magnetic Resonance analysis of stable micro-bubbles is applied to studies of vertical bubbly flows. A very fast dispersion (diffusion) of water in bubbly flows extends the fast diffusion limit down to short (2-10 ms) measurement times, permitting the use of the simplified analytical expression to extract the micro-bubble size information both in bulk and spatially resolved. The observed strong bubble-induced reduction in T(2)(*) necessitates the use of very short encoding times and pure phase encoding methods to accurately measure the void fraction. There was an expected underestimation of bubble sizes at faster flow rates due to the limitations of the theory derived for small bubble sizes and non-interacting spherical bubbles (low void fractions and slow flow rates). This approach lends itself to studies of bubbly flows and cavitating media characterized by small bubble sizes and low void fractions.

Quantitative density profiling with pure phase encoding and a dedicated 1D gradient.

A new centric scan imaging methodology for density profiling of materials with short transverse relaxation times is presented. This method is shown to be more robust than our previously reported centric scan pure phase encode methodologies. The method is particularly well suited to density imaging of low gyro-magnetic ratio non-proton nuclei through the use of a novel dedicated one-dimensional magnetic field gradient coil. The design and construction of this multi-layer, water cooled, gradient coil is presented. Although of large diameter (7.62 cm) to maximize sample cross section, the gradient coil has an efficiency of several times that offered by conventional designs (6 mT/m/A). The application of these ideas is illustrated with high resolution density-weighted proton (1H) images of hazelnut oil penetration into chocolate, and lithium ion (7Li) penetration into cement paste. The methods described in this paper provide a straightforward and reliable means for imaging a class of samples that, until now, have been very difficult to image.

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping.

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.

Water content profiles with a 1D centric SPRITE acquisition.

The purpose of this work is to develop a rapid MRI method amenable to profiling with minimal or no T(1) relaxation weighting. The behavior of a signal during a centric SPRITE acquisition is analyzed. It is shown that the technique can be made immune to a broad range of T(1) changes. In a properly executed measurement, only T(2)* and proton density parameters define the image intensity. A T(2)* mapping technique can be easily applied, separating T(2)* and proton density contributions to the image. A drying soil sample with low initial water content is experimentally studied as a demonstration of the technique. A characteristic baseline artifact is easily removed from the profiles by a simple operation.

Imaging of heterogeneous materials with a turbo spin echo single-point imaging technique.

A magnetic resonance imaging method is presented for imaging of heterogeneous broad linewidth materials. This method allows for distortionless relaxation weighted imaging by obtaining multiple phase encoded k-space data points with each RF excitation pulse train. The use of this method, turbo spin echo single-point imaging-(turboSPI), leads to decreased imaging times compared to traditional constant-time imaging techniques, as well as the ability to introduce spin-spin relaxation contrast through the use of longer effective echo times. Imaging times in turboSPI are further decreased through the use of low flip angle steady-state excitation. Two-dimensional images of paramagnetic doped agarose phantoms were obtained, demonstrating the contrast and resolution characteristics of the sequence, and a method for both amplitude and phase deconvolution was demonstrated for use in high-resolution turboSPI imaging. Three-dimensional images of a partially water-saturated porous volcanic aggregate (T(2L) approximately 200 ms, Deltanu(1/2) approximately 2500 Hz) contained in a hardened white Portland cement matrix (T(2L) approximately 0.5 ms, Deltanu(1/2) approximately 2500 Hz) and a water-saturated quartz sand (T(2) approximately 300 ms, T(2)(*) approximately 800 microseconds) are shown.

Increased MR image contrast by correlation of tissue relaxation properties.

An image postprocessing technique that is based on the relaxation properties of tissues and that can produce MR images with increased contrast is proposed. The technique involves no a priori assumptions concerning the form of the relaxation decay. An arbitrary number of postprocessed images, each emphasizing a selected tissue type, is obtained from the original images of a multiecho acquisition. It is shown with examples that the technique allows more complete utilization of relaxation information for tissue differentiation.