RESUMEN

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.

RESUMEN

Eddy currents caused by pulsed field gradients in magnetic resonance measurements of high-speed flow cause the magnetic field gradient amplitude waveform experienced by the sample to be different from the waveform demanded of the magnetic field gradient amplifiers. By measuring and using the system impulse response, pre-equalization magnetic field gradient waveform correction can be used to counteract the resulting errors in measured signal phase at the cost of minimal additional experimental time. The effectiveness of the pre-equalization method of magnetic field gradient waveform correction is tested with a motion-sensitized (pulsed field gradient) version of the SPRITE imaging pulse sequence which requires extreme gradient slew rates in excess of 1000 T/m/s in a 6.7-cm-bore set of gradient windings. Pre-equalized, motion-sensitized SPRITE is used to create velocity maps of high-speed (c. 4 m/s) water flow through a pipe constriction.

RESUMEN

The pressure variations experienced by a liquid flowing through a pipe constriction can, in some cases, result in the formation of a bubble cloud (i.e., hydrodynamic cavitation). Due to the nature of the bubble cloud, it is ideally measured through the use of non-optical and non-invasive techniques; therefore, it is well-suited for study by magnetic resonance imaging. This paper demonstrates the use of Conical SPRITE (a 3D, centric-scan, pure phase-encoding pulse sequence) to acquire time-averaged void fraction and velocity information about hydrodynamic cavitation for water flowing through a pipe constriction.

Asunto(s)

RESUMEN

MRI image resolution is proportional to the maximum k-space value, i.e. the temporal integral of the magnetic field gradient. High resolution imaging usually requires high gradient amplitudes and/or long spatial encoding times. Special gradient hardware is often required for high amplitudes and fast switching. We propose a high resolution imaging sequence that employs low amplitude gradients. This method was inspired by the previously proposed PEPI (π Echo Planar Imaging) sequence, which replaced EPI gradient reversals with multiple RF refocusing pulses. It has been shown that when the refocusing RF pulse is of high quality, i.e. sufficiently close to 180°, the magnetization phase introduced by the spatial encoding magnetic field gradient can be preserved and transferred to the following echo signal without phase rewinding. This phase encoding scheme requires blipped gradients that are identical for each echo, with low and constant amplitude, providing opportunities for high resolution imaging. We now extend the sequence to 3D pure phase encoding with low amplitude gradients. The method is compared with the Hybrid-SESPI (Spin Echo Single Point Imaging) technique to demonstrate the advantages in terms of low gradient duty cycle, compensation of concomitant magnetic field effects and minimal echo spacing, which lead to superior image quality and high resolution. The 3D imaging method was then applied with a parallel plate resonator RF probe, achieving a nominal spatial resolution of 17 µm in one dimension in the 3D image, requiring a maximum gradient amplitude of only 5.8 Gauss/cm.

RESUMEN

Monitoring the pore system in sedimentary rocks with MRI when fluids are introduced is very important in the study of petroleum reservoirs and enhanced oil recovery. However, the lengthy acquisition time of each image, with pure phase encode MRI, limits the temporal resolution. Spatiotemporal correlations can be exploited to undersample the k-t space data. The stacked frames/profiles can be well approximated by an image matrix with rank deficiency, which can be recovered by nonlinear nuclear norm minimization. Sparsity of the x-t image can also be exploited for nonlinear reconstruction. In this work the results of a low rank matrix completion technique were compared with k-t sparse compressed sensing. These methods are demonstrated with one dimensional SPRITE imaging of a Bentheimer rock core plug and SESPI imaging of a Berea rock core plug, but can be easily extended to higher dimensionality and/or other pure phase encode measurements. These ideas will enable higher dimensionality pure phase encode MRI studies of dynamic flooding processes in low magnetic field systems.