MRI Microflow Visualization
The goal of this project is to design and construct a portable and cost effective magnetic resonance imaging (MRI) instrument that is capable of resolving features at microscale to image flow fields of complex fluids in capillary tubes.
The behaviour of fluids at the microscale can differ from 'macrofluidic' behavior in that factors such as surface tension and viscosity start to dominate the system. The presence of cells, biological molecules or other substances typically transported through lab-on-a-chip devices results in very complex fluid flows with unique properties, especially when these objects reach the dimensions of their flow environment. In order to design effective lab-on-a-chip microfluidic systems, these fluids need to be visualized for careful characterization at these length scales. Current standard instrumentation, such as microPIV (micro Particle Image Velocimetry), do not achieve sufficiently high spatial resolution and are intrusive techniques that affect the flow through the need of seed particles. High resolution Magnetic Resonance Imaging (MRI) represents a powerful technology to characterize microflows in a non-intrusive manner in 3D. The development of a portable and low-cost high-resolution MRI velocimetry tool would be a logical next step in this area, and it would provide an important capability for the growing area of academic and industrial microfluidic research.
A prototype for a desktop high-resolution MRI velocimetry instrument to characterize flow fields in a capillary tube was demonstrated. This inexpensive compact system was achieved with 0.6 T permanent magnetic configuration (Larmor frequency of 25 MHz) and temperature compensation using off-the-shelf NdFeB permanent magnets. A triaxial gradient module with micro-fabricated copper coils using a lithographic fabrication process was developed. This gradient module was capable of generating fast switching gradients with sensitivities up to 1.7 T/m using custom made current amplifiers, and was optimized for microflow imaging. The RF probe was integrated with the gradient module and was driven by custom electronics. A 2D cross-sectional static image of the inside of a capillary tube with an inner diameter of 1.67 mm is acquired at an in-plane spatial resolution of better than 40 µm, as shown in images below. Time-of-flight flow measurements were also obtained using this MRI system to measure the velocity profile of water flowing at average velocities of above 40 mm/s. The flow profile for slower flow velocities was obtained using phase-encoded techniques, which provides quantitative velocity information in 2D, as shown in the bottom image.