MRI Current Imaging Project

MRI Current Imaging Project

2015 Winter Capstone Project

MAE 156B

Jacobs School of Engineering

University of California, San Diego

Sponsored by Lawrence Frank, Ph.D.

                                                                                                                    Figure 1: Final design for the phantom                                              Figure 2: From left to right: Mark Bennett, Adrian Ponce-Rojas,

                                                                                                                                                                                                                         Huy Nguyen, Junyoung Kwon, Dr. Lawrence Frank and Dr. Ben Regner 

Executive Summary

Project Background:

    According to Dr. Lawrence Frank and the Center for Scientific Computation in Imaging (CSCI), it has been hypothesized that the subtle currents generated by neural activity in the human brain may be detectable via the use of Magnetic Resonance Imaging (MRI). This would thereby allow direct imaging of neural activity, rather than the current method of functional MRI (fMRI) that detects blood flow and oxygenation changes, which has poorer spatial and temporal resolution and specificity for brain activity. However, to date, no conclusive evidence for such effects have been shown. 

    As such, Professor Lawrence Frank and CSCI have undertaken a project to investigate the physics of the interaction between neural currents and the MRI signal. To accomplish this, the use of both numerical simulation and experiments on idealized phantoms (as the ones above and below) in dense magnetic flux's (3T) will be implemented to quantitative the hypothesized effects and ultimately to design more sensitive methods for their detection in humans.

Project Objectives:

Primary Requirements:

• • Be capable of passing a 10-100 μA, 50-100 Hz controlled sine wave signal through phantom.

  • Air-tight, fluid-filled bundle of straws immersed in fluid to mimic neurons.

  • Cannot include magnetic materials.

Secondary Considerations:

• • Shielding from induced currents in wire.

  • Not produce excessive noise that distorts MRI images.  

  • Adjustable frequency and current is preferable.

Figure 3: (a) Example of sample phantom as designed by the CSCI (b) Schematic of MRI Phantom  

Final Design:

Figure 4: CAD Model of individual components of the final design

    The Final Design of the Phantom consists of two polycarbonate plates, seven tubes with their corresponding plugs, and a soft PVC sleeve. The tubes of the inner core are held in place by the plates and each tube is filled with the same salt water solution in order to allow current to pass through each tube. Each plug has a wire that has been glued in with acrylic cement passes through it allowing the current to be passed into and out of the tube. Additionally, a soft PVC sleeve allows the tubes to be immersed in a separate salt water solution. In this way, MRI tests may be conducted to observe how the current interacts with both the liquid it passes through, as well as its surrounding fluid, much like how neural currents are passed through axons. Finally, all of these components are combined together with silicone RTV glue in order to create an air and water tight seal for the separate tubes and outer shell of the phantom.

Figure 5: Block Diagram of the electrical signal path

    In addition to just the MRI Current Imaging Phantom, the necessary current signal is generated by an FG085 Function Generator, which has a soldered PC board with a 10 kΩ that modulates the current to the desired current amplitude. From there, the current follows a set of wires into the patch panel, through coaxial cables into an acrylic box, where the current is split into seven equal parts in order to pass through each individual tube in the phantom. The coaxial cables and acrylic box are shielded to minimize the effect of Radio Frequency noise that may distort MRI scans of the phantom. From the tubes, the current exits the phantom and follows a similar path back into the ground of the PC board and signal generator.

Summary of Performance Results:

MRI Performance:

Figure 6: (a) Original fast imaging employing steady-state acquisition (FIESTA) scan in MRI of phantom with 50 Hz, 5 Vpp signal going through tubes (b) MRI Image of phantom with 1 Hz, 5 Vpp signal with noise from solid portions subtracted and normalized (c) MRI image of phantom with 50 Hz, 5 Vpp signal with noise from solid portions subtracted and normalized.

    The MRI current phantom is scanned by wrapping the MRI scan around the axis of the cylindrical phantom. In addition, scans were taken with and without current. Afterwards the no-current-images were subtracted from the current-images and normalized to the no-current image, before being filtered for RF noise. As a result, Figures 6b and 6c are produced, whereby the current filled tubes of the phantom as well as the surrounding liquid noticeably stand out from the surrounding solid portions of the phantom and the environment. In addition, the scans of the images were analyzed by Dr. Ben Regner and exhibited a 1% difference from the no current case to the 68 μA 1Hz case and a 1.5% difference from the no current to the 68 μA 50Hz.