Project 3: Continuous Wave Electron Paramagnetic Resonance Imaging. Summary of Progress: We have modified our existing 300 MHz CW machine by supplementing a field sweep coil that can work at high AC frequencies. We tested the existing gradient coils for high frequency operation, and found that they can be used only up to a frequency of 1 kHz. By employing field sweeps at 0.33 kHz, and rotating gradients at 1 kHz (by applying sinusoidal gradients in two orthogonal gradient coils we could accomplish a left- or right-rotating gradient current corresponding to 2G/cm amplitude) required for 2D imaging. The lower frequency rapid scan signal, the three higher frequency gradient signals, and the trigger signals for data acquisition were generated using National Instruments LabView software and National Instrument PCI-6289 multifunction data acquisition card. The amplitudes of the gradient signals are adjusted to result in a circularly polarized gradient field rotating in the XZ plane at the selected frequency. The phase of the X and Z gradients can then be incremented programmatically relative to the rapid scan field at a desired step interval. The EPR signals from the RF bridge are down-converted to base band and amplified using a low noise low frequency amplifier and captured using a commercial digitizer-cum-summer Acqiris AP100 (Acqiris USA, Monroe, CA) dual channel Signal. By further applying a sinusoidal current in the third orthogonal coil, one can tilt the rotating gradient plane in equal tilt angles to cover an entire hemisphere of gradient trajectories centered on the resonator that is required for 3D spatial encoding. Since the projections are collected with the sweep field and simultaneous gradient rotation, one has to reorganize the observed signals by a simple matrix shuffling program to give the conventional Cartesian projections which are then used to generate the image by filtered back-projection. Even with this modest sweep frequency each projection could be carried out in less than 15 ms. We could collect 2D and 3D projection data in this so-called Rapid-scan-rotating gradient modality at least 50-100 times faster than the conventional CW approach. The invention has been protected by a US application.
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