NMR Imaging with shorted coaxial line probes

At frequencies below 1 GHz, resonant sections of coaxial lines have long been used in CW-Electron Paramagnetic Resonance (EPR) with the sample placed at the position of maximum B₁ at a short circuited end. Here, we show that because of the excellent separation of the B and E fields, the shield of the line can be removed in the region of the truncated end without greatly perturbing the RF properties of the line. The open region of the shield provides an aperture for local imaging in MRI. The B₁ fields can be shaped by contouring the inner conductor and outer shield, and the image aperture is controlled by the shape of the shield cutout. The shield opening can range from a narrow longitudinal slit up to a full 360° section that has only a few conducting strips of the shield remaining. Imaging with probes having shield diameters from 2 mm to 10 cm have been demonstrated. For imaging the useful depth is limited to approximately three to four times the probe's outer radius. Alternately, a relatively sharp cutoff at only a mm depth can be obtained by controlling the region of the shield removed, the RF power applied, and the probe diameter. The probes described here can be resonant or nonresonant. Because of the inherent broad bandwidth of the nonresonant truncated line probes, they have the potential for use in FT-EPR and FT-EPR imaging as well as other applications that require minimizing dead times.