Resonators for In Vivo Imaging: Practical Experience

Abstract

Resonators for preclinical electron paramagnetic resonance imaging have been designed primarily for rodents and rabbits and have internal diameters between 16 and 51 mm. Lumped circuit resonators include loop-gap, Alderman-Grant, and saddle coil topologies and surface coils. Bimodal resonators are useful for isolating the detected signal from incident power and reducing dead time in pulse experiments. Resonators for continuous wave, rapid scan, and pulse experiments are described. Experience at the University of Chicago and University of Denver in design of resonators for in vivo imaging is summarized.

250 MHz Rapid Scan Cross Loop Resonator

“…The CLR is similar, in design, to a 19 mm i.d. CLR designed for pulse imaging [2,14], which used four loops of copper foil for the sample resonator and an Alderman-Grant design for the driven resonator.…”

“…For comparison with prior low-frequency resonators for pulsed EPR imaging, the Alderman-Grant resonator described in Ref. [2] and [14] with 23 mm i.d. is the closest to our resonator.…”

“…Application of a correction to the effective center field for each projection that depended quadratically on the magnitude of the applied gradient ( Figure 8C) offsets the effects of the concomitant gradients. The expression for the correction was ΔB 0 = 2×10 −2 (gradient) 2 . The resulting corrected image shown in Figure 8D is in good agreement with the image ( Figure 7) for the phantom that was properly centered in the vertical dimension.…”

“…Resonator experience in the Denver and Chicago labs at 250 MHz was summarized in Ref. [2], and a recent paper implemented in vivo rapid scan imaging of nitroxide radicals using a cross loop resonator (CLR) [3]. Examples of resonators that have been used for imaging and physiology studies in other labs at frequencies below 500 MHz include a 221 MHz saddle coil with 13.5 mm diameter and 23.5 mm long [4]; a 300 MHz parallel coil resonator with ca.…”

Background correction in rapid scan EPR spectroscopy

“…Resonators have been designed to contain less metal to cut down on eddy currents. A Sorbothane ™ casing surrounding the scan coils of the 258 MHz cross-loop resonator has been shown in some cases to reduce the mechanical movements of the assembly [8], but may also change the symmetry of the response to the modulating field. Off-resonance subtraction is one way to remove background, but this method is limited by the fact that the background is field dependent.…”