Parallel Coil Resonators for Time-Domain Radiofrequency Electron Paramagnetic Resonance Imaging of Biological Objects N. Devasahayam, S. Subramanian, R. Murugesan, J. A. Cook, M. Afeworki, R. G. Tschudin, J. B. Mitchell, and M. C. Krishna Radiation Biology Branch, Division of Clinical Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892 Received June 3, 1999; accepted September 3, 1999 Resonators suitable fortime-domain electron paramagnetic res- onance spectroscopy and imaging at a radiofrequency capable of accommodating experimental animals such as mice are described. Design considerations included B 1 field homogeneity, optimal Q, spectral bandwidth, resonator ring-down, and sensitivity. Typi- cally, a resonator with 25-mm diameter and 25-mm length was constructed by coupling 11 single loops in parallel with a separa- tion of 2.5 mm. To minimize the resonator ring-down time and provide the necessary spectral bandwidth for in vivo imaging experiments, the Q was reduced predominantly by overcoupling. Capacitative coupling was utilized to minimize microphonic ef- fects. The B 1 field in the resonator was mapped both radially and axially and found to be uniform and adequate forimaging studies. Imaging studies with phantom objects containing a narrow-line spin probe as well as in vivo objects administered with the spin probe show the suitability of these resonators forvalid reproduc- tion of the spin probe distribution in three dimensions. The fabri- cation of such resonators is simple and can be scaled up with relative ease to accommodate largerobjects as well. © 2000 Academic Press INTRODUCTION Electron paramagnetic resonance (EPR) spectroscopy is a direct and sensitive technique for detecting paramagnetic spe- cies such as free radicals. With the increasing importance of free radicals in biology, EPR spectroscopy is used to under- stand the basic mechanisms of several disease processes with putative free radical pathways (1). EPR imaging can be imple- mented for in vivo biological studies (2), using physical prin- ciples similar to those of magnetic resonance imaging (MRI) (3). Recent studies have reported on the EPR imaging of intact objects such as isolated organs as well as experimental animals to detect endogenously generated free radicals as well as the spatial distribution of exogenously administered free radicals (4–9). Such experiments have established the capability of EPR imaging to provide valuable physiological information such as tissue oxygen status as well as tissue redox status (5, 10, 11). Obtaining such information noninvasively is of importance in characterizing pathological states as well as guiding effective treatments, making EPR imaging a poten- tially useful tool in diagnostic radiology (10). Several studies have obtained useful physiological information using spin probes such as nitroxides and particulates such as glucose char and India ink, (8, 12–14). The linewidths of these compounds are in the 0.5–5 G range requiring the use of continuous wave (CW) EPR detection since the transverse relaxation time T 2 is in the order of 250 –50 ns. With the recent availability of biologically compatible, non- toxic, water-soluble spin probes exhibiting a single-line EPR spectrum with oxygen-dependent linewidth in the range of 50 –200 mG, time-domain EPR techniques are receiving in- creasing attention because of the sensitivity inherent to the pulsed EPR techniques as well as minimal artifacts associated with object motion (15–19). In addition, substantially shorter times for a three-dimensional imaging experiment (5 min) compared to CW EPR imaging (25–30 min) make it possible to collect several three-dimensional images after administration of the spin probe and thereby allow monitoring of pharmaco- kinetic data. The feasibility of performing in vivo imaging experiments using time-domain radiofrequency (RF) EPR has been shown at 300 MHz by administering water-soluble tri- arylmethyl (TAM)-based paramagnetic spin probes to mice and examining regional anatomies (16). Subsequent efforts have been directed to: (1) provide capability to study whole animals (17); and (2) evaluate receiver configurations and data acquisition methodologies to enhance sensitivity (20). In this paper, design considerations for a resonator capable of accom- modating an experimental animal such as a mouse are de- scribed with emphasis on B1 field homogeneity, minimization of dead time, efficient utilization of RF power, and optimal spectral bandwidth. Imaging data from phantom objects and from in vivo experiments are presented. RESONATOR DESIGN CONSIDERATION Resonator Ring-Down Time, Sensitivity, and Spectral Bandwidth For the detection of a paramagnetic spin probe with pulsed EPR, the time-domain response after a pulsed excitation should Journal of Magnetic Resonance 142, 168 –176 (2000) Article ID jmre.1999.1926, available online at http://www.idealibrary.com on 168 1090-7807/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.