A BROAD-BAND PULSED RADIO-FREQUENCY ELECTRON-PARAMAGNETIC-RESONANCE SPECTROMETER FOR BIOLOGICAL APPLICATIONS

A time-domain radio frequency (rf) electron paramagnetic resonance (EP R) spectrometer/imager (EPRI) capable of detecting and imaging free ra dicals in biological objects is described, The magnetic field was 10 m T which corresponds to a resonance frequency of 300 MHz for paramagnet ic species. Short pulses of 20-70 ns from the signal generator, with r ise times of less than 4 ns, were generated using high speed gates, wh ich after amplification to 283 Vpp, were deposited into a resonator co ntaining the object of interest. Cylindrical resonators containing par allel loops at uniform spacing were used for imaging experiments. The resonators were maintained at the resonant frequency by tuning and mat ching capacitors. A parallel resistor and overcoupled circuit was used to achieve Q values in the range 20-30. The transmit and receive arms were isolated using a transmit/receive diplexer. The dead time follow ing the trailing edge of the pulse was about 450 ns. The first stage o f the receive arm contained a low noise, high gain and fast recovery a mplifier, suitable for detection of spin probes with spin-spin relaxat ion times (T-2) in the order of mu s. Detection of the induction signa l was carried out by mixing the signals in the receiver arm centered a round 300 MHz with a local oscillator at a frequency of 350 MHz. The a mplified signals were digitized and summed using a 1 GHz digitizer/sum mer to recover the signals and enhance the signal-to-noise ratio (SNR) . The time-domain signals were transformed into frequency-domain spect ra, using Fourier transformation (FT). With the resonators used, objec ts of size up to 5 cm(3) could be studied in imaging experiments. Spat ial encoding of the spins was accomplished by volume excitation of the sample in the presence of static field gradients in the range of 1.0- 1.5 G/cm. The spin densities were produced in the form of plane integr als and images were reconstructed using standard back-projection metho ds. The image resolution of the phantom objects containing the spin pr obe surrounded by lossy biologic medium was better than 0.2 mm with th e gradients used. To examine larger objects at local sites, surface co ils were used to detect and image spin probes successfully. The result s from this study indicate the potential of rf FT EPR for in vivo appl ications. In particular, rf FT EPR may provide a means to obtain physi ologic information such as tissue oxygenation and redox status. (C) 19 98 American Institute of Physics.