A quadrature excitation coil for longitudinally detected electron spin resonance

Longitudinally detected electron spin resonance (LODESR) is an alternative method to conventional continuous wave (cw) ESR with transverse detection. LODESR has some practical advantages over cw-ESR for in vivo experiments ca rried out with excitation at radio frequencies. Sample losses, however, can be a problem and generally limit the LODESR signal to noise ratio for biol ogical samples. We have developed a LODESR coil system with quadrature exci tation circuit in order to decrease the sample power deposition. The coil s ystem consists of a high pass birdcage excitation coil operated in quadratu re (320 MHz) and a solenoidal coil (325 kHz) for signal detection. Methods of decoupling the excitation and detection circuits are described. Results reported here show the effectiveness of the LODESR coil system with quadrat ure excitation in reducing the sample power deposition by a factor of 2. Th e quadrature excitation coil assembly we have described here could be used, without further modification, for LODESR imaging. The coil assembly, inclu ding rf screen, could be placed inside the field gradient coils. LODESR ima ging, in contrast to nuclear magnetic resonance imaging, uses continuous ra ther than pulsed gradient wave forms and the copper foil used for the rf sc reen would not significantly distort the gradients. (C) 2001 American Insti tute of Physics.