Compensating for Effects of Motion of Subjects Being Studied
by In Vivo EPR by an New
Automatic Matching System for Electronically Tunable Surface-Coil-Type
Resonators
Hiroshi Hirata*, Tadeusz Walczak, and Harold M. Swartz
EPR Center for the Study of Viable Systems, Department of Diagnostic
Radiology, Dartmouth Medical
School, Hanover, NH 03755, USA
* On leave from Yamagata University, Japan
INTRODUCTION: The use of electron paramagnetic resonance (EPR)
spectroscopy in vivo has
made it possible to use the unique capabilities of EPR spectroscopy in the
fully functional state.
One of the potential limiting factors in performing in vivo EPR is the
occurrence of motions due
to physiological functions and voluntary movements. The motion can result in
mismatches in the
usual detection method (i.e. between the resonator and the 50-ohm
transmission line). These
mismatches can occur due to the fact that the distance between the animal
and the surface-coil
detector can change rapidly. Such mismatches can create unacceptable
distortion of EPR spectra
due to increased noise and shifts in spectral baseline. To correct such
distortions we have
developed a novel automatic tuning and matching system using varactor
diodes.
METHODS: We designed, built, and tested a new surface coil resonator
using varactor diodes, for
tuning the resonance frequency to a fixed frequency oscillator and for
capacitive matching of the
resonator to the 50-ohm transmission line. The electronically tunable
resonator consists of a
surface coil, a parallel transmission line, a half-wave line balun, and
tuning and matching circuits
controlled with external voltages. The automatic matching circuit compares
incident and
reflected microwaves using phase sensitive detection and generates an error
signal to correct the
mismatching. The final resonator system was found to have a time constant
better than 1 ms.
RESULTS AND CONCLUSIONS: The performance of the automatic matching
system was tested in vivo
by measuring EPR spectra of lithium phthalocyanine (LiPc) implanted in rats.
Stability and
sensitivity of the spectrometer were appraised by measuring EPR spectra with
and without the
automatic matching system active. The overall experimental performance of
the spectrometer
was found to significantly improve during in vivo experiments using the
automatic matching
system. Increases in signal to noise ratio of at least 30% were observed.
Excellent matching
between the 50-ohm transmission line and the resonator was maintained under
all experimental
circumstances that were tested. This should allow us now to carry out
experiments that
previously were not possible. The approach should be adaptable to other
types of resonators as
well.
This study was presented at the EPR Workshop on In Vivo EPR and Related
Studies, Dartmouth College,
NH in September 1998. A manuscript was published in the Journal of Magnetic
Resonance: H. Hirata, T.
Walczak and H.M. Swartz, “Electronically Tunable Surface-Coil-Type Resonator
for L-Band EPR Spectroscopy,” J
Magn Reson 142:159-167 (2000).