The Implantable Microresonators
Tadeusz Walczak and Harold M. Swartz
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, HB7785, Hanover, NH, USA, 03755
INTRODUCTION: The use of in vivo EPR oximetry based on particlulate
oxygen sensitive materialsin
larger animals or human subjects is potentially very productive because of
the ability to make
repeated measurements from the same site(s). When the site is within 10 mm
of the surface a
surface resonator can be used effectively, making the procedure entirely
non-invasive once the
paramagnetic material is placed at the site of interest. One then can use a
whole-body or a
surface-coil resonator, depending on the size of the subject. With a
surface-coil, however, there
is a rapid drop of sensitivity with distance. To overcome this limitation we
previously have
developed needle-catheter resonators, which can be inserted through an
opening in the body and
can then extend to a depth of 8 cm or more. This approach, however, does
have the limitation of
requiring a persistent opening in the body while implanted and it also is
moderately bulky. To
overcome these limitations we have developed implantable resonators that can
be entirely
implanted and then connected to the spectrometer through magnetic coupling
to a subcutaneous
loop to a surface resonator.
METHOD: We investigated the influence of various resonator
configurations and coupling circuits
on the stability, sensitivity, and performance of the AFC system. Also, a
theoretical approach
was developed to estimate the operating characteristics of an implantable
resonator.
RESULTS: We have found that the best results can be obtained using a
resonance structure that is
formed of two loops connected by a micro-coaxial cable (0.5-0.8 mm in
diameter). A section of
the micro-coaxial cable is terminated on one end with a micro-loop (0.6 mm
in diameter) that is
filled with a paramagnetic material. The material is protected from
interactions with the
surrounding tissue by a layer of gas-permeable plastic coating. The second
end of the cable is
terminated with a larger loop, 5 mm in diameter. The length of the coaxial
cable for a given
frequency can be calculated using the equation:
l = [ p- arctan(wL/Z0)]/b
where L is the inductance of the larger loop, Z0 is the characteristic
impedance of the
transmission line, b is the phase constant 2p/l, and w is the operation
angular frequency 2p f.
The presence of the small loop in the circuit decreases the resonant
frequency less than 10% due
to the very low inductance of the loop. The whole resonator is implanted,
with the small
detection loop placed at the site to be investigated, and the bigger
"transmitting" loop in the
superficial layer of the skin. The bigger loop is then magnetically coupled
with an external
surface coil through the intact skin. Figure 1 shows the response of this
type of resonator when
implanted into a rat muscle.
Figure 1 The response of the implantable
micro-resonator to a cycle of "baseline-ligature-recovery".
The
resonator was implanted into rat muscle; spectra were taken each 60sec. during
the recovery phase.
DISCUSSION: We feel that this resonator may be a key to the rapid and
effective use of EPR clinically
for those applications in which it is feasible to implant a resonator: this
is the situation with
radiation therapy of tumors.