In Vivo EPR Dosimetry: Opportunities and Progress
Minoru Miyake*, Ke J. Liu, Tadeusz M. Walczak and Harold M. Swartz.
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, Hanover, NH 03755, USA
*On leave from Kagawa Medical University
The intrinsic nature of accidents establishes the need for "retrospective
dosimetry"
because often in accidental exposures to ionizing radiation conventional
dosimeters are not in
place. The recent development of in vivo electron paramagnetic resonance (EPR)
makes it
feasible to determine whether this powerful technique could be used in vivo
in human subjects.
Previously it had been shown that radiation induces substantial amounts of
paramagnetic species
which are detectable by EPR and while most of these decay too rapidly to be
useful, if they are
induced in areas with substantial amounts of water such as clothing, hair,
bone, and teeth, EPR
spectroscopy can be used to estimate the dose. The most sensitive dosimetry
studies have been
based on teeth and have been carried out at conventional EPR frequencies
(e.g. 9 GHz) for
higher sensitivity on extracted teeth. With lower frequency EPR ( e.g. 1
GHz) L-band, it might
be possible to assess the amount of irradiated dose in vivo because of the
greater tolerance to the
presence of water and the relatively large sample volume that can be
studied.
To determine the feasibility of in vivo dosimetry of relatively low levels
of ionizing
radiation (e.g. .25 Gy.) we have carried out measurements of
radiation-induced signals from the
teeth of living rats and isolated whole human teeth, using 1 GHz EPR . The
purpose of the in
vivo experiments in rats was to determine if adequate sensitivity could be
achieved in vivo and to
follow the stability of the radiation-induced EPR signals while the teeth
remained in place in a
living subject. The studies in rats also provided the essential data on the
relationship between the
sensitivity in isolated teeth and similar teeth in vivo. The purpose of the
use of isolated human
teeth was to determine the dose response relationships in human teeth and
then to use the data
from the rats to estimate the feasibility of achieving useful dosimetry in
human subjects.
In the initial experiments with rats the signal intensity from rat teeth in
vivo was quite
weak due to their very small mass, but after developing a new resonator
specially designed for
rat teeth, we were able to obtain signals whose intensity, when corrected
for differences in mass,
was similar to that which we obtained from isolated human teeth. The current
lower limit for
these measurements in isolated human teeth is 0.2 Gy. The data from the
experiments in rats
indicates that similar sensitivity should be obtainable when the
measurements are made in situ in
vivo. Because the limiting factor appears to be background signals rather
than signal/noise. It
should be feasible to reduce the effect of the background signal, thereby
making it feasible to
measure even lower doses. Instrumental developments to make the measurements
in teeth in
human subjects in situ seem quite achievable.
We conclude that a practical in vivo dosimeter for human subjects which
meets the needs
for measuring accidental exposures is achievable in the near term. This will
be based on the use
of a 1 GHz EPR spectrometer with a specially designed resonator which will
make
measurements on teeth in situ. The sensitivity should be more than
sufficient for the most critical
need for such dosimeters, the ability to determine reliably whether a
subject has received a dose
of radiation that is likely to lead to acute clinical symptoms. It also
seems likely that the
technique will be able to provide sufficiently accurate estimates of low
doses so that long term
studies of the effects of ionizing radiation can be carried out on persons
receiving accidental
exposures.
M. Miyake, K.J. Liu, T. Walczak, and H.M. Swartz, “In Vivo EPR Dosimetry of
Accidental Exposures to
Radiation: Experimental Results Indicating the Feasibility of Practical Use
in Human Subjects,” Appl.
Rad. & Isotopes 52:1031-1038 (2000).