research activities in 2007 - CSEM

Scintillating Fiber Probes for Neurophysiology
R. Eckert, B. Weber • , A. Buck • , M. Liley, R. Ischer, R. P. Stanley
CSEM has been working with the University Hospital Zurich to develop optical probes for neurophysiology. The results of this development is a
range of probes going from thin probes to be inserted into tissue to large area probes to be used externally for long term measurements.
Radioimaging is widely used as a diagnostic tool in medicine
as it allows the direct imaging of internal structures such as
organs as well as their activity. Of these tools, positron
emission tomography (PET) can give highly accurate images
of the human body. PET has become extremely useful
because compounds used in metabolism can be radiolabelled
so that the signal strength is related to internal metabolic
activity.
Tracing the evolution of these same compounds in time can
also give an insight into the dynamics of metabolism. PET is
too slow for this, a better option is to use a small probe that
can take local measurements in real time. Together with the
University Hospital Zurich and Swisstrace, CSEM has
developed a range of fiber optic based probes for making local
physiological measurements.
All fiber probes have two parts, a short part made from special
fluorescent fibers and a standard glass or plastic optical fiber
to transport the signal to the detection systems. The short
fluorescent fibers convert ・ particles (electrons or positrons)
coming from radioactive decay that penetrate the fiber, into
light. The intensity of the light depends on the energy of the
beta particles. Only a very small quantity of light is produced –
some tens of photons per beta particle.
This light has to be guided without loss to a very low noise
photodetection system. The fibre probes that have been
developed have three different diameters, 250 µm, 500 µm
and 3 mm. The thinnest fibers can be inserted into tissue
without causing damage, while the 3 mm probe has been
designed as a surface probe for chronic (long term)
measurements.
The signal from the thinnest fiber is in the range of 30 counts
per second (CPS) under standard operating conditions.
However, ambient light can also enter the fiber, producing an
unwanted background about ten orders of magnitude larger
than the actual signal. In order to avoid working in complete
darkness, the fibers have to be perfectly sealed against stray
light. Making the fibers light tight is very challenging because
the coating has to be completely opaque to light – even a
single micron-sized pinhole is unacceptable – while being thin
enough to allow the beta particles to reach the scintillating
fiber.
In order to achieve total light tightness, an opaque coating is
applied in a layered fashion and is carefully tested after each
layer to ensure specifications are achieved.
The large 3 mm probes pose additional challenges. Due to
their large area their cross section for ・ rays – which come
from the disintegration of positrons (・ + particles) – is rather
large. As the ・ rays can travel long distances, they are not a
local signal and are therefore an unwanted noise. Optically it
is impossible to tell the difference between fluorescence signal
from the fiber coming from ・‘s or ・’s. Given that the
absorption length for ・ particles is much shorter than that of
・ rays, the ratio between ・‘s and ・’s can be improved by
thinning the fluorescent fiber to a thin disc 3 mm in diameter
but less than 0.2 mm thick. These have been successfully
used to monitor the differences in activity between the left and
right hemispheres of the brain in real time.
A hardware platform was built to read the low light signal from
the scintillating fibers (see Figure 1). This includes ultra-low
noise detectors that are shielded from stray radiation by
integrated shielding. The system is interfaced to a computer
via a USB port.
Finally, the technology developed for the large fiber probes
can also be used to track any radiolabelled material including
nanoparticles. The feasibility of using these kinds of
measurements to check transport of nanoparticles through
biological tissue has been investigated. Initial calculations
show that the systems developed can have already
sensitivities in the 10-13 Molar range which is much better than
competing technologies.
Figure 1: The detection system developed by CSEM including ultralow
noise uncooled photodetectors. The detectors are protected from
background radiation by integrated lead shielding. The system
connects to a computer via a USB port.
This work was funded partly by University Hospital Zurich and
the EU-Project Nanosafe.
•
University Hospital Zurich, PET Centrum, Rigistrasse 56, 8006
Zürich, Switzerland.
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