Whirling-fiber endoscope allows two-photon imaging
Two-photon imaging could be a useful medical
technique if a suitable in vivo probe were available. Such a probe must be capable
of rapidly scanning across a reasonably large area, and it must effectively deliver
the excitation radiation and, equally, effectively collect the fluorescence signal.
In addition, it must be flexible and small enough to be practical in vivo.
Recently, scientists from the department of engineering
at the University of Washington in Seattle demonstrated a probe that meets these
criteria. It consists of an ∋8-mm length of optical fiber mounted on a tubular
piezoelectric (PZT) transducer whose outer surface is divided into four quadrants
(Figure 1). By applying sinusoidal, out-of-phase voltages to opposite pairs of PZT
electrodes, the scientists caused the fiber to whirl in a spiral pattern, scanning
the emitted beam across the surface to be imaged. The entire device, together with
a gradient-index lens, was housed in a 2.4-mm-diameter package at the end of an
Figure 1. Scanning was accomplished in the endoscope by a tubular
piezoelectric transducer that whirled the short length of fiber in a circular spiral
pattern (left). The fiber, transducer and a gradient-index (GRIN) lens were housed
in a 2.4-mm-diameter package at the end of the endoscope (right).
Because the instantaneous position
of the fiber lagged behind the voltage that was applied to the PZT electrodes, the
image deciphered from the drive voltage was distorted. The magnitude of the lag
depended on the instantaneous radial position of the fiber tip, and the scientists
were able to insert a reliable correction factor to eliminate the distortion in
Single-mode excitation radiation was
delivered to the sample through the core, and multimode fluorescence was collected
into both the core and the inner cladding of the same fiber. The double-clad fiber,
from Fibercore Ltd. of Southampton, UK, had a core diameter of 3.6 μm and a
0.19 NA. The 90-μm-diameter inner cladding had a 0.23 NA. Because the double-clad
fiber provided both a larger area and a better numerical aperture than a conventional
single-mode fiber, it collected fluorescence with much better efficiency.
A home-built ultrafast Ti:sapphire
laser provided the excitation signal in an experimental demonstration of the probe.
To minimize pulse broadening caused by material dispersion in the fiber, the scientists
applied a negative chirp to the pulses with a grating-based pulse stretcher before
inserting them into the fiber. A dichroic mirror separated the outgoing excitation
radiation from the returning fluorescence signal. The signal was then detected with
a photomultiplier tube, digitized and analyzed to generate an image of the sample.
They used fluorescent beads, breast
cancer cells stained with a fluorescent dye and live breast cancer cells targeted
with fluorescein-labeled monoclonal antibodies as samples in an in vitro demonstration
(Figure 2). The frame rate for these images was about 2.6 Hz, and each frame consisted
of 512 rings with 521 pixels per ring.
Figure 2. The endoscope was used to take images of fluorescent beads
(A,B), stained breast cancer cells (C) and live breast cancer cells targeted with
fluorescein-labeled monoclonal antibodies (D). The blurriness in the image of the
2.2-μm beads indicates that the endoscope is approaching its lateral resolution
To develop their prototype into an
effective medical tool, the scientists plan to redesign the optics to enhance
delivery and collection of light at the distal end of the endoscope. They
also intend to replace the current detector with one that is better suited for the
Optics Letters, April 15, 2006, pp. 1076-1078.
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