Magnifying endoscopes generally fall short of cellular resolution due to
basic limits of conventional wide-field optics (depth of field and out-of-focus
blur). However, confocal microscopes are common laboratory instruments which
can image intact tissue in subcellular detail by scanning a laser and restricting
the light path from the focal plane to the detector using pinholes. The limitation
is that they are normally bulky.
So how is it
possible to achieve confocal microscopic cellular and subcellular imaging
in vivo, using an otherwise normal endoscope, within the context of a conventional
endoscopic procedure?
Optical Fibre technologies can be used to achieve this by remotely linking
bulky components to the imaging end of the device.
Various approaches can be used to achieve the required miniaturisation.
The approach which has yielded the highest resolution to date involves
the use of a single optical fibre acting as both the illumination and detection
pinhole function in a confocal microscope. This is illustrated in the accompanying
animation.
The system works as follows:
A single optical fibre is used to project a focused pinpoint of laser light
via the flexible endoscope into the tissue.
Light emanating from the focused point (which is normally produced by fluorescence)
is recaptured back into the same optical fibre. This return light is focused
back through the optical fibre via the endoscope, and back to the detection
system on the endoscope trolley.
However light from outside the focused spot is not recaptured into the
fibre and is rejected.
In short, the detector can only “see” the focal plane illuminated
by the fibre. That is how the imaging plane is isolated.
The image is then built up by scanning the fibre tip under control of a
computer which also collects and displays the detector signal, mapping a
picture onto the screen in real time as the laser spot scans the tissue.
The result is a live image containing microscopic detail from the focal
plane, which can be at the surface or just beneath the surface of the tissue.
