wavelength-to-spatial-coordinate mapping
as the spectral shower performing the imaging if it is coupled with the imaging optics via a beam combiner and passes
through the same spatial disperser.
“By tuning the wavelength of the laser,”
he said, “the beam can be directed to any
arbitrary position on the sample to perform laser surgery [ablation], without any
mechanical movement of the probe to
steer the laser beam or the movement of
the sample.
“Hence, high-precision microsurgery
can be performed by computer-controlled
tuning of the laser wavelength according
to a preprogrammed pattern.”
scopic probe capable of both imaging and
laser surgery through the use of MEMS
scanners for beam steering. “SECOMM
has the advantages over such endoscopic
probes,” Tsia said, “as it eliminates the
need for mechanical scanning.”
Grating-VIPA arrangements have been
used in the past for demultiplexing in telecom applications and for spectroscopy, he
added. But his team used the 2-D spatial
disperser for imaging and microsurgery.
Advantages and challenges
Most currently employed endoscopes
rely on a CCD or a fiber bundle to capture
images, sometimes combined with a scal
pel or other surgical instrument to provide
simultaneous imaging and surgery.
For minimally invasive procedures, an endoscope must be very
flexible, and it must have a very
small diameter. “In CCD-based
probes, the size of the chip placed at
the distal tip limits the minimum diameter to about a few millimeters,”
Tsia noted, “and their electrical cables limit their flexibility.” He added
that fiber-bundle technology is limited because obtaining a high pixel
count requires a large number of
fibers, resulting in mechanical rigidity. Because SECOMM uses a single
fiber, it is both small and flexible.
Right now, SECOMM is hindered
by its spatial resolution – around 4
to 10 μm – and by the number of
pixels it can capture.
“Nevertheless,” Tsia said, “these
are not the inherent limitations of
this technique because the number
of pixels can significantly be increased by using an optical source
with larger bandwidth or a spectrometer with higher spectral resolution.”
A winning team
Tsia worked on the SECOMM project
with Keisuke Goda, a postdoctoral researcher in the electrical engineering department at UCLA, and with Bahram
Jalali, a professor of electrical engineering
at the university.
These same three researchers recently
made headlines when they developed “the
fastest camera in the world.” That camera
is known as STEAM, which stands for
“serial time-encoded amplified microscopy.” It allows real-time imaging with up
to 6 million fps, thanks to ultrashort laser
pulses; “optical” image gain enables high
detection sensitivity. STEAM, according to
Tsia, can overcome the trade-off between
speed and sensitivity that occurs in existing
CCD/CMOS cameras. And its speed makes
it useful for capturing rapid biological
processes and events such as the firing of a
neuron. It also can be used with SECOMM
for laser surgery.
In fact, Tsia said, SECOMM originated
from the work they did on STEAM.
“One of the key features in STEAM is
spectrally encoded imaging, which maps
the spatial information onto the spectrum of
an ultrashort laser pulse. We borrowed this
idea for SECOMM and realized that laser
surgery can also benefit from the same
wavelength-space mapping idea.”
The two can be combined to perform simultaneous ultrafast real-time imaging and laser surgery. “This is particularly useful to monitoring the laser
ablation dynamic in the tissue,” Tsia
said. “The optical amplification feature in STEAM can also be applied to
SECOMM, in which the detection
sensitivity can be greatly enhanced.”
First of its kind
Others have been working on designing similar probes in recent
years. A Harvard Medical School
group demonstrated a 1-D spectrally
encoded endoscope, but the technique requires mechanical scanning
to capture a whole 2-D image. Another group demonstrated an endo-
This demonstration of SECOMM’s ability to perform laser microsurgery
and simultaneous monitoring shows the images captured (a) before and
(b) after performing laser ablation on a bovine tissue sample. The “L”
pattern (outlined in dots) is carved out of the tissue by tuning the wavelength of the CW laser in the manner shown in the inset of the figure
(see arrow). The Z-axis represents the normalized reflectivity of the
sample.
Concept proved
The next step for the SECOMM
project is to design and build the
miniaturized SECOMM probe. This
will consist of off-the-shelf miniaturized optics, including a diffraction
grating and a gradient-index lens.
The preliminary design showed
that SECOMM can be miniaturized
into a submillimeter-diameter probe
with spatial resolution of 1.4 ; 2
μm, with a field of view of 280 ;
70 mm. “The present imaging technique can also be further extended
to three-dimensional volumetric imaging by employing an interferometric configuration, which enables
the acquisition of the depth information of the sample,” Tsia said,
which will be good news for future
patients undergoing delicate procedures.
“SECOMM can be applicable to
any area where high-precision, small
and flexible probes are required,
such as brain tumor, pediatric and
endovascular surgeries,” he said.
