wavelength-to-spatial-coordinate mappingas the spectral shower performing the imaging if it is coupled with the imaging optics via a beam combiner and passesthrough the same spatial disperser.
“By tuning the wavelength of the laser,”he said, “the beam can be directed to anyarbitrary position on the sample to perform laser surgery [ablation], without anymechanical movement of the probe tosteer the laser beam or the movement ofthe sample.
“Hence, high-precision microsurgerycan be performed by computer-controlledtuning of the laser wavelength accordingto a preprogrammed pattern.”
Advantages and challenges
Most currently employed endoscopesrely on a CCD or a fiber bundle to captureimages, sometimes combined with a scalpel or other surgical instrument to providesimultaneous imaging and surgery.
For minimally invasive procedures, an endoscope must be veryflexible, and it must have a verysmall diameter. “In CCD-basedprobes, the size of the chip placed atthe distal tip limits the minimum diameter to about a few millimeters,”Tsia noted, “and their electrical cables limit their flexibility.” He addedthat fiber-bundle technology is limited because obtaining a high pixelcount requires a large number offibers, resulting in mechanical rigidity. Because SECOMM uses a singlefiber, it is both small and flexible.
Right now, SECOMM is hinderedby its spatial resolution – around 4to 10 μm – and by the number ofpixels it can capture.
“Nevertheless,” Tsia said, “theseare not the inherent limitations ofthis technique because the numberof pixels can significantly be increased by using an optical sourcewith larger bandwidth or a spectrometer with higher spectral resolution.”
First of its kind
Others have been working on de-
signing similar probes in recent
years. A Harvard Medical School
group demonstrated a 1-D spectrally
encoded endoscope, but the tech-
nique requires mechanical scanning
to capture a whole 2-D image. An-
other group demonstrated an endo-
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 tele-
com applications and for spectroscopy, he
added. But his team used the 2-D spatial
disperser for imaging and microsurgery.
A winning team
Tsia worked on the SECOMM projectwith Keisuke Goda, a postdoctoral researcher in the electrical engineering department at UCLA, and with BahramJalali, a professor of electrical engineeringat 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 micros-
copy.” 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 originatedfrom 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 si-
multaneous ultrafast real-time imag-
ing and laser surgery. “This is particu-
larly useful to monitoring the laser
ablation dynamic in the tissue,” Tsia
said. “The optical amplification fea-
ture in STEAM can also be applied to
SECOMM, in which the detection
sensitivity can be greatly enhanced.”
The next step for the SECOMMproject is to design and build theminiaturized SECOMM probe. Thiswill consist of off-the-shelf miniaturized optics, including a diffractiongrating and a gradient-index lens.
The preliminary design showedthat SECOMM can be miniaturizedinto a submillimeter-diameter probewith spatial resolution of 1.4 ; 2
μm, with a field of view of 280 ;
70 mm. “The present imaging tech-
nique can also be further extended
to three-dimensional volumetric im-
aging by employing an interfero-
metric configuration, which enables
the acquisition of the depth infor-
mation of the sample,” Tsia said,
which will be good news for future
patients undergoing delicate proce-
“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.
This demonstration of SECOMM’s ability to perform laser microsurgeryand 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 thesample.