PARS-OCT Forward-Imaging Probe
Over the past
decade, the development of various endoscopic OCT probes has greatly extended
the application
range of this high-resolution biomedical imaging
technique. One of our research interest is a new design for a forward-imaging
OCT needle probe – the Paired Angle Rotation Scanning OCT (PARS-OCT)
probe. This probe design utilizes a pair of angle-cut rotating GRIN lenses
to deflect and scan the OCT probe beam across the forward region ahead of the
probe tip. In this design, the scan actuation system may be located away from
the probe tip, much like in the case of a side-imaging OCT probe, enabling
easy miniaturization of the actual probe. Further, this probe design can achieve
a large forward scan arc length to probe diameter ratio. This parameter is
especially relevant for clinical probe considerations, as a clinician will
desire as wide a scan range as possible with the smallest possible probe size.
The PARS-OCT probe
channels the input OCT probe light from a single mode fiber through the first
GRIN lens [see the following figure (a) and (b)]. The light
beam exits from the other face of the GRIN lens which is cut at an angle 
.
The beam then enters the second GRIN lens through an identically angle-cut
face of the GRIN lens. Finally, the beam exits the second GRIN lens and focuses
at a point ahead of the probe. The exact focal point is determined by the pitch
of the two GRIN lenses.
We define the orientations of the two GRIN lenses by angles 
1
and
2, which
are defined as the angles between the projections of vectors 
and
, respectively,
in the image plane and the x-axis [see figure (a)]. An analytical expression
of the 
as
a function of 1, 2
(under small angle approximation) can be derived as:
(1)
where
(2)
and 
are
the on-axis refractive index and the index gradient constant of the GRIN
lens, respectively. Z is the length of the second GRIN
lens and d is the diameter of the GRIN lens.
A fan sweep of the output beam in xz-plane [shown vertical in figure (d)]
can be performed by simply rotating the two GRIN lenses in opposite directions
at the same angular speed.
The PARS-OCT probe design is capable of performing volumetric scans with very
little modifications. By simply incrementally off-shifting the relative orientation
of the two GRIN lenses while performing B-scans, we can acquire volumetric
scans. A simpler implementation will be to introduce a slight offset to the
relative rotation scan velocities. In this case, the acquired B-scans will
automatically sweep through the entire volume scan space. Our acquisition of
orthogonal B-scans with the prototype probe demonstrates the simplicity by
which volumetric scans may be performed.
In the demonstration,
we rotated the two needles with equal and opposite angular speed (~21 rpm),
and acquired a single B-scan image from the specimen. We then
rotated both needles by 45º increment and acquired the second, third,
and fourth B-scan image [see the following figures (b)]. Fig. (a) shows the
photograph of the needle and the tadpole when acquiring the images. The scanned
locations are shown in Fig. (b). The acquired images are displayed in Fig.
(c)-(f). Each image has 350 A scan lines and is acquired in 1.4 s. We can clearly
discern the gill pockets in the images. The scan depth in the image is 2.3
mm and the largest scan half-angle is 19º.
The probe can be potentially used in needle guidance or biopsy to provide high-resolution
3-D tomographic images of the targets forward of the probe.
REFERENCE:
Jigang Wu, Michael Conry, Chunhui Gu, Fei Wang, Zahid Yaqoob, and Changhuei
Yang. “Paired-angle-rotation scanning optical coherence tomography
forward-imaging probe”, Optics Letters, 31, 1265, (2006). pdf
