Research

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 Ψ. 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 ζ1 and ζ 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 angle θ as a function of ζ1, ζ2 (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.

References