Fig 1.
(a) Thorlabs OCS1050SS swept-source OCT system is treated as a black box. The fiber optic rotary joint (FORJ) is connected to the OCS1050SS light source via a fiber optic patch cable. The electronic x-galvanometer scanner control signal from the OCS1050SS is connected to a custom inverting differentiator and rectifier (IDR) circuit that converts the sawtooth input wave to a square wave with a high signal during the period of data acquisition along the fast x-axis of the raster scan pattern. The IDR output is connected to an Arduino Uno that detects the rising edge and outputs a short digital pulse to signal the motor drivers to begin movement of the rotational and linear motors. The endoscope is connected directly to the FORJ. The rotational motor is connected to the FORJ through meshed gears. The rotational motor, FORJ, and endoscope are mounted on a linear translation stage that is controlled by the linear motor. The fast rotational motion and slow linear motion result in a helical scanning geometry. (b) A photograph of the sample arm, including the motors and translation stage. The rotational motor (RM) spins the fiber optic rotary joint (FORJ) and both are mounted on a linear translation stage that is moved by the linear actuator (LA). Motor drivers send control signals (CS) to the control the motors.
Table 1.
Comparison of Estimated Penetration Depth and Axial Resolution in Colon Tissue for Four Common OCT Wavelengths.
Fig 2.
(a) Optical layout showing rays traced through the glass spacer, gradient-index lens, 41° prism, glass window, and tissue. (b) Spot diagram at beam focus. The cylindrical glass window induces astigmatism, but the traced spot is still smaller than the diffraction limited spot radius of 6.14 μm (denoted by the black circle). (c) Photograph of the distal portion of the endoscope. (d) Diagram showing dimensions in mm. Both the spacer and GRIN lens have a diameter of 1.0 mm. The prism is cylindrical with a rectangular exit face. The radius of the circular entrance face is 0.85mm with the square exit face cut a distance of 0.7mm from the center of the circular entrance face. The wall thickness of the glass envelope is 100 μm and a length of about 35 mm. The GRIN lens is characterized by a numerical aperture of 0.5 and a pitch of 1.4.
Fig 3.
Inverting differentiator and rectifier circuit schematic.
This circuit converts the x-galvanometer waveform (Vgalvo) from a sawtooth wave with a peak-to-peak voltage of ±10V to a square wave that is 4V during the slow sweep of the x-galvanometer mirror and 0V otherwise. The Vgalvo waveform is characterized by three parts. Data are acquired during the low-magnitude negative slope portion of the wave. The rapid return of the x-galvanometer mirror occurs during the high-magnitude positive slope portion. In between these two sweeps, the raw OCT fringe data is saved to the PC and the Vgalvo waveform is kept constant (zero slope). The purpose of this circuit is to detect the beginning of the negative slope portion of the waveform and communicate this to an Arduino Uno, which can safely read input voltages from 0V to 5V. Resistor R2 attenuates high-frequency ringing in the Differentiator. The Rectifier diode holds the negative portion of the Differentiator output at 0V. The optional Clipper prevents the output Vo from exceeding 4.3V (the sum of the bias voltage V3 and the forward voltage drop of D2). The input impedance of the Arduino I/O pins is represented by R3.
Fig 4.
Oscilloscope traces of motor control signals.
A few cycles of the electronic signals used in motor synchronization are displayed. The x-galvanometer control signal is yellow, the output of the inverting differentiator and rectifier circuit is green, and the Arduino output that serves as the digital pulse to trigger the motors is purple.
Algorithm 1.
Numerical dispersion compensation.
Fig 5.
OCT images of saline-treated mouse colon.
(a) Cross-sectional image (B-scan) showing high contrast between the many layers of the colon wall (labelled are colonic mucosa (CM), submucosa (SM), muscularis propria (MP), and serosa (S)). The horizontal axis is circumferential and the vertical axis is depth. (b) En face standard-deviation projection of the colon with the anus located at the top. The crypt structure can be seen throughout. (c) Enlarged region denoted by the square inset in (b) showing the crypts. The colon circumference (horizontal axis in (a) and (b)) is 6.3mm and the length (vertical axis in (b)) is 15 mm. The horizontal dashed line in (b) indicates the location of the B-scan displayed in (a). All scale bars are 1mm.
Fig 6.
OCT images of AOM-treated mouse colon.
(a) Cross-sectional image (B-scan) showing adenoma (AD), characterized by rapid attenuation and loss of tissue boundary visibility. The horizontal axis is circumferential and the vertical axis is depth. (b) En face standard-deviation projection of the colon with the anus located at the top. Adenoma are bounded by dashed curves. Crypt structure can also be visualized. (c) Enlarged region denoted by the square inset in (b) showing crypts. Notice the non-uniform crypt density and crypt elongation compared to Fig 5c. The colon circumference (horizontal axis in (a) and (b)) is 6.3mm and the length (vertical axis in (b)) is 15 mm. The horizontal dashed line in (b) indicates the location of the B-scan displayed in (a). All scale bars are 1mm.