Precision depth control of bone resection is necessary for safe surgical procedures in the spine. In this paper, we compare the control and quality of cutting bovine tail bone, as an ex vivo model of laminectomy and bony resection simulating spinal surgery, planned with micro-CT data and executed using two approaches: (a) mechanical milling guided by optical topographical imaging (OTI) and (b) optical milling using closed-loop inline coherent imaging (ICI) to monitor and control the incision depth of a high-power 1070 nm fiber laser in situ. OTI provides the in situ topology of the 2-dimensional surface of the bone orientation in the mechanical mill which is registered with the treatment plan derived from the micro-CT data. The coregistration allows the plan to be programmed into the mill which is then used as a benchmark of current surgical techniques. For laser cutting, 3D optical land marking with coaxial camera vision and the ICI system is used to coregister the treatment plan. The unstable, carbonization-mediated ablation behaviour of 1070 nm light and the unknown initial geometry of bone leads to unpredictable ablation which substantially limits the depth accuracy of open-loop cutting. However, even with such a non-ideal cutting laser, we demonstrate that ICI provides in situ high-speed feedback that automatically and accurately limits the laser’s cut depth to effectively create an all-optical analogue to the mechanical mill.
A key issue in laser surgery is the inability for the human operator to stop the laser irradiation in time while
cutting/ablating delicate tissue layers. In the present work, we forward-image through the laser machining front in
complex biological tissue (dense bovine bone) to monitor the incision's approach to subsurface interfaces in real-time
(47-312 kHz line rate). Feedback from imaging is used to stop the drilling process within 150 micron of a targeted
interface. This is accomplished by combining the high temporal and spatial resolution of infrared optical coherence
tomography (OCT) with a robust, turn-key, high brightness fiber laser. The high sensitivity of the imaging system (~100
dB) permit imaging through the rapidly changing beam path even with the additional scattering caused by the thermal
cutting process. In spectral-domain OCT, the imaging acquisition period is easily locked to the machining laser
exposure. Though motion-induced artifacts reduce interface contrast, they do not introduce incorrect depth measurements as found in other OCT variants. Standard tomography imaging of the tissue (B-scans) is also recorded in situ before and after laser processing to highlight morphology changes.
It has been shown that 30 ns FWHM duration pulses from a MOPA fiber laser (wavelength: 1064 nm) cleanly
micromachines silicon with little cracking or heat-affected zone1. In this paper, we show that similar results can be
achieved using a 1070 nm quasi-continuous wave laser pulsed with a 6.6 μs duration (average power: 2.8 W) in
combination with coaxially delivered nitrogen assist gas. The holes are cut at a 5 kHz repetition rate with a resulting
diameter on the order of 15 μm and an etch rate of up to 18 μm/pulse. Hole size is increased for longer pulses and
the heat-affected zone broadens to greater than 25 μm with no assist gas. By combining low coherence microscopy
with machining, we depth image the machining front and obtain in situ images during and after the drilling process
showing rich cut dynamics in real time.
The stochastic effects of assist gas in QCW and pulsed laser machining (percussion drilling) in steel are measured with a
novel in situ high speed low coherence imaging system. Real-time imaging is delivered coaxially with machining energy
and assist gas revealing relaxation and melt flow dynamics over microsecond timescales and millimeter length scales
with ~10 micrometer resolution. Direct measurement of cut rate and repeatability avoids post cut analysis and iterative
process development. Feedback from the imaging system can be used to overcome variations in relaxation and guides
blind hole cutting.
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