Moving from biomedical to industrial applications: OCT Enables Hi-Res ND Depth Analysis Patrick Merken a,c, Hervé Copin a, Gunay Yurtsever b, Bob Grietens a a Xenics NV, Leuven, Belgium b UGENT, Ghent, Belgium c RMA, Brussels, Belgium
Contents Introduction - OCT : what is it, and how is it done? Infrared based OCT Sensor requirements Applications - Medical and Industrial IR ROIC Conclusions Image : Heidelberg Engineering
Introduction : What is OCT (I) Optical Coherence Tomography Invented in 1991 - MIT Prof James Fujimoto Carl Zeiss Research Award - Two previous Carl Zeiss honorees won the Nobel Prize This research at MIT - In collaboration with investigators from - Harvard Medical School - Tufts University School of Medicine - Produced a host of valuable OCT applications. Prof James Fujimoto
Introduction : What is OCT (II) Very similar to ultrasound - Use fs light pulses instead of sound - Perform optical ranging Provides high resolution images - Opaque Samples - Depth information (6 mm) - Generate image cubes (ND) Non-Destructive technique Bridges gap between confocal microscopy and MRI
Principle : LCI Low Coherence Interferometry - Two distinct beams Reference Beam - Passes through optical delay-line Sample Beam - Reflected by Reflector Sample Interference @ Detector - Pattern contains information about layer composition
Principle : Spectral Interferometry Identical principle - Two distinct beams - But now mirror is fixed Spectral response can be captured - Using detector line-array Fourrier transform - contains information about layer composition
Principle : 2D surface imaging With either technique - Time domain or Spectrum based - Provides information on 1 point Add scanning motion - Build Line or 2D depth profile Some limitations : - Axial resolution can be good (~ 10 µm) - Lateral resolution is limited (20 µm) - Axial scanning must be rapid (fringe blurring) - Lateral scanning is necessarily slower (~ 1 second per scan)
Infrared Based OCT Most surfaces cannot be penetrated with optical rays But : Appropriate wavelength can - Use of Infra-Red light (1550 nm) - Detectable with appropriate detector technology (InGaAs) CMOS and CCD devices not suited InGaAs : - Linear and 2D detectors and cameras - Highly sensitive and fast - Key technologies in OCT for depth imaging.
FT IR OCT Fourier Transform Infra-Red OCT Using high density line array
Full Field IR OCT (I) 2D sensor camera allows full-field OCT - Tomographic images orthogonal to the optical axis - Entire field of the image is illuminated - One arrangement based on a Michelson interferometer
Full Field IR OCT (II) Full-field OCT is essentially an interference microscope - Illuminated by a broadband spectrum - Very short interference lengths give high axial resolution (1-2 μm) - Short acquisition time of each image frame is essential At present only one camera is compatible with FF OCT - Xenics 640 x 512 pixels, Cheetah-640CL (1,730 fps) - High speed cameras transform OCT into a dynamic, real-time control method for biomedical and industrial processes. - Reduced dark current provided with TEC
Fast and large sensors As has been discussed : - Fast axial scanning is a must - Fast 2D Camera s are an asset High spectral resolution - Using large line arrays - Up to 2048 elements - Small pitch (high spectral contant) Lynx and Cheetah - Worlds fastest 2D camera - High density line arrays
Examples : Medical Imaging (I) Perfect to image structures in the eye - Non-invasive optical imaging with - Millimeter penetration - Submicron resolution Real time imaging possible Very popular in eye clinics - 20 000 ophtalmic OCT systems - 2 000 new installations each year
Examples : Medical Imaging (II) Blood vessel imaging High resolution images - Using catheter - Detect coronary artery diseases OCT - Expected to replace ultrasound - Provides 10 x better resolution (4 20 μm vs 110 μm) Clear and detailed image shows - Layered structure of the blood vessel wall - Intima (I), Media (M), and adventitia (A) Image : Thor Labs
Examples : Medical Imaging (III) OCT in Dentistry Detecting failing composite - Voids and fractures visible Image comparison - X-ray (left) - OCT in background - Plain view (right) Image : Lantis Laser
Examples : Medical Imaging (IV) In the near future : - Handheld OCT device to visualize and map non-melanoma skin cancer (NMSC) under the skin - Early detection and treatment possible Image : www.octnews.org
OCT for industrial applications (I) New and innovative for quality assurance - Non-Destructive inspection technique - On-the-fly monitoring - Inspect manufacturing and assembly processes Delivers cross-sectional images MEMS pressure sensor - Several cross sections - At various depth levels
OCT for industrial applications (II) Fraunhofer-Institut ILT - OCT to investigate material properties - Test on plastic wall samples of a liquids container - 3 layers thick, including 100 µm diffusion barrier Achievements : - Thickness and uniformity of each layer - In real time during a production - Higher process security, Potential of large savings in production facilities
OCT for industrial applications (III) Many materials are compatible with IR OCT As another example - OCT cross-section of an orange peel
Infrared ROIC (I) Not only do we need - Highly sensitive InGaAs PDAs - With low dark current ROIC fulfills crucial role to - Deliver low noise signals - Provide high dynamic range - Provide high speed ROIC front-ends are optimized as detector interfaces - Parameter values are software settable - Can cover various pixel sizes and application demands All these parameters are extremely important - To provide deep and motion artifact free OCT imaging
Infrared ROIC (II) Most ROIC provide multi-stage signal pre-processing First : I-V converter - Keep InGaAs PD at constant bias - 5 integration capacitors (5 ff 2 pf) - Selectable at runtime - Allows imaging materials with different reflectivities. Then follows a CDS stage - Eliminate all offset variations - Reduce ktc noise - Often combined with S/H stage
Infrared ROIC (III) The Sample and Hold (S/H) stage - Decouples integration and read-out. - Allows IWR An analog multiplexer and pad driver - Transfers all pixel values via the IC output to an external ADC
Conclusions Today s InGaAs camera s are scalable OCT platforms - Different sensor types, ROICs with various parameter settings - Multi-stage thermoelectric cooling Particularly suited for - Scientific, industrial and medical applications - Direct imaging and spectroscopy They can deliver - Resolutions one micrometer - Penetration depths of over 6 mm They allow to implement OCT - Meaningful and detailed analysis tool - Increase quality, throughput and yield