Intracoronary Optical Coherence Tomography (2)

Intracoronary Optical Coherence Tomography

Biophotonics

585.634.31

Joseph Strakna

Coronary Artery Disease

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• Prevalence: approximately 15 million • Myocardial Infarctions almost 8 million

Mortality • CAD 375 thousand • MI 120 thousand in 2011 AMI results from the blockage of coronary arteries that supply blood to the heart. Caused by thrombus formed by the rupture or erosion of vulnerable plaques within the artery wall. Treatment to restore the blood flow • Thrombolytic therapy • Angioplasty • Coronary artery bypass A method to identify vulnerable plaques prior to eruption or erosion is needed Optical Coherence Tomography offers a method to obtain images with sufficient resolution to characterize the structure of the wall of the coronary artery.

Optical Coherence Tomography

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Angiography X-ray with contrast Two dimensional view

Coronary artery imaging modalities

Intravascular ultrasound Doppler sound Resolution limited to 100-200 μm

OCT Speed of light High resolution: 15μm Detailed structure


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OCT is an imaging modality that forms an image from backscattered light History • First described by Huang in 1991 • Earliest application in retinal imaging • Brezinski et al. demonstrated the method could work in non-transparent medium • ICOCT systems available in 2010 in the US • Used in cardiac catheterization labs today


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OCT is based on Michaelson interferometer Light as e field from a low coherence source is split • Es over sample path length ls • Er over reference path length lr • Sum Ed=Er+Es at detector

Ed =Er0*exp(i(2krlr-ωt)) and Es=Es0*exp(i(2ksls-ωt)) Where: • ω-angular frequency • Ks-source propagation constant • Kr-reference propagation constant

Intensity at detector I(t)=<|Er+ES|>^2= Er0^2+Es0^2+Es0*Er0*cos(2ks*Is-2kr*lr) If we let kl=kr=2πn/λ0 interference term= cos(4πn/λ0*Is-lr) • ls-lr is the optical path length difference between the two arms • Introduces a difference in time of arrival at the detector between the 2 waves • Interference signal amplitude varies with difference in path length • Sample scatterers will interfere if within coherence length • If reference mirror is scanned the axial location scatterers in the sample is scanned )

Time delay OCT system OCT tissue scan

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• Coherence length defines resolution • Reference mirror scanned to bring different sample portions within coherence length • Inference signal amplitude determined by scattering intensity • Sample scanned laterally to produce 2D image Fourier domain systems • Fixed reference mirror • Laser light source sweeps and multiple wavelengths • Axial depth information from frequency content of backscatter • Simultaneous acquis on of backscatter information from all depths • Increase in speed


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TD-OCT FD-OCT IVUS

Energy Source Near Infrared Light Near Infrared Light Ultrasound 20-44MHZ

Wavelength 1.3μm 1.3μm 35-80 μm

Axial Resolution 15μm 15μm 100-200μm

Lateral Resolution 40μm Varies with wall proximity 200-300μm

Frame rate 15-20 frames/s 100 (180*) frames/s 30 frames/s

Pull-back rate 1-3 mm/s 20 mm/s 0.5-1 mm/s

Max scan diameter 7 mm 10 mm 15 mm

Tissue penetration 1.5-2mm 2mm 10mm

OCT vs IVUS: higher resolution but lower penetration Sound vs light OCT λ 1300nm avoids peaks of hemoglobin TD OCT vs FD OCT TD speed required vessel occlusion to eliminate blood FD no mechanically activated mirror-increase in speed Simple catheter flush


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Dragonfly Duo ICOCT catheter (ST. Jude Medical)

Optical and drive motor Flush connector Wire with 155μm SM core Micro-lens focus 90 °deflector Tiox coated cover


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Tissue Type Image Characteristics

Fiber Homogenous

High Reflectivity

Low Attenuation

Calcium Sharp Edges

Low Reflectivity

Low Attenuation

Lipid Diffuse Edges

High Reflectivity

High Attenuation

Red Thrombus Medium Reflectivity

(RBC) High Attenuation

White thrombus Medium Reflectivity

(WBC) Low Attenuation

OCT Image Characteristics


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Normal Coronary Artery

Three layer structure Lamina-signal rich with low attenuation Muscular media-dark, low reflectivity


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Classify plaque composition Plaques with different compositions A. Fibrous-high reflectivity, low attenuation, homogenous B. Calcific-low reflectivity, low attenuation, defined edges C. Lipids-high reflectivity, high attenuation, decreasing

gradient D. Thrombi-high absorber, high attenuation Asses level of stenosis (blockage) Guide Intervention • Thrombolytic (clot dissolving medications) • Stent

Guide placement of stent • Stent-supporting cage inserted into artery • Metal struts-high reflectivity, total attenuation • Level of tissue prolapse between struts • Malopposed strut at 7 o’clock • Asses bio-absorbable stent


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Future developments Gray scale image analysis requires experience and time Image post-processing • Color-coding • Real-time 3-D image • Algorithms to characterize composition

Combined OCT and IVUS More precise information on composition and extent of plaque • Leverage OCT detail with IVUS penetration • Combining transducer images difficult • Dual transducer catheter under study OCT and NIRF Microstructure, composition, and molecular processes • Identify areas of inflammation • Assessment of molecular remodeling after intervention • indocyanine green indicator


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Intracoronary Optical Coherence Tomography (2)

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