OPTICAL COHERENCE TOMOGRAPHY (OCT)

OPTICAL COHERENCE TOMOGRAPHY (OCT)DR NILESH TAWADEJHRC MUMBAI

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INTRODCUTIONOPTICAL LIGHT COHERENCE SPECEFIC PROPERTY OF LIGHT (MONOCHROMATC LIGHT )TOMOGRAPHY LOOKING THE TISSUE IN SLICES (CROSS SECTIONS )

Milestones in OCT Development

White light interferometrydemonstrated (1881)

Single-mode fiber invented (1970)1st commercial OCT eye scanner(1997)

1st OCT images of biomedical tissue reported (1991)

Fiber-optic OCDR introduced for telecom (1987)High-speed endoscopic OCT

(1998)Doppler OCTPolarization-sensitive- OCT

1st clinical application of intravascular OCT (2002)1st commercial FD-OCT scanner introduced (2007)

Intravascular OCT

Flexible fiberoptic catheter used for light delivery

Catheter rotates to create image frames

Catheter pulls back to map vessel segment

Lesion analysis, stent planning, post-stent assessment, follow-up

Image GenerationMeasure echo time delay of reflected light waves

One pixel 5 x 19 um

One axial line 1024 pixels

One frame 500 axial lines

Optical resolution 15um axial, 20 to 40 um transverse

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axial

transverse

Pullback GenerationOne pullback 270 frames

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Image DisplayL-Modelongitudinal view

B-Modecross-sectional view

9Basic Physics of OCT

Time Domain OCTLight is too fast for direct echo measurement interferometryCompare path length between known reference arm and sampleMechanical reference arm motion limits imaging speed

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intensityaxial distance

Demod

AmpBroadbandSource

D

TissueMirrorReflections

Frequency Domain OCTMeasurement of interference pattern spectrum + Fourier transformSignal generated from all depths simultaneouslyFaster image acquisition without loss of quality

11Swept Laser

`

D

intensity

intensitydistanceFFT

Amp

Frequency Domain OCT12

+

=Optical fields summed at detector (ES+ER)

(delayed) optical field from sample armresulting intensity (power) on detectorI= (ES+ER)2

optical field fromreference armESER

Frequency encodes sample depth, amplitude encodes brightness12

TD-OCT

FD-OCT

TD-OCT FD-OCT13

FD-OCT vs TD-OCT14

C7XRM3Dr. Francesco Prati, San Giovanni-Addolorata Hospital, Rome, Italy.Prof. Eliosa Arbustini, Centro Malattie Genetiche Cardiovasc.Lab. di Anat. Pat.-Area Trapiantologica, Pavia, Italy100 fps, 20 mm/s pullback20 fps, 1 mm/s pullbackPostmortem, formalin fixed artery

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ParameterDeterminesControlled ByC7XR ValueImaging SpeedAcquisition timeRequired flush volumeLaser sweep rateCatheter rotation ratePullback speed100Hz50,000 axial lines/s20 mm/sSensitivityMinimum detectable tissue reflectionImage contrastElectrical and optical system designBetter than -100 dBImaging RangeMaximum vessel diameterLaser linewidthElectrical and optical system design10 mm (in contrast)ResolutionMinimum detectable tissue featureSpeckle size and image granularityLaser tuning range (axial)Catheter focusing optics (lateral)15 mcm (axial

20 40 mcm (lateral)Tissue PenetrationVisible depth into vessel wallScattering and absorption of tissue1 2 mm

Key Specifications15

TD-OCT vs FD-OCT SummaryTime Domain OCT (TD-OCT)Mechanically scans a reference mirrorSlow imaging, moderate image quality

Frequency Domain OCT (FD-OCT)Electronically scans the laser wavelengthFast imaging, exceptional image quality16

Mention that most previous studies done with TD-OCT16

Performance Evolution19

1999-2001

PTCA balloon + ImageWireTM

R&D prototypes

Not commercially availableInside PTCA balloonSnapshot flush imaging2007M3 System

CE mark20 fps / 240 linesOcclusion + flush

2004

Soft occlusion balloon + ImageWireTMM2 System

CE mark15 fps / 200 linesOcclusion + flush

2009C7XR System

CE mark, FDA cleared100 fps / 500 lines

Occlusion-free DragonflyTM

NO OCCLUSION

We have long history of OCT, competition is new to the fieldNO OCCLUSION19

C7XR System /FD-OCTBalloon occlusion not requiredFast flush, spiral pullback acquisition5 cm arterial segment in 2.5 secRapid exchange (Rx) imaging catheter20

100 frames/s, 500 lines/frame15 mm axial resolution10 mm scan diameter

Call acquisitions spiral pullback, not 3D20

OPTICAL COHERENCE TOMOGRAPHY (OCT)DR NILESH TAWADEJHRC MUMBAI

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OCT Applications in PCIPre-InterventionPlaque characterization (lipid, calcium, fiberous, cap thickness, thrombus) Intervention planning and stent sizing (lesion diameter, length)Post-InterventionStent strut malappositionIntimal dissectionFollow-Up AssessmentNeointimal growth (no coverage, thin coverage, restenosis)In-stent thrombosis22

FD-OCT vs IVUS23

Edge dissection during stent implantationNeointimal growth on previously implanted stent at follow-up

Performance Comparison, FD-OCT vs IVUSAxial Resolution12 - 15 mm100 - 200 mmBeam Width20 40 mm200 300 mmFrame Rate100 frames/s30 frames/sPullback Speed20 mm/s0.5 - 1 mm/sMax. Scan Dia.10 mm15 mmTissue Penetration1.0 - 2.0 mm10 mmLines per Frame500256Lateral Sampling (3mm Artery)19 mm225mmBlood ClearingRequiredNot Required

IVUSC7XR

Say that IVUS penetration is best-case25

FD-OCT /SWAPT SOURCE In all the new generation ofOCTsystems the optical probe is integrated in a catheterThe profile ranges from 2.4 Fr to 3.2 Fr but all of them are compatible with 6 Fr guiding catheter. The usable length is around 140 cm. The position and number of the radiopaque markers varies for the different systemsDuring the pullback the optic fiber probe is pulled along the catheter sheathAll the systems have dedicated pullback devices and consoles that allow the processing and storage of the data.

In all the new generation ofOCTsystems the optical probe is integrated in a catheterThe profile ranges from 2.4 Fr to 3.2 Fr but all of them are compatible with 6 Fr guiding catheter. The usable length is around 140 cm. The position and number of the radiopaque markers varies for the different systems. During the pullback the optic fiber probe is pulled along the catheter sheath. All the systems have dedicated pullback devices and consoles that allow the processing and storage of the data.27

FD-OCTPrepare the catheter purging it with salineCalibrate the optical probe. Advance the catheter over 0.014 guidewire till the optical probe is distal to the region of interest. The different systems have different radiopaque markers to guide the catheter positioningStart contrast or diluted contrast injection through the guiding catheter (with a pump injector).Begin pullback when vessel is visible inOCTimage.

With probe integrated in the catheter systems1. Prepare the catheter purging it with saline.2. Calibrate the optical probe.3. Advance the catheter over 0.014 guidewire till the optical probe is distal to the region of interest. The different systems have different radiopaque markers to guide the catheter positioning5. Start contrast or diluted contrast injection through the guiding catheter (with a pump injector).7. Begin pullback when vessel is visible inOCTimage.

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Image Interpretation And Tissue Characterization

OCT Image Interpretation TerminologyBackscatter

The reflection of light waves off the tissue and back to the Dragonfly catheterHigh backscatter means a brighter pixel Also described as a signal rich regionLow backscatter means a darker pixel Also described as a signal poor region

AttenuationThe reduction in intensity of the light waves as they pass through tissue due to absorption or scatteringHigh attenuation means the light cannot penetrate very deepLow attenuation means the light can pass through to allow visualization of deeper tissue

Scattering: This occurs when light strikes an object and is redirected, like when one pool ball strikes another. Scattered photons may be redirected at any angle. In OCT, photons that are scattered directly backwards (backscattered) are captured by the catheter optics, guided back to the engine, and are detected by the electronics. These backscattered photons help to form the image. Photons that are scattered at other angles are not captured by the imaging system, and can also be thought of as being lost. Scattering is heavily dependent on the size and shape of the particles that make up the sample. Red blood cells have extremely high scattering, which is the main reason that we cannot see through blood with OCT. You can observe scattering by placing a laser pointer against your finger. The diffuse blob of light that seems to emanate from your skin is heavily scattered.

Attenuation: This refers to the decrease in light intensity as it propagates through a sample. Attenuation is caused by a combination of absorption and scattering. A strongly attenuating tissue type will appear to have a low penetration depth under OCT imaging, since a large portion of the light is lost as it propagates into the tissue.

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OCT Image Interpretation TerminologyCompositionHomogeneousUniform in structureHeterogeneousStructure consists of dissimilar elementsTextureCoarseFine33

Absorption: This occurs when light strikes an object (such as tissue) and vanishes into the material. The absorbed photons do not propagate beyond the object and are lost forever. Absorption is heavily dependent on the wavelength of light. Lipid is an example of a material that has very high absorption at OCT wavelengths. Calcium, on the other hand, does not heavily absorb light. Black material becomes hot in the sunlight because black absorbs visible wavelengths.

Reflection: This occurs when light strikes a solid, shiny material (such as a stent strut or catheter sheath) and a portion of the light bounces off the sample. Unlike scattering, the reflection angle is determined entirely by geometry...if you shine light off a mirror that is perpendicular to the light beam, it will reflect directly backwards. If you tilt the mirror 45 degrees, it will reflect light at 90 degrees.

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Image OrientationNormal coronary arteryUniform silhouette3 layers visible in vessel wall

Data on file at LLI

Imaging catheterGuidewire shadow

AdventitiaMediaIntima34

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OCT Image Interpretation TerminologyEdge/BorderThe creation of a border is due to the interface between different tissue typesOne of the parameters used to differentiate plaque types

CalciumLipid

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Fibrous Plaque36

Calcified Plaque37

Data on file at LLI

Lipid Plaque38

Data on file at LLI

Plaque Types - Recognition FibrousBright pixelsFinely texturedDeep penetrationHomogeneousLipidDark pixelsDiffuse edgeLow penetrationHomogeneousCalciumDark pixelsSharp edgeDeep penetrationHeterogeneous

Fibrous

Lipid

Calcium

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Vessel Wall DamageEdge dissectionA disruption of the vessel luminal surface in the edge regionEasy to interpret using cross-sectional and longitudinal views

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Vessel Wall DamageIntimal tearClear visualization of even small irregularities

42Data on file at LLI

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Thrombus Red (Acute)Thrombus redAbsorbs near-infrared lightHigh backscatter on surface due to signal attenuationAppears as a bright massShadow (cannot see behind it)

43Data on file at LLI

Red thrombusData on file at LLI

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White thrombusThrombus White (Chronic)Thrombus whiteHigh backscatterLow attenuationAble to see behind it44

Data on file at LLI

Neointimal HyperplasiaIn-stent restenosisThick layer between stent struts and lumen

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Data on file at LLIStent struts

In-Stent Restenosis46 Not to be distributed or reproduced

Fibrous Rich Plaque, HomogeneousEEM47 Not to be distributed or reproduced

Post Stent Implantation Overlapping Stents48 Not to be distributed or reproduced

CalciumFibrous 49 Not to be distributed or reproducedCalcium and Fibrous Plaque

Stent Strut Coverage2 months

7 months

50Data on file at LLIImages: Dr. Suzuki, Toyohashi Heart Center

Data on file at LLI

Obtaining Clear Images

Obtaining Clear ImagesBlood in Dragonfly Imaging catheter lumen52Data on file at LLI

Blood in catheter lumenCatheter lumen purged of blood.

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Blood Swirls Turbulent flow between flush and blood Flush not filling lumen, perhaps going into other arteriesOR End of flush bolus

Data on file at LLI

Blood Speckle

Red blood cells mixed into flush fluid

Flush contrast diluted with saline, less viscous, does not flush all RBC

Data on file at LLI

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>> Area < +/- 1%. No discernible effect of blood on lumen area measurements when boundary of vessel wall is visibleSwirlsSpecklesBlood in LumenData on file at LLI

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Artifacts

Data on file at LLIGuidewire Shadow

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Original position of arteryShifted position of artery

Radial scan lines

Image

Cause: Rapid movement of the artery or Dragonfly during a single frameEffect: Slight elliptical distortion with visible stitch.Sew Up ArtifactData on file at LLI

More common with M3 than C758

Ghost imagesof stent strut

Actual stent strutGhost Reflections ArtifactCauseLight reflected multiple times between two surfacesEffectBlurry replica appears at a fixed distance away from the primary image of an objectHigh reflecting objects, like metallic stent struts, can produce a series of ghost reflections

Data on file at LLI

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Saturation

Cause: Very high reflections, usually from metallic stent struts, saturate the system following signal amplificationEffect: Spoke of signal-rich reflectionsSaturation ArtifactData on file at LLI

Clinical ApplicationsD i a g n o s t i c a s s e s s m e n t o f c o r o n a r y a t h e r o s c l e r o s i s .

80% of clinically evident plaque rupture originates within an inflamed thin-capped fibroatheromas

Thin-capped fibroatheromas are characterized by 3 essential components: a lipid core, inflammatory cell cap infiltration, and a thin fibrous cap

While OCT does not currently have the depth to quantify large lipid cores, itshigh resolution allows precise visualization and quantification of the thin fibrous cap

Stent strut apposition/coverageA proposed 3-point classification defines stent strut apposition as embedded (when the leading edge is buried within the intima by more than one-half its thickness),protrusion (when the stent strut is apposed but not embedded), malapposed (when there is no intimal contact)

Another classification, pertinent for follow-up studies examining the degree of neointima formation, describes whether or not the stent strut appearscovered with tissue, and whether struts are well apposed or malapposed

Classification Stent struts are, therefore, classified as1) well apposed and covered; 2) well apposed and not covered;3) malapposed and not covered; and 4) malapposed but covered

Tissue prolapseTissue prolapse, or protrusion of tissue between stent struts without apparent surface disruption, is defined as occurring if the depth of protrusion is > 50 mcm . This high frequency is similar to postmortem findings (94%) ,but significantly higher than IVUS-verified prolapse of 18% to 35%. suggesting OCT is both sensitive and specific. The clinical significance of such prolapse is, however, unclear, given that it occurs so frequently, and has not been associated with early clinical events nor examined in relation to late clinical events

Stent coverage

Strut malapposition was determined when the negative value of SIT was higher than the strut thickness (plus the thickness of abluminal polymer for PES), according to the stent manufacturer specifications (140 m for TAXUS Express2and 132 m for BMS Express2), corrected for strut blooming thickness (distance between inner and outer strut reflection boundaries, 378 m, as calculated from 2250 struts).The final cut-off value for malapposition was 158 m for PES and 150 m for BMS81

The Future OutlookThe future of OCT will include advancements in anatomical and functional assessment of lesions for the interventional cardiologist.Anatomical assessment: An advanced edge detection algorithm that enables automated stent strut identificationTexture analysis of OCT images may also help facilitate tissue characterization,F u n c t i o n a l a s s e s s m e n t .

Detection of Doppler-like signals may permit integration of physiology and anatomical assessment using a single device.

CONCLUSIONOCT enhances imaging resolution that may permit the evaluation of clinical (e.g., luminal measurements during PCI) and research (e.g., fibrous cap thickness and strut levelanalysis) parameters for the interventional cardiologist. The versatility of the physical properties of light position OCT as an imaging modality could be useful for improving our understanding of the vascular biology of atherothrombosis. and assisting in our performance of PCI procedures. However, routine clinical use of OCT will require further clinical trials to validate the technology, establish standard definitions/measurements, and to test its safety and utility in improving clinical outcomes like IVUS