Intracoronary Optical Coherence Tomography(OCT)Dr. Md. Mashiul AlamMBBS, ECFMG certified (USA)Phase B residentUniversity Cardiac CenterBangabandhu Sheikh Mujib Medical Univeristy

1

INTIMAMediaAdventatiaCoronary artery in cross sectionLumen

Intracoronary artery Imaging techniques:IVUSVirtual HistologyOptical coherence tomographyAngioscopy

Intravascular Ultrasound

PlaqueCatheter tipLumen

Virtual histologyby IVUS

Angioscopy (Endoscopy procedure)

Optical coherencetomography

Optical Coherence tomographyHigh-resolution cross sectional imaging.

Analogous to IVUS imaging, uses light instead of sound.

Provides cross sectional images of tissue structure on the micron (m) scale

A type of Optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination, OCT can provide images of tissue in situ and in real time.

OCT can be used where standard excisional biopsy is hazardous or impossible, to reduce sampling errors associated with excisional biopsy, and to guide interventional procedures.

Principles of operationHuang et al. in 1991.

The light source used for OCT imaging is in the Near-infrared range, around 1,300-nm wavelength, selected to achieve both penetration and delineation of vascular structures[tissue penetration is limited to 1 to 3 mm as compared with 4 to 8 mm achieved by intravascular ultrasound].

Cross-sectional images are generated by measuring the Echo time delay and Intensity of light that is reflected or backscattered from internal structures in tissue .

The Echo time delay cannot be measured directly [speed of light (3x108 m/s) as light is much faster than that of sound (1,500 m/s)

Correlation or Interferometry techniques.

Interferometry measures the echo time delay and intensity of backscattered light by interfering it with light that has travelled a known reference path length and time delay.

The interferometer splits the emitted light source into a Reference and Sample beam; the reference beam is directed to a reference mirror at known distance, the sample beam is directed to the structures of interest (Retina or vessel wall)

The backscattered light from the sample (Retina or vessel wall) is interfered with reflected light from the reference arm and their interference fringes are detected by a photodetector.

When the back-reflected optical intensity of the two arms (interference signal) is measured and compared, the optical properties of the tissue can be deduced

The intensity of the back-reflected light can be measured and quantified digitally in grey scale, enabling the creation of a digital image.

Retina or vessel wall

OCT Image AcquisitionBlood strongly scatters light-- intravascular OCT requires a blood-free field lasting several seconds to allow imaging.Time domain OCT (TD OCT)

Injecting continuous saline/ contrast flushes through the guiding or delivery catheters.

Proximal balloon occlusion of the vessel with distal saline/contrast injection.

Time-consuming

Require a high degree of operator expertiseFourier Domain OCT (FD OCT)

FD OCT systems do not require proximal occlusion

Bolus injection of saline, contrast, or otherSolution, injected at rates of 2 to 4 ml/s, and an automated 20 mm/s pullback within a monorail rapid exchange catheter allows imaging of a 6-cm-long coronary segment during a 3-s injection

Procedure

Conventional angioplasty guide wire (0.014-inch) inserted using an over-the-wire balloon catheter (Helios).

The Helios balloon--- maximum external diameter of 1.5 mm---- compatible with large 6-F guiding catheters (0.071-inch inner diameter).

Advanced distally to the segment of interest.

The guidewire is exchanged with the OCT Image Wire---occlusion balloon is pulled back ------ repositioned in a healthy proximal segment.

The balloon is inflated at pressure that allows totally clean imaging from blood, usually between 0.4 to 0.7 atm ----- dedicated inflator.

FD-OCT systems

Imaging --- without balloon occlusion.

The pullback speed can reach up to 20 to 40 mm/s and is performed during contrast injection (4 cc/s) to assure complete blood clearance.

Imaging of 4 to 6 cm of coronary artery segments can be achieved with 50micron m.

Observed universally (97.5%) at some point along the stented segment

IVUS-verified prolapse of 18% to 35%, suggesting OCT is both sensitive and specific.

Though clinical significance of tissue prolapse---- unclear

Tissue prolapsed through stent struts

Stent coverage- detected by OCT

A. Well apposed and coveredB. Well apposed but not coveredC. Malapposed and not coveredD. Malapposed but covered.

Immediate post procedural evaluationMalapposition

b. Dissection

c. Intrastrut prolapse

d. Thrombus

Edge dissection at different level of coronary artery after stent placement

Follow-up Stent coverage after DES

Matsumoto et al studied 34 patients (57 SES) with IVUS and OCT at 6 months follow-up. The authors reported that 64% of the struts were covered by thin neointima undetectable by IVUS .

DES inhibits neointimal proliferation to such an extent that it may not be detectable by IVUS. The higher resolution of OCT allows the visualisation and measurement of tiny layers of tissue covering the stent struts.

A study in a carotid rabbit model evaluated the usefulness of OCT for identifying strut coverage after stenting. No differences in the mean neointimal thickness measured by histology and OCT.

Assessment of restenosis

Useful in the evaluation of the causes that contribute to restenosis after DES implantation, such as incomplete coverage of lesion or gaps between stents.

Stent fracture (defect of local drug delivery) has been related to restenosis in DES and could be visualised with OCT.

Non-uniform distribution of stent struts could affect the drug delivery and therefore have an influence on restenosis in DES.

OCT has not been able to distinguish if the tissue covering the struts is neointimal tissue or fibrin.

the presence of tissue covering the strut does not prove that normal endothelial function in the area has been restored.

Pathological and functional studies are needed to understand the real meaning of the OCT findings in strut coverage

Artifacts in OCT image

Residual blood --- Attenuates the OCT light beam ---- defocus the beam if red cell density is high.

This reduces brightness of the vessel wall, especially at large radial distances from the Image Wire.

Mistakenly labelling residual blood artifact as thrombus or some other specific intra-vascular finding.

Artifacts related to eccentric wire position

Eccentricity of the image wire in the vessel lumen image influence interpretation.

The reflection from metallic stent struts align toward the imaging wire, akin to sunflowers aligning to the sun or a sunflower effect or merry-go-round effect .

Saturation Artifact

Occurs when light reflected from a highly specular surface (usually stent struts) produces signals with amplitudes that exceed the dynamic range of the data acquisition system

Sew-up Artifactis

Result of rapid artery or imaging wire movement leading to misalignment of the lumen border

Bubble Artifact

Result of air bubbles in the Image Wire sheath.

It attenuates the signal along a region of the vessel wall, and images with this artifact are not suitable for tissue characterization.

Fold-over Artifactis

specific to the new generation of FD-OCT.

The longitudinal view demonstrates that the cross section is located at the level of a side branch.

when structure signals are reflected from outside the systems field of view. Typical examples are side branch and large vessels.

LimitationsNeed to displace blood or dilute the hematocrit, either with saline or contrast flush injection, or a combination of the two.

Shallow image penetration of 1 to 2.5 mm. This prevents assessments of cross-sectional plaque area ---- OCT has only a limited role in the assessment of left main stem and Saphenous vein graft atherosclerosis severity.

The differentiation of calcific areas from lipid pools can be problematic . both result in a low attenuation signal.

Image artifacts

Future Trends

Combination of IVUS and OCT

Fusion of IVUS and OCT would provide ideal imaging of luminal and vessel wall pathology.

IVUS ---Increased penetration allow assessment of plaque burden and identification of positive or negative remodeling. High-resolution OCT ---- Permits assessment of luminal morphology, accurate estimation of fibrous cap thickness, identification of thrombus, and detection of plaque erosion and rupture.

The combined information provided by both modalities would permit a more precise characterization of the type of plaque.

59

A catheter combining an IVUS with an OCT probe would also be useful in planning and assessing the outcome of percutaneous coronary intervention.

IVUS would provide information about the correct stent diameter (on the basis of the media-adventitia dimensions) ----- OCT would permit a detailed evaluation of the final result and detection of dissections, stent malapposition, or the presence of thrombus.

There is currently no such catheter for clinical applications.

THANK YOU