Diffuse Reflectance Near Infrared Spectroscopy of Atherosclerotic Plaque,

Progress With an Intracoronary Device

(Part Two)

Vulnerable Plaque Research Program,University of Texas Houston and

Texas Heart Institute

TOC: Electromagnetic Spectrum and

Spectroscopy Emission Spectroscopy (Thermography) Diffuse Reflectance Spectroscopy Raman, and fluorescence Spectroscopy Structural/chemical Imaging vs.

Functional Imaging (pH, lactate, free radicals…)

The goal of combined “Photonic Catheter”

What is electromagnetic radiation? Electromagnetic radiation is a form of energy, sometimes called optical energy. The most familiar form of electromagnetic radiation is visible light. However, there are many other forms of electromagnetic radiation including:

•Gama rays•X-rays

•Ultraviolet Light•Infrared Light•Microwaves

•Radio Waves

Spectroscopy BasicsIn general, spectroscopy is the use of the electromagnetic spectrum to perform physical or chemical analysis

E=hc/

Spectroscopy = The interaction of light with various materials

Energy is either absorbed, transmitted, or reflected by molecules present in sample

1) Non-ionizing radiation (light) is used to interrogate sampleExample wavelengths:

Visible 0.4 – 0.7 microns

Near-Infrared 0.7 – 2.5 microns

Mid-Infrared 2.5 – 10 microns

2) Wavelengths are separated for detection

3) Detector converts intensity to voltage signal as a function of wavelength

The human eye is a crude reflectance spectrometer

A modern spectrometer, however, can measure finer details over a broader wavelength range and with greater precision. Thus, a spectrometer can measure absorptions due to more processes than can be seen with the eye.

Light can reveal much about tissue without ever damaging or changing it’s structure.

Light can be delivered/collected via optical fibers which can access remote sites within the body via endoscopic catheters.

Visible light penetrates only a few mm through tissues. Near infrared light penetrates only a few cm through tissues.

This is both a strength and a weakness. It is a weakness because light can only interrogate limited volumes of tissues.

It is a strength because much of the body consists of thin tissue layers, therefore optical techniques are well-suited for localized interrogation of tissue layers.

In our case, studying arterial wall and atherosclerotic plaque which are well within millimeters, it works perfectly.

Near Infrared Spectroscopy has come to be widely used to determine the composition of a variety of materials ranging from human and animal feeds to foods.

Quality control of products, e.g. lean from fat, fake arts and antiques from originals and so many other industrial applications…

Old Technique New Application

•Oximetry to assess blood oxygenation. •Monitoring hyperbilirubinemia in jaundiced neonates using reflectance. •Locating early cancer in the lung, colon, cervix, and other tissues using fluorescence. •Assessing blood perfusion and oxygenation of the brain during child birth. •Measuring glucose by optical measurements of skin. •Detecting a pneomothorax in neonates. •Detecting atheromatous plaque in blood vessels using NIR, fluorescence, and IR Raman.

Combining spectroscopy with imaging yields a spectrally weighted image that is used or functional mappings:

•Mapping blood perfusion •Mapping brain hemorrhage •Mapping tissue oxygenation •Mapping the redox potential of tissues

Hyperspectral Imaging

California Institute of Technology

Pioneering works:Focus on lipid, calcium, and collagen analysis: Feld et.al. (MIT)

– Raman spectroscopy: currently working on building Raman fiber optical catheter

– FTIR spectroscopy: only ex-vivo pathological identification

Lodder et al. (Univ. of Kentucky)– NIR Catheter to study in-vivo rabbits

cholesterol contents;

In vivo determination of the molecular composition of artery wall by intravascular Raman spectroscopy.Buschman et al. The Netherlands

Intravascular ultrasound combined with Raman spectroscopy to localize and quantify cholesterol and calcium salts in atherosclerotic coronary arteries.Romer et al, The Netherlands

Optical detection of triggered atherosclerotic plaque disruption by fluorescence emission analysis.Christov et al Ontario, Canada.

Time-resolved Fluorescence reflectance spectroscopy

Grundfest et al, UCLA

NIR spectroscopy and Partial Least Squares for total cholesterol

Jaross et.al., Germany

Photo Dynamic Therapy “Photo-Angioplasty”!

Pharmacyclics Inc.

Others:

Fluorescence spectrum analysis of atherosclerotic plaque using doxycycline.

Miyagi M et al. Japan

Small branch starches –dextran and other photonic dies yet to come

What about photonic contrast media?

NIR Spectroscopic Survey of 30 Human Carotid Endartherectomized Plaques

Near Infrared Spectroscopy

Sensitivity 0.002 and Accuracy 0.016 pH unit

J Clin Monit 1996 Sep;12(5):387-95

Non-invasive measurement of tissue pH using

near-infrared reflectance spectroscopy.

Soller et al.

University of Massachusetts Medical Center, Worcester 01655, USA.

Optical measurement of tissue pH: Patent #5,813,403

NIR Spectrum of Lactate

0.15

0.2

0.25

0.3

0.35

0.4

0.45

1700 1800 1900 2000 2100 2200 2300 2400 2500

wavelength (nm)

arbi

trary

uni

ts (a

bsor

banc

e)

Diffuse Reflectance NIR Spectroscopy Absorption is due to

combinations and overtones of fundamental vibrations

Reflectance Mode: path length varies for different tissues and wavelengths

Catheter geometry and optical coupling important

Small source-detector separations: light penetrates tissue while restricting volume interrogated

plaqueinterface

to spectrometer

~3 mm

Tissue Penetration Study NIR reflectance off mirror 100% signal Tissue stacks placed on probe end Incremental increase in signal with mirror

~50 um slices Aortic tissue

Mirror-Enhanced Reflectance

Tissue Absorption & Scattering

Mirror

FiberProbe

Plaque Measurements Full spectrum absorbance data (400-2500 nm, FOSS

NIRSystems) 24 gauge needle thermistors (Cole-Parmer model 8402-20) 750 m diameter pH electrodes (Microelectrodes, MA) Punch needle biopsy 1 – 5 mg pieces for lactate assay Measurements on plaque in 37 incubator Histology on the rest of the plaque

°C

pH

spectrometer

10-20

mm

~2 mm

Size ~3F

Length 1.5m

Fiber Material Low OH polyimide coated

silica fibers

Fiber Diameter 100-140 x 10^-6 m

Total Number of Fibers 39

Illumination Fibers 13

Receiving Fibers 26

Side Looking Tip 0.5 mm width

Near Infrared Spectroscopy Catheter (Prototype I)

Catheter Tipped with a Side Looking Silvered Conical 0.5 mm Mirror

cm0.5mm

Small Diameter Probe Preliminary visible/NIR

spectra (UMass Worcester)Visible Spectra

0.1

0.15

0.2

0.25

0.3

0.35

0.4

400 500 600 700 800 900 1000 1100Wavelength

Fibrofatty Ulcerated FTNIR Plaque

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

w avelength

A.U

.

f ibrofatty ulcerated area

Large Diameter Probe: Full Spectrum of Rabbit Aortic Arch

Aortic Arch

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

400 550 700 850 1000 1150 1300 1450 1600 1750 1900 2050 2200 2350 2500

wavelength

abso

rban

ce

aortic arch WHHL 0.9 mm NZ aortic arch

WHHL

NewZealand

Area 2: Fibrous

Area 1: Thrombus

Area 3: Calcified/Mixed

Spectral ClassificationSpectral Classification Full range spectra: 400-2500 nm classification of Full range spectra: 400-2500 nm classification of

spectral signaturespectral signature Cluster analysis Cluster analysis

Principal components analysis for data and noise Principal components analysis for data and noise reductionreduction

K-means algorithmK-means algorithm KK clusters, clusters, mm objects, objects, nn variables variables Mahalanobis distance metric (co-variance taken into Mahalanobis distance metric (co-variance taken into

account)account)

PC1

PC2

x

211 b)(ab)(a CovD T

Yesterday’s Dream, Today’s Plan, Tomorrow’s Catheter!

“Infrared Catheter” providing the following critical information about plaque:

1- Temperature (IR) 2- physio-chemical properties (pH, lactate, free radicals, oxidized lipids, oxidized collagen… with NIR) 3- physio-pathological features (with photonic

contrast media and tracers)

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Research @ the Speed of Thought