Tissue simulating phantoms for optical coherence tomography

https://doi.org/10.4224/21270494

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Kennedy, Brendan; Lamouche, Guy; Bisaillon, Charles-Etienne; Kennedy, Kelsey; Curatolo, Andrea; Campbell, Gord; Sampson, David

Tissue simulating phantoms for optical

coherence tomography

Brendan Kennedy*, Guy Lamouche**, Charles-Etienne Bisaillon**, Kelsey

Kennedy*, Andrea Curatolo*, Gord Campbell**, and David Sampson*,_***

*Optical+Biomedical Engineering Laboratory, The University of Western Australia **National Research Council Canada

Photonics West, 2nd _{February 2013}

OCT phantoms

2

Background

• Phantoms: an important enabling technology in OCT

…for development of techniques and applications …for inter and intra system comparison

• Main focus → optical properties

• Structural and mechanical properties increasingly important

Kennedy et al, phantoms for OCT – 8583-19

Bisaillon et al Biomed. Opt. Express (2011)

Techniques

Agrawal et al, Biomed Opt. Express (2012)

Elastography

Dermatology

3D structured phantom

De Bruin et al, Biomed.

Opt. Express (2011)

Nanoparticles Laser microinscription

Resolution/sensitivity measurement

Endoscopy

Rowe Technical Design

Applications

Ophthalmology

Tomlins et al, Biomed. Opt. Express (2011)

Focussed on 3 materials: • Silicone

• Fibrin

• Poly(vinyl alcohol)

Compared materials based on: • Optical properties

• Mechanical properties • Structural properties

Photonics West, 2nd _{February 2013 3}

Kennedy et al, phantoms for OCT – 8583-19

Focus of this presentation: • Silicone • Fibrin • Poly(vinyl alcohol) • Optical properties • Mechanical properties • Structural properties • Chemical properties

Silicone phantoms

– chemical properties

siloxane backbone

Polydimethylsiloxane (PDMS)

PDMS terminated with trimethylsilyloxy

• Silicone elastomers: crosslinked polymer chains in a 3D network • Most common: Polydimethylsiloxane (PDMS)

dimethylsiloxane

PDMS polymer chain

Silicone phantoms

– chemical properties

• Silicone elastomers: crosslinked polymer chains in a 3D network • Most common: Polydimethylsiloxane (PDMS)

dimethylsiloxane

Repeating Polymer chain

Kennedy et al, phantoms for OCT – 8583-19

PDMS polymer chain

PDMS oil

Photonics West, 2nd _{February 2013}

Silicone phantoms

– chemical properties

• Cross-linking required to convert PDMS oil into solid elastomer • Cross-link: chemical bond between polymer chains

polymer chains crosslinks

• Addition curing → two components: Part A: PDMS oil, Part B curing agent • Catalyser added to either A or B to accelerate crosslinking

• Pot time: time before A + B mixture is unpourable

6

• Optical

• Mechanical

• Structural

Phantom properties

• Optical

• Mechanical

• Structural

Silicone phantoms

– optical properties

• PDMS transparent → Micro/nanoparticles added to control optical properties • Most common: Titanium dioxide, aluminum oxide, silica microspheres

7

SEM - titanium dioxide

Key goals:

• Uniformly distributed particles → sonication • Removal of air bubbles → degassing

• Both goals hindered by silicone viscosity → thinning, e.g., hexane

Possible simplification: dilute pigment pastes supplied by manufacturers

Kennedy et al, phantoms for OCT – 8583-19

OCT- transparent PDMS OCT – scattering PDMS

Particle “clumping” Air bubbles

Photonics West, 2nd _{February 2013}

Silicone phantoms

– optical properties

Goal: measure attenuation coefficient

• Attenuation coefficient, mt: extracted from:

)

2

exp(

1

1

)

(

z

z

z

z

z

R

m















 



z = physical path length z_cf = confocal gate position

z_R = apparent Rayleigh length

OCT measurements from silicone phantoms

• Range of mt reported for soft tissue (near-infrared)*: 0-20 mm-1

Characterisation of optical properties of OCT phantoms

9

• Optical

• Mechanical

• Structural

Phantom properties

Silicone phantoms - mechanical properties

• Mechanical property of interest:

- Elasticity

• Control of mechanical properties important for:

-

- Applications:

Cardiology

OCT elastography Magnetomotive OCT Needle OCT

Silicone phantoms - mechanical properties

l

l

A

F

Strain

Stress

E







Reduced crosslinking → Increased weight between crosslinks → Lower elasticity

Sample Compression plate

Rigid plate

Elasticity → Elastic modulus, E:

M

RT

E



3



How can the PDMS elasticity be controlled?

How is E measured? • Compression test • Indentation

• Dynamic mechanical analysis

Kennedy et al, phantoms for OCT – 8583-19

Compression test

Silicone phantoms - mechanical properties

Three mechanisms to vary elasticity:

1. Vary ratio between silicone and curing agent* 2. Add non-crosslinking PDMS oil**

3. Choose PDMS designed for low elasticity***

Combination of 3 mechanisms recommended

1. Vary ratio

2. PDMS oil

3. Selection of PDMS

*Bisaillon et al, Phys. Med. Biol., 53 N237 (2008) **Oldenburg et al, Opt. Express, 13(17) 6597, 2005

Silicone phantoms - mechanical properties

Summary:

• PDMS: wide range of elasticity's in range of tissue

…..we’ve characterised phantoms in the range 1 kPa - ~5 MPa

Kennedy et al, phantoms for OCT – 8583-19 13

Elasticity range

Caution: optical scatterers reduce crosslinking density → reduces elasticity

• Typical values in soft tissue:

Adipose: 10 kPa*

Tumour (breast): ~1 MPa**

**Krouskop et al, Ultrason. Imaging 20(4) 260 (1998) *Samani et al, Phys. Med. Biol. 48(14) 2183 (2003)

• Optical

• Mechanical

• Structural

Silicone phantoms - structural properties

15

Breast elastography Complex tissue structures

- kidney

Resolution

Structured ultrasound phantoms commercially available

Why are structural properties important?

• Organs imaged in OCT → tissues assembled in complex structures ….can simulate using phantoms

• 2D and 3D structures → validation of system performance and techniques

Structural properties - 2D skin phantoms

100 mm 100 mm

• Skin-like structures formed in silicone by sequential molding

• Stacking layers with different optical properties → added and cured one layer at a time*

• Positive replica of human skin** • Fibre embedded at depth ~0.5 mm

• Surface corrugations → imaging artefacts

• Real biological structure used to make phantom

**Liew et al, J. Biomed. Opt., 16(11) 116018, 2011

Skin imaging artefacts phantom

*de Bruin et al, J. Biomed. Opt., 15(2) 025001, 2010

100 mm

• Soft-lithographic technique → replica molding • Based on a two-stage casting technique

…first casting: mold produced by UV photolithography

…second casting: poured over first casting • Suitable for image quality assessment, resolution

measurement, image registration algorithms

3D phantoms

Curatolo et al, Opt. Express, 19(20) 19480, 2011

Optical profilometry of 1st _casting

Kennedy et al, phantoms for OCT – 8583-19 17

Structured phantoms difficult to realise with PVA and fibrin

B-scan 3D Rendering

Summary

• Silicone phantoms for use in OCT

• Chemical properties

• Optical properties

• Mechanical properties

22

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