Tissue-mimicking phantoms

Section 1. Tissue-mimicking phantoms

KelseyMakingPhantoms

Development and validation of new optical imaging methods require that we perform measurements on artificial imaging targets, known as phantoms, with controllable optical, mechanical, and structural properties. It is necessary to alter these properties to mimic a range of tissue properties.

Silicone is a convenient base material for flexible and straightforward fabrication of phantoms. It is readily compatible with a wide range of suitable scatterers for adjustment of the optical properties. The mechanical properties can be adjusted over a wide range by controlling the amount of cross-linking within the silicone formulation and can be done independently of incorporating optical scatterers. Silicone can also be used to fabricate phantoms with complex structures due to its low viscosity prior to curing and high toughness.

1) Structured phantoms for OCT

We have developed a novel structured 3D phantom for use in OCT, fabricated from silicone and titatnium dioxide particles (for optical scattering) in a two-stage casting process using a photolithography master. The features in the phantom spell out the letters “OBEL.” We have demonstrated the utility of the phantom by quantitatively evaluating the spatial resolution and speckle contrast in a focus compounding speckle reduction scheme. We expect such phantoms to prove invaluable in providing quantitative reproducible benchmark standard imaging targets for OCT.

(a) Schematic representation of the structured 3D phantom design (not to scale). The yellow dashed line represents an OCT B-scan; (b) A photograph of the phantom with the feature location indicated by the black arrowhead and an Australian 5 cent coin; (c) Profilometry of the phantom after feature casting. (d-e) Photo-micrographs of the letters.

(a) Schematic representation of the structured 3D phantom design (not to scale). The yellow dashed line represents an OCT B-scan; (b) A photograph of the phantom with the feature location indicated by the black arrowhead and an Australian 5 cent coin; (c) Profilometry of the phantom after feature casting. (d-e) Photo-micrographs of the letters.

1.1. Download movie: 3D OCT image of the OBEL phantom.

2) Phantoms for elastography

Careful control of the mechanical properties, and in particular, the elasticity, of the phantom materials used in optical coherence elastography (OCE) is imperative for validating new techniques and analysing mechanical contrast. To obtain an assortment of phantoms mimicking a range tissue stiffness values for use in OCE, we have acquired several commercially available silicones and measured their Young’s modulus for varying combinations of cross-linker and catalyst. Table 1 illustrates the effect of varying the ratio of cross-linker to catalyst for a particular commercially available silicone, Wacker Elastosil® 601. Using this silicone, a Young’s modulus range of ~100 kPa to ~5 MPa is achievable, which covers a wide range of tissue stiffness. However, very soft tissues such as adipose, brain and liver, have Young’s modulus below 10 kPa. To further reduce the stiffness of silicone phantoms to be in the range of very soft tissues, silicone fluid such as polydimethylsiloxane (PDMS) oil may be added prior to curing. Silicone fluid does not participate in the curing process, but remains as a fluid within the cross-linked polymer network of the cured silicone, resulting in a softer material. Table 2 lists the measured Young’s modulus of Wacker Elastosil® 601 silicone with varying concentrations of silicone fluid. By adding PDMS oil, a Young’s modulus as low as 10 kPa was achieved.

Table 1. Young’s modulus of various mixing ratios (cross-linker : catalyst) for Wacker Elastosil® 601 silicone phantoms.

Mixing ratio Elastic modulus (kPa)
1:5 4910
1:15 3060
1:20 1483
1:30 1008
1:40 286
1:50 127

Table 2. Young’s modulus of various mixing ratios (cross-linker : catalyst : PDMS oil) for Wacker Elastosil® 601 silicone and Wacker AK50 PDMS silicone fluid.

Mixing ratio Elastic modulus (kPa)
1:10:10 316
1:10:20 122
1:10:30 81
1:10:40 38
1:10:50 23
1:10:60 10

These silicones can then be structured into phantoms containing rigid inclusions, such as the star-shaped inclusion below, and imaged using OCE to analyse the mechanical contrast in the images against the expected contrast.

A stiff, star-shaped inclusion to be embedded in a soft phantom for testing the elastography system

A stiff, star-shaped inclusion to be embedded in a soft phantom for testing the elastography system

Elastography image of the star above embedded in soft silicone

Elastography image of the star above embedded in soft silicone