In optical coherence tomography (OCT), it is vitally important to develop tissue-simulating phantoms for the validation of new methods, technologies and applications [1
]. To date, the main focus has been on developing phantoms with optical properties in the range of those of tissues [3
], but other important properties include durability, mechanical behavior and three-dimensional structure. The continued advancement of OCT methods, such as optical coherence elastography (OCE) [4
] and magneto-motive OCT [5
], technologies, such as needle OCT [6
], and applications, such as intravascular OCT [7
], will depend upon the availability of phantoms simulating the optical and mechanical properties and the three-dimensional structure of tissues. Such phantoms are needed to form the basis for durable, transferable standard imaging targets.
In this paper, we review progress on the development of phantoms that simulate the optical and mechanical properties as well as the complex structures found in tissue, whilst concentrating on materials that provide durability of at least one month.
Many early OCT phantoms were based on hydrogels, two of the most common of which are agar [8
] and gelatin [9
]. These semisolid matrices allow for the inclusion of both organic and non-organic additives as optical scatterers. The tissue-like mechanical properties of these hydrogels have also been utilized in early OCE experiments [4
]. However, there are several major issues with hydrogel phantoms. They have a short durability on the order of a week and are not rigid at room temperature [1
], which makes forming them into complex shapes impractical. Resin phantoms, by contrast, are very durable; they can be used for years whilst maintaining their optical properties [10
] and have the potential to be fabricated into complex shapes. However, resin phantoms are much stiffer than soft tissue, which limits their utility in intravascular OCT [7
] and elastography [4
In this paper, we focus on phantoms based on three materials: silicone [13
]; fibrin [14
]; and poly(vinyl alcohol) cryogels (PVA-C) [15
], which we believe to be the best candidates for the development of versatile tissue-simulating phantoms.
Silicone is a convenient base material for flexible and straightforward fabrication of phantoms. It provides ready compatibility 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. Silicone is also well suited for fabrication of phantoms with complex structures due to its low viscosity prior to curing and high toughness, i.e., resistance to fracture.
A disadvantage is that silicone is not compatible with organic materials, such as tissue constituents. In simulating the optical properties of tissue, a convenient option is to employ material systems to which tissue constituents may be readily added [1
]. Fibrin phantoms [14
] meet this need, providing a transparent organic matrix to which both organic and inorganic scatterers and absorbers may be added. Fibrin is a naturally occurring protein in humans that provides structural support for blood clots [16
]. Fibrin is readily synthesised and has a shelf life of up to one month.
Poly(vinyl alcohol) cryogels (PVA-C), the other class of phantom materials we consider, have been used extensively to fabricate phantoms for other medical imaging modalities, particularly ultrasound and magnetic resonance imaging [15
]. PVA-C is especially attractive for its mechanical properties, which are readily controllable over the range found in tissue. In biomedical optics, PVA-C phantoms have been used for photoacoustic imaging [17
], diffuse optical imaging [18
] and optical elastography [19
]. However, their use in OCT has been less extensive [20
In the following, we review in detail the development of phantoms based on the three candidate material systems. The structure of the remainder of the paper is as follows: in Section 2, we review the optical properties of OCT phantoms, with particular emphasis on backscatter amplitude and attenuation; in Section 3, the mechanical properties of phantoms are discussed and reviewed in the context of both elasticity and viscoelasticity; in Section 4, the development of phantoms with complex shapes is reviewed; and in Section 5, we summarize and draw some conclusions.