In many tissue pathologies such as fibrosis, edema and cancers in breast, liver and prostate, change of mechanical properties [1
] is a common phenomenon observed by clinicians or patients themselves using hand palpation. However, the hand palpation method is qualitative, at most semi-quantitative (such as the scoring system from the palpation impression), thus limiting its use in quantitative and objective studies. In the last two decades, in order to find optimal modalities for disease diagnosis and tissue assessment, more and more researchers have put efforts in developing quantitative and objective approaches in the field of elasticity measurement and imaging based on ultrasound [5
], MRI [16
] or optical [17
] measurements with the help of some kind of mechanical disturbances caused to the tissues including compression, indentation, suction, vibration or acoustic radiation.
Indentation is currently one of the most frequently used techniques to measure the biomechanical properties of soft tissues [23
]. Indentation, due to a small contact area with the tested objective, has the advantages of no necessity to excise the particular tissue, which is almost impossible for a standard uniaxial compression test, thus allowing its especial use for in vivo
applications. A traditional indentation system used a rigid indenter to compress the tissue in order to characterize the mechanical properties of soft tissue using the relationship between force and deformation. However, usually no thickness information could be obtained directly from the indentation test. Tissue thickness is an important parameter for the diagnosis of some tissue pathologies such as cartilage degeneration [25
], and is also an important factor for the calculation of tissue stiffness because this is included in the boundary condition of the theoretical analysis of the indentation test [23
]. The thickness was usually measured by extra post-test methods such as the needle probe penetration [26
]. To address this issue, Zheng and Mak [7
] developed a portable ultrasound indentation system, which used the ultrasound transducer itself as an indenter. The system is capable of measuring both the initial thickness and stiffness. Due to its easy operation and a relatively compact profile fit for clinical operations, this ultrasound indentation system has been successfully applied in the assessment of a variety of tissues in vivo
, including muscular tissues [27
], residual limb tissues [28
], diabetic foot plantar tissues [29
], neck tissue fibrosis induced by radiotherapy [30
], hypertrophic scar tissues [32
] and carpal tunnel ligament [34
]. Similar ultrasound indentation has also been used for the assessment of articular cartilage [35
Typical ultrasound indentation uses an ultrasound transducer with the central frequency of 2–10 MHz for which the resolution is quite limited for small specimens such as the skin and articular cartilage. For transducers with a higher frequency, the end is normally concave for better energy focus and larger penetration depth. In this case, the transducer tip cannot be used directly as an indenter. Introduction of a bolster at the transducer tip for a planar indentation surface may possibly induce a poor coupling of the ultrasound signal and significantly attenuate the ultrasound signal, which severely affects the ultrasound measurement [36
]. In addition, a rigid indenter is limited in achieving a fast scanning speed because of the requirement of pointwise measurement. To address these issues, a water jet indentation system was developed [37
]. In this system, the water jet serves as both the indenter and the coupling medium, thus significantly improving the speed in a C-scan test, i.e. the imaging plane is perpendicular to the ultrasound beam [38
]. Phantom tests showed that this system was capable of measuring the soft tissue elasticity quantitatively and reliably and also providing a fast C-scan mapping of the tissue elasticity. It was further applied for the study of cartilage degeneration in a bovine patella model [39
] and bone–tendon junction healing in a rabbit model [40
] in vitro
. The noncontact fluid indentation method also has the advantage of lowering the potential or risk of causing damage to the tested soft tissue, especially in tissues such as in scars or wounds where the contact indenter may cause inflammatory effects. Almost at the same time, another group reported a similar system using a water jet for the assessment of articular cartilage [19
]. They used an optical technique to detect the deformation applied by the water jet on the cartilage surface. Briefly, an optical beam with a constant intensity was illuminated onto the cartilage surface, and reflecting light was collected and its intensity was related to the local surface curvature, which was caused by the compression of the water jet.
One inconvenience in using the water jet indentation is that the water will spill all over during the test. If it is used for tissues located inside a body, the water or saline needs to be removed by another instrument. Another important factor for the widespread applications of prototype biomedical instrumentations to clinical situations is the potential of device miniaturization [41
]. Even though miniaturization of ultrasound transducers is possible, such as those used for an intra-vascular ultrasound imaging system, it is very expensive and the resolution is inherently limited. The water jet optical measurement system [19
] used the intensity of the reflecting light to measure tissue deformation, but it is not accurate, particularly when the deformation is large or the tissue surface has different colors and textures.
In various optical methods, optical coherence tomography (OCT) is a recent and fast developing technique that has acquired more and more widespread use in biomedical research [42
]. The principle of OCT itself is analogous to that of pulse-echo ultrasound imaging. It collects backscattered signals from the optically scattering tissues for the purpose of cross-sectional imaging. The difference with respect to ultrasound is that OCT uses the optical interferometric rather than the absolute time of flight technique to resolve the spatial information. OCT uses an optical interferometric method with a small coherence length of the light source and can achieve a high resolution. Axial and lateral resolutions in a range of several microns can be achieved, thus making the optical biopsy a unique and attractive characteristic of this technique [43
]. Similar to the ultrasound signals, the OCT can measure the thickness of tissue layers where normally at the interfaces the refractive index is abruptly changed. For example, one of the most important applications of OCT is pachymetry, i.e., to measure the corneal thickness in vivo
for the use of diagnosis or surgery guide [46
]. OCT-based elasticity imaging for tissues has also been widely investigated using contact compression [18
]. In comparison with ultrasound elastography, the OCT-based technique can provide higher resolutions. However, these elasticity imaging techniques can normally provide contrast of local strains but not an absolute value of tissue elasticity.
The integration of an OCT probe with the air jet indentation was realized in the current study to develop a novel system to measure the mechanical properties of soft tissues. First, the construction of the system is described in the next section, then experimental and data analysis methods on phantoms and in vivo hand soft tissues, and the corresponding results are presented to demonstrate the utility of the current system, and finally further issues related to the limitations, further improvement and applications of the current system in biomedical engineering are discussed before the study is concluded.