Elastography can noninvasively map the elasticity distribution in biological tissue, which can potentially be used to reveal disease conditions. In this paper, we have demonstrated photoacoustic elastography by using a linear-array photoacoustic computed tomography system. The feasibility of photoacoustic elastography was first demonstrated by imaging the strains of single-layer and bilayer gelatin phantoms with various stiffness values. The measured strains agreed well with the theoretical values, with an average error of less than 5.2%. Next,

Elastic properties of biological tissues can reflect pathological conditions [

Photoacoustic (PA) tomography (PAT) is a hybrid imaging technique that combines rich optical absorption contrast and high ultrasonic spatial resolution [

The photoacoustic elastography was developed based on a linear-array PACT system [

In our photoacoustic elastography system, an aluminum compression plate larger than the object exerted a small axial compressive force on the object [

Here, σ is the compression stress, g is the acceleration of gravity, m_{a} and m_{b} are the scale readings before and after compression, and A is the area on which the compression force is applied.

To demonstrate quantitative elasticity measurement, photoacoustic elastography was first used to image four homogeneous gelatin phantoms with respective gelatin concentration of 40, 60, 80, and 100 g/L. To provide absorption contrast for photoacoustic imaging, 50 μm microspheres were mixed in the gelatin phantoms at a concentration of ~5 microspheres per mm^{3}. Each gelatin phantom was imaged with the photoacoustic elastography system before and after compression with an external stress of 53 Pa [

Here, ε is the strain of the gelatin phantom, σ is the stress applied to the phantom, K is a constant factor, and C is the gelatin concentration. Note K is affected by the equilibrium temperature, the temperature and duration of the gelatin mixing process, and the molecular weight of gelatin.

Photoacoustic elastography was then used to image a bilayer gelatin phantom with different gelatin concentrations in each layer. The top layer had a gelatin concentration of 50 g/L and a thickness of 2.5 mm.^{3} The bottom layer had a gelatin concentration of 100 g/L and a thickness of 2.0 mm. Again, 50 μm microspheres were mixed in the gelatin phantom at the concentration of 5 microspheres per mm^{3}. The bilayer phantom was imaged by the photoacoustic elastography system before and after compression with a stress of 98 Pa [

A mouse leg was then imaged

Compared to previous studies, our photoacoustic elastography technique based on a linear-array photoacoustic computed tomography has the following distinctive features [

In summary, we have demonstrated photoacoustic elastography on gelatin phantoms and

We would like to point out that the motivation of this work is not to prove that photoacoustic elastography is superior to ultrasound elastography. Instead, the major motivation is to demonstrate the feasibility of elasticity measurement by using PAT as an independent device: Not all the photoacoustic imaging systems have the capability of ultrasound transmission, and thus ultrasound elastography is not always available. Photoacoustic elastography can be implemented on existing photoacoustic imaging systems, as an additional function, to provide more comprehensive information about the tissue’s mechanical and functional information.

Further, photoacoustic tomography can potentially measure elasticity concurrently with other functional parameters, including the oxygen saturation of hemoglobin, which may provide more comprehensive information for disease diagnosis and treatment evaluation [

The authors appreciate Prof. James Ballard’s close reading of the manuscript. L. V. Wang has a financial interest in Endra, Inc., and Microphotoacoustics, Inc., which, however, did not support this work.

Schematic of the photoacoustic elastography system. (a) Side view of the photoacoustic elastography system. (b) Top view of the compression plate with the imaging window at the center.

Strain measurement on single-layer gelatin phantoms by photoacoustic elastography. (a–b) Cross-sectional PA images of a gelatin phantom (40 g/L gelatin concentration) mixed with 50 μm microspheres acquired (a) before and (b) after compression. (c) Displacement image obtained from (a) and (b). (d) Average displacement versus the depth. The strain of the phantom was estimated as the slope of the linear fitting. (e) Measured strains of gelatin phantoms with 4%, 6%, 8%, and 10% concentration in weight. The data was fitted to a quadratic model that describes the relationship between the strain and the gelatin concentration [

Strain measurement of a bilayer gelatin phantom by photoacoustic elastography. (a–b) PA images of a bilayer gelatin phantom mixed with 50 μm microspheres acquired (a) before and (b) after compression. (c) Displacement image obtained from (a) and (b). (d) Average displacement versus depth. The data was fitted by a linear function for each layer.

Photoacoustic elastography of a mouse leg