Deep photoacoustic calcification imaging: Feasibility study

Calcification is one of the important indicators for early breast cancer detection. Although X-ray mammography is the well-established gold standard method for identifying microcalcifications (i.e., smaller than 0.5 mm), it is with ionizing radiation and thus there is inevitably carcinogenic risk. Ultrasound (US) imaging is a powerful adjunct to X-ray mammography for imaging of breast lesions. However, it is challenging for US imaging of microcalcifications because speckle noise results in low ultrasound contrast between breast tissues and micro-calcifications. The nonionizing radiation and speckle-free nature of photoacoustic (PA) imaging potentially overcomes the drawbacks of the above two tools. In this study, we explore the feasibility of deep PA calcification imaging based on a 5-MHz PA array imaging system. An optimal near-infrared excitation wavelength is investigated where the PA signal of granulated calcium hydroxyapatite (HA), which is the major chemical composition of micro-calcifications associated with malignant breast cancer, rivals that of blood. Intralipid gelatin and chicken breast phantoms with a blood tube and different-sized HAs (i.e., 0.3 mm, 0.5 mm and 1 mm) embedded are imaged to verify the imaging capability and penetration depth of calcifications. Experimental results demonstrated that PA imaging is capable of visualizing 0.3-0.5 mm HA particles at the depth of 30 mm. In the US image, HA particles are barely identified because of speckle noises. On the contrary, the HA particles and blood tube can be clearly imaged from the speckle free PA image. For the 0.3-mm HA particle at the depth of 30 mm, the signal-to-noise ratio is about 19 dB, indicating deeper penetration is feasible. Currently, the penetration depth is about adequate for imaging of breast calcifications, most of which appear at the depth from 15 mm to 35 mm. It is promising for PA imaging as a real-time diagnosis and biopsy guidance tool of breast micro-calcifications. Future work will focus on spectroscopic PA signal processing and PA/US dual-modal imaging for the differentiation of calcifications from blood.