Abstract
In this study, based on a medical ultrasound array imaging platform, we developed a high penetration photoacoustic (PA) array imaging system for visualization of breast calcifications. Phantom studies were used to verify the imaging capability and penetration depth of the developed PA system for calcification imaging. In our phantom studies, intralipid and chicken breast phantoms embedded with different-sized hydroxyapatite (HA) particles, major components of breast calcification associated with malignant breast cancer, were imaged. A laser at 700 nm was used for photoacoustic excitation and imaging was performed in sideward mode and backward mode. A PA transducer made by integrating the fiber bundle with the ultrasound transducer was applied for backward-mode photoacoustic signal detection; its configuration was optimized through Monte Carlo simulation. Currently, the axial, lateral, and elevational resolution of this developed PA array imaging system are 0.54 mm, 0.35 mm, and 1.25 mm, respectively, and its highest frame rate is 10 frames/sec, which is limited by the laser pulse rate. The image was reconstructed by delay-and-sum algorithm while contrast enhancement was performed by coherence factor weighting. Experimental results demonstrated that this system is capable of calcification imaging of 0.3 - 0.5 mm HA particles. For the 0.5 mm HA particles at depth of 3 cm, the imaging signal-to-noise ratio was about 14 dB, comparable to that of blood. We then developed a dual-modal PA and ultrasound imaging to further enhance the calcification imaging capability, which may facilitate needle biopsy guidance for clinical use. Additionally, we analyzed the signal-to-noise ratio of HA and calculated the capable detection depth of 0.5-mm HA in human breast at approximately 3.0 - 3.5 cm. This is compatible with clinical applications, as calcifications are usually found at a depth of 0.6 – 3.0 cm. Moreover, based on the distinct optical absorption spectra of blood and HA in the near infrared wavelength range, we can also use PA signals from two selected wavelengths to differentiate the HA from the blood. Future work will focus on further validation of photoacoustic imaging of clinical breast calcifications. In addition, we wish to distinguish between calcium oxalate (COD) and HA, which are the components of breast calcifications associated with benign and malignant breast tumor, respectively. We also want to provide blood-flow information on our image by using Doppler ultrasound.