Nonlinear photoacoustic and optical properties of hydroxyapatite and calcium phosphate. Towards a new method for the densitometry of bones

Abstract

Abstract In this work, a new bone densitometry method is disclosed based on the photoacoustic response of chicken bones under the illumination of a continuous blue wave laser beam. First, calcium phosphate and hydroxyapatite are dispersed in an aqueous solution and hemoglobin, as the main optical-sensitive component of the blood, is added to this solution to simulate the real in vivo conditions around the bone. Herein, we used a technique which combines the photoacoustic Z-scan and conventional optical Z-scan, which is called OPAZ-scan. Then, the obtained photoacoutic and optical transmission signals are measured simultaneously in a single experiment. The performed OPAZ-scan results showed that the origin of PA signals has been due to the hemoglobin; but, the frequency and amplitude of the generated PA signals are directly dependent to the bone density. Thereafter, the experiments are repeated on real chicken bone samples and the results are compared. Studies are performed on three chicken bone samples including keel, tibia, and humerus bones. Interestingly, the outputs of nonlinear optical and PA response of the bone samples are well in accordance with those attained with the artificial above mixtures. In addition, a direct relation is found between the bone density and frequency of the generated photoacoustic sounds that can be used as a facile and reliable method to the bone densitometry and evaluation of the mineral content of bones. The optical absorption, FWHM, amplitude, and peak of PA signals are studied for the bones and their dependencies to the bone mineral content are studied. Using a numerical curve fitting, values of nonlinear photoacoustic indices are obtained. Moreover, the main frequency and amplitude of the generated PA signals are directly dependent to the bone density. Also, keel bone with more pores shows a wider range of the amplified PA signals. We hope to extend the results of this study for humans by using simple X-ray photos and without needing to any further pre-treatments on bones under the in vivo conditions.