An 890-nm spectral-domain OCT system with a 20-kHz acquisition rate, as shown in Fig. 1, was used for vibration detection. The axial and lateral resolutions of the system were measured to be 3.5 and 14.8 μm, respectively. The signal-noise-ratio (SNR) of the system was 100 dB with 650 μW of sample arm power and a 50-μs A-line rate. The minimum detectable phase for this system was measured to be 1.5 mrad, which corresponded to a velocity sensitivity of 2.13 μm/s. For the purposes of elastography, the sample was placed on the surface of a focused ultrasonic transducer, where the acoustic force was measured as roughly uniform by a highly accurate hydrophone. The transducer featured a resonant frequency of 4 MHz and was driven by a square wave modulated RF signal (50% duty cycle amplitude modulation) to generate periodical acoustic radiation pushes in the longitudinal direction on to the sample through a thin layer of ultrasound gel. The modulation frequency and amplitude were chosen for each sample in such a way that the phase difference induced between adjacent A-lines was large, to enhance the sensitivity, but less than , to avoid phase wrapping. The phase difference was obtained from consequent A-lines while the system was scanning continuously. The oversampled data points were averaged by a factor of 4 and then used to calculate the phase shift for each A-line. From the phase shift information, the instantaneous axial velocity can be extracted as Display Formula
(1)where the light source center wavelength is 890 nm, and the adjacent A-line time interval is 50 μs. and represent the lateral and axial location, denotes the phase shift between two adjacent A-lines, and is the tissue refraction index, which is assumed to be 1.4 in this paper. Within a certain customized time window, the displacement and the axial strain of the sample are expressed asDisplay Formula
(3)where is the original thickness of the sample prior to ultrasonic stimulation. Young’s modulus is an important parameter that characterizes the elastic properties of the sample. It is defined by Display Formula
(4)where is the axial (or normal) stress (force per area) acting on the sample, is the acoustic radiation force out of the ultrasonic transducer, and is the sample surface area. Since the acoustic radiation force was uniform and the OCT probe beam size is much less than the ultrasound beam waist, the strain applied in the experiment is small enough (less than 0.1) that the material can be assumed as elastic, and Eq. (4) can be used as the first-order approximation to quantify the relative dynamic Young’s modulus in our dynamic OCE method. In this experiment, ARF-OCE of both the phantom and the human coronary artery samples were constructed while the samples were under vibration excitation.