Optical resolution photoacoustic microscopy (OR-PAM) provides high optical contrast and lateral resolution. However, the resolution of a typical OR-PAM using an objective lens is limited to not exceeding Abbe's optical diffraction limit. In this study, a lensless shear force scanning PAM is presented. Instead of a lens, the system uses an imaging probe that combines a non-coated tapered fiber with a quartz tuning fork (QTF). A shear force feedback mechanism is used to maintain a tens of nm (near-field) distance between the fiber tip and the sample. With the system, PA signals generated in the near field of a gold sputtered glass sample were successfully acquired. We also performed 2D PA scanning experiments and obtained PA images of gold cube samples with high lateral resolution. This study demonstrates the existence of a near field PA signal and shows its potential for super-resolution scanning PAM.
Sharing the same acoustic imaging principle, photoacoustic (PA) imaging is available to be done simultaneously with ultrasound (US) imaging, while still discrepancy between two modalities exists as PA imaging relatively suffers with higher depth attenuation and low signal contrast compared to the US imaging. In order to balance the PA to the level of US, we designed an application-specific integrated circuit (ASIC) preamplifier built in with selective switching protocol to amplify only the PA signal. The preamplifier accepts two distinct triggers: synchronized to the Q-switch trigger given with actual beam emission, preamplifier switches and amplifies the PA signal with customized gain. On the other hand, within the flashlamp trigger given before the Q-switch trigger (approx. 300 μs), the preamp is switched off and the US acquisition bypasses amplification. The preamp affords single channel, and we implemented the designed preamplifier to the acoustic-resolution 3D PAUS scanner installed with 5-MHz single-element focused transducer. The signal-to-noise ratio (SNR) and according penetration depth enhancement was well validated by imaging both carbon leads and wire phantoms under optically turbid media (5% diluted milk), measured to be ~ 16.8 dB at 25 mm and 10.6 dB at 33.7 mm, respectively. To fully demonstrate the improvement of PA images under practical circumstance, the bimodal whole-body image of a healthy anesthetized nude Balb/c mouse was acquired with and without the preamplifier. As a result, the organs (spleen, liver, cecum) and vasculatures lying down in the deeper region were unveiled from preamplifier-applied PA images. Above all, our proposed switchable preamplifier well preserved PA signal of weak level against sequentially acquired with high intensity US signals, extending the penetration depth and increasing the PA image contrast. Further extension toward multichannel application would be helpful to translate array US transducer-based simultaneous PA/US imaging toward clinical practice.
Calcium signaling is a second messenger that triggers physiological changes at the cellular level, such as proliferation, differentiation, or apoptosis. It was recently found that calcium dynamics play a vital role in many studies, including cancer, Alzheimer’s disease, and Parkinson’s disease, and we studied how intracellular signaling pathways work by ultrasound mechanotransduction. However, since ultrasound mechanotransduction does not yet have many experimental results by the quantified ultrasound parameters, little is known about the mechanism between ultrasound parameters and calcium dynamics. We investigate calcium level changes using different frequencies of ultrasound to study intracellular signal pathways of fibroblasts, which may function as one of the contributing factors of tissue repair. We quantified a few major ultrasound stimulation parameters, i.e., operating frequency, beam width, and acoustic pressure. Three 40 MHz ultrasound transducers with different f-numbers (0.8, 1.0, and 1.5) were designed and fabricated. During the cell stimulation, ultrasound waves with different frequencies (36, 45, and 69 MHz) but the same beam width and same acoustic pressure were exerted on the cells. The cell lines used were NIH/3T3 fibroblasts. At the fixed acoustic pressure and beam width, intracellular calcium level increased more rapidly at higher frequencies, which shows that the intracellular signal pathways of fibroblasts may be mainly dependent upon the frequency used for stimulation.
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