Beamforming for large-area scan and improved SNR in array-based photoacoustic microscopy.

Beamforming enhances the performance of array-based photoacoustic microscopy (PAM) systems for large-area scan. In this study, we quantify the imaging performance of a large field-of-view optical-resolution photoacoustic-microscopy system using an phased-array detector. The system combines a low-cost pulsed-laser diode with a 128-element linear ultrasound probe. Signal-to-noise ratio (SNR) and generalized contrast-to-noise ratio (gCNR) are quantified using the phased-array detector and applying three beamforming strategies: a no-beamforming method equivalent to a single-element flat transducer, a fixed focus beamforming method that mimics a single-element focused transducer, and a dynamic focus beamforming using a delay-and-sum (DAS) algorithm. The imaging capabilities of the system are demonstrated generating high-resolution images of tissue-mimicking phantoms containing sub-millimetre ink tubes and an ex vivo rabbit's ear. The results show that dynamic focus DAS beamforming increases and homogenizes SNR along 1-cm2 images, reaching values up to 15 dB compared to an unfocused detector and up to 30 dB compared to out-of-focus regions of the fixed focus configuration. Moreover, the obtained values of gCNR using the DAS beamformer indicate an excellent target visibility, both on phantoms and ex vivo. This strategy makes it possible to scan larger surfaces compared to standard configurations using single-element detectors, paving the way for advanced array-based PAM systems.

Nonlinear change of on-axis pressure and intensity maxima positions and its relation with the linear focal shift effect.

A comprehensive experimental, analytical and numerical study of the true focal region drift relative to the geometrical focus (focal shift effect) in acoustic focused beams and its nonlinear evolution is presented. For this aim, the concept of Fresnel number, proportional to the linear gain, is introduced as a convenient parameter for characterizing focused sources. It is shown that the magnitude of the shift is strongly dependent on the Fresnel number of the source, being larger for weakly focused systems where a large initial shift occurs. Analytical expressions for axial pressure distributions in linear regime are presented for the general case of truncated Gaussian beams. The main new contribution of this work is the examination of the connection between the linear and nonlinear stages of the focal shift effect, and its use for the estimation of the more complicated nonlinear stage. Experiments were carried out using a continuous-wave ultrasonic beam in water, radiated by a focused source with nominal frequency f=1 MHz, aperture radius a=1.5 cm and geometrical focal distance R=11.7 cm, corresponding to a Fresnel number N(F)=1.28. The maximum measured shifts for peak pressure and intensity were 4.4 and 1.1cm, respectively. The evolution of the different maxima with the source amplitude, and the disparity in their axial positions, is interpreted in terms of the dynamics of the nonlinear distortion process. Analytical results for the particular case of a sound beam with initial Gaussian distribution are also presented, demonstrating that the motion of peak pressure and peak intensity may occur in opposite directions.

Telomerase and telomere dynamics in ageing and cancer: current status and future directions.

This review will focus on the clinical utilities of telomerase for human cancer diagnosis and prognosis. Much attention has been focused on control of telomerase activity in early and late stage tumours. Telomerase stabilisation may be required for cells to escape replicative senescence and to proliferate indefinitely. Because of a very strong association between telomerase and malignancy, both clinicians and pathologists expect this molecule to be a useful diagnostic and prognostic marker and a new therapeutic target. These data have greatly inspired the development of various strategies to target telomere and telomerase for cancer therapy. Finally, evidence is now emerging that G-quadruplex ligands produce rapid senescence and selective cell death. A summary of recent experimental works with new small molecules as potential inhibitors of telomerase is presented.

Telomerase and telomere dynamics in ageing and cancer: current status and future directions

This review will focus on the clinical utilities of telomerase for human cancer diagnosis and prognosis. Much attention has been focused on control of telomerase activity in early and late stage tumours. Telomerase stabilisation may be required for cells to escape replicative senescence and to proliferate indefinitely. Because of a very strong association between telomerase and malignancy, both clinicians and pathologists expect this molecule to be a useful diagnostic and prognostic marker and a new therapeutic target. These data have greatly inspired the development of various strategies to target telomere and telomerase for cancer therapy. Finally, evidence is now emerging that G-quadruplex ligands produce rapid senescence and selective cell death. A summary of recent experimental works with new small molecules as potential inhibitors of telomerase is presented (AU)

Self-pulsing dynamics of ultrasound in a magnetoacoustic resonator.

A theoretical model of parametric magnetostrictive generation of ultrasound is considered, taking into account magnetic and magnetoacoustic nonlinearities. The stability and temporal dynamics of the system is analyzed with standard techniques revealing that, for a given set of parameters, the model presents a homoclinic or saddle-loop bifurcation, which predicts that the ultrasound is emitted in the form of pulses or spikes with arbitrarily low frequency.