Stepped frequency ultrasound computed tomography with waveform inversion.

Routine US (ultrasound) scans for breast imaging run on a conventional console suffer from machine and operator dependence and are subject to personal interpretation. Recently, the framework of USCT (ultrasound computed tomography) has emerged as a safe, powerful and operator independent alternative to diagnostic US scans and x-rays mammography. The most known systems employ one circular array or a combination of transmitters and receivers by exploiting reflection, diffraction and transmission data. These systems are based on a pulsed transmission. Following propagation in tissue, the signals are usually recorded with a direct RF sampling scheme and stored as digital time-series. Image reconstruction is performed in the frequency domain in the 400 kHz-1 MHz bandwidth over a limited number of discrete frequencies. In this paper, we propose a new architecture based on the stepped frequency continuous waveform (SFCW) principle. In this scheme, the transmission is a continuous one and the received waveforms undergo a homo-dyne stage. By sequentially transmitting single tones at different frequencies, data can be collected directly in the frequency domain at specific frequencies, with programmable frequency steps and with any desired SNR. We describe in detail the transmitter and the receiver paths and compare with a conventional pulsed USCT architecture. Finally, we highlight the benefits of a SFCW-USCT device and comment on SNR, absorbed power, data fidelity and data storage.

Activity-Dependence of Synaptic Vesicle Dynamics.

The proper function of synapses relies on efficient recycling of synaptic vesicles. The small size of synaptic boutons has hampered efforts to define the dynamical states of vesicles during recycling. Moreover, whether vesicle motion during recycling is regulated by neural activity remains largely unknown. We combined nanoscale-resolution tracking of individual synaptic vesicles in cultured hippocampal neurons from rats of both sexes with advanced motion analyses to demonstrate that the majority of recently endocytosed vesicles undergo sequences of transient dynamical states including epochs of directed, diffusional, and stalled motion. We observed that vesicle motion is modulated in an activity-dependent manner, with dynamical changes apparent in ∼20% of observed boutons. Within this subpopulation of boutons, 35% of observed vesicles exhibited acceleration and 65% exhibited deceleration, accompanied by corresponding changes in directed motion. Individual vesicles observed in the remaining ∼80% of boutons did not exhibit apparent dynamical changes in response to stimulation. More quantitative transient motion analyses revealed that the overall reduction of vesicle mobility, and specifically of the directed motion component, is the predominant activity-evoked change across the entire bouton population. Activity-dependent modulation of vesicle mobility may represent an important mechanism controlling vesicle availability and neurotransmitter release.SIGNIFICANCE STATEMENT Mechanisms governing synaptic vesicle dynamics during recycling remain poorly understood. Using nanoscale resolution tracking of individual synaptic vesicles in hippocampal synapses and advanced motion analysis tools we demonstrate that synaptic vesicles undergo complex sets of dynamical states that include epochs of directed, diffusive, and stalled motion. Most importantly, our analyses revealed that vesicle motion is modulated in an activity-dependent manner apparent as the reduction in overall vesicle mobility in response to stimulation. These results define the vesicle dynamical states during recycling and reveal their activity-dependent modulation. Our study thus provides fundamental new insights into the principles governing synaptic function.