The relationship between center frequency downshift and transmitted bandwidth was investigated for a pulse with Gaussian amplitude spectrum propagating through a lossy medium with power law frequency dependence of attenuation. Tissue equivalent material was characterized by multiple narrowband attenuation measurements, via a substitution method. Power law curves were fitted to the data. The parameters of the curves were used to predict the behavior of the center frequency downshift vs. transmitted bandwidth obtained from a second experiment. The results confirm the mathematical model.
In applications requiring a liquid which is acoustically well matched to biological tissues, it is often difficult to find a material which is matched well in terms of both the acoustic impedance and speed of sound propagation in it; changing one parameter invariably affects the other. A three component liquid system is described, which allows independent adjustment of these two acoustic parameters over a wide range. This range encompasses the soft tissues of the body. Results of parameter measurements are presented in the form which allows simple determination of the mixture required to match any combination of acoustic impedance and speed of sound propagation over a given range.
A novel ultrasonic ring transducer and special control electronics have been developed for scattering and imaging studies. The transducer contains 2048 rectangular elements with a center frequency of 2.4 MHz and a −6-dB bandwidth of 70%. At the center frequency, the element size is 0.29 wavelength ×40 wavelength and the spacing is 0.37 wavelength. A multiplexer provides access to any contiguous 128 elements for transmission and any contiguous 16 elements for simultaneous reception. The transmit electronics have independently programmable waveforms. The receive electronics have time-varied gain functions independently programmable over the range 15–55 dB. Each receive channel includes a 20-MHz, 12-bit A/D converter. The electronics permit synthesis of arbitrary transmit and receive apertures. A novel ultrasonic wavefront design method has been implemented to determine element excitations using backpropagation of a user-specified field pattern. Pulse-echo compound images using constant f/1.0 transmit and receive apertures have been obtained for model scattering objects and an anthropomorphic breast phantom. Scattering measurements have been analyzed to obtain frequency- and angle-dependent average differential scattering cross sections of random media. The system is a useful facility for measurements of ultrasonic scattering for characterization of tissue, development of adaptive beam-formation techniques, and implementation of quantitative image reconstruction methods.
Experimental work using a quantitative C-scan technique to measure in situ enhancement of hepatic backscatter resulting from the administration of gelatin microspheres to dogs is reported. We have found that reproducible enhancement of hepatic backscatter on the order of 2 dB followed the peripheral administration of gelatin within minutes, the magnitude of the effect was not sensitive to the rate of infusion, but was likely to be dose dependent, changes in the overlying tissue attenuation had a pronounced effect on the measurement, and the effect was sustained for up to one half hour or longer.
Minimally invasive, miniature (2.2- × 50-mm aperture, 3.3-mm diameter) dual-mode linear arrays have been developed into low-cost disposable probes with high acoustic power output (120 W/cm2 at the source), high transmit efficiency (>65% typical), and good imaging performance (50% fractional bandwidth, >100-mm-deep field of view). These therapy/imaging probes have been integrated into a flexible intense ultrasound surgery platform which also includes conventional diagnostic imaging probes. A system architecture has been developed which includes a 64-channel therapy driver with software selection of array aperture and phasing (λ/16), frequency (0.5–8 MHz), drive amplitude (5 W/channel, nominal), rotational steering (±180 deg), and temporal sequencing/switching of imaging/therapy/monitoring modes. System software includes graphical and text-based script mode control of therapeutic treatment. Real-time monitoring of electric power per channel, temperature sensors, and thermal effects provide a range of feedback and safety. Numerous system and probe technological issues such as electrical interconnect and matching, acoustic coupling, thermal control, and maintaining probe efficiency have been addressed. The array-based imaging/therapy system has produced encouraging results in preclinical studies of bulk tissue ablation and imaging.
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