Volume 117, Issue 1, January 2005
Index of content:
- BIOACOUSTICS 
117(2005); http://dx.doi.org/10.1121/1.1810191View Description Hide Description
Ultrasonic backscattered signals contain frequency-dependent information that is usually discarded to produce conventional B-mode images. It is hypothesized that parametrization of the quantitative ultrasound frequency-dependent information (i.e., estimating scatterer size and acoustic concentration) may be related to discrete scattering anatomic structures in tissues. Thus, an estimation technique is proposed to extract scatterer size and acoustic concentration from the power spectrum derived from a three-dimensional impedance map (3DZM) of a tissue volume. The 3DZM can be viewed as a computational phantom and is produced from a 3D histologic data set. The 3D histologic data set is constructed from tissue sections that have been appropriately stained to highlight specific tissue features. These tissue features are assigned acoustic impedance values to yield a 3DZM. From the power spectrum, scatterer size and acoustic concentration estimates were obtained by optimization. The 3DZM technique was validated by simulations that showed relative errors of less than 3% for all estimated parameters. Estimates using the 3DZM technique were obtained and compared against published ultrasonically derived estimates for two mammary tumors, a rat fibroadenoma and a 4T1 mouse mammary carcinoma. For both tumors, the relative difference between ultrasonic and 3DZM estimates was less than 10% for the average scatterer size.
Controlled ultrasound tissue erosion: The role of dynamic interaction between insonation and microbubble activity117(2005); http://dx.doi.org/10.1121/1.1828551View Description Hide Description
Previous studies showed that ultrasound can mechanically remove tissue in a localized, controlled manner. Moreover, enhanced acoustic backscatter is highly correlated with the erosion process. “Initiation” and “extinction” of this highly backscattering environment were studied in this paper. The relationship between initiation and erosion, variability of initiation and extinction, and effects of pulse intensity and gas saturation on time to initiation (initiation delay time) were investigated. A 788-kHz single-element transducer was used. Multiple pulses at a 3-cycle pulse duration and a 20-kHz pulse repetition frequency were applied. values between 1000 and 9000 W/cm2 and gas saturation ranges of 24%–28%, 39%–49%, and 77%–81% were tested. Results show the following: (1) without initiation, erosion was never observed; (2) initiation and extinction of the highly backscattering environment were stochastic in nature and dependent on acoustic parameters; (3) initiation delay times were shorter with higher intensity and higher gas saturation (e.g., the mean initiation delay time was 66.9 s at of 4000 W/cm2 and 3.6 ms at of 9000 W/cm2); and (4) once initiated by high-intensity pulses, the highly backscattering environment and erosion can be sustained using a significantly lower intensity than that required to initiate the process.
117(2005); http://dx.doi.org/10.1121/1.1828851View Description Hide Description
Auditory systems of cetaceans are considered highly specialized for underwater sound processing, whereas the extent of their hearing capacity in air is still a point of issue. In this study, the sensitivity to airborne sound in a male tucuxi (Sotalia fluviatilis guianensis) was tested by means of a go/no go response paradigm. Auditory thresholds were obtained from 2 to 31.5 kHz. Compared to the hearing thresholds of other dolphins as well as of amphibian mammals, the sensitivity to airborne sound of the test subject is low from 2 to 8 kHz, with the highest threshold at 4 kHz. Thresholds at 16 and 31.5 kHz reveal a sharp increase in hearing sensitivity. Thus, although not obtained in this study, the upper aerial hearing limit is in the ultrasonic range. A comparison of the present data with the underwater audiogram of the same test subject referred to sound intensity indicates that the sensitivity of Sotalia to underwater sound is generally better than to airborne sound.
Are high perches in the blackcap Sylvia atricapilla song or listening posts? A sound transmission study117(2005); http://dx.doi.org/10.1121/1.1828805View Description Hide Description
Birds often sing from high perches referred to as song posts. However, birds also listen and keep a lookout from these perches. We used a sound transmission experiment to investigate the changes for receiving and sending conditions that a territorial songbird may experience by moving upwards in the vegetation. Representative song elements of the blackcap Sylvia atricapilla were transmitted in a forest habitat in spring using a complete factorial design with natural transmission distances and speaker and microphone heights. Four aspects of sound degradation were quantified: signal-to-noise ratio, excess attenuation, distortion within the sounds determined as a blur ratio, and prolongation of the sounds with “tails” of echoes determined as a tail-to-signal ratio. All four measures indicated that degradation decreased with speaker and microphone height. However, the decrease was considerably higher for the microphone than for the speaker. This suggests that choosing high perches in a forest at spring results in more benefits to blackcaps in terms of improved communication conditions when they act as receivers than as senders.
117(2005); http://dx.doi.org/10.1121/1.1823213View Description Hide Description
Simulations of iterative transmit-beam aberration correction using a time-delay and amplitude filter have been performed to study the convergence of such a process. Aberration in medicalultrasonic imaging is usually modeled by arrival-time and amplitude fluctuations concentrated on the transducer array. This is an approximation of the physical aberration process, and may be applied to correct the transmitted signal using a time-delay and amplitude filter. Estimation of such a filter has proven difficult in the presence of severe aberration. Presented here is an iterative approach, whereby a filter estimate is applied to correct the transmit-beam. This beam induces acoustic backscatter better suited for arrival-time and amplitude estimation, thus facilitating an improved filter estimate. Two correlation-based methods for estimating arrival-time and amplitude fluctuations in received echoes from random scatterers were employed. Aberration was introduced using eight models emulating aberration produced by the human abdominal wall. Results show that only a few iterations are needed to obtain corrected transmit-beam profiles comparable to those of an ideal aberration correction filter. Furthermore, a previously developed focusing criterion is found to quantify the convergence accurately.