High Acoustic Impedance and Attenuation Backing for High-Frequency Focused P(VDF-TrFE)-Based Transducers.

Backing materials with tailored acoustic properties are beneficial for miniaturized ultrasonic transducer design. Whereas piezoelectric P(VDF-TrFE) films are common elements in high-frequency (>20 MHz) transducer design, their low coupling coefficient limits their sensitivity. Defining a suitable sensitivity-bandwidth trade-off for miniaturized high-frequency applications requires backings with impedances of >25 MRayl and strongly attenuating to account for miniaturized requirements. The motivation of this work is related to several medical applications such as small animal, skin or eye imaging. Simulations showed that increasing the acoustic impedance of the backing from 4.5 to 25 MRayl increases transducer sensitivity by 5 dB but decreases the bandwidth, which nevertheless remains high enough for the targeted applications. In this paper, porous sintered bronze material with spherically shaped grains, size-adapted for 25-30 MHz frequency, was impregnated with tin or epoxy resin to create multiphasic metallic backings. Microstructural characterizations of these new multiphasic composites showed that impregnation was incomplete and that a third air phase was present. The selected composites, sintered bronze-tin-air and sintered bronze-epoxy-air, at 5-35 MHz characterization, produced attenuation coefficients of 1.2 and >4 dB/mm/MHz and impedances of 32.4 and 26.4 MRayl, respectively. High-impedance composites were adopted as backing (thickness = 2 mm) to fabricate focused single-element P(VDF-TrFE)-based transducers (focal distance = 14 mm). The center frequency was 27 MHz, while the bandwidth at -6 dB was 65% for the sintered-bronze-tin-air-based transducer. We evaluated imaging performance using a pulse-echo system on a tungsten wire (diameter = 25 µm) phantom. Images confirmed the viability of integrating these backings in miniaturized transducers for imaging applications.

Inkjet-printed P(VDF-TrFE) film for high-frequency annular array.

An innovative processing to deposit P(VDF-TrFE) film on silicon wafers by an inkjet printing method was used to fabricate high-frequency annular array prototype. This prototype has a total aperture of 7.3 mm and 8 active elements. A polymer-based lens with low acoustic attenuation was added to the flat deposition on the wafer, setting the geometric focus to 13.8 mm. With a thickness of around 11 µm, the electromechanical performance of P(VDF-TrFE) films was evaluated with an effective thickness coupling factor of 22%. Electronics allowing all elements to simultaneously emit as a single element transducer was developed. In reception, a dynamic focusing, based on eight independent amplifying channels, was preferred. The center frequency of the prototype was 21.3 MHz, the insertion loss was 48.5 dB and the -6 dB fractional bandwidth was 143%. The trade-off sensitivity/bandwidth has rather favored the large bandwidth. Dynamic focusing on reception was applied and allowed to improvements in the lateral-full width at half maximum as shown on images obtained with a wire phantom at several depths. The next step, for a fully operational multi-element transducer, will be to achieve a significant increase of the acoustic attenuation in the silicon wafer.

Quantification of muscle volume by echography: comparison with MRI data on subjects in long-term bed rest.

The objective of this study was to determine the accuracy of an echographic method for measuring the change in leg muscle volume against the gold standard, magnetic resonance imaging (MRI). Leg muscle volume was measured using an echographic scanner, which consisted of two metallic rails on which a probe holder moved via an electric engine. Ten to 20 transverse muscle views were collected along the area scanned, and the muscle cross-sectional area (CSA, cm2) was measured on each of them. The integration of all the CSAs along the scanned area provided the muscle volume (cm3). Echographic results were compared with MRI data on 24 subjects undergoing 60 d of bed rest (8 control "Con," 8 with exercise countermeasures "Ex" and 8 with nutrition countermeasures "Nut"). The vastus intermedius (VI) and the vastus medialis (VM) volumes decreased significantly and similarly in both Con and Nut (VI, -17%; VM, -21%; p < 0.02). In the Ex group, the VI and VM did not change significantly. The correlation coefficient between the muscle volume change measured with the echographic and MRI methods was 0.78. The present study confirms that the echographic scanner is sufficiently accurate for assessing muscle volume changes and detects the effect of exercise countermeasures on muscle volume during long-term bed rest.

Use of a robotic arm to perform remote abdominal telesonography.

OBJECTIVE: The purpose of this study was to design and validate a method of performing sonography between an expert center and an isolated site. A sonography probe is held on the patient by a robotic arm and remotely controlled from the expert's center. The robotic arm reproduces all the movements of the expert's dummy probe on the patient probe. The system was tested on 87 patients at our hospital. CONCLUSION: Robotic telesonography can be used for reliable diagnosis without moving the patient. No false diagnoses were made in this study. A bandwidth of 250 Kbps via integrated services digital network or satellite is required for reliable diagnosis. Such a system can provide diagnostic information that is currently unavailable in isolated or inaccessible areas and on rescue vehicles.

Vascular distribution of an ultrasound contrast agent used to simulate decompression bubbles.

OBJECTIVE: The objectives of this preliminary work were to evaluate the distribution of an ultrasound contrast agent (UCA) (microbubbles) in different arterial regions (brain, kidney, lower limbs) using the Doppler spectrum brightness analysis and to discuss the results in the context of decompression physiology. METHOD: There were four patients who instrumented with two pulsed Doppler sensors in order to monitor in real-time the spectrum of the middle cerebral femoral arteries. Renal arteries were investigated using an echo-Doppler probe handled by a sonographer. Measurements of the systolic mean, diastolic frequencies, qualitative, and quantitative analysis of the spectrum brightness intensity were performed before and after intravenous injection of a UCA. RESULTS: All of the systolic mean and diastolic arterial frequencies remained constant during the experiment. Some seconds after the first injection, the cerebral spectrum was heterogeneously enhanced with strong flashes over the entire spectrum. The renal and femoral spectrums were homogeneously reinforced. The spectrum brightness patterns did not change during the first 10 min. DISCUSSION: This work shows that the distribution of the UCA microbubbles within the vessel sections changed according to the distance from the heart, as suggested by the spectrum recorded at different sites. By mixing the blood, the heart could re-aggregate the UCA particles, even when they returned from the distal vascular regions where they were homogeneously distributed. The circulating microbubbles and their distribution within the arteries should be considered in decompression procedures with repetitive dives.

Calf venous volume during stand-test after a 90-day bed-rest study with or without exercise countermeasure.

The objectives to determine both the contribution to orthostatic intolerance (OI) of calf venous volume during a stand-test, and the effects of a combined eccentric-concentric resistance exercise countermeasure on both vein response to orthostatic test and OI, after 90-day head-down tilt bed-rest (HDT). The subjects consisted of a control group (Co-gr, n = 9) and an exercise countermeasure group (CM-gr, n = 9). Calf volume and vein cross-sectional area (CSA) were assessed by plethysmography and echography during pre- and post-HDT stand-tests. From supine to standing (post-HDT), the tibial and gastrocnemius vein CSA increased significantly in intolerant subjects (tibial vein, +122% from pre-HDT; gastrocnemius veins, +145%; P < 0.05) whereas it did not in tolerant subjects. Intolerant subjects tended to have a higher increase in calf filling volume than tolerant subjects, in both sitting and standing positions. The countermeasure did not reduce OI. Absolute calf volume decreased similarly in both groups. Tibial and gastrocnemius vein CSA at rest did not change during HDT in either group. During the post-HDT stand-test, the calf filling volume increased more in the CM-gr than in the Co-gr both in the sitting (+1.3 +/- 5.1%, vs. -7.3 +/- 4.3%; P < 0.05) and the standing positions (+56.1 +/- 23.7% vs. +1.6 +/- 9.6%; P < 0.05). The volume ejected by the muscle venous pump increased only in the CM-gr (+38.3 +/- 21.8%). This study showed that intolerant subjects had a higher increase in vein CSA in the standing position and a tendency to present a higher calf filling volume in the sitting and standing positions. It also showed that a combined eccentric-concentric resistance exercise countermeasure had no effects on either post-HDT OI or on the venous parameters related to it.