Nonlinear native propagation effects of diagnostic ultrasound computed and measured in blood.

Nonlinear propagation effects produced by focused pulses in blood were measured over a 20-cm range, being inspired by diagnostic applications in cardiology. The initial and maximum pressures applied during measurements in blood were equal to 0.40 MPa(pp) and 0.76 MPa(pp), while the pressure estimated at the patient body surface equalled 0.70 MPa(pp). Measurements of the frequency characteristic and the linearity of the ultrasonic probe used in experiments were performed in water. A numerical procedure developed previously was applied in blood to calculate the pressure distribution of its first and second harmonics along the beam axis. The comparison of numerical and measured distributions in blood at a temperature of 37 degrees C showed rather good agreement. Using numerical methods, a proportional growth of the second harmonic with the increased applied initial pressure was first observed, and finally the maximum limiting effect was found. In this way, much higher level of harmonics could be obtained. However, there arise the questions of the transmitting system construction and of the nonuniform resolution in the case of harmonic imaging when increasing the applied initial pressure.

Sensitivity of ultrasonic hydrophone probes below 1 MHz.

Frequency responses of different PVDF polymer hydrophone probes, including membrane and needle designs, were measured and are presented in terms of end-of-cable voltage sensitivity versus frequency over a wide, 4.5 octave bandwidth ranging from 0.25 to 2.5 MHz. The probes are seldom, if at all, characterized in this frequency range due to the difficulties associated with a lack of adequate and readily implementable calibration techniques. To this end, a technique, which uses a combination of swept frequency chirp and reciprocity, so that both the relative and absolute plots of sensitivity versus frequency can be obtained, was developed and tested. Salient features of the technique including the design of a 6 octave auxiliary acoustic source are described. The experimental data indicate that a majority of the PVDF membrane hydrophones exhibit a relatively uniform (to within +/- 2 dB) response. While, in general, this is not the case for commercially available needle hydrophone probes, it is evidenced that a careful attention to the PVDF probe design results in frequency characteristics fairly close to those achievable with a membrane design. The overall uncertainty of the calibration technique was estimated to be better than +/- 1.5 dB in the considered frequency range. The results of this work are important to implement procedures for adequate determination of the Mechanical Index (MI) of ultrasound imaging devices. MI is widely accepted as a predictor of potential bioeffects associated with cavitation phenomena. Current efforts are focused on extending the applicability of the technique to frequencies below 100 kHz.

PSpice modelling of ultrasound transducers: comparison of software models to experiment.

This paper presents a complete PSpice model of an ultrasound single-element transducer, including electrical and mechanical matching as well as the focusing lens. By using this model, it is possible to obtain a relation between the electrical driving source and the acoustic velocity on the transducer surface. This boundary condition then allows the acoustic field to be calculated by numerical methods. Experimental data obtained with two different transducers are in good agreement with results predicted by the related models.

The diagnosis of pleural effusion by ultrasonic and radiologic techniques.

The value of the A-mode ultrasonic technique and the radiologic method in the diagnosis of pleural effusion was assessed in 116 patients with diseases of the pleura. Ultrasonic and radiologic examinations, as well as needle punctures, were performed, and the results were compared statistically. The pleural fluid was detected by ultrasound in 93 percent (74) and by radiologic examination in 83 percent (66) of the 80 cases with such fluid. The absence of fluid was established by ultrasound in 89 percent (32/36) and by radiologic examination in 61 percent (22/36). For the first time the superiority of the ultrasonic method over the radiologic one was demonstrated, and the difference was most obvious in cases of small pleural effusion. Ultrasound permitted the detection of very small amounts (even 3 to 5 ml) of loculated pleural fluid. In contrast to the radiologic method, ultrasound permitted easy differentiation between loculated pleural fluid and pleural thickenings. The ultrasonic method appeared especially useful in the accurate localization and precise indicating of the site for needle aspiration of even the smallest fluid collections. It made possible thoracocentesis in 94 percent (154) of 163 instances. The practical value of the ultrasonic method, both in establishing diagnosis and in treatment, is emphasized.