Nonlinear native propagation effects of diagnostic ultrasound computed andmeasured in blood

Nonlinear propagation effects produced by focused pulses in blood were meas ured over a 20-cm range, being inspired by diagnostic applications in cardi ology. The initial and maximum pressures applied during measurements in blo od were equal to 0.40 MPapp and 0.76 MPapp, while the pressure estimated at the patient body surface equalled 0.70 MPapp. Measurements of the frequenc y characteristic and the linearity of the ultrasonic probe used in experime nts were performed in water. A numerical procedure developed previously was applied in blood to calculate the pressure distribution of its first and s econd harmonics along the beam axis. The comparison of numerical and measur ed distributions in blood at a temperature of 37 degreesC showed rather goo d agreement. Using numerical methods, a proportional growth of the second h armonic with the increased applied initial pressure was first observed, and finally the maximum limiting effect was found. In this way, much higher le vel of harmonics could be obtained. However, there arise the questions of t he transmitting system construction and of the nonuniform resolution in the case of harmonic imaging when increasing the applied initial pressure. (C) 2001 World Federation for Ultrasound in Medicine & Biology.