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Ultrasonic pulse echo imaging in inhomogeneous media suffers from significant lateral and contrast resolution losses due to the defocusing effects of the inhomogeneities. The losses in lateral and contrast resolution are associated with increases in the width of the mainbeam and increases in sidelobe levels, respectively. These two forms of resolution loss represent significant hurdles to improving the clinical utility of biomedical ultrasonic imaging. A number of research efforts are currently under way to investigate the defocusing effects of tissue and to consider corrective measures. All of these efforts assume linear propagation, and base the image-formation process on the reception of the transmitted pulse. A novel pulse echo imaging scheme in which the image is formed using the finite amplitude distortion components of the received pulse is considered here. Alternatively, this could be described as image formation using the nonlinearly-generated higher harmonics. In homogeneous beam propagations, it has been established that the sidelobes of nonlinearly-generated higher harmonics are much lower than their linear counterparts. Computations considered here suggest that this relationship also holds for the case of propagations through abdominal wall and breast wall tissue. These computations also suggest that the lateral resolution limits imposed by abdominal wall and breast wall tissue are slightly smaller for nonlinearly-generated second harmonics than for their linear counterparts. The resulting potential of these higher harmonics to improve image resolution is investigated.

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The computational predictions for the imaging potential of the second harmonic produced by finite amplitude distortion were investigated with a simple experiment. A focused transducer containing concentric 2.5 MHz and 5.0 MHz elements was used to obtain a sequence of radio-frequency (r-f) backscattered signals using a tissue equivalent phantom. The 2.5 MHz element was used as the transmitter and the 5.0 MHz element was used as the receiver. At 0.68 cm in front of the geometric focal point of the transducer, the phantom contained a 0.6 cm diameter cylindrical volume which contained no scatterers. Each of these r-f signals was then processed to produce the corresponding fundamental (2.5 MHz-centered) and second harmonic (5.0 MHz-centered) envelopes. The contrast resolution obtained for the scatterer-free or cyst region of the envelopes was compared against the computed prediction and good agreement was obtained. The results of this experiment also suggest that the simple one-pulse scheme may be adequate for second harmonic imaging.

The computational modeling of a Dornier HM3 electrohydraulic, extracorporeal shock wave lithotripter is considered. In order to produce large amplitude shock waves for the purpose of pulverizing renal and ureteric calculi (stones), the HM3 uses a hemi-ellipsoidal bowl to focus the spherical field generated by a high-voltage spark gap. The initial propagation of the bowl-focused field is accomplished using a recently developed nonplanar source algorithm. An updated version of an existing nonlinear acoustic beam propagation model is then used to consider the subsequent propagation of the field. The resulting modeling sequence accounts for the effects of diffraction, attenuation, dispersion, nonlinearity, and (planar) reflection and refraction. The water path computed predictions agree well with existing measurements. The computed in vivo predictions suggest that the Dornier HM3's clinical performance is not significantly different than its water path performance.