A new frequency compounding technique for super harmonic imaging

Second harmonic imaging is currently the de-facto standard in commercial echographic systems for diagnosis because of its improved resolution and contrast to tissue ratio. An emerging technique called super harmonic imaging is based on a combination of multiple frequency components generated during the propagation of sound in tissue. This combination of third to fifth harmonic has the potential to further enhance resolution and image quality of echographic pictures. In order to fulfill the bandwidth requirements of super harmonic imaging, a special interleaved phased array transducer has been developed. Currently, the achievable bandwidth for phased array elements used in transmission is close to 80%, which involves that generated harmonics will be separated by gaps in the frequency domain. That will introduce specific artifacts visible as ripples in the echo image. We propose a two-pulse technique that reduces the ripple artifacts and recovers the axial resolution. Method. This technique consists in firing two lines with a 15% frequency shift for the second firing. Summing the echoes of those two lines will result in filling the gaps in the frequency band of the distorted signal. The optimal choice for the frequency of the second pulse can be derived analytically. Standard detection methods applied to this two-pulse technique will strongly minimize artifacts encountered with envelope detection on super harmonic signals. Results. Theoretical calculations show an improvement in axial resolution by a factor of 2.7 at the -15 dB level compared to second harmonic imaging. For a fair comparison, the super harmonic signal will be compared with the lowest frequency component of its spectrum, which is the third harmonic. A shortening of the pulse is visible in the enclosed figure where axial point spread function simulations of third harmonic and super harmonic pulses are compared. Experimentally, this method shortens the pulse by a factor 2.3 at the -15 dB level compared to second harmonic, and 1.9 compared to third harmonic. Conclusion. Super harmonic imaging quality can be further improved by frequency compounding techniques such as the two-pulse method described here.