In vivo comparison of fundamental and harmonic lateral transmit beam shapes
Harmonic imaging has been shown to provide superior image quality in clinical studies, but few quantitative analyses of image quality have been performed. To better understand the improvements in image quality attained in vivo using harmonic imaging, we have measured the lateral transmit beam shape using two methods: parallel receive beamforming and application of the van Cittert-Zernike theorem. In the first method, parallel receive beamforming methods are applied to individual channel rf-echo data acquired using a single transmit pulse. Data is beamformed at multiple lateral increments around the transmit beam to form a B-mode image. The lateral beam shape can be directly estimated at different ranges from the resultant B-mode image. In the second method, the lateral transmit beam intensity is approximated as the Fourier transform of the complex spatial covariance of the backscattered echo signal at multiple ranges. Simulations, phantom experiments, and in vivo breast studies were performed to evaluate these two estimation methods, and to compare the lateral transmit beam shapes of fundamental and harmonic signals. Simulation results illustrate the accuracy of both methods in estimating lateral transmit beam shape. Results from phantom experiments indicate improved transmit beamforming for harmonic signals, with and without the presence of a transmit aberrator. Preliminary results from clinical studies suggest that estimation of lateral transmit beam shape in vivo improves when phase aberration correction is implemented on receive echo data.
