| In this dissertation, wave propagation and limited-diffraction beams are further studied to gain a deep understanding of their principles and applications in ultrasonic imaging. With the concept of angular spectrum, the ultrasound fields generated by array transducers are mapped as the summation of limited-diffraction beams. A new method of spatial impulse response based on simple algebraic operations instead of complex geometrical considerations for rectangular arrays is derived. Numerical and experimental results show that the method developed has a high accuracy and efficiency.;Based on the knowledge of previous studies, a general theory of Fourier based imaging method is developed from the diffraction tomography theory that solves the inhomogeneous Helmholtz equation under the Born approximation. The object function defined in this theory is more naturally linked to the physical properties of the object, such as the relative change of local compressibility and density. With this treatment, limited-diffraction array beam and broad-band steered plane wave transmissions studied previously are included, in addition to other previously studied imaging methods. A relationship between the Fourier transform of the echo data and that of the object function is established. The theory is developed directly in 3D. Computer simulations, imaging experiments for wire targets, tissue-mimicking phantoms, and in vivo kidney and heart are carried out to verify the theory using the high frame rate imaging system.;To study various methods on wave propagations, limited diffraction beams, and high frame rate imaging, logics and programs are designed and implemented for a general-purpose high-frame-rate ultrasound imaging system. The system has 128 independent transmit and receive channels, each has a high-speed, high-precision A/D, D/A, and a large storage. The system is flexible for various ultrasound experiments. |
