Frequency domain compensation for inhomogeneous layers in ultrasound holography

Phase shift migration for imaging layered objects and objects immersed in water

This paper proposes the use of phase shift migration for ultrasonic imaging of layered objects and objects immersed in water. The method, which was developed in reflection seismology, is a frequency domain technique that in a computationally efficient way restores images of objects that are isotropic and homogeneous in the lateral direction but inhomogeneous in depth. The performance of the proposed method was evaluated using immersion test data from a block with side-drilled holes with an additional scatterer residing in water. In this way, the method's capability of simultaneously imaging scatterers in different media and at different depths was investigated. The method was also applied to a copper block with flat bottom holes. The results verify that the proposed method is capable of producing high-resolution and low-noise images for layered or immersed objects.

Forward planar projection through layered media

A planar forward projection algorithm is combined with ray theory to describe longitudinal propagation through an arbitrary number of randomly oriented isotropic layers. This method first measures the space-time pressure field in a plane, then uses wavevector frequency-domain methods to project the field through layered media and to an arbitrary new plane, not necessarily parallel to the initial plane. The approach is valid for longitudinal propagation through liquid layers and in solids, such as soft tissues, that can be approximated as viscous liquids. The algorithm is verified by propagating the field from a 0.5 MHz planar transducer through a combination of rubber, plastic, and water layers. Hydrophone measurements indicate correlation between measured and simulated fields for angles below the longitudinal critical angles of the layered materials.

Ultrasound holographic B-scan imaging

An ultrasound holographic B-scan (UHB) imaging apparatus comprising a minicomputer system, a data acquisition unit, and a special 64-element UHB transducer has been developed. Simulation studies and real experiments with a tissue-equivalent phantom show that lateral and longitudinal resolution (-6 dB) of about 1 mm was achieved in the entire image. Furthermore, results from clinical evaluation, including diagnostic and neurosurgical imaging, suggest that the UHB imaging method is operational and has some special advantages in patient diagnosis. Theoretically, the addition of phase information to the ultrasound images can result in enhanced tissue characterization, which is extremely important in tumor diagnosis and treatment.

Circular-array ultrasound holography imaging using the linear-array approach

A circular-array, pulse-echo, ultrasound holography imaging method is presented. The configuration of the measurement system is much simpler than that of traditional computed-tomography (CT) and compound beta-scan imaging. Wide-beam insonification is used. The geometrical differences of the circular array compared with a linear array system are corrected in the frequency domain of the hologram. The computation time of the reconstruction process for a circular image is practically the same as that of a linear array imaging process using the backward propagation (BP) principle. A fast Fourier transform (FFT) algorithm is directly used in the reconstruction process. Computer simulation shows that the resolution of this imaging method can surpass that of a linear-array system. In the preliminary experiments, 2-mm resolution was achieved in both the lateral and radial directions with an ultrasound frequency of 2 MHz.