An Ultrasonic Adaptive Beamforming Method and Its Application for Trans-Skull Imaging of Certain Types of Head Injuries; Part II: Reception Mode and Adaptive Imaging.

OBJECTIVE: Background theory and a new algorithm for single-point adaptive focusing in transmission mode through ultrasonic barriers via one-dimensional phased arrays were reported in part I. In this paper the algorithm is further extended and implemented into a full adaptive beamforming process, including complete transmission and reception modes. METHODS: Corrected time delay patterns, adapted to the local acoustical and geometrical properties of the barrier, are calculated and applied in both modes. Further, an adaptive imaging process is also developed that implements the proposed beamforming process for two-dimensional imaging through randomly shaped multilayered phase-aberrating structures. The method is optimized for the case of human skull as the ultrasound barrier and its application for transcranial imaging is discussed. RESULTS: Laboratory results of adaptive imaging through realistic skull-mimicking phantoms are presented. The algorithms are implemented on a 64-channel ultrasound open-source phased array platform controlling a standard 128-element biomedical phased array. Irregularly shaped reflectors with characteristic dimensions of the order of ∼0.5 mm to ∼4.5 mm were used as targets behind the skull phantoms in our experiments. Minimum and maximum distortional target displacements of 2.2 mm and 25.3 mm (in 12 cm depth) were observed in sonograms when uncompensated time delays were used. By contrast, the positioning errors ranged from 0.0 to 0.9 mm when our algorithm was employed. CONCLUSION AND SIGNIFICANCE: The adaptive imaging results demonstrate strong potential of the proposed technique for diagnostic imaging of acoustically reflective head injuries directly through intact human skull.

Ultrasonic imaging of foreign inclusions and blood vessels through thick skull bones.

We report a new progress in the development of a portable ultrasonic transcranial imaging system, which is expected to significantly improve the clinical utility of transcranial diagnostic ultrasound. When conventional ultrasonic phased array and Doppler techniques are applied through thick skull bones, the ultrasound field is attenuated, deflected, and defocused, leading to image distortion. To address these deficiencies, the ultrasonic transcranial imaging system implements two alternative ultrasonic methods. The first method improves detection of small foreign objects, such as bone fragments, pieces of shrapnel, or bullets, lodged in the brain tissue. Using adaptive beamforming, the method compensates for phase aberration induced by the skull and refocuses the distorted ultrasonic field at the desired location. The second method visualizes the blood flow through intact human skull using ultrasonic speckle reflections from the blood cells, platelets, or contrast agents. By analyzing these random temporal changes, it is possible to obtain 2D or 3D blood flow images, despite the adverse influence of the skull. Both methods were implemented on an advanced open platform phased array controller driving linear and matrix array probes. They were tested on realistic skull bone and head phantoms with foreign inclusions and blood vessel models.

Ultrasonic imaging of static objects through an aberrating layer using harmonic phase conjugation approach.

The main goal of this study is to develop a new image reconstruction approach for the ultrasonic detection of small objects (comparable to or smaller than the ultrasonic wavelength) behind an aberrating layer. Instead of conventional pulse-echo experimental setup we used through transmission, as the backscattered field after going twice through the layer becomes much weaker than the through-transmitted field. The proposed solution is based on the Harmonic Phase Conjugation (HPC) technique. The developed numerical model allows to calculate the amplitude and phase distributions of the through-transmitted acoustic field interacting with the objects and received by a linear transducer array either directly or after passing through an additional aberrating layer. Then, the digitized acoustic field received by the array is processed, phase-conjugated, and finally, numerically propagated back through the medium in order to reconstruct the image of the target objects. The reconstruction quality of the algorithm was systematically tested on a numerical model, which included a barrier, a medium behind it, and a group of three scatterers, by varying scatterer distances from the source transducer, their mutual arrangement, and the angle of the incident field. Subsequently, a set of laboratory experiments was conducted (at transmit frequency of 2 MHz) to verify the accuracy of the developed simulation. The results demonstrate feasibility of imaging multiple scattering objects through a barrier using the HPC method with better than 1mm accuracy. The results of these tests are presented, and the feasibility of implementing this approach for various biomedical and NDT imaging applications is discussed.

An Ultrasonic-Adaptive Beamforming Method and Its Application for Trans-skull Imaging of Certain Types of Head Injuries; Part I: Transmission Mode.

A new adaptive beamforming algorithm for imaging via small-aperture 1-D ultrasonic-phased arrays through composite layered structures is reported. Such structures cause acoustic phase aberration and wave refraction at undulating interfaces and can lead to significant distortion of an ultrasonic field pattern produced by conventional beamforming techniques. This distortion takes the form of defocusing the ultrasonic field transmitted through the barrier and causes loss of resolution and overall degradation of image quality. To compensate for the phase aberration and the refractional effects, we developed and examined an adaptive beamforming algorithm for small-aperture linear-phased arrays. After accurately assessing the barrier's local geometry and sound speed, the method calculates a new timing scheme to refocus the distorted beam at its original location. As a tentative application, implementation of this method for trans-skull imaging of certain types of head injuries through human skull is discussed. Simulation and laboratory results of applying the method on skull-mimicking phantoms are presented. Correction of up to 2.5 cm focal point displacement at up to 10 cm depth under our skull phantom is demonstrated. Quantitative assessment of the method in a variety of temporal focusing scenarios is also reported. Overall temporal deviation on the order of a few nanoseconds was observed between the simulated and experimental results. The single-point adaptive focusing results demonstrate strong potential of our approach for diagnostic imaging through intact human skull. The algorithms were implemented on an ultrasound advanced open-platform controlling 64 active elements on a 128-element phased array.