Kiyanoosh Shapoori

Locating an acoustic point source scattered by a skull phantom via time reversal matched filtering

This paper examines the utilization of the time reversal matched filtering method to resolve the location of an acoustic point source beneath a skull phantom (variable thickness layer), without the removal of this layer. This acoustical process is examined experimentally in a water tank immersion system containing an acoustic source, a custom-made skull phantom, and a receiving transducer in a pitch-catch arrangement. The phantom is designed to approximately model the acoustic properties of an average human skull bone (minus the diploe layer), while the variable thickness of the phantom introduces a variable time delay to the acoustic wave, relative to its entry point on the phantom. This variable delay is measured and corrected for, and a matched filtering time reversed process is used to determine the location of the point source. The results of the experiment are examined for various positions of the acoustic source behind the phantom and compared to the reference cases with no phantom present. The average distance between these two cases is found to be 4.36 mm, and within the expected deviation in results due to not accounting for the effects of refraction.

Adaptive beamforming for ultrasonic phased array focusing through layered structures

Successful realization of ultrasonic imaging through a multilayered composite barrier is hampered by scattering, attenuation, and multiple reflections of acoustic waves at and inside the barrier. These effects tend to distort the beam pattern produced by conventional phased arrays, defocusing the ultrasonic field transmitted through the barrier and causing image quality degradation and resolution loss. To compensate for the refraction and multiple reflection effects, we developed an adaptive beamforming algorithm for small-aperture linear phased arrays. After assessing the barriers local geometry, the method calculates a new timing distribution to refocus the distorted beam at its original location. The procedure is in fact a construction of a matched filter that automatically adapts the transmission pattern of the phased array to the local geometry of the barrier and cancel its distorting effect In this work, the adaptive beamforming algorithms, in transmission mode, for the barriers in the form of a flat homogeneous layer, a layer with a smooth, randomly curved back surface and a two-layered combination of the above have been developed and experimentally verified on custom-engineered samples with prescribed acoustical properties. The algorithms were implemented on ULA-OP, an ultrasound advanced open-platform (University of Florence), controlling 64 active elements on a 128-elements phased array. Experimental measurements of original, distorted and corrected beam profiles confirm the ability of our algorithms to refocus the beam after passing through a scattering and refractive sample. Different excitation signals and windowing options introduced through ULA-OP were examined and compared.

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 barriers 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.

Development of a method to image blood flow beneath the skull or tissue using ultrasonic speckle reflections

The interest of our study is the in-vivo transcranial visualization of blood flow without removal of the skull. The strong attenuation, scattering, and distortion by the skull bones (or other tissues) make it difficult to use currently existing methods. However, blood flow can still be detected by using the ultrasonic speckle reflections from the blood cells and platelets (or contrast agents) moving with the blood. The methodology specifically targets these random temporal changes, imaging the owing region and eliminating static components. This process analyzed over multiple exposures allows an image of the blood flow to be obtained, even with negative acoustic effects of the skull in play. Experimental results show this methodology is able to produce both 2D and 3D images of the owing region, and eliminates those regions of static acoustic sources as predicted. Images produced of the owing region are found to agree with the physical size of the vessel analogues, and also found to provide a qualitative measure on the amount of flow through the vessels.

Resolving the Location of Acoustic Point Sources Scattered Due to the Presence of a Skull Phantom

This paper considers resolving the location of a foreign object in the brain without the removal of the skull bone by detecting and processing the acoustic waves emitted from the foreign object modeled as point source. The variable thickness of the skull bone causes propagation acoustic waves to be scattered in such a manner that the acoustic wave undergoes a variable time delay relative to its entry point on the skull. Matched filtering can be used to detect the acoustic wave front, the time delay variations of the skull can be corrected for, and matched filtering time reversal algorithms can then detect the location of the acoustic source. This process is examined experimentally in a water tank system containing an acoustic source, custom-made skull phantom, and receiver. The apparatus is arranged in transmission mode so that the acoustic waves are emitted from the source, scattered by the phantom, and then received by a second transducer. The skull phantom has been designed so that the acoustic properties (velocity, density, and attenuation correspond approximately to those of a typical human skull. In addition, the phantom has been molded so that the surface closest to the acoustic source has smoothly oscillating ridges and valleys and a flat outer surface, approximately modeling a real-world skull bone. The data obtained from the experiment is processed to detect and extract the scattered acoustic wave front and correct for the time of flight variations in the skull. This re-creates the approximate wave front of a point source, whose location can be resolved via a matched filtering time reversal algorithm. The results of this process are examined for cases where there is no phantom present (no scattering), and with the phantom present. Comparison of these results shows a correlation between the calculated locations of the acoustic source and the expected location.

2D noninvasive acoustical image reconstruction of a static object through a simulated human skull bone

A new method for 2D visualization of foreign objects in the brain tissue, such as bone fragments, bullets, pieces of shrapnel, etc. is presented. The method uses acoustic ray tracing approach to model the propagation of ultrasonic waves through the skull bone and the brain tissue. The mathematical theory of the method, the preliminary results of computer modeling and laboratory testing are presented. A simulation has been developed to take into account the scattering of acoustical fields transmitted through a human skull bone. The experimental data is processed and an image showing the position of the foreign object is reconstructed. The new algorithm has been designed to work with a linear array of 128 receivers. The model consists of a simulated skull bone (scattering medium) and a reflector as a secondary source of ultrasound. To experimentally check the validity of the algorithm, a skull phantom was prepared for use in the laboratory tests. After passing through the phantom layer, the secondary ultrasound field originated from the reflector is recorded by the array of receivers. Then, the detected field distribution is signal-processed to compensate for the distortion by the scattering layer and to reconstruct an image containing data about the reflectors position. This method opens the possibility to non-invasively visualize and characterize the inclusions in the brain tissue through the skull.

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.

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.

Transmission mode adaptive beamforming for planar phased arrays and its application to 3D ultrasonic transcranial imaging

A new adaptive beamforming method for accurately focusing ultrasound behind highly scattering layers of human skull and its application to 3D transcranial imaging via small-aperture planar phased arrays are reported. Due to its undulating, inhomogeneous, porous, and highly attenuative structure, human skull bone severely distorts ultrasonic beams produced by conventional focusing methods in both imaging and therapeutic applications. Strong acoustical mismatch between the skull and brain tissues, in addition to the skulls undulating topology across the active area of a planar ultrasonic probe, could cause multiple reflections and unpredictable refraction during beamforming and imaging processes. Such effects could significantly deflect the probes beam from the intended focal point. Presented here is a theoretical basis and simulation results of an adaptive beamforming method that compensates for the latter effects in transmission mode, accompanied by experimental verification. The probe is a custom-designed 2 MHz, 256-element matrix array with 0.45 mm element size and 0.1mm kerf. Through its small footprint, it is possible to accurately measure the profile of the skull segment in contact with the probe and feed the results into our ray tracing program. The latter calculates the new time delay patterns adapted to the geometrical and acoustical properties of the skull phantom segment in contact with the probe. The time delay patterns correct for the refraction at the skull-brain boundary and bring the distorted beam back to its intended focus. The algorithms were implemented on the ultrasound open-platform ULA-OP (developed at the University of Florence).