Aberrating Layer Research Articles

SLODAR method. Part 2: The use of point sources

The SLOpes Detection And Ranging (SLODAR) method is supplemented with the theory of the measurement of the positions of aberration layers for the case of the use of point reference sources having converging/diverging wavefronts. We determine the dependence of the shift in the correlation peak on the distance to the measured aberration layer, the measurement error, and the maximum measured distance to the layer. Numerical experiments are performed to determine the position of one and several screens under conditions close to experimental ones.

Energy-based adaptive focusing of waves: application to noninvasive aberration correction of ultrasonic wavefields

An aberration correction method based on the maximization of the wave intensity at the focus of an emitting array is presented. The potential of this new adaptive focusing technique is investigated for ultrasonic focusing in biological tissues. The acoustic intensity is maximized noninvasively through direct measurement or indirect estimation of the beam energy at the focus for a series of spatially coded emissions. For ultrasonic waves, the acoustic energy at the desired focus can be indirectly estimated from the local displacements induced in tissues by the ultrasonic radiation force of the beam. Based on the measurement of these displacements, this method allows determination of the precise estimation of the phase and amplitude aberrations, and consequently the correction of aberrations along the beam travel path. The proof of concept is first performed experimentally using a large therapeutic array with strong electronic phase aberrations (up to 2pi). Displacements induced by the ultrasonic radiation force at the desired focus are indirectly estimated using the time shift of backscattered echoes recorded on the array. The phase estimation is deduced accurately using a direct inversion algorithm which reduces the standard deviation of the phase distribution from sigma = 1.89 radian before correction to sigma = 0.53 radian following correction. The corrected beam focusing quality is verified using a needle hydrophone. The peak intensity obtained through the aberrator is found to be -7.69 dB below the reference intensity obtained without any aberration. Using the phase correction, a sharp focus is restored through the aberrator with a relative peak intensity of -0.89 dB. The technique is tested experimentally using a linear transmit/receive array through a real aberrating layer. The array is used to automatically correct its beam quality, as it both generates the radiation force with coded excitations and indirectly estimates the acoustic intensity at the focus with speckle tracking. This technique could have important implications in the field of high-intensity focused ultrasound even in complex configurations such as transcranial, transcostal, or deep seated organs.

Measurement of electrophoretic mobility of cardiomyocytes

The electrophoretic mobility (EPM) of rat cardiomyocytes with or without the treatment of neuraminidase was studied by cell electrophoresis. The EPM was found to change over a range from 0 to 8.67 microm s(-1)/V cm(-1), depending on ionic strength, transmembrane potential, pH value, and/or surface charges. It is interesting that zero EPM was observed but reverse of the mobility was not. These results suggested that the negative charges carried on the cardiomyocyte surface might comprehensively consist of surface sialic acid, plasmalemma proteins, phospholipids, and transmembrane potential. The aberrant electrical double layer formed between the carried negative charges and adions had a big adsorption layer and a diffusion layer whose sizes changed circularly, making only negative charges be carried on the surface of living cardiomyocytes. The special structures on the surface of cardiomyocytes probably play a considerable role in the process of cardiac electrical activity.

Dissociation of Corticothalamic and Thalamocortical Axon Targeting by an EphA7-Mediated Mechanism

Molecular mechanisms generating the topographic organization of corticothalamic (CT) circuits, which comprise more than three-quarters of the synaptic inputs onto sensory relay neurons, and their interdependence with thalamocortical (TC) axon development are unknown. Using in utero electroporation-mediated gene transfer, we show that EphA7-mediated signaling on neocortical axons controls the within-nucleus topography of CT projections in the thalamus. Notably, CT axons that mis-express EphA7 do not shift the relative positioning of their pathway within the subcortical telencephalon (ST), indicating that they do not depend upon EphA7/ephrin-A signaling in the ST for establishing this topography. Moreover, mis-expression of cortical EphA7 results in disrupted topography of CT projections, but unchanged inter- and intra-areal topography of TC projections. Our results support a model in which EphA/ephrin-A signaling controls independently the precision with which CT and TC projections develop, yet is essential for establishing their topographic reciprocity.

Visualization of phase conjugate ultrasound waves passed through inhomogeneous layer

Compensation of phase distortions of ultrasound beams by means of parametric phase conjugation is visualized. Quasi-plane and focused primary beams were distorted by a polymer aberration layer introduced between the primary wave source and the wave phase conjugator. It is demonstrated acousto-optically that, while the acoustic field is strongly irregular in the area between aberration layer and conjugator, the phase conjugate wave visibly reproduces the primary beams in the area between the layer and the primary wave source. The phenomenon is observed in supercritical mode of parametric amplification when intensity of phase conjugate wave is high enough for manifestations of acoustic nonlinearities in water.

Effect of aberration on the acoustic field in tissue harmonic imaging (THI)

A numerical simulation was used to study the impact of an aberrating layer on the generation of the fundamental and second-harmonic (SH) field in a tissue harmonic imaging scenario. The simulation used a three-dimensional time-domain code for solving the KZK equation and accounted for arbitrary spatial variations in all acoustic properties. The aberration effect was modeled by assuming that the tissue consisted of two layers where the interface has a spatial variation C that acted like an effective phase screen. Initial experiments were carried out with sinusoidal-shaped interfaces. The sinusoidal interface produced grating lobes which were at least 6 dB larger for the fundamental signal than the SH. The energy outside of the main lobe was found to increase linearly as the amplitude of the interface variation increased. The location of the grating lobes was affected by the spatial period on the interface variation. The inhomogeneous nature of tissue was modeled with an interface with a random spatial variation. With the random interface the average sidelobe level for the fundamental was −30 dB whereas the SH had an average sidelobe level of −36 dB. [Work supported by the NSF through the Center for Subsurface Sensing and Imaging Systems.]

Laminar disorganisation of mitral cells in the olfactory bulb does not affect topographic targeting of primary olfactory axons

Primary olfactory neurons expressing the same odorant receptor protein typically project to topographically fixed olfactory bulb sites. While cell adhesion molecules and odorant receptors have been implicated in guidance of primary olfactory axons, the postsynaptic mitral cells may also have a role in final target selection. We have examined the effect of disorganisation of the mitral cell soma layer in mutant mice heterozygous for the β-subunit of platelet activating factor acetylhydrolase (Lis1 −/+) on the targeting of primary olfactory axons. Lis1 −/+ mice display abnormal lamination of neurons in the olfactory bulb. Lis1 −/+ mice were crossed with the P2-IRES-tau:LacZ line of transgenic mice that selectively expresses β-galactosidase in primary olfactory neurons expressing the P2 odorant receptor. LacZ histochemistry revealed blue-stained P2 axons that targeted topographically fixed glomeruli in these mice in a manner similar to that observed in the parent P2-IRES-tau:LacZ line. Thus, despite the aberrant organisation of postsynaptic mitral cells in Lis1 −/+ mice, primary olfactory axons continued to converge and form glomeruli at correct sites in the olfactory bulb. Next we examined whether challenging primary olfactory axons in adult Lis −/+ mice with regeneration would affect their ability to converge and form glomeruli. Following partial chemical ablation of the olfactory neuroepithelium with dichlobenil, primary olfactory neurons die and are replaced by newly differentiating neurons that project axons to the olfactory bulb where they converge and form glomeruli. Despite the aberrant mitral cell layer in Lis −/+ mice, primary olfactory axons continued to converge and form glomeruli during regeneration. Together these results demonstrate that the convergence of primary olfactory axons during development and regeneration is not affected by gross perturbations to the lamination of the mitral cell layer. Thus, these results support evidence from other studies indicating that mitral cells do not play a major role in the convergence and targeting of primary olfactory axons in the olfactory bulb.

Adaptive ultrasonic imaging using SONOLINE Elegra

Adaptive correction of the effects of propagation through inhomogeneous tissue is critical to the improvement of current ultrasonic imaging systems. Currently, estimation and correction of time-delay errors is more feasible than other more sophisticated approaches. Data acquisition, time-delay estimation and compensation have been implemented on the SONOLINE Elegra system using the system CPU, the Crescendo image processor, and the existing front-end electronics. Experimental results with this implementation will be reported. The effects of compensating the transmit beam is studied using the waveform similarity factor and single transmit imaging. On an RMI404 phantom plus a 1-D aberration layer with a rms time fluctuation of 40 ns and correlation length of 5 mm, the waveform similarity factor of randomly scattered waveforms improved from 0.362 to 0.477 by iteration. Correspondingly, the −20 dB lateral resolution improved from 1.62 to 0.77 mm, and the image contrast improved by 8.5 dB (speckle region is 6 dB brighter while echo-free region is 2.5 dB darker). Experiments with a 2-D aberration layer and with a special phase aberration phantom showed less image improvements. Preliminary body scans with adaptive imaging showed improved image contrast and details in some cases but the results are mixed and influenced by such factors as isoplanatic patch size and complex scattering structures. [Study partially supported by NIH R29 CA81688.]

Focal translation by frequency shift in free space

Variable focusing in range from a transmit array can be accomplished by a frequency shift in an acoustic waveguide. In this paper, a general algorithm is derived in free space that can move the focal spot of a linear transmit array both transversely and horizontally. In particular, for variable range focusing, the prescribed frequency shift is the same for all elements of the array, whereas, for transverse focal shifts, the frequency shift varies with element location in the array. The method described in this paper is especially suitable for varying the location of the focal spot of a time-reversal mirror in the presence of an aberrating layer in that no knowledge of the medium properties is necessary. Simulation results demonstrating the proposed method are presented in comparison with those from a conventional steering method.

Application of the parabolic equation method to medical ultrasonics

The application of the parabolic equation (PE) method is extended to hyperthermia and imaging problems in medical ultrasonics. It is shown that the PE provides an accurate solution of the acoustic field generated by a pre-focused array through an inhomogeneous medium. To investigate hyperthermia therapy, the PE is coupled with the heat equation. Simulations provide evidence that this interaction defocuses the acoustic field and degrades the efficiency of the hyperthermia treatment. We propose a procedure to maintain the focus in the zone of interest using two arrays and phase-conjugation invariance properties. For medical imaging, an algorithm is developed to move the focal spot of a linear array in any direction using a frequency shift in a time-reversal mirror (TRM). Simulations demonstrate that the proposed algorithm can be applied to an inhomogeneous medium in the presence of an aberrating layer without the knowledge of the medium properties.

Regulation of Purkinje Cell Alignment by Reelin as Revealed with CR-50 Antibody

Cerebellar Purkinje cells are generated in the ventricular zone, migrate outward, and finally form a monolayer in the cortex. In reeler mice, however, most Purkinje cells cluster abnormally in subcortical areas. Reelin, the candidate reeler gene product recognized by the CR-50 monoclonal antibody, is concentrated in a cortical zone along which Purkinje cells are aligned linearly, implying that it may regulate their alignment. We used an in vitro system and a transplantation approach to analyze the function of Reelin. Explant culture for 7 d of cerebella isolated from wild-type and reeler mice at embryonic day 13 (E13) reproduced in a phenotype-dependent manner the two distinct arrangement patterns (linear vs clustered) of Purkinje cells. Extensive CR-50 binding to wild-type explants converted the linear pattern into a reeler-like, clustered pattern. On the other hand, when reeler explants lacking Reelin were crowned with an artificial layer of Reelin+ granule cells, some Reelin molecules were distributed into a superficial zone of the reeler explants, and Purkinje cells formed a linear pattern along the Reelin-rich overlay. This "rescue" effect was also inhibited by CR-50. Hence, Reelin is involved in the Purkinje cell alignment, and the lack of this activity may explain the malformation in reeler cerebella. We further injected Reelin+ granule cells into the fourth ventricle of E12-13 mice. Extensive incorporation of the injected Reelin+ cells into the ventricular zone, but not of Reelin- cells, forced Purkinje cells of the host cerebella to form an aberrant layer, suggesting that premigratory Purkinje cells may already be responsive to Reelin or Reelin-related signals.

Aberration correction in ultrasonic medical imaging with time-reversal techniques

Degradation of image quality is currently observed with ultrasonic scanners owing to distortion of the ultrasonic beams through inhomogeneous tissue layers. Adaptive time-delay focusing techniques allow an efficient correction of these effects when the inhomogeneous layer is close to the transducer array. If the aberrating layers are far from the array, these techniques are no longer appropriate to correct the propagation effects between the layer and the transducer array. In this article we show that time-reversal mirrors can solve this problem. In the matched-filter approach that extends the concept of time reversal mirrors, the Green's function of a dominant scatterer available in the medium is recorded in digital memory and used to focus on the scatterer in both transmit and receive modes. An extension of this technique is also presented to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, new Green's functions matched to the new point of interest are calculated. The algorithm uses the concept of time-reversal propagation, and theoretical and experimental results obtained with this technique are presented. The calculation of each Green's function matched to each new desired focal point allows us to realize a B-scan image of the zone surrounding the reflector. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 110–125, 1997

Ultrasonic beam steering through inhomogeneous layers with a time reversal mirror

Adaptive time delay focusing techniques allow an efficient correction of the effects due to an inhomogeneous layer close to the transducer array. If the layer is far from the array, these techniques are no longer appropriate to correct the diffraction effects between the layer and the transducer array. This problem was overcome by the use of acoustic time reversal mirrors. In this technique, the Green's function of a dominant scatterer available in the medium is recorded in digital memories and used to focus on the scatterer in both transmit and receive modes. We present in this paper an extension of this technique to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it in order to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, we calculate the new Green's function matched to the new point of interest. The algorithm uses the concept of time reversal propagation, and we shall present here theoretical and experimental results obtained with this technique. Finally, the knowledge of each Green's function matched to each new desired focal point allows the realization of a B-scan image of the zone surrounding the reflector.

Mathematical Simulation of Pressure Pulse Propagation in Biological Tissues

The propagation of pressure pulses of arbitrary time form in inhomogeneous attenuating media with characteristics similar to real biological tissues is considered. A mathematical model using the quasi-optical approximation is developed to solve the wave equation. Such a model proves to be convenient for computer realization. A spectral analysis of known experimental data on frequency dependent backscatter is realized to separate coherent and incoherent components of backscatter from tissues and determine their parameters. The characteristics of the scattering are utilized for digital simulations of ultrasound propagation from an interrogated tissue volume. The results presented describe the expected space distribution of the pressure amplitude and phase near the transducer focus in the presence of an aberrating layer with parameters characteristic of abdominal imaging.

Mathematical Simulation of Pressure Pulse Propagation in Biological Tissues

The propagation of pressure pulses of arbitrary time form in inhomogeneous attenuating media with characteristics similar to real biological tissues is considered. A mathematical model using the quasi-optical approximation is developed to solve the wave equation. Such a model proves to be convenient for computer realization. A spectral analysis of known experimental data on frequency dependent backscatter is realized to separate coherent and incoherent components of backscatter from tissues and determine their parameters. The characteristics of the scattering are utilized for digital simulations of ultrasound propagation from an interrogated tissue volume. The results presented describe the expected space distribution of the pressure amplitude and phase near the transducer focus in the presence of an aberrating layer with parameters characteristic of abdominal imaging.

Axial imaging through an aberrating layer of water in confocal microscopy

Axial imaging in confocal microscopy is investigated by considering the effect of spherical aberration caused by focusing a beam of light deeply into a specimen, which is simplified as a layer of water. It is found that the axial response consists of a series of signals reflected from different interfaces. Aberration compensation by altering the effective tube length of the objective is also studied in order to obtain an optimum axial response. Corresponding experimental results are obtained, which are well in agreement with the theoretical predictions.

Phase aberration correction in medical ultrasound using speckle brightness as a quality factor.

Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off-peak response of the imaging system's point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phased array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the correction technique used by Muller and Buffington [J. Opt. Soc. Am. 64 (9), 1200-1209 (1974)] to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two-dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.