Aberration Correction Techniques Research Articles

Full-wave acoustic and thermal modeling of transcranial ultrasound propagation and investigation of skull-induced aberration correction techniques: a feasibility study.

BackgroundTranscranial focused ultrasound (tcFUS) is an attractive noninvasive modality for neurosurgical interventions. The presence of the skull, however, compromises the efficiency of tcFUS therapy, as its heterogeneous nature and acoustic characteristics induce significant distortion of the acoustic energy deposition, focal shifts, and thermal gain decrease. Phased-array transducers allow for partial compensation of skull-induced aberrations by application of precalculated phase and amplitude corrections.MethodsAn integrated numerical framework allowing for 3D full-wave, nonlinear acoustic and thermal simulations has been developed and applied to tcFUS. Simulations were performed to investigate the impact of skull aberrations, the possibility of extending the treatment envelope, and adverse secondary effects. The simulated setup comprised an idealized model of the ExAblate Neuro and a detailed MR-based anatomical head model. Four different approaches were employed to calculate aberration corrections (analytical calculation of the aberration corrections disregarding tissue heterogeneities; a semi-analytical ray-tracing approach compensating for the presence of the skull; two simulation-based time-reversal approaches with and without pressure amplitude corrections which account for the entire anatomy). These impact of these approaches on the pressure and temperature distributions were evaluated for 22 brain-targetsResultsWhile (semi-)analytical approaches failed to induced high pressure or ablative temperatures in any but the targets in the close vicinity of the geometric focus, simulation-based approaches indicate the possibility of considerably extending the treatment envelope (including targets below the transducer level and locations several centimeters off the geometric focus), generation of sharper foci, and increased targeting accuracy. While the prediction of achievable aberration correction appears to be unaffected by the detailed bone-structure, proper consideration of inhomogeneity is required to predict the pressure distribution for given steering parametersConclusionsSimulation-based approaches to calculate aberration corrections may aid in the extension of the tcFUS treatment envelope as well as predict and avoid secondary effects (standing waves, skull heating). Due to their superior performance, simulationbased techniques may prove invaluable in the amelioration of skull-induced aberration effects in tcFUS therapy. The next steps are to investigate shear-wave-induced effects in order to reliably exclude secondary hot-spots, and to develop comprehensive uncertainty assessment and validation procedures.

Stability in computed optical interferometric tomography (Part II): in vivo stability assessment.

Stability is of utmost importance to a wide range of phase-sensitive processing techniques. In Doppler optical coherence tomography and optical coherence elastography, in addition to defocus and aberration correction techniques such as interferometric synthetic aperture microscopy and computational/digital adaptive optics, a precise understanding of the system and sample stability helps to guide the system design and choice of imaging parameters. This article focuses on methods to accurately and quantitatively measure the stability of an imaging configuration in vivo. These methods are capable of partially decoupling axial from transverse motion and are compared against the stability requirements for computed optical interferometric tomography laid out in the first part of this article.

Direct structure analysis of advanced nanomaterials by high-resolution electron microscopy

AbstractHigh-resolution electron microscopy (HREM) analysis has contributed to the direct structure analysis of advanced nanostructured materials, of which the properties of these materials are strongly dependent on the atomic arrangements. In the present article, the direct structure analysis of nanostructured materials such as boride and oxide materials was described and the high-resolution imaging methods were applied to boron nitride nanomaterials such as nanotubes and nanoparticles. An aberration correction technique is also expected as an advanced nanostructure analysis with higher resolution. The HREM image of TlBa2Ca3Cu4O11 was taken with the incident beam parallel to the a axis together with a structure model after image processing.

Combination of scene-based and stochastic measurement for wide-field aberration correction in microscopic imaging

We report on a novel aberration correction technique that uses the sequential combination of two different aberration measurement methods to correct for setup-induced and specimen-induced aberrations. The advantages of both methods are combined and, thus, the measurement time is strongly reduced without loss of accuracy. The technique is implemented using a spatial-light-modulator-based wide-field microscope without the need for additional components (e.g., a Shack-Hartmann sensor). The aberrations are measured without a reference object by directly using the specimen to be imaged. We demonstrate experimental results for technical as well as biological specimens.

Adaptive aberration correction based on ant colony algorithm for solid-state lasers: numerical simulations

We describe an adaptive aberration correction technique based on an ant colony algorithm for solid-state lasers and a general class of other adaptive optics systems. We show that it is possible to compensate phase aberrations without wavefront sensing in this approach, which iteratively adjusts the control voltages of a deformable mirror to maximize certain system performance metrics of the far-field intensity distribution of the laser beam. The effectiveness of this approach is analyzed numerically by use of a 37-element piezoelectric deformable mirror and a variation of the Strehl ratio as the metric. Results demonstrate that this approach can effectively compensate the phase distortions of laser beams and significantly improve beam quality. A comparison indicates that this approach is much faster than a genetic algorithm while achieving almost the same beam quality.

Time reversal for ultrasonic transcranial surgery and echographic imaging

High‐intensity focused ultrasound (HIFU) is able to induce non‐invasively controlled and selective destruction of tissues by focusing ultrasonic beams within organs, analogous to a magnifying glass that concentrates enough sunlight to burn a hole in paper. The brain is an attractive organ in which to perform ultrasonic tissue ablation, but such an application has been hampered by the strong defocusing effect of the skull bone. Our group has been involved in this topic for several years, providing proofs of concept and proposing technological solutions to this problem. Thanks to a high‐power time‐reversal mirror, presented here are in vivo thermal lesions induced through the skull of 12 sheep. Thermal lesions were confirmed by T2‐weighted magnetic resonance post‐treatment images and histological examination. These results provide striking evidence that noninvasive ultrasound brain surgery is feasible. A recent approach for high‐resolution brain ultrasonic imaging will also be discussed with a skull aberration correction technique based on twin arrays technology. The correction of transcranial ultrasonic images is implemented on a new generation of time‐reversal mirrors relying on a fully programmable transmit and receive beamformer.

Analysis and correction of ultrasonic wavefront distortion based on a multilayer phase-screen model.

A model is introduced that incorporates the cumulative wavefront distortion effects caused by spatial heterogeneities along the path of propagation, and a corresponding model-based wavefront distortion-correction method is presented. In the proposed model, a distributed heterogeneous medium is lumped into a series of parallel phase screens. The distortion effects can be compensated--without a priori knowledge of the distorting structure--by backpropagation of received wavefronts through hypothetical multiple phase screens located between the imaging system and targets, while each pointwise time shift is adjusted iteratively to maximize a specified image quality factor at the final layer. Theoretical analyses indicate that the mean speckle brightness decreases monotonically with the root-mean-square value of distributed phase distortions; therefore, the speckle brightness can be used as an image quality factor. Experimental one-dimensional (1-D) array data with simulated distortion effects based on a real 2-D abdominal-tissue map were used to evaluate the performance of the proposed method and existing aberration-correction techniques. The simulated characteristics of wavefront distortion and relative performance of existing correction techniques were similar to reports based on abdominal-wall data and breast data. This investigation shows that the proposed method provides better compensation for wavefront distortion.

Aberration correction using a multi-segment mirror with feedback interferometry

A novel real-time aberration correction system using feedback interferometry and a multi-segment mirror for the wavefront corrector is described. The result is a simple and high-speed adaptive optics system demonstrating that feedback interferometry may potentially be an alternative method to conventional aberration correction techniques. Its high-speed processing and simplicity makes real-time correction practical. Our results show that the system can respond up to 37 Hz for sinusoidal phase variations with amplitude of one wavelength and improves Strehl ratio significantly.

Performance of an Off-axis Holographic Lens in the Presence of Aberrations Induced by Construction-reconstruction Wavelength Shift: Extended Object Imaging

In this paper investigations have been carried out on the imaging of extended objects by an off-axis holographic lens operating at a wavelength different from its recording. Aberration balancing and correction techniques have been applied to improve the quality of imaging coherent as well as incoherent bar and edge objects. The light intensity distribution in the Gaussian image plane has been computed numerically and the results have been presented in graphical form. The image quality of the aberration-balanced holo-lens has been found to be comparable to that of the ideal holo-lens.

Adaptive focusing in scattering media through sound-speed inhomogeneities: The van Cittert Zernike approach and focusing criterion

An original formalism that allows a thorough understanding of cross-correlation-based phase aberration correction techniques in a scattering medium is developed. This formalism is based on the analysis of the second-order statistics of the pressure field scattered by a random distribution of scatterers. One of the major interests of this analysis is the ability it provides to evaluate and monitor the convergence of phase aberration correction techniques. This is achieved by the computation of a single parameter C that serves as a focusing criterion. The theoretical developments are validated experimentally.

Imaging of extended objects using an off-axis holographic lens: effect of wavelength shift from construction to reconstruction

In this paper investigations on the imaging of extended objects by an off-axis holographic lens reconstructed at a wavelength different from its recording have been carried out. An aberration correction technique has been applied to minimise the aberrations introduced by the wavelength shift. The performance of a corrected holo-lens has been evaluated by computing the images of coherent as well as incoherent bar and edge objects and compared with that of the aberrated holo-lens. Light intensity distribution in the image plane has been evaluated numerically and the results have been presented in graphical form.

Two dimensional ultrasonic beam distortion in the breast: In vivo measurements and effects

Two dimensional arrival time data was obtained for the propagation of ultrasound across the breasts of 7 female volunteers. These profiles were extracted through the use of cross-correlation measurements and a simulated annealing process that maintained phase closure while aligning the data. The phase aberration measured in two dimensions had a larger magnitude than previously reported phase aberration measured in one dimension in the breast. A point spread function generation computer program was used to demonstrate the system response degrading effects of the measured phase aberration and the usefulness of current one dimensional phase aberration correction techniques. The results indicate that two dimensional correction algorithms are necessary to restore the system performance losses due to phase aberration.

Two dimensional ultrasonic beam distortion in the breast: in vivo measurements and effects.

Two dimensional arrival time data was obtained for the propagation of ultrasound across the breasts of 7 female volunteers. These profiles were extracted through the use of cross-correlation measurements and a simulated annealing process that maintained phase closure while aligning the data. The phase aberration measured in two dimensions had a larger magnitude than previously reported phase aberration measured in one dimension in the breast. A point spread function generation computer program was used to demonstrate the system response degrading effects of the measured phase aberration and the usefulness of current one dimensional phase aberration correction techniques. The results indicate that two dimensional correction algorithms are necessary to restore the system performance losses due to phase aberration.

A statistical analysis of phase aberration correction using image quality factors in coherent imaging systems

A fundamental analysis of phase aberration correction techniques which use speckle brightness-based image equality factors (QFs) to iteratively reduce phase errors is presented. Phase aberration arises from the spatial inhomogeneity of acoustic velocity in human tissue and degrades the performance of diagnostic ultrasonic imaging systems. A theoretical analysis is presented indicating that the mean speckle brightness decreases with root-mean-square (RMS) phase error. A general definition of QFs is given using the probability of error as a criterion of performance. The QF is optimized through minimization of the probability of error under different conditions. The analysis provides a theoretical framework for the current correction technique using QFs under a variety of conditions, and is a useful tool to evaluate new QFs and correction techniques.