25319584
Accurate simulation of transcranial ultrasound propagation for ultrasonic neuromodulation and stimulation.
Non-invasive, focal neurostimulation with ultrasound is a potentially powerful neuroscientific tool that requires effective transcranial focusing of ultrasound to develop. Time-reversal (TR) focusing using numerical simulations of transcranial ultrasound propagation can correct for the effect of the skull, but relies on accurate simulations. Here, focusing requirements for ultrasonic neurostimulation are established through a review of previously employed ultrasonic parameters, and consideration of deep brain targets. The specific limitations of finite-difference time domain (FDTD) and k-space corrected pseudospectral time domain (PSTD) schemes are tested numerically to establish the spatial points per wavelength and temporal points per period needed to achieve the desired accuracy while minimizing the computational burden. These criteria are confirmed through convergence testing of a fully simulated TR protocol using a virtual skull. The k-space PSTD scheme performed as well as, or better than, the widely used FDTD scheme across all individual error tests and in the convergence of large scale models, recommending it for use in simulated TR. Staircasing was shown to be the most serious source of error. Convergence testing indicated that higher sampling is required to achieve fine control of the pressure amplitude at the target than is needed for accurate spatial targeting.
35720999
Bilateral Subthalamic Nucleus Stimulation for Parkinson's Disease: A Systematic Review of the Clinical Literature
OBJECTIVE: To evaluate the benefits and adverse effects of bilateral subthalamic nucleus stimulation in the treatment of Parkinson's disease (PD) by systematically reviewing the published literature. METHODS: A search of the PubMed database using the key words subthalamic, nucleus, and stimulation yielded 624 articles published between 1966 and December 2003. Only articles that included original, nonduplicated descriptions of patients with PD treated with bilateral subthalamic nucleus stimulation were selected for further analysis. RESULTS: A total of 38 studies from 34 neurosurgical centers in 13 countries were identified for critical review. The outcomes for 471 patients with PD treated with bilateral subthalamic nucleus stimulation were assessed according to the Unified Parkinson's Disease Rating Scale in both on-medication and off-medication conditions. With stimulation, Unified Parkinson's Disease Rating Scale motor scores in the off-medication condition improved by 50% after 6 months, 56% after 12 months, 51% after 2 years, and 49% after 5 years compared with preoperative off-medication scores. At 12 months of subthalamic nucleus stimulation, the mean improvement in tremor was 81%, in rigidity was 63%, in bradykinesia was 52%, in gait was 64%, and in postural instability was 69% when compared with preoperative off-medication subscores. On-medication dyskinesias were reduced by 94%, as assessed 12 months after stimulation using the Unified Parkinson's Disease Rating Scale IV complications of therapy score. There was an overall 52% reduction in the l-dopa-equivalent dose intake after 12 months of stimulation. Most adverse effects were mild to moderate. There was a 1 to 2% incidence of severe adverse effects (death or permanent neurological deficits related to intracerebral hemorrhages). Nineteen percent of the patients had adverse effects related to stimulation that could be reversed by changing stimulation parameters. There was a 9% incidence of adverse effects related to the hardware (infections, lead and pulse generator problems). CONCLUSION: Bilateral subthalamic nucleus stimulation is effective in the treatment of PD. Further refinements in patient selection and surgical technique may lessen the incidence of complications associated with this procedure.
1398283
Transcranial Pulsed Ultrasound Stimulates Intact Brain Circuits
Electromagnetic-based methods of stimulating brain activity require invasive procedures or have other limitations. Deep-brain stimulation requires surgically implanted electrodes. Transcranial magnetic stimulation does not require surgery, but suffers from low spatial resolution. Optogenetic-based approaches have unrivaled spatial precision, but require genetic manipulation. In search of a potential solution to these limitations, we began investigating the influence of transcranial pulsed ultrasound on neuronal activity in the intact mouse brain. In motor cortex, ultrasound-stimulated neuronal activity was sufficient to evoke motor behaviors. Deeper in subcortical circuits, we used targeted transcranial ultrasound to stimulate neuronal activity and synchronous oscillations in the intact hippocampus. We found that ultrasound triggers TTX-sensitive neuronal activity in the absence of a rise in brain temperature (<0.01 degrees C). Here, we also report that transcranial pulsed ultrasound for intact brain circuit stimulation has a lateral spatial resolution of approximately 2 mm and does not require exogenous factors or surgical invasion.
17122211
Probing and Regulating Dysfunctional Circuits Using Deep Brain Stimulation
Despite the best available medical treatments, many patients continue to be disabled by neurologic and psychiatric disorders, resulting in a large unmet need. Advances in imaging and neurophysiology over the last two decades have led to a reinterpretation of some neurologic and psychiatric conditions as primarily disorders of circuit function, or "circuitopathies." These developments have been accompanied by advances in neurosurgical techniques with the increasingly widespread utilization of deep brain stimulation (DBS) to recalibrate dysfunctional circuits. The versatility of DBS as both a probe and modulator of neural circuits is making it a powerful tool to study the human brain, helping provide important details of the pathophysiology of circuit dysfunction. We are currently in a phase of active investigation to determine which circuits and disorders could be treated with DBS. Here we review recent advances in the DBS field and discuss potential future directions in targeted intracranial neuromodulation.
19307766
Frequency Dependence of Ultrasound Neurostimulation in the Mouse Brain.
Ultrasound neuromodulation holds promise as a non-invasive technique for neuromodulation of the central nervous system. However, much remains to be determined about how the technique can be transformed into a useful technology, including the effect of ultrasound frequency. Previous studies have demonstrated neuromodulation in vivo using frequencies <1 MHz, with a trend toward improved efficacy with lower frequency. However, using higher frequencies could offer improved ultrasound spatial resolution. We investigate the ultrasound neuromodulation effects in mice at various frequencies both below and above 1 MHz. We find that frequencies up to 2.9 MHz can still be effective for generating motor responses, but we also confirm that as frequency increases, sonications require significantly more intensity to achieve equivalent efficacy. We argue that our results provide evidence that favors either a particle displacement or a cavitation-based mechanism for the phenomenon of ultrasound neuromodulation.
205436259
Transcranial focused ultrasound modulates the activity of primary somatosensory cortex in humans
Improved methods of noninvasively modulating human brain function are needed. Here we probed the influence of transcranial focused ultrasound (tFUS) targeted to the human primary somatosensory cortex (S1) on sensory-evoked brain activity and sensory discrimination abilities. The lateral and axial spatial resolution of the tFUS beam implemented were 4.9 mm and 18 mm, respectively. Electroencephalographic recordings showed that tFUS significantly attenuated the amplitudes of somatosensory evoked potentials elicited by median nerve stimulation. We also found that tFUS significantly modulated the spectral content of sensory-evoked brain oscillations. The changes produced by tFUS on sensory-evoked brain activity were abolished when the acoustic beam was focused 1 cm anterior or posterior to S1. Behavioral investigations showed that tFUS targeted to S1 enhanced performance on sensory discrimination tasks without affecting task attention or response bias. We conclude that tFUS can be used to focally modulate human cortical function.
18493808
Image-Guided Transcranial Focused Ultrasound Stimulates Human Primary Somatosensory Cortex
Focused ultrasound (FUS) has recently been investigated as a new mode of non-invasive brain stimulation, which offers exquisite spatial resolution and depth control. We report on the elicitation of explicit somatosensory sensations as well as accompanying evoked electroencephalographic (EEG) potentials induced by FUS stimulation of the human somatosensory cortex. As guided by individual-specific neuroimage data, FUS was transcranially delivered to the hand somatosensory cortex among healthy volunteers. The sonication elicited transient tactile sensations on the hand area contralateral to the sonicated hemisphere, with anatomical specificity of up to a finger, while EEG recordings revealed the elicitation of sonication-specific evoked potentials. Retrospective numerical simulation of the acoustic propagation through the skull showed that a threshold of acoustic intensity may exist for successful cortical stimulation. The neurological and neuroradiological assessment before and after the sonication, along with strict safety considerations through the individual-specific estimation of effective acoustic intensity in situ and thermal effects, showed promising initial safety profile; however, equal/more rigorous precautionary procedures are advised for future studies. The transient and localized stimulation of the brain using image-guided transcranial FUS may serve as a novel tool for the non-invasive assessment and modification of region-specific brain function.
23089481
Experimental demonstration of noninvasive transskull adaptive focusing based on prior computed tomography scans.
Developing minimally invasive brain surgery by high-intensity focused ultrasound beams is of great interest in cancer therapy. However, the skull induces strong aberrations both in phase and amplitude, resulting in a severe degradation of the beam shape. Thus, an efficient brain tumor therapy would require an adaptive focusing, taking into account the effects of the skull. In this paper, we will show that the acoustic properties of the skull can be deduced from high resolution CT scans and used to achieve a noninvasive adaptive focusing. Simulations have been performed with a full 3-D finite differences code, taking into account all the heterogeneities inside the skull. The set of signals to be emitted in order to focus through the skull can thus be computed. The complete adaptive focusing procedure based on prior CT scans has been experimentally validated. This could have promising applications in brain tumor hyperthermia but also in transcranial ultrasonic imaging.
20446194
Ultrashort echo-time MRI versus CT for skull aberration correction in MR-guided transcranial focused ultrasound: In vitro comparison on human calvaria.
PURPOSE
Transcranial magnetic resonance-guided focused ultrasound (TcMRgFUS) brain treatment systems compensate for skull-induced beam aberrations by adjusting the phase and amplitude of individual ultrasound transducer elements. These corrections are currently calculated based on a preacquired computed tomography (CT) scan of the patient's head. The purpose of the work presented here is to demonstrate the feasibility of using ultrashort echo-time magnetic resonance imaging (UTE MRI) instead of CT to calculate and apply aberration corrections on a clinical TcMRgFUS system.


METHODS
Phantom experiments were performed in three ex-vivo human skulls filled with tissue-mimicking hydrogel. Each skull phantom was imaged with both CT and UTE MRI. The MR images were then segmented into "skull" and "not-skull" pixels using a computationally efficient, threshold-based algorithm, and the resulting 3D binary skull map was converted into a series of 2D virtual CT images. Each skull was mounted in the head transducer of a clinical TcMRgFUS system (ExAblate Neuro, Insightec, Israel), and transcranial sonications were performed using a power setting of approximately 750 acoustic watts at several different target locations within the electronic steering range of the transducer. Each target location was sonicated three times: once using aberration corrections calculated from the actual CT scan, once using corrections calculated from the MRI-derived virtual CT scan, and once without applying any aberration correction. MR thermometry was performed in conjunction with each 10-s sonication, and the highest single-pixel temperature rise and surrounding-pixel mean were recorded for each sonication.


RESULTS
The measured temperature rises were ∼ 45% larger for aberration-corrected sonications than for noncorrected sonications. This improvement was highly significant (p < 10(-4)). The difference between the single-pixel peak temperature rise and the surrounding-pixel mean, which reflects the sharpness of the thermal focus, was also significantly larger for aberration-corrected sonications. There was no significant difference between the sonication results achieved using CT-based and MR-based aberration correction.


CONCLUSIONS
The authors have demonstrated that transcranial focal heating can be significantly improved in vitro by using UTE MRI to compute skull-induced ultrasound aberration corrections. Their results suggest that UTE MRI could be used instead of CT to implement such corrections on current 0.7 MHz clinical TcMRgFUS devices. The MR image acquisition and segmentation procedure demonstrated here would add less than 15 min to a clinical MRgFUS treatment session.
23222210
Targeting accuracy of transcranial magnetic resonance-guided high-intensity focused ultrasound brain therapy: a fresh cadaver model.
OBJECT
This work aimed at evaluating the accuracy of MR-guided high-intensity focused ultrasound (MRgHIFU) brain therapy in human cadaver heads.


METHODS
Eighteen heads of fresh human cadavers were removed with a dedicated protocol preventing intracerebral air penetration. The MR images allowed determination of the ultrasonic target: a part of the thalamic nucleus ventralis intermedius implicated in essential tremor. Osseous aberrations were corrected with simulation-based time reversal by using CT data from the heads. The ultrasonic session was performed with a 512-element phased-array transducer system operating at 1 MHz under stereotactic conditions with thermometric real-time MR monitoring performed using a 1.5-T imager.


RESULTS
Dissection, imaging, targeting, and planning have validated the feasibility of this human cadaver model. The average temperature elevation measured by proton resonance frequency shift was 7.9°C ± 3°C. Based on MRI data, the accuracy of MRgHIFU is 0.4 ± 1 mm along the right/left axis, 0.7 ± 1.2 mm along the dorsal/ventral axis, and 0.5 ± 2.4 mm in the rostral/caudal axis.


CONCLUSIONS
Despite its limits (temperature, vascularization), the human cadaver model is effective for studying the accuracy of MRgHIFU brain therapy. With the 1-MHz system investigated here, there is millimetric accuracy.
8669087
Non-invasive transcranial ultrasound therapy based on a 3D CT scan: protocol validation and in vitro results.
A non-invasive protocol for transcranial brain tissue ablation with ultrasound is studied and validated in vitro. The skull induces strong aberrations both in phase and in amplitude, resulting in a severe degradation of the beam shape. Adaptive corrections of the distortions induced by the skull bone are performed using a previous 3D computational tomography scan acquisition (CT) of the skull bone structure. These CT scan data are used as entry parameters in a FDTD (finite differences time domain) simulation of the full wave propagation equation. A numerical computation is used to deduce the impulse response relating the targeted location and the ultrasound therapeutic array, thus providing a virtual time-reversal mirror. This impulse response is then time-reversed and transmitted experimentally by a therapeutic array positioned exactly in the same referential frame as the one used during CT scan acquisitions. In vitro experiments are conducted on monkey and human skull specimens using an array of 300 transmit elements working at a central frequency of 1 MHz. These experiments show a precise refocusing of the ultrasonic beam at the targeted location with a positioning error lower than 0.7 mm. The complete validation of this transcranial adaptive focusing procedure paves the way to in vivo animal and human transcranial HIFU investigations.
41908338
Time-reversal transcranial ultrasound beam focusing using a k-space method.
This paper proposes the use of a k-space method to obtain the correction for transcranial ultrasound beam focusing. Mirroring past approaches, a synthetic point source at the focal point is numerically excited, and propagated through the skull, using acoustic properties acquired from registered computed tomography of the skull being studied. The received data outside the skull contain the correction information and can be phase conjugated (time reversed) and then physically generated to achieve a tight focusing inside the skull, by assuming quasi-plane transmission where shear waves are not present or their contribution can be neglected. Compared with the conventional finite-difference time-domain method for wave propagation simulation, it will be shown that the k-space method is significantly more accurate even for a relatively coarse spatial resolution, leading to a dramatically reduced computation time. Both numerical simulations and experiments conducted on an ex vivo human skull demonstrate that precise focusing can be realized using the k-space method with a spatial resolution as low as only 2.56 grid points per wavelength, thus allowing treatment planning computation on the order of minutes.
118008175
Spectral Methods in MATLAB
Preface 1. Differentiation matrices 2. Unbounded grids: the semidiscrete Fourier transform 3. Periodic grids: the DFT and FFT 4. Smoothness and spectral accuracy 5. Polynomial interpolation and clustered grids 6. Chebyshev differentiation matrices 7. Boundary value problems 8. Chebyshev series and the FFT 9. Eigenvalues and pseudospectra 10. Time-stepping and stability regions 11. Polar coordinates 12. Integrals and quadrature formulas 13. More about boundary conditions 14. Fourth-order problems Afterword Bibliography Index.
43224697
A k-space method for coupled first-order acoustic propagation equations.
A k-space method for large-scale simulation of ultrasonic pulse propagation is presented. The present method, which solves the coupled first-order differential equations for wave propagation in inhomogeneous media, is derived in a simple form analogous to previous finite-difference methods with staggered spatial and temporal grids. Like k-space methods based on second-order wave equations, the present method is exact for homogeneous media, unconditionally stable for "slow" [c(r) < or = c0] media, and highly accurate for general weakly scattering media. In addition, unlike previous k-space methods, the form of the method allows straightforward inclusion of relaxation absorption and perfectly matched layer (PML) nonreflecting boundary conditions. Numerical examples illustrate the capabilities of the present k-space method. For weakly inhomogeneous media, accurate results are obtained using coarser temporal and spatial steps than possible with comparable finite-difference and pseudospectral methods. The low dispersion of the k-space method allows accurate representation of frequency-dependent attenuation and phase velocity associated with relaxation absorption. A technique for reduction of Gibbs phenomenon artifacts, in which compressibility and exponentially scaled density functions are smoothed by half-band filtering, is introduced. When employed together with this smoothing technique, the k-space method provides high accuracy for media including discontinuities, high-contrast inhomogeneities, and scattering structures smaller than the spatial grid resolution.
23453034
Modeling nonlinear ultrasound propagation in heterogeneous media with power law absorption using a k-space pseudospectral method.
The simulation of nonlinear ultrasound propagation through tissue realistic media has a wide range of practical applications. However, this is a computationally difficult problem due to the large size of the computational domain compared to the acoustic wavelength. Here, the k-space pseudospectral method is used to reduce the number of grid points required per wavelength for accurate simulations. The model is based on coupled first-order acoustic equations valid for nonlinear wave propagation in heterogeneous media with power law absorption. These are derived from the equations of fluid mechanics and include a pressure-density relation that incorporates the effects of nonlinearity, power law absorption, and medium heterogeneities. The additional terms accounting for convective nonlinearity and power law absorption are expressed as spatial gradients making them efficient to numerically encode. The governing equations are then discretized using a k-space pseudospectral technique in which the spatial gradients are computed using the Fourier-collocation method. This increases the accuracy of the gradient calculation and thus relaxes the requirement for dense computational grids compared to conventional finite difference methods. The accuracy and utility of the developed model is demonstrated via several numerical experiments, including the 3D simulation of the beam pattern from a clinical ultrasound probe.
38646896
A k-space Green's function solution for acoustic initial value problems in homogeneous media with power law absorption.
An efficient Green's function solution for acoustic initial value problems in homogeneous media with power law absorption is derived. The solution is based on the homogeneous wave equation for lossless media with two additional terms. These terms are dependent on the fractional Laplacian and separately account for power law absorption and dispersion. Given initial conditions for the pressure and its temporal derivative, the solution allows the pressure field for any time t>0 to be calculated in a single step using the Fourier transform and an exact k-space time propagator. For regularly spaced Cartesian grids, the former can be computed efficiently using the fast Fourier transform. Because no time stepping is required, the solution facilitates the efficient computation of the pressure field in one, two, or three dimensions without stability constraints. Several computational aspects of the solution are discussed, including the effect of using a truncated Fourier series to represent discrete initial conditions, the use of smoothing, and the properties of the encapsulated absorption and dispersion.
120263575
Finite Difference Schemes and Partial Differential Equations
Preface to the second edition Preface to the first edition 1. Hyperbolic partial differential equations 2. Analysis of finite difference Schemes 3. Order of accuracy of finite difference schemes 4. Stability for multistep schemes 5. Dissipation and dispersion 6. Parabolic partial differential equations 7. Systems of partial differential equations in higher dimensions 8. Second-order equations 9. Analysis of well-posed and stable problems 10. Convergence estimates for initial value problems 11. Well-posed and stable initial-boundary value problems 12. Elliptic partial differential equations and difference schemes 13. Linear iterative methods 14. The method of steepest descent and the conjugate gradient method Appendix A. Matrix and vectoranalysis Appendix B. A survey of real analysis Appendix C. A Survey of results from complex analysis References Index.
11338797
Focusing of therapeutic ultrasound through a human skull: a numerical study.
A numerical model was developed which can use digitized layer interfaces to calculate ultrasound wave absorption, diffraction, reflection, and refraction. This model was used to evaluate the feasibility of ultrasound therapy and surgery through a human skull. A digitized human skull profile was obtained from magnetic resonance (MR) images and used to calculate the ultrasound field in the brain of a volunteer from a spherically curved phased array. With no phase correction, the focus of the array was shifted and defocused. The phased array technique was used to correct focal shift, reduce side lobes, and enhance focal amplitude. The optimum source element width was estimated for each frequency to obtain a near optimium focus, and an appropriate frequency range for transskull ultrasound therapy and surgery was determined. Acoustic pressure amplitude on the skull surfaces was examined, and it was shown that the skull heating problem could be overcome. Despite high attenuation, complex interface shape, and nonuniform thickness of a human skull, a sharply focused transskull ultrasound field can be generated for noninvasive ultrasound therapy and surgery in the brain.
17405268
Numerical study of a simple transcranial focused ultrasound system applied to blood-brain barrier opening
In this paper, we investigate the focalization properties of single-element transducers at low frequencies (300 to 1000 kHz) through primate and human skulls. The study addresses the transcranial targeting involved in ultrasound- induced blood-brain barrier (BBB) opening with clinically relevant targets such as the hippocampus and the basal ganglia, which are typically affected by early Alzheimer's and Parkinson's disease, respectively. A finite-difference, timedomain simulation platform is used to solve the 3-D linear acoustic wave equation with CT-based acoustic maps of the skulls. The targeted brain structures were extracted from 3-D brain atlases registered with the skulls and used to virtually position and orient the transducers. The effect of frequency is first investigated and the targeting of the different structures is then tested. The frequency of 500 kHz provided the best tradeoff between phase aberrations and standing wave effects in the human case, whereas the frequency of 800 kHz was most suitable in the case of the primate skull. A fast periodic linear chirp method was developed and found capable of reducing the standing wave effects. Such a simple, affordable, and convenient system is concluded to be feasible for BBB opening in primates and humans and could thus allow for its broader impact and applications.
61917466
A perfectly matched layer for the absorption of electromagnetic waves
A new technique of free-space simulation has been developed for solving unbounded electromagnetic problems with the finite-difference time-domain method. Referred to as PML, the new technique is based on the use of an absorbing layer especially designed to absorb without reflection the electromagnetic waves. The first part of the paper presents the theory of the PML technique. The second part is devoted to numerical experiments and to numerical comparisons with the previously used techniques of free-space simulation. These comparisons show that the PML technique works better than the others in all cases; using it allows us to obtain a higher accuracy in some problems and a release of computational requirements in some others.
140569733
Accuracy of heterogeneous staggered-grid finite-difference modeling of Rayleigh waves
Heterogeneous finite-difference (FD) modeling assumes that the boundary conditions of the elastic wavefield between material discontinuities are implicitly fulfilled by the distribution of the elastic parameters on the numerical grid. It is widely applied to weak elastic contrasts between geologic formations inside the earth. We test the accuracy at the free surface of the earth. The accuracy for modeling Rayleigh waves using the conventional standard staggered-grid (SSG) and the rotated staggered grid (RSG) is investigated. The accuracy tests reveal that one cannot rely on conventional numerical dispersion discretization criteria. A higher sampling is necessary to obtain acceptable accuracy. In the case of planar free surfaces aligned with the grid, 15 to 30 grid points per minimum wavelength of the Rayleigh wave are required. The widely used explicit boundary condition, the so-called image method, produces similar accuracy and requires approximately half the sampling of the wavefield compared to heterogeneous free-surface modeling. For a free-surface not aligned with the grid (surface topography), the error increases significantly and varies with the dip angle of the interface. For an irregular interface, the RSG scheme is more accurate than the SSG scheme. The RSG scheme, however, requires 60 grid points per minimum wavelength to achieve good accuracy for all dip angles. The high computation requirements for 3D simulations on such fine grids limit the application of heterogenous modeling in the presence of complex surface topography.
15532229
Improving intersubject image registration using tissue-class information benefits robustness and accuracy of multi-atlas based anatomical segmentation
Automatic anatomical segmentation of magnetic resonance human brain images has been shown to be accurate and robust when based on multiple atlases that encompass the anatomical variability of the cohort of subjects. We observed that the method tends to fail when the segmentation target shows ventricular enlargement that is not captured by the atlas database. By incorporating tissue classification information into the image registration process, we aimed to increase the robustness of the method. For testing, subjects who participated in the Oxford Project to Investigate Memory and Aging (OPTIMA) and the Alzheimer's Disease Neuroimaging Initiative (ADNI) were selected for ventriculomegaly. Segmentation quality was substantially improved in the ventricles and surrounding structures (9/9 successes on visual rating versus 4/9 successes using the baseline method). In addition, the modification resulted in a significant increase of segmentation accuracy in healthy subjects' brain images. Hippocampal segmentation results in a group of patients with temporal lobe epilepsy were near identical with both approaches. The modified approach (MAPER, multi-atlas propagation with enhanced registration) extends the applicability of multi-atlas based automatic whole-brain segmentation to subjects with ventriculomegaly, as seen in normal aging as well as in numerous neurodegenerative diseases.
16952215
Tetrahedral mesh generation from volumetric binary and grayscale images
We report a general purpose mesh generator for creating finite-element surface or volumetric mesh from 31) binary or gray-scale medical images. This toolbox incorporates a number of existing free mesh processing utilities and enables researchers to perform a range of mesh processing tasks for image-based mesh generation, including raw image processing, surface mesh extraction, surface re-sampling, and multi-scale/adaptive tetrahedral mesh generation. We also implemented robust algorithms for meshing open-surfaces and sub-region labeling. Atomic meshing utilities for each processing step can be accessed with simple interfaces, which can be streamlined or executed independently. The toolbox is compatible with Matlab or GNU Octave. We demonstrate the applications of this toolbox for meshing a range of challenging geometries including complex vessel network, human brain and breast.
40283503
Comparison of analytical and numerical approaches for CT-based aberration correction in transcranial passive acoustic imaging.
Computed tomography (CT)-based aberration corrections are employed in transcranial ultrasound both for therapy and imaging. In this study, analytical and numerical approaches for calculating aberration corrections based on CT data were compared, with a particular focus on their application to transcranial passive imaging. Two models were investigated: a three-dimensional full-wave numerical model (Connor and Hynynen 2004 IEEE Trans. Biomed. Eng. 51 1693-706) based on the Westervelt equation, and an analytical method (Clement and Hynynen 2002 Ultrasound Med. Biol. 28 617-24) similar to that currently employed by commercial brain therapy systems. Trans-skull time delay corrections calculated from each model were applied to data acquired by a sparse hemispherical (30 cm diameter) receiver array (128 piezoceramic discs: 2.5 mm diameter, 612 kHz center frequency) passively listening through ex vivo human skullcaps (n  =  4) to emissions from a narrow-band, fixed source emitter (1 mm diameter, 516 kHz center frequency). Measurements were taken at various locations within the cranial cavity by moving the source around the field using a three-axis positioning system. Images generated through passive beamforming using CT-based skull corrections were compared with those obtained through an invasive source-based approach, as well as images formed without skull corrections, using the main lobe volume, positional shift, peak sidelobe ratio, and image signal-to-noise ratio as metrics for image quality. For each CT-based model, corrections achieved by allowing for heterogeneous skull acoustical parameters in simulation outperformed the corresponding case where homogeneous parameters were assumed. Of the CT-based methods investigated, the full-wave model provided the best imaging results at the cost of computational complexity. These results highlight the importance of accurately modeling trans-skull propagation when calculating CT-based aberration corrections. Although presented in an imaging context, our results may also be applicable to the problem of transmit focusing through the skull.
121747962
Focus Control Aided by Numerical Simulation in Heterogeneous Media for High-Intensity Focused Ultrasound Treatment
A focus control method based on time reversal aided by numerical simulation was implemented to correct focal errors in heterogeneous media by using a 56-element phased array transducer at a frequency of 2 MHz. Two types of acrylic phantom were employed to mimic the acoustic heterogeneities in the human body. Focal errors were significantly reduced by applying the phase correction. Moreover, the effects of amplitude correction, array configuration of the transducer, and transverse wave propagation in an elastic body were examined to improve the focal quality. The results suggested that the ultrasound was effectively converged on the target by correcting the amplitude of the ultrasound, and the accuracy of the focus control was enhanced by changing the array configuration and taking into account the shear elasticity of the elastic body in the simulation.