Influence of bone microstructure on ultrasound loss through skull‐mimicking digital phantoms

Medical Physics · 2025-10-30 · 3 citations

BACKGROUND: Transcranial focused ultrasound treatments rely on precisely delivering ultrasound through the inhomogeneous human skull. Full-wave ultrasound simulations are a means to predict and correct the resulting ultrasound aberrations and attenuation. To do this, the acoustic properties of the skull, including phase velocity and attenuation, must be determined. A common approach relates computed tomography (CT) Hounsfield Units (HU) to these acoustic properties. In the trabecular regions of skulls, the CT HU values will depend on the fraction of bone and marrow within an image volume element, but they are typically insensitive to the microstructure of the bone and marrow. PURPOSE: This study explores the influence of bone/marrow microstructures on determining the relationship of acoustic properties, particularly loss, to CT HUs. The typical clinical CT resolution (0.5 mm) cannot resolve fine trabecular bone microstructure, suggesting the relationship of attenuation to HU may be ill-determined. METHODS: The ultrasound insertion loss was found through various skull-mimicking digital phantoms consisting of two constituent materials (red marrow and cortical bone) from 0% to 75% porosity. The phantoms were assigned one of six pore diameters ranging from 0.2 to 1.0 mm. Ultrasound simulations were computed using k-Wave with a continuous 230 or 650 kHz uniform pressure source. The insertion loss with and without absorption was defined as the mean pressure through the phantom with respect to the mean pressure in a water-only reference. RESULTS: The simulations at 230 kHz showed that the loss changed with porosity, but specific microstructure had little effect. However, in both nonabsorbing and absorbing 650 kHz source simulations, the insertion loss depended on porosity and pore diameter. Larger pore diameter phantoms generally had higher losses than smaller pore diameter phantoms at the same porosity. In the nonabsorbing phantoms, the maximum range in insertion loss was 2%-52% over the range of pore diameters, which occurred at 20% porosity. Absorbing phantoms increased the loss by an average of 8.2%, with the greatest increase of 13% occurring for the smallest pore diameter at 2.5% porosity. Coherent multiple reflections from the phantom's planar interfaces influenced the loss within smaller pore diameter phantoms. The phase coherence of these reflections was disrupted by increased scattering within the larger pore diameter phantoms. CONCLUSION: The results suggest that the relationship between attenuation and clinical HUs is ill-determined at 650 kHz, since the insertion loss depends on both porosity and pore diameter. The demonstrated uncertainty has important implications for developing CT-derived acoustic models of skull bone, as no single attenuation value can be related to HUs comprised of variable microstructures. Generally, our results show larger pore diameters (coarse microstructures) have higher loss than smaller pore diameters (fine microstructures) at the same porosity, which is consistent with scattering theory.

The choice of viscous or viscoelastic models affects attenuation and velocity determination in simplified skull-mimicking digital phantoms

ArXiv.org · 2025-12-09

Simulation-guided transcranial focused ultrasound therapies rely on estimating skull acoustic properties from pretreatment imaging. Typical clinical resolution (0.5 mm isotropic) cannot resolve bone microstructure, making the acoustic properties underdetermined and sensitive to modeling assumptions. Here, we examine how viscous and viscoelastic models predict changes in attenuation and phase velocity due to microstructure. Using viscous and viscoelastic k-Wave implementations, we simulated transmission of a broadband 625 kHz tone burst (250 kHz-1 MHz) through skull-mimicking digital phantoms. The phantoms contained spherical pores (0.1-1.0 mm diameter) randomly embedded within cortical bone (2.5%-90% porosity). Virtual sensors measured attenuation and phase velocity using a time-distance matrix approach. Both models predict increased attenuation with increasing pore size at a fixed porosity, but differ in the strength and porosity dependence of this relationship. The viscoelastic model generally predicts attenuation peaks at higher porosities than the viscous model. For 1.0 mm pores, the viscous peak (1.98 Np/cm) occurs at 20% porosity, while the viscoelastic peak (2.98 Np/cm) occurs at 70%. Phase velocity decreases with pore size for both models, though the viscoelastic predictions are less sensitive to pore size. These results demonstrate that viscous and viscoelastic models exhibit distinct attenuation and phase-velocity behavior for idealized bone microstructures. While both indicate that microstructure has a strong impact on attenuation, it has a lesser effect on phase velocity for the viscoelastic model compared to the viscous model. This work highlights the importance of acoustic model choice when estimating skull acoustic properties from computed tomography images. Future work will identify which acoustic model best represents ultrasound propagation through skull microstructure.

The speed of sound in skull-mimicking digital phantoms depends on the microstructure

Physics in Medicine and Biology · 2025-11-03 · 1 citations

Abstract Objective. Transcranial focused ultrasound therapies depend on accurately focusing the ultrasound beam through the skull. Simulated phase aberration correction with properties derived from computed tomography (CT) can partially restore the focus. However, the typical clinical CT resolution (0.5 mm isotropic) cannot resolve the bone microstructure, introducing uncertainty in the velocity relationship to CT Hounsfield units (HUs), which reduces focusing precision. Approach. To demonstrate this, we simulated through-transmission measurements through skull-mimicking digital phantoms consisting of cortical bone and marrow with porosities ranging from 0% to 80%. The phantoms comprised spherical marrow pores (0.1–0.6 mm diameter) randomly placed into a cortical background, forming fine-to-coarse microstructures. Using k-Wave, we simulated pulsed and continuous planar sources at four center frequencies (250 kHz, 500 kHz, 750 kHz, 1 MHz). Group and phase velocities are reported for each pore diameter and porosity. The steady-state phase is reported through representative phantoms. Main results. The velocity varies with pore diameter and porosity, with smaller pores yielding faster velocities than larger pores at the same porosity. At 25% porosity and 500 kHz, group velocity ranges from 3147 to 2211 m s −1 and phase velocity from 3168 to 2345 m s −1 across 0.1–0.6 mm pore diameters. The steady state phase depends on the pore diameter and frequency, with the variation across the measurement plane broadening as both increase, indicating dependence on the microstructure’s pore distribution. Significance. The results indicate that the velocity relationship to CT HUs is ill-determined due to the unresolved microstructure. The variation in group velocity impacts pulsed sources, such as those used for histotripsy, while variation in phase velocity affects quasi-continuous sources, including those used for neuromodulation and thermal ablation. Our results emphasize the need to account for the skull microstructure for safer and more effective transcranial focusing.

MR-Compatible Ultrasound Through Transmission for Focused Ultrasound Thermal Therapy

2025-09-15

Focused ultrasound (FUS) therapies for cancer provide non-invasive precise thermal treatment to tissues. Current FUS treatment systems rely on magnetic resonance imaging (MRI), B-mode ultrasound imaging, or harmonic motion imaging for guidance. MRI has the advantage of quantitatively measuring temperature but the high cost and limited availability of MRI limit ultimate impact. An MR-compatible Ultrasound Through Transmission (UTT) system that can quantitatively assess tissue changes caused by temperature is being investigated as a replacement for the current standard of guidance. Two FUS 256-element transducers are mounted in a container with a sample placed at their coinciding geometric center. A FUS transmitter system is connected to the transmitter transducer, while a FUS research system samples the signal at the receiver transducer. A UTT protocol of 256 sequential single-element transmissions with reception on 256 elements is performed on three different samples: homogeneous gelatin phantoms, gelatin phantoms with attenuative inclusions, and porcine meat samples before and after thermal ablation. Hybrid Angular Spectrum (HAS) acoustic simulations are performed on the unheated gelatin samples segmented into regions with measured properties. Simulations are also done on the heated porcine sample segmented into regions with ablative temperatures based on MR temperature imaging. Measured UTT datasets are compared to HAS-predicted transmission data. Complex regression shows good agreement between the UTT-measured and HAS-predicted datasets. On homogeneous samples, the average complex correlation coefficient across all receiver elements is 0.8897. In 1 cm and 2 cm diameter inclusion samples, the correlation is 0.7994 and 0.6934, respectively. On the porcine meat sample, the pre-ablation average correlation is 0.6636, with a post-ablation average correlation of 0.6595. The data indicate that ablation of the tissue causes measurable changes in the received signal, but our simplified model is inadequate to capture the true tissue changes. Future work is investigating this discrepancy with more advanced modeling. The ultimate goal is to use physics-informed neural networks to predict tissue changes from the UTT received signal.

The choice of viscous or viscoelastic models affects attenuation and velocity determination in simplified skull-mimicking digital phantoms.

PubMed · 2025-12-09

Objective: Simulation-guided transcranial focused ultrasound therapies rely on accurately estimating the skull's acoustic properties with pretreatment imaging. The typical imaging resolution (0.5 mm isotropic) is insufficient to resolve the bone microstructure, and as a result, the acoustic properties are underdetermined. Consequently, the determination of acoustic properties is sensitive to methodology. Here, we investigate how viscous or viscoelastic models predict variations in attenuation and phase velocity due to microstructure. Approach: Using viscous and viscoelastic k-Wave implementations, we simulated the transmission of a broadband 625 kHz tone burst ( -6 dB bandwidth: 250 kHz-1 MHz) through skull-mimicking digital phantoms. The phantoms consisted of spherical marrow pores (0.1 mm-1.0 mm diameter) randomly placed within a solid cortical background (2.5%-90% porosity). Virtual sensors measured the attenuation and phase velocity by the time-distance matrix approach. Results: Both models predict that attenuation increases with increasing pore size for a fixed porosity, but the strength of the relationship and its dependence on porosity depend on the simulation technique applied. The viscoelastic model generally predicts that the attenuation peaks at larger porosity values than those predicted by the viscous model. For example, for 1.0 mm pore phantoms, the viscous attenuation peak (1.98 Np/cm) occurs at 20% porosity, while the viscoelastic peak (2.98 Np/cm) occurs at 70%. In general, the phase velocity decreases for larger pore phantoms for both techniques, but the phase velocity/porosity relationship predicted by the viscoelastic model is less sensitive to pore size compared to the viscous model. Significance: Viscous and viscoelastic models predict different attenuation and phase velocity behavior for these idealized bone models. While both approaches predict that properties of bone microstructure have a large impact on acoustic attenuation within the skull, the viscous model indicates that bone microstructure impacts phase velocity within the skull, whereas the viscoelastic model suggests that phase velocity is less sensitive to microstructure. This study highlights the acoustic model's role in predicting transcranial propagation and the need to identify which approach most accurately reflects the physics of the human skull.

Evaluation of acoustic-thermal simulations of in vivo magnetic resonance guided focused ultrasound ablative therapy

International Journal of Hyperthermia · 2024-01-17 · 11 citations

To evaluate numerical simulations of focused ultrasound (FUS) with a rabbit model, comparing simulated heating characteristics with magnetic resonance temperature imaging (MRTI) data collected during in vivo treatment. Methods: A rabbit model was treated with FUS sonications in the biceps femoris with 3D MRTI collected. Acoustic and thermal properties of the rabbit muscle were determined experimentally. Numerical models of the rabbits were created, and tissue-type-specific properties were assigned. FUS simulations were performed using both the hybrid angular spectrum (HAS) method and k-Wave. Simulated power deposition patterns were converted to temperature maps using a Pennes' bioheat equation-based thermal solver. Agreement of pressure between the simulation techniques and temperature between the simulation and experimental heating was evaluated. Contributions of scattering and absorption attenuation were considered. Results: Simulated peak pressures derived using the HAS method exceeded the simulated peak pressures from k-Wave by 1.6 2.7%. The location and FWHM of the peak pressure calculated from HAS and k-Wave showed good agreement. When muscle acoustic absorption value in the simulations was adjusted to approximately 54% of the measured attenuation, the average rootmean-squared error between simulated and experimental spatial-average temperature profiles was 0.046 0.019 C/W. Mean distance between simulated and experimental COTMs was 3.25 1.37 mm. Transverse FWHMs of simulated sonications were smaller than in in vivo sonications. Longitudinal FWHMs were similar. Conclusions: Presented results demonstrate agreement between HAS and k-Wave simulations and that FUS simulations can accurately predict focal position and heating for in vivo applications in soft tissue.

The influence of bone model geometries on the determination of skull acoustic properties

International Journal for Numerical Methods in Biomedical Engineering · 2023-10-04

In this study, we investigated the impact of various simulated skull bone geometries on the determination of skull speed of sound and acoustic attenuation values via optimization using transmitted pressure amplitudes beyond the bone. Using the hybrid angular spectrum method (HAS), we simulated ultrasound transmission through four model sets of different geometries involving sandwiched layers of diploë and cortical bone in addition to three models generated from CT images of ex-vivo human skull-bones. We characterized cost-function solution spaces for each model and, using optimization, found that when a model possessed appreciable variations in resolvable layer thickness, the predefined attenuation coefficients could be found with low error (RMSE < 0.01 Np/cm). However, we identified a spatial frequency cutoff in the models' geometry beyond which the accuracy of the property determination begins to fail, depending on the frequency of the ultrasound source. There was a large increase in error of the attenuation coefficients determined by the optimization when the variations in layer thickness were above the identified spatial frequency cutoffs, or when the lateral variations across the model were relatively low in amplitude. For our limited sample of three CT-image derived bone models, the attenuation coefficients were determined successfully. The speed of sound values were determined with low error for all models (including the CT-image derived models) that were tested (RMSE < 0.4 m/s). These results illustrate that it is possible to determine the acoustic properties of two-component models when the internal bone structure is taken into account and the structure satisfies the spatial frequency constraints discussed.

Influence of cerebrospinal fluid on power absorption during transcranial magnetic resonance‐guided focused ultrasound treatment

Medical Physics · 2023-04-20 · 5 citations

BACKGROUND: Ultrasound beam aberration correction is vital when focusing ultrasound through the skull bone in transcranial magnetic resonance-guided focused ultrasound (tcMRgFUS) applications. Current methods make transducer element phase adjustments to compensate for the variation in skull properties (shape, thickness, and acoustic properties), but do not account for variations in the internal brain anatomy. PURPOSE: Our objective is to investigate the effect of cerebrospinal fluid (CSF) and brain anatomy on beam focusing in tcMRgFUS treatments. METHODS: Simulations were conducted with imaging data from 20 patients previously treated with focused ultrasound for disabling tremor. The Hybrid Angular Spectrum (HAS) method was used to test the effect of including cerebral spinal fluid (CSF) and brain anatomy in determining the element phases used for aberration correction and beam focusing. Computer tomography (CT) and magnetic resonance imaging (MRI) images from patient treatments were used to construct a segmented model of each patient's head. The segmented model for treatment simulation consisted of water, skin, fat, brain, CSF, diploë, and cortical bone. Transducer element phases used for treatment simulation were determined using time reversal from the desired focus, generating a set of phases assuming a homogeneous brain in the intracranial volume, and a second set of phases assigning CSF acoustic properties to regions of CSF. In addition, for three patients, the relative effect of separately including CSF speed of sound values compared to CSF attenuation values was found. RESULTS: We found that including CSF acoustic properties (speed of sound and attenuation) during phase planning compared to phase correction without considering CSF increased the absorbed ultrasound power density ratios at the focus over a range of 1.06 to 1.29 (mean of 17% ± 6%) for 20 patients. Separately considering the CSF speed of sound and CSF attenuation showed that the increase was due almost entirely to including the CSF speed of sound; considering only the CSF attenuation had a negligible effect. CONCLUSIONS: Based on HAS simulations, treatment planning phase determination using morphologically realistic CSF and brain anatomy yielded an increase of up to 29% in the ultrasound focal absorbed power density. Future work will be required to validate the CSF simulations.

Benchmark problems for transcranial ultrasound simulation: Datasets for intercomparison of compressional wave models

Zenodo (CERN European Organization for Nuclear Research) · 2022-02-09 · 1 citations

This dataset contains the skull maps and modeling results associated with the forthcoming publication "Benchmark problems for transcranial ultrasound simulation: Intercomparison of compressional wave models".

Benchmark problems for transcranial ultrasound simulation: Intercomparison of compressional wave models

arXiv (Cornell University) · 2022-02-09 · 4 citations

Computational models of acoustic wave propagation are frequently used in transcranial ultrasound therapy, for example, to calculate the intracranial pressure field or to calculate phase delays to correct for skull distortions. To allow intercomparison between the different modeling tools and techniques used by the community, an international working group was convened to formulate a set of numerical benchmarks. Here, these benchmarks are presented, along with intercomparison results. Nine different benchmarks of increasing geometric complexity are defined. These include a single-layer planar bone immersed in water, a multi-layer bone, and a whole skull. Two transducer configurations are considered (a focused bowl and a plane piston), giving a total of 18 permutations of the benchmarks. Eleven different modeling tools are used to compute the benchmark results. The models span a wide range of numerical techniques, including the finite-difference time-domain method, angular-spectrum method, pseudospectral method, boundary-element method, and spectral-element method. Good agreement is found between the models, particularly for the position, size, and magnitude of the acoustic focus within the skull. When comparing results for each model with every other model in a cross comparison, the median values for each benchmark for the difference in focal pressure and position are less than 10\% and 1 mm, respectively. The benchmark definitions, model results, and intercomparison codes are freely available to facilitate further comparisons.