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

The effects of image homogenisation on simulated transcranial ultrasound propagation

Transcranial transmission of ultrasound is increasingly used in a variety of clinical and research applications, including high intensity ablation, opening the blood brain barrier, and neural stimulation. Numerical simulations of ultrasound propagation in the head are used to enable effective transcranial focusing and predict intracranial fields. Such simulations require maps of the acoustic properties of the head, which can be derived from clinical CT images. However, the spatial resolution of these images is typically coarser than the scale of heterogeneities within the skull bone, which are known to exert a major influence on ultrasound propagation. 
 
 In the present work, the impact of image related homogenisation on transcranial transmission from a single element transducer is examined using a dataset of co-registered clinical resolution CT and micro-CT images of skull sections. Reference acoustic property maps are derived from micro-CT images of cortical bone tissue. The influence of imaging resolution is examined by progressively downsampling the segmented acoustic property maps, and through comparison with maps derived from co-registered clinical CT images. The influence of different methods of segmenting the bone volume from the clinical CT images, and for resampling the clinical and micro-CT data are also examined. 
 
 Image related homogenisation is demonstrated to have a substantial effect on the transcranial transmission of ultrasound, resulting in underestimations of simulated transmission loss and time-of flight. Effects on time-of flight are due to the loss of the internal scattering microstructure of the skull, while changes in transmitted ultrasound amplitude are due to both loss of microstructure and other smoothing effects. Inflating the simulated attenuation coefficient of the skull layer reduces the error in transmitted pressure amplitude to around 40%, however this is unable to correct fully for errors in time of flight and the pressure distribution of the transmitted field.&#13

Staircase-free acoustic sources for grid-based models of wave propagation

The k-Wave MATLAB toolbox is widely used to conduct medical ultrasound simulations. It uses a Fourier collocation method to numerically solve the governing model equations, and introduces sources by adding acoustic pressure at points on an orthogonal grid. This approach introduces two errors when sources don't exactly align with the grid. These are phase errors arising from shifting source points to nearby grid nodes, and amplitude errors arising from an angular dependence in the density of source points. These two errors are collectively referred to as `staircasing'. Staircasing errors can be overcome by considering the band-limited representation of sources that arises from the use of a Fourier collocation method. To do so, sources are discretised by convolving a band-limited point source with the desired source geometry. To validate this approach, a comparison is made with current k-Wave source algorithms and with the FOCUS ultrasound simulation code. The new sources are shown to eliminate staircasing errors

Safety of Nonsteroidal Anti-inflammatory Drugs in Major Gastrointestinal Surgery: A Prospective, Multicenter Cohort Study

Background Significant safety concerns remain surrounding the use of nonsteroidal anti-inflammatory drugs (NSAIDs) following gastrointestinal surgery, leading to wide variation in their use. This study aimed to determine the safety profile of NSAIDs after major gastrointestinal surgery. Methods Consecutive patients undergoing elective or emergency abdominal surgery with a minimum one-night stay during a 3-month study period were eligible for inclusion. The administration of any NSAID within 3 days following surgery was the main independent variable. The primary outcome measure was the 30-day postoperative major complication rate, as defined by the Clavien–Dindo classification (Clavien–Dindo III–V). Propensity matching with multivariable logistic regression was used to produce odds ratios (OR) and 95 % confidence intervals. Results From 9264 patients, 23.9 % (n = 2212) received postoperative NSAIDs. The overall major complication rate was 11.5 % (n = 1067). Following propensity matching and adjustment, use of NSAIDs were not significantly associated with any increase in major complications (OR 0.90, 0.60–1.34, p = 0.560). Conclusions Early use of postoperative NSAIDs was not associated with an increase in major complications following gastrointestinal surgery