Browsing by Subject "Cavitation"

In this dissertation work, the aim was to garner better mechanistic understanding of how shock wave lithotripsy (SWL) breaks stones in order to guide design improvements to a modern electromagnetic (EM) shock wave lithotripter. To accomplish this goal, experimental studies were carefully designed to isolate mechanisms of fragmentation, and models for wave propagation, fragmentation, and stone motion were developed. In the initial study, a representative EM lithotripter was characterized and tested for in vitro stone comminution efficiency at a variety of field positions and doses using phantom kidney stones of variable hardness, and in different fluid mediums to isolate the contribution of cavitation. Through parametric analysis of the acoustic field measurements alongside comminution results, a logarithmic correlation was determined between average peak pressure incident on the stone surface and comminution efficiency. It was also noted that for a given stone type, the correlations converged to an average peak pressure threshold for fragmentation, independent of fluid medium in use. The correlation of average peak pressure to efficacy supports the rationale for the acoustic lens modifications, which were pursued to simultaneously enhance beam width and optimize the pulse profile of the lithotripter shock wave (LSW) via in situ pulse superposition for improved stone fragmentation by stress waves and cavitation, respectively. In parallel, a numerical model for wave propagation was used to investigate the variations of critical parameters with changes in lens geometry. A consensus was reached on a new lens design based on high-speed imaging and stone comminution experiments against the original lens at a fixed acoustic energy setting. The results have demonstrated that the new lens has improved efficacy away from the focus, where stones may move due to respiration, fragmentation, acoustic radiation forces, or voluntary patient movements. Using traditional theory of brittle fragmentation and newfound understanding of average peak pressure correlation to stone comminution, the entire set of stone comminution data for lens comparison was modeled using a Weibull-style distribution function. This model linked both the average peak pressure and shock wave dose to efficacy, including their respective threshold parameters, and demonstrated correlation of coefficients to cavitation activity. Subsequently, this model was used in prediction of stone comminution efficiency from mimicked respiratory motions in vitro, which compared favorably to actual simulated motion studies using both the new and original lenses. Under a variety of mimicked respiratory motions, the new lens produced statistically higher stone comminution efficiency than the original lens. These results were confirmed in vivo in a swine model, where the new lens produced statistically higher stone comminution after 1,000 and 2,000 shocks. Finally, a mechanistic investigation into the effects of cavitation with the original lens was conducted using an integrated, self-focusing annular ring transducer specially designed for tandem pulse lithotripsy. It was found that cavitation and stone comminution efficiency are progressively enhanced by tandem pulsing as source energies of both the primary LSW and trailing pressure pulse increase, which suggests cavitation and stress waves act synergistically enhance the efficacy in kidney stone fragmentation.

Traumatic brain injuries (TBIs) are a major public health concern and socioeconomic burden worldwide. In recent years, brain injuries in US service personnel have focused attention on TBI affecting the military population (Bass et al., 2012). Blast injuries have become the most common cause of mortality and morbidity in soldiers returning from Iraq and Afghanistan (Owens et al., 2008, Warden, 2006). The frequency of blast-related sequelae found in allied forces has led some to call it the ‘signature wound’ of the wars abroad.

The growing incidence of TBI has spurred an increase in research efforts within the neurotrauma community to define TBI etiology. Identification of the critical injury mechanisms underlying TBI is an area of greatest need. Our understanding of TBI etiology, physical damaging mechanisms, and pathophysiology remains inadequate. The ability to design specific countermeasures and targeted prevention strategies is restricted by an incomplete understanding of the underlying damaging mechanisms.

Cavitation, the formation of vapor filled cavities in a liquid medium, has been proposed as a damaging mechanism of TBI in both blunt impacts (Ward et al., 1948, Gross, 1958) and blast-induced neurotrauma (Moore et al., 2008, Panzer et al., 2012c). The cavitation hypothesis of TBI centers on observation that high energy events such as high-explosive blast impingement onto the head generate large pressure transients in and around the brain. Localized areas of low pressure may surpass the tensile limits of the cerebrospinal fluid vaporizing the fluid and forming cavitation bubbles. These voids grow, potentially displacing surrounding tissue. When the bubbles collapse, perhaps violently, jets of liquid with potentially large localized pressures and temperatures may be created, damaging surrounding tissue.

The main objective of this dissertation was to develop an experimental foundation and provide empirical evidence for cavitation as a damaging mechanism of blast-induced TBI. This dissertation uses biofidelic surrogate head models of blast and in vivo animal models of blast injury to address the unanswered questions surrounding cavitation and blast neurotrauma. Foremost, cavitation response was observed in the surrogate head form exposed to blast conditions associated with injury. The 50% risk of cavitation occurs at a blast level of 262 kPa incident overpressure and 1.96 ms duration. This blast dosage represents a 62% chance of mild intracranial bleeding from scaled ferret experiments (Rafaels et al., 2012). Cavitation onsert, growth, and collapse were confirmed through high-speed imaging of the fluid layers of the contrecoup, while strong acoustic emission signatures associated with cavity collapse were captured and time matched with the video. Near-harmonic frequencies at 64 kHz, 126 kHz, and 267 kHz were associated with the energetic collapse of the bubbles. Our results provide compelling evidence that primary blast alone may induce cavitation that leads to TBI.

Evidence of cavitation was recorded in live porcine specimen exposed to blast. Acoustic sensors mounted to the skull of each specimen recorded acoustic emissions during blast exposure. Scaled spectral analysis revealed acoustic energy in higher frequencies bands with peaks at 64 kHz, 139 kHz, and 251 kHz, closely matching the spectral peaks associated with void collapse in surrogate experiments. To our knowledge, this study is the first to present evidence of blast-induced cavitation in a live animal model in the field of cavitation TBI research.

The results presented in this dissertation also greatly improve our understanding of how mechanical loads are imparted onto the head during a blast exposure and how this loading leads to cavitation onset. Strain analysis of the surrogate head indicates wall compliance from skull deformation and shear wave propagation through the skull as significant physical factors driving the tensile fluid responses in the head. Future design considerations for preventative measures should account for these physical mechanisms.

This dissertation also makes important contributions to blast injury research by presenting a clinically relevant murine model of blast TBI. Murine blast lethality risk and functional behavior outcomes before and after blast injury are presented. We provide guidelines for small animal blast testing, along with methodological recommendations for benchtop shock tube design and specimen placement in relation to the shock tube.

The contributions of this dissertation further serve as an important methodological guide to the neurotrauma and biomechanics community studying blast-related TBI and cavitation as a damaging mechanism. The developed surrogate head system and cavitation detection techniques provide a research template and are a springboard to future research efforts elucidating the damaging effects of cavitation during TBI.

Traumatic Brain Injury (TBI) has become a marquee injury of this generation, prevalent in both military and civilian populations (Meaney 2014). Blunt impacts to the head are the known cause of approximately 1.7 million of TBI hospitalizations per year (Meaney 2014), and while mild TBI has the highest incidence (approximately 75% of TBIs) the injuries range from mild concussions to life threating severe bleeding within the brain (Meaney 2014).Due to wide spread prominence, blunt impact TBI has garnered a wealth of academic research interest focusing on the full spectrum of the biological scale, from subcellular and cellular response, to global human body modeling. The foundational theory of current blunt impact TBI research is neurological tissue damage by simple shear strain caused by motion of the skull (Cullen 2016, Alshareef 2020). While this likely contributes to tissue damage, its global perspective does not provide a satisfactory solution to the focal symptomology of TBI etiology. This is most likely because there are less appreciated mechanisms of injury contributing to TBI such as shear shock formation or cerebrospinal fluid (CSF) cavitation. The focus of this dissertation is to unpack the role of CSF cavitation in blunt impact TBI and contribute an important piece missing from the mechanistic understanding of TBI. This work develops an acoustic biomarker that indicates transient cavitation collapse, uses this biomarker to investigate cavitation mechanisms, observes cavitation in fresh, non-frozen, full body pig cadaver blunt impact testing, and provides clinical implications for transient cavitation through a reanalysis of live subhuman primate seminal data. It takes advantage of the large magnitude wideband acoustic emission of transient cavitation collapse, advanced acoustic sensor technology, and novel acoustic analysis methods to uncover a piece of the mechanistic mystery surrounding blunt impact TBI. There are five major conclusions reached in this dissertation. 1: The blunt impact head kinematics that induce cavitation are not significantly influenced by neck strength or cervical muscle activation. 2: Broadband acoustic emissions can be used as an acoustic biomarker to detect the incidence of transient cavitation collapse through the skull. 3: Compliance of the vessel containing a cavitating medium significantly influences the levels at which cavitation occurs during a blunt impact. 4: Blunt impact CSF cavitation occurs in a fresh, non-frozen, uncompromised pig cadaver head at impact levels below catastrophic injury thresholds. 5: Brain contusions are a potential clinical implication of transient cavitation collapse. Due to a lack of tools and technology, previous work on blunt impact cavitation was restricted to experimentation with limitations prohibiting the direct study of intracranial transient CSF cavitation. This innovative work provides direct observation of blunt impact CSF cavitation that benefits tools, injury risk functions, safety device design, and detection methodologies.

Nano Pulse Lithotripter (NPL) is a new technology in intracorporeal lithotripsy. It utilizes a high-voltage spark discharge of about 30-nanosecond duration, released at the tip of a flexible probe under endoscopic guidance, to break up kidney stones into fine powders for spontaneous discharge. Several primary damage patterns have been observed during the NPL treatment of hard and soft BegoStones: crater formation near the probe tip, crack development from the distal wall of the stone, and crack initiation in the form of radial and ring-shape circumferential fracture in the proximal surface of stones of small sizes. Compared to the traditional intracorporeal lithotripsy technologies (laser and EHL), NPL has been shown to comminute kidney stones with higher efficiency, especially with hard stones, although its working mechanisms are largely unclear. Multiple potential contributory factors have been proposed: direct dielectric breakdown in the stone material near the NPL probe tip, shock wave induced by the spark discharge in the fluid, and cavitation and resultant asymmetric collapse of bubbles. Various experiments have been carried out to correlate each of the proposed mechanisms with the damage patterns observed. Comparison between micro-CT images of the damage initiation sites and COMSOL simulation of the stress field in the stone indicate that the observed cracks are most likely to be produced by the locally intensified tensile stresses associated with the surface acoustic waves (SAW) generated by the incidence of the spark-generated, spherically divergent shock wave on the proximal surface of the stone, and/or their interactions with bulk acoustic waves (P or S) upon reflection from the stone boundaries. Dielectric breakdown is found to potentially contribute to crater formation, especially for soft BegoStones. However, the contribution of cavitation to stone fragmentation in NPL appears to be minimal.