Speed of Sound and Attenuation Temperature Dependence of Bovine Brain: Ex Vivo Study

Ultrasound Mediated Polymerization for Cell Delivery, Drug Delivery, and 3D Printing

Safe and accurate in situ delivery of biocompatible materials is a fundamental requirement for many biomedical applications. These include sustained and local drug release, implantation of acellular biocompatible scaffolds, and transplantation of cells and engineered tissues for functional restoration of damaged tissues and organs. The common practice today includes highly invasive operations with major risks of surgical complications including adjacent tissue damage, infections, and long healing periods. In this work, a novel non-invasive delivery method is presented for scaffold, cells, and drug delivery deep into the body to target inner tissues. This technology is based on acousto-sensitive materials which are polymerized by ultrasound induction through an external transducer in a rapid and local fashion without additional photoinitiators or precursors. The applicability of this technology is demonstrated for viable and functional cell delivery, for drug delivery with sustained release profiles, and for 3D printing. Moreover, the mechanical properties of the delivered scaffold can be tuned to the desired target tissue as well as controlling the drug release profile. This promising technology may shift the paradigm for local and non-invasive material delivery approach in many clinical applications as well as a new printing method - "acousto-printing" for 3D printing and in situ bioprinting.

Tissue specific considerations in implementing high intensity focussed ultrasound under magnetic resonance imaging guidance

High-intensity focused ultrasound can ablate a target permanently, leaving tissues through which it passes thermally unaffected. When delivered under magnetic resonance (MR) imaging guidance, the change in tissue relaxivity on heating is used to monitor the temperatures achieved. Different tissue types in the pre-focal beam path result in energy loss defined by their individual attenuation coefficients. Furthermore, at interfaces with different acoustic impedances the beam will be both reflected and refracted, changing the position of the focus. For complex interfaces this effect is exacerbated. Moreover, blood vessels proximal to the focal region can dissipate heat, altering the expected region of damage. In the target volume, the temperature distribution depends on the thermal conductivity (or diffusivity) of the tissue and its heat capacity. These are different for vascular tissues, water and fat containing tissues and bone. Therefore, documenting the characteristics of the pre-focal and target tissues is critical for effective delivery of HIFU. MR imaging provides excellent anatomic detail and characterization of soft tissue components. It is an ideal modality for real-time planning and monitoring of HIFU ablation, and provides non-invasive temperature maps. Clinical applications involve soft-tissue (abdomino-pelvic applications) or bone (brain applications) pre-focally and at the target (soft-tissue tumors and bone metastases respectively). This article addresses the technical difficulties of delivering HIFU effectively when vascular tissues, densely cellular tissues, fat or bone are traversed pre-focally, and the clinical applications that target these tissues. The strengths and limitations of MR techniques used for monitoring ablation in these tissues are also discussed.

Two-dimensional mapping of the ultrasonic attenuation and speed of sound in brain

Brain is inhomogeneous due to its composition of different tissue types (gray and white matter), anatomical structures (e.g. thalamus and cerebellum), and cavities in the brain (ventricles). These inhomogeneities lead to spatial variations in the ultrasonic properties of the organ. The goal of this study is to characterize the spatial variation of the speed of ultrasound and frequency slope of attenuation in fixed sheep brain. 1-cm-thick slices of tissue from the coronal, sagittal and transverse cardinal planes were prepared from 12 brains. Ultrasonic measurements were performed using broadband transducers with center frequencies of 3.5, 5.0, 7.5 and 10 MHz. By mechanically scanning the transducers over the specimens, two-dimensional maps of the speed of sound (SOS) and frequency slope of attenuation (FSA) were produced. Measured values for the spatial mean and standard deviation of FSA ranged between 0.59 and 0.81 dB/cm·MHz and 0.29-0.60 dB/cm·MHz, respectively, depending on the specimen and transducer frequency. Measured values for the spatial mean and standard deviation of SOS ranged from 1532-1541 m/s and 10-14 m/s, respectively. Detailed, two-dimensional maps of FSA and SOS were produced, representing the first such characterization of the spatial variation of the ultrasonic properties of normal mammalian brain.

Ultrasonic Thermal Monitoring of the Brain Using Golay-Coded Excitations-Feasibility Study

Thermal monitoring during focused ultrasound (FUS) transcranial procedures is mandatory and commonly performed by MRI. Transcranial ultrasonic thermal monitoring is an attractive alternative. Furthermore, using the therapeutic FUS transducer itself for this task is highly desirable. Nonetheless, such application is challenged by massive skull-induced signal attenuation and aberrations. This study examined the feasibility of implementing the Golay-coded excitations (CoE) for temperature monitoring in bovine brain samples in the range of 35 °C-43 °C (hyperthermia). Feasibility was assessed using computer simulations, water-based phantoms, and ex vivo bovine brain white-matter samples. The samples were gradually heated to about 45 °C and sonicated during cool down with a 1-MHz therapeutic FUS implementing Golay CoE. Initially, a calibration curve correlating the normalized time-of-flight (TOF) changes and the temperature was generated. Next, a bovine bone was positioned between the FUS and the brain samples, and the scanning process was repeated for different fresh samples. The calibration curve was then used as a mean for estimating the temperature, which was compared to thermocouple measurements. The simulations demonstrated a substantial improvement in signal-to-noise ratio (SNR) and suggested that the implementation of 4-bit sequences is advantageous. The experimental measurements with bone demonstrated good temperature estimation with an average absolute error for the water phantoms and brains of 1.46 °C ± 1.22 °C and 1.23 °C ± 0.99 °C, respectively. In conclusion, a novel noninvasive method utilizing the Golay CoE for ultrasonic thermal monitoring using a therapeutic FUS transducer is introduced. This method can lead to the development of an acoustic tool for brain thermal monitoring.