Published: Feb. 25, 2021 By

Matteo Mazzotti is the first author on two new studies that measure the dynamic response of the human skull, potentially providing a new and non-invasive way to monitor the cranial bone and brain.

Mazzotti is a research associate in the Paul M. Rady Department of Mechanical Engineering as part of Professor Massimo Ruzzene’s lab. His research focus is on the high-frequency dynamics of solids, fluids, and metamaterials, with particular emphasis on ultrasonic guided waves applications for nondestructive evaluation. The work was published in IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control and Ultrasonics this year.

We asked him about potential applications in medicine and where the research goes from here.

Question: How would you describe the work and results of this paper? What are the applications in the real world?
Answer: The best way to visualize our work on cranial guided waves is to think of the brain and skull as the Earth’s core and crust – respectively – with guided waves playing the role of an earthquake. 

Geophysicists have been able to use information from earthquakes propagating on the surface of the Earth to gain understanding on the composition of its core. That is how they established at how many miles below our feet the core of the Earth becomes fluid, for example. In our papers, we followed a similar principle, which consists in generating imperceptible, high-frequency “earthquakes,” or in this case cranial guided waves, on the external surface of the skull. By measuring the dynamic response of the skull and the fluid medium around it, we were able to characterize the material properties of the cranial bone in a totally non-invasive manner and to observe how the cranial vault absorbs and re-transmits an external dynamic input to the brain. 

Our studies indicate that cranial guided waves can potentially be used in clinical applications as a complementary tool to focused ultrasound. For example, they can potentially be employed to monitor changes in the cranial bone marrow from health disorders, or to efficiently transmit acoustic signals through the skull-brain barrier, which can then be leveraged in the treatment of neurological conditions such as the essential tremor. 
Q: Is this a research topic or area you were interested in before joining the lab? 
A: I had been working for many years with guided ultrasonic waves before joining professor Ruzzene’s lab as this is a very active research and applied topic in the fields of mechanical and civil engineering, which is where I come from. However, it is still considered as a sort of niche topic in the biomedical field, where it has been mainly tested to diagnose osteoporosis and improve the healing process of fractures in long bones. The study of guided waves propagation in the skull and brain has only emerged in recent years.

Massimo in the lab with computer
Ruzzene demonstrates lab equipment used in the paper. The findings have implications on the diagnosis of conditions related to brain injuries and may enable new ultrasonic imaging procedures.

Q: What is it like working in the Ruzzene Lab? 
A: I have been working at CU Boulder for a year and a half and what I find to be very exciting is the diversity in research opportunities that the Ruzzene lab, and the Department of Mechanical Engineering in general, can offer. This helped me to expand my career and research goals while interacting and engaging with other research groups on many fascinating projects.

Q: Was there a particular aspect of this work that was hard to complete?
A: The most difficult part of the work was certainly the interpretation of the experimental signal data recorded on the external surface of the skull and understanding how these can be fed to mathematical models that can translate them into useful information.

To visualize this problem, one can think to a signal as a musical composition. Just like music is composed of different fundamental sounds, a signal resulting from cranial guided waves propagating across the skull is composed of several fundamental waves. Each possesses interesting (and sometimes unique) properties that can be leveraged to a different extent depending on the specific application. For example, some fundamental waves radiate energy from the skull to the brain efficiently, whereas others do not. The difficult – but also most intriguing – part of this process was establishing a mathematical “decoder” that could help in classifying and characterizing each fundamental wave.

Q: What research questions are still to be answered in this area?
A: We are currently working on different aspects that span from a more fundamental understanding of the wave propagation process to the improvement of our mathematical models and the design of ad-hoc sensors for wave generation and recording. A question that we are trying to answer is how the mechanical energy is transmitted across cranial sutures. A direct implication of this study would involve the capability to generate and record a signal at two diametrically opposed locations of the skull, which can potentially be leveraged in the measurement of the intracranial pressure from head injuries or tumors.

“Experimental Identification of High Order Lamb Waves and Estimation of the Mechanical Properties of a Dry Human Skull” and “Radiation Characteristics of Cranial Leaky Lamb Waves” were supported by National Science Foundation CMMI Award No. 1933158 on Coupling Skull-Brain Vibroacoustics and Ultrasound Toward Enhanced Imaging, Diagnosis, and Therapy. Other CU Boulder authors on these papers include Massimo Ruzzene with additional support from Mohit Gupta.