SUBJECT: Ph.D. Proposal Presentation
BY: Michael Gray
TIME: Monday, February 13, 2012, 12:00 p.m.
PLACE: Love Building, 109
TITLE: Convergent Field Elastography
COMMITTEE: Dr. Peter Rogers, Chair (ME)
Dr. Mardi Hastings (ME)
Dr. David Ku (ME)
Dr. Mark Prausnitz (CHBE)
Dr. Darlene Ketten (Woods Hole Oceanographic Institution)


An ultrasound-based system for non-invasive determination of the in vivo, low frequency, complex-valued shear speed of whale and dolphin head tissues is investigated. The system is intended to provide material parameters needed to model the response of the head to sound, as well as a diagnostic capability for use by marine mammal stranding responders. The design of the system is intended to allow estimation of shear speed in soft tissues exterior to skeletal structures, as well as tissues occluded by compact bone in certain portions of the mandible and the skull. The system concept builds upon previous work in soft tissue elastography and ultrasonic vibrometry while introducing several novel design concepts that are intended to overcome the problems and limitations of existing systems.

The central design feature is a sectored annular ultrasonic source that remotely creates a ring-like forcing field. The distributed force generates a shear wave field, part of which converges to the center of the force field. A second confocal transducer nested inside the shear wave generation source is used to measure the component of the converging shear wave field motion that is parallel to the forcing axis. Shear speed is found by calculating the change in tissue vibration phase as the forcing ring radius is dilated by a carrier frequency downshift. It is hypothesized that this concept for convergent field elastography will significantly improve the overall ability to estimate soft tissue shear speeds in thick, complex tissues while keeping within FDA-mandated ultrasound exposure limits.

The scope of the proposed thesis is to model, design and test a prototype ultrasound system that will allow estimation of complex shear speed at frequencies below 1 kHz. The performance of the system will be assessed through laboratory experiments with tissue-mimicking materials whose elastic properties shall be independently ascertained. In order to quantify the impact of intervening bone, the experiments will be augmented to include cranial and mandibular bone samples obtained through a Marine Mammal Stranding Network. Finally, the prototype system will be tested on one or more captive bottlenose dolphins under animal use permits granted by the appropriate regulatory agencies.