100 MHz sub-millimeter size fiber optic pressure sensors: a luxury or necessity?

Poster presented at Biomedical Technology Showcase 2006, Philadelphia, PA. Retrieved 18 Aug 2006 from http://www.biomed.drexel.edu/new04/Content/Biomed_Tech_Showcase/Poster_Presentations/Lewin_1.pdf.In the past decade, medical diagnostic ultrasound has become the primary noninvasive imaging modality because unlike CT or PET-scanners it does not use the ionizing radiation. In addition it is inexpensive in comparison with the MRI imaging and last but not least ultrasound imaging provides real-time information on the moving anatomical structures. Although diagnostic ultrasound safety record is impeccable and no side effects were reported in clinical applications, in general, the ultrasound exposure may lead to undesirable biological effects. Therefore, the acoustic output of the diagnostic ultrasound devices is regulated and cannot exceed prescribed limits. In the USA, these limits are established by the Food and Drug Administration's Center for Devices and Radiological Health, which requires the safety indicators such as Mechanical Index (MI) and Thermal Index (TI) to be displayed on the ultrasound imaging systems. Determination of these two indices requires precise characterization and measurements of the acoustic pressure-time waveforms produced by the imaging transducer. The objective of the research described here is to develop and optimize the calibration techniques for ultrasonic hydrophone probes capable of measuring acoustic fields at the frequencies beyond 20 MHz in particular beyond 60 MHz. Such techniques are currently unavailable and these high megahertz frequencies are gaining attention in skin, eye and intraluminal imaging as they offer enhanced sub-millimeter resolution. These objectives will be accomplished by development and implementation of two independent (acoustic and optic) measurement techniques that are capable of providing sensitivity versus frequency response of miniature ultrasonic probes over a wide, 100 MHz bandwidth. The innovative elements of the proposed research include implementing a 100 MHz fiber optic (FO) hydrophone probe with an active diameter of about 11 μm (microns) that will eliminate the need for spatial averaging correction and is sufficiently robust to measure fields generated by High Intensity Focused Ultrasound (HIFU) transducers. The intrinsically rugged characteristics of the fiber constitute an attractive feature as the existing probes are fragile and, in practice, cannot be used in therapeutic HIFU fields. Preliminary data indicate that once fully developed and calibrated, the acousto-optic system will form an important breakthrough in acoustic measurements of both diagnostic and therapeutic fields