Аннотация:Background, Motivation and Objective:
The advent of therapeutic applications of medical ultrasound has highlighted the need for precise control of the acoustic fields delivered to tissue. While measurement standards are currently being developed to characterize high-intensity fields, metrological tools are needed to effectively meet these standards. Toward this end, acoustic holography is a technique that can be used to capture 3D field information by measuring pressure magnitude and phase over a 2D surface. For low-intensity, linear fields, a measured 2D hologram can efficiently capture the entire acoustic field, including focusing characteristics and side lobes. For high-intensity fields, a measured hologram can be used to define vibratory boundary conditions at the source's surface so that the full nonlinear field can be modeled. The objective here is to assess the practical feasibility of using holography to characterize both therapeutic and diagnostic transducers.
Statement of Contribution/Methods:
Acoustic holography was used to characterize three separate ultrasound sources: a focused, single-element transducer; a C5-2 imaging probe; and a two-dimensional therapeutic array. In each case, continuous-wave pressure measurements over a 2D surface in front of the transducer were acquired using an Onda lipstick hydrophone. These recorded measurements were analyzed within a case-specific time window to identify the steady-state pressure magnitude and phase. The acoustic field defined by the measured hologram was then numerically back-propagated to define the source's surface vibrations.
Results/Discussion:
Holographic reconstruction of the single-element transducer demonstrates that angular misalignment of the measurement coordinate system relative to the transducer is easily identified and corrected from phase information. Moreover, these data clearly demonstrate the capability of to capture non-ideal transducer behaviors. Reconstructions of the diagnostic and therapeutic transducer arrays demonstrate that element patterns can be identified, and a non-operational element for the therapeutic array is clearly seen. In addition, source holograms for the arrays clearly capture phase variations associated with beam steering and/or focusing by a lens. Results confirm the applicability of holography techniques for medical ultrasound transducers. Holography data can be used effectively as boundary conditions for numerical models, measurements of source output power, and quality assurance records. While holography shows promise as a metrological tool, advances in the practical implementation of the technique are still needed (e.g., faster scanning times, quantitative error estimates for particular measurement parameters). [Work supported by NIH EB007643, RFBR 09-02-01530, and NSBRI through NASA NCC 9-58].