Overview

The possibility of forming ultrasound images similarly to radiography and photography-- i.e. Acoustography-- has eluded researches for more than half a century.  The fundamental reason holding back this development has been the lack of a sound sensitive detector screen/film.  While many attempts were made to develop an ultrasonic analog of a radiographic/photographic film and detectors, none had all the features needed to make Acoustography a viable method.

Under a NSF-funded SBIR project, Santec Systems, Inc. has successfully developed a novel 2D Acousto-Optic (AO) sensor offering the combined features sought after by many researchers.  This fundamental advancement has made Acoustography a viable ultrasonic imaging method for applications in various fields such as nondestructive testing, medical ultrasound and military underwater surveillance. 

Basic Principles...how Acoustography works...

Acoustography* is a full-field ultrasonic imaging process where a novel, exceptionally high resolution 2D Acousto-Optic (AO) sensor is employed to directly convert the ultrasound into a visual image in real time.  Acoustic images can be formed in through transmission mode (Fig. 1) or in reflection mode (Fig. 2). Acoustic image can be formed using acoustic lens analogous to photography or video camera (Fig. 3). 

Fig 1 : Through transmission mode Acoustography

In the through-transmission shadow mode of Acoustography*, usually suited for nondestructive testing of components during manufacturing, ultrasound is passed through the test component where it is absorbed, reflected, and scattered by material structure and any anomalies therein.  The projection image of the material structure and anomalies is created by the ultrasound as it exits the test component.  This projection image is directly converted into a corresponding visual image by the AO sensor in real time. 

Fig 2 : Reflection mode Acoustography

In reflective shadow mode of Acoustography, usually suitable for in-service nondestructive testing of components, the sound source and the AO sensor are on the same side of the test component.  Ultrasound enters the component and is reflected from the back surface.  Upon exiting the component, the ultrasound image is directly converted into a corresponding visual image by the AO sensor in real time.

Fig 3 : Acoustic Camera

In the acoustic lens mode of Acoustography, usually suited for military underwater surveillance and medical imaging, the area of interest is ultrasonically illuminated.  Ultrasound reflected and scattered by the object is received by an acoustic lens that forms a 2D image, which is directly converted into a corresponding visual image by the AO sensor in real time.  

NOTE:  Acoustography-based ultrasound images can be digitized for computer storage and image enhancement using commercially available video cameras and frame grabbers. 

Direct Acousto-Optic Conversion

The AO sensor converts ultrasound image directly into a visual image due to the inherent acousto-optic effect of a proprietary "mesophase" material contained in the sensor.  This circumvents the need for complex electronics necessary in conventional C-scan and 2D piezoelectric arrays used to convert the ultrasonic data into a visual map (see image above)

High Resolution

The AO sensor offers exceptionally high pixel resolution because ultrasound is sensed by a continuous layer of a proprietary "mesophase" material with sensing molecules that are on the order of 20 Angstroms.  Accounting for the correlation length between the molecules, the effective pixel resolution of the AO sensor is on the order of microns.  This inherent property of the AO sensor offers a major advantage over conventional point-by-point ultrasonic C-scan systems, where the lateral resolution depends on index spacing between data points.  To match the AO sensor's resolution, the C-scan will require index spacing on the order of few micron that will take prohibitively long time to scan large areas.

Wide Area

The AO sensor can be fabricated to have a large sensing area without compromising its high resolution.   Therefore, large areas (e.g. 6"x 6") of the test component can be imaged instantly.

*Nondestructive Testing Handbook, 2nd Edition: vol. 9, Special Nondestructive Testing Methods, Columbus, OH, American Society for Nondestructive Testing (1995), p278-284.

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