Jaswinder S. Sandhu

Acoustography for rapid ultrasonic inspection of composites

Acoustography provides an alternative to the conventional point-by-point scanning approach employed for ultrasonic inspection of composites. In acoustography, an acousto-optic area detector is employed for direct ultrasonic imaging of composites in near real time. In this work, we will report on the application of this approach for inspecting composites where the point-by-point approach may not be practical and/or cost-effective.

Full-field acoustomammography using an acousto-optic sensor.

In this Letter the authors introduce a wide-field transmission ultrasound approach to breast imaging based on the use of a large area acousto-optic (AO) sensor. Accompanied by a suitable acoustic source, such a detector could be mounted on a traditional mammography system and provide a mammographylike ultrasound projection image of the compressed breast in registration with the x-ray mammogram. The authors call the approach acoustography. The hope is that this additional information could improve the sensitivity and specificity of screening mammography. The AO sensor converts ultrasound directly into a visual image by virtue of the acousto-optic effect of the liquid crystal layer contained in the AO sensor. The image is captured with a digital video camera for processing, analysis, and storage. In this Letter, the authors perform a geometrical resolution analysis and also present images of a multimodality breast phantom imaged with both mammography and acoustography to demonstrate the feasibility of the approach. The geometric resolution analysis suggests that the technique could readily detect tumors of diameter of 3 mm using 8.5 MHz ultrasound, with smaller tumors detectable with higher frequency ultrasound, though depth penetration might then become a limiting factor. The preliminary phantom images show high contrast and compare favorably to digital mammograms of the same phantom. The authors have introduced and established, through phantom imaging, the feasibility of a full-field transmission ultrasound detector for breast imaging based on the use of a large area AO sensor. Of course variations in attenuation of connective, glandular, and fatty tissues will lead to images with more cluttered anatomical background than those of the phantom imaged here. Acoustic coupling to the mammographically compressed breast, particularly at the margins, will also have to be addressed.

Real-time full-field ultrasonic inspection of composites using acoustography

Many small composite parts undergo manual pulse-echo scan because 1) the set-up time for and automated scan is unjustifiably long and 2) the automated scan does not provide the flexibility to cope with frequent angle changes in a complex geometry part. Manual scans can be time consuming, laborious, and are prone to errors due to operator fatigue and subjectivity. What is required is a full-field ultrasonic inspection system analogous to real time radiography that allows the operator to perform ultrasonic inspection by manipulating the part under a systems field of view. In this paper, we will present an acoustography-based ultrasonic inspection system developed under a SBIR (Small Business Innovation Research) award that is bringing this vision to into reality. Acoustography is the ultrasonic analog of radiography and photography. A unique, wide area 2D detector, called acousto-optic (AO) sensor, is used to directly convert ultrasound into visual images; much like a fluorescent screen is able to convert x-rays into visual images. It offers the potential for providing the NDT engineer with a large field of view (e.g. 6”x 6” or larger) and a capability to inspect complex shaped parts in real time.

Acoustography: a side-by-side comparison with conventional ultrasonic scanning

Acoustography is being developed as an alternative to conventional point-by-point ultrasonic scanning commonly employed for composite inspection. In acoustography, an area detector is used to produce full-field images of the test component in near real time, which makes the method suitable for providing rapid ultrasonic inspection of composites. Although the possibility of using acoustography to inspect composites has recently been demonstrated, a side-by-side comparison of acoustography with conventional ultrasonic scanning has not been made. In this work we will report on studies conducted toward establishing a direct comparison between acoustography and conventional ultrasonic scanning.

Approaches for non-uniformity correction and dynamic range extension for acoustography

Acoustography is a full-field ultrasonic imaging process where a high resolution 2D acousto-optic sensor based on liquid crystal technology is employed to directly convert the ultrasound into a visual image in near real time. Unprocessed acoustography images typically suffer from non-uniformity due to spatial variations in the optical brightness response of the acousto-optic sensor field to ultrasonic intensity. Additionally, dynamic range of the acousto-optic sensor is limited to approximately 20 to 30 db. The nonuniformity and dynamic range limitation can result in difficulty in acoustography image interpretation, impracticality for large field application, and difficulty for use on samples having a wide range of attenuation. The approach of this ongoing study is to apply various methodologies that address these limitations in hopes of extending the usefulness and applications of acoustoography for nondestructive testing. This article shows initial results of methodologies developed to correct for image non-uniformity and explains the proposed approach to extend the dynamic range of acoustography images.

Acoustography: It could be a practical ultrasonic NDE tool for composites

Acoustography is a full field, large area ultrasonic imaging method where a novel, wide area acousto-optic (AO) sensor is employed to form ultrasonic images similarly to real-time x-ray imaging. The AO sensor converts ultrasound directly into a visual image due to the inherent acousto-optic property of a proprietary mesophase material contained in the AO sensor. The AO sensor also offers exceptionally high pixel resolution, as a continuous layer of the mesophase material, with sensing molecules on the order of 20 Angstroms in size, senses the ultrasound. This paper will report on progress being made under a SBIR project to develop acoustography as an efficient and economical alternative to conventional point-by-point ultrasonic scanning (e.g. A-scan, C-scan).

Ultrasonic inspection of tight-radii in composites using acoustography

Acoustography provides an alternative to the conventional point-by-point ultrasonic scanning approach commonly used for composite inspection. In acoustography, an area detector (sonoplate) is employed for near real-time imaging of composites. In this paper, we report on the application of acoustography for ultrasonic inspection of tight-radii in composite components/specimens, where conventional point-by- point ultrasonic scanning may not be practical and/or cost- effective.

SU-D-134-05: Using Monochromatic Sources to Obtain Depth Information in Multispectral Transmission Ultrasound Breast Imaging.

PURPOSE To investigate the feasibility of using a limited number of multispectral transmission ultrasound images acquired with a novel full-field liquid crystal ultrasonic detector to estimate the sizes of cystic and malignant breast lesions. METHODS In our prototype ultrasound imaging system, a high-resolution liquid crystal detector measures the intensity of the acoustic field transmitted through the compressed breast. Projection images can be acquired at multiple transducer frequencies with several monochromatic sources. Assuming normal breast parenchyma containing either a simple breast cyst or infiltrating duct carcinoma, image data acquired at two or more transducer frequencies can potentially be used to estimate the size of the lesion present. The presence of electronic Gaussian noise precludes an exact lesion thickness determination; the lesion thickness can only be estimated with some uncertainty. We have used estimation theory to derive the Cramer-Rao lower bound on the uncertainty of the thickness estimate for cystic and malignant lesions of variable sizes. RESULTS For a 1-cm simple breast cyst and SNR of 50, an uncertainty in the estimated cyst thickness of 0.095 cm can be obtained using two transmission ultrasound breast images acquired with transducer frequencies of 5 and 5.508 MHz. For a malignant breast lesion of the same size and SNR of 50, an uncertainty in the lesion thickness estimate of 0.197 cm can be obtained using two breast images acquired with frequencies of 5 MHz and 5.462 MHz. In general, the lower bound on the precision of the thickness estimate is found to improve with increasing SNR and lesion size. CONCLUSION For the cases considered, the Cramer-Rao lower bound on the uncertainty of the thickness estimate is significantly less than the actual lesion size. Furthermore, the precision of the thickness estimate can be improved by using lower transducer frequencies, although diffraction artifacts might then become prohibitive. Department of Defense (DOD) Breast Cancer Research Program IDEA Award W81XWH-11-1-0332.

Acoustography-based ultrasonic testing

Acoustography is the ultrasonic analog of radiography and photography. A unique 2D detector, called acousto-optic (AO) sensor, is used that is capable of directly converting ultrasound into visual images; much like a fluorescent screen is able to convert x-rays into a visual image. The AO sensor offers exceptionally high resolution and can be fabricated to have a large area. This allows image formation through simple shadow casting (analogous to x-ray image formation) or with acoustic lense (analogous to a photographic or video camera). This paper will report on several new developments, which could allow acoustography to provide a simpler more cost-effective alternative to conventional ultrasonic testing.

TH-AB-209-06: An Investigation of the Feasibility of Malignant Lesion Detection During Routine Breast Cancer Screening Using a Prototype Acousto-Optic Transmission Ultrasound Imaging System

PURPOSE To investigate the feasibility of malignant lesion detection during routine breast cancer screening using an acousto-optic (AO) transmission ultrasound imaging system, particularly in premenopausal women with high breast density. METHODS A full-field, single-projection ultrasound imaging system was developed that uses a high-resolution AO detector to convert the acoustic intensity transmitted through the compressed breast into a visual image by virtue of the AO effect in nematic liquid crystals. In this work, a comprehensive system model was proposed to describe the AO imaging process, including the generation of the incident acoustic field by the transducer, the propagation of the field through the breast tissue, and the conversion of the transmitted acoustic field intensity into a visual image. Validation of the imaging model was achieved through comparison of actual AO breast phantom images with simulated images based on the proposed model. Malignant lesion detectability studies were subsequently performed in simulation using homogeneous and heterogeneous numerical breast phantoms. RESULTS Comparison of actual AO breast phantom images with simulated images based on the proposed system model showed strong agreement, with an RMSE less than 4%. Lesion detectability studies using homogeneous numerical breast phantoms demonstrated excellent visibility for breast lesions as small as 0.5 cm and source frequencies on the order of 4 MHz. Though lesion detectability proved immune to spatial variations in mass density (2%) and attenuation (15%) in the breast parenchyma, minor variations in parenchymal sound speed (1.3%) resulted in substantial refraction artifacts that compromised the diagnostic utility of the prototype system. CONCLUSIONS Differences in acoustic absorption may overcome refraction and enhance lesion visibility at conventional breast ultrasound frequencies (10 to 15 MHz). However, provided the FDA limit on the incident acoustic intensity (1 W/cm2 ) is employed, such high-frequency imaging will require improvements in the current AO detector sensitivity (10-6 W/cm2 ). Department of Defense (DOD) Breast Cancer Research Program IDEA Award W81XWH-11-1-0332; National Institutes of Health (NIH) Grant T32 EB002103-24 from the National Institute of Biomedical Imaging and Bioengineering (NIBIB).