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Electronic Imaging & Signal Processing

2D source-detector arrays enhance spatial information in diffuse imaging

A new approach based on multi-element source-detector arrays enhances the spatial resolution and depth-discrimination capability of diffuse optical imaging.
13 March 2006, SPIE Newsroom. DOI: 10.1117/2.1200602.0062

Diffuse optical imaging is a noninvasive technique for the study of highly-scattering media. One important example is near-infrared imaging of biological tissue, such as functional mapping of the brain,1 optical mammography,2 and optical oximetry in various tissues. The high sensitivity of diffuse optical imaging to changes in blood distribution and oxygenation within tissue means that the technique has significant potential for diagnostic and functional studies.3 Unfortunately, the strong diffusion of light in tissue reduces spatial information and complicates the quantitative measurement of tissue properties.

Previously-proposed methods of enhancing spatial resolution have used time-domain4 and frequency-domain5,6 approaches. The issue of depth discrimination has been tackled by introducing off-axis detection,7 by applying two-layer or multilayer models,8 or by fully-fledged solutions of the inverse imaging problem.9 Here, we propose a multi-element phased-array method that extends the concept of two-element phased-arrays previously proposed,5 and that does not rely on any a-priori information about boundary conditions or spatial features of tissue inhomogeneities. This new approach achieves an enhanced spatial resolution10 and depth discrimination11 with respect to a single-source/single-detector imaging scheme.

Let's consider an array of three collinear continuous-wave light sources and a single optical detector, as shown in Figure 1. We indicate the detected intensity associated with each source as Ii. Our phased-array approach consists of normalizing Ii by the background intensity (I0i) and introducing individual amplitude (Ai) and phase (αi) factors to yield a phased-array intensity (IP-A) according to Equation 1.

Figure 1. Representative phased-array consisting of three sources having the indicated amplitude (Ai) and phase (αi) factors as defined in Eq. (1). The detector faces the central source at a distance of 6cm, while the inter-source separation is 1cm. The dashed lines represent a section of the null surfaces (where the phased-array intensity is zero).

The distance between source and detector (6cm) represents a typical source-detector distance used in optical studies of thick tissues.

The improvement in spatial resolution is illustrated in Figure 2, which shows linear scans of single-source and phased-array intensities within a turbid medium (milk with black India ink). Two black cylinders were equidistant (3.0cm) from the source array and the collection fiber.Figure 2 shows that the two cylinders are not resolved by the single-source intensity trace, but are resolved by the phased-array trace with two separate peaks associated with each cylinder.

Figure 2. Line scans of the intensity recorded with a single fiber (center-fiber intensity) and by combining the intensities from a three-element source array (phased-array intensity). The two absorbing objects, separated by 10mm, are resolved only by the phased-array intensity.

To test the depth-discrimination potential, we used the experimental setup illustrated in Figure 3, which uses a superposition of three linear source arrays along different directions. We calculated the phased-array intensity associated with each linear array (according to Equation 1) and took the maximum to optimize sensitivity to directional structures. Three black cylinders were placed between the source array and the detector fiber, with their axes parallel to the plane of the source array and at different depths.Figure 4shows that the single-source intensity is equally sensitive to the top and bottom cylinders, while the intensity with the phased array on top (bottom) is more sensitive to the top (bottom) cylinder.

Figure 3. Experimental setup for the depth discrimination study. PMT: photomultiplier tube.

Figure 4. Comparison of the images of the three cylinders obtained with (a) a single source, (b) the phased array on top, and (c) the phased array on the bottom. The top cylinder (horizontal) appears with higher contrast when the phased array is on top, while the bottom cylinder (diagonal) appears with high contrast when the phased array is on the bottom.

Enhancement in spatial resolution (Figure 2) and depth-discrimination capability (Figure 4) finds important applications in diffuse optical imaging. In particular, the approach presented here directly lends itself to implementations in optical mammography, where it is possible to employ a transmission geometry similar to the one used in these studies.

Detecting tumors, identifying localized tissue areas associated with specific functional activities, or assessing local changes in tissue metabolism are applications within reach of near-infrared imaging of tissue. While the intrinsic optical contrast associated with the blood spatial distribution and with its oxygenation is high, the spatial-information content of diffuse optical imaging is intrinsically limited by the diffusive nature of near-infrared light propagation in tissue. Our proposed phased-array approach has the potential to enhance the spatial resolution and to afford depth discrimination, thus improving the spatial information content of diffuse optical images. Our next steps will be to explore different sets of amplitude and phase coefficients in Equation 1, to implement a dual-source array and detector array, and to study the feasibility of applying these principles to a reflectance geometry.

This research was supported by the National Science Foundation (Award BES-93840) and by the National Institutes of Health (Grants DA14178 and CA95885).

Ning Liu, Angelo Sassaroli, Sergio Fantini
Department of Biomedical Engineering, Bioengineering Center, Tufts University
Medford, MA
Ning Liu is a research assistant and PhD candidate in the Biomedical Engineering Department at Tufts University, where she is working in the field of optical imaging of tissue. She graduated from Sichuan University (China) with a BS in physics, and an MS in radio electronics. She received a second masters degree in electrical engineering from Tufts University. In addition, Ning Liu has written several articles for SPIE conferences: these include Optical Technologies in Biophysics and Medicine and Lasers and Electro-Optics, 2004, where she also presented.
Angelo Sassaroli, received his physics degree from the University of Florence in 1996. He received his PhD from the University of Electro-Communications, Tokyo, Japan, in 2002. Since then he has been a research associate at Tufts University in the field of NIRS and diffuse optical imaging of tissues. In addition, he has authored and co-authored of several papers in SPIE's Optical tomography and Spectroscopy of Tissue.
Prof. Sergio Fantini received his doctoral degree in physics from the University of Florence, Italy. From 1993 to 1999, he held postdoctoral and faculty appointments at the University of Illinois at Urbana-Champaign. In 1999, he joined Tufts University where he is currently an associate professor of biomedical engineering. His research interests include the development of new optical approaches to medical diagnostics and functional monitoring \emph{in vivo.} In addition, he has moderated SPIE's BiOS Hot Topics session for the last three years, and has presented numerous papers at SPIE's conferences onOptical Tomography and Spectroscopy of Tissue, and Optical Techniques in Brain Imaging.

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