IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 9, NO. 2, MARCH/APRIL 2003 257 Angular Domain Imaging of Objects Within Highly Scattering Media Using Silicon Micromachined Collimating Arrays Glenn H. Chapman, Member, IEEE, Maria Trinh, Nick Pfeiffer, Gary Chu, and Desmond Lee Abstract—Optical imaging of objects within highly scattering media, such as tissue, requires the detection of ballistic/quasi-bal- listic photons through these media. Recent works have used phase/coherence domain or time domain tomography (fem- tosecond laser pulses) to detect the shortest path photons through scattering media. This work explores an alternative, angular domain imaging, which uses collimation detection capabilities of small acceptance angle devices to extract photons emitted aligned closely to a laser source. It employs a high aspect ratio, microma- chined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200: 1) containing 51 1000 m etched channels with 102- m spacing over a 10-mm silicon width. With precise array alignment to a laser source, unscattered light passes directly through the channels to the charge coupled device detector and the channel walls absorb the scattered light at angles 0.29 . Objects within a scattering medium were scanned quickly with a computer-controlled axis table. High-resolution images of 100- m-wide lines and spaces were detected at scat- tered-to-ballistic ratios of 5 10 : 1, with objects located near the middle of the sample seen at even higher levels. At 10 :1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction. Monte Carlo simulation programs designed to test the angular domain imaging concept showed that the collimator detects the shortest path length photons, as in other optical tomography methods. Furthermore, the collimator acts as an optical filter to remove scattered light while preserving the image resolution. Simulations suggest smaller channels and longer arrays could enhance detection by 100. Index Terms—Angular domain imaging, lasers, micromachined optics, optical tomography, tissue optics. I. INTRODUCTION R ESEARCHERS have spent many years seeking to de- velop optical detection techniques that will supplement or replace X-rays for imaging objects within tissue. Medical optical tomography techniques depend on the fact that light can penetrate tissue quite deeply, where some (but not much) is absorbed and most becomes heavily scattered. The key to successful optical imaging is separating the components of the light into: a) unscattered or slightly scattered light, which carries information about the structure of the tissue through which it passes, and b) highly scattered light, which is many Manuscript received November 18, 2002; revised February 10, 2003. This work wsa sponsored by the Natural Science and Engineering Research Council of Canada. The authors are with the Simon Fraser University, School of Engineering Sci- ence, Burnaby, BC V5A 1S6, Canada (e-mail: glennc@cs.sfu.ca). Digital Object Identifier 10.1109/JSTQE.2003.811286 orders of magnitude greater and from which it is much more difficult to extract the structural information. The value in exploring optical imaging techniques is due to the fact that light has several important advantages over X-rays for noninvasive imaging of interior body structures. 1) Light is nonionizing at wavelengths in the visible to near- infrared range ( 500-1200 nm). Thus, optical techniques could allow for greater monitoring frequency, enhancing early detection of cancer in areas such as mammography. 2) Unlike X-rays, the optical characteristics of tissue can be measured at varying wavelengths, providing important biomedical and functional information. 3) Optical imaging techniques are compatible with com- puter-aided tomography. 4) The advent of high-power laser diodes at a wide range of wavelengths offers the potential to exploit optical methods to create a small, portable, low-power scanning system. This paper investigates the use of a new type of optical tomog- raphy detection system. Angular domain imaging uses a silicon micromachined collimating array to restrict photons based on the source angle. We discuss the fabrication of the collimators, its testing with scattering mediums, and the computer simula- tion of the underlying principles. II. EXISTING OPTICAL TOMOGRAPHY RESEARCH Most optical tomography uses collimated laser beams as the light source to illuminate the tissue. As noted, light entering the tissue undergoes both absorption and scattering. In its sim- plest form, the laser beam intensity follows an exponentially de- caying Beer–Lambert Law along its path through the media where, for typical mammography values, the absorption coefficient is cm , the scattering coefficient cm , and the depth cm [1]. Light that is unscattered becomes “ballistic photons.” For this example, the ratio of scattered to ballistic photons (scattering ratio or level) is 6.7 10 : 1. Fortunately, most of the light is not scattered uniformly in all directions, but, instead, tends to divert mostly toward the laser beam’s direction of motion. This forward scattering creates an effective absorption anisotropic coefficient, cm for the so called “quasi-ballistic or snake photons” (the ones that are mostly scattered forward). Since these quasi-ballistic photons also contain desired optical 1077-260X/03$17.00 © 2003 IEEE