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MRS BULLETIN • VOLUME 30 • JANUARY 2005 5 Resolution Optical Tomography for Imaging of Biological Tissues Imaging of soft biological tissues using noninvasive techniques is of crucial interest in biomedicine as a satisfactory technique would greatly aid in detecting abnormalities and functions within such tissues. In the December 1, 2004, issue of Optics Letters (p. 2770), S. Sakad ic and L.V. Wang (Optical Imaging Laboratory at Texas A&M University) have reported the realization of high-resolution ultrasound-modulated optical tomography based on optical contrast for imaging soft biological tissues. Ultrasound-modulated optical tomography is a hybrid technique that combines ultrasonic resolution with optical contrast. Ultrasound generates collective motions of the optical scatterers in a tissue and also leads to periodic changes in the optical index of refraction. This leads to fluctuations in the intensity of the speckles formed by multiple scattered light. Thus, the ultrasound-modulated component of light contains information about the tissue from the region of interaction between the optical and ultrasonic waves. Measurement techniques such as parallel speckle detection combined with ultrasound frequency sweep and computer tomography are commonly used to detect the modulated component of light produced by continuous-wave ultrasound. However, pulsed ultrasound provides direct resolution along the optical axis and is more compatible with conventional ultrasound imaging. In Sakad ic and Wang’s study, a longcavity confocal Fabry–Perot interferometer (CFPI) is used to achieve high-resolution ultrasound-modulated optical imaging. A CFPI is advantageous due to its high etendue, which is defined as the geometric capability of an optical system to transmit radiation; more specifically, it is the product of the opening size and solid angle from which the system accepts light. The CFPI is able to detect the propagation of high-frequency ultrasound pulses in real time and tolerate speckle decorrelation. Ultrasound frequency of 15 MHz was used to detect ~100-μm-sized objects placed ~3 mm below the surface of chicken breast tissue samples. Two chicken breast tissue samples were investigated, each with a radius of curvature of ~3 mm. The 100-μm-thick black latex objects were placed in the centers of curvature of the samples, with their wide sides parallel to the ultrasound beam. As shown in Figure 1, estimated axial and lateral resolutions of 70 μm and 120 μm were measured, respectively. The researchers said this study demonstrates the feasibility of high-resolution ultrasound-modulated optical tomography for imaging biological tissue to a depth of several millimeters. The researchers also said that the resolution can be improved upon by using higher ultrasound frequencies. ADITI S. RISBUD