Diffuse Optical Tomography

33 Volume images of the body are an indispensable clinical diagnostic tool. For visualizing and examining complex body organs such as the brain, doctors depend on the information 3D imaging modalities, such as x-ray computed tomography (CT) and magnetic resonance imaging (MRI), provide. Although these mature techniques have developed into reliable systems delivering high-quality images, some drawbacks remain. MRI requires a large and expensive instrument with considerable maintenance costs, while CT uses ionizing radiation, which is potentially harmful to patients. Today, the drive is toward relatively inexpensive, noninvasive, and portable imaging systems that can run at a patient’s bedside and produce data continuously over a long period of time. Although CT and MRI provide primarily anatomical information about tissue structure, interest is shifting toward functional imaging modalities that provide information about physiologically relevant parameters and parameter changes (tissue oxygenation level, for example) and can be used in applications like brain activation monitoring. Diffuse optical tomography (DOT) can potentially meet these criteria. Like CT, it uses electromagnetic radiation to probe the body, but at a much lower energy level and therefore doesn’t damage tissue. DOT operates at wavelengths at the red end of the visible spectrum and in the near-infrared range, around 650 to 900 nanometers. This spectral range is called a window of transparency because it lets light propagate relatively deeply into the tissue before being absorbed. At longer wavelengths the absorption spectrum of water rises steeply, whereas toward lower wavelengths, blood becomes a strong absorber, essentially blocking all light transmission within a few millimeters. Researchers have known for several decades that light can propagate through thick sections of body organs.1 The exciting feature of infrared light is that its absorption by blood and tissue depends on their oxygenation state,2 which lets us directly measure the body’s oxygenation levels. Spectroscopic applications such as pulse oximetry,3 which clinicians routinely use, exploit this feature. The use of infrared light in tomographic imaging makes DOT a functional imaging modality with potentially significant applications in brain function imaging and early tumor detection. (See articles by David Boas and colleagues4 and Arjun Yodh and Britton COMPUTATIONAL ASPECTS OF DIFFUSE OPTICAL TOMOGRAPHY