Optical imaging encompasses a host of light-based imaging modalities, including near-infrared spectroscopy, diffuse correlation spectroscopy, optical coherence tomography, two-photon microscopy and more.
Diffuse optical imaging and spectroscopy
Optics @ Martinos has played a major role in the development of near-infrared spectroscopy (NIRS). NIRS takes advantage of the optical properties of light to image inside the body. If you shine a flashlight on your hand, you’ll see that light can travel through centimeters of tissue and still be detected. Based on our understandings of light migration through tissue we can use this detected light to measure changes in hemodynamics – changes in blood volume or blood oxygenation, for example – which can tell us a great deal about what’s going on in the body.
Thus NIRS can contribute to a variety of applications.
In the brain, for example, it can detect and localize important events such as ischemic/hemorrhagic stroke or hyper-/hypoxia. It can also detect and localize vascular responses to brain activation – and in this way help to advance brain mapping efforts. Because a tight coupling exists between vascular and metabolic responses to activation, these measurements can reveal considerable information about the latter and may even provide some clues as to the underlying neural responses.
We are also exploring the potential of NIRS for optical breast imaging. Here, the technique can reveal changes in blood volume and oxygen saturation that are specific to early stages of cancer. Because it focuses on these functional changes, it can – in theory – identify cancers before they are structurally evident (that is, before they are visible on X-ray or discernable by palpation). In addition to potential detection of cancers, NIRS offers a means to explore the physiology of the breast. The literature includes, for example, studies on changes in the optical properties associated with age, exogenous hormone levels and menopausal status.
Optical recording offers several attractions over other imaging methods, including simplicity, low cost and portability. NIRS systems can consist of little more than a probe with sources and detectors, or a piece of dedicated hardware about the size of a small suitcase. (Systems can be much larger, depending primarily on the type of laser source employed, but the approach generally offers a degree of portability unobtainable with many other modalities.) For this reason, the technique could be ideally suited to clinical applications such as bedside monitoring of cerebral oxygenation.
We have executed extensive studies that cross-validated fNIRS with fMRI, which simultaneously confirmed that fNIRS measures the same spatio-temporal hemodynamic response to brain activity as fMRI, and confirmed the biophysical underpinnings of fMRI. This validation has had significant impact in convincing users from wide ranging disciplines to adopt fNIRS for their studies.
For studies we have available several continuous wave (CW) (Techen) and frequency-domain (FD) (ISS, Inc.) multichannel systems. Several home built diffuse correlation spectroscopy (DCS) systems. We also have home built FD-NIRS systems and a home-built time-domain (TD) NIRS and DCS systems.
We are exploring use of OCT and two photon microscopy to obtain volumetric histological information of the human brain ex vivo to enable identification of whole brain cytoarchitectural maps. This will revolutionize neuro-anatomy by enabling the routine acquisition and analysis of whole brain cytoarchitectural maps leading to the study of variation within a population and alterations caused by different diseases and injury.