text: Diffuse correlation spectroscopy (DCS) is a type of medical imaging and optical technique that utilizes near-infrared light to directly and non-invasively measure tissue blood flow. The imaging modality was created by David Boas and Arjun Yodh in 1995.
  Blood flow is one the most important factors affecting the delivery of oxygen and other nutrients to tissues. Abnormal blood flow is associated with many diseases such as stroke and cancer. Tumors from cancer can generate abnormal tumor blood flow compared to the surrounding tissue. Current treatments attempt to decrease blood flow to cancer cells. Therefore, there is an urgent need for a way to measure blood flow. However, blood flow is difficult to measure because of sensitivity and stability of the measurement as it depends on magnitude of flow, location, and the diameter of individual vessels.
  Current imaging modalities used to measure blood flow include Doppler ultrasound, PET, and MRI. Doppler ultrasound is limited to large vessels. PET requires arterial blood sampling and exposure to ionizing radiation. MRI cannot be used for patients with pacemakers and those with metal implants. All together, these imaging modalities have large and costly instrumentation and are not conducive to continuous measurements.
  With these considerations in mind, the first methodology used to measure blood flow is near-infrared spectroscopy (NIRS). It is based on a well known spectral window that exists in the near-infrared (NIR, 700-900 nm) where tissue absorption is relatively low so that light can penetrate into deep/thick volumes of tissue, up to several centimeters. It provides a fast and portable alternative to measure deep tissue hemodynamics. However, it has a poor spatial resolution and is a ‘static’ method. This means that it measures the relatively slow variation in tissue absorption and scattering. In other words, it measures the changes in the amount of scattering rather than the motion of the scatter.
  This led to the ‘dynamic’ NIRS technique or Diffuse correlation spectroscopy. It measures the motions of the scatters while also maintaining the advantages of NIRS. The primary moving scatterers are red blood cells. The main advantages of this method is no ionizing radiation, no contrast agents, high temporal resolution, and large penetration depth. The utility of DCS technology has been demonstrated in tumors, brains, and skeletal muscles. The general approach with DCS is that the temporal statistics of the fluctuations of the scattered light within a speckle area or pixel is monitored. Then, the electric field temporal autocorrelation function is measured. A model for photon propagation through tissues, the measured autocorrelation signal is used to determine the motion of blood flow.