Coherent light (e.g., laser light) is emitted into a tissue sample through an optical fiber. The tissue sample diffuses the coherent light. Different blood flow quantities generate different coherent light interference patterns. An image of a coherent light interference pattern is captured with an image sensor coupled to an optical fiber. The speckle contrast of the image quantifies coherent light interference pattern. The speckle contrast is determined and is mapped to blood flow quantities using one or more data models. A quantity of blood flow is identified in a tissue sample at least partially based on the speckle contrast value of the captured image.