The application features methods, devices, and systems for measuring blood flow in a subject. The computer-implemented methods include receiving functional magnetic resonance imaging (fMRI) data that provides information on at least one of volume or oxygenation of blood at one or more locations in a body over a first predetermined length of time. The methods also include receiving near-infrared spectroscopic (NIRS) imaging or measurement data representing at least one of blood concentration or oxygenation at a first portion of the body over a second predetermined length of time. The methods further include deriving, from the fMRI data corresponding to a second portion of the body, a time varying data set representing changes in blood oxygenation or volume or both blood oxygenation and volume at the second portion over the first predetermined length of time and determining, by a computing device, a time delay and a value of a similarity metric corresponding to a part of the spectroscopic imaging data that most closely matches the time varying data set. The time delay represents a difference between a first time in which blood flows from a third portion in the body to the first portion and a second time in which blood flows to the second portion from the third portion. The value of the similarity metric represents an amount of blood at the second portion. An estimate of a characteristic of at least one of blood flow or blood volume in the second portion at a given time is determined based on the time delay and the value of the similarity metric.
