Medical diagnostic apparatus and methods are disclosed.
Ultrasound radiation pressure selectively modulates a target area within a body. One or more pulses of
radiation containing temporally correlated groups of photons are generated. The photons are characterized by two or more different wavelengths that are selected to have specific interaction with a target
chromophore. The two or more different wavelengths are also selected to have substantially similar scattering cross-sections and
anisotropy parameters in the target and its surroundings. The pulses of
radiation are injected into the body
proximate the target area being modulated by the
radiation pressure field.
Photon groups at each of the different wavelengths that are backscattered from the target area are detected in temporal
coincidence. Time-gated background-free amplification of the return
signal is used to exclude photons which could not by virtue of their
arrival time have interacted with the radiation-pressure-modulated target.
Photon groups are selected with a modulation component at the modulation frequency of the
radiation pressure modulation field, or at a
harmonic of the modulation frequency. From the arrival rate of the detected temporally correlated
photon pairs or multiplets, chemical information about the target area, such as an
oxygenation or
pH level can be inferred.
Cardiac output may be computed from measurements of venous and / or arterial
oxygenation using this technique.