A system for measuring material properties with ultrasound of a material subject to varying deformation, the system comprising: an ultrasound transducer assembly for transmitting an acoustic ultrasound signal into a material and receiving the acoustic ultrasound signal after reflection and modification by the material; and a processor communicating with the ultrasound transducer assembly to receive an electrical ultrasound signal corresponding to the received acoustic ultrasound signal as modified by the material; wherein the processor is constructed and arranged to process the electrical ultrasound signal according to a stored program to: (i) measure a time of travel of the waves of the acoustic ultrasound signal between a first and a second boundary, characterized by the further steps of (ii) measure a reflection strength of reflected waves of the acoustic ultrasound signal in the material at each of the first and second boundaries of the material for each of a first and second deformation of the material, wherein the reflection strength is a measure of the proportion of the power of the acoustic ultrasound signal that is reflected at the boundary relative to the power of the acoustic ultrasound signal incident on the boundary; (iii) combine the reflection strengths measured in step (ii) and the time of travel measured in step (i) to deduce at least one property selected from the group consisting of material strain, material stiffness at one state of deformation, stiffness gradient of the material with respect to strain, material density, material Poisson's ratio, material tangential modulus, target tissue thickness as a function of applied strain and wave attenuation of the material as a function of applied strain. an ultrasound transducer assembly for transmitting an acoustic ultrasound signal into a material and receiving the acoustic ultrasound signal after reflection and modification by the material; and a processor communicating with the ultrasound transducer assembly to receive an electrical ultrasound signal corresponding to the received acoustic ultrasound signal as modified by the material; wherein the processor is constructed and arranged to process the electrical ultrasound signal according to a stored program to: (i) measure a time of travel of the waves of the acoustic ultrasound signal between a first and a second boundary, characterized by the further steps of (ii) measure a reflection strength of reflected waves of the acoustic ultrasound signal in the material at each of the first and second boundaries of the material for each of a first and second deformation of the material, wherein the reflection strength is a measure of the proportion of the power of the acoustic ultrasound signal that is reflected at the boundary relative to the power of the acoustic ultrasound signal incident on the boundary; (iii) combine the reflection strengths measured in step (ii) and the time of travel measured in step (i) to deduce at least one property selected from the group consisting of material strain, material stiffness at one state of deformation, stiffness gradient of the material with respect to strain, material density, material Poisson's ratio, material tangential modulus, target tissue thickness as a function of applied strain and wave attenuation of the material as a function of applied strain. (i) measure a time of travel of the waves of the acoustic ultrasound signal between a first and a second boundary, characterized by the further steps of (ii) measure a reflection strength of reflected waves of the acoustic ultrasound signal in the material at each of the first and second boundaries of the material for each of a first and second deformation of the material, wherein the reflection strength is a measure of the proportion of the power of the acoustic ultrasound signal that is reflected at the boundary relative to the power of the acoustic ultrasound signal incident on the boundary; (iii) combine the reflection strengths measured in step (ii) and the time of travel measured in step (i) to deduce at least one property selected from the group consisting of material strain, material stiffness at one state of deformation, stiffness gradient of the material with respect to strain, material density, material Poisson's ratio, material tangential modulus, target tissue thickness as a function of applied strain and wave attenuation of the material as a function of applied strain.