A method for examining interior material of an object from a proximal surface of the object using ultrasound, wherein the method comprises the steps of:
- a. transmitting at least a first ultrasound signal, by at least a first ultrasound transmitter of a predetermined group of ultrasound transmitters, to the interior material of the object, wherein in the interior material of the object reflections and/or diffractions of the first ultrasound signal occur,
- b. receiving reflections and/or diffractions of the first ultrasound signal from the interior material of the object using a plurality of ultrasound receivers of a predetermined group of ultrasound receivers, the plurality of ultrasound receivers being acoustically coupled to the proximal surface of the object at positions that are distributed in at least one dimension of the proximal surface of the object, and wherein, with each of the plurality of ultrasound receivers, a receiving signal is generated from the received reflections and/or diffractions of the at least first ultrasound signal from the interior material of the object, and
- c. processing in combination the receiving signals generated by the plurality of the ultrasound receivers in order to determine, according to the principle of inverse wave field extrapolation, where in the interior material of the object, reflections and/or diffractions of the transmitted first ultrasound signal occurred, wherein the processing is based on a data representation using at least one predetermined virtual grid of positions,
- wherein grid positions of each virtual grid of positions, of the at least one predetermined virtual grid of positions, are each at a predetermined area of interest that includes a part of the interior material of the object,
- wherein each virtual grid of positions, of the at least one predetermined virtual grid of positions, is defined along a series of grid lines that extend along respective directions,
- wherein at least a first direction, of the respective directions, corresponds to an at least approximate local direction of a spatial gradient of ultrasound travel time from the at least first ultrasound transmitter to one or more of the plurality of ultrasound receivers, and
- wherein the spatial gradient of ultrasound travel time at least partly results from different grid positions, of the grid positions of each virtual grid of positions, being associated with different travel times from the at least first ultrasound transmitter to the one or more of the plurality of ultrasound receivers.