Method for determining a physical characteristic on a punctual location inside a medium, a method for determining an image of a medium, and an apparatus implementing said methods

1. A method for determining an image of a region inside a medium, the image being composed of a plurality of pixels, each pixel having a pixel value corresponding to a physical characteristic representing the nature of the medium, and wherein the method comprises for a one or more locations inside the region, the following steps:

sending an emitted sequence of ultrasound waves towards one location of the one or more locations into the region, the emitted sequence comprising at least two emitted pulses of different amplitudes;

receiving a received sequence of ultrasound waves from the location of the step of sending an emitted sequence of ultrasound waves, the received sequence comprising at least two received pulses corresponding to said emitted pulses;

calculating a phase difference between the phase of each of the at least two received pulses and the phase of the corresponding emitted pulses; and determining the pixel values of the image on the bases of said phase difference, each pixel corresponding to a location in the medium,

wherein the succession of the sending an emitted sequence of ultrasound waves, receiving a received sequence of ultrasound waves, and calculating a phase difference steps are repeated several times at each location for providing a plurality of phase differences for each location of the one or more locations inside the region.

2. The method according to claim 1 , wherein the step of calculating a phase difference comprises the following sub-steps:

c1) determining a phase for each one of the received pulses in the received sequence, and

c2) calculating the phase difference by combining said phases.

3. The method according to claim 1 , wherein the step of calculating a phase difference comprises the following sub-steps:

c3) received signals corresponding to said received pulses are weighted by weighting factors and summed for producing a combined signal, said weighting factors determined to compensate the different amplitudes of the emit pulses, and

c4) the phase difference is a phase of said combined signal.

4. The method according to claim 1 , wherein:

during the determining the pixel values step, the pixel values are determined by calculating a mean or a standard deviation or a variance of said plurality of phase differences.

5. The method according to claim 1 , wherein the emitted sequence comprises a number greater than two emitted pulses, the received sequence comprises at least the same number of received pulses, and wherein:

during calculating a phase difference step, a plurality of phase differences between couples of corresponding to two received and emitted pulses is calculated, each couple of emitted pulses having a different amplitude, and

during the determining the pixel values step, the pixel values are determined by calculating a mean or a standard deviation or a variance of said plurality of phase differences.

6. The method according to claim 1 , wherein:

the region comprises a plurality of sub-regions,

between the steps of calculating a phase difference and determining the pixel values, a step of calculating a sub-region phase difference is performed, said sub-region phase difference being a mean value of the plurality of phase differences of one or more locations belonging to said sub-region, and

each pixel of the plurality of pixels in the image corresponds to a sub-region.

7. The method according to claim 1 , wherein two emitted pulses of the emitted sequence have an amplitude ratio of greater or equal to two.

8. The method according to claim 1 , wherein before the step of sending an emitted sequence of ultrasound waves, a contrast agent is introduced into the medium.

9. The method according to claim 8 , wherein the contrast agents comprises microbubbles.

10. An apparatus for determining an image of a region inside a medium, the image being composed of a plurality of pixels, each pixel having a pixel value corresponding to a physical characteristic representing the nature of the medium, said apparatus comprising

a probe comprising an ultrasound transducer or a transducer array for sending an emitted sequence of ultrasound waves towards a location of one or more locations into the region, the emitted sequence comprising at least two emitted pulses of different amplitudes;

receiving a received sequence of ultrasound waves from each location of the one or more locations of the step of sending an emitted sequence of ultrasound waves, the received sequence comprising at least two received pulses corresponding to said emitted pulses,

a processing unit comprising a computer for processing signals from the received sequence and for calculating a phase difference between the phase of each of the at least two received pulses and the phase of the corresponding emitted pulses; and determining the pixels values of the image on the bases of said phase difference, each pixel corresponding to a location in the medium,

wherein the succession of the sending an emitted sequence of ultrasound waves, receiving a received sequence of ultrasound waves, and calculating a phase difference steps are repeated several times at each location for providing a plurality of phase differences for each location of the one or more locations inside the region.