An ultrasound system comprising: - a probe adapted for being put into contact with a medium and comprising at least one transducer (T i) adapted for emitting and receiving ultrasound waves in said medium, and - a processing unit associated to said probe and adapted for processing signals from the probe, wherein - the probe is not axisymmetric but is a linear probe extending according to a longitudinal direction, - the probe comprises at least one portion comprising along the longitudinal direction a plurality of probe sections having a width that decrease from a first end to a second end of said portion so as to behave as a Fresnel lens in that the probe focusses the ultrasound waves towards and from a focal point inside the medium, said focal point being different for each frequency of said ultrasound waves, and - the processing unit analyses signals received from said probe at a plurality of frequencies (f j) around a nominal frequency (f n) for sensing the medium at a plurality of focal points (FP j) situated in the medium, and - the processing unit builds an image of the medium on the bases of a plurality of focal points sensed in the medium, the image being composed of a grid of pixels, the pixels aligned in the depth direction perpendicular to the longitudinal direction being determined by the level of the signals received for each time of flight corresponding to a distance between the probe and one of the plurality of focal points (FPj) in the medium, and the ultrasound system being characterized in that the pixels aligned in the longitudinal direction being determined by the signals at each one of the plurality of frequencies. - a probe adapted for being put into contact with a medium and comprising at least one transducer (T i) adapted for emitting and receiving ultrasound waves in said medium, and - a processing unit associated to said probe and adapted for processing signals from the probe, wherein - the probe is not axisymmetric but is a linear probe extending according to a longitudinal direction, - the probe comprises at least one portion comprising along the longitudinal direction a plurality of probe sections having a width that decrease from a first end to a second end of said portion so as to behave as a Fresnel lens in that the probe focusses the ultrasound waves towards and from a focal point inside the medium, said focal point being different for each frequency of said ultrasound waves, and - the processing unit analyses signals received from said probe at a plurality of frequencies (f j) around a nominal frequency (f n) for sensing the medium at a plurality of focal points (FP j) situated in the medium, and - the processing unit builds an image of the medium on the bases of a plurality of focal points sensed in the medium, the image being composed of a grid of pixels, the pixels aligned in the depth direction perpendicular to the longitudinal direction being determined by the level of the signals received for each time of flight corresponding to a distance between the probe and one of the plurality of focal points (FPj) in the medium, and the ultrasound system being characterized in that the pixels aligned in the longitudinal direction being determined by the signals at each one of the plurality of frequencies. the pixels aligned in the depth direction perpendicular to the longitudinal direction being determined by the level of the signals received for each time of flight corresponding to a distance between the probe and one of the plurality of focal points (FPj) in the medium, and the ultrasound system being characterized in that the pixels aligned in the longitudinal direction being determined by the signals at each one of the plurality of frequencies.