An ultrasound system comprising: an ultrasound unit including an ultrasound probe for acquiring an anatomical image of a human body and for locating a medical instrument with respect to said image, the ultrasound probe including a first set of imaging transducer elements (550-1, 550-2...) and a second set of localisation transducer elements (551-1a, 551-2a, 551-1b...), wherein the first set of imaging transducer elements are distinct and disjoint from the second set of localisation transducer elements, wherein: the first set of imaging transducer elements are configured to: (i) produce ultrasound imaging transmissions into the human body, wherein the ultrasound imaging transmissions are focussed into an image scan plane, and (ii) receive reflections of the ultrasound imaging transmissions for generating a two-dimensional anatomical image corresponding to the image scan plane; and wherein the second set of localisation transducer elements are configured to produce ultrasound localisation transmissions into the human body for locating the medical instrument with respect to the anatomical image, wherein the ultrasound localisation transmissions extend outside the image scan plane, and wherein localisation transducer elements (551-1a, 551-2a) from said second set are spaced from one another so as to be arranged at four different offsets from the image scan plane; and a sensor console for receiving the signals from a transducer in said medical instrument that correspond to the localisation transmissions; wherein the ultrasound system is configured to process the received signals to determine the location of the medical instrument within the human body relative to the ultrasound probe based on the received signals. an ultrasound unit including an ultrasound probe for acquiring an anatomical image of a human body and for locating a medical instrument with respect to said image, the ultrasound probe including a first set of imaging transducer elements (550-1, 550-2...) and a second set of localisation transducer elements (551-1a, 551-2a, 551-1b...), wherein the first set of imaging transducer elements are distinct and disjoint from the second set of localisation transducer elements, wherein: the first set of imaging transducer elements are configured to: (i) produce ultrasound imaging transmissions into the human body, wherein the ultrasound imaging transmissions are focussed into an image scan plane, and (ii) receive reflections of the ultrasound imaging transmissions for generating a two-dimensional anatomical image corresponding to the image scan plane; and wherein the second set of localisation transducer elements are configured to produce ultrasound localisation transmissions into the human body for locating the medical instrument with respect to the anatomical image, wherein the ultrasound localisation transmissions extend outside the image scan plane, and wherein localisation transducer elements (551-1a, 551-2a) from said second set are spaced from one another so as to be arranged at four different offsets from the image scan plane; and a sensor console for receiving the signals from a transducer in said medical instrument that correspond to the localisation transmissions; wherein the ultrasound system is configured to process the received signals to determine the location of the medical instrument within the human body relative to the ultrasound probe based on the received signals.