A system comprising: an ultrasound transducer for transmitting ultrasound into a subject's body and receiving ultrasound echoes from the body; a tissue-penetrating medical tool comprising a needle or cannula and having an insertion end for insertion into the body of the subject; a first electrode provided at the insertion end of the tissue-penetrating medical tool, exposed for electrical contact with the subject's body; a second electrode in electrical contact with the subject's body and positioned at or near the insertion end of the tissue-penetrating medical tool; and an electrical power supply and an impedance meter electrically-connected to the first electrode and the second electrode for measuring the impedance between the first electrode and the second electrode; a processor and display for analysing the ultrasound echoes and displaying an ultrasound image of the body; characterized by: the tissue-penetrating medical tool being magnetized; and by the system further comprising: a magnetic position detection system (12, 14) comprising an array of magnetometric sensors positioned on the ultrasound transducer for detecting the magnetic field originating from the magnetized tissue-penetrating medical tool and adapted to calculate the position of the magnetised tissue-penetrating medical tool in the body of the subject relative to the ultrasound transducer; and wherein the processor and display are adapted to display on the ultrasound image the position of the tissue-penetrating medical tool as detected by the position detection system (12, 14). an ultrasound transducer for transmitting ultrasound into a subject's body and receiving ultrasound echoes from the body; a tissue-penetrating medical tool comprising a needle or cannula and having an insertion end for insertion into the body of the subject; a first electrode provided at the insertion end of the tissue-penetrating medical tool, exposed for electrical contact with the subject's body; a second electrode in electrical contact with the subject's body and positioned at or near the insertion end of the tissue-penetrating medical tool; and an electrical power supply and an impedance meter electrically-connected to the first electrode and the second electrode for measuring the impedance between the first electrode and the second electrode; a processor and display for analysing the ultrasound echoes and displaying an ultrasound image of the body; characterized by: the tissue-penetrating medical tool being magnetized; and by the system further comprising: a magnetic position detection system (12, 14) comprising an array of magnetometric sensors positioned on the ultrasound transducer for detecting the magnetic field originating from the magnetized tissue-penetrating medical tool and adapted to calculate the position of the magnetised tissue-penetrating medical tool in the body of the subject relative to the ultrasound transducer; and wherein the processor and display are adapted to display on the ultrasound image the position of the tissue-penetrating medical tool as detected by the position detection system (12, 14). the tissue-penetrating medical tool being magnetized; and by the system further comprising: a magnetic position detection system (12, 14) comprising an array of magnetometric sensors positioned on the ultrasound transducer for detecting the magnetic field originating from the magnetized tissue-penetrating medical tool and adapted to calculate the position of the magnetised tissue-penetrating medical tool in the body of the subject relative to the ultrasound transducer; and wherein the processor and display are adapted to display on the ultrasound image the position of the tissue-penetrating medical tool as detected by the position detection system (12, 14). a magnetic position detection system (12, 14) comprising an array of magnetometric sensors positioned on the ultrasound transducer for detecting the magnetic field originating from the magnetized tissue-penetrating medical tool and adapted to calculate the position of the magnetised tissue-penetrating medical tool in the body of the subject relative to the ultrasound transducer; and wherein the processor and display are adapted to display on the ultrasound image the position of the tissue-penetrating medical tool as detected by the position detection system (12, 14).