| Many medical procedures require the detection, tracking and guidance of (biopsy) needles. The detection of the position of the needle can be challenging because of specular reflection which deflects the sound in a direction away from the transducer surface. To visualize the tip of small needles often motion is introduced to the discomfort of the patient. Vilkomerson and co-workers suggested in 1981 the placement of an ultrasound receiver close to the needle tip. The received echoes are detected by add-on hardware. The maximum echo is assumed to originate from a beam directly above the detector, and the time of flight determines the distance to the transducer. The feasibility of the method was demonstrated by the same group in-vivo. The precision of the method has not been previously discussed in literature. This paper introduces two methods for estimation of the position of the needle tip and investigates their precision. The first method uses conventional imaging. Instead of detecting the maximum echo, as previously suggested, the center of mass is found both across beams and along the received signals, thus decreasing the sensitivity to noise. The second method is based synthetic aperture (SA) scanning. The position of the tip is found via triangulation which involves solving a system of linear equations. The robustness to noise is ensured through averaging a number of estimates. The sensor is a ring of piezo-electric film making it possible to receive waves from any direction. The results were obtained using simulations in Field II. The center frequency is 7 MHz. The transducer array is mechanically focused in elevation plane at 25 mm while the height of the elements is 4.5 mm. The transducer pitch is 202 microns. The sensor is a ring with height of 1 mm and 2 mm diameter. Positions were varied from 10 to 120 mm in depth and from 0 to 20 mm in lateral and elevation direction. The mean error of position estimation is for the case of conventional and SA imaging is 0.2 and 0.05 mm, respectively. The precision of two methods to determine the position of the needle tip is investigated. Using spherical transmissions yields higher accuracy. Both methods can be extended to 3D if the transmissions originate from transducer elements that are not placed on a line. Needles equipped with receivers can be used for deploying brachytherapy seeds ensuring high precision of the procedure. |