Estimation of needle tip location using ultrasound image processing and hypoechoic markers

Ultrasound guidance is a common method used in procedures such as peripheral nerve blocks and biopsies. Accurate determination of the location of the needle tip is critical in many such procedures. Visual graduations on the needle may give an external estimate of the depth of the needle tip, but the tip itself may be difficult to find in the ultrasound image even with in-plane imaging. The needle tip echo may also be occluded by other hyperechoic regions in the image. The objective of the current work was to develop a simple technique to accurately estimate the location of the needle tip in an ultrasound image, using a set of hypoechoic reference markings on the shaft of the needle, with image processing to improve accuracy and minimize error. A unique set of discrete hypoechoic markings was made along the shaft of a 14-gauge (2.1 mm nominal outer diameter) needle to provide a number of needle location reference points in the ultrasound image. Ultrasound scans were obtained of the needle in agar-based tissue mimicking phantoms using an 8.0 MHz linear array. The resulting images were cropped and binarized using an intensity filter before morphological operations were applied to isolate the needle echo in the image. A straight line was fitted to the top edge of the needle echo and up to three adjacent reference markings were found along this line. These were then used to estimate the actual position of the tip in the image. The accuracy of the location was affected by the resolution of the ultrasound image and the probe frequency. Redundancy in the number of markings used increased the robustness of the technique.