Real-time length measurement of epidural Tuohy needle during insertion

This study presents a novel technique for the measurement of insertion depth of an epidural Tuohy needle in real-time. A wireless camera is used which transmits video images during insertion to a host computer. The computer contains the image processing algorithm to detect the visible needle in the image and measures the length. The measurement is done by HSV background removal, colour comparison and using RGB intensity profile to locate 10 mm markings on the Tuohy needle shaft. The visible length is then subtracted from the known length of the needle to calculate the depth of the needle tip. The camera can be placed in the operating theatre between 50 and 100 cm away from the needle insertion site. Wireless camera is beneficial since it minimises disturbance to the anaesthetist and the patient and avoids ethical and sterility concerns which would be caused by bringing additional equipment into the hospital room. The speed of the image processing technique is 10 frames per second with a maximum error of ±3 mm. The image processing failure rate was 3 frames out of 150 which gave an overall reliability of 97.8% during insertion. Also this enables continuous needle depth monitoring during insertion by storing measurements into a data file. The purpose of measuring needle depth in real time is to precisely place the needle in the epidural space.