Design and validation of an ultrasound array optimised for epidural needle guidance

Frequent use is made of lumbar epidural and combined spinal epidural anaesthesia and analgesia for pain relief during labour and childbirth. However, the conventional use of palpation to identify the position of the relevant interspinous space can be difficult in obese parturients. The safety, low cost and capability for ultrasound to be used in real time at the point of care make it a suitable clinical imaging modality to use as an alternative. With a conventional biomedical ultrasound imaging array, with or without a needle guide, the needle cannot be made to lie exactly in the imaging plane because of the physical geometry and with lengths up to more than 100 mm for lumbar epidural procedures, it is easy for the needle to leave the imaging plane altogether. The work reported here has been directed towards a practical solution to this problem via custom array design. This takes into account the need for ergonomic manipulation by the anaesthetist, insertion of the needle exactly parallel to the ultrasonic beam, and sterility. The array incorporates two separate electroacoustic components with a space for needle insertion in between. Using standard array modules has allowed this novel configuration to be connected to a conventional imaging system with only minimal software modifications. The array is operated with its body parallel to the obese parturient's back and with an optimised clinical procedure. The procedure first involves use of the array to identify the relevant interspinous space, followed by insertion of the needle through the array, which is subsequently held in place to guide the needle tip until contact is established with the spine or can be withdrawn once the needle path is well established.