Measurement of bio-impedance with a smart needle to confirm percutaneous kidney access

Purpose: The traditional method of percutaneous renal access requires freeh and needle placement guided by C-arm fluoroscopy, ultrasonography, or compu terized tomography. This approach provides limited objective means for veri fying successful access. We developed an impedance based percutaneous Smart Needle system and successfully used it to confirm collecting system access in ex vivo porcine kidneys. Materials and Methods: The Smart Needle consists of a modified 18 gauge per cutaneous access needle with the inner stylet electrically insulated from t he outer sheath. Impedance is measured between the exposed stylet tip and s heath using Model 4275 LCR meter (Hewlett-Packard, Sunnyvale, California). An ex vivo porcine kidney was distended by continuous gravity infusion of 1 00 cm. water saline from a catheter passed through the parenchyma into the collecting system. The Smart Needle was gradually inserted into the kidney to measure depth precisely using a robotic needle placement system, while i mpedance was measured continuously. Results: The Smart Needle was inserted 4 times in each of 4 kidneys. When t he needle penetrated the distended collecting system in 11 of 16 attempts, a characteristic sharp drop in resistivity was noted from 1.9 to 1.1 ohm in . Entry into the collecting system was confirmed by removing the stylet and observing fluid flow from the sheath. This characteristic impedance change was observed only at successful entry into the collecting system. Conclusions: A characteristic sharp drop in impedance signifies successful entry into the collecting system. The Smart Needle system may prove useful for percutaneous kidney access.