Quantitative analysis of coated needles for ultrasound guided intervention

For ultrasound guided interventions, the visibility of needles is of crucial importance for accurate positioning. Standard needles are only visible when the angle between transmitted ultrasound and needle is around 90 degrees. Therefore, needles were coated with a polyethersulfone coating containing specially developed polymeric microcapsules. Three different sizes of capsules were used, 10-25 m, 25-50 m and 50-75 m. Ultrasound data were acquired using a Philips iE33 echo apparatus equipped with an L11-3 linear array transducer. A fixed control setting with known flat depth-gain control and linear post-processing look-up table was used to allow transformation of the image gray levels into a dB scale of echo levels. The needles were positioned in a water tank filled with degassed water and mounted in a rotation arm. Data were acquired for angles from 90° down to 10° in steps of 10°. Data for each angle were rotated to 90° and averaged over all image lines to calculate an average reflection profile. The maximum of this profile was taken as the reflected, or backscattered echo level whereas the distance between the -6 dB fall-off points was taken as the thickness. The coated, as well as the normal needle, are clearly visible in the water tank. However, quantitative analysis demonstrates that the echogenicity of the coated needle is independent of the angle: the echo level is equal for all angles. This effect is observed for all used sizes of the microcapsules. The uncoated needle shows a decay of the echo level for angles smaller than 80° with a decrease of 8 dB for angles smaller than 60°. The thickness is overestimated at 90°, but decreases to the actual value for angles smaller than 60°. Additionally, a normal and coated needle were inserted in a chicken breast at an angle of 45° to demonstrate differences in performance under clinical circumstances. The normal needle was not visible at all, wh- reas the coated needle could be clearly depicted. In conclusion, coating needles with microcapsules improves the ultrasound visibility and makes the echo level angle independent.