Precision of MR-guided needle placement - Experimental results.

Purpose: To estimate the accuracy of the determination of tip position of M R compatible biopsy needles based on signal voids and artifacts in MR guide d biopsies. Method: In a nylon grid phantom an MR compatible 18G needle (E- Z-EM) was measured in a 1.0T imager using TSE- and FFE-sequences of 20 s an d 40 s duration in 34 different orientations of needle versus B-0, frequenc y and slice selection gradient. 4 radiologists with no experience in the ev aluation of signal void artifacts estimated the needle tip positions from n eedle tip artifacts. The readers determined the needle tip before and after a 15 minute training session based on high resolution images with explanat ion of size and shape of specific artifacts of biopsy needles in 12 differe nt orthogonal or parallel orientations to B-0 and frequency encoding gradie nt that are possible if the needle lies parallel to the slice, i.e. orthogo nal to the slice selection gradient. The values obtained before and after t he training session were compared to the real position of the needle tip. R esults: Mean distance of actual needle tip and tip position as determined f rom images was 1.8+/-2.3 mm in TSE- versus 2.5+/-1.2 mm in FFE-images, with the needle length overestimated. After a 15 minute training session the po sitioning error decreased significantly to 0.2+/-1.8 mm for TSE-sequences a nd to 1.0+/-1.8 mm for FFE-sequences. A higher accuracy of tip localization was obtained with TSE sequences. Conclusion: In MR guided biopsies using F FE- and TSE-sequences the needle position can be more accurately determined if the reader is familiar with the 12 orthogonal or parallel positions of the needle with respect to B-0 and frequency encoding gradient and the corr esponding artifacts.