Continuous impedance monitoring during CT-guided stereotactic surgery: relative value in cystic and solid lesions

Continuous monopolar impedance monitoring was performed during CT-guided stereotactic procedures involving 46 probe passages in 33 patients. Four readings were obtained during each passage to the lesion corresponding to the gray matter (A), subcortical white matter (B), perilesional tissue (C), and the target (D). The mean impedance of low-grade gliomas (442.7 +/- 96.1 SD ohm) was lower than that of high-grade gliomas (675 +/- 67.3 SD ohm) (p = 0.01). But because of considerable overlap of values in the 400-700 ohm range only impedances of less than 400 ohm and greater than 700 ohm are capable of predicting pathology, the former being associated with low-grade gliomas and the latter with high-grade gliomas. The mean change in impedance from the perilesional tissue to the target (C-D) was higher for cystic lesions compared with the solid lesions (127.5 +/- 131.5 SD versus 78.9 +/- 72.4 SD ohm) but was not statistically significant (p = 0.148). Impedance monitoring was found to be particularly useful in determining the entry into thick-walled cysts. It also helped elucidate the structures encountered in the probe track. Overall this procedure is a simple and useful adjunct, which can enhance the accuracy of selected CT-guided procedures without unduly increasing the length of the procedure or altering the morbidity.