J. Neurosurg. / Volume 32 / March, 1970

A Simplified Method for Electrical Impedance Monitoring in Brain Tumor Localization* Technical Note

DONALD P. BECKER, M.D., CAVETT M. ROBERT, JR., M.D., AND JOHN SEELIG Division o] Neurosurgery, Harbor General Hospital, Torrance, California

LECTRICAL impedance of cerebral tis- 500,000 cps. Most impedance studies have sue has been utilized effectively to been performed at much lower current fre- E localize brain tumors. Grant 2 demon- quencies as significant shunt capacitances strated, in 1923, that neoplastic lesions usu- occur in the apparatus at very high frequen- ally show an impedance approximately one- cies. However, relative impedance measure- half that of normal tissue. Recently, Organ, ments for normal and abnormal tissue can et al., :~ were able to localize brain tumors in be defined with this unit, and it is thus suffi- 13 of 14 patients during surgical procedures cient for our present purpose. Current levels using an impedance probe. However, their produced by the generator are also high circuitry was moderately extensive and not (milliampere range), so care must be taken ordinarily available to the clinical neurosur- to use levels that can be effectively measured geon. This paper describes a technique for but not create a lesion by heating effects. safely measuring impedance through the ad- Safe and appropriate parameters for mea- vancing exploring needle at the time of needle suring impedance without creating a lesion biopsy or craniotomy when the tumor loca- were initially determined in 10 cats. Normal tion is not apparent at the cortical surface. It impedance levels with this apparatus and de- utilizes a standard commercial radiofrequency sign were determined for cat brain employ- generator which is readily available and eas- ing 24 penetrations in four animals. ily operated by the neurosurgeon. We have These experiments were performed in ani- previously used this same unit for impedance mals anesthetized with pentobarbital (Dia- measurements in cerebrospinal fluid and spi- butal) and placed in a stereotaxic frame. The nal cord for target electrode localization in electrodes were advanced in brain with a mi- stereotaxic cordotomy for intractable pain? cromanipulator. At the conclusion of each experiment the animal was given 5 cc of try- Material and Methods pan blue intravenously to stain any potential The materials needed for this technique lesions, and then killed with an overdose of are the same as those used for percutaneous intravascular sodium pentobarbital. The cordotomy as described by Rosomoff, et al2 fixed brains were later sectioned and exam- A No. 18 thin-walled lumbar puncture needle ined for lesions other than those of the elec- is used as the carrying needle. The elec- trode tract. trode with 3 mm exposed tip may consist of In both animals and patients, modified bi- the Teflon-insulated steel wire commercially polar and unipolar methods for impedance availablet or a No. 25 stainless steel wire measurement were used. For the bipolar sharpened electrolytically and insulated with technique, the indifferent electrode was the PE 50 polyethylene. carrying lumbar puncture needle. With uni- The electrical source was the Radion- polar measurements, the indifferent electrode ics, Inc. Model RFG 3A radiofrequency consisted of a 2
89inch No. 18 needle fully generator with an internal impedance of 330 thrust into the anesthetized scalp at the time ohms. This unit has a fixed frequency of of needle biopsy or at craniotomy. A constant current was applied intermit- Received for publication May 29, 1969. tently as the advancing electrode was di- * This study was supported by Special Grant 444 from the California Division of the American rected toward the site of the suspected le- Cancer Society. sion. The voltmeter determined the voltage ? Codman and Shurtleff, Inc. at each depth measurement and relative 375 376 D. P. Becker, C. M. Robert, Jr. and J. Seelig TABLE 1 they lower the observed impedance and thus Current vs size of lesion created make the change from normal brain to lesion (200 trials at 20 sec) less distinct. It must be recognized that electrodes of Milliamperes Watts Volume of different size, type, and configuration will (mA) (aver.) Lesion give different impedance readings. The ma- jority of our animal and human measure- 10 0.10 none 20 0.25 none ments were made with a No. 25 stainless 30 0.45 none steel electrode insulated with polyethylene. 40 0.80 3.0 mm'~ With this electrode, the average bipolar 50 1.30 14.0 mm~ impedance in normal cat brain at 20 mA was 630 ohms. The commercial Teflon- impedance was immediately determined. Ac- coated electrode resulted in impedance mea- cording to Ohm's law, resistance equals volt- surements 200 to 300 ohms higher, depend- age divided by current. With knowledge of ing on the size of the exposed tip. the voltage and current at each level of The average impedance measured in cat penetration, the relative impedance can be brain (200 trials) at 20 mA of current uti- derived on a predesigned graph or slide rule. lizing the modified bipolar and unipolar A sudden fall in voltage noted on the volt- techniques is given below: meter will define entry into a soft lesion, Average bipolar impedance, 630 (~ 105) whereas a rise will usually signify entry into ohms a firm or fibrous tumor. When the lesion is Average unipolar impedance, 831 (+_ 81 ) thus localized by a significant impedance ohms change, a satisfactory biopsy may be ob- tained through the carrying lumbar puncture A higher impedance occurs with unipolar needle. measurements, and the variation in measure- ments is less (note smaller standard devia- Results tion). The wider variation seen with bipolar Safe current levels that can be used when measurements may relate to a variable obtaining cerebral impedance measurements amount of tissue fluid accumulation around in the range that normal brain tissue pos- the electrode tip and probe shaft, which can sesses are shown in Table 1. We found that create an electrical shunting effect. Of fur- 30 mA of current or less could be safely ad- ther interest is the fact that impedance mea- ministered for up to 20 sec without creating surements in normal human brain tissue with a lesion larger than the needle tract itself. the present apparatus were in a range similar In fact, at 20 mA (delivering 0.12 W) no to that of the cat brain. The vast majority of lesion was observed even up to 150 sec of measurements were made in white matter, continued current administration. When a and no attempt was made to differentiate current of 40 mA or more is used for 20 sec impedance levels between gray and white or longer, the power is sufficient to create a matter. lesion from heating effects. As only several As seen in Table 2 all five of our patients seconds of current are needed to obtain a TABLE 2 measurement at each depth checked, work- ing in the range of 10 to 30 mA is safe. Brain tumor impedance With the present apparatus, the greater 'Lesions Imped- / the intensity of the current, the lower the ob- Case Pathology ance at 20 mA ~ Ratio to served impedance. Since comparative mea- No. (ohms) ~ Normal surements must be made with a constant current, we chose 20 mA as the level best glioblastoma 350 0:25 utilized for human measurements with the astrocytoma III 260 0:43 present generator. Lower levels of current astrocytoma III 240 0:40 are difficult to read accurately on the genera- cholesteotoma 1900 3:40 tor's ammeter. Higher levels make it difficult meningioma 425 0:71 to measure the observed impedance, since