An electrodeless measuring technique for determining conductivity of biological tissues at radio frequencies

A multifrequency magnetic induction tomography system using planar gradiometers: data collection and calibration

We developed a 14-channel multifrequency magnetic induction tomography system (MF-MIT) for biomedical applications. The excitation field is produced by a single coil and 14 planar gradiometers are used for signal detection. The object under measurement was rotated (16 steps per turn) to obtain a full data set for image reconstruction. We make measurements at frequencies from 50 kHz to 1 MHz using a single frequency excitation signal or a multifrequency signal containing several frequencies in this range. We used two acquisition boards giving a total of eight synchronous channels at a sample rate of 5 MS s(-1) per channel. The real and imaginary parts of DeltaB/B(0) were calculated using coherent demodulation at all injected frequencies. Calibration, averaging and drift cancellation techniques were used before image reconstruction. A plastic tank filled with saline (D = 19 cm) and with conductive and/or paramagnetic perturbations was measured for calibration and test purposes. We used a FEM model and an eddy current solver to evaluate the experimental results and to reconstruct the images. Measured equivalent input noise voltage for each channel was 2 nV Hz(-1/2). Using coherent demodulation, with an integration time of 20 ms, the measured STD for the magnitude was 7 nV(rms) (close to the theoretical value only taking into account the amplifier's thermal noise). For long acquisition times the drift in the signal produced a bigger effect than the input noise (typical STD was 10 nV with a maximum of 35 nV at one channel) but this effect was reduced using a drift cancellation technique based on averaging. We were able to image a 2 S m(-1) agar sphere (D = 4 cm) inside the tank filled with saline of 1 S m(-1).

Measurement of surgical diathermy current during trans-urethral resection of the prostate gland

A system is described that has been developed to measure the surgical diathermy current passing through the patient during each cut of a trans-urethral resection of the prostate gland (TURP), with 1.5% glycine solution as irrigant. The immediate aim was to distinguish capsular tissue of the male prostate gland from the hyperplastic adenomatous tissue within it. The ultimate aim was to prevent resection into the surgical capsule which can easily result in excessive blood loss and absorption of irrigant. The system was used in a conventional theatre setting with a commercial diathermy system (Eschmann TD 411-S) of known output characteristics. Measurements were made in 12 patients during resection with a standard loop, involving typically 75 cuts in each patient (60 in adenoma and 15 in capsule). In 8 of these patients this was followed by roller ball placement on typically 10 sites each of capsule or residual adenoma at very low diathermy settings. The results showed great variability both between patients, and between different cuts in the same patient, with no consistent difference between tissues identified under direct vision as adenoma and surgical capsule. Physical reasons are suggested for the observed variability, and it is concluded that it is not possible to use this technique to guide tissue resection in the prostate.