Design and characterization of an electromagnetic probe for distinguishing morphological differences in soft tissues.

We present a method for designing and optimizing an in-house designed electromagnetic probe for distinguishing morphological differences in biological tissues. The probe comprises concentric multi-wound coils, the inner being the primary coil and the outer being the detector coil. A time-varying voltage is imposed on the primary coil, resulting in an induced current in the detector coil. For highly conductive samples, eddy currents are induced in the sample and inductively couple with the electromagnetic probe. However, in weakly conducting samples, the primary coupling mechanism is found to be capacitive though there can be a non-negligible inductive component. Both the mutual inductive coupling and the capacitive coupling between the sample and the probe are detected as a change in the induced voltage of the detector coil using lock-in detection. The induced voltage in the detector coil is influenced more by the morphological structure of the specimen rather than by changes in electrical conductivity within different regions of the sample. The instrument response of the lock-in amplifier is also examined with simulated input voltage signals to relate its output to specific changes in inductive and capacitive coupling, in order to relate sample characteristics to a single voltage output. A circuit element model is used to interpret the experimental measurements. It is found that the sensitivity of the measurement for a given set of probe characteristics (resistances, inductances, and capacitances) can be optimized by adding a small amount of capacitance in the external circuit in parallel with the detector coil. Illustrative measurements are presented on animal (porcine and bovine) tissue and on human liver tissue containing a metastatic tumor to demonstrate the capabilities of the probe and measurement method in distinguishing different tissue types despite having similar electrical conductivities. Since biological tissues are multi-scale, heterogeneous materials comprising regions of differing conductivity, permittivity, and morphological structure, the electromagnetic method presented here has the potential to examine structural variations in tissue undergoing physical changes due to healing or disease.

Ex vivo electrical impedance measurements on excised hepatic tissue from human patients with metastatic colorectal cancer.

Point-wise ex vivo electrical impedance spectroscopy measurements were conducted on excised hepatic tissue from human patients with metastatic colorectal cancer using a linear four-electrode impedance probe. This study of 132 measurements from 10 colorectal cancer patients, the largest to date, reports that the equivalent electrical conductivity for tumor tissue is significantly higher than normal tissue (p < 0.01), ranging from 2-5 times greater over the measured frequency range of 100 Hz-1 MHz. Difference in tissue electrical permittivity is also found to be statistically significant across most frequencies. Furthermore, the complex impedance is also reported for both normal and tumor tissue. Consistent with trends for tissue electrical conductivity, normal tissue has a significantly higher impedance than tumor tissue (p < 0.01), as well as a higher net capacitive phase shift (33° for normal liver tissue in contrast to 10° for tumor tissue).