Dijana Popovic

A large-scale study of the ultrawideband microwave dielectric properties of normal, benign and malignant breast tissues obtained from cancer surgeries

The development of microwave breast cancer detection and treatment techniques has been driven by reports of substantial contrast in the dielectric properties of malignant and normal breast tissues. However, definitive knowledge of the dielectric properties of normal and diseased breast tissues at microwave frequencies has been limited by gaps and discrepancies across previously published studies. To address these issues, we conducted a large-scale study to experimentally determine the ultrawideband microwave dielectric properties of a variety of normal, malignant and benign breast tissues, measured from 0.5 to 20 GHz using a precision open-ended coaxial probe. Previously, we reported the dielectric properties of normal breast tissue samples obtained from reduction surgeries. Here, we report the dielectric properties of normal (adipose, glandular and fibroconnective), malignant (invasive and non-invasive ductal and lobular carcinomas) and benign (fibroadenomas and cysts) breast tissue samples obtained from cancer surgeries. We fit a one-pole Cole-Cole model to the complex permittivity data set of each characterized sample. Our analyses show that the contrast in the microwave-frequency dielectric properties between malignant and normal adipose-dominated tissues in the breast is considerable, as large as 10:1, while the contrast in the microwave-frequency dielectric properties between malignant and normal glandular/fibroconnective tissues in the breast is no more than about 10%.

A large-scale study of the ultrawideband microwave dielectric properties of normal breast tissue obtained from reduction surgeries.

The efficacy of emerging microwave breast cancer detection and treatment techniques will depend, in part, on the dielectric properties of normal breast tissue. However, knowledge of these properties at microwave frequencies has been limited due to gaps and discrepancies in previously reported small-scale studies. To address these issues, we experimentally characterized the wideband microwave-frequency dielectric properties of a large number of normal breast tissue samples obtained from breast reduction surgeries at the University of Wisconsin and University of Calgary hospitals. The dielectric spectroscopy measurements were conducted from 0.5 to 20 GHz using a precision open-ended coaxial probe. The tissue composition within the probes sensing region was quantified in terms of percentages of adipose, fibroconnective and glandular tissues. We fit a one-pole Cole-Cole model to the complex permittivity data set obtained for each sample and determined median Cole-Cole parameters for three groups of normal breast tissues, categorized by adipose tissue content (0-30%, 31-84% and 85-100%). Our analysis of the dielectric properties data for 354 tissue samples reveals that there is a large variation in the dielectric properties of normal breast tissue due to substantial tissue heterogeneity. We observed no statistically significant difference between the within-patient and between-patient variability in the dielectric properties.

Precision open-ended coaxial probes for in vivo and ex vivo dielectric spectroscopy of biological tissues at microwave frequencies

Hermetic stainless-steel open-ended coaxial probes have been designed for precision dielectric spectroscopy of biological tissue, such as breast tissue, over the 0.5-20-GHz frequency range. Robust data-processing techniques have also been developed for extracting the unknown permittivity of the tissue under test from the reflection coefficient measured with the precision probe and a vector network analyzer. The first technique, referred to as a reflection-coefficient deembedding method, converts the reflection coefficient measured at the probes calibration plane to the desired aperture-plane reflection coefficient. The second technique uses a rational function model to solve the inverse problem, i.e., to convert the aperture-plane reflection coefficient to the tissue permittivity. The results of the characterization and validation studies demonstrate that these precision probes, used with the prescribed measurement protocols and data-processing techniques, provide highly accurate and reliable in vivo and ex vivo biological tissue measurements, including breast tissue spectroscopy.

Sensing volume of open-ended coaxial probes for dielectric characterization of breast tissue at microwave frequencies

To achieve accurate measurements, the tissue sample should be homogeneous within a volume large enough so that the measured reflection coefficient is identical to that of a sample filling the entire half-space. Thus, the question arises of the appropriate size of the tissue sample. Sensing volume guidelines have been previously investigated in terms of somewhat arbitrarily chosen constraints on the relative errors in the measured reflection coefficient of tissue-equivalent liquids. In this paper we report sensing volume guidelines that have been developed by first choosing acceptable levels of error in the permittivity and then deriving the appropriate constraints on the errors in the measured reflection coefficient.

Effective permittivity at the interface of dispersive dielectrics in FDTD

A simple treatment of E-field component tangential to dispersive media interfaces in FDTD is introduced. The method uses concepts from the auxiliary differential equations method to average the constitutive parameters. The cases of a wave propagating in a coaxial line and of an open-ended coaxial line radiating into the dispersive media are investigated. Results show that the simulations could be significantly erroneous if the interface is not handled properly.

Effects of mechanical flaws in open-ended coaxial probes for dielectric spectroscopy

A detailed study of dielectric properties of breast tissue in the 0.1 to 20 GHz frequency range currently under way uses open-ended teflon coaxial probes as sensors. This letter quantifies the effects of small mechanical imperfections at the probe aperture on the measured reflection coefficient. The mechanical flaws in the probe can lead to significant errors, thus probes for dielectric spectroscopy of breast tissue have to be carefully manufactured.

Response characterization of the precision open-ended coaxial probe for dielectric spectroscopy of breast tissue

Large-scale dielectric spectroscopy of healthy and diseased breast tissue in the 0.1 to 20 GHz frequency range is currently underway. Open-ended coaxial probes are used as sensors to record the reflection coefficient of the tissue samples. The measured reflection coefficient is converted to the dielectric properties data through a suitable inverse technique. The collected data will aid in further advances of the microwave technology for early breast cancer detection. Thus, it is crucial to ensure the highest possible accuracy and reliability of the tissue measurements.

The dielectric properties of normal and malignant breast tissue at microwave frequencies: analysis, conclusions, and implications from the wisconsin/calgary study

The clinical efficacy of emerging microwave breast cancer detection and hyperthermia treatment techniques (see [X. Li et al., 2005], [M. Converse et al., 2004] and references therein) depend on the microwave dielectric properties of normal, malignant, and benign breast tissues. Knowledge of these properties has been limited by gaps and discrepancies in previously published small-scale studies reporting the dielectric properties of normal and malignant breast tissues obtained from cancer surgeries [L. Sha et al., 2002]. To address these limitations, we have conducted a large-scale joint study at the Universities of Wisconsin and Calgary to experimentally characterize the wideband dielectric properties at microwave frequencies (from 0.5 to 20 GHz) of freshly excised normal, benign, and malignant breast tissues obtained from breast reduction as well as cancer surgeries. In our presentation, we will highlight the conclusions from all aspects of our completed study. Due to space limitations in this conference paper summary, here we focus on the results of a comparison of the dielectric properties of normal breast tissues obtained from both reduction and cancer surgeries.

Precision open-ended coaxial probe for dielectric spectroscopy of breast tissue

Research suggests that there exists a significant contrast between the dielectric properties of cancerous and normal breast tissue at microwave frequencies. Open-ended coaxial probes have been extensively used and analyzed in dielectric spectroscopy. There has been an extensive numerical and experimental analysis of these probes to ensure the highest possible accuracy in the dielectric-property estimates. For simple probes made of sections of semi-rigid cable, small geometrical changes at the probe aperture, which can result from manufacturing, handling and/or usage, may have very significant effects on the observed reflection coefficient. Consequently, a precision, 3.0 mm, stainless steel open-ended coaxial probe has been designed. The probe is built out of sections of high quality coaxial line, using dielectrics whose thermal properties are similar to those of the metal. Tolerances on probe aperture dimensions and flatness are better than 10 /spl mu/m. Borosilicate glass, hermetically sealed to the metal conductors, is used as the dielectric in the final section of the probe. We present preliminary experimental and numerical reflection coefficient estimates using this probe. The carefully manufactured open-ended coaxial probe is reliable and yields accurate results, and is suitable as a sensor for high fidelity in vivo tissue measurements and characterization.

Volume sensing properties of open-ended coaxial probes for dielectric spectroscopy of breast tissue

An ultrawideband microwave imaging approach to early breast cancer detection is based on the hypothesis that there exists a significant dielectric-properties contrast between cancerous and normal breast tissue at microwave frequencies. Definitive knowledge of the dielectric properties of breast tissue in the RF/microwave frequency range (0.1-20 GHz) is required for the development of novel technologies related to the detection and treatment of breast cancer. Coaxial probes have been extensively analyzed and used in dielectric spectroscopy. A careful analysis of the sensing volume of the probes used in our measurements on excised specimens is needed to ensure the highest possible accuracy in the dielectric properties estimates of human breast tissue. We present preliminary estimates of the sensing volume of small diameter coaxial probes. Results were obtained for 2.20 mm and 3.58 mm diameter probes, both without flanges. De-ionized water, methanol, and butanol were chosen as test liquids because their dielectric properties cover the expected range of permittivities of breast tissue. Our results suggest that relatively small tissue samples are sufficient to establish accurately their dielectric properties in the microwave frequency range up to 20 GHz, using small-diameter open-ended coaxial probes.