Absolute Temperature Monitoring Using RF Radiometry in the MRI Scanner

Temperature detection using microwave radiometry has proven value for noninvasively measuring the absolute temperature of tissues inside the body. However, current clinical radiometers operate in the gigahertz range, which limits their depth of penetration. We have designed and built a noninvasive radiometer which operates at radio frequencies (64 MHz) with ~100-kHz bandwidth, using an external RF loop coil as a thermal detector. The core of the radiometer is an accurate impedance measurement and automatic matching circuit of 0.05 Omega accuracy to compensate for any load variations. The radiometer permits temperature measurements with accuracy of plusmn0.1degK, over a tested physiological range of 28degC-40 degC in saline phantoms whose electric properties match those of tissue. Because 1.5 T magnetic resonance imaging (MRI) scanners also operate at 64 MHz, we demonstrate the feasibility of integrating our radiometer with an MRI scanner to monitor RF power deposition and temperature dosimetry, obtaining coarse, spatially resolved, absolute thermal maps in the physiological range. We conclude that RF radiometry offers promise as a direct, noninvasive method of monitoring tissue heating during MRI studies and thereby providing an independent means of verifying patient-safe operation. Other potential applications include titration of hyper- and hypo-therapies     

Vesicoureteral reflux in children: a phantom study of microwave heating and radiometric thermometry of pediatric bladder

We have investigated the use of microwave heating and radiometry to safely heat urine inside a pediatric bladder. The medical application for this research is to create a safe and reliable method to detect vesicoureteral reflux, a pediatric disorder, where urine flow is reversed and flows from the bladder back up into the kidney. Using fat and muscle tissue models, we have performed both experimental and numerical simulations of a pediatric bladder model using planar dual concentric conductor microstrip antennas at 915 MHz for microwave heating. A planar elliptical antenna connected to a 500 MHz bandwidth microwave radiometer centered at 3.5 GHz was used for noninvasive temperature measurement inside tissue. Temperatures were measured in the phantom models at points during the experiment with implanted fiberoptic sensors, and 2-D distributions in cut planes at depth in the phantom with an infrared camera at the end of the experiment. Cycling between 20 s with 20 Watts power for heating, and 10?s without power to allow for undisturbed microwave radiometry measurements, the experimental results show that the target tissue temperature inside the phantom increases fast and that the radiometer provides useful measurements of spatially averaged temperature of the illuminated volume. The presented numerical and experimental results show excellent concordance, which confirms that the proposed system for microwave heating and radiometry is applicable for safe and reliable heating of pediatric bladder.     

Microwave radiometry in living tissue: what does it measure?

The sensitivity of microwave radiometry for detecting subcutaneous targets was studied both experimentally and theoretically. The radiometer used a dielectric loaded rectangular waveguide antenna in contact with a lossy dielectric medium. A cylindrical target with dielectric properties and/or temperature different from that of the surrounding medium was located beneath this surface. For most of the studies, the target and the surrounding medium were maintained at constant, but unequal, temperatures (i.e., heat conduction effects were insignificant). The received radiometric signal was calculated as the location and dielectric properties of the target were varied. Finally, the radiometer signal was calculated for the situation with the target maintained at constant temperature but with the surrounding medium modeled by the bioheat equation. Experimental studies were performed using a radiometer operating at 4.7 GHz. The target was a thin walled tube through which a temperature controlled liquid was circulated, located in a temperature controlled fluid tank. The results indicate that microwave radiometry (as used in this study) responds to the temperature averaged over the field pattern of the antenna with very strong weighting of regions near the surface. A simple quasi-static analysis provides a good indication of the sensitivity of the technique for detecting cylindrical targets whose dielectric properties are different from those of the surrounding medium. A simple estimate of thermal conduction around the target suggest that thermal effects greatly increase the apparent size of the target.     

Frequency-Dependent Absorption of Electromagnetic Energy in Biological Tissue

The frequency-dependent absorption of electromagnetic energy in biological tissue is illustrated by use of the Debye equations, model calculations for different irradiation conditions, and measured electrical properties (conductivity and permittivity) of different tissues. Four separate irradiation conditions are treated for calculating the power absorbed. in a given tissue when it forms a flat interface or is surrounded by another tissue. The calculations show that the greatest differential absorption occurs at frequencies between the dominant relaxation frequencies in the two tissues. From rat mammary gland tumor data, the calculations show an optimum frequency range of about 100-500 MHz for microwave hyperthermia treatment of at least these types of tumor.     

Theoretical Analysis of the Biological Thermal Effect of Millimeter Waves in Layered-Dielectric-Slabs

The theory is presented for one method of determining the biological thermal effect of millimeter waves in microwave radiometry. It has been studied theoretically that millimeter waves propagation and absorption in a human body. The model is a plane straticulate homogeneous slab of tissues under the irradiance of normal incidence plane wave. It has been discussed by obtaining the electromagnetic field, absorbent power, specific absorption rate, temperature field and their distributions in the human trunk model. Also, the principle of thermal therapeutics of millimeter waves to cancer has been discussed preliminarily.     

Optimal steady-state temperature distribution for a phased arrayhyperthermia system

A method is presented for the evaluation of optimal amplitude and phase excitations for the radiating elements of a phased array hyperthermia system, in order to achieve desired steady-state temperature distributions inside and outside of malignant tissues. Use is made of a detailed electromagnetic and thermal model of the heated tissue in order to predict the steady-state temperature at any point in tissue. Optimal excitations are obtained by minimizing the squared error between desired and model predicted temperatures inside the tumor volume, subject to the constraint that temperatures do not exceed an upper bound outside the tumor. The penalty function technique is used to solve the constrained optimization problem. Sequential unconstrained minima are obtained by a modified Newton method. Numerical results for a four element phased array hyperthermia system are presented     

Multiangle method for temperature measurement of biological tissues by microwave radiometry

A new approach for deriving the temperature distribution in biological tissues of microwave radiometry is proposed. It consists in the measurement of the thermal radiation of the body, at a given frequency, as a function of the observation angle, for two mutually orthogonal polarizations. Theoretically, this method yields results comparable to those obtained with the multispectral method. In order to derive the relations between the body temperature and the emitted thermal signal, the biological body is modeled by a set of parallel planar layers, each characterized by constant permittivity and temperature. It is demonstrated that for all practical purposes the radiation pattern of the antenna may be approximated by that of an unbounded plane wave.<>     

微波热疗天线在生物组织中温度分布的模拟

模拟微波肿瘤热疗条件下生物组织中的温度分布,对临床治疗中微波热疗天线的设计、选择及治疗方案的确定具有重要意义.本文结合电磁场的时域有限差分(FDTD)和温度场的有限差分方法模拟了微波热疗天线在生物组织中产生的温度分布.通过单极子天线对等效组织模型的加热温度模拟结果与实验测量结果比较,对微波热疗天线在生物组织中产生的温度场模拟程序进行了验证.     

Image restoration in chirp-pulse microwave CT (CP-MCT)

Chirp-pulse microwave computed tomography (CP-MCT) is a technique for imaging the distribution of temperature variations inside biological tissues. Even if resolution and contrast are adequate to this purpose, a further improvement of image quality is desirable. In this paper, we discuss the blur of CP-MCT images and we propose a method for estimating the corresponding point spread function (PSF). To this purpose we use both a measured and a computed projection of a cylindrical phantom. We find a good agreement between the two cases. Finally the estimated PSF is used for deconvolving data corresponding to various kinds of cylindrical phantoms. We use an iterative nonlinear deconvolution method which assures nonnegative solutions and we demonstrate the improvement of image quality which can be obtained in such a way.     

Microwave radiometry for continuous non-contact temperature measurements during microwave heating.

Temperature measurement during microwave heating in industrial and commercial processes can improve quality, throughput, and energy conservation. Conventional ways of measuring temperature inside a microwave oven cavity are costly, inconvenient, or unsuitable for high-volume industrial applications. In this paper, we describe the theory of microwave radiometry as applied to the measurement of temperature during microwave heating. By extending the theory of radiative transfer to the case of thermal microwave radiation inside a cavity, we show that the same characteristics which make a microwave cavity suitable for heating materials also assist in obtaining meaningful temperature data with microwave radiometry. We present experimental data from the heating of liquid and solid materials which confirm the essential features of the theory, and show agreement between this method and more conventional methods of +/-4 degrees C.     

Through-Wall Sensing With Multifrequency Microwave Radiometry: A Proof-of-Concept Demonstration

A proof-of-concept demonstration of through-wall sensing with microwave radiometry is described. A multifrequency microwave radiometer with 37 channels from 2.1 to 17.35 GHz was used in two experiments to observe objects through a cinder block wall of approximately 20-cm thickness. Measured data show the clear ability of the radiometer to detect thermal contrasts on the interior of the wall. A discussion of the basic physical processes involved in the measurement is provided. When compared with active systems, microwave radiometry for through-wall sensing faces significant challenges, including limitations in ranging, horizontal resolution, and corruption by radio-frequency interference, but also provides complementary capabilities, particularly with regard to thermal information. Further consideration of microwave radiometry appears warranted for applications where thermal information is of interest.     

Radiometry in the Submillimeter Region Using the Interferometric Modulator

Radiometry in the submillimeter and far infrared regions involves problems of a type not encountered in the centimeter region which require solutions using techniques different from those used in centimeter-wavelength radiometry. The nonlinear variation of the magnitude of the black-body radiation spectral density with temperature and wavelength, the limitation of antenna beamwidth by factors connected with the size of the noncoherent detector and the antenna focal length (rather than by diffraction effects and the antenna aperture) and the heavy absorption of submillimeter radiation by atmospheric water vapor are typical of the problems normally not encountered in centimeter radiometry. The unavailability of microwave techniques (i.e., waveguides, coherent receivers, etc.) makes necessary the use of quasi-optical techniques in this wavelength region. The interferometric modulator, which has already been used in far infrared spectrometers, is proposed in this paper as the major component of a practical submillimeter radiometer. Its use as the wave-number-selection device in a radiometer is analyzed and estimates are obtained for the sensitivity of this submillimeter radiometer. It is estimated that a 0.2/spl deg/ minimum detectable temperature differential is achievable with this radiometer. Also discussed are the effects of atmospheric water vapor absorption and the sensitivity of a number of different types of radiation detectors suitable for use in the submillimeter-wavelength region.     

FDTD electromagnetic and thermal analysis of interstitialhyperthermic applicators

The electromagnetic and thermal behavior of interstitial applicators was analyzed by using the finite-difference time-domain method. Two configurations were considered: a simple insulated dipole antenna radiating in a layered tissue, and an air cooled applicator radiating in a tissue-equivalent phantom. The proposed approach allows a detailed modeling of the complete structure of the applicator. Furthermore, specific absorption rate and temperature distributions can be determined considering real clinical or experimental conditions. The temperature distribution for the air cooled applicator has been compared with experimental results     

乳腺红外热图的临床分析

本文分析２７６例红外热图，发现热图分析仅反映乳腺血管的分布情况，与年龄有关，并无临床意义。出现温差则提示有病理改变。乳癌与小叶增生的热图各具特征，温差界限较明显，与临床和病理的符合率较高，但良性肿瘤的检出率并不理想。笔者认为：红外遥测技术，对机体无痛无损，能提供精确的物理参数，确有实用价值，但有其局限性和片面性，应视为综合检查手段之一。结合临床，综合分析，可提高诊断的正确率。     

Detection of vesicoureteral reflux using microwave radiometry-system characterization with tissue phantoms

Microwave (MW) radiometry is proposed for passive monitoring of kidney temperature to detect vesicoureteral reflux (VUR) of urine that is externally heated by a MW hyperthermia device and thereafter reflows from the bladder to kidneys during reflux. Here, we characterize in tissue-mimicking phantoms the performance of a 1.375 GHz radiometry system connected to an electromagnetically (EM) shielded microstrip log spiral antenna optimized for VUR detection. Phantom EM properties are characterized using a coaxial dielectric probe and network analyzer (NA). Power reflection and receive patterns of the antenna are measured in layered tissue phantom. Receiver spectral measurements are used to assess EM shielding provided by a metal cup surrounding the antenna. Radiometer and fiberoptic temperature data are recorded for varying volumes (10-30 mL) and temperaturesg (40-46°C) of the urine phantom at 35 mm depth surrounded by 36.5°C muscle phantom. Directional receive pattern with about 5% power spectral density at 35 mm target depth and better than -10 dB return loss from tissue load are measured for the antenna. Antenna measurements demonstrate no deterioration in power reception and effective EM shielding in the presence of the metal cup. Radiometry power measurements are in excellent agreement with the temperature of the kidney phantom. Laboratory testing of the radiometry system in temperature-controlled phantoms supports the feasibility of passive kidney thermometry for VUR detection.     

Analysis of Thermal Radiation from an Inhomogeneous Cylindrical Human Body Model

The thermal radiation from a cylindrical human body model at microwave frequencies is treated analytically. The human body model is taken to be a homogeneous cylinder at temperature T having a localized internal thermal inhomogeneity at temperature T + Delta T. The mean energy density for the near field outside the cylinder is determined by employing the dyadic Green's function of the homogeneous cylinder and the fluctuation-dissipation theorem. Analytical results are derived for the contributions of the homogeneous cylinder and the inhomogeneity region. Numerical results are presented for several geometries at low microwave frequencies where a reasonable transparency of tissues is expected. The possibility of using microwave radiometry techniques to measure temperature distributions in depth is discussed in relation to hyperthermia and the development of noninvasive diagnostic techniques. It is shown that the emission from surrounding tissues limits the detectability of thermal inhomogeneities inside the body and that by using low microwave frequencies (~1 GHz), temperature measurement at depths up to 2 cm can be performed.     

Time-multiplexed beamforming for noninvasive microwave hyperthermia treatment

A noninvasive microwave beamforming strategy is proposed for selective localized heating of biological tissue. The proposed technique is based on time multiplexing of multiple beamformers. We investigate the effectiveness of the time-multiplexed beamforming in the context of brain hyperthermia treatment by using a high-fidelity numerical head phantom of an adult female from the Virtual Family (IT'IS Foundation) as our testbed. An operating frequency of 1 GHz is considered to balance the improved treatment resolution afforded by higher frequencies against the increased penetration through the brain afforded by lower frequencies. The exact head geometry and dielectric properties of biological tissues in the head are assumed to be available for the creation of patient-specific propagation models used in beamformer design. Electromagnetic and thermal simulations based on the finite-difference time-domain method are used to evaluate the hyperthermia performance of time-multiplexed beamforming and conventional beamforming strategies. The proposed time-multiplexing technique is shown to reduce the unintended heating of healthy tissue without affecting the treatment temperature or volume. The efficacy of the method is demonstrated for target locations in three different regions of the brain. This approach has the potential to improve microwave-induced localized heating for cancer treatment via hyperthermia or heat-activated chemotherapeutic drug release.     

A tool for the quantitative spatial analysis of complex cellular systems.

Spatial events largely determine the biology of cells, tissues, and organs. In this paper, we present a tool for the quantitative spatial analysis of heterogeneous cell populations, and we show experimental validation of this tool using both artificial and real (mammary gland tissue) data, in two and three dimensions. We present the refined relative neighborhood graph as a means to establish neighborhood between cells in an image while modeling the topology of the tissue. Then, we introduce the M function as a method to quantitatively evaluate the existence of spatial patterns within one cell population or the relationship between the spatial distributions of multiple cell populations. Finally, we show a number of examples that demonstrate the feasibility of our approach.     

A new radiation balance microwave thermograph for simultaneous and independent temperature and emissivity measurements.

In the past, biomedical temperature measurements by microwave radiometry suffered from variable mismatch (emissivity less than 1) between the specimen under test and the receiving antenna. We have developed an improved radiometer, which simultaneously measures temperature and emissivity, independent by of a possible mismatch. Comparative measurements demonstrate the superiority of the new system as compared to conventional ones.     

Comments on ;Electromagnetic field distribution and developed losses inside a finite cylinder and sphere under MRI conditions' by G. Sergiadis et al

In the above-named work by G. Sergiadis et al. (see ibid., vol.7, p.381-5, 1988), an exact solution to the electromagnetic field distribution inside a conductive cylinder of finite length was proposed for estimation of the thermal losses in biological tissues under MRI conditions. The commenters claim to show that such a solution is untrue for a finite-length cylinder, and that the related numerical treatments in the work of Sergiadis et al. are for an infinite cylinder, not a finite cylinder.