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

Review of the Capacity to Accurately Detect the Temperature of Human Skin Tissue Using the Microwave Radiation Method

Microwave radiometry (MWR) is instrumental in detecting thermal variations in skin tissue before anatomical changes occur, proving particularly beneficial in the early diagnosis of cancer and inflammation. This study concisely traces the evolution of microwave radiometers within the medical sector. By analyzing a plethora of pertinent studies and contrasting their strengths, weaknesses, and performance metrics, this research identifies the primary factors limiting temperature measurement accuracy. The review establishes the critical technologies necessary to overcome these limitations, examines the current state and prospective advancements of each technology, and proposes comprehensive implementation strategies. The discussion elucidates that the precise measurement of human surface and subcutaneous tissue temperatures using an MWR system is a complex challenge, necessitating an integration of antenna directionality for temperature measurement, radiometer error correction, hardware configuration, and the calibration and precision of a multilayer tissue forward and inversion method. This study delves into the pivotal technologies for non-invasive human tissue temperature monitoring in the microwave frequency range, offering an effective approach for the precise assessment of human epidermal and subcutaneous temperatures, and develops a non-contact microwave protocol for gauging subcutaneous tissue temperature distribution. It is anticipated that mass-produced measurement systems will deliver substantial economic and societal benefits.

Medical Antennas for Microwave Radiothermometry of Biological Objects

The results of an analytical review of various antennas used in medicine are presented. Issues of modern microwave radiothermometry related to the development of new antennas are discussed, as well as possible ways to solve them. The tasks of further research aimed at creating new designs for conformal antennas and antenna arrays providing a significant expansion of the functionality and improvements in the characteristics of medical radiothermographs are formulated. The study was supported by the Russian Science Foundation (Grant No. 22-19-00113), https://rscf.ru/project/22-19-00113/.

Microminiaturization of Multichannel Multifrequency Radiographs

Due to the COVID-19 epidemic, the challenge of introducing methods for investigating patients reducing or eliminating the probability of infection of medical staff is currently relevant. This article provides an analytical review of new technological approaches to organizing the work of medical personnel in carrying out auscultation of patients with COVID-19. The development and approval of such technologies is shown to have started around the world. The ubiquitous and large-scale introduction of these methods into medical practice therefore seems expedient.

Design of a rapid diagnostic model for bladder compliance based on real-time intravesical pressure monitoring system

OBJECTIVE

The diagnosis of bladder dysfunction for children depends on the confirmation of abnormal bladder shape and bladder compliance. The existing gold standard needs to conduct voiding cystourethrogram (VCUG) examination and urodynamic studies (UDS) examination on patients separately. To reduce the time and injury of children's inspection, we propose a novel method to judge the bladder compliance by measuring the intravesical pressure during the VCUG examination without extra UDS.

METHODS

Our method consisted of four steps. We firstly developed a single-tube device that can measure, display, store, and transmit real-time pressure data. Secondly, we conducted clinical trials with the equipment on a cohort of 52 patients (including 32 negative and 20 positive cases). Thirdly, we preprocessed the data to eliminate noise and extracted features, then we used the least absolute shrinkage and selection operator (LASSO) to screen out important features. Finally, several machine learning methods were applied to classify and predict the bladder compliance level, including support vector machine (SVM), Random Forest, XGBoost, perceptron, logistic regression, and Naive Bayes, and the classification performance was evaluated.

RESULTS

73 features were extracted, including first-order and second-order time-domain features, wavelet features, and frequency domain features. 15 key features were selected and the model showed promising classification performance. The highest AUC value was 0.873 by the SVM algorithm, and the corresponding accuracy was 84%.

CONCLUSION

We designed a system to quickly obtain the intravesical pressure during the VCUG test, and our classification model is competitive in judging patients' bladder compliance.

SIGNIFICANCE

This could facilitate rapid auxiliary diagnosis of bladder disease based on real-time data. The promising result of classification is expected to provide doctors with a reliable basis in the auxiliary diagnosis of some bladder diseases prior to UDS.

Pre-Clinical Testing of Microwave Radiometer and a Pilot Study on the Screening Inflammation of Knee Joints

This article presents the pre-clinical evaluation of our custom-built, single-band microwave radiometer centered at 1.3 GHz for deep tissue thermometry, and a pilot study on volunteers for passive detection of inflammation in knee joints. The electromagnetic (EM) compatibility of the battery-operated radiometer for clinical use was assessed as per International Special Committee on Radio Interference (CISPR) 22 standard. The ability to detect inflammation in knee joints was assessed using a substrate integrated waveguide antenna connected to the radiometer. EM compatibility tests carried out in the laboratory indicated device immunity to intentional radiated interference up to -20 dBm injected power in the global system for mobile communication frequency band, and pre-compliance to CISPR 22 standard. Radiometer temperature measurements recorded at the lateral and medial aspects of both knees of 41 volunteers indicated mean temperature greater than 33°C for the diseased sites compared with the mean temperature of 28°C measured for the healthy sites. One-way analysis of variance statistics indicated significantly (P < 0.005) higher radiometer temperature at the diseased sites unlike the healthy sites. Thus, the EM pre-compliance of the device and the potential to measure deep tissue inflammation were demonstrated. Bioelectromagnetics. 2019;40:402-411. © 2019 Bioelectromagnetics Society.

Open-Ended Coaxial Probe Technique for Dielectric Measurement of Biological Tissues: Challenges and Common Practices

Electromagnetic (EM) medical technologies are rapidly expanding worldwide for both diagnostics and therapeutics. As these technologies are low-cost and minimally invasive, they have been the focus of significant research efforts in recent years. Such technologies are often based on the assumption that there is a contrast in the dielectric properties of different tissue types or that the properties of particular tissues fall within a defined range. Thus, accurate knowledge of the dielectric properties of biological tissues is fundamental to EM medical technologies. Over the past decades, numerous studies were conducted to expand the dielectric repository of biological tissues. However, dielectric data is not yet available for every tissue type and at every temperature and frequency. For this reason, dielectric measurements may be performed by researchers who are not specialists in the acquisition of tissue dielectric properties. To this end, this paper reviews the tissue dielectric measurement process performed with an open-ended coaxial probe. Given the high number of factors, including equipment- and tissue-related confounders, that can increase the measurement uncertainty or introduce errors into the tissue dielectric data, this work discusses each step of the coaxial probe measurement procedure, highlighting common practices, challenges, and techniques for controlling and compensating for confounders.

The effect of using a dielectric matching medium in focused microwave radiometry: an anatomically detailed head model study

Microwave radiometry is a passive technique used to measure in-depth temperature distributions inside the human body, potentially useful in clinical applications. Experimental data imply that it may provide the capability of detecting in-depth local variations of temperature and/or conductivity of excitable tissues at microwave frequencies. Specifically, microwave radiometry may allow the real-time monitoring of brain temperature and/or conductivity changes, associated with local brain activation. In this paper, recent results of our ongoing research regarding the capabilities of focused microwave radiometry for brain intracranial applications are presented. Electromagnetic and thermal simulation analysis was performed using an anatomically detailed head model and a dielectric cap as matching medium placed around it, in order to improve the sensitivity and the focusing attributes of the system. The theoretical results were compared to experimental data elicited while exploring that the sensing depth and spatial resolution of the proposed imaging method at 2.1 GHz areas located 3 cm deep inside the brain can be measured, while at 2.5 GHz, the sensing area is confined specifically to the area of interest. The results exhibit the system's potential as a complementary brain imaging tool for multifrequency in-depth passive monitoring which could be clinically useful for therapeutic, diagnostic, and research applications.

Imaging of Conductivity Changes of Excitable Tissues Based on Focused Passive Microwave

Aims:

Modeling of ionic distribution fluctuations of excitable tissues based on data elicited using focused microwave radiometry.

Methodology:

Focused Microwave Radiometry implemented to carry out measurements of in depth body temperature distributions, may provide the capability of sensing local electrical conductivity fluctuations during the cycle of actions potentials in the case of brain excitable cell clusters. An analog beamformer consisting of a conductive inner-surface ellipsoidal cavity is used to focus the chaotic-black body radiation emerging from human tissues by providing convergence of the electromagnetic energy from one focus area where the phantom or subject is placed, to the other where the antennas of sensitive radiometric receivers are positioned. During the past 10 years numerous phantom, animal and human volunteer experiments have been performed with the focused radiometry imaging system. The results show that the detected changes of the output radiometric voltage are attributed to temperature and/or conductivity changes that occur locally concentrated at the areas of interest under measurement. Theoretical and experimental studies are continuously carried out at various frequency bands in conjunction with the use of matching materials placed around the human head or phantom to improve focusing and detection depth. It seems that the manipulation of the focusing area in the tissue in terms of detection depth and spatial resolution is feasible depending on the suitable combination of operation frequencies and matching material. In this paper, theoretical analysis of ion charge diffusion during the cycle of action potentials, propagating along the axons in case of measurements of specific cortical regions is presented. The ion charge diffusion modeling is based on electromagnetic diffusion analogies in the effort to explain the observed experimental results obtained under various psychophysiological conditions in the case of human volunteer measurements.

Results:

By implementing an analysis based on the continuity equations of ionic charges it is concluded that the microwave radiometry output voltage is not affected by the temporal and spatial average fluctuations of Na⁺, K⁺, and Cl^- ions of neural cell axons.

Conclusion:

The analysis of conductivity fluctuations in the central neural system in conjunction with the electromagnetic analysis of the system, leads to the interpretation of the previously acquired experimental data. The application of this technique with other brain functional mapping methods, may provide complementary knowledge to the understanding of the functional organization of psychophysiological processes.

Non-invasive measurement of brain temperature with microwave radiometry: demonstration in a head phantom and clinical case

This study characterizes the sensitivity and accuracy of a non-invasive microwave radiometric thermometer intended for monitoring body core temperature directly in brain to assist rapid recovery from hypothermia such as occurs during surgical procedures. To study this approach, a human head model was constructed with separate brain and scalp regions consisting of tissue equivalent liquids circulating at independent temperatures on either side of intact skull. This test setup provided differential surface/deep tissue temperatures for quantifying sensitivity to change in brain temperature independent of scalp and surrounding environment. A single band radiometer was calibrated and tested in a multilayer model of the human head with differential scalp and brain temperature. Following calibration of a 500MHz bandwidth microwave radiometer in the head model, feasibility of clinical monitoring was assessed in a pediatric patient during a 2-hour surgery. The results of phantom testing showed that calculated radiometric equivalent brain temperature agreed within 0.4°C of measured temperature when the brain phantom was lowered 10°C and returned to original temperature (37°C), while scalp was maintained constant over a 4.6-hour experiment. The intended clinical use of this system was demonstrated by monitoring brain temperature during surgery of a pediatric patient. Over the 2-hour surgery, the radiometrically measured brain temperature tracked within 1-2°C of rectal and nasopharynx temperatures, except during rapid cooldown and heatup periods when brain temperature deviated 2-4°C from slower responding core temperature surrogates. In summary, the radiometer demonstrated long term stability, accuracy and sensitivity sufficient for clinical monitoring of deep brain temperature during surgery.

Thermal modelling using discrete vasculature for thermal therapy: A review

Reliable temperature information during clinical hyperthermia and thermal ablation is essential for adequate treatment control, but conventional temperature measurements do not provide 3D temperature information. Treatment planning is a very useful tool to improve treatment quality, and substantial progress has been made over the last decade. Thermal modelling is a very important and challenging aspect of hyperthermia treatment planning. Various thermal models have been developed for this purpose, with varying complexity. Since blood perfusion is such an important factor in thermal redistribution of energy in in vivo tissue, thermal simulations are most accurately performed by modelling discrete vasculature. This review describes the progress in thermal modelling with discrete vasculature for the purpose of hyperthermia treatment planning and thermal ablation. There has been significant progress in thermal modelling with discrete vasculature. Recent developments have made real-time simulations possible, which can provide feedback during treatment for improved therapy. Future clinical application of thermal modelling with discrete vasculature in hyperthermia treatment planning is expected to further improve treatment quality.

Numerical 3D modeling of heat transfer in human tissues for microwave radiometry monitoring of brown fat metabolism

BACKGROUND

Brown adipose tissue (BAT) plays an important role in whole body metabolism and could potentially mediate weight gain and insulin sensitivity. Although some imaging techniques allow BAT detection, there are currently no viable methods for continuous acquisition of BAT energy expenditure. We present a non-invasive technique for long term monitoring of BAT metabolism using microwave radiometry.

METHODS

A multilayer 3D computational model was created in HFSS™ with 1.5 mm skin, 3-10 mm subcutaneous fat, 200 mm muscle and a BAT region (2-6 cm3) located between fat and muscle. Based on this model, a log-spiral antenna was designed and optimized to maximize reception of thermal emissions from the target (BAT). The power absorption patterns calculated in HFSS™ were combined with simulated thermal distributions computed in COMSOL® to predict radiometric signal measured from an ultra-low-noise microwave radiometer. The power received by the antenna was characterized as a function of different levels of BAT metabolism under cold and noradrenergic stimulation.

RESULTS

The optimized frequency band was 1.5-2.2 GHz, with averaged antenna efficiency of 19%. The simulated power received by the radiometric antenna increased 2-9 mdBm (noradrenergic stimulus) and 4-15 mdBm (cold stimulus) corresponding to increased 15-fold BAT metabolism.

CONCLUSIONS

Results demonstrated the ability to detect thermal radiation from small volumes (2-6 cm3) of BAT located up to 12 mm deep and to monitor small changes (0.5 °C) in BAT metabolism. As such, the developed miniature radiometric antenna sensor appears suitable for non-invasive long term monitoring of BAT metabolism.

Radiometric temperature reading of a hot ellipsoidal object inside the oral cavity by a shielded microwave antenna put flush to the cheek

A new scheme for detection of vesicoureteral reflux (VUR) in children has recently been proposed in the literature. The idea is to warm bladder urine via microwave exposure to at least fever temperatures and observe potential urine reflux from the bladder back to the kidney(s) by medical radiometry. As a preliminary step toward realization of this detection device, we present non-invasive temperature monitoring by use of microwave radiometry in adults to observe temperature dynamics in vivo of a water-filled balloon placed within the oral cavity. The relevance of the approach with respect to detection of VUR in children is motivated by comparing the oral cavity and cheek tissue with axial CT images of young children in the bladder region. Both anatomical locations reveal a triple-layered tissue structure consisting of skin-fat-muscle with a total thickness of about 8-10 mm. In order to mimic variations in urine temperature, the target balloon was flushed with water coupled to a heat exchanger, that was moved between water baths of different temperatures, to induce measurable temperature gradients. The applied radiometer has a center frequency of 3.5 GHz and provides a sensitivity (accuracy) of 0.03 °C for a data acquisition time of 2 s. Three different scenarios were tested and included observation through the cheek tissue with and without an intervening water bolus compartment present. In all cases, radiometric readings observed over a time span of 900 s were shown to be highly correlated (R ~ 0.93) with in situ temperatures obtained by fiberoptic probes.

Non-invasive vesicoureteral reflux detection: heating risk studies for a new device

OBJECTIVE

To investigate a novel non-invasive device developed to warm bladder urine and to measure kidney temperature to detect vesicoureteral reflux.

MATERIALS AND METHODS

Microwave antennas focused energy within the bladder. Phantom experiments measured the results. The heating protocol was optimized in an in-vivo porcine model, and then tested once, twice and three times consecutively in three pigs followed by pathologic examinations.

RESULTS

Computer simulations showed a dual concentric conductor square slot antenna to be the best. Phantom studies revealed that this antenna easily heated a bladder phantom without over heating intervening layers. In-vivo a bladder heating protocol of 3 min with 30 W each to two adjacent antennas 45 s on 15 s off followed by 15 min of 15 s on and 45 s off was sufficient. When pigs were heated once, twice and three times with this heating protocol, pathologic examination of all tissues in the heated area showed no thermal changes. More intensive heating in the animal may have resulted in damage to muscle fibers in the anterior abdominal wall.

CONCLUSIONS

Selective warming of bladder urine was successfully demonstrated in phantom and animals. Localized heating for this novel vesicoureteral reflux device requires low-power levels and should be safe for humans.

Improved radiometric performance attained by an elliptical microwave antenna with suction

We present a new way to securely mount a medical microwave antenna onto the human body for improved in vivo temperature measurements by microwave radiometry. A low cost and simple vacuum pressure source is used to provide suction (negative pressure) on the aperture of an elliptical antenna with vacuum chamber cavity backing. The concept offers improved electromechanical coupling between the antenna surface and the skin of the body. The proposed solution is evaluated experimentally to test repeatability of radiometric temperature measurements by remounting the antenna many times in one sequence on a given anatomical location. Four representative locations (hand, belly, hip, and chest) were used to test the suction antenna concept against anatomical curvature and load variations. Statistical analysis shows a marked decrease in the standard deviation of measured temperatures with the use of suction compared to conventional manual fixation. At repeated measurements, the vacuum antenna produces less uncertainty and improved estimate of the true lossy load temperature. During body movement, the antenna mounted at bone-filled areas shows greatest potential for improved performance.

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.

EVOLUTION OF ANTENNA PERFORMANCE FOR APPLICATIONS IN THERMAL MEDICNE

This presentation provides an overview of electromagnetic heating technology that has proven useful in clinical applications of hyperthermia therapy for cancer. Several RF and microwave antenna designs are illustrated which highlight the evolution of technology from simple waveguide antennas to spatially and temporally adjustable multiple antenna phased arrays for deep heating, conformal arrays for superficial heating, and compatible approaches for radiometric and magnetic resonance image based non-invasive thermal monitoring. Examples of heating capabilities for several recently developed applicators demonstrate highly adjustable power deposition that has not been possible in the past.