Øystein Klemetsen

DESIGN OF MEDICAL RADIOMETER FRONT-END FOR IMPROVED PERFORMANCE

We have investigated the possibility of building a singleband Dicke radiometer that is inexpensive, small-sized, stable, highly sensitive, and which consists of readily available microwave components. The selected frequency band is at 3.25-3.75 GHz which provides a reasonable compromise between spatial resolution (antenna size) and sensing depth for radiometry applications in lossy tissue. Foreseen applications of the instrument are non-invasive temperature monitoring for breast cancer detection and temperature monitoring during heating. We have found off-the-shelf microwave components that are sufficiently small (< 5 mm × 5 mm) and which offer satisfactory overall sensitivity. Two different Dicke radiometers have been realized: one is a conventional design with the Dicke switch at the front-end to select either the antenna or noise reference channels for amplification. The second design places a matched pair of low noise amplifiers in front of the Dicke switch to reduce system noise figure.Numerical simulations were performed to test the design concepts before building prototype PCB front-end layouts of the radiometer. Both designs provide an overall power gain of approximately 50 dB over a 500 MHz bandwidth centered at 3.5 GHz. No stability problems were observed despite using triple-cascaded amplifier configurations to boost the thermal signals. The prototypes were tested for sensitivity after calibration in two different water baths. Experiments showed superior sensitivity (36% higher) when implementing the low noise amplifier before the Dicke switch (close to the antenna) compared to the other design with the Dicke switch in front. Radiometer performance was also tested in a multilayered phantom during alternating heating and radiometric reading. Empirical tests showed that for the configuration with Dicke switch first, the switch had to be locked in the reference position during application of microwave heating to avoid damage to the active components (amplifiers and power meter). For the configuration with a low noise amplifier up front, damage would occur to the active components of the radiometer if used in presence of the microwave heating antenna. Nevertheless, this design showed significantly improved sensitivity of measured temperatures and merits further investigation to determine methods of protecting the radiometer for amplifier first front ends.

Improved Detectability in Medical Microwave Radio-Thermometers as Obtained by Active Antennas

Microwave radiometry is a spectral measurement technique for resolving blackbody radiation of heated matter above absolute zero. The emission levels vary with frequency and are at body temperatures maximized in the infrared spectral band. Medical radio-thermometers are mostly noninvasive short-range instruments that can provide temperature distributions in subcutaneous biological tissues when operated in the microwave region. However, a crucial limitation of the microwave radiometric observation principle is the extremely weak signal level of the thermal noise emitted by the lossy material (-174 dBm/Hz at normal body temperature). To improve the radiometer SNR, we propose to integrate a tiny, moderate gain, low-noise preamplifier (LNA) close to the antenna terminals as to obtain increased detectability of deep seated thermal gradients within the volume under investigation. The concept is verified experimentally in a lossy phantom medium by scanning an active antenna across a thermostatically controlled water phantom with a hot object embedded at 38 mm depth. Three different setups were investigated with decreasing temperature contrasts between the target and ambient medium. As a direct consequence of less ripple on the raw radiometric signal, statistical analysis shows a marked increase in signal-to-clutter ratio of the brightness temperature spatial scan profiles, when comparing active antenna operation with conventional passive setups.

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.

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.

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 young children: a study of radiometric thermometry as detection modality using an ex vivo porcine model.

Microwave radiometry is evaluated for renal thermometry tailored to detect the pediatric condition of vesicoureteral urine reflux (VUR) from the bladder through the ureter into the kidney. Prior to a potential reflux event, the urine is heated within the bladder by an external body contacting a hyperthermia applicator to generate a fluidic contrast temperature relative to normal body temperature. A single band, miniaturized radiometer (operating at 3.5 GHz) is connected to an electromagnetic-interference-shielded and suction-coupled elliptical antenna to receive thermal radiation from an ex vivo porcine phantom model. Brightness (radiometric) and fiberoptic temperature data are recorded for varying urine phantom reflux volumes (20-40 mL) and contrast temperatures ranging from 2 to 10 °C within the kidney phantom. The kidney phantom itself is located at 40 mm depth (skin-to-kidney center distance) and surrounded by the porcine phantom. Radiometric step responses to injection of urine simulant by a syringe are shown to be highly correlated with in situ kidney temperatures measured by fiberoptic probes. Statistically, the performance of the VUR detecting scheme is evaluated by error probabilities of making a wrong decision. Laboratory testing of the radiometric system supports the feasibility of passive non-invasive kidney thermometry for the detection of VUR classified within the two highest grades.

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.

Low-cost and small-sized medical microwave radiometer design

In the past decades research in microwave radiometry has been conducted for use in medical applications [1]. Detection of breast cancer, hyperthermia, and measurment the temperature brain temperature of newborn babies are areas here this technique have been used. Russian scientists have developed a radiometer which is sold under the name of RTM-01 for the detection of breast cancer. The technical details of this radiometer is not known, and are therefore difficult to be tested. The detection procedure is to make symmetrical point measurements for each breast. Total power radiometer consists of a high gain low noise amplifier followed by a square law detector and integrator. This radiometer is prone to amplification drift. Dicke radiometer uses a switch in front between the sensor antenna and a reference. Whitch can remove the system drift. The goal of the present design is to determine whether it is possible to create small sized frontstage to a radiometer, using inexpensive commercial surface mount device (SMD) components. The tradeoff between low- and high frequency radiometer band must also be considered, as the first in general requires a larger antenna, but can penetrate deeper into the body tissue [2]. The emissivity and spatial resolution does also increases with higher frequency.