Bone dielectric property variation as a function of mineralization at microwave frequencies

Transmission-Based Vertebrae Strength Probe Development: Far Field Probe Property Extraction and Integrated Machine Vision Distance Validation Experiments

We are developing a transmission-based probe for point-of-care assessment of vertebrae strength needed for fabricating the instrumentation used in supporting the spinal column during spinal fusion surgery. The device is based on a transmission probe whereby thin coaxial probes are inserted into the small canals through the pedicles and into the vertebrae, and a broad band signal is transmitted from one probe to the other across the bone tissue. Simultaneously, a machine vision scheme has been developed to measure the separation distance between the probe tips while they are inserted into the vertebrae. The latter technique includes a small camera mounted to the handle of one probe and associated fiducials printed on the other. Machine vision techniques make it possible to track the location of the fiducial-based probe tip and compare it to the fixed coordinate location of the camera-based probe tip. The combination of the two methods allows for straightforward calculation of tissue characteristics by exploiting the antenna far field approximation. Validation tests of the two concepts are presented as a precursor to clinical prototype development.

ROS production in response to high-power microwave pulses induces p53 activation and DNA damage in brain cells: Radiosensitivity and biological dosimetry evaluation

Background: Pulsed high-power microwave (HPM) has many applications and is constantly being researched to expand its uses in the future. As the number of applications grows, the biological effects and safety level of pulsed HPM become a serious issue, requiring further research. Objective: The brain is regarded as the most vulnerable organ to radiation, raising concerns about determining an acceptable level of exposure. The effect of nanosecond pulses and the mechanisms underlying HPM on the brain has not been studied. For the first time, we observed the effect of pulsed 3.5 GHz HPM on brain normal astrocytes and cancer U87 MG cells, as well as the likely mechanisms involved. Methods: To generate 3.5 GHz HPM, an axial virtual cathode oscillator was constructed on pulsed power generator "Chundoong". The cells were directly exposed to HPM (10, 25, 40, and 60) pulses (1 mJ/pulse), with each pulse delivered after 1 min of charging time to evaluate the dose dependent effects. Results: A strong electric field (∼23 kV/cm) of HPM irradiation primarily causes the production of reactive oxygen species (ROS), altering cell viability, mitochondrial activity, and cell death rates in U87 and astrocytes at certain dosages. The ROS generation in response to HPM exposure was primarily responsible for DNA damage and p53 activation. The hazardous dosage of 60 pulses is acknowledged as having damaging effects on brain normal cells. Interestingly, the particular 25 pulses exhibited therapeutic effects on U87 cells via p53, Bax, and Caspase-3 activation. Conclusion: HPM pulses induced apoptosis-related events such as ROS burst and increased oxidative DNA damage at higher dosages in normal cells and specific 25 pulses in cancer U87. These findings are useful to understand the physiological mechanisms driving HPM-induced cell death, as well as the safety threshold range for HPM exposure on normal cells and therapeutic effects on cancer U87. As HPM technology advances, we believe this study is timely and will benefit humanity and future research.

Open-Ended Transmission Coaxial Probes for Sarcopenia Assessment

We developed a handheld, side-by-side transmission-based probe for interrogating tissue to diagnose sarcopenia—a condition largely characterized by muscle loss and replacement by fat. While commercial microwave reflection-based probes exist, they can only be used in a lab for a variety of applications. The penetration depth of these probes is only in the order of 0.3 mm, which does not even traverse the skin layer, and minor motion of the coaxial feedlines can completely dismantle the calibration. Our device builds primarily on the transmission-based concept that allows for substantially greater signal penetration depth operating over a very broad bandwidth. Additional features were integrated to further improve the penetration, optimize the geometry for a more focused planar excitation, and juxtapose the coaxial apertures for more controlled interrogation. The larger coaxial apertures further increased the penetration depth while retaining the broadband performance. Three-dimensional printing technology made it possible for the apertures to be compressed into ellipses for interrogation in a near-planar geometry. Finally, fixed side-by-side positioning provided repeatable and reliable performance. The probes were also not susceptible to multipath signal corruption due to the close proximity of the transmitting and receiving apertures. The new concept worked from 100 MHz to over 8 GHz and could sense property changes as deep as 2–3 cm. While the signal changes due to deeper feature aberrations were more subtle than for signals emanating from the skin and subcutaneous fat layers, the large property contrast between muscle and fat is a sarcopenic indication that helps to distinguish even the deepest objects. This device has the potential to provide needed specificity information about the relevant underlying tissue.

Monitoring Bone Density Using Microwave Tomography of Human Legs: A Numerical Feasibility Study

A major cause of bone mass loss worldwide is osteoporosis. X-ray is considered to be the gold-standard technique to diagnose this disease. However, there is currently a need for an alternative modality due to the ionizing radiations used in X-rays. In this vein, we conducted a numerical study herein to investigate the feasibility of using microwave tomography (MWT) to detect bone density variations that are correlated to variations in the complex relative permittivity within the reconstructed images. This study was performed using an in-house finite-element method contrast source inversion algorithm (FEM-CSI). Three anatomically-realistic human leg models based on magnetic resonance imaging reconstructions were created. Each model represents a leg with a distinct fat layer thickness; thus, the three models are for legs with thin, medium, and thick fat layers. In addition to using conventional matching media in the numerical study, the use of commercially available and cheap ultrasound gel was evaluated prior to bone image analysis. The inversion algorithm successfully localized bones in the thin and medium fat scenarios. In addition, bone volume variations were found to be inversely proportional to their relative permittivity in the reconstructed images with the root mean square error as low as 2.54. The observations found in this study suggest MWT as a promising bone imaging modality owing to its safe and non-ionizing radiations used in imaging objects with high quality.

Application of a Neural Network Classifier to Radiofrequency-Based Osteopenia/Osteoporosis Screening

Objective: There is an unmet need for quick, physically small, and cost-effective office-based techniques that can measure bone properties without the use of ionizing radiation. Methods: The present study reports the application of a neural network classifier to the processing of previously collected data on very-low-power radiofrequency propagation through the wrist to detect osteoporotic/osteopenic conditions. Our approach categorizes the data obtained for two dichotomic groups. Group 1 included 27 osteoporotic/osteopenic subjects with low Bone Mineral Density (BMD), characterized by a Dual X-Ray Absorptiometry (DXA) T-score below – 1, measured within one year. Group 2 included 40 healthy and mostly young subjects without major clinical risk factors such as a (family) history of bone fracture. We process the complex radiofrequency spectrum from 30 kHz to 2 GHz. Instead of averaging data for both wrists, we process them independently along with the wrist circumference and then combine the results, which greatly increases the sensitivity. Measurements along with data processing require less than 1 min. Results: For the two dichotomic groups identified above, the neural network classifier of the radiofrequency spectrum reports a sensitivity of 83% and a specificity of 94%. Significance: These results are obtained without including any additional clinical risk factors. They justify that the radio transmission data are usable on their own as a predictor of bone density. This approach has the potential for screening patients at risk for fragility fractures in the office, given the ease of implementation, small device size, and low costs associated with both the technique and the equipment.

Identification of Bone Density Changes Applying Impedance Spectroscopy with a Piezo-Device Coupled to a Human Tooth

Bone tissue is a calcium deposit and supporting structure of the human body, it is exposed to several pathologies that modify its mineral content. To determine these changes, different diagnostic procedures are performed with techniques using invasive ionizing radiation, which are limited by the negative effects in the long term on human health. A methodology is explored that could be applicable in the diagnosis of pathologic variations in bone mineral density, using structural monitoring tools. The proposed technique estimates changes in bone conditions by applying impedance spectroscopy with a tooth-borne piezo-device. Bone-tooth samples were prepared to simulate a section of maxillary bone and subsequently treated with chemical agents, simulating pathologic decalcification. The piezo-device is inserted in the slot of an orthodontic bracket, previously bonded to the crown of the tooth, in order to transmit vibration to surrounding bone. The variations in bone micro-architecture were computed by image processing analyzed with samples prepared in transparent resin, allowing the measurement of morphometry before and after the induced changes in mineral content. Using vibrational bone response, impedance measurements allowed to observe the variations in bone mass as the samples were progressively decalcified. In the 5-50kHz spectrum, it was demonstrated the sensitivity of the electro-mechanical impedance during the bone alteration procedure since the electrical resistance signals of the piezo-device consistently changed in the frequency spectrum (5-50kHz). The piezo-device shows to be sensitive to the changes produced by the bone alterations, which were caused by the stiffness variations made in the sample during the decalcifying. These changes were statistically correlated to demonstrate that in a less invasive way, bone alterations could be monitored from the teeth. This result opens the door to search for a new way to diagnose bone density changes in real applications.

Discrete Dipole Approximation-Based Microwave Tomography for Fast Breast Cancer Imaging

This paper describes a fast microwave tomography reconstruction algorithm based on the two-dimensional discrete dipole approximation. Synthetic data from a finite-element based solver and experimental data from a microwave imaging system are used to reconstruct images and to validate the algorithm. The microwave measurement system consists of 16 monopole antennas immersed in a tank filled with lossy coupling liquid and a vector network analyzer. The low-profile antennas and lossy nature of system make the discrete dipole approximation an ideal forward solver in the image reconstructions. The results show that the algorithm can readily reconstruct a 2D plane of a cylindrical phantom. The proposed forward solver combined with the nodal adjoint method for computing the Jacobian matrix enables the algorithm to reconstruct an image within 6 seconds. This implementation provides a significant time savings and reduced memory requirements and is a dramatic improvement over previous implementations.

Expansion of the Nodal-Adjoint Method for Simple and Efficient Computation of the 2D Tomographic Imaging Jacobian Matrix

This paper focuses on the construction of the Jacobian matrix required in tomographic reconstruction algorithms. In microwave tomography, computing the forward solutions during the iterative reconstruction process impacts the accuracy and computational efficiency. Towards this end, we have applied the discrete dipole approximation for the forward solutions with significant time savings. However, while we have discovered that the imaging problem configuration can dramatically impact the computation time required for the forward solver, it can be equally beneficial in constructing the Jacobian matrix calculated in iterative image reconstruction algorithms. Key to this implementation, we propose to use the same simulation grid for both the forward and imaging domain discretizations for the discrete dipole approximation solutions and report in detail the theoretical aspects for this localization. In this way, the computational cost of the nodal adjoint method decreases by several orders of magnitude. Our investigations show that this expansion is a significant enhancement compared to previous implementations and results in a rapid calculation of the Jacobian matrix with a high level of accuracy. The discrete dipole approximation and the newly efficient Jacobian matrices are effectively implemented to produce quantitative images of the simplified breast phantom from the microwave imaging system.

Electromechanical impedance measurements for bone health monitoring through teeth used as probes of a Piezo-device

Bone is a dynamic biological tissue that acts as the primary rigid support of the body. Several systemic factors are responsible for pathologies that negatively affect its structural attributes. Although the bone is in continuous renewal by osteogenesis, metabolic diseases are the most common affectations that alter its natural equilibrium. Different techniques based on ionizing radiation are used for the bone diagnosis restrictively. However, if these are not used adequately, the application could present risks for human health. In this paper, it is proposed and explored a new technique to apply an early-stage diagnosis of bone variations. The technique evaluates bone structural conditions from the teeth (used as probes) by applying a structural health monitoring (SHM) methodology. An experimental procedure is described to identify the stiffness variations produced by mechanical drillings done in prepared bone samples. The identification is carried out applying the electromechanical impedance technique (EMI) through a piezo-actuated device in the frequency spectrum 5-20kHz. Three bone samples with incorporated teeth (three teeth, two teeth, and one tooth) were prepared to emulate a mandibular portion of alveolar bone-PDL (periodontal ligament)-tooth system. Piezo-device was attached to the crown of the tooth with an orthodontic bracket allowing the teeth to act as probes. The electrical resistance measurements were computed with an electrical decoupling approach that improved the detection of the drillings; it was due to the increment of the sensitivity of the signals. The results showed that the bone mass reduction is correlated with statistical indices obtained in specific frequency intervals of the electrical resistance. This work suggests the possibility of a future application addressed to a bone diagnosis in a non-invasive way.

Microwave Bone Imaging: A Preliminary Investigation on Numerical Bone Phantoms for Bone Health Monitoring

Microwave tomography (MWT) can be used as an alternative modality for monitoring human bone health. Studies have found a significant dielectric contrast between healthy and diseased human trabecular bones. A set of diverse bone phantoms were developed based on single-pole Debye parameters of osteoporotic and osteoarthritis human trabecular bones. The bone phantoms were designed as a two-layered circular structure, where the outer layer mimics the dielectric properties of the cortical bone and the inner layer mimics the dielectric properties of the trabecular bone. The electromagnetic (EM) inverse scattering problem was solved using a distorted Born iterative method (DBIM). A compressed sensing-based linear inversion approach referred to as iterative method with adaptive thresholding for compressed sensing (IMATCS) has been employed for solving the underdetermined set of linear equations at each DBIM iteration. To overcome the challenges posed by the ill-posedness of the EM inverse scattering problem, the L2-based regularization approach was adopted in the amalgamation of the IMATCS approach. The simulation results showed that osteoporotic and osteoarthritis bones can be differentiated based on the reconstructed dielectric properties even for low values of the signal-to-noise ratio. These results show that the adopted approach can be used to monitor bone health based on the reconstructed dielectric properties.

Dielectric characterization of diseased human trabecular bones at microwave frequency

The objective of this study is to determine whether in vitro dielectric properties of human trabecular bones, can distinguish between osteoporotic and osteoarthritis patients' bone samples. Specifically this study enlightens intra-patient variation of trabecular bone microarchitecture and dielectric properties, inter-disease comparison of bone dielectric properties, and finally establishes the correlation to traditional bone histomorphometry parameter (bone volume fraction) for diseased bone tissue. Bone cores were obtained from osteoporotic and osteoarthritis patients (n = 12). These were scanned using microCT to examine bone volume fraction. An open-ended coaxial probe measurement technique was employed to measure dielectric properties over the 0.5 - 8.5 GHz frequency range. The dielectric properties of osteoarthritis patients are significantly higher than osteoporotic patients; with an increase of 41% and 45% for relative permittivity and conductivity respectively. The dielectric properties within each patient vary significantly, variation in relative permittivity and conductivity was found to be greater than 25% and 1.4% respectively. A weak correlation (r  =  0.5) is observed between relative permittivity and bone volume fraction. Osteoporotic and osteoarthritis bones can be differentiated based on difference of dielectric properties. Although these do not correlate strongly to bone volume fraction, it should be noted that bone volume fraction is a poor predictor of fracture risk. The dielectric properties of bones are found to be influenced by mineralization levels of bones. Therefore, dielectric properties of bones may have potential as a diagnostic measure of osteoporosis.

Microwave tomography with phaseless data on the calcaneus by means of artificial neural networks

The aim of this study is to use a multilayer perceptron (MLP) artificial neural network (ANN) for phaseless imaging the human heel (modeled as a bilayer dielectric media: bone and surrounding tissue) and the calcaneus cross-section size and location using a two-dimensional (2D) microwave tomographic array. Computer simulations were performed over 2D dielectric maps inspired by computed tomography (CT) images of human heels for training and testing the MLP. A morphometric analysis was performed to account for the scatterer shape influence on the results. A robustness analysis was also conducted in order to study the MLP performance in noisy conditions. The standard deviations of the relative percentage errors on estimating the dielectric properties of the calcaneus bone were relatively high. Regarding the calcaneus surrounding tissue, the dielectric parameters estimations are better, with relative percentage error standard deviations up to ≈ 15%. The location and size of the calcaneus are always properly estimated with absolute error standard deviations up to ≈ 3 mm. Microwave tomography of the calcaneus using phaseless data. Simulations were inspired in Computed Tomography images from real heels (above). Inverse problem was solved using Multilayer Perceptron Artificial Neural Network (below).

Pulsed high-power microwaves do not impair the functions of skin normal and cancer cells in vitro: A short-term biological evaluation

•

Pulsed high power microwave (MW) at a frequency 3.5 GHz was generated.

•

MW did not induce cell death in skin fibroblast normal cells and melanoma cells.

•

MW did not alter the morphology of melanoma cells.

•

Gene expression related to ATP synthesis and proliferation can get altered by MW.

•

MW selectively stimulated viability and proliferation of only melanoma cells.

Over the past few decades, microwave (MW) radiation has been widely used, and its biological effects have been extensively investigated. However, the effect of MW radiation on human skin biology is not well understood. We study the effects of pulsed high-power microwaves (HPMs) on melanoma (G361 and SK-Mel-31) and normal human dermal fibroblast (NHDF) cells. A pulsed power generator (Chundoong) was used to generate pulsed HPMs (dominant frequency: 3.5 GHz). For treatment 1, 5, 15, and 45 shots are given to cells in which the electromagnetic energy of 0.6 J was delivered to the cells at each trigger shot. Cell viability, proliferation rate, apoptosis, cell death, metabolic activity, and oxygen-free radical regulation were evaluated after the MW exposure at low and high doses. MW exposure increased the viabilities and proliferation rates of both melanoma cell lines in a dose-dependent manner, while no significant effects on the fibroblast cells were observed. We found an elevated level of ATP and mitochondrial activity in melanoma cells. Also, it was observed that MW exposure did not affect cell death in melanoma and fibroblast cells. A polymerase chain reaction analysis indicated that the MWs induced dose-dependent proliferation markers without affecting the cell cycle and apoptotic genes in the melanoma cells. Our findings show the differential effects of the MW radiation on the melanoma cells, compared to those on the fibroblast cells.

Biophysical and biochemical roles of Moringa oleifera leaves as radioprotector

It has been found that medicinal plants have chemical and/or therapeutic effects on different diseases related to oxidative damage. This work investigates the use of ethanolic Moringa oleifera leaves extract; as a protective and/or therapeutic agent against damage induced by high acute dose of ionizing radiation. Also, this study aims to explore the associations of electrical properties (relaxation time and DC conductivity of bone marrow) with biochemical markers (SOD, CAT and GSH) to detect and prognosticate radiation effects. Biophysical and biochemical data revealed that Moringa extract can improve the electrical properties of bone marrow and the antioxidants levels in the blood. They also showed that the feeding of Moringa leaves extract post irradiation is preferred to recover rapidly and continuously from radiation effects.

Two-step inversion with a logarithmic transformation for microwave breast imaging

PURPOSE

The authors have developed a new two-step microwave tomographic image reconstruction process specifically designed to incorporate logarithmic transformed microwave imaging algorithms as a means of significantly improving spatial resolution and target property recovery. Log transform eliminates the need for a priori information, but spatial filtering often integrated as part of the regularization required to stabilize image recovery, generally smooths image features and reduces object definition. The new implementation begins with this smoothed image as the first step, but then utilizes it as the starting estimate for a second step which continues the iterative process with a standard weighted Euclidean distance regularization. The penalty term of the latter restricts the new image to a multi-dimensional location close to the original but allows the algorithm to optimize the image without excessive smoothing.

METHODS

The overall approach is based on a Gauss-Newton iterative scheme which incorporates a log transformation as a way of making the reconstruction more linear. It has been shown to be robust and not require a priori information as a condition for convergence, but does produce somewhat smoothed images as a result of associated regularization. The new two-step process utilizes the previous technique to generate a smoothed initial estimate and then uses the same reconstruction process with a weighted Euclidean distance penalty term. A simple and repeatable method has been implemented to determine the weighting factor without significant computational burden. The reconstructions are assessed according to conventional parameter estimation metrics.

RESULTS

We apply the approach to phantom experiments using large, high contrast canonical shapes followed by a set of images recovered from an actual patient exam. The image improvements are substantial in regards to improved property recovery and feature delineation without inducing unwanted artifacts. Analysis of the residual vector after the reconstruction process further emphasizes that the minimization criterion is efficient with minimal biases.

CONCLUSIONS

The outcome is a novel synergism of an established stable reconstruction algorithm with a conventional regularization technique. It maintains the ability to recover high quality microwave tomographic images without the bias of a priori information while substantially improving image quality. The results are confirmed on both phantom experiments and patient exams.

Including spatial information in nonlinear inversion MR elastography using soft prior regularization

Tissue displacements required for mechanical property reconstruction in magnetic resonance elastography (MRE) are acquired in a magnetic resonance imaging (MRI) scanner, therefore, anatomical information is available from other imaging sequences. Despite its availability, few attempts to incorporate prior spatial information in the MRE reconstruction process have been reported. This paper implements and evaluates soft prior regularization (SPR), through which homogeneity in predefined spatial regions is enforced by a penalty term in a nonlinear inversion strategy. Phantom experiments and simulations show that when predefined regions are spatially accurate, recovered property values are stable for SPR weighting factors spanning several orders of magnitude, whereas inaccurate segmentation results in bias in the reconstructed properties that can be mitigated through proper choice of regularization weighting. The method was evaluated in vivo by estimating viscoelastic mechanical properties of frontal lobe gray and white matter for five repeated scans of a healthy volunteer. Segmentations of each tissue type were generated using automated software, and statistically significant differences between frontal lobe gray and white matter were found for both the storage modulus and loss modulus . Provided homogeneous property assumptions are reasonable, SPR produces accurate quantitative property estimates for tissue structures which are finer than the resolution currently achievable with fully distributed MRE.

Clinical microwave tomographic imaging of the calcaneus: a first-in-human case study of two subjects

We have acquired 2-D and 3-D microwave tomographic images of the calcaneus bones of two patients to assess correlation of the microwave properties with X-ray density measures. The two volunteers were selected because each had one leg immobilized for at least six weeks during recovery from a lower leg injury. A soft-prior regularization technique was incorporated with the microwave imaging to quantitatively assess the bulk dielectric properties within the bone region. Good correlation was observed between both permittivity and conductivity and the computed tomography-derived density measures. These results represent the first clinical examples of microwave images of the calcaneus and some of the first 3-D tomographic images of any anatomical site in the living human.