[Show abstract][Hide abstract] ABSTRACT: A biomedical microwave tomography system with 3D-imaging capabilities has been constructed and
translated to the clinic. Updates to the hardware and reconfiguration of the electronic-network layouts
in a more compartmentalized construct have streamlined system packaging. Upgrades to the data acquisition
and microwave components have increased data-acquisition speeds and improved system
performance. By incorporating analog-to-digital boards that accommodate the linear amplification
and dynamic-range coverage our system requires, a complete set of data (for a fixed array position
at a single frequency) is now acquired in 5.8 s. Replacement of key components (e.g., switches and
power dividers) by devices with improved operational bandwidths has enhanced system response
over a wider frequency range. High-integrity, low-power signals are routinely measured down to
−130 dBm for frequencies ranging from 500 to 2300 MHz. Adequate inter-channel isolation has
been maintained, and a dynamic range >110 dB has been achieved for the full operating frequency
range (500–2900 MHz). For our primary band of interest, the associated measurement deviations are
less than 0.33% and 0.5◦ for signal amplitude and phase values, respectively. A modified monopole
antenna array (composed of two interwoven eight-element sub-arrays), in conjunction with an updated
motion-control system capable of independently moving the sub-arrays to various in-plane
and cross-plane positions within the illumination chamber, has been configured in the new design
for full volumetric data acquisition. Signal-to-noise ratios (SNRs) are more than adequate for all
transmit/receive antenna pairs over the full frequency range and for the variety of in-plane and crossplane
configurations. For proximal receivers, in-plane SNRs greater than 80 dB are observed up to
2900 MHz, while cross-plane SNRs greater than 80 dB are seen for 6 cm sub-array spacing (for frequencies
up to 1500 MHz). We demonstrate accurate recovery of 3D dielectric property distributions
for breast-like phantoms with tumor inclusions utilizing both the in-plane and new cross-plane data.
Review of Scientific Instruments 12/2014; 85(12):124704. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have developed a simple mechanism incorporating feedline bends and rotary joints to enable motion of a monopole antenna within a liquid-based illumination chamber for tomographic imaging. The monopole is particularly well suited for this scenario because of its small size and simplicity. For the application presented here a full set of measurement data is collected from most illumination and receive directions utilizing only a pair of antennas configured with the rotating fixture underneath the imaging tank. Alternatively, the concept can be adapted for feed structures entering the tank from the sides to allow for measurements with vertically and horizontally polarized antennas. This opens the door for more advanced imaging applications where anisotropy could play an important role such as in bone imaging.
International Journal of Antennas and Propagation 06/2014; 2014(431602):8. · 0.68 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Tissue dielectric properties are specific to physiological changes and consequently have been pursued as imaging biomarkers of cancer and other pathological disorders. However, a recent study (Phys Med Biol 52:2637-2656, 2007; Phys Med Biol 52:6093-6115, 2007), which utilized open-ended dielectric probing techniques and a previously established sensing volume, reported that the dielectric property contrast may only be 10% or less between breast cancer and normal fibroglandular tissue whereas earlier data suggested ratios of 4:1 and higher may exist. Questions about the sensing volume of this probe relative to the amount of tissue interrogated raise the distinct possibility that the conclusions drawn from that study may have been over interpreted.
[Show abstract][Hide abstract] ABSTRACT: Purpose: Breast magnetic resonance imaging is highly sensitive but not very specific for the detection of breast cancer. Opportunities exist to supplement the image acquisition with a more specific modality provided the technical challenges of meeting space limitations inside the bore, restricted breast access, and electromagnetic compatibility requirements can be overcome. Magnetic resonance (MR) and microwave tomography (MT) are complementary and synergistic because the high resolution of MR is used to encode spatial priors on breast geometry and internal parenchymal features that have distinct electrical properties (i.e., fat vs fibroglandular tissue) for microwave tomography.Methods: The authors have overcome integration challenges associated with combining MT with MR to produce a new coregistered, multimodality breast imaging platform-magnetic resonance microwave tomography, including: substantial illumination tank size reduction specific to the confined MR bore diameter, minimization of metal content and composition, reduction of metal artifacts in the MR images, and suppression of unwanted MT multipath signals.Results: MR SNR exceeding 40 dB can be obtained. Proper filtering of MR signals reduces MT data degradation allowing MT SNR of 20 dB to be obtained, which is sufficient for image reconstruction. When MR spatial priors are incorporated into the recovery of MT property estimates, the errors between the recovered versus actual dielectric properties approach 5%.Conclusions: The phantom and human subject exams presented here are the first demonstration of combining MT with MR to improve the accuracy of the reconstructed MT images.
Medical Physics 10/2013; 40(10):103101. · 2.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: INTRODUCTION: Microwave tomography recovers images of tissue dielectric properties, which appear to be specific for breast cancer, with low-cost technology that does not present an exposure risk, suggesting the modality may be a good candidate for monitoring neoadjuvant chemotherapy. METHODS: Eight patients undergoing neoadjuvant chemotherapy for locally advanced breast cancer were imaged longitudinally five to eight times during the course of treatment. At the start of therapy, regions of interest (ROIs) were identified from contrast-enhanced magnetic resonance imaging studies. During subsequent microwave examinations, subjects were positioned with their breasts pendant in a coupling fluid and surrounded by an immersed antenna array. Microwave property values were extracted from the ROIs through an automated procedure and statistical analyses were performed to assess short term (30 days) and longer term (four to six months) dielectric property changes. RESULTS: Two patient cases (one complete and one partial response) are presented in detail and demonstrate changes in microwave properties commensurate with the degree of treatment response observed pathologically. Normalized mean conductivity in ROIs from patients with complete pathological responses was significantly different from that of partial responders (P value = 0.004). In addition, the normalized conductivity measure also correlated well with complete pathological response at 30 days (P value = 0.002). CONCLUSIONS: These preliminary findings suggest that both early and late conductivity property changes correlate well with overall treatment response to neoadjuvant therapy in locally advanced breast cancer. This result is consistent with earlier clinical outcomes that lesion conductivity is specific to differentiating breast cancer from benign lesions and normal tissue.
Breast cancer research: BCR 04/2013; 15(2):R35. · 5.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have implemented a soft prior regularization technique for microwave tomographic imaging that exploits spatial prior information from alternative imaging modalities. We have previously demonstrated in both simulation and phantom experiments that this approach is capable of improved property recovery compared with our conventional Tikhonov regularized method. An important concern with this type of integration is that the spatial information could be implemented in such a way as to overly influence or even bias the final results. For this reason, we have performed a rigorous quantitative analysis of this approach using both simulation and phantom experiments to investigate the sensitivity to inaccurate or even false spatial information. The majority of cases tested here involved simple targets to easily assess problems when the prior shapes are incorrect with respect to size and location or even when there is an extra target that does not exist in the actual imaging situation. In addition, we have also performed a simulated experiment utilizing anthropomorphic breast structural information to explore the capabilities in more challenging situations. The results are encouraging and demonstrate that the soft prior regularization can be a powerful tool, especially where the goal is specificity instead of sensitivity.
IEEE Transactions on Microwave Theory and Techniques 03/2013; 61(5):2129-2136. · 2.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This paper presents a full-wave, whole-system modeling of microwave imaging tomography (MWT) systems, to be used as the forward model in reconstruction (inverse) algorithms. The full geometry including antennas and their ports is simulated via a finite element method (FEM) approach. A new technique is used to compute the antenna operation in the system, which provides a general method to enforce the excitation as a specific modal distribution and to extract the voltage and current from the employed antenna. We report results for a complete microwave imaging (MWI) system with comparison between measured and simulated data.
[Show abstract][Hide abstract] ABSTRACT: We have acquired 2D and 3D 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 6 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 CT-derived density measures. These results represent the first clinical examples of microwave images of the calcaneus and some of the first 3D tomographic images of any anatomical site in the living human.
IEEE transactions on bio-medical engineering 07/2012; · 2.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microwave tomographic image quality can be improved significantly with prior knowledge of the breast surface geometry. The authors have developed a novel laser scanning system capable of accurately recovering surface renderings of breast-shaped phantoms immersed within a cylindrical tank of coupling fluid which resides completely external to the tank (and the aqueous environment) and overcomes the challenges associated with the optical distortions caused by refraction from the air, tank wall, and liquid bath interfaces.
The scanner utilizes two laser line generators and a small CCD camera mounted concentrically on a rotating gantry about the microwave imaging tank. Various calibration methods were considered for optimizing the accuracy of the scanner in the presence of the optical distortions including traditional ray tracing and image registration approaches. In this paper, the authors describe the construction and operation of the laser scanner, compare the efficacy of several calibration methods-including analytical ray tracing and piecewise linear, polynomial, locally weighted mean, and thin-plate-spline (TPS) image registrations-and report outcomes from preliminary phantom experiments.
The results show that errors in calibrating camera angles and position prevented analytical ray tracing from achieving submillimeter accuracy in the surface renderings obtained from our scanner configuration. Conversely, calibration by image registration reliably attained mean surface errors of less than 0.5 mm depending on the geometric complexity of the object scanned. While each of the image registration approaches outperformed the ray tracing strategy, the authors found global polynomial methods produced the best compromise between average surface error and scanner robustness.
The laser scanning system provides a fast and accurate method of three dimensional surface capture in the aqueous environment commonly found in microwave breast imaging. Optical distortions imposed by the imaging tank and coupling bath diminished the effectiveness of the ray tracing approach; however, calibration through image registration techniques reliably produced scans of submillimeter accuracy. Tests of the system with breast-shaped phantoms demonstrated the successful implementation of the scanner for the intended application.
Medical Physics 06/2012; 39(6):3102-11. · 2.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microwave breast imaging (using electromagnetic waves of frequencies around 1 GHz) has mostly remained at the research level for the past decade, gaining little clinical acceptance. The major hurdles limiting patient use are both at the hardware level (challenges in collecting accurate and noncorrupted data) and software level (often plagued by unrealistic reconstruction times in the tens of hours). In this paper we report improvements that address both issues. First, the hardware is able to measure signals down to levels compatible with sub-centimeter image resolution while keeping an exam time under 2 min. Second, the software overcomes the enormous time burden and produces similarly accurate images in less than 20 min. The combination of the new hardware and software allows us to produce and report here the first clinical 3-D microwave tomographic images of the breast. Two clinical examples are selected out of 400+ exams conducted at the Dartmouth Hitchcock Medical Center (Lebanon, NH). The first example demonstrates the potential usefulness of our system for breast cancer screening while the second example focuses on therapy monitoring.
IEEE transactions on medical imaging. 05/2012; 31(8):1584-92.
[Show abstract][Hide abstract] ABSTRACT: Microwave imaging for biomedical applications, especially for early
detection of breast cancer and effective treatment monitoring, has
attracted increasing interest in last several decades. This fact is due
to the high contrast between the dielectric properties of the normal and
malignant breast tissues at microwave frequencies. The available range
of dielectric properties for different soft tissue can provide important
functional information about tissue health. Nonetheless, one of the
limiting weaknesses of microwave imaging is that unlike conventional
modalities, such as X-ray CT or MRI, it inherently cannot provide
high-resolution images. The conventional modalities can produce highly
resolved anatomical information but often cannot provide the functional
information required for diagnoses. Previously, we have developed a
regularization strategy that can incorporate prior anatomical
information from MR or other sources and use it in a way to refine the
resolution of the microwave images, while also retaining the functional
nature of the reconstructed property values. In the present work, we
extend the use of prior structural information in microwave imaging from
2D to 3D. This extra dimension adds a significant layer of complexity to
the entire image reconstruction procedure. In this paper, several
challenges with respect to the 3D microwave imaging will be discussed
and the results of a series of 3D simulation and phantom experiments
with prior structural information will be studied.
[Show abstract][Hide abstract] ABSTRACT: Microwave imaging techniques are prone to signal corruption from unwanted multipath signals. Near-field systems are especially vulnerable because signals can scatter and reflect from structural objects within or on the boundary of the imaging zone. These issues are further exacerbated when surface waves are generated with the potential of propagating along the transmitting and receiving antenna feed lines and other low-loss paths. In this paper, we analyze the contributions of multi-path signals arising from surface wave effects. Specifically, experiments were conducted with a near-field microwave imaging array positioned at variable heights from the floor of a coupling fluid tank. Antenna arrays with different feed line lengths in the fluid were also evaluated. The results show that surface waves corrupt the received signals over the longest transmission distances across the measurement array. However, the surface wave effects can be eliminated provided the feed line lengths are sufficiently long independently of the distance of the transmitting/receiving antenna tips from the imaging tank floor. Theoretical predictions confirm the experimental observations.
International Journal of Biomedical Imaging 01/2012; 2012:697253.
[Show abstract][Hide abstract] ABSTRACT: We are developing a microwave imaging system for interrogating the calcaneus bone as a way of monitoring bone health progression in diseases such as osteoporosis. We have previously shown that the dielectric properties of trabecular bone correspond well to the associated bone volume fraction which can be used as a good indicator of fracture risk. We present 3D results showing that the technique is sophisticated enough to recover the pertinent structures and that ultimately this technique will be sufficiently sensitive for disease monitoring.
Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: The complex dielectric properties can be quite unique for different substances. Non-invasive microwave imaging which recovers the associated properties of commercial products could prove useful in numerous settings such as assessing the integrity of solids and testing for spoilage of food stuffs. We have adapted our microwave breast imaging system for testing arbitrarily shaped targets with varying properties. This exploits a new soft prior regularization strategy which only requires spatial geometry information along with the measured field data to reconstruct accurate property measurements of the target under test (TUT). Refining the target interface and optimizing the reconstruction code for this specific application could facilitate near real-time assessment for a range of targets.
Antennas and Propagation Society International Symposium (APSURSI), 2012 IEEE; 01/2012
[Show abstract][Hide abstract] ABSTRACT: We have imaged several breast cancer patients at multiple intervals during her neoadjuvant chemotherapy to assess the capability of microwave tomography as a therapy monitoring device. For the patient discussed here, we illustrate the spectral behavior of our tomographic approach in the context of a complex imaging situation with a large scattering tumor along with less frequently encountered structures such as thickened skin in the tumor vicinity. These results demonstrate that the microwave technology is sensitive to dielectric property perturbations associated with treatment-induced physiological changes. In addition, it also confirms previously hypothesized notions that the lower frequency images provide lower resolution but useful counterparts to the enhanced resolution, higher frequency images. This spectral data can be instructive for both UWB radar approaches and multi-frequency or time-domain tomographic approaches. The chemotherapy patients are unique with respect to breast cancer imaging cases in that they usually involve electrically large tumors along with other non-standard features such as extra-thick skin (easily as thick as 1 cm). These large, high contrast features are quite challenging for all types of microwave imaging (radar and tomography-based) and form an important benchmark for testing imaging techniques. For our efforts, we have developed a log transformation as part of the image reconstruction process which we have shown to have superior convergence behavior (i.e. no local minima) while retaining phase wrapping information that is generally lost when only considering more classical minimization criteria. As we will show, this technique requires broadband scattering data which we provide from measurements using our ultrawideband monopole antennas.
Antennas and Propagation in Wireless Communications (APWC), 2012 IEEE-APS Topical Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: We have developed a new 3D forward solution for use in microwave tomography based on the discrete dipole approximation (DDA). Given that the forward problem at each iteration is the primary computational load, heavy emphasis has been placed on reducing this cost by the research community. Some of the more advanced approaches include techniques such as finite difference time domain; but even these can require multiple hours to complete even using advanced hardware such as GPU's and parallel processors. The DDA is infrequently used in the microwave modeling setting because once metallic scatterers are introduced, the number of dipoles needed increases substantially and negates the computational gain. However, because of our approach's low profile antennas and highly lossy coupling medium, the non-transmitting antennas barely impact propagation allowing for an efficient DDA implementation. The gains using this technique are impressive with full 3D clinical reconstructions taking under 10 minutes using a Matlab code operating on a 2.66GHz Intel Core i7 processor.
[Show abstract][Hide abstract] ABSTRACT: Microwave imaging has shown clinical value based on its ability to detect a wide range of dielectric properties. Tomographic microwave images are traditionally reconstructed using a uniform mesh. Dartmouth College's microwave tomographic imaging system requires that imaged volumes be submerged in the system's coupling medium. Due to the fact that the electrical properties of the coupling medium are known, we have investigated a conformal imaging technique that deploys the reconstruction mesh directly to the imaged zone, such that all reconstruction nodes lie on or within the volume of interest. The broadband nature of the system [.5-3 GHz] allows the use of a frequency-hopping, phase-unwrapping technique, where low frequency phase information is used to guide the phase unwrapping at higher frequencies. In this paper we present the results from a simulation and phantom experiment to verify the use of conformal microwave imaging with our current imaging system.
Ultra-Wideband (ICUWB), 2012 IEEE International Conference on; 01/2012
[Show abstract][Hide abstract] ABSTRACT: A critical need exists for new imaging tools to more accurately characterize bone quality beyond the conventional modalities of dual energy X-ray absorptiometry (DXA), ultrasound speed of sound, and broadband attenuation measurements. In this paper we investigate the microwave dielectric properties of ex vivo trabecular bone with respect to bulk density measures. We exploit a variation in our tomographic imaging system in conjunction with a new soft prior regularization scheme that allows us to accurately recover the dielectric properties of small, regularly shaped and previously spatially defined volumes. We studied six excised porcine bone samples from which we extracted cylindrically shaped trabecular specimens from the femoral heads and carefully demarrowed each preparation. The samples were subsequently treated in an acid bath to incrementally remove volumes of hydroxyapatite, and we tested them with both the microwave measurement system and a micro-CT scanner. The measurements were performed at five density levels for each sample. The results show a strong correlation between both the permittivity and conductivity and bone volume fraction and suggest that microwave imaging may be a good candidate for evaluating overall bone health.
International Journal of Biomedical Imaging 01/2012; 2012:649612.