[Show abstract][Hide abstract] ABSTRACT: Microwave imaging (MI) for early breast cancer diagnosis is emerging as a potential alternative and/or complementary to conventional modalities such as MRI. This is mainly due to the fact that MI is able to exploit significant contrast between the dielectric properties, namely the permittivity and conductivity, of malignant and normal breast tissue. Despite providing such valuable functional information, MI inherently suffers from sub-centimeter resolution. In order to overcome this shortcoming, we have previously developed an image reconstruction technique called soft prior (SP) regularization, which incorporates prior anatomical information from high-resolution imaging modalities, such as MRI, into our original MI reconstruction algorithm. This technique has been tested rigorously in 2D, and has been extended to 3D with promising preliminary results. Since the incorporated spatial information could be implemented in such a way as to overly effect or even bias the reconstructed images, this work presents a set of comprehensive experiments to assess the sensitivity and robustness of the algorithm.
37th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Milan, Italy; 08/2015
[Show abstract][Hide abstract] ABSTRACT: In many antenna array applications, the beam pattern is defined as a product of the individual element pattern and the array factor generated by the spacing, orientation and relative phasing and apodization of the elements. In this paper we explore the interaction between the two beam patterns of associated antennas in a circular tomographic imaging configuration and demonstrate that the effective zones of sensitivity are relatively narrow cigar-shaped swathes between the transmit and receive antennas in spite of the isotropic beam patterns of the individual monopole antennas. In this case, the rows of the Jacobian matrix used in the image reconstruction process provide the maps of the sensitivity within the propagation zone which become the bi-static array factor. This analysis further informs antenna design and the overall impact on signal-to-noise considerations.
[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.
(SNRs) are more than adequate for all transmit/receive antenna pairs over the full frequency range and for the variety of in-plane and cross-plane 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.
[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. DOI:10.1155/2014/431602 · 0.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background
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.
We performed open-ended dielectric probe measurements in two-layer compositions consisting of a background liquid and a planar piece of Teflon that was translated to predetermined distances away from the probe tip to assess the degree to which the probe produced property estimates representative of the compositional averages of the dielectric properties of the two materials resident within a small sensing volume around the tip of the probe.
When Teflon was in contact with the probe, the measured properties were essentially those of pure Teflon whereas the properties were nearly identical to those of the intervening liquid when the Teflon was located more than 2 mm from the probe tip. However, when the Teflon was moved closer to the probe tip, the dielectric property measurements were not linearly related to the compositional fraction of the two materials, but reflected nearly 50% of those of the intervening liquid at separation distances as small as 0.2 mm, and approximately 90% of the liquid when the Teflon was located 0.5 mm from the probe tip.
These results suggest that the measurement methods reported in the most recent breast tissue dielectric property study are not likely to return the compositional averages of the breast tissue specimens evaluated, and thus, the conclusions reached about the expected dielectric property contrast in breast cancer from this specimen study may not be correct.
BMC Medical Physics 06/2014; 14(3). DOI:10.1186/1756-6649-14-3
[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.
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.
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%.
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. DOI:10.1118/1.4820361 · 2.64 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.
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.
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).
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. DOI:10.1186/bcr3418 · 5.49 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. DOI:10.1109/TMTT.2013.2247413 · 2.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Approximately 1 in 8 women will develop breast cancer in their lifetime. Estimates suggest 230,500 new cases of invasive breast cancer in 2011, resulting in approximately 40,000 deaths. Traditional screening technologies, such as X-ray mammography use ionizing radiation and suffer from high false-positive and false-negative rates. Due to the high contrast that exists between the dielectric properties of normal and abnormal breast tissue, microwave-imaging spectroscopy has proven an attractive breast cancer imaging modality. We have shown that the incorporation of a volume's internal structural information into our image reconstruction algorithm can increase the accuracy of recovered dielectric properties. Additionally, image reconstruction has benefited from the use of a custom reconstruction mesh generated from the imaged volume's perimeter boundary. This information is used in a conformal microwave image (CMI) reconstruction process, and has increased the accuracy of recovered high contrast regions within the volume's perimeter without the use of prior internal spatial information. In simulation and phantom experiments with regular geometries, boundary information is obtained through spatial measurements. For irregularly shaped boundaries, alternative means are necessary for accurate boundary extraction. In this paper we demonstrate the MR-guided CMI reconstruction process for an irregularly shaped boundary; boundary information extracted from MR images will be used to generate a custom boundary-derived mesh for microwave image reconstruction. Results from images reconstructed using the MR-guided CMI reconstruction process will be compared with uniformly reconstructed images, highlighting the increased accuracy of high contrast features within the volume without the use of prior internal spatial information.
Proceedings of SPIE - The International Society for Optical Engineering 02/2013; DOI:10.1117/12.2007939 · 0.20 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 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: 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.
[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. DOI:10.1118/1.4711799 · 2.64 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Microwave approaches to breast imaging include the measurement of signals transmitted through and reflected from the breast. Prototype systems typically feature sensors separated from the breast, resulting in measurements that include the effects of ...
International Journal of Biomedical Imaging 05/2012; 2012:252093. DOI:10.1155/2012/252093
[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.
[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 04/2012; 2012:649612. DOI:10.1155/2012/649612
[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 04/2012; 2012:697253. DOI:10.1155/2012/697253