Ultrasound-guided microwave imaging of breast cancer: Tissue phantom and pilot clinical experiments

Generation of Prior Information in a Dual-Mode Microwave-Ultrasound Breast Imaging System

A new breast imaging system capable of obtaining ultrasound and microwave scattered-field measurements with minimal or no movement of the breast between measurements has recently been reported. In this work, we describe the methodology that has been developed to generate prior information about the internal structures of the breast based on ultrasound data measured with the dual-mode system. This prior information, estimating both the geometry and complex-valued permittivity of tissues within the breast, is incorporated into the microwave inversion algorithm as a means of enhancing image quality. Several techniques to map reconstructed ultrasound speed to complex-valued relative permittivity are investigated. Quantitative images of two simplified dual-mode breast phantoms obtained using experimental data and the various forms of prior information are presented. Though preliminary, the results presented herein provide an understanding of the impacts of different forms of prior information on dual-mode reconstructions of the breast and can be used to inform future work on the subject.

Breast tissue mimicking phantoms for combined ultrasound and microwave imaging

We present a new formulation for a breast tissue-mimicking phantom for combined microwave and ultrasound imaging to assist breast cancer detection. Formulations based on coconut oil, canola oil, agar and glass beads were used to mimic skin and fat tissues. First, 36 recipes were fabricated, and properties were measured to determine the relationship and possible interaction between ingredients with the ultrasound and microwave properties. Based on these results, the formulae were developed to mimic different tissues found in breast, including skin, fat, fibroglandular, and tumour tissues. All phantoms contained a base of agar and glass beads at different proportions depending on the tissue mimicked. Tumour and fibroglandular tissues were best mimicked by adding polyvinylpyrrolidone (PVP), while using coconut oil for skin and canola oil for fat produced the best results. Five final phantoms with different internal structures were fabricated and imaged using B-mode ultrasound and a microwave transmission system. Microwave permittivity maps were obtained from the microwave system and compared to ultrasound images. The structure and composition of the phantoms were all confirmed through this microwave and ultrasound imaging.

Bimodal microwave and ultrasound phantoms for non-invasive clinical imaging

A precise and thorough methodology is presented for the design and fabrication of bimodal phantoms to be used in medical microwave and ultrasound applications. Dielectric and acoustic properties of human soft tissues were simultaneously mimicked. The phantoms were fabricated using polyvinyl alcohol cryogel (PVA-C) as gelling agent at a 10% concentration. Sucrose was employed to control the dielectric properties in the microwave spectrum, whereas cellulose was used as acoustic scatterer for ultrasound. For the dielectric properties at microwaves, a mathematical model was extracted to calculate the complex permittivity of the desired mimicked tissues in the frequency range from 500 MHz to 20 GHz. This model, dependent on frequency and sucrose concentration, was in good agreement with the reference Cole-Cole model. Regarding the acoustic properties, the speed of sound and attenuation coefficient were employed for validation. In both cases, the experimental data were consistent with the corresponding theoretical values for soft tissues. The characterization of these PVA-C phantoms demonstrated a significant performance for simultaneous microwave and ultrasound operation. In conclusion, PVA-C has been validated as gelling agent for the fabrication of complex multimodal phantoms that mimic soft tissues providing a unique tool to be used in a range of clinical applications.

On the Use of Focused Incident Near-Field Beams in Microwave Imaging

We consider the use of focused incident near-field (NF) beams to interrogate the object of interest (OI) in NF microwave imaging (MWI). To this end, we first discuss how focused NF beams can be advantageously utilized to suppress scattering effects from the neighbouring objects whose unknown dielectric properties are not of interest (i.e., undesired scatterers). We then discuss how this approach can also be helpful in reducing the required measured data points to perform imaging. Driven by the relation between the electromagnetic inverse source and inverse scattering problems, our approach emphasizes the importance of tailoring the induced contrast sources in the imaging domain through the utilized incident NF beams. To demonstrate this idea, we consider two recently-proposed NF beams, and simulate them for imaging applications. The first one is a subwavelength focused NF beam generated by a passive NF plate, and the other is a Bessel beam generated by a leaky radial waveguide. Simple imaging examples are considered to explore the potential advantages of this approach, in particular, toward mainly seeing the object of interest, and not the unknown undesired scatterers. The scope of this paper is limited to homogeneous dielectric objects for which the induced total field distributions in the interrogated objects are similar to the incident field distributions (e.g., those that satisfy the Born approximation). Simple inversion results for focused and non-focused beams are presented accompanied by discussions comparing the achieved reconstructed values.

Preliminary Results of a New Auxiliary Mechatronic Near-Field Radar System to 3D Mammography for Early Detection of Breast Cancer

Accurate and early detection of breast cancer is of high importance, as it is directly associated with the patients’ overall well-being during treatment and their chances of survival. Uncertainties in current breast imaging methods can potentially cause two main problems: (1) missing newly formed or small tumors; and (2) false alarms, which could be a source of stress for patients. A recent study at the Massachusetts General Hospital (MGH) indicates that using Digital Breast Tomosynthesis (DBT) can reduce the number of false alarms, when compared to conventional mammography. Despite the image quality enhancement DBT provides, the accurate detection of cancerous masses is still limited by low radiological contrast (about 1%) between the fibro-glandular tissue and affected tissue at X-ray frequencies. In a lower frequency region, at microwave frequencies, the contrast is comparatively higher (about 10%) between the aforementioned tissues; yet, microwave imaging suffers from low spatial resolution. This work reviews conventional X-ray breast imaging and describes the preliminary results of a novel near-field radar imaging mechatronic system (NRIMS) that can be fused with the DBT, in a co-registered fashion, to combine the advantages of both modalities. The NRIMS consists of two antipodal Vivaldi antennas, an XY positioner, and an ethanol container, all of which are particularly designed based on the DBT physical specifications. In this paper, the independent performance of the NRIMS is assessed by (1) imaging a bearing ball immersed in sunflower oil and (2) computing the heat Specific Absorption Rate (SAR) due to the electromagnetic power transmitted into the breast. The preliminary results demonstrate that the system is capable of generating images of the ball. Furthermore, the SAR results show that the system complies with the standards set for human trials. As a result, a configuration based on this design might be suitable for use in realistic clinical applications.

Review of breast screening: Toward clinical realization of microwave imaging

Microwave imaging (MI) technology has come a long way to introduce a noninvasive, inexpensive, fast, convenient, and safe screening tool for clinical breast monitoring. However, there is a niche between the existing understanding of MI by engineers versus clinicians. Our manuscript targets that niche and highlights the state of the art in MI technology compared to the existing breast cancer detection modalities (mammography, ultrasound, molecular imaging, and magnetic resonance). The significance of our review article is in consolidation of up-to-date breast clinician views with the practical needs and engineering challenges of a novel breast screening modality. We summarize breast tissue abnormalities and highlight the benefits as well as potential drawbacks of the MI as a cancer detection methodology. Our goal is to present an article that MI researchers as well as practitioners in the field can use to assess the viability of the MI technology as a competing or complementary modality to the existing means of breast cancer screening.

Preliminary imaging results and SAR analysis of a microwave imaging system for early breast cancer detection

Currently X-ray-based imaging systems suffer from low contrast between malignant and healthy fibrous tissues in breast. Microwave Near-field Radar Imaging (NRI) shows a higher contrast between the aforementioned tissues and therefore can enhance tumor detection and diagnosis accuracy. In this work, we present the first imaging results of our developed NRI system that is equipped with a pair of Antipodal Vivaldi Antennas. We used a metal bearing ball immersed in oil as our object of interest, to keep the first measurement configuration simple. Moreover, to demonstrate the safety of our system for human subject tests, we simulated the Specific Absorption Rate (SAR) in a realistic breast tissue model and compared the resulted values with both the USA and Europe standards. The results show that firstly the imaging results from the measurements and simulations are comparable, and secondly the antennas radiations meet the SAR criteria.

Breast monitoring via time-domain microwave radar: early clinical trial study

This work describes early results from our firststage clinical trial involving the monitoring of healthy volunteers with our time-domain microwave breast screening system. The system is composed of a 16-sensor multistatic array that records the electromagnetic energy scattered off of the breast tissue. All measurements are performed in the timedomain. We present here the system setup, patient-interface considerations, volunteer criteria and initial results from breast monitoring.

3D parallel-detection microwave tomography for clinical breast imaging

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 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.

Surface wave multipath signals in near-field microwave imaging

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.

Thermoacoustic molecular tomography with magnetic nanoparticle contrast agents for targeted tumor detection

PURPOSE

The primary feasibility steps of demonstrating the ability of microwave-induced thermoacoustic (TA) in phantoms have been previously reported. However, none were shown to target a diseased site in living subjects in thermoacoustic tomography (TAT) field so far. To determine the expressions of oncogenic surface molecules, it is quite necessary to image tumor lesions and acquire pathogenic status on them via TAT.

METHODS

Compared to biological tissues, iron oxide nanoparticles have a much higher microwave absorbance. Fe3O4/polyaniline (PANI) nanoparticles were prepared via polymerization of aniline in the Fe304 superparamagnetic fluids. Then Fe3O4/PANI was conjugated to folic acid (FA), which can bind specifically to the surface of the folate receptor used as a tumor marker. FA-Fe3O4/PANI targeted tumor was irradiated by pulsed microwave at 6 GHz for thermoacoustic detection and imaging.

RESULTS

The effect of the Fe3O4/PANI superparamagnetic nanoparticles for enhancing TAT images was successfully investigated in ex vivo human blood and in vivo mouse tail. Intravenous administration of the targeted nanoparticles to mice bearing tumors showed fivefold greater thermoacoustic signal and much longer elimination time than that of mice injected with nontargeted nanoparticles in the tumor. The specific targeting ability of FA-Fe3O4/PANI to tumor was also verified on fluorescence microscopy.

CONCLUSIONS

Fabricated iron oxide nanoparticles conjugated with tumor ligands for targeted TAT tumor detection at the molecular level was reported for the first time. The results indicate that thermoacoustic molecular imaging with functionalized iron oxide nanoparticles may contribute to targeted and functional early cancer imaging. Also, the modified iron oxide nanoparticles combined with suitable tumor markers may also be used as novel nanomaterials for targeted and guided cancer thermal therapy.

In vivo detection and imaging of low-density foreign body with microwave-induced thermoacoustic tomography

Radiography or computed tomography is the most widely available imaging tool for foreign body detection. However, the detectability of low-density substances by x ray is very poor when located in soft tissues. Various dielectric loss factors of foreign bodies contribute great microwave absorption heterogeneity compared with the surrounding tissue. A fast thermoacoustic tomography system at 1.2 and 6 GHz was developed to detect foreign targets in small animals. The 6 GHz system had a much higher signal-to-noise ratio in near-surface imaging but smaller imaging depth than the 1.2 GHz system. The effects of microwave distribution inhomogeneity on nonuniform excitation of acoustic pressure were studied and a corresponding calibration algorithm for image distortion was provided and experimentally examined. Thermoacoustic images of radiolucent objects including glass fiber, wood, and bamboo hidden in phantom and residual in living mice were compared with radiography and ultrasonography. Good contrast was obtained between the foreign bodies and the tissue surrounding it, and the location and size of the lesion targets in thermoacoustic images were in good agreement with the actual sample. The experimental results demonstrate that thermoacoustic tomography may become the ideal modality for radiolucent foreign body detection and imaging in animals and human.

Anniversary paper: evolution of ultrasound physics and the role of medical physicists and the AAPM and its journal in that evolution

Ultrasound has been the greatest imaging modality worldwide for many years by equipment purchase value and by number of machines and examinations. It is becoming increasingly the front end imaging modality; serving often as an extension of the physician's fingers. We believe that at the other extreme, high-end systems will continue to compete with all other imaging modalities in imaging departments to be the method of choice for various applications, particularly where safety and cost are paramount. Therapeutic ultrasound, in addition to the physiotherapy practiced for many decades, is just coming into its own as a major tool in the long progression to less invasive interventional treatment. The physics of medical ultrasound has evolved over many fronts throughout its history. For this reason, a topical review, rather than a primarily chronological one is presented. A brief review of medical ultrasound imaging and therapy is presented, with an emphasis on the contributions of medical physicists, the American Association of Physicists in Medicine (AAPM) and its publications, particularly its journal Medical Physics. The AAPM and Medical Physics have contributed substantially to training of physicists and engineers, medical practitioners, technologists, and the public.

Development of anatomically realistic numerical breast phantoms with accurate dielectric properties for modeling microwave interactions with the human breast

Computational electromagnetics models of microwave interactions with the human breast serve as an invaluable tool for exploring the feasibility of new technologies and improving design concepts related to microwave breast cancer detection and treatment. In this paper, we report the development of a collection of anatomically realistic 3-D numerical breast phantoms of varying shape, size, and radiographic density which can readily be used in finite-difference time-domain computational electromagnetics models. The phantoms are derived from T1-weighted MRIs of prone patients. Each MRI is transformed into a uniform grid of dielectric properties using several steps. First, the structure of each phantom is identified by applying image processing techniques to the MRI. Next, the voxel intensities of the MRI are converted to frequency-dependent and tissue-dependent dielectric properties of normal breast tissues via a piecewise-linear map. The dielectric properties of normal breast tissue are taken from the recently completed large-scale experimental study of normal breast tissue dielectric properties conducted by the Universities of Wisconsin and Calgary. The comprehensive collection of numerical phantoms is made available to the scientific community through an online repository.

Characterization of an Implicitly Resistively-Loaded Monopole Antenna in Lossy Liquid Media

Microwave tomographic imaging of the breast for cancer detection is a topic of considerable interest because of the potential to exploit the apparent high-dielectric property contrast between normal and malignant tissue. An important component in the realization of an imaging system is the antenna array to be used for signal transmission/detection. The monopole antenna has proven to be useful in our implementation because it can be easily and accurately modeled and can be positioned in close proximity to the imaging target with high-element density when configured in an imaging array. Its frequency response is broadened considerably when radiating in the liquid medium that is used to couple the signals into the breast making it suitable for broadband spectral imaging. However, at higher frequencies, the beam patterns steer further away from the desired horizontal plane and can cause unwanted multipath contributions when located in close proximity to the liquid/air interface. In this paper, we have studied the behavior of these antennas and devised strategies for their effective use at higher frequencies, especially when positioned near the surface of the coupling fluid which is used. The results show that frequencies in excess of 2 GHz can be used when the antenna centers are located as close as 2 cm from the liquid surface.