Passive millimeter wave (mmW) imagers have improved in terms of resolution sensitivity and
frame rate. Currently, the Office of Naval Research (ONR), along with the US Army Research,
Development and Engineering Command, Communications Electronics Research Development
and Engineering Center (RDECOM CERDEC) Night Vision and Electronic Sensor Directorate
(NVESD), are investigating the current state-of-the-art of mmW imaging systems. The focus of
this study was the performance of mmW imaging systems for the task of small watercraft / boat
identification field performance. First mmW signatures were collected. This consisted of a set of
eight small watercrafts; at 5 different aspects, during the daylight hours over a 48 hour period in
the spring of 2008. Target characteristics were measured and characteristic dimension, signatures,
and Root Sum Squared of Target's Temperature (RRSΔT) tabulated. Then an eight-alternative,
forced choice (8AFC) human perception experiment was developed and conducted at NVESD.
The ability of observers to discriminate between small watercraft was quantified. Next, the task
difficulty criterion, V50, was quantified by applying this data to NVESD's target acquisition
models using the Targeting Task Performance (TTP) metric. These parameters can be used to
evaluate sensor field performance for Anti-Terrorism / Force Protection (AT/FP) and navigation
tasks for the U.S. Navy, as well as for design and evaluation of imaging passive mmW sensors for
both the U.S. Navy and U.S. Coast Guard.
It is well documented that photoacoustic imaging has the capability to differentiate tissue based on the spectral
characteristics of tissue in the optical regime. The imaging depth in tissue exceeds standard optical imaging techniques,
and systems can be designed to achieve excellent spatial resolution. A natural extension of imaging the intrinsic optical
contrast of tissue is to demonstrate the ability of photoacoustic imaging to detect contrast agents based on optically
absorbing dyes that exhibit well defined absorption peaks in the infrared. The ultimate goal of this project is to
implement molecular imaging, in which Herceptin<sup>TM</sup>, a monoclonal antibody that is used as a therapeutic agent in breast
cancer patients that over express the HER2 gene, is labeled with an IR absorbing dye, and the resulting in vivo bio-distribution
is mapped using multi-spectral, infrared stimulation and subsequent photoacoustic detection. To lay the
groundwork for this goal and establish system sensitivity, images were collected in tissue mimicking phantoms to
determine maximum detection depth and minimum detectable concentration of Indocyanine Green (ICG), a common IR
absorbing dye, for a single angle photoacoustic acquisition. A breast mimicking phantom was constructed and spectra
were also collected for hemoglobin and methanol. An imaging schema was developed that made it possible to separate
the ICG from the other tissue mimicking components in a multiple component phantom. We present the results of these
experiments and define the path forward for the detection of dye labeled Herceptin<sup>TM</sup> in cell cultures and mice models.
There are over 2 million reported burn injuries each year in the United States with 75,000 of these incidents resulting in
hospitalization. Current medical imaging modalities have limited capabilities to assess initial burn damage and monitor
healing progress. Some of these limitations can be attributed to modality occlusion from bandages, dried tissue and/or
blood and inflammation. Since terahertz radiation can see through textiles and bandages<sup>1</sup>, previous studies<sup>2,3</sup> suggested
that terahertz radiation, in a reflectance configuration, could be used for non-invasive analysis of tissue thermal damage
and healing status. In this study, we perform an analysis of the terahertz absorption and reflection properties of the
tissue constituents comprising a wound area, and provide a feasibility assessment of the capabilities of terahertz imaging
to provide a clinical tool for initial burn analysis and healing progress.
We have developed a first generation of electro-optic polymer modulators, designed specifically for passive millimeter-wave
detection. The advantages of utilizing electro-optic polymers for modulator fabrication are their economical and
simple fabrication, potential for large scale array fabrication, and well matched RF and optical indices, which provide
the potential for an excellent high-frequency response. The current drawbacks of these devices include long term device
stability due to oxidation and the relative immaturity of the RF designs for the modulator and interconnects, which lead
to unacceptable internal losses and low sensitivity. These are both items we expect remedied in the upcoming year. We
provide a brief overview on the opto-electronic method of detecting millimeter waves and our design and fabrication of
the polymer modulator. Current measured results for the modulator response at 95GHz are presented and an analysis of
the required performance for imaging is presented.
We have developed a mm wave/terahertz imaging simulation package from COTS graphic software and custom
MATLAB code. In this scheme, a commercial ray-tracing package was used to simulate the emission and reflections of
radiation from scenes incorporating highly realistic imagery. Accurate material properties were assigned to objects in the
scenes, with values obtained from the literature, and from our own terahertz spectroscopy measurements. The images
were then post-processed with custom Matlab code to include the blur introduced by the imaging system and noise levels
arising from system electronics and detector noise. The Matlab code was also used to simulate the effect of fog, an
important aspect for mm wave imaging systems. Several types of image scenes were evaluated, including bar targets,
contrast detail targets, a person in a portal screening situation, and a sailboat on the open ocean. The images produced by
this simulation are currently being used as guidance for a 94 GHz passive mm wave imaging system, but have broad
applicability for frequencies extending into the terahertz region.
We have utilized a prototype Thermoacoustic Computed Tomography Small Animal Imaging System to acquire images of athymic mice with bilateral tumors implanted in the cranial mammary fat pads. The breast tumor cell lines used in the study, which are MCF7, and MCF7 transfected with Vascular Endothelial Growth Factor (VEGF), exhibit distinctly contrasting levels of vascularization. Three dimensional images of the mice, acquired using pulses of NIR stimulating light, demonstrate the ability of the system to generate high resolution images of the vascular system up to one inch deep in tissue, and at the same time, differentiate tissue types based on the infrared absorption properties of the tissue; a property related in part to blood content and oxygenation levels. We have processed images acquired at different stimulating wavelengths to generate images representative of the distribution of oxygenated and deoxygenated hemoglobin throughout the tumors. The images demonstrate the in vivo capabilities of the imaging system and map system structure as well as the total, oxygenated and deoxygenated hemoglobin components of the blood.
We have completed the design and testing of a thermoacoustic computed tomography scanner for whole-breast imaging. We report on the technical changes in this design form our previous TCT scanner, and how these design changes have improved image quality. Improvements to the design include: greater angular coverage of TCT measurements, increased sensitivity of the ultrasound detector array, and improved delivery of radio wave energy. These improvements resulted in higher fidelity 3D reconstructions, reduced scan time, and fewer image artifacts. These improvements were documented by imaging simple, 3D phantoms, formulated from salinated agar spheres. We confirmed improvements in breast image quality by comparing images of patient volunteers taken with our previous TCT scanner and this new TCT scanner.
In order to assess the potential clinical utility of using thermoacoustic computer tomography (TCT) to image the breast, we conducted a retrospective pilot study of 78 patients. We recruited patients in three age groups (<40,40-50,>50 years). The study population was further segregated into normal and suspicious based on the results of the previous x-ray mammography and ultrasound. Image quality was evaluated qualitatively by consensus of two trained mammographers using a 4-point scale. The appearance of normal anatomy, cysts, benign disease and cancer was noted. Patients were also asked to rate the comfort of the TCT exam and to indicate a personal preference for x-ray mammography or TCT. Analysis of the data indicated that TCT image quality was dependent upon both patient age and breast density, improving with both increasing breast density and decreasing patient age. Fibrocystic disease was well seen, cysts appearing as areas of low RF absorption. Fibroadenomas did not demonstrate contrast enhancement with the exception of one patient with associated atypical hyperplasia. Cancer displayed higher RF absorption than surrounding tissues in 4/7 patients in whom cancer was confirmed, including one patient with a 7-mm ductal carcinoma in situ (DCIS).
We have developed instrumentation for measuring the tissue- absorption properties of radio waves in the human body using thermoacoustic interactions. The imaging principles upon which this instrumentation is based are applicable to other irradiation sources, such as visible and IR. We present the imaging reconstruction methodology that we have developed for mapping radiation absorption pattern sin 3D. Both simulated and experimental data are used to illustrate imaging principles.
We have previously developed instrumentation for performing thermoacoustic computed tomography (TCT) of the human breast using 434 MHz radio waves. Recently, we have modified our original TCT scanner design in a number of important ways. We have increased the number of ultrasound detectors and decreased their size, and we have replaced our single RF wave- guide with a phased array of eight wave-guides. These modifications have led to increased spatial resolution, increased imaging field of view, and decreased scan time. Here we report the design considerations that led to these improvements.
Numerical simulations are a valuable tool in the development of complex systems. They provide the ability to determine the effects of individual parameters on system functionality, and in the case of electronic systems, the ability to examine the system without the limitations introduced by electronic noise. The Thermoacoustic Computed Tomography (TACT) system under development was a natural candidate for numerical analysis. Early versions of the system exhibited exceptional promise, but final image quality was limited by a variety of confounding geometrical and electronic factors. The simulations described in this paper were used to generate the transducer signals that would theoretically be collected by the actual TACT imaging system when a sample was exposed to a pulse of electromagnetic radiation. The simulated data streams were then fed into the actual image reconstruction software to provide images of the 'virtual' phantoms. These images were analyzed and quantified to provide a measure of the system parameters responsible for the image blurs that limit system spatial resolution.
Acoustic pressure waves are induced in soft tissue whenever time-varying radiation is absorbed. By recording these time- dependent pressure waves over a sufficient number of angles surrounding the tissue being imaged, it is possible to reconstruct the pattern of radiation absorption within the tissue in three dimensions with spatial resolution that is independent of the carrier frequency of the irradiating energy. We recently constructed the world's first thermoacoustic computed tomography (TACT) scanner, which exploits this physical interaction. Initial in vivo imaging of a human breast was performed using safe levels of 434 MHz radiation. Good soft tissue differentiation with 2 - 5 mm spatial resolution to a depth of 40 mm was achieved. The absorption properties of the breast and the irradiation pattern within the breast determined the TACT image contrast. The length of the RF pulse, the size of the transducers and their frequency response, the geometry of the detector array, and the reconstruction algorithm that was used determined the spatial resolution. We conclude that TACT imaging may have application to breast cancer detection.