Millimeter wave imaging system for land mine detection

Improved-resolution millimeter-wave imaging through structured illumination

A resolution-improved active millimeter-wave (MMW) imaging structure, based on the theory of structured illumination, is proposed in this paper. The structured illumination is a well-defined concept for surpassing the diffraction limit in optical microscopy, where imposing grating patterns on the targeted object could help in moving the object frequency spectrum along the desired direction in the spatial frequency domain. This frequency shift helps in passing different parts of the object's frequency spectrum through the diffraction filter. The combination of resultant images provides a framework to pass a wider frequency band of the object, thereby achieving super-resolution. This idea has not yet been employed for MMW image resolution improvement due to practical limitations in producing the desired grating patterns. Therefore, a desired fringe pattern is produced here and tailored for a MMW imaging system through antenna array synthesis. In the proposed scheme, the structured illumination has been implemented for improving the MMW image resolution. Furthermore, an adaptive approach has been proposed in order to generalize the structure for resolution improvement in all required directions in a very fast manner. Electromagnetic simulation results show at most twofold improvement in the image resolution through the proposed MMW imaging structure.

Range resolved mode mixing in a large volume for the mitigation of speckle and strategic target orientation requirements in active millimeter-wave imaging

In spite of many reports of active millimeter-wave imaging in the literature, speckle and requirements for cooperative target orientation significantly reduce its practical usefulness. Here we report a new technique, range resolved mode mixing (RRMM), which significantly mitigates both of these issues. It also provides a three-dimensional (3D) image. RRMM accomplishes this by combining multimode illumination (which eliminates the requirement for cooperative target orientation) with range resolution (which provides statistical independence of speckle patterns for averaging and the 3D image). The use of a 5W extended interaction klystron amplifier results in large signal margins in the 50 m scale atrium of the Physics Department at Ohio State University. It appears that there are a number of scenarios out to a range of 1 km for which this approach is useful to provide 3D images, with minimal speckle, and no requirement for cooperative target orientation.

Structured covariance principal component analysis for real-time onsite feature extraction and dimensionality reduction in hyperspectral imaging

Presented in a three-dimensional structure called a hypercube, hyperspectral imaging suffers from a large volume of data and high computational cost for data analysis. To overcome such drawbacks, principal component analysis (PCA) has been widely applied for feature extraction and dimensionality reduction. However, a severe bottleneck is how to compute the PCA covariance matrix efficiently and avoid computational difficulties, especially when the spatial dimension of the hypercube is large. In this paper, structured covariance PCA (SC-PCA) is proposed for fast computation of the covariance matrix. In line with how spectral data is acquired in either the push-broom or tunable filter method, different implementation schemes of SC-PCA are presented. As the proposed SC-PCA can determine the covariance matrix from partial covariance matrices in parallel even without prior deduction of the mean vector, it facilitates real-time data analysis while the hypercube is acquired. This has significantly reduced the scale of required memory and also allows efficient onsite feature extraction and data reduction to benefit subsequent tasks in coding and compression, transmission, and analytics of hyperspectral data.

Remote detection of buried land-mines and IEDs using LWIR polarimetric imaging

We report results of an ongoing study designed to assess the ability for enhanced detection of recently buried land-mines and/or improvised explosive devices (IED) devices using passive long-wave infrared (LWIR) polarimetric imaging. Polarimetric results are presented for a series of field tests conducted at various locations and soil types. Well-calibrated Stokes images, S0, S1, S2, and the degree-of-linear-polarization (DoLP) are recorded for different line-of-sight (LOS) slant paths at varying distances. Results span a three-year time period in which three different LWIR polarimetric camera systems are used. All three polarimetric imaging platforms used a spinning-achromatic-retarder (SAR) design capable of achieving high polarimetric frame rates and good radiometric throughput without the loss of spatial resolution inherent in other optical designs. Receiver-operating-characteristic (ROC) analysis and a standardized contrast parameter are used to compare detectability between conventional LWIR thermal and polarimetric imagery. Results suggest improved detectability, regardless of geographic location or soil type.

Automatic image segmentation for concealed object detection using the expectation-maximization algorithm

We address an image segmentation method to detect concealed objects captured by passive millimeter wave (MMW) imaging. Passive MMW imaging can create interpretable imagery on the objects concealed under clothing, which gives the great advantage to the security system. In this paper, we propose the multi-level expectation maximization (EM) method to separate the concealed objects from the other area in the image. We apply the EM method to obtain a Gaussian mixture model (GMM) of the acquired image. In the experiments, we evaluate the performance by the average probability of error. We will show that the consecutive EM processes separates the object area more accurately than the conventional EM method.

Terahertz spectroscopy techniques for explosives detection

Spectroscopy in the terahertz frequency range has demonstrated unique identification of both pure and military-grade explosives. There is significant potential for wide applications of the technology for nondestructive and nonintrusive detection of explosives and related devices. Terahertz radiation can penetrate most dielectrics, such as clothing materials, plastics, and cardboard. This allows both screening of personnel and through-container screening. We review the capabilities of the technology to detect and identify explosives and highlight some of the critical works in this area.

Intraoperative bile duct visualization using near-infrared hyperspectral video imaging

BACKGROUND

Current methodologies for imaging the biliary system during cholecystectomy are cumbersome and do not eliminate the risk of bile duct injury. We describe an approach to intraoperative biliary imaging that will enable surgeons to see through the hepatoduodenal ligament and visualize the anteriorly placed biliary system.

METHODS

A laparoscopic-capable, near-infrared, hyperspectral imaging system was built. Reflected light passes through a liquid crystal filter that is continuously tunable in the near-infrared spectrum (650-1,100 nm). Spectroscopic image data are collected from laparoscopic surgery images onto array detectors formatted into a 3-dimensional hyperspectral data cube having spatially resolved images in the x-y plane and wavelength data in the z plane. Deconvoluting and color-coding the spatial and spectral information provides an image representative of inherent chemical properties to the imaged tissue.

RESULTS

Images of porcine biliary structures were obtained. The common duct-reflected spectra displayed a characteristic lipid shoulder at 930 nm and a strong water peak at 970 nm. Venous structures had absorption peaks at 760 nm (deoxyhemoglobin), 800 nm (oxyhemoglobin), and 970 nm (water). Arterial vessels had absorption peaks at 800 nm and 970 nm that would be expected for oxyhemoglobin and water.

CONCLUSIONS

We have designed and constructed a device to significantly enhance intraoperative biliary imaging. This system should enable surgeons to see through the hepatoduodenal ligament and image the anteriorly placed biliary system without the need for dissection of the cystic duct, as is needed with intraoperative cholangiography. Because the biliary system can be seen before any dissection is performed, this dimensional imaging technology has the potential for eradicating bile duct injury.