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Detection of explosives using image analysis Dr Charles A Bouman, Eri Haneda, Aarthi Balachander, Krithika Chandrasekar, Govind Manian, Charvaka Mattaparthy, Aziza Satkhozhina

School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana

Overview Flowchart of Algorithm Results (cont.)

References

Contact information

 

Danger DetectorSchool of Electrical and Computer EngineeringPurdue University West Lafayette, Indiana

All questions or correspondence related to this document should be addressed to Dr Charles A Bouman – bouman@purdue.eduMs Eri Haneda – ehaneda@purdue.eduAarthi Balachander – abalacha@purdue.eduKrithika Chandrasekar – kchandra@purdue.eduGovind Manian – vmanian@purdue.eduCharvaka Mattaparthy – cmattapa@purdue.eduAziza Satkhozhina - asatkhoz@purdue.edu

1Committee on the Review of Existing and Potential Standoff Explosives Detection Techniques, National Research Council, (2004). Existing and Potential Standoff Explosives Detection Techniques. National Academies Press. EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX-Rays/ucm115318.htm 2FDA. (2009, June 18). What is Computed tomography?. Retrieved from http://www.fda.gov/Radiation- Herman, G.T. (2009). 3Hu, Y., Huang, P., Guo, L., Wang, X., & Zhang, C. (2006). Terahertz spectroscopic investigations of explosives. Physics Letters A, 359, 728-32.4Singh, S., & Singh, M. (2003). Explosives detection systems (EDS) for aviation security. Signal Processing, 83, 31-55. 5UTCT. (2009). About High-resolution x-ray ct. Retrieved from http://www.ctlab.geo.utexas.edu/overview/index.php6Computed tomography for airport security. (2010.). http://www.analogic.com/about-us-overview.htm

COMPUTED TOMOGRAPHY• Airports use computed tomography (CT) to detect thin objects such as sheet explosives.

• CT scanners are commonly used for non-invasive medical imaging and have 3 steps

1. The object is passed through a radiation source and detector and 2D images of the object are taken2.Assembles a 3D reconstruction from 2D images about a single axis of rotation.2

3.The images are reconstructed using a computer-based process.6

• The images are analyzed using Fourier Slice Theorem

EXPLOSIVES • Thresholding, binarization, and image segmentation are involved in explosive detection.

• An explosive is a substance that contains a great amount of stored energy that can produce an explosion, or sudden expansion of the material

• Airport security is threatened by explosives called high explosives (HE),1

• Explosives are classified by sensitivity, velocity, composition and other properties.4

• Two key properties are used to detect explosives 1. Geometry: the presence of metallic detonator and associated

wires can be detected using image shape analysis.4 2. Elemental composition and material density: the explosive consists of oxidant and reductant.

• A large percentage of nitrogen and oxygen can be a sign of an explosive device

• Currently, optical density and effective atomic numbers are used to detect explosives. 1

• Explosives have higher optical densities than non-explosive material s

Review of Background

Input: CT slices of scanned baggage

Step 1: Otsu Thresholding (OUTPUT: image with two distinct classes of pixels)

Step 2: Connected Component analysis to group together pixel regions of interest (OUTPUT: connected regions of pixels for each CT slice)

Step 3: Region Growing across multiple slices to obtain the 3D object

Step 4: Feature vector determination for K-means clustering

Step 5: K means clustering to determine regions of change in linear attenuation co-efficient

Step 6: Comparison with attenuation co-efficient of known explosives

Fig 2 Flow Diagram of Algorithm

Fig 3. CT Slice 20 of Pan Am data set (Input)

Fig 4. CT Slice after applying Otsu’s Method (Step 1 of algorithm)

ResultsFig 5. Pixel intensity histogram of slice 20 (graphical verification of Otsu threshold). The threshold value found to minimize intra-class variance for this slice is 0.1255.

Output: Location of explosive in baggage

Recent security threats in airports have resulted in the need for sophisticated explosive detection techniques. This project aims to write an algorithm based on image analysis techniques to detect explosives in baggage and eliminate false alarms in screening equipment at airports. The algorithm will focus on analyzing differences in density distribution across the 3D volume of objects assembled using region growing.

Fig 1. 3D scanned image of baggage6

• The algorithm takes CT slices of screened baggage, as its input.

• The image slices are individually converted to grayscale and Otsu thresholding is performed on them.

• Connected component analysis is performed on the slices to find regions of connected pixels.

• The slices are assembled to obtain the 3D object using region growing.

• Individual objects are compared using feature vector analysis to check if they have a uniform density distribution.

• Significant changes in density distribution are detected.

• K-means clustering is performed on the object.

• A match to known explosives is found by comparing the attenuation co-efficient.

Methods and Approach

Fig 1. Absorption of explosives3