Multivariate analysis of energy dispersive X-ray diffraction data for the detection of illicit drugs in border control
© The Author(s) 2017
Received: 10 February 2016
Accepted: 25 December 2016
Published: 19 January 2017
A system using energy dispersive X-ray diffraction has been tested to detect the presence of illicit drugs concealed within parcels typical of those which are imported into the UK via postal and courier services. The system was used to record diffraction data from calibration samples of diamorphine (heroin) and common cutting agents and a partial least squares regression model was established between diamorphine concentration and diffraction spectra. Parcels containing various crystalline and amorphous materials, including diamorphine, were then scanned to obtain multiple localised diffraction spectra and to form a hyperspectral image. The calibration model was used for the prediction of diamorphine concentration throughout the volume of parcels and enabled the presence and location of diamorphine to be determined from the visual inspection of concentration maps. This research demonstrates for the first time the potential of an EDXRD system to generate continuous hyperspectral images of real parcels from volume scanning in security applications and introduces the opportunity to explore hyperspectral image analysis in chemical and material identification. However, more work must be done to make the system ready for implementation in border control operations by bringing down the procedure time to operational requirements and by proving the system’s portability.
A major threat to UK security in recent years has been the importation of illicit drugs via routes such as the postal system and courier services (Coleman 2011; Dhani 2014). The UK Border Force is responsible for ensuring that imported parcels are free from illegal items such as illicit drugs, firearms, explosives and dangerous chemicals before they can be allowed into the country. Manual searching of parcels deemed suspicious on external visual inspection may be time consuming, subject to significant error, and risks damage to the handled parcels. There is therefore a motivation for the development of an automated and non-destructive method to investigate the contents of parcels before they are selected for manual examination (Drakos 2015). The need for the development of efficient and relocatable scanners has been expressed by both the European Commission (Magnusson 2003; Lipoti 2003; Rothschild 2003) and by previous research in the field (Cook et al. 2007, 2009; Griffiths 2008; Koutalonis 2009; Pani 2009). In the interests of efficiency and to minimise disruption through false-positive results, the UK Border Force requires that the duration of initial screening does not exceed 5 min per parcel and be capable of detecting illicit drugs at a minimum threshold of 40% drug purity.
Cocaine and diamorphine are both class A drugs, classified according to the Misuse of Drugs Act 1971 (MDA), and are the most commonly seized drugs in the UK with approximately 17,000 and 8500 seizures in 2013/2014, respectively (Dhani 2014). Due to the limited availability of cocaine during the experimental phase of the research, this study focuses on the detection of diamorphine.
Energy dispersive X-ray diffraction (EDXRD) is a non-destructive technology which has previously been employed in laboratory settings to scan the contents of letters, parcels, boxes, suitcases and palletised goods to identify illicit drugs and explosive materials (Luggar 1995, 1996a, b; Strecker 1993; Speller 1996, 2001). A portable prototype for use in border control, which operates at room temperature, has been developed during the experimental phase to scan large volumes containing amorphous and crystalline materials (Drakos 2015). The objective of this research is to test the system for its feasibility in detecting materials of interest using EDXRD in the context of fast-parcel screening. The study predicts the presence of seized diamorphine, which had subsequently been concealed in parcels typical of those encountered at border control. The research introduces continuous hyperspectral imaging of parcel cross sections and employs the well-established partial least squares regression method on the resulting hypercubes to predict illicit drug concentration locally through the parcel volume. The results of this study motivate further research using spectroscopic image analysis and chemometric methods to advance the exploration and prediction methodology from hyperspectral diffraction images in security applications (Amigo et al. 2015). The research presented here is a proof of concept of full-volume parcel screening and future work is required to bring down the current 30 min procedure time to meet the time constraints set by UK Border Force. To this end, data acquisition times may be reduced by increasing the X-ray flux of the system, or through image analysis performed on lower resolution diffraction images.
Varying the angle of detection using a monochromatic X-ray source—a method known as angular dispersive X-ray diffraction (ADXRD);
Using a polychromatic X-ray source and detecting scatter at a fixed angle—a method known as energy dispersive X-ray diffraction (EDXRD).
In EDXRD, a collimated energy-resolving detector at a fixed angle is used to measure forward scatter events from a polychromatic source of X-rays, as shown in Fig. 1. Diffraction from molecular planes alters the incident spectrum forming a diffraction profile of the materials as measured by the detector. Focusing on low angle (3°–7°) forward scatter, where photons are scattered coherently, produces an intensity distribution containing information on both the chemical composition and the crystallinity (Cook 2008). Furthermore, the scatter fields correspond to a well-defined volume within the material, which enables volume imaging, improves localisation and has the potential to increase the detection rate by increasing the specificity of a system (Luggar and Gilboy 1999).
A tungsten target (W) X-ray source tube (Monoblock®, Spellman High Voltage Electronics Corporation) with maximum output potential settings of 140 kV and 5 mA was used for all the experiments. The source had an inherent filtration of 0.8 mm Be and 1.5 mm glass. Additional filtration of 12–15 mm (max) oil (Shell Diala A), 0.41 mm Al and 0.04 mm Cu was included on the X-ray tube casing and the source had a nominal focal spot size of 2.5 × 2.5 mm. The scattered photons were detected at a nominal angle of 5° using twenty off-the-shelf CdZnTe detectors with crystal dimensions of 5 × 5 × 5 mm (SPEAR, eV Products) and energy resolution of 6% at 59.6 keV (Drakos 2015). Twenty detectors were used in order to interrogate 20 discrete scatter volumes of 10 × 10 × 10 mm within the parcels simultaneously. High voltage gain amplifiers and multichannel analysers (ORTEC®) were used to retrieve the measured diffraction profile by binning the voltage pulses to one of 512 channels in a format (Maestro, ORTEC®) that could be read by both The Unscrambler (v.9.5, CAMO) and MATLAB® (v.2014b, MathWorks) software packages.
The incident beam was collimated by a purpose-built 10 mm thick lead structure operating in two modes: (i) transmission mode with dimensions 2 mm wide x 108 mm tall; and (ii) diffraction mode with 20 slits (corresponding to the 20 individual CdZnTe detectors) each with dimensions of 1 mm wide x 2.12 mm tall and configured as depicted in Fig. 1 (Drakos 2015). The use of a primary collimator ensures that the incident X-ray beam is collimated to a narrow beam. The collimation of the scattered photons was provided by a set of 20 soller-slit collimators of 17 mm width, 17 mm height and 55 mm depth placed at a nominal scattering angle of 5° from the scattering centre. These collimators could be interchanged such that the slits were either perpendicular or parallel to the X-ray beam field. Lastly, a line-scan X-ray detector (C9750-10FC, Hamamatsu) was chosen to capture transmission images of interrogated objects.
The concentrations of diamorphine and cutting agents in the calibration samples
Diamorphine concentration (%)
Results and discussion
Three parcels were scanned and analysed to test the system with their contents given in Table 2. The parcel contents were chosen to test the capability of the system to identify regions containing diamorphine (Parcel 1), to disregard regions containing crystalline materials other than diamorphine (Parcel 2) and to be able to identify diamorphine in parcels containing the drug in several locations and with reflective surfaces (Parcel 3).
The concentration map of Parcel 1 (Fig. 6a) identifies the sample of diamorphine at 73% purity which can be seen in the transmission image in the lower central area of the image. Decreased transmission intensity is also observable in the lower right region of the image corresponding to a book. The concentration map correctly identifies the diamorphine sample and disregards the book and all other materials within the parcel leading to a true positive result.
The contents and results of the three analysed parcels to test for diamorphine
Book, textiles, lead gloves and sample of 73% diamorphine and unknown cutting agents
Textiles and sodium bicarbonate
Sample of 69% diamorphine placed in textile samples, sample of 69% diamorphine stuffed within soft toys, and three DVDs
Parcel 3 (Fig. 6c) contains samples of diamorphine at 69% purity found in two locations. A large sample was wrapped in textiles and can be observed in the central right region of the transmission image. The concentration map identifies diamorphine correctly within this region. A smaller sample of diamorphine concealed within a soft toy is observable in the top left of the transmission image, above the DVD cases. A strong prediction of diamorphine concentration is determined at this location. However, the position of the sample coincides with the malfunctioning detectors, resulting in false positive results in the same rows to the right of the concentration map. The three DVDs observable in the lower left of the tranmission image are not mistakenly identified as diamorphine. The volume elements corresponding to the malfunctioning detectors notwithstanding, this parcel has a true positive result for identifying diamorphine.
The multivariate analysis technique of partial least squares regression has been demonstrated to be capable of identifying the illicit drug diamorphine in parcels containing a variety of crystalline and amorphous materials using the portable EDXRD system described previously (Drakos 2015). In the three parcels investigated the presence or absence of diamorphine could be accurately identified from visual inspection of concentration maps. To fully characterise and assess the sensitivity and specificity of the system the process is to be expanded to 75 parcels. In order to accurately identify diamorphine from parcel diffraction spectra the volume elements were scanned for 28 s leading to a total scanning time per parcel in excess of 30 min. To satisfy the UK Border Force requirements of a maximum scanning time of 5 min, improvements to the system design, signal processing or calibration model are required and will be the focus of future work. Recent methodology developed for the analysis of hyperspectral images will be explored to motivate improvements in drug detection performance (Amigo 2015).
This project was funded by the Engineering and Physical Sciences Research Council. Acknowledgements should be given to the Home Office Centre for Applied Science and Technology who prepared the samples used in the investigation.
ID set up the portable EDXRD system and took the measurements. ID and PK analysed the data. PK drafted the manuscript. RS is the principal investigator for the project. TF consulted on the statistical analysis. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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