Detecting contraband by employing interactive multiprobe tomography

An inspection system for detecting a specific material of interest in items of baggage or packages includes a multi-view X-ray inspection probe and one or more material sensitive probes. The multi-view X-ray inspection probe employs X-ray radiation transmitted through or scattered from an examined item to identify a suspicious region inside the item. An interface is used to receive from said X-ray inspection probe X-ray data providing spatial information about the suspicious region and to provide this information to a selected material sensitive probe. The material sensitive probe then acquires material specific information about the previously-identified suspicious region and provides it to a computer. The computer uses a high level detection algorithm to identify presence of the specific material in the suspicious region. The material sensitive probe may be a directional probe such as a coherent scatter probe, or a non-directional probe such as a Compton scatter probe or an NQR probe. The detection algorithm can automatically employ the different probes operating in a preferred geometry. The inspection system may also include a graphical interface and an operator interface that enable interactive communication with the detection algorithm.

The present invention relates to interactive, flexible geometry, multiprobe 
inspection systems for examination of packages or baggage. 
BACKGROUND OF THE INVENTION 
Over the past several years, X-ray baggage inspection systems have evolved 
from simple x-ray imaging systems that were completely dependent on 
interpretation by an operator to more sophisticated automatic systems that 
can automatically recognize certain types of contraband. The more 
sophisticated inspection systems have employed single energy or dual 
energy X-ray radiation transmitted through or scattered from the examined 
baggage. Some systems have used a single view source detector arrangement, 
others have utilized a dual view or multi-view arrangements. The single or 
dual view systems usually scan baggage, as it moves on a conveyor, using a 
fan beam or a scanning pencil beam of X-rays in a fixed geometry. The 
multiview, Computed Tomography (CT) type systems usually scan stationary 
baggage in a fixed geometry of scan angles and process data corresponding 
to absorption of X-rays to reconstruct selected slices of the baggage. 
At airports, the baggage inspection procedure is divided into at least 
three levels of inspection. A `level 1` system processes baggage rapidly, 
ideally at a rate of up to 1500 bags per hour. This system is located at a 
first inspection station and inspects all baggage; as such, it represents 
the first line of defense. The system rapidly scans baggage and 
automatically makes a decision based on its particular modes of detection 
and methodology. This methodology may be object based, e.g., mass density, 
effective atomic number (Z.sub.eff), or Compton X-ray scatter, or it may 
be bag-based, such as Nuclear Quadrupole Resonance (NQR), ion mass 
spectroscopy, vapor chemi-luminescence, or other techniques. Such systems 
are able, to some degree, to rapidly eliminate "clean" and non-suspicious 
baggage from the stream of passenger baggage and have been proven 
effective in detecting real threats. The number of bags that cannot be 
cleared at level 1 can range from 10% to 50% of the total number of bags. 
The clearing efficiency depends on the particular detection methodology 
and threat thresholds used in the system. 
The rejected (i.e., non-cleared) bags are automatically sent to a `level 2` 
area. In the `level 2` area, an operator usually visually inspects an 
x-ray image of the rejected bag and attempts to determine whether a 
suspicious object inside the bag can be cleared based on its obvious 
shape. The operator searches the image for characteristic objects such as 
weapons, timing and detonation devices, wires, or other characteristics 
associated with contraband. The operator at this station can clear most, 
but not all of the rejected bags. The remaining baggage, which is usually 
0.1% to 0.5% of the initial stream, is then sent on to a `level 3` 
inspection station which is usually a slower inspection device that uses a 
different technology. 
Vapor or trace detectors (also called `sniffers`) and CT scanners have been 
used as `level 3` devices. A vapor or trace detector does not employ a 
penetrating type of radiation, but looks for traces of characteristic 
materials. It was suggested that, by careful placement inside a bag, a 
relatively large amount of explosives can pass undetected by the trace 
device. On the other hand, a CT scanner, which of course employs 
penetrating X-ray radiation, is usually successful in identifying 
explosives inside a bag especially when present in a large amount. The CT 
scanner basically measures a single parameter, that is, the mass density 
of the examined object. The CT scanner can be set up to communicate with 
the `level 1` device in order to interrogate a specific object identified 
within the baggage by that device. If a `level 1` X-ray scanner identifies 
threats on the basis of effective atomic number (Z.sub.eff) the additional 
density information from the CT scanner can significantly reduce the false 
alarm rate, but may not be able to eliminate it completely. However, CT 
scanners can be very expensive considering the low utilization at a `level 
3` station. 
On average, `level 3` devices clear less than half of the inspected 
objects, yielding 0.05% to 0.25% of the baggage to be sent to `level 4`. 
`Level 4` is defined as reconciliation with the owner. The reconciliation 
is often difficult, if not impossible. When the reconciliation is not 
possible the bag is confiscated, which causes a complaint from an unhappy 
passenger. 
While the above system can perform successfully, there is still a need for 
a relatively low cost X-ray inspection device that can reliably detect 
various explosives (or other contraband) having different shapes and being 
located anywhere in the examined baggage. Such a device should have a high 
confidence and a low false alarm rate. 
SUMMARY OF THE INVENTION 
In general, the inspection system detects different types of contraband 
(e.g., explosives, drugs, money) in items of baggage or packages by one or 
more material sensitive probes deployed on an arm or scanner that is 
positioned mechanically to examine the item of luggage with many different 
and programmable views including radial and axial views. The inspection 
system includes an X-ray probe that has an x-ray source and a detection 
system with one or more detector arrays. The x-ray source and the detector 
arrays are constructed to move together in a coordinated fashion. Thus the 
X-ray probe can take CT type data and can also reconstruct cross-sectional 
and mass density information. The X-ray probe also includes programmable 
collimation and filtering to generate filtered pencil beams for Rayleigh 
and Compton scattering measurements. Dual energy x-ray projection data can 
be taken by setting one of several fan beam collimators and scanning along 
a linear axis. The detection system includes an array of dual energy 
transmission detectors and Rayleigh scattering detectors with programmable 
collimation. The detection system may also include arrays of Compton 
scatter detectors mounted on the scanning head and under the table holding 
the inspected item. 
The inspection system includes several different modes of detection 
operating in conjunction with an expert system of computer software and 
detection algorithms to effect a very high confidence x-ray scanner for 
explosives detection in luggage. The inspection system may also include a 
graphical interface and an operator interface that enable interactive 
communication with the detection algorithm. 
In general, an X-ray inspection method of detecting a specific material of 
interest in items of baggage or packages includes employing X-ray 
radiation transmitted through or scattered from a examined item to obtain 
multi-view spatial information about the examined item, identifying from 
the spatial information a suspicious region inside the item, employing a 
material sensitive probe to acquire material specific information about 
the suspicious region, and identifying, based on computer-processing, 
presence of the specific material in the suspicious region. 
The material sensitive probe may utilize a Compton scattered X-ray 
radiation, a Raman spectrum, an infrared spectrum, the nuclear quadrupole 
resonance effect, a wave of microwave energy modified by dielectric 
material property, or reflected millimeter wave (microwave) field. The 
material sensitive probe may have directional properties such as a 
coherent scatter probe. 
X-ray radiation is employed by exposing the examined item at multiple 
locations to a fan beam of X-rays, detecting X-ray radiation transmitted 
through or scattered from the examined item, and processing the detected 
X-ray data to identify the suspicious region. The detection is performed 
by detecting X-ray radiation transmitted through, back-scattered or 
forward-scattered from the examined item. 
The introduced X-ray radiation may be a fan beam or a pencil beam generated 
at least one energy. 
The detection of coherently scattered X-rays may be performed by employing 
a position sensitive or an energy sensitive X-ray detector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an X-ray inspection system 10 includes a level one 
x-ray inspection device 12, such as VIVID Rapid Detection System 
(available from Vivid Technologies, Inc., Waltham, Mass.), which examines 
items of baggage being transported on a conveyor 16. When device 12 
examines an item of baggage 14A and evaluates the item as free of regions 
that could contain contraband, the item is automatically directed by a 
baggage pusher 18 to proceed further on conveyor 20. If device 12 detects 
possible presence of contraband, pusher 18 directs baggage 14B to conveyor 
22 that transports the baggage to an X-ray inspection device 30, such as a 
modified version of a QDR 4500 scanner (available from Hologic, Inc., 
Waltham, Mass.). An operator located at a remote location 32 can oversee 
the entire inspection process, evaluate data detected and processed by 
inspection device 12 and direct operation of inspection device 30. 
Referring to FIGS. 2 and 3, X-ray inspection device 30, enclosed in 
shielding 28 (FIG. 1), includes several material sensitive probes. These 
probes are located on a C-arm 40, which utilizes displacement subsystems 
42 and 44 to move linearly along or to rotate relative to a moving 
platform 34 and a frame 35. A first probe utilizes polychromatic X-ray 
radiation to scan baggage 14B at several angles to locate suspicious 
objects or regions. Based on multi-view X-ray transmission data, the first 
probe creates images of article surrounding the suspicious objects or 
regions and also identifies the best examination geometry for a second 
stage probe. 
The system uses several different probes with directional or 
non-directional properties in the second stage of examination. A preferred 
directional probe employs the X-ray diffraction technique using the 
geometry provided by the first probe. Other second stage probes utilize 
Raman spectroscopy, infrared spectroscopy, NQR, or microwave imaging as 
will be described below. 
The first, multiview, polychromatic X-ray probe includes an x-ray source 
subsystem 46, an X-ray detector subsystem 50 and the corresponding 
electronics shown diagrammatically FIG. 4A. X-ray source subsystem 46 has 
an X-Ray tube, a collimator and an X-ray controller 48 that includes an 
X-Ray tube filament transformer, a high voltage rectifier circuit, and 
sensing circuits that monitor the high voltage applied to the x-ray tube 
and the beam current. The X-ray controller, which can operate both in 
pulsed mode or in a continuous mode, triggers the X-ray source (e.g., a 
tungsten X-ray tube) that can produce a beam of X-rays at high and low 
energy bands. (A suitable X-ray controller is disclosed in U.S. Pat. No. 
5,319,547, which is incorporated by reference.) The emitted X-ray 
radiation is collimated by the collimator to create a fan beam of X-ray 
radiation 47. 
X-ray detector subsystem 50 includes 72 scintillating CdWO.sub.4 crystals, 
each optically coupled to one photodiode. Each photodiode detects light 
emitted the corresponding crystal and converts the light into current, 
which is amplified in a current-to-voltage converter. Output from the 
amplifier is applied to an integrator through analog switches. The analog 
switches, which operate in parallel, are turned on during the X-ray pulse 
and turned off during the integrator hold time to prevent integrating 
noise into the data. The control signal that turns these switches on and 
off is supplied from an integrator/multiplexor board 114. 
Integrator/multiplexor board 114 sends the analog signals to an 
analog-to-digital (ADC) board 112 for conversion to a digital format. The 
analog signals from integrator/multiplexor board 114 are applied to 
differential amplifiers on ADC board 112 in four groups of up to 64 
channels. Outputs from the differential amplifiers are combined in a final 
multiplexor consisting of four analog switches. The multiplexed signals 
pass through a programmable gain amplifier and summing amplifier before 
being applied to the A/D converter. The A/D converter converts the analog 
signal into 16 bit parallel data for processing by a digital signal 
processor, e.g., Motorola 56000. The digital signal processor generates 
all the control signals necessary for the detector array assembly. This 
processor also provides a high speed serial data link to computer 70. 
Also referring to FIG. 4, inspection device 30 also includes a computer 70, 
which controls and commands the hardware. Computer 70 is connected to a 
distribution board 80, which provides interconnections between an 
operator's console 33, C-arm interface 110 (FIGS. 4A, 4B or 4C), control 
panel controller 84 and displacement subsystem 42 and 44. Control panel 
controller 84 interfaces a control panel 86 to operator's console 33. 
Control panel 86 provides switches including visual indicators for moving 
C-arm 40 and platform 34. Linear displacement subsystem 42 includes TX and 
TY stepper motors and a rotational subsystem 44 includes C-arm rotation 
motor 98 and a C-arm carriage motor 102. TX stepper motor driver 88 
controls the motion of platform 34 using a driver motor 90 based on 
commands from computer 70. A position encoder associated with motor 90 
provides position monitoring. TY stepper motor 94 and associated driver 92 
provides the motion of platform 34 in the direction of baggage movement 
and its position is tracked by an associated position encoder. An AR 
stepper motor driver 96 controls the rotation of C-arm 40, which is moved 
by a C-arm rotation motor 98 and its position controlled with an 
associated position and coder. A C-arm 40 carriage motor 102 moves C-arm 
40 linearly and an associated position encoder provides position 
monitoring. The left or right movement of C-arm 40 is controlled by an AY 
stepper motor driver 100. A platform motor raises or lowers platform 34 
and the corresponding position encoder provides position monitoring. The 
motion of motor 106 is controlled by a DZ drive motor controller 104. 
Device 30 is interfaced to a ISA Bus computer to control table and C-arm 
movement and X-ray generation, perform all necessary calculations, and 
manage baggage and database information. The system uses a computer with a 
pentium processor, a 16 Mb of memory and a super VGA video card. 
Referring to FIG. 5, the second probe, which is a coherent (Rayleigh) x-ray 
scatter probe 140 is also located on C-arm 40. Coherent x-ray scatter 
probe 140 includes a polychromatic X-ray source 142 emitting X-ray 
radiation collimated by a pencil beam collimator 144. X-ray source 142 
shares the tungsten X-ray tube with X-ray source 142 of the first probe. 
(Alternatively, X-ray source 142 uses a separate X-ray tube.) Collimator 
144 is located on and controlled by aperture motor and sensor 118 (FIG. 
4A). Pencil beam 146 is filtered by an erbium filter 148 or a thulium Ross 
filter 150 before it irradiates the examined object (155). Filtered X-ray 
beam 152 is scattered by the examined object and the characteristic 
radiation (158) is detected by a coherent scatter X-ray detection 
subsystem 160. Detection subsystem 160 includes a collimator 162 limiting 
the view angle of the X-ray detector. The X-ray detector includes a 
position sensitive PMT 166, which detects spatial location of optical 
radiation emitted from a NaI scintillating crystal 164. The detection 
geometry was selected to substantially avoid interaction of other 
obscuring, objects (154a, 154b) with the probe beam. 
This probe uses a technique utilizing as a Ross filter, also called a 
balanced filter, and was described by B. D. Cullity in "Elements of X-Ray 
Diffraction" Addison-Wesley Publishing Company, Inc., 1978. Since the 
filters (148 and 150) are made of two different substances differing in 
atomic number by one, then they have the same relative attenuation at 
every wavelength except for the narrow band of wavelengths between the K 
edges of the two substances. When the diffraction pattern from a substance 
is measured first with one filter and then with the other filter, the 
difference between the two measured diffraction patterns yields the 
diffraction pattern corresponding to a nearly monochromatic beam with an 
energy at the K edge. Advantageously, this measurement does not require 
determination of the photon energy of the coherently scattered X-rays. The 
employed filters have K edges respectively just below (57.43 Kev) and just 
above (59.32 Kev) the characteristic K.sub..alpha. of tungsten (58.87 
Kev). Thus, their band pass region coincides precisely with the strongest 
emission band of tungsten. The detailed balancing of attenuation between 
the two filters is accomplished by a physical balancing of the relative 
thickness of the two filters or by the detection software. 
In another embodiment, the coherent x-ray scatter probe 140 measures 
accurately the energy spectrum of the scattered photons at several 
different locations. The probe measures the entire energy spectrum or only 
selected energies corresponding to selected materials. It utilizes 
different types of coherent x-ray scatter measurements described in the 
prior art (see (1) Strecker, H., Harding, G., Bomsdorf, H., Kanzenbach, 
J., Linde, R., Martens, G. "Detection of explosives in airport baggage 
using coherent X-ray scatter." SPIE vol.2092 Substance Detection Systems 
399-410 1993; (2) Luggar, R. D., Gilboy, W. B., MacCuaig, N., "Industrial 
potential of Rayleigh scattered X-rays for identification of low-Z 
materials", SPIE Vol. 2092 Substance Detection Systems, pp. 378-386, 1993; 
(3) Speller, R. D., Horrock, J. A., Lacey, R., "X-Ray scattering 
signatures for material identification" SPIE vol. 2092 Substance Detection 
Systems, pp. 366-377, 1993; and (4) Luggar, R. D., Horrocks, J. A., 
Speller, R. D., Royle, G. J., Lacey, R. "Optimization of a low angle x-ray 
scatter system for explosive detection" all of which are incorporated by 
reference). 
FIG. 4B shows diagrammatically an interface for both coherent (Rayleigh) 
scatter probe 140 and a Compton scatter probe 275. A switchable 
filter/collimator subsystem includes a filter/collimator motor and sensor 
262 controlled by a filter/collimator driver 260. The filter/collimator 
subsystem selectively position K-edge filtration and a foil collimation 
system into X-ray beam 146 to enable optimum detection of a selected 
material. The scattered radiation is detected by detection subsystem 160 
that includes position sensitive detector (i.e., scintillating crystal 164 
and PMT 166 of FIG. 5) with the corresponding electronics 270 and a 
position/height discriminator 264. Rayleigh detection subsystem 160 is 
capable of discriminating the position (268) and energy (266) of each 
interacting event. The position and energy data are sent to a computer via 
C-aram interface 110. 
Compton scatter probe 275 includes a array of PMT detectors and the 
corresponding electronics (272) mounted underneath table 34. The scatter 
array, constructed as described in the above-referenced patent 
application, is moved to a desired position by motor 280 controlled by 
driver 282. The scatter array programmably scans along the axial axis of 
the examined item to reconstruct a 2-dimensional map of back-scattered 
X-ray radiation. Forward-scattered X-ray radiation is detected by an 
additional forward-scatter array, or the Rayleigh detection subsystem is 
set to detect Compton scatter. 
FIG. 6 shows diagrammatically the operation of inspection device 30. First, 
the multiview, polychroimatic X-ray probe acquires projection image data 
(absorption and scatter X-ray data H and L, and transformed data B and I) 
in a similar way as is done by a `level 1` X-ray scanner (200). For 
example, see U.S. patent application Ser. No. 08/533,646, filed Sep. 25, 
1995, entitled "DETECTING EXPLOSIVES OR OTHER CONTRABAND BY EMPLOYING 
TRANSMITTED AND SCATTERED X-RAY" which is incorporated by reference as if 
fully set forth herein). By employing the detection algorithms described 
in the above-cited application, the polychromatic X-ray probe detects 
objects within the Z.sub.eff window that could contain a threat material 
(202) and also runs sheet algorithms (204) to identify sheets of a threat 
material. Then the probe identifies the coordinates of the individual 
objects and performs a recognition on each object (206). Objects that are 
identified as free of any threat (or contraband) are cleared and the 
remaining objects are further examined depending on their shape (208). 
The probe further examines the bulk objects that were rejected (i.e., 
uncleared) by taking selected radial views (210). The algorithm forms a 
reconstruction image and locates the objects in the image. Then the 
algorithm measures the density of these objects (212). The threat decision 
is automatically made based on a selected density threshold (214). Objects 
outside of this threshold are cleared (216) and the others (218) need 
further processing by a second probe, such as the coherent scatter probe 
(236). This bulk algorithm is faster than a standard CT algorithm since 
the probe takes only a limited number of the radial views of objects that 
could not be cleared by the `level 1` examination (steps 200 to 208). The 
cleared objects are no longer examined. 
The probe also examines the identified sheet objects by taking high 
resolution side views of these objects (220). Usually, the sheet objects 
are found in the walls of a examined suitcase or bag. Then the algorithm 
applies a selected threat routine (230) to clear objects free of 
explosives (or other investigated contraband). Alternatively, the X-ray 
probe can here employ only the Compton Scatter technique to examine the 
walls of a examined suitcase, as is described in the above-referenced 
patent application. 
As described above, the second probe employs coherent (Rayleigh) X-ray 
scatter to further inspect the objects that could not be cleared (236). 
The coherent scatter probe receives coordinates of suspicious objects and 
other obscuring objects and identifies the best geometry for examination. 
The probe accumulates X-ray data at different positions and of different 
energies characteristic of the irradiated material. The probe makes a 
threat decision by comparing the acquired data to a database of explosives 
(or other contraband) (238). If there is an object that could not be 
cleared, another probe such as a Raman probe, an infrared probe, a 
microwave probe, or an NQR probe may be employed (240). 
Referring to FIG. 4C, Raman probe 300 may be employed instead of coherent 
scatter probe 140 to probe surfaces of the examined items, which could not 
be cleared by the coherent scatter probe. Raman probe 300 has a laser 
source subsystem 305, an optical detector subsystem and a processor. Laser 
source subsystem 305 includes a laser, a scan motor 304 with an associated 
scan driver 302, and a focus motor 308 with an associated focus driver 
306. Laser source subsystem 305 can dynamically modify the focus relative 
to the surface of an irregular item. The reflected/scattered optical 
radiation is directed to a Raman detector 305, which includes a PMT. A 
programmable set of optical filters are positioned in front of the PMT by 
a filter motor 318 controlled by a driver 316. The optical filters for the 
appropriate wavelengths are used for detection of specific explosives (or 
other contraband). Alternatively, the reflected/scattered radiation is 
directed to several parallel sets of optical detectors, each having a 
different substance specific filter; this arrangement can detect 
simultaneously multiple substances. Raman probe 300 utilizes different 
types of optical measurements described in the prior art (see (1) D. N. 
Batchelder, C. Cheng, I. P. Hayward, R. J. Lacey, G. D. Pitt and T. G. 
Sheldon, "Raman Microscopy and Direct 2-D Imaging of Explosives and 
Drugs", Contraband and Cargo Inspection technology International Symposium 
pp. 73-75. Office of National Drug Control Policy, Conference Proceedings 
Washington, D.C. 1992. K. Carleton, P. Nebolsine, S. Davis, J. Lakovits 
and R. Van Duyne, "Detection of Narcotics and Explosives by 
Surface-Enhanced Raman Spectroscopy," Contraband and cargo Inspection 
Technology International Symposium pp. 401-407. Office of National Drug 
Control Policy, Conference Proceedings Washington, D.C. 1992. 
Alternative Embodiments 
Alternatively, an infrared probe can be employed instead of the Raman 
probe. The infrared probe includes a IR laser source subsystem, an optical 
detector subsystem and a processor. An advantage of the infrared probe is 
a deeper penetration of the IR laser beam than the visible laser beam of 
the Raman probe. A suitable infrared measurement was described by D. O. 
Henderson and E. Silberman in "Fourier-Transform Infrared Spectroscopy 
Applied To Explosive Vapor Detection", Proceedings of the First 
International Symposium on Explosive Detection Technology, pp. 604-617, 
Nov. 13-15, 1991. 
Alternatively, a dielectrometer probe can be employed. The dielectrometer 
probe uses a microwave antenna (described in U.S. Pat. Nos. 4,234,844 and 
4,318,108) that excites a standing (constant or pulsed) wave of microwave 
energy in a fixed field of configuration directed to the suspicious 
objects. The microwave energy penetrates non-metallic objects and produces 
a volumetric reflection coefficient. Based on the reflection coefficient, 
a dielectric constant and loss tangent which are material characteristics. 
This type of dielectrometric measurements was described by D. C. Steward 
and T. Yukl in "Explosive Detection Using Dielectrometry, " Proceedings on 
the First International Symposium on Explosive Detection Technology, S. M. 
Khan, Ed., Nov. 13-15, 1991; and in the publications cited therein. 
Alternatively, a millimeter wave (microwave) probe can be employed, instead 
of the coherent scatter probe, to create a two dimensional scanned image 
of the suspicious object. The probe uses a millimeter wave (10 GHz to few 
hundred GHz depending on the desired penetration) imaging system that 
gathers amplitude and phase data of reflected waves to reconstruct the 
wavefronts and create an image. This type of detection is described in 
detail by D. M. Sheen, D. L. McMakin, H. D. Collins, and T. E. Hall in 
"Near-field Millimeter Wave Imaging for weapon Detection" SPIE Vol. 1824, 
p. 223, 1992; and in the references cited therein. 
Alternatively, the system employs an X-ray probe using an algorithm which 
interactively focuses inspection dependent on information gleaned from 
previous views and the bag topology. The algorithm involves selectively 
`looking around` the bag under inspection to measure certain material 
parameters such as Z.sub.eff with much more accuracy. To achieve higher 
accuracy, the system selectively sets the beam angle to avoid clutter 
detected in previous views, and integrates the data for longer time 
periods to minimize statistical noise. The system `looks around` by 
processing the projection image to select slice planes that have the least 
amount of clutter, as measured with a `clutter` algorithm that is 
sensitive to rapid material property variations over a small area. These 
slice planes are then scanned to find a closely placed pair of pencil beam 
coordinates, which see different amounts of threat materials. The pencil 
beam coordinates are in close proximity and relatively uncluttered and 
thus can be used for a higher precision measurement of Z.sub.eff of the 
selected object. 
The system inspects certain suspicious areas with much higher resolution 
and also places material-specific filters selectively in front of the 
source or detectors. In this type of a `directed search`, the results of 
previous scans point to the next area and mode of inspection. For example, 
in one routine, if a projection scan detects that a part of the lining 
looks abnormally thick with respect to the rest of the examined bag (or as 
compared to a stored set of bag models), the machine specifically targets 
this area. The geometry of the subsequent scan is chosen to maximize the 
cross-sectional attenuation; this greatly improves measurement of 
Z.sub.eff and estimated mass and density. Density is then calculated by 
measuring the areal density in the cross-sectional view divided by the 
length of the thickened portion wherein the length data are measured 
obliquely by a perpendicular scan. By concentrating the X-ray views on the 
targeted object as opposed to uniformly distributed angular views around 
the entire inspection region; the algorithm is more accurate or faster 
than a general CT-type scan which performs a standard cross-sectional 
reconstruction. 
Another algorithm interactively searches for specific modalities based on 
shape, density, or Zef.sub.f. The algorithm directs a sequence of material 
property scans with appropriately selected spatial resolutions and 
positioning to enable a flexible and targeted material property sensitive 
inspection. Each additional scan adds information and modifies parameters 
for subsequent scans for effecting a rapid convergence to a decision on a 
particular object. This process is performed iteratively. The algorithm 
can also divide spatial information of a larger bag into several regions 
and then iteratively process the regions in parallel. 
The above-described fully automatic mode, called `smart x-ray eyes`, is an 
analogue to a human operator with x-ray eyes who can `look around` the bag 
to notice `abnormalities` and see and understand each indicated threat 
object. Alternatively, the automatic mode is enhanced by providing input 
from an operator at different stages of the examination. Here, the 
inspection system includes a graphical interface and a user interface. At 
different stages, the operator receives one or more images that may also 
include a color overlay indicating properties of the located objects or 
regions. Then the operator is prompted to provide optimal selection of 
inspection geometries for the subsequent scan. This way the algorithm 
utilizes the experience of a human in an interactive manner. 
Another embodiment of the system utilizes a single dual energy x-ray probe 
and a data interface to the level 1 inspection machine. This enables the 
system to use the data from the level 1 device to direct the inspection 
process, thereby shortening the time to execute the `level 3` inspection 
process. 
Other embodiments are within the following claims: