U.S. patent number RE28,544 [Application Number 05/493,868] was granted by the patent office on 1975-09-02 for radiant energy imaging with scanning pencil beam.
This patent grant is currently assigned to American Science & Engineering, Inc.. Invention is credited to Jay A. Stein, Roderick Swift.
United States Patent |
RE28,544 |
Stein , et al. |
September 2, 1975 |
Radiant energy imaging with scanning pencil beam
Abstract
A pencil beam of X-rays scans an object along a line of
direction before an X-ray detector to produce an image of the line
along a picture tube. By relatively displacing the object scanned
and the line of scan in a direction transverse to the line of scan,
a sequence of lines appear on the display to produce an image of
concealed objects, such as guns.
Inventors: |
Stein; Jay A. (Newton, MA),
Swift; Roderick (Belmont, MA) |
Assignee: |
American Science & Engineering,
Inc. (Cambridge, MA)
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Family
ID: |
26856830 |
Appl.
No.: |
05/493,868 |
Filed: |
August 2, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
160363 |
Jul 7, 1971 |
03780291 |
Dec 18, 1973 |
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Current U.S.
Class: |
378/146; 378/57;
250/369 |
Current CPC
Class: |
G03B
42/028 (20130101); G01N 23/043 (20130101); G01V
5/0016 (20130101); G21K 1/043 (20130101) |
Current International
Class: |
G03B
42/02 (20060101); G01N 23/02 (20060101); G01N
23/04 (20060101); G01V 5/00 (20060101); G01T
001/20 () |
Field of
Search: |
;250/358,363,359,360,71.5R,71.5S,77,83.30,105,52 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Willis; Davis L.
Attorney, Agent or Firm: Hieken; Charles Cohen; Jerry
Claims
What is claimed is:
1. Radiant energy imaging apparatus comprising a source of a pencil
beam of .Iadd.X-ray .Iaddend.radiant energy
radiant energy detecting means defining a curve in fixed
relationship to said source,
means for scanning with said pencil beam said radiant energy
detecting means along said curve to provide an image signal
representative of the radiant energy response of the medium in a
region traversed by said pencil beam along a path to said detecting
means,
means for relatively displacing said region and an assembly
comprising said source and said detecting means to establish
relative translating motion in a direction transverse to a line
joining said source and said detecting means to produce a sequence
of image siganls representative of the radiant energy response of
said region in two dimensions,
and means responsive to said image signals for producing an image
representative of said response. .[.2. Radiant energy imaging
apparatus in accordance with claim 1 wherein said radiant energy
comprises X-rays.
.]. 3. Radiant energy imaging apparatus in accordance with claim 1
wherein said source of a pencil beam comprises,
a source of said radiant energy,
means for collimating said radiant energy into a slit-like
beam,
and means defining an aperture for intercepting said slit-like beam
to provide said pencil beam,
said means for scanning comprising means for relatively moving
said
aperture and said slit-like beam to effect said scanning. 4.
Radiant energy imaging apparatus in accordance with claim 3 wherein
said source of an uncollimated beam of said radiant energy
comprises an X-ray tube,
said means for collimating comprises a plate of X-ray opaque
material formed with a slit of X-ray transparent material,
said means defining an aperture comprises a radial slit that is
X-ray transparent in an X-ray opaque disc,
and said means for relatively moving comprises means for rotating
said disc
to move said radial slit along said first-mentioned slit. 5.
Radiant energy imaging apparatus in accordance with claim .[.2 .].
.Iadd.1 .Iaddend.wherein,
said detecting means comprises means for converting incident X-ray
energy into light energy,
and photodetecting means responsive to the latter light energy for
providing an electrical image signal that is amplitude modulated in
proportion to the instantaneous X-ray flux incident upon said
detecting
means. 6. Radiant energy imaging apparatus in accordance with claim
5 and further comprising a television display system responsive to
said image
signal for displaying a corresponding image. 7. Radiant energy
imaging apparatus in accordance with claim 5 wherein said means is
a crystal from the group consisting of sodium iodide and cesium
iodide,
and said photodetecting means comprises photomultipliers at each
end of
said crystal means. 8. Radiant energy imaging apparatus in
accordance with claim 6 and further comprising means for relatively
displacing a region to be scanned and said curve to display a
two-dimensional image of the X-ray
response of said region being scanned. 9. Radiant energy imaging
apparatus in accordance with claim 7 and further comprising means
for relatively displacing a region to be scanned and said curve to
provide a two-dimensional image signal of the X-ray response of
said region being
scanned. 10. Radiant energy imaging apparatus in accordance with
claim 1 wherein said radiant energy comprises X-rays,
said source of a pencil beam comprising,
a source of said radiant energy,
said radiant energy detecting means defining a line,
means including a plate of X-ray opaque material formed with a
linear slit of X-ray transparent material for collimating said
radiant energy into a slit-like beam embracing a plane
substantially including said slit and said straight line, an X-ray
tube comprising a source of the uncollimated beam of said radiant
energy,
an X-ray opaque disc formed with at least one radial slit that is
X-ray transparent,
and means for rotatably supporting said disc with its plane
generally perpendicular to the plane of said slit-like beam so that
rotation of said disc causes said radial slit to transmit
contiguous portions of said slit-like beam to said detecting means
to effectively provide said pencil
beam scanning said straight line from one end to the other. 11.
Radiant energy imaging apparatus in accordance with claim 10
wherein said means for relatively displacing comprises means for
translating an object to be imaged transverse to and across said
plane substantially including said
straight line and said slit-like beam. 12. Radiant energy imaging
apparatus in accordance with claim 10 wherein said means for
relatively displacing comprises means for moving said source and
said detecting means together while said region remains stationary.
Description
BACKGROUND OF THE INVENTION
The present invention relates in general to radiant energy imaging
and more particularly concerns novel apparatus and techniques for
displaying a visual image of concealed objects with sufficient
resolution to identity the object while keeping the intensity of
radiation relatively low. The system is reliable, relatively
economical and may be operated by relatively unskilled
personnel.
The problem of detecting contraband concealed in packages and on
persons is a serious one. X-ray equipment is useful for assisting
in the discovery of concealed contraband. Conventional X-ray
equipment is costly, requires operation by skilled personnel and
may well subject personnel and parcels to undesired excessive
dangerous radiation.
Accordingly, it is an important object of this invention to provide
an X-ray imaging system that overcomes one or more disadvantages of
conventional systems.
It is an important object of this invention to provide an X-ray
imaging system for displaying an image of concealed devices without
exposing personnel or parcels to excessive radiation.
It is a further object of the invention to achieve one or more of
the preceding objects with apparatus that is relatively inexpensive
and capable of being operated by relatively unskilled
personnel.
It is a further object of the invention to achieve one or more of
the preceding objects with apparatus that operates reliably and is
relatively easy to manufacture.
SUMMARY OF THE INVENTION
According to the invention, there is means for scanning a radiation
sensitive detector along a curve with a pencil beam of radiation to
provide a line image signal characteristic of radiant energy
response between the source of the pencil beam and the radiation
sensitive detector, and means for displaying the image represented
by the image signal. The radiation sensitive detector and the
source are in fixed relationship. The detector may be positioned
for receiving direct and/or reflected or scattered radiation.
Preferably there is means for relatively displacing the curve
scanned and an object to produce a sequence of image signals
representative of the radiant energy response of the object in two
dimensions. There is means for relatively displacing the region
embracing the object and an assembly comprising the source and
radiation sensitive detector or detecting means to establish
relative translating motion in a direction transverse to a line
joining the source and the detecting means. Preferably the curve is
a line with the relative displacement between object and line being
in a direction orthogonal to the line. Preferably the detector
comprises a sodium iodide or cesium iodide crystal that produces a
visible manifestation of the intensity of the incident radiation
that may be sensed by a photodetector to provide a characteristic
electrical output signal that may be applied to a television
display system that may incorporate a storage tube.
Numerous other features, objects and advantages of the invention
will become apparent from the following specification when read in
connection with the accompanying drawing in which:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a pictorial representation of a parcel inspection system
according to the invention; and
FIG. 2 is a pictorial representation of an exemplary embodiment of
the invention for inspecting personnel.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
With reference now to the drawing and more particularly FIG. 1
thereof, there is shown a pictorial representation of a system
according to the invention for scanning parcels. A parcel 11 is
scanned by the invention to produce an image 12 of contraband on
the video storage and display unit 13. An X-ray tube 14 provides a
generally conical beam of X-rays 15 that are collimated into a fan
beam 16 by slit collimator 17 oriented generally vertically as
shown and incident upon the rotating collimation disc 18 formed
with an array of peripheral radial slits, such as 21, for
intercepting fan beam 16 to produce pencil beam 23. Pencil beam 23
scans parcel 11 and radiation sensitive detector 25 from top to
bottom as rotating disc 18 rotates in the direction of arrow 24 to
provide an image signal over output line 26 that is transmitted to
video storage and display unit 13 to produce the image 12 of the
parcel scanned as conveyor 27 carries parcel 11 in the direction of
arrow 28 across the line being scanned.
Since most specific elements of the system are known to those
skilled in the art who can practice the invention from an
examination of FIG. 1 and the accompanying description, minute
specific details are omitted so as to avoid obscuring the
invention.
The geometry and timing of the system is arranged so that each slit
21 causes a new pencil beam to strike the top of detector 25 just
after the previous pencil beam has swept past the bottom of the
detector. That is to say, the height of fan beam 16 corresponds
substantially to the separation between adjacent ones of slits 21
at substantially the maximum radial distance from the edge of disc
18 where the slits intercept fan beam 16. While FIG. 1 shows the
elements that provide the scanning pencil beam source in exploded
form to better illustrate the principles of the invention, the
elements 14, 17 and 18 are preferably housed relatively close
together in an enclosure that shields radiation so that the only
significant radiant energy that escapes is that in pencil beam
23.
As parcel 11 move past the line being scanned, it differentially
attenuates the X-rays in pencil beam 23 incident upon detector 25
so that the electrical signal provided on output line 26 is
amplitude modulated in proportion to the instantaneous X-ray flux
incident upon it. This signal thus corresponds to a vertical line
image of the transmissivity of parcel 11 and is analogous to one
scan line of a television video signal. As parcel 11 moves
horizontally past the line being scanned, sequential pencil beams
intercept slightly displaced regions of parcel 11 so that the
corresponding electrical signals from detector 25 may be
appropriately displayed line-by-line to produce a two dimensional
image of parcel 11 in X-rays analogous to the display of a picture
on a television monitor as formed by line-by-line images. The
output of detector 25 may thus be processed in accordance with the
same storage and display techniques used in conventional video
systems to store and display single raster images. Since these
techniques are well known in the art, further discussion of them is
unnecessary here.
Although detector 25 is shown behind the object being scanned for
responding to the radiant energy transmitted through the object
being scanned, it is within the principles of the invention to
position the detector in the region between the radiant energy
source and the object being scanned to respond to the scattered
energy. This arrangement helps the apparatus detect concealed
objects having different scattering characteristics from their
surrounding. Moreover, a system according to the invention may
include both detecting means before and behind the object being
scanned for simultaneously providing signals representative of both
radiant energy transmission and scattering. Appropriately combining
such signals may help increase the ability of the system to detect
a wide variety of concealed objects.
Referring to FIG. 2, there is shown a pictorial diagram
illustrating the logical arrangement of a system according to the
invention for personnel inspection. This system embodies the
principles of the system of FIG. 1; however, the pencil beam scans
horizontally, and the scanning system and person relatively move
vertically to produce a two dimensional image of the person. Like
elements in the system of FIG. 2 are designated by corresponding
reference numerals.
A vertically movable platform 41 supports the pencil beam source
comprising X-ray tube 14, fan beam collimator 17 and rotating
collimation disc 18 to scan person 42 along a sequence of
horizontal lines as detector 25 and platform 41 move down together.
Detector 25 is also supported for vertical displacement.
Details of specific means for vertically displacing detector 25 and
platform 41 are well within the skill of one having ordinary skill
in this art and are omitted so as to avoid obscuring the principles
of the invention. They might, for example, be guided by vertical
shafts, at least one of which was a rotating feedscrew supporting
detector 25 and platform 41 rotating in synchronism so that
platform 41 and detector 25 move together. Numerous other
techniques could be employed for effecting vertical scanning. For
example, the person being scanned could be placed upon a platform
that was raised and lowered. This approach would be especially
convenient where a person entered the scanning area on one level
and left it on another, an especially convenient arrangement,
where, for example, an airline passenger might enter at ground
level and leave closer to boarding ramp level. Video storage and
display unit 13 then displays image 12 which, in this embodiment,
is an image formed of a sequence of horizontal lines as
distinguished from the sequence of vertical lines forming the image
in FIG. 1.
Considering now specific parameters for a parcel examining system
of FIG. 1, such a system could examine parcels with dimensions up
to 32 .times. 20 .times. 16 inches provided that parcels with
dimensions exceeding 20 inches are oriented with their long axes
parallel to their direction of travel and all parcels are guided
close to detector 25 with a distance between source and detector of
approximately 6 feet and the height of detector 25 about 24 inches.
Then the maximum distortion caused by differences in magnification
of the front and back surfaces of parcels will never exceed .+-. 19
percent from the average magnification and would occur only rarely.
Objects with overall depths less than 20 inches along the direction
of the scanning beam would have proportionately less
distortion.
Resolution capabilities of 1 millimeter square are readily
obtainable for identification of most objects having characteristic
dimensions of several inches. With 1 mm resolution a 20 inch object
could be covered in 500 scans without gaps or overlap, larger
parcels being covered by more scans or greater spacing between
scans. In either case the image could be displayed on a standard
512-line television monitor with negligible loss of detail.
For a nominal conveyor speed of 10 inch/second (250 mm/second), 250
scans/second (or 4 milliseconds/scan) achieves 1 millimeter
resolution where each scan covers the full 24 inches height of the
detector so that a 20-inch long parcel could be scanned in 2
seconds.
With X-ray tube 14 conventional and operating at moderate voltage
and current (60-100 kv, 10 ma,) it typically produces a flux at 6
feet (the distance to detector 25) of millions of X-rays per
mm.sup.2 per second. A filtered tungsten target tube operating at
100 kv and 15 ma provides typically an X-ray flux at detector 25 of
about 10.sup.7 X-rays/mm.sup.2 /sec. with a broad energy spectrum
extending from 20 to 100 kev. Generating 250,000 resolution
elements in 2 seconds results in each resolution element being
irradiated for about 2/250,000 seconds or 8 microseconds. With an
X-ray flux at the detector 25 of 10.sup.7 X-ray mm.sup.2 /second,
each resolution element would (in the absence of an X-ray absorbing
object) receive about 80 X-rays per exposure. Taking into account
the absorption by packing material of low energy X-rays, 10-20
X-rays/resolution element would typically be detected during a 2
second total exposure, about the statistically significant number
of X-rays required to distinguish white from black in adjacent
resolution elements so that the proposed 2-second exposure time is
appropriate to achieve 1 .times. 1 mm resolution.
A feature of the invention is that the X-ray detection process is
ideal. The X-ray quantum efficiency of the detector 25 is close to
100 percent. X-rays will produce output pulses several times larger
than photomultiplier noise (dark) pulses so that the latter can be
completely eliminated by threshold discrimination. Moreover, since
the detector can be made very narrow, the background contribution
from radiation scattered by a parcel is negligibly small so that
the invention may use minimum X-ray dosage for 1 mm resolution,
typically less than 0.003 mrads per image compared with the daily
dosage received from cosmic rays and naturally occurring radio
activity of about 0.3 mrads and to the dosage required to expose
X-ray film to a barely detectable 0.01 density unit above
background fog which requires at least 0.1 mrads. Thus, the
invention may be safely used for inspecting personnel and parcels
without using harmful radiation levels.
Preferably the X-ray tube and associated power supply are
conventional. Preferably X-ray tube 14 is operable at variable
voltages up to 150 kv to optimize image quality. Preferably X-ray
tube 14 is operated water cooled with a peak voltage of 150 kv,
peak current 5-10 ma, a 100 percent duty cycle, the power being at
constant potential and the focal spot size of 0.4 mm, all these
characteristics being readily available.
For the dimensions discussed above and a source spot size of 0.4
mm, a slit 21 width of 0.3 mm will provide 1 mm resolution. If disc
18 were moved closer to detector 25, a wider slit could be used,
but the disc diameter would increase proportionately. Conversely, a
smaller disc could be used if it were moved closer to X-ray tube
25, but the slit size would have to be reduced. A 2-foot diameter
disc with 0.3 millimeter slits located midway between tube 14 and
detector 25 is a satisfactory compromise between rotation of a
larger disc at higher speeds and fabrication of smaller slits. The
slits themselves are shaped to collimate the beam along all pencils
comprising the fan and may be fabricated from tungsten inserts
installed in the disc. The rate of rotation of disc 18 is related
to the time available for a full exposure. For 500 scan lines in 2
seconds, disc 18 generating six scans per revolution rotates at
250/6 revolutions per second or 2,500 rpm, a rate readily achieved
with standard motors.
A preferred form for detector 25 comprises a sodium iodide crystal
that detects X-rays below 200 kev with 100 percent efficiency. Such
a detector with dimensions 1 .times. 1 .times. 24 inches can be
readily fabricated from two or three shorter pieces of standard
material. The energy of each X-ray interacting in sodium iodide is
converted to light sufficiently large to be easily detected by a
photomultiplier. By optically coupling a 1 inch end window
photomultiplier to each end of the sodium iodide crystal, there is
complete and uniform light collection for X-ray interactions
occurring at any position along the length of the detector. The
summed currents from the two photomultipliers are proportional to
the instantaneous X-ray flux striking the detector to produce an
image signal analogous to an ordinary video signal that, after
amplification, may be stored and displayed by techniques known in
the art.
It is preferred that the amplifier for the summed photomultiplier
output currents have a bandwidth from d-c to about 1MHz to retain
all information in a 500 scan exposure that may be completely
transparent (or opaque) to X-rays for one parcel and may contain
structure at the limits of resolution (at 8 microseconds per
resolution element) for another parcel, preferably being low noise
so as to not limit system sensitivity and providing an output
signal at high enough level to be stored. By utilizing as much
amplification as practical from the photomultiplier itself, many
commercially available amplifiers, such as commonly available
oscilloscope preamplifiers are adequate.
In an actual working embodiment of the invention, the X-ray image
was reconstructed by employing suitably triggered time-base units
to provide successive vertical sweeps, each slightly displaced from
its neighbor to produce a television-like raster scan and using the
analog signal derived from the photomultiplier tube to intensity
modulate the CRT electron beam on a storage oscilloscope to produce
an image that was retained long enough for visual inspection.
It is preferred to use a scan converted storage tube of a type well
known in the art in order to produce a better image, such systems
being commercially available and of the type used to convert slow
radar scans to a continuously displayed television picture that is
updated at every successive radar scan.
The invention has numerous uses, including medical applications,
and may take many different forms. For example, there may be a
number of detectors and fan beams arranged for providing a
multiplicity of scanning beams. Other techniques may be employed
for providing the scanning beams of radiant energy and for
detecting transmitted and/or scattered energy.
There has been described a novel radiant energy imaging system
characterized by relatively high resolution, low radiation dosage,
ease of operation and numerous other features. It is evident that
those skilled in the art may now make numerous uses and
modifications of and departures from the specific embodiments
described herein without departing from the inventive concepts.
Consequently, the invention is to be construed as embracing each
and every novel feature and novel combination of features present
in or possessed by the apparatus and techniques herein
disclosed.
* * * * *