Dual energy imaging

Dual energy imaging develops simultaneous signals from transmitted energy of different characteristics. The detector includes two serially arranged detectors. A first detector preferentially absorbs lower level radiant energy and the second detector preferentially absorbs higher radiant energy. The detectors may be identical in which case the preferential characteristic is developed by reason of the detector's position. Alternatively, the characteristics of the detectors may be selected to produce the desired effect, e.g. a gaseous detector may be pressurized in accordance with its function, a scintillating screen may be tilted or its length may be selected to produce the desired absorption characteristic.

DESCRIPTION 
1. Field of the Invention 
The invention relates to improvements in imaging using penetrating radiant 
energy, for example x-rays. 
2. Background Art 
Mistretta and Brody have made improvements in what is here termed dual 
energy imaging. These improvements allow illumination of a complex object 
(one including two or more different substances such as different 
elements, compounds or tissues) to allow a selective display related to 
some but not all of the different components of the object. This is a very 
significant improvement in the field of imaging, since in the real world 
most objects which are imaged are complex objects, and many problems arise 
because the display is cluttered with the inclusion of images of 
irrelevant components. Aside from merely cluttering the display, 
irrelevant images may actually hide or obscure desired images. It is thus 
a prime object of the invention to improve dual energy imaging. 
Two of the problems evidenced in the prior art dual energy imaging 
techniques relate to the type of illumination employed and the lack of 
simultaneity in the data collected. 
The dual energy imaging techniques known to the prior art all require 
control of the illumination energy. In one of the techniques (Mistretta), 
three different exposures are made using illumination of three different 
energies, each of which are monochromatic or nearly so. Typically, nearly 
monochromatic illumination is obtained by filtering energy from an 
uncontrolled source. This filtering obviously reduces the incident 
illumination on the object and has the concommitant disadvantage that the 
resulting image is photon limited. 
While Brody's work is less severely limited, i.e. he uses two 
illuminations, each with a relatively broader band of illumination 
energies; the two illuminations are still filtered to the extent that the 
illumination energies do not "overlap too much". While the results of 
Brody's work are not as photon limited as Mistretta's, the filtering 
necessary to obtain the desired characteristics in the illumination energy 
still reduces the illumination intensity. 
A further disadvantage, which is common to all the prior art work in this 
field is that the different exposures employed are time sequential, i.e. 
the data collected during the several exposures lack simultaneity. 
Obviously, images taken at different times will only be identical if 
motion is absent, and many interesting subjects are inherently subject to 
motion. 
It is therefore one object of the present invention to provide a method and 
apparatus for dual energy imaging which collects data simultaneously thus 
eliminating motion artifacts in the images. It is another object of the 
present invention to provide a method and apparatus for dual energy 
imaging which does not require filtering or processing of the illumination 
energy and is not subject to being photon limited, as is the prior art. 
SUMMARY OF THE INVENTION 
These and other objects of the invention are met by providing an imaging 
apparatus including a source of penetrating radiant energy, means for 
directing the penetrating radiant energy emitted by the source so as to 
travel a path toward a target area, detector means located beyond the 
target area along the path of said radiant energy for producing signals 
representative of radiant energy reaching the detector means. The 
invention is characterized in that the detector means includes at least a 
first energy detector for converting radiant energy of a first 
characteristic to a first signal and a second energy detector for 
converting radiant energy of a second characteristic, different from the 
first characteristic, to a second signal. As is described hereinafter, in 
accordance with preferred embodiments of the invention, the sensitivity of 
the first energy detector falls off at a relatively lower level energy as 
compared with the sensitivity of the second energy detector which is 
responsive to radiant energy of a relatively higher energy level. 
For shorthand purposes, the first energy detector of the preferred 
embodiments can be called a `thin` detector and the second energy detector 
can be called a `thick` detector. The adjective `thin` implies an 
increasing inability to absorb harder and harder radiant energy, i.e. it 
is most sensitive to low energy radiant energy. The `thick` adjective 
implies that the detector characteristic falls off at a higher energy 
level than the characteristic of the `thin` detector. However as will be 
explained, the detectors may have the identical characteristic. In some 
cases the order of the thin and thick detectors may be reversed. 
Detector means as is recited above include first and second energy 
detectors located with the first energy detector in front of the second 
energy detector along the path of travel of the radiant energy. In the 
preferred embodiments, the first energy detector has increased sensitivity 
to and therefore preferentially absorbs lower level radiant energy, and 
thus higher level radiant energy passes through the first energy detector 
and is incident on the second energy detector. The second energy detector 
may be arranged to respond to illumination of a higher energy level than 
the first energy detector. 
Preferred embodiments of the invention described hereinafter may employ 
either gaseous, liquid or solid state radiant energy detectors. In the 
case of a gaseous energy detector, the different energy responsive 
characteristics may be provided by pressurizing the gas in the first 
energy detector at a relatively lower pressure than the gas in the second 
energy detector. 
In the case of solid state radiant energy detectors, a single continuous 
scintillating material is common to one form of the first and second 
energy detectors. The first energy detector includes a portion of the 
scintillating crystal located nearer the source of illumination than the 
portion of the scintillating crystal associated with the second energy 
detector. The first and second energy detectors further include 
opto-electronic transducers located adjacent respective portions of the 
scintillating crystal so that the opto-electronic transducer (or diode) 
forming part of the first energy detector is responsive to light energy 
emitted by that portion of the scintillating crystal associated with the 
first energy detector and correspondingly the opto-electronic transducer 
(or diode) associated with the second energy detector is located adjacent 
that portion of the scintillating crystal which is associated with the 
second energy detector. 
In other embodiments of the invention, first and/or second energy detectors 
may comprise a scintillating screen. The energy responsive characteristics 
of the screen responsive can be varied by merely varying the angle a 
normal to the screen makes with the path of the radiant energy. By 
increasing the angle (from zero) the effective "length" of the screen is 
increased making it responsive to penetrating radiant energy of higher and 
higher energy levels. 
The first and second signals produced by the first and second detectors can 
then be processed, stored and displayed in accordance with conventional 
dual energy imaging. Furthermore, the dual energy imaging described in my 
co-pending application, incorporated herein by this reference, entitled 
"Improvements in Imaging" , filed Sept. 29, 1982 and assigned to the 
assignee of this application, may be used. 
The `thin` and `thick` characteristics of the detectors can be obtained by 
tailoring the device or by its relative positioning. 
In special cases, the `thick` detector may precede the `thin` detector, but 
in this case the device characteristics must be tailored as will be 
described below. 
The dual energy imaging in accordance with the invention can be achieved 
using conventional flying spot scanning illumination. With flying spot 
illumination each of the detectors can be a line detector. However, the 
invention is also applicable to fan beam illumination. In this case, each 
of the detectors consists of a series of `point` detectors. Finally the 
invention is applicable to area illumination. In this last case each of 
the detectors consists of a matrix of `point` detectors. 
While an X-ray illumination source is described, other types of radiant 
energy can be used with appropriate detectors.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
As shown in FIG. 1, the apparatus of the invention includes, in the main, 
three components. A source of penetrating radiant energy 10 in one 
component. The penetrating radiant energy 15 emitted by the source 10 (for 
example, such as an x-ray tube) impinges on apparatus 30 to direct the 
radiant energy toward the target area A. In addition to directing (perhaps 
collimating) the energy emitted by the source 10, the apparatus 30 may 
also form a flying spot beam, for repeatedly scanning a line in space. 
Since the apparatus for shaping the penetrating radiant energy 15 emitted 
by the source 10, and in those cases in which a flying spot beam is 
desired, for forming such a spot beam, is well known to those skilled in 
the art, the apparatus 30 need not be further described herein. Reference 
is made to U.S. Pat. Nos. 4,031,401; RE 2,544; 3,790,799; 4,242,583 and 
4,260,898. 
Assuming that the apparatus 30 forms a flying spot beam, then as shown in 
FIG. 1 the penetrating radiant energy at any instant takes the form of a 
ray R, which, as a function of time, sweeps repeatedly back and forth in 
the region illustrated in the drawing. A target to be illuminated is 
located in the target area A, and finally the radiant energy transmitted 
through the object in the target area A impinges on detector means 20. 
For purposes of developing the dual energy imaging, the detector means 20 
can take a variety of forms, three different embodiments of which are 
illustrated respectively in FIGS. 2, 4 and 5. 
Referring now to FIG. 2, the typical ray R is shown impinging on the 
detector means 20 which, in the embodiment of FIG. 2 includes a first 
energy detector 21 and a second energy detector 22. As seen in FIG. 2, the 
first and second energy detectors 21 and 22 (shown in cross-section in 
FIG. 2) are located serially in the path R taken by the penetrating 
radiant energy so that penetrating radiant energy intercepts the first 
energy detector 21 and energy passing through the first energy detector 21 
impinges on the second energy detector 22. In the embodiment illustrated 
in FIG. 2, the first and second energy detectors each may comprise a 
gaseous detector associated with a transducer to convert energy from 
absorbed X-rays (or other radiant energy such as gamma rays) to electrical 
signals. In accordance with the invention, as will be described 
hereinafter, the first energy detector 21 is responsive to radiant energy 
of a first characteristic, and the second energy detector 22 is responsive 
to radiant energy of a second characteristic different from the first 
characteristic. In particular, the first energy detector 21 has a 
sensitivity characteristic which falls off at radiant energy of lower 
energy level than the characteristic of energy detector 22. Referring 
briefly to FIG. 3 which is a plot of detection efficiency versus incident 
energy for the first and second energy detectors, the curve 40 identifies 
the characteristic of the first energy detector 21 and the curve 50 
identifies the characteristic of the second energy detector 22. 
Returning briefly to FIG. 2, the energy detectors 21 and 22 can comprise 
gas filled tubes, filled with an identical gas, with the energy detector 
22 pressurized to a pressure greater than that of the energy detector 21. 
Under these circumstances, the detection of efficiency of the first energy 
detector 21 drops off as the incident energy increases in energy level. 
While the second energy detector 22 illustrates the same characteristic 
(of falling efficiency as energy increases) because of the increased gas 
pressure in energy detector 22, the fall off in efficiency in the second 
energy detector occurs at a higher energy level than in the first. As a 
result, the second energy detector 22 has increased sensitivity to energy 
of a level higher than the energy to which energy detector 21 is 
responsive to. This result is also contributed to by the relative location 
of the two detectors, one in front of the other. Since the absorption of 
radiant energy is proportional to the incident flux, the detector 21, in 
absorbing lower energy radiant energy, reduces the lower energy radiant 
energy reaching detector 22. This reduces the apparent sensitivity of 
detector 22 to lower level radiant energy. 
FIG. 4 illustrates another embodiment of the detector means 20, and in 
contrast to FIG. 2 wherein the detector means 20 includes two individual 
energy detectors 21 and 22, the detector means 20 of FIG. 4 includes a 
single scintillating material such as crystal 60. Associated with the 
single scintillating crystal 60 is a pair of opto-electronic transducers 
or diodes 42 and 52. The diode 42 is associated with a portion 41 of the 
scintillating crystal 60, and the diode 52 is associated with a different 
portion 51 of the scintillating crystal 60. As shown in FIG. 4, the 
portion 41 of the scintillating crystal 60 is located closer to the target 
area A than is the portion 51, in the direction of travel of the radiant 
energy R. Accordingly, the detector means of FIG. 4 includes a first 
energy detector 31 and a second energy detector 32. Each of the energy 
detectors 31 and 32 includes its respective diode (42 or 52) and a portion 
of the scintillating crystal 41 or 51. It should be understood by those 
skilled in the art that the division of the crystal 60 (the dotted line) 
shown in FIG. 4 is theoretical; in practice there need be no physical 
division between portions 41 and 51 of the scintillating crystal 60. 
Because of the location of the diodes 42 and 52, however, each diode is 
responsive to optical energy emitted by one portion or the other (41 or 
51) of the scintillating crystal 60. By selecting the scintillating 
material 60 and the length (in the direction of the ray R) of the portions 
41 and 51, we can assure that a given (high) percentage of the energy 
below a selected energy E.sub.1 is absorbed in portion 41 and a given 
(high) percentage of the energy below a selected, higher, energy E.sub.2 
(E.sub.2 &gt;E.sub.1) is absorbed in portion 51. In this fashion, the optical 
emissions of the portion 41 and 51 are related to different energy levels. 
Those skilled in the art will appreciate that the scintillating material 
may be uniform and that the foregoing characteristic follows from the fact 
that the energy flux absorbed over any small incremental distance of the 
scintillating material is proportional to the incident flux. Thus, lower 
energy is preferentially absorbed over portion 41 and portion 51 sees a 
higher proportion of the higher energy level. This effect requires little 
or no light migration along the energy path, R. This is easily arranged in 
conventional fashion. This preferential absorption can be accentuated as 
shown in FIG. 6. 
FIG. 6 is identical to FIG. 4 except that the material 60 is now composed 
of three portions, 41, 51 and 61, with portion 61 located between 41 and 
51. Since the diodes 42 and 52 remain adjacent portions 41 and 51, 
respectively, the energy absorbed in portion 61 increases the preferential 
absorption of portion 51 to higher level radiant energy. Note that with 
either embodiments of FIG. 4 or 6 the preferential absorption may be 
achieved merely by preferential location since in other respects the 
detectors 31 and 32 are identical. The use of detectors with identical 
characteristics is of course not confined to the solid state detectors 
shown in FIGS. 4 and 6. 
Finally, in still another embodiment of the invention, shown in FIG. 5, the 
first and second energy detectors comprise first and second scintillating 
screens 71 and 72, respectively. The manufacture of scintillating screens 
are well known to those skilled in the art and therefore a specific 
description of the parameters of the screen 71 and 72 is not necessary. 
However, those skilled in the art are aware that the stopping power of a 
scintillating screen depends on the length of the screen in the direction 
of travel of the radiant energy. Screens of identical dimensions can be 
arranged to have different energy responsive characteristics, by selecting 
the angle the screen makes with the direction of the radiant energy. 
More particularly, it may be desired to have the first energy detector with 
sensitivity limited to lower level radiant energy, and the second energy 
detector with a higher sensitivity to higher level radiant energy. Screens 
of identical dimensions can be located to achieve these goals by arranging 
the second screen so that a normal to the screen makes a greater angle to 
the direction of the radiant energy R, than the angle a normal to the 
first screen makes with the same direction. Typically, a normal to the 
first screen will be parallel to the direction R, so the desired effect 
can be produced by tilting the second screen. By reason of this angular 
orientation, the "length" of the screen seen by the radiant energy is 
greater for the second screen than for the first. This increased length 
increases the sensitivity of the second screen to higher energy levels as 
compared to the sensitivity of the first screen. 
Thus, FIG. 5 shows two identical screens 71 and 72, each of length L, in 
the direction of the ray R, with other dimensions A and B, respectively. 
Since screen 72 is "tilted" with respect to the direction R by the angle 
a, the length of screen 72 seen by the radiant energy is L/cos a. Since a 
is by definition .noteq.0, L/cos a&gt;L. 
Of course, if desired, the two scintillating screens 71 and 72 could be 
arranged to make a common angle with the direction of radiant energy R, 
wherein the characteristics (dimensions, material, etc.) of this second 
screen are varied with respect to the first so as to increase the "length" 
or stopping power of the second screen in the direction of the radiant 
energy R, as compared to the "length" or stopping power of the first 
screen. 
Finally, the preferential absorption can be solely a result of preferential 
location, i.e. the two screens 71 and 72 may be identical in size and 
angle. 
In the several embodiments of the invention described with reference to 
FIGS. 2, 4 and 5, the description has been in terms of a pair of gaseous 
radiant energy detectors, a pair of scintillating crystal radiant energy 
detectors or a pair of scintillating screen radiant energy detectors. 
These embodiments of the invention have been described for convenience, 
and those skilled in the art will be aware that there is no requirement 
that the first and second energy detectors be of the same type, i.e. 
gaseous and solid state energy detectors can be used together. What is 
essential is that the first energy detector has a particular sensitivity 
characteristic, that it allows some flux to pass therethrough, and that 
the second energy detector be sensitive to the flux emitted by the first 
energy detector. In practice, this almost always requires that the two 
detectors be serially arranged with the first (or thin) energy detector 
located "in front" of the second (or thick) energy detector. 
The preceding discussion, given in terms of a flying spot scanner, requires 
only two detectors, a raster image being produced by the motion of the 
flying spot and the (slower) indexing of the target perpendicular to the 
plane described by the flying spot (if necessary). However, the art also 
uses a line of detectors (usually in conjunction with a fan beam), made up 
of a linear array of detector cells as well as an area detector (used in 
conjunction with a beam having a cross-section of similar form) made up of 
a two dimensional array of detector cells. Those skilled in the art will 
readily perceive that the invention can be applied to linear detector 
arrays as well as two dimensional detector arrays. In either case a second 
detector, identical to the first, is located in the radiant energy beam 
path. 
The specific embodiments of the invention described here consisted of two 
energy detectors. However, the invention can be applied to more than two 
energy detectors. For example, in FIG. 6 there may be added one or more 
additional photo-diode(s) adjacent the region 61. 
While the usual case includes the detectors in a serial arrangement with 
the lower energy (or `thin`) detector leading the higher energy (or 
`thick`) detector, that is also not essential. A high energy (or `thick`) 
detector can lead the lower energy (or `thin`) detector so long as the 
`thick` detector passes sufficient lower energy radiant energy to be 
detected by the `thin` detector. 
For example, this can be achieved by using the edge transition in the 
stopping material of the first detector. The first detector is chosen so 
its characteristic has an absorption edge at an energy just below the 
lowest energy of diagnostic interest. At greater energies the absorption 
increases leaving a low energy "window" which can be detected by the 
second energy detector. The second energy detector has an absorption edge 
at a different energy than the edge of the material of the first energy 
detector. 
For example, the detector combination of any of FIGS. 2, 4, 5 or 6 is used. 
The first or `thick` detector has a sensitivity characteristic 70 (see 
FIG. 7), the second or `thin` detector has the sensitivity characteristic 
80. 
Those skilled in the art will realize that a characteristic like the 
characteristic 70 is available from many materials, the particular 
detector will be chosen based on the composition of the object being 
imaged. For example, tungsten has an edge at 69.5 kv and any iodinated 
scintillator has an edge at 33 kv. 
It should be apparent from the foregoing that the signals produced by the 
two detectors can be processed and displayed to produce the advantages of 
dual energy imaging. In contrast to the prior art, the images are 
simultaneously obtained, thus eliminating motion artifacts. Furthermore, 
there are no artificial constraints on the illumination such that photon 
limiting is not expected.