Electronic x-ray recording

A method for increasing the efficiency of gas ionization detectors for diagnostic x-rays by matching a detector gas absorption edge with the energy level of an x-ray source to produce photons of only slightly higher energy. Sequential x-ray images using different matched gas absorption edges and x-ray source energies allows contrast information to be obtained to distinguish calcium deposits or dye from bone, for example. Specific ionization detectors useful for practicing the inventive methods are also described. One such detector uses ultrafast electronics associated with individual wires of a multiwire two dimensional proportional counter. Another detector employs a conductor backed sheet of insulating material to immobilize positive gas ions in a density distribution conforming to the x-ray image. The density distribution may subsequently be read out by electromechanical means for sensing the potential differences on the sheet. The potential sensing means may be located within or outside the detector chamber.

DESCRIPTION 
1. Background of the Invention 
This is a continuation-in-part application of copending patent application 
Ser. No. 239,313, filed Mar. 2, 1981, entitled ELECTRONIC X-RAY RECORDING. 
This invention relates to electronic means for recording x-ray shadowgrams. 
A shadowgram is defined as a two-dimensional spatial picture of the 
shadow, i.e., the absorption of x-rays by some object placed between an 
x-ray source and the recording system. 
Since the discovery of x-rays at the end of the 19th century, there have 
only been two principal methods available for recording x-ray shadowgrams 
in medical applications. These two methods are fluoroscopy, in which the 
x-rays strike selected chemical compounds causing them to emit visible 
light, and photography, in which x-rays directly interact with grains of 
silver halide in a photographic emulsion rendering them developable. 
For the last 40 years, the principal tool for xray generation for medical 
x-ray diagnosis has been the hot cathode-tungsten anode x-ray tube. 
Refinements have been made, such as rotating anodes, which increase the 
intensity of the x-ray flux available and thereby reduce exposure times. 
However, due to the low sensitivity of photographic and fluoroscopic 
materials and their lack of differentiation of response to x-ray photons 
of different energies, little work has been done on materially different 
x-ray tube sources. 
In contrast to the low sensitivity of photographic and fluoroscopic 
methods, ionization detectors are very sensitive at x-ray energy levels. 
When the energy of an x-ray photon is the same as the energy required just 
to eject one of the electrons from an atom of the gas, then the gas 
becomes strongly absorbing, i.e., the x-ray photon energy is equal to an 
absorption edge energy of the gas. Photons having an energy below the 
absorption edge of the gas are absorbed weakly and, therefore, are not 
detected. Photons having an energy above the absorption edge of the gas 
emit secondary photons, as well as electrons, upon collision with a gas 
atom. If the incoming x-ray photon is of sufficiently high energy, the 
secondary photon may itself be absorbed and cause electron emission, 
possible from a different level. Electrons produced by secondary photons 
degrade x-ray images, and specialized techniques must be used to reduce 
their effect. 
Electronic detectors utilizing gas ionization chambers have been used with 
x-ray beams in CAT scanning devices, but these devices are extremely 
expensive and cannot be considered as a practical substitute for the 
ordinary two-dimensional shadowgram. Ionization chamber detectors based on 
multi-wire proportional counters have also been described for gamma ray 
and nuclear particle detection and have been used in other applications as 
x-ray detectors. These latter detectors, however, if used to obtain x-ray 
shadowgrams, would require x-ray exposure periods on the order of 
fractions of a minute because the electronic read-out time is substantial. 
Over-exposure of the patient can be avoided by reducing the intensity of 
the x-ray flux, but practical limits on the time a patient may remain free 
of voluntary and involuntary movement dictate that the exposure time be 
only a few seconds at most. Ideally, exposure time would be significantly 
less than one second so that heart movement could be frozen. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention provides a method and structure which will permit the 
formation of x-ray shadowgram images of a quality comparable to that of 
photographic or fluoroscopic techniques while, at the same time, reducing 
patient exposure to harmful high energy xray radiation. The method is a 
technique for optimizing the efficiency of position sensitive ionization 
detectors. Ionization detector structures which make routine diagnostic 
shadowgrams practical by acquiring image information with an exposure of 
only fractions of a second are also part of the present invention. 
Ionization detectors employing high electric field gradients, such as those 
employing a fine wire positive electrode, have the advantage that an 
electron released from a gas atom by an x-ray photon, once in the vicinity 
of the wire electrode, is strongly accelerated. This high energy electron 
collides with other gas atoms and ionizes them, thereby creating a shower 
of electrons and positively charged gas ions. The path of the electrons 
ends at the wire, whereas the positive ions return to the negative 
electrode. This charge multiplication characteristic makes it much easier 
to detect single x-ray photons and thereby offers the potential of 
providing the same information as that of conventional photographic or 
fluoroscopic techniques while maintaining patient exposure at a much lower 
level. 
One aspect of the present invention is a method of maximizing the 
efficiency of the x-ray ionization detectors by matching the detector 
absorbing gas and x-ray source so that the energy level of the x-rays 
emitted by the source is just above the absorption edge of the gas. This 
keeps patient exposure to a minimum while achieving optimal detector 
efficiency. Furthermore, the problem of secondary photon emission causing 
false images is minimized, any secondary photons emitted having too low an 
energy to be strongly absorbed by the detector gas. This method of 
matching the detector gas and the x-ray source, in addition to lowering 
patient dosage by maximizing the efficiency of the detector, also permits 
lower patient exposures by use of lower energy x-rays for images of the 
patient's thinner extremities where body absorption of lower energy 
radiation is insufficient to seriously reduce the radiation flux reaching 
the detector. 
A variation of the inventive method involves providing two or more matched 
gas/source combinations so that successive images obtained from x-rays of 
different energy levels can be compared to enhance contrast, as for 
instance, between bone and soft tissue. It is also possible to 
differentiate by this means between bone and either dye or calcium 
deposits. By employing subtractive techniques on images from two energy 
levels, it is possible to produce a deep body image of soft tissue in 
which the interfering bone structure can be significantly suppressed, if 
not eliminated. Of particular utility would be the production of images of 
the thorax (heart, lungs and medial stinum) with minimal obstruction of 
ribs and vertabrae. This type of image could reduce the need of multiple 
x-ray views (i.e., lateral and oblique) when trying to detect soft tissue 
abnormalities. 
Several different two-dimensional ionization detectors have also been 
invented to solve the problem of slow image read-out which has heretofore 
prevented practical application of ionization detectors to production of 
two-dimensional diagnostic shadowgrams. A first detector has a multi-wire 
two-dimensional proportional counter which utilizes simultaneous read-out 
and storage of onedimensional x-ray collision information from each wire 
followed by processing of the information from all wires into a 
two-dimensional image. This parallel read-out technique substantially 
decreases the amount of time necessary to obtain detailed image 
information. For instance, for an image of 256.times.256 dots, the 
acquisition time is approximately 0.125 seconds. 
Other detectors are electro-mechanical ionization chamber detectors which 
create an ion density distribution, corresponding to the x-ray shadow 
image, on an insulating sheet and then electronically read the image after 
the x-ray exposure has ceased. 
Three embodiments of this latter detector are described which have the 
image read-out means located within the detector. The second is a simple 
variation of the first, having two insulating sheets and two read-out 
structures so that contrast information may be obtained by taking 
sequential images at different photon energies. In the third embodiment, 
the insulating sheet is provided in belt form. The belt is driven past the 
read-out structures to simplify mechanization of the read-out operation. 
Some embodiments of the electro-mechanical detector are portable cassettes 
which are free of external encumbrances or electrical connections. The 
read-out means may be external to and separate from the cassette so that 
multiple cassettes may be employed with a single read-out machine. Such a 
cassette may be exposed in much the same manner as x-ray film cassettes 
currently used.

DETAILED DESCRIPTION OF THE INVENTION 
Improved Method and Apparatus for Obtaining X-ray Shadowgrams 
In its broadest form, the present invention includes the method of 
minimizing patient x-ray exposure when producing a two-dimensional 
shadowgram image by detecting the image in an ionization gas chamber 
detector and matching the x-ray source energy with the ionization gas 
absorption edge so that the x-ray energy level is just above an absorption 
edge of the ionization gas. An example of such a combination is a 
Praesodymium anode and Xenon detector gas. The x-rays produced by the 
Praesodymium anode have an energy level just above the k absorption edge 
of the Xenon gas. Another example of such a gas/anode combination is 
Krypton gas with a Yttrium anode x-ray tube. Again, the Yttrium anode 
produces fluorescent k x-ray emissions with energies just above the k 
x-ray absorption edge of the Krypton detector gas. 
The methods of the present invention need not be limited to matched 
combinations of x-ray sources and elemental gases. Molecular gases may 
also be useful. Neutron counting using Boron trifluoride is an established 
technique, as is the use of hydrocarbons, such as isobutane. However, no 
such literature exists for gaseous compounds of high Z materials. Of 
particular interest would be a gaseous compound of Erbium which would be 
the resonant material for Tungsten fluorescent x-ray emission (Tungsten is 
the standard x-ray tube target material). Such a compound must not 
disassociate too easily with x-ray radiation and, more importantly, it 
should not form negative ions which would trap the initial electron and 
prevent multiplication (i.e., act as a quenching gas). Prospective gases 
can be tested for these characteristics by observation of their behavior 
in a proportional counter. 
If two different x-ray sources are provided, successive images at different 
x-ray photon energies can be taken and compared electronically, by 
conventional means, to enhance image contrast or the degree of 
calcification, etc. For maximum efficiency, a second anode source is 
matched with a different cathode gas or different absorption edge of the 
same detector gas. For example, using a Praesodymium anode/Xenon 
combination to produce one image and a Yttrium anode/Krypton combination 
for a second image allows calcium, for which the ratio of absorption at 
the two energies is about 1:80, to be distinguished from carbon, for which 
the absorption ratio is 1:13. This enables image subtraction to be used 
for enhancing or suppressing bone tissue (calcium) relative to the soft 
tissue image (carbon). 
This technique can also be used to enhance the identification of the cause 
of abnormal density changes in the soft tissue. For example, the 
aggregation of calcium in cancerous cell colonies will enable their direct 
differentiation from similar density benign cell tumors. It has been 
reported that the nuclei of breast cancer cells have such a high calcium 
concentration. It may be possible, therefore, to identify aggregates of 
breast cancer cells which are not visible by conventional mamography or 
palpation. 
Although detection efficiency increases with increased concentration of gas 
atoms, it is not necessary to change detector gases between exposures when 
employing the method of comparing images obtained from different energy 
levels. If one is willing to operate the detector at higher pressure or to 
sacrifice some detector efficiency, mixtures of ionizing gases may be 
used. Alternatively, two absorption edges of a single gas, e.g. the k and 
1 edges of Xenon, may be used in some applications. 
When using mixed gases, the respective concentrations can be adjustd so 
that their absorption is approximately equal at the two x-ray energies of 
the target gases. In the case of Xenon and Krypton, the Krypton 
concentration need only be about 10% of the total gas concentration. 
To change x-ray sources, it is contemplated that a x-ray tube would be 
provided with a turret target having a mechanical rotation device to 
facilitate rapid charging of x-ray sources. Other means for changing 
energies may also be employed including demountable tubes or a plurality 
of different x-ray tubes which may be successively directed at the 
subject. 
It is most preferred that the method of the present invention be employed 
with one of the ionization detectors described hereinafter. In different 
ways, each of these detectors permits an electronic image to be obtained 
while exposing the patient to the x-ray source for a minimal length of 
time. 
Multi-Wire Proportional Counter with Ultrafast Electronic Read-out 
Two dimensional x-ray detectors of the proportional counter type have, in 
the past, used either a continuous position sensing element (for example, 
a long wire) or a two-dimensional matrix. Such counters are economical to 
realize, but have the basic defect that only one measurement can be made 
within the counter resolving time over the whole area of the counter. This 
restriction makes its application to medical radiography impractical as 
the needed exposure times to produce an x-ray image are unreasonably long. 
A prior art two-dimensional type matrix type counter is shown in FIG. 1. An 
ionization chamber is constructed using a frame of insulating material 10 
across which is strung a set of closely spaced fine wires 12. This chamber 
is closed and made gas tight by two plates 14 and 16 made of insulating 
material and carrying on their inner side a closely spaced array of 
conducting strips 18. One of these plates, 16, is made thin in order to 
permit x-rays (originating from a source shown generally as box 17) to 
enter the chamber. In operation, this counter is filled with gas, usually 
at high pressure, and a positive potential applied to wires 12. 
Incoming x-rays cause a sequence of events to occur, as is shown in FIGS. 2 
and 3. Incoming x-ray g interacts at point x with an atom of the counter 
gas filling, releasing a free electron e and possible a further photon h 
of reduced energy. The free electron is attracted to the nearest wire 12a 
by the positive potential applied and on its way, in the vicinity of the 
wire, creates a shower of further electrons e' by collision ionization 
processes. The electrical outcome of this process is a pulse of current on 
wire 12a and on the metal strip 18 opposite the location of the event. 
These two pulses provide the electronic information of the X, Y location 
of the x-ray event, and can then be processed by standard digital 
electronics. 
FIG. 4 shows the inventive multi-wire detector used to record x-ray 
information in this application. As before, in an insulating frame 20 a 
set of fine (25 micron) conductors 22 are strung at 1 mm intervals. These 
spacings of the closely spaced conductors are intended to be illustrative 
of the best mode known to the inventors, for example, a 2 mm spacing could 
also provide suitable results. In this case, however, the "wire" is made 
of a highly resistive material, namely, a carbon film deposited on a 
quartz core. Electrically completing the counter are two sheets of 
aluminized Mylar (a polyester film marketed by DuPont) 24, 26 which are 
assembled with their conducting sides toward the wires and mechanical end 
plates 28 and 30 which render the chamber gas-tight. Plate 28 is made thin 
to allow the unrestricted entry of x-rays, while plate 30 is thick to 
absorb x-rays which have failed to interact in the counter. Holes 32 admit 
gas to the counter, in this case Xenon, at a pressure of 3 atmospheres 
together with a 1 percent addition of "Freon 13B1." 
The operatin of this chamber is identical to that of the prior art in that 
an incoming x-ray produces an electron e which flows at a given location 
to the nearest wire, being repelled from the Mylar sheets 24, 26 by virtue 
of a negative potential placed on them. As before, close to the wire 22a 
the electron generates a cloud of electrons e' and ions due to collision 
ionization. This process is effective instantaneously, taking 10-11 
seconds or less, and results electrically in a charge being placed 
instantaneously at a given point on the wires 22a. 
This charge is not, however, able to flow away instantaneously to the wire 
ends, as the wire resistance, together with the capacitance between the 
wire and the aluminized Mylar plates, forms a distributed RC time 
constant. The RC product will be proportional to the distance of the event 
from each end of the wire. Determination of those time constants will, 
therefore, determine one coordinate of the event, the other being 
determined by the physical location of the wire. 
It should be noted that, providing independent event processing is 
available for each wire, the data acquisition rate is now determined by 
the time to process N events simultaneously, where N is the number of 
wires in the counter (in the first realization 256). Assuming each 
position on the wire must process an average of 500 events and, for 
symmetry, the wire is divided into 256 distinct cells, then the 
acquisition time is 500.times.256.times.p, where p is the event processing 
time. In this realization, p is in the order of one microsecond, making 
the acquisition time for an x-ray image approximately 0.125 seconds. 
The elctronics affecting the acquisition of position information is shown 
in FIG. 6. Two high speed operational amplifiers operating in inverting 
mode (A.sub.1 A.sub.2) present to the wire W virtual grounds at their 
inputs. As charge flows from wire W into A.sub.1 and A.sub.2, a potential 
pulse is developed at their outputs proportional to the magnitude of the 
incoming charge flow, the proportionality constant being determined by 
R.sub.1, R.sub.2. This signal is transferred directly and delayed by delay 
lines D.sub.1 D.sub.2 to the inputs of differential comparators A.sub.3 
A.sub.4. When the potential from the amplifiers A.sub.1 A.sub.2 is 
increasing, the direct signal to the comparator will be larger (more 
positive) than the delayed signal. When the majority of the charge has 
been collected, the amplifier signal will decrease, and the delayed signal 
will be larger than the direct signal. The comparator will thus become 
true for the time taken for the amplifier signal to reach its peak value 
and start to decline again, thus measuring the "rise time" of the signal 
at each end of the wire. 
These signals are converted to digital information by an 8-bit counter 
C.sub.1, C.sub.2 which is clocked by an external 325 MHz signal. When 
comparator A.sub.3 has gone true and returned to false, D type flip flop 
F.sub.1 will be set. The NAND gate G.sub.1 will, therefore, be satisfied, 
its enable will be low and the counter will start counting. Similarly, 
when A.sub.4 comparator goes through a false true false cycle, D type flip 
flop F.sub.2 will set, thus terminating the count. Delay line D.sub.3 is 
inserted in the signal from A.sub.4 to the flip flop so that the operation 
of the flip flop is delayed by 128 clock cycles. This enables the counter 
to work in the positive integer mode. When flip flop F.sub.2 is set, the 
AND gate G.sub.2 is satisfied and the signal STORE becomes true. At this 
point, the counter information is transferred through level translators 
T.sub.1, T.sub.2, thus releasing the analysis for another event. 
The latched information from T.sub.1 T.sub.2 is then processed by normal 
digital techniques (not shown). Associated with each wire is memory of 
capacity 256 words of 12 bits. Each STORE request results in a read, add 
one, replace (RAO) cycle being executed at the address determined by the 
latched values of A.sub.0 through A.sub.7. 
This memory which totals 64 K words is also organized on normal data bus 
techniques to be part of the memory space of the associated computer and 
also to refresh the memory of the video display. It should be noted that 
each wire of the counter corresponds to one line of information of the 
video display, hence a minimal processing is needed to initially display 
the acquired image. 
Components which may be used in the circuitry of FIG. 6 are as follows: 
A.sub.1, A.sub.2 Fairchild A 715 or equivalent (amplifier) 
A.sub.3, A.sub.4 Motorola MC 1650 or equivalent (comparator) 
D.sub.1, D.sub.2 Bel Fuse 100 100 or equivalent (delay line) 
C.sub.1 Motorola MC 1654 or equivalent (4-bit binary counter) 
C.sub.2 Motorola MC 10136 or equivalent (4-bit binary counter) 
As a total of 256.times.256.times.500=32.times.10.sup.7 photons are 
required to form an image in the embodiment discussed above, the total 
photon flux into the counter must be 2.56.times.10.sup.9 photons per 
second for 100 percent counter efficiency. As the calculated counter 
efficiency is 5 percent, the required flux is 5.12.times.10.sup.10 photons 
per second. The x-ray tube intensity requirements are, therefore, minimal. 
A conventional pierce electron gun system using an air cooled anode may be 
employed. The preferred anode materials are, however, unique being a 
pellet of Praesodymium or Yttrium imbedded in a carbon block. These 
materials are chosen as they produce fluorescent k x-rays of energy just 
above the required k absorption edge energy of Xenon and Krypton, 
respectively, the preferred chamber gas fillings. Usual filtering is 
employed to minimize the white background radiation. 
As previously described, the multi-wire detector of the present invention 
may be used to obtain contrasting images by making successive exposures 
with different source/detector gas combinations. To do so, however, will 
normally require doubling of the initial memory capability because the 
second exposure will be most advantageously made before the data from the 
initial exposure can be processed into a composite image. 
ELECTRO-MECHANICAL X-RAY IMAGE CHAMBER 
The ionization chamber used in this embodiment is shown in FIG. 7. It 
consists of a frame 40 supporting taut closely strung wires 42. Wires 42 
are typically 2 mil gold plated Tungsten strung at 10 mil intervals. On 
one side of this frame is a sheet of metal backed insulating material 44 
which is typically copper-backed Mylar. Sheet 44 is separated from the 
wires by a small space, typically 0.5 inches. The whole assembly is 
mounted in a gas-tight container comprising rigid back plate 50, thin 
window 52 and gas retaining enclosing walls (not shown). The wire 
frame/insulating sheet assembly must remain in mechanical alignment 
without distortion during pressurization of the chamber and, for this 
reason, this assembly is not attached rigidly to the container. 
Within the container and resting normally below the wire frame/insulating 
sheet assembly is a bar 56 mounted on guides 58 and 60, as shown in FIG. 
8. Bar 56 may be mechanically moved along guides 58 and 60 by means of 
drive cable 62. Although not shown in FIG. 7, guides 58 and 60 and drive 
cable 62 extend upwardly between the wire frame 40 and plate 44. The 
thickness of bar 56 is such that it will pass between the wires and the 
insulating plate 44. 
The ionization chamber is filled with a suitable gas (typically Xenon) 
under pressure, to which may be added a stabilizing agent, such as Freon 
13B1 (the combination being known in the literature as "magic gas"). The 
choice of front window thickness and gas pressure is chosen in order to 
minimize the fraction of x-ray photons absorbed in the window and to 
maximize the fraction absorbed in the Xenon gas filling. 
In recording the x-rays, a high positive potential (typically plus 2000 V) 
is applied to the wires 42 with respect to the conductive rear surface of 
plate 44. In this embodiment, the wires are not insulated from each other 
and are, therefore, energized together. When x-ray photons are passed 
through window 52 into the detector chamber, ions and electrons are formed 
as previously described. The electrons are attracted to the wire, while 
the positive gas ions (Xenon) are attracted to the plate. As before, in 
the vicinity of the wire the high local electric field causes the incoming 
electrons to produce more electron ion pairs by collision with the gas 
atoms. Typcially, each arriving electron generates 1-10,000 further pairs. 
The positive gas ions are repelled from the wire to the plate where they 
come to rest on the surface of the insulating layer of plate 44. Due to 
the insulating properties of this layer, these ions are immobilized at 
their landing site and, therefore, remain as a density distribution which 
reflects the x-ray intensity distribution, i.e., the x-ray image desired. 
The ion density distribution is read out by means of bar 56. On the face of 
bar 56 is a close packed line of plates 66, each plate being insulated 
from its neighbors. Each plate, therefore, interrogates one line element 
of the insulated sheet 44. In a typical embodiment, each plate 66 is about 
25 mils square and is held by bar 56 about 10 mils from the insulator 
surface of plate 44. 
Insulated plates 66 are each connected on the interior side, that is, the 
side remote from the insulating surface of sheet 44, to a current to 
voltage amplifier. Alternatively, the interior sides of plates 66 form a 
set of contacts which connect to a group of current to voltage amplifiers 
which are guided down the bar. In principle, and in fact, this operation 
is similar to the beam and carriage operation of an X-Y recorder. The 
output of these amplifiers is transferred by flexible leads to connections 
passing through the pressure vessel. 
A drive motor (not shown), associated with drive cable 62, causes the bar 
to traverse the insulator surface. 
In order to sense the vertical position of the bar, a displacement 
transducer (typically a linear potentiometer) is mounted along side bar 
guide 60. 
Underneath bar 56 a slit cylindrical tube 70, containing a sharp-edged 
electrode 74, is mounted. The function of this device is to establish a 
uniform charge on the insulating surface of sheet 44 prior to x-ray 
exposure. 
To read out the x-ray image created by the ion density distribution on 
insulating sheet 44, the high potential on wires 42 is removed and bar 56 
is traversed at constant speed over the insulting surface. As the sensing 
capacitance plates 66 scan the surface of sheet 44, displacement currents 
flow as the charge shares between the capacitance formed by the sheet 44 
surface to its metallic backing and the capacitance formed by the sheet 44 
surface to the sensing electrode or plates 66. These currents, which are 
the differential of the charge pattern of the plate, are converted to 
potentials and led out of the chamber. 
In the most luxurious realization of this detector, each pick-up plate 66 
has its own amplifier, and an on-bar multiplexer enables all line images 
to be read out in one pass. In a more economical realization, a group of 
amplifiers is mechanically switched to a group of pick-up plates, and 
several passes are made over the insulating surface. These details, 
however, merely affect the speed of the read-out. 
When the read-out is finished, a high potential is applied from a constant 
current supply to the sharpedged electrode 74, establishing a steady 
corona discharge between it and the surrounding tube 70. This discharge 
causes a conducting plasma to protrude from the tube slit 78 and contact 
the insulating surface of sheet 44. If this discharge is now traversed 
over the insulating layer of sheet 44 by moving bar 66, ions or electrons 
will land on it preferentially until the insulator surface is at plasma 
potential. In this manner, a uniform nearly zero charge is deposited on 
the insulator before commencing the next x-ray exposure. 
In practice, the insulating sheet 44 may not be discharged evenly, and the 
gains and offsets of the amplifiers for the individual plates 65 may not 
be matched. This is overcome by reading first a discharged surface and 
then a surface after a uniform x-ray exposure. The value so obtained need 
not be retained by the associated computer and memory, but can be written 
out onto mass storage for later use as normalizing parameters. In this 
matter, systematic noise can be removed by renormalization of the data 
after recording. 
The correct "exposure" can be obtained by integrating the displacement 
current to the metallic backing of the sheet 44 during the time the x-ray 
tube is emitting. This feature can be used to automatically control the 
x-ray flux to ensure an acceptabe image with a minimal patient dose. 
For a 0.062 inch Mylar sheet 44, an ion pair multiplication ratio of 10,000 
at the wire will ensure that 500 detected photons will produce a surface 
potential of 60 volts. At this potential, the uncertainty fluctuation 
noise of the photons (about 500 to the 1/2 power) is greater than the 
electrical current noise of normal 741 series operational amplifiers when 
operated at a band width determined as follows. Assuming an 11.times.16 
inch insulating sheet read in a 16-inch direction and with maximum 
television resolution, i.e., 512 elements in the 11-inch and 768 in the 
16-inch direction, the reading time will be 
512.times.768.times.100.times.10.sup.-6 =40 seconds, if a normal 8-bit 
successive approximation ADC of 100 microseconds conversion time is used 
and all pick-up plates are read at one passage. During this time, any 
individual line is read 768 times, i.e., at 20 times a second. A band 
width of 100 Hz is, therefore, adequate to ensure settling to 2 percent 
accuracy between the readings. The reading speed limit is probably in the 
order of one second, assuming a typical slew rate for the bar of 20 inches 
per second. This would involve a 2.5 microsecond conversion rate and a 
current to voltage amplifier band width of about 2000 Hz, both of which 
are easily within the reach of modest electronics. 
If a contrast image, obtained by comparing two images obtained from x-rays 
of different energy levels, is desired, the chamber of this embodiment may 
be constructed using two insulating sheet read-out bar assemblies. The 
second metallic backed insulating sheet 80 shown in FIG. 10 is placed 
between the chamber window 52' and the wire frame 40. Likewise, a second 
read-out beam and guide bar assembly, as shown in FIG. 8, is placed 
between sheet 80 and frame 40'. To expose sheet 44', sheet 44' is 
grounded, while sheet 80 is maintained at a potential of the wires 42'. 
The positive ions formed in the vicinity of wires 42', when the x-rays 
enter the chamber, are attracted to sheet 44' and repelled from sheet 80. 
To obtain an image on sheet 80, the potential difference between the 
metallized surfaces of sheets 44' and 80' is reversed. That is, the 
metallic surface of sheet 80 is grounded, whereas the metallic surface of 
sheet 44' is maintained at the wire potential. 
This double insulating sheet/read-out bar structure permits recording of 
two x-ray images, closely spaced in time, for subsequent processing for 
contrast information as previously described. 
Portable Cassette Form of Electro-Mechanical X-ray Imaging Chamber 
The portable cassette described in this section is an alternate embodiment 
of the electro-mechanical image chamber just described. This embodiment is 
a transportable cassette which can replace film x-rays almost completely 
as the recording device has no electrical connections or other 
encumbrances. It may be extended to dental and industrial applications. 
The image recording chamber is separate from the image reading machine, 
thus permitting a reading machine to be shared over multiple cassettes. 
The basic construction of the cassette is shown in FIG. 11. It consists of 
an enclosed gas-tight metal box having a metal front window 84, a frame 86 
comprising side walls of the chamber, a grid or harp of fine wires 88 
carried on the frame and an insulating rear window 90 transparent to 
ultraviolet light. The wire grid is insulated from the box and provided 
for the connection to the outside. In normal use, a metallic back cover 92 
is secured to the back of the box in order to provide protection for the 
rear window and to insure that the box is completely electrically 
conducting on the outer surface. The gas in this chamber, however, is 
retained by the transparent window, not the back cover. A high quality 
capacitor 94 (polystyrene or similar dielectric), shown schematically in 
FIG. 12, is connected between the wire grid and the outer ground potential 
surface of the box. 
To operate this chamber, the back cover is attached by any suitable means, 
and a high voltage power supply is connected through a high impedance, 
such as 1 Megohm resistor 95 to the wire grid, and the high quality 
capacitor charged. The high impedance provides both safety for the 
operator and prevents large currents from flowing. The power supply is 
then disconnected. The cassette is then ready for use. A polystyrene 
capacitor has a leakage time of approximately 50 years, so the wire grid 
will remain charged to operating potential for an extended period. 
The cassette is exposed in the same manner as a film x-ray cassette, i.e., 
placed behind the subject being x-rayed. The electrical function is the 
same as described previously, the x-rays producing electrons, 
multiplication taking place near to the wires and positively charged ions 
accumulating on the inside surface of the transparent insulating sheet 90 
where they are immobilized. Because the front window 84 is maintained at 
the same potential, i.e., ground potential, as the insulating sheet 90, it 
is preferred that the wire grid be as close to window 84 as possible 
without causing arcing between the grid and window 84. The potential on 
the grid typically will be on the order of 2 kilovolts. Moving the wire 
grid closer to front window 84 than to rear window 90 results in most 
photon-gas atom collisions occurring behind the wire mesh so that the 
positive ions resulting therefrom migrate toward insulating sheet 90, 
rather than toward metal front window 84. 
The cassette is read out by removing the back cover 92 and placing the 
cassette with the exposed insulating plate over read-out bar equipment 
similar to that shown in FIG. 8. The read-out equipment contains a movable 
read-out bar 56' essentially identical to bar 56 described in FIG. 8. 
However, this equipment does not have the corona discharge tube 70, shown 
in FIG. 8. Bar 56' traverses in close proximity to the outer surface of 
the insulating sheet and reads the displacement currents flowing to a row 
of sensors in a manner similar to that described for the 
electro-mechanical chamber. 
Once read-out is complete, the high voltage on the wire grid is removed by 
discharging the storage capacitor through the high impedance, and 
ultraviolet light is projected through the transparent insulating window 
90. This light produces photoelectrons which are attracted to the 
positively charged ions immobilized by the inner surface of window 90 and 
continues to neutralize them until the inner surface is uniformly charged 
slightly negative, thus preventing further electrons from landing. 
The removable back is then reattached, the wire grid recharged and the 
cassette is again ready for use. 
A suitable material for the insulating is quartz, as this is both of high 
resistivity and transparent to ultraviolet light. However, materials 
satisfying both criteria may also be used. Since the read-out equipment 
must sense the capacitance differentials, through the thickness of window 
90, this window must be maintained at a minimum thickness, preferably 
about 10 mils or less. 
The wire harp is preferably supported from the side of the enclosure by 
polystyrene insulators, not shown, as it is necessary to minimize leakage. 
At present, the absence of a backing plate in front of insulating sheet 90, 
i.e., inside the chamber, reduces the available signal currents by an 
estimated factor of 10. Because of this, it may be necessary to introduce 
a fine mesh secondary or screen grid 96 immediately in front of the 
insulating plate. If this grid is introduced, it should be maintained 
slightly positive with respect to the insulating sheet, e.g. about 100 
volts, so that the positively charged gas ions will migrate through the 
mesh to insulating sheet 90. 
Whereas the schematic representation of capacitor C.sub.1 and resistor 
R.sub.11 shown in FIG. 12 shows these components on the exterior of the 
cassette chamber, it is contemplated that, in practice, these elements 
will be imbedded in the frame 86 of the cassette. A recessed access pot 97 
can be used to provide means for making the necessary electrical 
connection to charge and discharge the capacitor. 
Another access port 98 can be used to flush and fill the chamber with 
ionizing gas. 
FIG. 13 shows a top cut away view of another embodiment of the detector 
which may be more easily read out than the detectors shown in FIGS. 7, 10 
and 11. In this embodiment of the invention, the transparent insulating 
window is formed into a continuous belt 90' supported by rollers 98 and 
100 at both sides of the window in frame 86. Belt 90' is on opposite sides 
of and surrounds the grid or harp of wires 88. the outside surface of belt 
90' is divided into two conductive portions positioned with one on each 
side of grid 88 when the belt is positioned to receive an x-ray 
shadowgram. Rollers 98 and 100 have a conductive rubber surface which 
discharges any portion of the insulating surface which discharges any 
portion of the insulating surface of belt 90' brought into direct contact 
therewith. 
The improved embodiment of FIG. 13 is particularly suited to take two 
images in close time sequence. This feature is important when it is 
desired to take identical views of a subject using different intensities 
to differentiate between bone and dye or calcium. 
As was the case with the detector embodiment in FIGS. 11 and 12, the wire 
grid 88 is insulated from the remainder of the box and may be held at high 
potential by being connected to one of the conductive portions of belt 90' 
with a high quality capacitor 94 which has been charged through a high 
impedance resistor 95. The other conductive portion is connected to the 
box reference potential to minimize interference with the ions in the 
detector. The detector is then exposed by being placed behind the subject 
being x-rayed, and the inside dielectric surface of the lower portion of 
belt 90' receives the stored charge in accordance with the received 
radiation intensity to define an image. A second image can be placed on 
the outer insulating surface of the belt by establishing the grid 
potential with respect to the other conductive backing and exposing the 
detector to the x-ray source. 
In this embodiment, the read-out of the image is accomplished by rotating 
the drums 98 and 100 in a clockwise manner, as viewed in FIG. 14, to move 
the inner surfaces of the dielectric sheet 90' past the read-out heads 
56A" and 56B" which are located in the vicinity of the roller 98. The 
read-outs 56A" and 56B" are the equivalent of the read-outs 56 and 56' in 
FIGS. 8 and 12, but differ from them in that they remain fixed in a 
stationary manner relative to frame 86 during the entire read-out process, 
while the dielectric surfaces of belt 90' are transported past them by 
rotation of rollers 98 and 100. As the portion of the insulating surface 
bearing the deposited charge come into contact with the conductive surface 
of the nearest roller, the surface of the roller returns the dielectric 
surface to a nearly uncharged zero potential condition so that further 
images can be applied to the film as soon as it is read out. 
It can be seen that both the top and bottom inner surfaces of the 
dielectric belt 90' can be exposed at about the same time by selectively 
energizing the appropriate back conductor of the dielectric as desired 
before the read-out is accomplished. The two inner surfaces of the belt 
are read out simultaneously by the detector arrays 56A" and 56B" which are 
mounted on frame 86 in proximity with the inner face of belt 90'. The two 
images are read and the belt is erased by drums 98 and 100 in a 
coordinated one-step operation as the information is stored in the storage 
means for further processing or use. 
Instead of utilizing the secondary grid 96 mounted separately on frame 86 
in front of the dielectric sheet 90, the improved embodiment of FIG. 13 
utilizes a secondary grid 96' which is deposited directly on the inner 
dielectric surface of belt 90'. FIG. 15 shows, in amplified detail, the 
surface of a portion of belt 90' with the conductive elements 96 of the 
secondary grid directly deposited on the inner surface of belt 90' on the 
opposite side of the belt from the conductive back portion 91. In 
operation, the potential between the conductive grid 88 and the conductive 
back plate 91 of belt 90' is approximately 2000 volts with a potential 
maintained between the secondary grid and the back plate of approximately 
100 volts. The seconday grid 96 produces a potential distribution 
indicated by dotted lines 101 in FIG. 14, thus preventing a charge 
build-up on the dielectric layer from repelling incoming charged particles 
and deflecting them off their ballistic path. A charged particle is shown 
on its trajectory 97 in FIG. 14. A deflection due to charge build-up would 
result in an undesirable defocussing of the image. The secondary grid also 
acts to provide a drain for excess charge build-up on the dielectric 
surface in the event of high intensity flow of charged particles. 
Excessive charge build-up can cause blooming of the x-ray image and a 
resultant loss of detail. 
The net effect of applying the secondary grid 96 to the surface of the 
dielectric medium is to linearize the exposure versus charge relationship 
until the surface potential is equal to the voltage maintained on the 
secondary grid, at which point the charge versus exposure characteristic 
becomes flattened with virtually no further increase in the charge 
deposited on the dielectric after this threshold is reached. This prevents 
the common and undesirable blooming effects in prior art x-ray devices 
where high intensity radiation passing around a small object being 
x-rayed, for example, "blooms" and obliterates all detail in the perimeter 
of the object being illuminated. In prior art systems, it was necessary to 
use water bags and other techniques to reduce the difference in 
transmissibility to x-ray radiation between the object being illuminated 
and the background. The improved linearity and sharp cut-off of the 
exposure characteristics avoids the need of taking precautions to avoid 
large density differences between the object to be x-rayed and the 
background.