X-ray detector for automatic exposure control of an imaging apparatus

An x-ray detector for an automatic exposure control system has a substrate of carbon composite material with a first layer of conductive material on a major surface of the substrate. A second layer of homogeneous semiconductive material, such as CdTe, CdZnTe or amorphous silicon, is deposited on the first layer and has an electrical characteristic, such as conductivity, that varies in response to impingement of x-rays. A third layer of conductive material is formed on the surface of the semiconductor layer and is divided into a plurality of electrode elements which define a plurality of regions in the layer of semiconductive material. By sensing the conductivity between the first layer and each of the electrode elements, the intensity of x-rays striking the different regions can be measured.

BACKGROUND OF THE INVENTION 
The present invention relates to automatic exposure control systems for 
x-ray imaging apparatus; and more particularly to x-ray detectors utilized 
in such exposure control systems. 
Conventional fluoroscopic x-ray imaging equipment include a source for 
projecting a beam of x-rays through an object being imaged, such as a 
medical patient. The portion of the beam which passes through the patient 
impinges upon an x-ray detector which converts the x-rays attenuated by 
the patient into photons which then are converted into an electric image 
signal. The x-ray detector may comprise an image intensifier tube and a 
video camera, or a combination of a scintillator in front of a 
two-dimensional photodetector array. The electrical image signal from 
either type of detector is processed to display an image of the patient on 
a video monitor. 
The image signal also is applied to the input of a feedback loop which 
responds to the level of the image signal by producing a control signal 
which regulates the x-ray exposure and thus brightness of the image on the 
video monitor. An example of this type of feedback control is described in 
U.S. Pat. No. 4,573,183. The control signal regulates the biasing of the 
x-ray tube and the gain of the video camera. This feedback control loop 
ensures that the image produced in the video monitor has sufficient 
brightness and contrast for proper viewing by an x-ray technician or 
physician. 
SUMMARY OF THE INVENTION 
A general object of the present invention is to provide a separate x-ray 
detector for the automatic exposure control which is not part of the image 
detector utilized to produce the image displayed on the video monitor. 
Another object of the present invention is to provide an x-ray detector 
which can be placed in front of or behind the image detector with respect 
to the source of x-rays. 
A further object of the present invention is to provide such a detector 
which has minimal x-ray attenuation variations so as to minimize inducing 
visible image artifacts. 
These and other objects are fulfilled by an x-ray detector for an automatic 
exposure control system that comprises a substrate having a major surface 
on which a first electrode layer is formed. A layer of semiconductive 
material is deposited on the first electrode and has an electrical 
characteristic which varies in response to impingement of x-rays. In the 
preferred embodiment, this latter layer comprises homogeneous 
semiconductive material, such as CdTe, CdZnTe or amorphous silicon. A 
second electrode layer is formed on the layer of semiconductor material 
remote from the first electrode layer. One of the first and second 
electrode layers is divided into a plurality of conductive elements 
thereby defining a plurality of regions in the layer of semiconductive 
material adjacent to each of the plurality of conductive elements. 
The semiconductive layer acts as a solid state ion chamber and can be 
operated in a photovoltaic mode in which an electrical current is produced 
in this layer in response to x-ray bombardment or operated in a 
photoconductivity mode in which the conductivity of the semiconductive 
layer is altered by the x-rays. In either mode, the intensity of x-rays 
striking the various regions can be measured by sensing the electric 
current flow between each of the conductive elements and the other 
electrode layer.

DETAILED DESCRIPTION OF THE INVENTION 
With initial reference to FIG. 1, an x-ray imaging system 14 includes an 
x-ray tube 15 which emits an x-ray beam 17 when excited by power supply 
16. As illustrated, the x-ray beam 17 is directed through a patient 18 
lying on an x-ray transmissive table 20 and impinge upon a detector 
assembly designated 22. The detector assembly 22 comprises an automatic 
exposure control (AEC) detector 23 and an imaging device 25 formed in this 
embodiments by a scintillator 24 in front of a two-dimensional 
photodetector array 26. The scintillator 24 converts the x-ray protons to 
lower energy photons in the visible spectrum and the photodetector array 
26, contiguous with the scintillator, converts the light photons into 
electrical signals. The scintillator 24 and photodetector array 26 are 
well-known components of conventional x-ray detector assemblies used in 
previous x-ray imaging systems. Alternatively, the AEC detector 23 can be 
located on the remote side of the detector assembly 22 from the x-ray tube 
15. The AEC detector 23 may be used with other types of imaging devices 
25, such as an image intensifier with a video camera, or even x-ray film. 
A detector controller 27 contains electronics for operating the 
photodetector array 26 acquire an image by reading a signal from each 
photodetector element in the two-dimensional array. The output signal from 
the photodetector array 26 is coupled to an image processor 28 that 
includes circuitry for processing and enhancing the x-ray image signal. 
The processed image then is displayed on a video monitor 32 and may be 
archived in an image Storage device 30. 
The overall operation of the x-ray imaging system 14 is governed by a 
system controller 36 which receives commands from the operator via an 
operator panel 38. 
The x-ray exposure is controlled automatically by a feedback loop that 
includes the AEC x-ray detector 23 and an exposure control circuit 34. The 
AEC x-ray detector 23 produces a plurality of electrical signals 
corresponding to the intensity of the x-rays impinging different regions 
of the detector. Those signals are applied to inputs of the exposure 
control circuit 34 along with signals from the system controller 36 
designating the desired x-ray dose selected by the operator. The exposure 
control circuit 34 responds to these input signals by producing a command 
signal for the x-ray tube power supply 16 which defines the bias voltage 
and filament current levels for the x-ray tube 15 to produce the desired 
x-ray dosage. During the x-ray exposure, the exposure control 34 responds 
to the AEC detector signal, which indicates the intensity of the x-rays 
passing through the patient 18, by commanding the power supply 16 to 
increase or decrease the bias voltage and filament current for the x-ray 
tube 15 to achieve an optimal x-ray exposure for a satisfactory image. 
The present invention relates to the unique multiple layer structure of the 
AEC detector 23 shown in FIG. 2. The active layers are formed on a surface 
of substrate 40 which preferably comprises a low density carbon composite 
material containing fibers in a matrix material. Use of such conventional 
carbon composites is particularly desirable when the AEC detector 23 is 
placed in front of the photodetector array 26, as the low density material 
is x-ray transmissive and provides uniform x-ray attenuation. As a 
consequence, the carbon composite does not induce artifacts in the signal 
produced by photodetector array 26 enabling the AEC x-ray detector 23 to 
be placed in front of the scintillator 24 and photodetector array 26. When 
the AEC detector 23 is placed on the remote side of photodetector array 
26, the x-ray transmission characteristics of the substrate 40 are not 
critical and other materials may be employed. 
A uniform first electrode layer 42 is applied over the entire major surface 
on one side of substrate 40 and serves as a high voltage bias electrode. 
The first electrode layer 42 is formed by either metallization or other 
conductive material, such as indium tin oxide, deposited by conventional 
techniques. The first electrode layer 42 is relatively thin so as to 
minimally attenuate x-rays passing therethrough. 
A thin semiconductive layer 44, such as molecular beam epitaxially 
deposited CdTe, CdZnTe, amorphous silicon or other semiconductor material, 
extends over the first electrode layer 42. The semiconductive layer 44 has 
a good response to x-rays, high efficiency in converting x-rays to an 
electrical signal, uniform x-ray transmission and long term x-ray 
stability. The layer 44 of semiconductive material preferably is 
homogeneous, i.e. p-n junctions are not formed. Thus the semiconductive 
material acts as a solid state ionization chamber in which charged ions 
are formed in proportion to the intensity of the x-rays which strike the 
layer. By collecting the ions to form an electrical signal the x-ray 
intensity can be measured. 
On top of the semiconductive layer 44 is a second electrode layer 46 
comprising a plurality of conductive elements 47 and 48 in a pattern which 
enables the sensing of x-rays impinging different regions of the 
semiconductive layer. The second electrode layer 46 is relatively thin so 
as to minimally attenuate x-rays passing therethrough. An exemplary 
pattern of conductive elements 51-66 is shown in FIG. 3. This pattern is a 
four-by-four matrix of rectangular elements on the surface of 
semiconductive layer 44. Four conductive elements 51, 55, 59, and 63 along 
one edge 71 of the AEC detector 23 are connected electrically to 
individual contact pads 68 in a first margin 70 along that edge 71. An 
additional set of four contact pads 72 is formed on the first margin 70 
with conductive stripes extending therefrom between the outer electrode 
elements to interior electrode elements 52, 56, 60 and 64. The opposite 
side 73 of the AEC detector 23 has a second margin 74 with for contact 
pads 76 connected to outer electrode elements 54, 58, 62 and 66 
immediately adjacent the second margin. Another set of four contact pads 
78 is formed within the second margin 74 with separate conductors 
extending therefrom between the outer electrode elements to four interior 
electrode elements 53, 57, 61, and 65. The second margin 74 has an 
additional contact pad 80 which extends through an opening in the 
semiconductive layer 44 to provide electrical contact with the first 
electrode layer 42. The plurality of contact pads 68, 72, 76, 78 and 80 
are connected by individual wires to separate inputs of the exposure 
control 34. 
Although a four-by-four matrix of rectangular electrode elements is 
illustrated in FIG. 3 other geometric matrix patterns and shapes of 
individual electrode elements of the second electrode layer 46 are within 
the purview of the present invention. In addition, the electrode elements 
do not have to be spaced over the entire surface of the substrate, but may 
be positioned periodically on the surface or only at the corners depending 
upon the degree of x-ray sensing desired for the automatic exposure 
control. 
The semiconductive layer 44 can be configured to operate in any common 
solid state mode, such as the photovoltaic mode or the photoconductivity 
mode. The exposure control 34 selectively senses the electrical signal 
produced in the semiconductor material between the first electrode layer 
42 and each of the electrode elements 51-66 of the second electrode layer 
46. In doing so, the exposure control 34 may apply a high voltage to the 
first electrode layer 42 to bias the semiconductive layer 44 for drift and 
collection of x-ray induced charges therein. The collection of charge 
produces an electrical signal between the first and second electrode 
layers 42 and 46 which can be locally sensed by detecting the signal at 
each of the electrode elements 51-66 of the second layer 46. 
Thus the exposure control is able to detect the intensity of x-rays 
striking different portions of the AEC detector 23 and determine from 
those various signal samples, the power supply command to produce the 
desired x-ray dosage for optimal image production. 
The foregoing description is directed primarily to preferred embodiments of 
the invention. Although some attention was given to various alternatives 
within the scope of the invention, it is anticipated that skilled artisans 
will likely realize additional alternatives that are now apparent from the 
disclosure of those embodiments. Accordingly, the scope of the invention 
should be determined from the following claims and not limited by the 
above disclosure.