X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-12-17
2001-09-11
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098800
Reexamination Certificate
active
06289078
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray examination apparatus, including an X-ray source for emitting an X-ray beam, an X-ray detector for detecting an X-ray image and converting it into an optical image, and a video extractor which is coupled to the X-ray detector via optical coupling means, the optical coupling means being provided with an optical pick up for feeding a fraction of the light flux to a photosensor which produces a control signal for adjusting the X-ray flux from the X-ray source.
Such an apparatus is known from the International patent application WO 96/20579-A1. The cited document describes an X-ray examination apparatus with an exposure control circuit which supplies a control signal for adjustment of an X-ray source. In case of strongly overexposed areas within a measuring field selected in an X-ray image, the X-ray flux from the X-ray source is reduced by means of said control circuit. In a specific embodiment the signal applied to the exposure control circuit is obtained by means of an optical pick up and a photodetector. The optical pick up and the photodetector, inserted in the collimated beam of the optical coupling means, form a linearly responsive detection unit which carries out the photosensing for X-ray control purposes. As the incorporated photodetector integrates the detected light flux over a selected measuring field, an average value for the X-ray detector output luminance will be found after amplification of the photodetector current. The problem arising in the control loop for the X-ray source is caused highlights, particularly direct radiation on the X-ray detector. As these highlights change in respect of area and amplitude, the control signal from the measuring field is greatly influenced by peak values that may reach values as much as 100 times larger than the signals of interest. The resultant image will be underexposed and relevant image information may be lost.
Depending on the specific applications, a dose from 1 nGy to 10 &mgr;Gy is applied. As already mentioned, with highlights and direct radiation the signal to be detected may vary with a factor 100, so that the photosensor must be sensitive for a signal amplitude range of 5 decades. The dynamic range of the known linear detection unit is then insufficient for pixelized sensors, e.g. CCDs.
Many proposals have been made to cope with this problem, particularly the application of sensor systems in which anatomically determined measuring fields are selected and which are provided with some intelligence to mitigate the consequences of highlights and black areas. In other proposals the optical image obtained by means of a fraction of the X-ray signal is analyzed; on the basis of these analyses the adverse effects of black and extreme white areas are suppressed, after which the examination is performed with full luminance. All of such systems are based on linear detection on a pixel basis; they all involve black and extreme white exclusion in a more or less intelligent way. The detectors are based on a spatially sampled system, i.e. on the basis of pixels, with CCDs.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an apparatus as described in the opening paragraph which is simpler and less expensive and in which the adverse effects of the occurrence of highlights and particularly direct radiation are strongly reduced, but accurate X-ray measurements are guaranteed nevertheless.
Therefore, the apparatus in accordance with the invention is characterized in that the photosensor is provided with an array of sensor elements, with weighting means for the signals detected in or by each of said pixels, and with means for determining a mean value signal from the detected and weighted signals, thus yielding a control signal which is fed back in order to adjust the X-ray flux from the X-ray source.
In other words, a control signal from the photosensor will be extracted, within a defined measuring field A, which signal, as a consequence of the weighting process, represents a compressed response to highlights or an expanded response to dark image parts. This control signal will be proportional to the mean value of the weighted signals from the individual pixels, while each of the weighted signals can be represented by f(E
ij
), i.e. a non-linear function of a pixel illuminance value E
ij
.
U.S. Pat. No. 4,674,108 discloses an X-ray examination apparatus, particularly for Digital Subtraction Angiography (DSA), wherein, after a video image is obtained, the video signal is amplified logarithmically for more effective use of the subsequent digital-analog converter. A control signal for the X-ray source is obtained from a video extractor.
Each weighting means in a first embodiment supplies a non-linear (sub-linear) amplified signal whereby, particularly, in each pixel respective weighting means are integrated so as to obtain a non-linear output characteristic of said integrated circuit. Therefore, each pixel includes a photodetector element and a non-linear amplifier element. For example, a pixel may be formed by a photodiode with a non-linear amplifier element such as a FET. In an actual embodiment the photosensor consists of a monolithic two-dimensional array of photodiodes and logarithmic amplifier elements, each of which is coupled to a photodiode. The mean value of the output signals of the logarithmic amplifier elements can be represented by the relation
S
=
S
o
N
∑
A
ln
(
E
ij
E
o
)
wherein S
o
is a suitable conversion factor, E
o
a reference illuminance value and N the number of pixels.
The logarithmic amplification will yield ample detector dynamic range. In comparison with the application of linear amplifier elements, the response to highlights and direct radiation is compressed. Upon calibration intended to determine the values of E
o
and S
o
, the detector can measure the encountered illuminance values in an absolute way and in the full range.
The measuring area A can be determined from the image content and hence related to anatomical shapes. In a preferred embodiment, in which the array of pixels is addressable in two dimensions, the possibility is opened for fast sensing of only a few pixels of interest; these pixels are representative of the measuring area A and yield a reliable control signal.
A consequence of the logarithmic output characteristic of the pixels is that the adjustment of the X-ray source is not realized with the same accuracy over the full range of the output signal of the means determining the mean value of the detected and weighted signals. In order to achieve a high and uniform control accuracy, the X-ray examination apparatus is provided with control accuracy enhancing weighting means for converting the mean value signal into a feedback signal for adjusting the X-ray source. More specifically, in the control accuracy enhancing weighting means the mean value signal is processed with a function which is the inverse of the weighting, thus producing a non-linear amplified signal for each sensor element. In the case of a logarithmic pixel output characteristic, the control accuracy enhancing weighting means produce an output signal, which can be represented by the relation
S
c
=
ke
S
S
o
wherein k is a constant. In other words, the latter weighting means yield a linear control signal
S
c
=
const
.
∏
i
,
j
E
ij
N
.
The output signal of each pixel in a further embodiment is applied to a processor in which the weighting and determination of the mean value of the detected signals are realized on the basis of a program. Every suitable weighting function can be easily applied on the basis of a program. For the exclusion of black or very dark and extreme white areas, a weighting “zero” can be assigned to the respective pixels. After weighting and determination of the mean value of the detected signals, the signals may be subjected to further weighting so as to enhance the control accuracy, particularly so as to obtain a substantially linear control characteristic as me
Linders Petrus W. J.
Nederpelt Christianus G. L. M.
Snoeren Rudolph M.
Kiknadze Irakli
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
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