Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-06-25
2003-08-19
Gagliardi, Albert (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S366000, C250S369000, C250S370140
Reexamination Certificate
active
06608311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to image detectors of the type using arrays of photosensitive sensors made from semiconductor materials. The invention applies in a particularly beneficial manner (but not exclusively) in the case of the detection of radiological images. Its aim is to facilitate the manufacture and to reduce the costs thereof, as well as to improve the stability of the measurements and the quality of the images obtained, in the case in particular of image detectors using a large number of photosensitive sensors.
2. Discussion of the Background
It is common practice to use arrays of sensors or photosensitive points made of semiconductor materials, in image acquisition techniques. The photosensitive points are made on semiconductor material, silicon for example, and they very often each comprise at least one photodiode. The photodiodes are sensitive in a band of wavelengths generally corresponding to visible radiation or near-visible radiation.
Depending on the applications for which they are intended, the photosensitive arrays may comprise highly variable numbers of pixels, that is to say of photosensitive points, from a few to several tens of thousands (and possibly as many as several millions of photosensitive points for dimensions of the order of for example 50 cm×50 cm).
One of the benefits of the images obtained with the aid of photosensitive points made of a semiconductor material lies in the fact that these images can be digitized, offering as advantages in particular ease of processing and of storage of the image. Of course, the advantages connected with images of digital type are just as important in the field of radiological image detection, and particularly in that of medical imaging using X-rays.
To apply photosensitive arrays such as described above to the detection of radiological images, it is well known to interpose a screen made of a scintillator substance between the X-ray radiation and each of the photosensitive points of the array. The scintillator substance is chosen so as to convert the X-ray radiation into light radiation, in the wavelength band to which the photosensitive points are sensitive.
According to one of the common operating modes, each photosensitive point comprises an element which functions as a switch, mounted in series with the photodiode. The line-by-line control of each of the switch elements (control effected after an exposure phase or integration phase, in the course of which the photodiode is exposed to a measurement light signal, that is to say to a signal corresponding to an image to be detected), makes it possible to transfer to a column electric charges produced by the corresponding photodiode during the integration phase; it should be noted that during the integration phase, the photodiode is fairly strongly reverse biased, so that it forms a capacitance in which the charges generated during this phase are stored.
Amplifiers and multiplexing circuits then make it possible to transfer the charges from the various photosensitive points to a readout and data processing circuit. An array each of whose photosensitive points comprises a photodiode cooperating with a switch element, as indicated above, is described together:with its operating mode in a French patent application No. 86.14.058 (publication No. 2,605,166).
In the operating example mentioned above, the charges are stored, as and when they are generated, in the zone of the photodiode by which they were produced. According to another fairly common operating mode, both as regards X-ray imaging and imaging based on visible radiation, the charges are transferred as soon as they are produced, so as to be stored outside the zone of the photodiode, for example at the level of an amplifier catering for an integrator function. This configuration, which may be encountered for example in X-ray CT scanners, poses in particular various problems which the invention aims to solve. However, it should nevertheless be noted that the solutions proposed by the invention find application also in the operating modes mentioned above.
CT scanners are X-ray apparatus which generally use a single source of X-rays and a detector assembly which may comprise a large number of photosensitive points. The assembly formed by the source and the detector assembly can rotate and/or move in translation with respect to the body of a patient, with a view to forming the internal image of a slice of the patient. Such an apparatus is described for example in the document U.S. Pat. No. 5 592 523.
FIG. 1
diagrammatically and in a simplified manner represents some of the essential elements of a CT scanner. The CT scanner comprises a source
1
producing X-ray radiation
2
, and a detector assembly
4
. The X-ray radiation
2
irradiates a body
3
of a patient interposed between the source
1
and the detector device
4
. The CT scanner rotates about an axis of rotation represented by a point
6
. It may furthermore comprise an auxiliary detector
5
, situated outside the X-ray field masked by the body
3
. The detector device
4
has a length which extends according to an arc of a circle. The detector assembly
4
comprises a multitude of photosensitive detection points arranged along the length and the width of the detector device
4
. The photosensitive points may be grouped together into detection modules MD.
A CT scanner can comprise for example up to several tens of detection modules, arranged side by side along the length of the detector device
4
. Each detection module MD comprises a scintillator material
71
superimposed on a photosensitive array
72
. The scintillator material has the function of converting the X-ray radiation into light radiation to which the photosensitive arrays are sensitive; the scintillator material is therefore situated on the source
1
side.
A photosensitive array
72
can comprise a grid of photodiodes (not represented in FIG.
1
), arranged along for example 32 rows and 16 columns, i.e. 512 diodes in number. In certain applications such as those relating to CT scanners, each photodiode is linked to a data acquisition amplifier which has the function in particular of receiving and integrating the charges as and when they are produced by the photodiode; this integration of the charges achieves in a certain manner the storage of the charges which, in the other operating mode mentioned previously, is performed at the level of the photodiode itself, whereas in the present case this storage is performed outside the photosensitive array.
In fact in this configuration the amplifiers which receive the signals from the photodiodes are not made on the same substrate as these latter, these amplifiers may be located in proximity to the photosensitive array.
FIG. 2
diagrammatically represents a photosensitive array
7
similar to an array
72
of
FIG. 1
, and each photosensitive point of which is formed by a photodiode. To simplify the description only four photodiodes Dp
1
to Dp
4
are represented. In the example, they are arranged in two rows L
1
, L
2
, and two columns cl
1
, cl
2
. The photodiodes Dp
1
to Dp
4
are made on a substrate Sb
1
and, as a constructional example, they are all linked electrically by one and the same end, their anode in the non-limiting example represented, to the said substrate and to ground.
The other end of each of the photodiodes, i.e. the cathode, is connected to an individual conductor
19
, by way of which each of these cathodes is connected to a first input E
1
of an amplifier a
1
to a
4
specific to each photodiode.
The amplifiers a
1
to a
4
are commonly constructed with the aid of operational amplifiers; however, it is also known practise to embody them as discrete components, with one or more transistors. These amplifiers intended for acquiring the data delivered by the photodiodes (in the form of electric charges), must meet very severe constraints: low noise, large dynamic range and high stability for the reproduction of the measurements.
In the conventional example repr
Munier Bernard
Roziere Guy
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