Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2001-12-20
2004-03-09
Fulton, Christopher W. (Department: 2859)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
Reexamination Certificate
active
06703617
ABSTRACT:
This invention relates to an imaging device assembly for imaging radiation and to an imaging system including such an imaging device assembly.
Imaging devices comprising an array of image elements of various types are known.
Charged coupled image sensors (also known as charged coupled devices (CCDs)) form one type of known imaging device. A CCD type device operates in the following way:
1. Charge is accumulated within a depletion region created by an applied voltage. For each pixel (image cell) the depletion region has a potential well shape and constrains electrons under an electrode gate to remain within the semiconductor substrate.
2. Voltage is applied as a pulse to the electrode gates of the CCD device to clock each charge package to an adjacent pixel cell. The charge remains inside the semiconductor substrate and is clocked through, pixel by pixel, to a common output.
During this process, additional charge cannot be accumulated.
Another type of imaging device which is known is a semiconductor pixel detector which comprises a semiconductor substrate with electrodes which apply depletion voltage to each pixel position and define a charge collection volume. Typically, simple buffer circuits read out the electric signals when a photon is photo-absorbed or when ionising radiation crosses the depletion zone of the substrate. Accordingly pixel detectors of this type typically operate in a pulse mode, the numbers of hits being accumulated externally to the imaging device. The buffer circuits can either be on the same substrate (EP-A-0 287 197) as the charge collection volumes, or on a separate substrate (EP-0 571 135) that is mechanically bonded to a substrate having the charge collection volumes in accordance with, for example, the well known bump-bonding technique.
An example of such imaging devices are flat panel sensors using amorphous silicon substrates. In such sensors, a layer of amorphous silicon is grown on a glass substrate. Each radiation sensitive element of the sensor is formed of a photodiode and has an associated thin film transistor (TFT) which acts as a simple switch for the display element to form an active matrix. An X-ray to light converter (scintillator) is placed above the amorphous silicon (a-Si) to produce light, which is detectable in the a-Si layer, corresponding to incident X-rays.
A drawback of such devices is that they have a relatively low X-ray to charge conversion factor and consequently electronic circuitry for amplifying or processing the charge must be placed close to the active screen. However, the glass substrate is unsuitable for supporting the necessary circuitry and hence it is placed around the periphery of the active screen.
A further type of device is described in International Application WO 95/33332. In WO 95/33332, an Active-pixel Semiconductor Imaging Device (ASID) is described. The ASID comprises an array of image elements including a semiconductor substrate having an array of image element detectors and a further array of image element circuits. The image element detectors generate charge in response to instant radiation. Each image element circuit is associated with a respective image element detector and accumulates charge resulting from radiation incident on the image element detector. The image element circuits are individually addressable and comprise circuitry which enables charge to be accumulated from a plurality of successive radiation hits on the respective image element detectors. The device operates by accumulating charge on the gate, for example, of a transistor. Accordingly, analogue storage of the charge value is obtained. At a determined time, the charge from the image element circuits can be read out and used to generate an image based on the analogue charge values stored in each of the image element circuits.
CCD devices suffer from disadvantages of limited dynamic range, due to the limited capacity of the potential well inside the semiconductor substrate, and also to the inactive times during which an image is read out. Pulse counting semiconductive pixel devices also have the disadvantage of limited dynamic range. As these devices read the pixel contact when a hit is detected, they suffer from saturation problems at high counting rates. The semiconductor image element device according to WO 95/33332 provides significant advantages over the earlier prior art by providing a large dynamic range for the accumulation of images.
It has been proposed to utilise the above-mentioned CCD and semiconductor devices to replace the film used in conventional radiation imaging systems, in order to provide real-time imaging and a more controlled lower dosage of radiation for a given exposure.
Conventional radiation imaging system utilise a film cassette
1
for holding the X-ray or high energy radiation sensitive film in place within the imaging system. The X-ray film takes up an area
2
within the cassette and is surrounded by a support area
3
, as illustrated in
FIG. 1
of the accompanying drawings.
The above-mentioned CCD and semiconductor devices with which it is proposed to replace the film used in conventional radiation bridging systems, are generally supported upon printed circuit boards and have the configuration substantially illustrated in FIG.
2
. The CCD or semiconductor device
6
which may include radiation sensitive detectors such as direct-conversion detectors and indirect-conversion detectors, such as a scintillator screen and CCD or a-Si, is supported on printed circuit board
8
and take up an area
10
. In the imaging device
6
illustrated in
FIG. 2
, the imaging area
10
comprises a 3×4 array of image device tiles, such as may be utilised with an ASID as referred to above. Each imaging device tile comprises a plurality of such ASIDs and provides for a mosaicing of a number of tiles in order to provide a relatively large imaging surface area. Optionally, a-Si detectors could be used. The printed circuit board
8
also supports electronic circuitry
13
for providing control signals to the device
6
and reading out signals from the device.
Conventionally, the electronic circuits are provided on the circuit board
8
to the sides of the imaging area
10
. An electrical connector
15
is attached to the printed circuit board
8
in order to transfer signals from the electronic circuitry
13
to signal processing electronics elsewhere in the imaging system. The regions
4
,
5
shown in broken line indicate areas to which the printed circuit board
8
may be extended to accommodate further electronic circuitry.
Conventionally, as the need for electronic processing of the raw data from the image elements increases, so does the amount of electronic circuitry,
13
, provided on the printed circuit board
8
. As can be seen from
FIGS. 1 and 2
, there is a significant amount of wasted, or non-imaging, space on printed circuit board
8
compared to the conventional film cassette. Consequently, imaging devices utilising semiconductor devices take up considerably more surface area and volume than a conventional film cassette having the same imaging surface area. An additional drawback is that semiconductor devices cannot be provided as “slot-in” or “plug-in” replacements for conventional film cassettes. Thus, it is necessary to provide new holders within the imaging radiation systems in order to accommodate CCD-and semiconductor-based radiation detectors.
Examples of imaging devices and systems in which the electronics takes up a significant surface area of the imaging device are the:
dpiX FlashScan 20 system which has an active imaging area of 570 mm×360 mm, yet dimensions of 250×250 mm, and the FlashScan 30 system which has an active imaging area of 293 mm×406 mm, yet dimensions of 500×366 mm; and
Trixell pixium 4600 which has an imaging area of 426.3 mm×432 mm, but a distance of 26 mm from the image area to top and bottom sides of the casing and 65 mm from the image area to left and right sides of the casing.
The surface area not used for imaging is generally used for ele
Eraluoto Markku
Jurthe Stefan
Spartiotis Konstantinos Evangelos
Fulton Christopher W.
Kenyon & Kenyon
Simage Oy
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