Title: Flat dynamic radiation detector
Abstract: The invention relates to a radiation detector for converting electromagnetic radiation (15) into electric charge carriers. The invention also relates to an X-ray examination apparatus provided with such a radiation detector, and to a method of manufacturing a radiation detector. In order to achieve a small building height of the radiation detector while nevertheless satisfying the same requirements as regards the resetting of the converter arrangement (16, 18) by means of an illumination device (6), it is proposed to provide a supporting layer (8) underneath a glass plate (2a) with a photosensor arrangement (2b), which supporting layer on the one hand provides uniform distribution of the light incident from below and on the other hand imparts the necessary stability to the radiation detector. In a further embodiment it is proposed to provide a scatter foil (14) instead of the supporting layer (8), which scatter foil provided the homogeneous light distribution while the supporting function is taken over by the illumination device.
Patent Number: 6,989,539 Issued on 01/24/2006 to Wischmann, et al.
| Inventors:
|
Wischmann; Hans-Aloys (Aachen, DE);
Wieczorek; Herfried Karl (Aachen, DE);
Busse; Falko (Aachen, DE);
Schmidt; Ralf (Aachen, DE)
|
| Assignee:
|
Koninklije Philips Electronics N.V. (Eindhoven, NL)
|
| Appl. No.:
|
189960 |
| Filed:
|
July 8, 2002 |
Foreign Application Priority Data
| Jul 06, 2001[DE] | 101 32 924 |
| Current U.S. Class: |
250/370.11; 250/361.R; 250/367; 250/370.08; 250/370.09; 250/374 |
| Current Intern'l Class: |
G01T 1/24 (20060101); G01T 1/20 (20060101); H01J 47/02 (20060101) |
| Field of Search: |
250/361 R,367,370.06,370.09,374,210,370.11,370.01
313/502
438/65
|
References Cited [Referenced By]
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| 2003/0155515 | Aug., 2003 | Moy et al.
| |
| Foreign Patent Documents |
| 44 20 603 | Jun., 1995 | DE.
| |
| 199 14 217 | Oct., 2000 | DE.
| |
| 19914217 | Oct., 2000 | DE.
| |
Primary Examiner: Font; Frank G.
Assistant Examiner: El-Shammaa; Mary
Attorney, Agent or Firm: McKnight; Douglas B.
Claims
What is claimed is:
1. A radiation detector for converting electromagnetic radiation into electric
charge carriers comprising:
at least one converter arrangement;
an illumination device, wherein the illumination device is located in the radiation
detector; and
a layer which supports at least the converter arrangement, the supporting layer
realizing a spatially homogeneous distribution of the light emitted by the illumination device.
2. The radiation detector of claim 1 wherein the supporting layer is provided
between the converter arrangement and the illumination device and the illumination
device is arranged to emit light in the wavelength range of from 300 to 900 nm
in the direction of the converter arrangement.
3. The radiation detector of claim 1 wherein the supporting layer comprises a
material having at least one of a low light absorption and a high thermal conductivity
and mechanical stability.
4. The radiation detector of claim 1 wherein that the supporting layer is connected
to the illumination device in a form-fit fashion.
5. The radiation detector of claim 1 wherein the supporting layer is provided
with cooling medium chambers containing a cooling medium in order to carry off heat.
6. The radiation detector of claim 1 wherein that the supporting layer contains TiO2.
7. A radiation detector for converting electromagnetic radiation into electric
charge carriers comprising:
a converter arrangement;
an illumination device, the illumination device being located in the detector; and
a scatter foil between the converter arrangement and the illumination device,
the scatter foil acts as a distribution layer for the homogeneous distribution
of light emitted by the illumination device, the illumination device supporting
the converter arrangements arranged over the illumination device.
8. The radiation detector of claim 1 wherein the converter arrangement includes:
a first converter layer for converting electromagnetic radiation into visible
light, and
a second converter layer for converting the visible light into electric charge carriers.
9. The radiation detector of claim 1 wherein the converter arrangement includes:
a directly converting converter layer for converting X-rays into electric charge carriers,
a back electrode,
a charge carrier sensor with a read-out, and
a storage component.
10. A radiation detector for converting x-rays into electric charge carriers,
the detector comprising:
a converter arrangement which includes one of:
a first converter layer for converting electromagnetic radiation into visible
light and a second converter layer for converting the visible light into the electric
charge carriers; and
a directly converting converter layer for converting the detected x-rays into
electric charge carriers and a charge carrier sensor with a readout;
a controllable light source optically coupled to the converter arrangement, the
light source being controllable to supply light to reset the converter arrangement
after receiving x-rays,
wherein the converter arrangement has an X-ray receiving face and an oppositely
disposed backside, the light source being optically coupled to the back side.
11. A radiation detector for converting x-rays into electric charge carriers,
the detector comprising:
a converter arrangement which includes one of:
a first converter layer for converting electromagnetic radiation into visible
light and a second converter layer for converting the visible light into the electric
charge carriers; and
a directly converting converter layer for converting the detected x-rays into
electric charge carriers and a charge carrier sensor with a readout;
a controllable light source optically coupled to the converter arrangement, the
light source being controllable to supply light to reset the converter arrangement
after receiving x-rays,
wherein the converter arrangement includes at least one amorphous silicon layer
formed on and supported by a light transmissive substrate, the light source being
optically coupled to the light transmissive substrate.
12. An X-ray examination apparatus comprising:
an X-ray source and
one or more radiation detectors, wherein at least one radiation detector comprises:
at least one converter arrangement;
an illumination device, wherein the illumination device is located in said at
least one radiation detector; and
a layer which supports at least the converter arrangement, the supporting layer
realizing a spatially homogeneous distribution of the light emitted by the illumination device.
13. The X-ray examination apparatus of claim 12 wherein the supporting layer
comprises a material having at least one of a low light absorption and a high thermal
conductivity and mechanical stability.
14. The X-ray examination apparatus of claim 12 wherein the supporting layer
is provided with cooling medium chambers containing a cooling medium in order to
carry off heat.
15. The X-ray examination apparatus of claim 12 wherein the converter arrangement includes:
a first converter layer for converting electromagnetic radiation into visible
light, and
a second converter layer for converting the visible light into electric charge carriers.
16. The X-ray examination apparatus of claim 12 wherein the converter arrangement includes:
a directly converting converter layer for converting X-rays into electric charge carriers,
a back electrode,
a charge carrier sensor with a read-out, and
a storage component.
Description
BACKGROUND
The invention relates to a radiation detector for converting electromagnetic
radiation into electric charge carriers. The invention also relates to an X-ray
examination apparatus provided with radiation detector of this kind and to a method
of manufacturing a radiation detector.
Radiation detectors are used notably in the medical field, that is, for
X-ray examinations, and serve to form radiation images of an object to be examined,
usually a patient, in the context of mostly a medical examination or therapy. An
image pick-up system which also includes the X-ray detector is used to form images
of the object to be examined which is exposed to the X-rays, said images being
output, for example, via a monitor. The X-rays incident on the X-ray detector are
converted into electric charge carriers in a converter arrangement. The electric
charge carriers generated in the converter arrangement are collected in associated
capacitances so as to be read out by a downstream electronic read-out circuit arrangement.
Generally speaking, a radiation detector is constructed in such a manner
that the electromagnetic radiation is incident on a converter arrangement. Depending
on the specific construction of the radiation detector, either a directly converting
converter layer in the converter arrangement converts the radiation into electric
charge carriers which are subsequently read out, or the radiation is first converted
into visible light by means of two converter layers and subsequently, that is,
in a second converter layer which is arranged therebelow, notably a photosensor
arrangement, into electric charge carriers so as to be read out.
In the case of radiation detectors provided with a converter arrangement which
includes two converter layers, the first converter layer is provided as a scintillator
layer of, for example, CsI:Ti. Underneath this first converter layer, that is,
viewed in the direction of the incident radiation, the second converter layer is
formed as a photosensor arrangement.
The individual photosensors detect the radiation converted into visible light,
said radiation then being read out one pixel after the other via the individual
photosensors. The conversion of radiation directly into electric charge carriers
in radiation detectors provided with a converter arrangement with only a single
converter layer is also referred to as direct conversion. The first converter layer
is then constructed as a directly converting semiconductor layer of, for example,
amorphous selenium. Radiation detectors with direct conversion in only a single
converter layer may also be realized by means of a PbO layer, the charge carriers
produced then being stored and subsequently read out.
Underneath the converter layer or layers (depending on the construction
of the radiation detector) there is provided an illumination device which serves
to reset the individual pixels of the photosensor arrangement in the context of
the preparation of the radiation detector for a further exposure. For radiation
detectors which include only a single converter layer for direct conversion it
is also effective to induce a charge carrier flood by way of a reset light pulse,
thus exerting a positive effect on the decay behavior of the converter layer so
as to enable a faster series of X-ray images and/or a better quality to be achieved
without image artifacts.
The converter layers mentioned thus far are supported by a substrate of, for
example, glass.
It has been found that the photosensor arrangement or second converter layer
exhibits
a slow decay which has an adverse effect on successive image exposures. Such a
decay behavior is detrimental notably when many images are acquired per unit of
time. The cause of such decay lies in physical processes which take place in the
photosensors upon incidence of optical photons. When a photon is incident on the
semiconductor material of the photosensor arrangement, an electron is moved from
the valence band to the conduction band and the electric charge thus produced is
stored on electrodes of the semiconductor layer which constitute a capacitance.
However, because so-called traps occur in the semiconductor layer of the photosensor
arrangement due to contaminations and grid defects, many electrons remain behind
in the semiconductor layer. Normally speaking the charge carriers present in the
traps are thermally emitted in the course of time and transferred to the electrodes,
be it that this may take a long period of time. Because of this quasi-thermal emission,
which also takes place when the photosensor arrangement has already been read out
and a second image is formed, so-called afterimages or image artifacts of the previously
acquired image will be visible in subsequently acquired images.
In order to solve this problem, it is known to read out the photosensor arrangement
after successful formation of an X-ray image and to make the illumination device
deliver subsequently at least one light pulse which acts on the second converter
layer. The light pulse floods the second converter layer with charge carriers and
the traps in all pixels are uniformly occupied. In order to achieve an as effective
as possible occupation of the traps by charge carriers, the illumination device
emits light of a given wavelength in the form of one or more separate light pulses
in rapid succession. For effective resetting, however, it is a prerequisite that
the light emitted by the illumination device is uniformly distributed in the direction
of the photosensor arrangement.
DE 199 14 217 describes an X-ray detector in which a scintillator arrangement
is arranged over a pixel matrix, both elements being arranged over a glass support
which supports the scintillator arrangement and the pixel matrix. Underneath the
glass support there is provided a layer of air and the light source or illumination
device is situated underneath said layer of air. This layer of air is necessary
to achieve a spatial distribution of the light emitted by the illumination device
and to distribute the light as homogeneously as possible. Direct arrangement of
the illumination device underneath the glass support, that is, without a corresponding
layer of air, is detrimental because in that case the required homogeneous light
distribution will not be achieved so that the resetting of the photosensor arrangement
and also of the scintillator or converter layer is not effective. The glass support,
serving notably for stabilizing the photosensor arrangement and the scintillator
or converter layer carried by the support, cannot realize such a homogeneous light
distribution. A further drawback of such a detector resides in its considerable
height which is due to the presence of the glass support layer and the layer of air.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide a radiation detector,
an X-ray examination apparatus provided with a radiation detector, and a method
of manufacturing a radiation detector which enable a small structural height of
the radiation detector to be achieved while satisfying the same high reset requirements nevertheless.
This object is achieved by means of a radiation detector for the conversion
of electromagnetic radiation into electric charges which includes at least one
converter arrangement and an illumination device and a layer which supports at
least the converter arrangement, the supporting layer realizing a spatially homogeneous
distribution of the light emitted by the illumination device.
The invention is based on the idea that the functions of the glass support which
supports the converter layers and the layer of air as known from the state of the
art can be advantageously combined, yielding a significant reduction of the structural
height in conjunction with further improvements and advantageous effects as will
be described in detail hereinafter.
To this end, it is proposed to provide a supporting layer between the converter
arrangement and the illumination device, which supporting layer on the one hand
provides a homogeneous light distribution and on the other hand has adequate mechanical
stability for stable accommodation and support of the converter arrangement provided
on the supporting layer. The homogeneous light distribution is required for effective
resetting of the converter arrangement and is realized by way of the layer of air
in conformity with the state of the art. The homogeneous light distribution is
achieved notably in that a layer of a synthetic material, for example, an acrylic
layer, is provided between the converter arrangement and the illumination device.
Such a layer has the necessary light absorption, thermal conductivity and also
mechanical stability to support the converter arrangements provided over such a layer.
The illumination device emits the reset light notably in the direction of the
overlying converter arrangement. In that case it is particularly effective to select
a wavelength which lies in the sensitivity range of the converter arrangement.
It is advantageous to choose light of a wavelength in the range of from 300 to
900 nm in such a case.
The supporting layer in a preferred embodiment of the invention is provided with
cooling medium chambers which can receive a cooling liquid so that heat can be
very effectively carried off in the radiation detector. A constant temperature
in the radiation detector has a positive effect on the converter arrangement as
well as on the electronic read-out circuitry. As the temperature is lower, for
example, the sensitivity of the first converter layer increases, that is, notably
when CsI:Ti is used as the scintillator material. A constant temperature enhances
the stability of the dark images.
The illumination device in a further preferred embodiment of the invention is
connected to a scatter foil so that the illumination device takes over the mechanical
stability or the mechanical support of the converter arrangement provided thereon.
This enables the construction of a radiation detector which is even flatter. The
scatter foil serves to realize a homogeneous distribution of the reset light which
is incident from below. In comparison with the state of the art, the foil enables
the heat to be carried off even better in the flatter radiation detector, so that
a water cooling system as required thus far can be dispensed with.
The acrylic layer or the scatter foil can be advantageously used to carry off
the heat in the radiation detector by thermal conduction instead of thermal radiation,
so that a cooling system as required thus far can operate more efficiently or can
even be dispensed with completely.
Said acrylic layer in a particularly advantageous embodiment of the invention
is molded directly together with the illumination device, so that the illumination
device is connected to the supporting layer in a form-fit fashion.
The converter arrangement in a first embodiment includes two converter layers.
The first converter layer is now formed by a scintillator arrangement. Said first
converter layer converts the radiation incident thereon into visible light. Underneath
the first converter layer there is arranged a second converter layer. The latter
converter layer converts the visible light into electric charge carriers, for example,
in a photosensor arrangement.
The converter arrangement in a further embodiment of the invention includes only
a directly converting converter layer. This layer converts the X-rays into electric
charge carriers which are read out via electrodes.
The photosensor arrangement is provided on a glass plate by means of a thin-film
technique. In conformity with the state of the art such a glass plate would be
bonded to the glass support arranged therebelow, thus necessitating the use of
an additional adhesive layer could contain, for example, bubbles which attenuate
the reset light from below or from the rear by as much as 10%. At the edges of
the bubbles inhomogeneities occur, that is, so-called Newton rings, which obstruct
the uniform resetting of the photosensor arrangement and also the spatially resolved
measurement of the gain effect, or the non-linearity of the photosensors arranged
thereabove, by means of the reset light.
A further advantage is achieved in that the vibrations of the glass plate are
attenuated,
for example, by the acrylic supporting layer in conjunction with the illumination
device when the latter is mechanically pressed against the glass plate on which
the photosensor arrangement is provided, thus mitigating microphony problems.
A directly converting layer of PbO or a scintillator arrangement of CsI:Ti can
be vapor deposited on the glass plate with the sensors or electrodes provided by
means of the thin-film technique.
In an advantageous embodiment an acrylic glass plate of a thickness of approximately
4 mm is arranged on the rear of the glass plate of a thickness of approximately
1 mm and provided with the photosensor arrangement, which acrylic glass plate is
mechanically pressed against the glass plate with the photosensor arrangement by
the illumination device.
The illumination device in a further advantageous embodiment of the invention
is pressed into a liquid acrylic mass directly after the mounting of, for example,
LEDs, so that the illumination device is connected to the supporting layer so as
to be directly coupled and has a plane surface. Consequently, optical losses otherwise
incurred because of the presence of an additional intermediate layer between the
illumination device and the supporting layer are avoided. The plane surface is
mechanically pressed against the converter arrangement.
It is advantageous to add, for example, titanium dioxide powder of a suitable
concentration to the supporting layer in both cases described above, so that the
distribution of the light emitted by the LEDs or the illumination device is spatially homogenized.
For improved dissipation of heat as well as attenuation of vibrations, the illumination
device of this embodiment can again be immersed in an acrylic mass after the mounting
of the components in order to ensure, by way of the plane surface, a large area
of contact with the glass supporting plate with the photosensor arrangement. In
that case, however, no additive will be required for light scattering.
The object is also achieved by means of an X-ray examination apparatus in which
an X-ray detector in accordance with the invention is arranged so as to face an
X-ray tube and the X-rays emitted by the X-ray tube traverse a patient or an object
to be examined and are incident on the X-ray detector in accordance with the invention.
The following description, claims and accompanying drawings set forth certain
illustrative embodiments applying various principles of the present invention.
It is to be appreciated that different embodiments applying principles of the invention
may take form in various components, steps and arrangements of components and steps.
These described embodiments being indicative of but a few of the various ways in
which some or all of the principles of the invention may be employed in a method
or apparatus. The drawings are only for the purpose of illustrating an embodiment
of an apparatus and method applying principles of the present invention and are
not to be construed as limiting the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other features and advantages of the present invention will
become apparent to those skilled in the art to which the present invention relates
upon consideration of the following detailed description of apparatus applying
aspects of the present invention with reference to the accompanying drawings, wherein:
FIG. 1 is a diagrammatic representation of the construction of a contemporary
flat dynamic X-ray detector.
FIG. 2 shows a first embodiment of the X-ray detector in accordance with the invention.
FIG. 3 shows a second embodiment of the X-ray detector in accordance with the invention.
FIG. 4 shows the construction of a directly converting radiation detector.
DETAILED DESCRIPTION
FIG. 1 shows the construction of a radiation detector in conformity with the
state of the art. The converter arrangement
16 thereof is formed essentially
by a first converter layer
1 and a second converter layer
2. The
converter layer
1 is connected, via an adhesive layer
11, to the
glass plate
2a with the photosensor arrangement
2b provided
thereon in the thin-film technique, for example, by amorphous silicon technology
11. Above the converter layer
1 there is provided a reflection layer
9 which reflects upwards reflected light back in the direction of the photosensor
arrangement. The glass support
4 carries the glass plate
2a with
the photosensor arrangement
2b provided thereon and the converter
layer
1 arranged thereabove. Underneath the glass support
4 there
is formed a layer of air
5 of a thickness of approximately 10 mm. The illumination
device
6 with light-emitting diodes (LEDs)
7 is provided underneath
said layer of air
5. The incident X-rays
15 pass the reflection layer
9 and are converted into visible light in the converter layer
1 which
is constructed as a scintillator. The visible light is converted into electric
charge carriers in the photosensor arrangement
2b, said charge carriers
being applied to an electronic read-out circuit
12 which is arranged underneath
the illumination device
6. The electronic read-out circuit is shielded from
X-rays by a shielding layer
13.
In FIG. 2 the converter layer
1 is arranged over the glass plate
2a
with the photosensor arrangement
2b. The glass plate
2a
and the photosensor arrangement
2b constitute the converter layer
2. The two converter layers
1 and
2 are supported by a supporting
layer
8 which is made of, for example, acrylic glass. Titanium dioxide is
added to the acrylic glass. However, any other material suitable for achieving
a homogeneous light distribution and a corresponding stability can be used. The
illumination device
6 is arranged underneath the supporting layer
8.
For example, a semiconductor layer of CsI is vapor deposited on the pixels with
the photosensors
2b and/or the capacitances for charge storage. The
adhesive layer
11 between the first and the second converter layer
1,
2 can thus be dispensed with.
FIG. 3 shows a further embodiment of the invention. The converter layer
1
therein is arranged over the glass plate
2a with the photosensor
arrangement
2b. Underneath the glass plate
2a with
the photosensor arrangement
2b there is provided a scatter foil
14
which is deposited directly on the surface of the illumination device
6.
The stabilizing function is taken over by the illumination device
6 in this embodiment.
FIG. 4 shows a radiation detector provided with a directly converting converter
arrangement
18. This arrangement includes a converter layer
19, for
example, of amorphous selenium or of PbO, a back electrode
20 and charge
carrier sensors
17. The charge carrier sensors
17 include electrodes
which store, by way of a capacitance, the charge carriers generated until they
can be read out. The generating of the charge carriers in the converter layer
19
takes place in the electric field between the back electrode
20 and pixel
electrodes (not shown) in the charge carrier sensors
17. The converter arrangement
18 is again arranged over the supporting layer
8 which itself is
arranged over the illumination device
6. The supporting layer
8 ensures
homogeneous distribution of the light emitted by the illumination device.
The FIGS. 2,
3,
4 clearly show that the radiation detector can
be constructed so as to have a height which is significantly smaller than that
which can be achieved in conformity with the state of the art as shown in FIG. 1.
The invention is of course not limited to the described or shown embodiments,
but generally extends to any embodiment, which falls within the scope of the appended
claims as seen in light of the foregoing description and drawings. While a particular
feature of the invention may have been described above with respect to only one
of the illustrated embodiments, such features may be combined with one or more
other features of other embodiments, as may be desired and advantageous for any
given particular application. From the above description of the invention, those
skilled in the art will perceive improvements, changes and modification. Such improvements,
changes and modification within the skill of the art are intended to be covered
by the appended claims.
*
