U.S. patent application number 09/732590 was filed with the patent office on 2001-09-27 for x-ray apparatus with filter comprising filter elements with adjustable x-ray absorption and x-ray absorption sensor.
Invention is credited to Herbert, Brian Kenneth.
Application Number | 20010024486 09/732590 |
Document ID | / |
Family ID | 8241789 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024486 |
Kind Code |
A1 |
Herbert, Brian Kenneth |
September 27, 2001 |
X-ray apparatus with filter comprising filter elements with
adjustable X-ray absorption and X-ray absorption sensor
Abstract
An X-ray device with an X-ray source to produce a beam of X-ray
radiation, an X-ray detector to detect this radiation and an X-ray
filter with filter elements which is positioned between the X-ray
source and the X-ray detector to attenuate the X-ray beam in each
independent filter element individually. Every filter element (213)
can contain a fluid (32) which is electrically conducting and X-ray
absorbing, which is supplied via a transport channel (30), whereas
the value of the X-ray absorption of each filter element can de
discretely adjusted by a stepwise adjustment of the level of the
fluid (32) within each filter element. Each filter element
comprises a first electrode, which is located in the wall of the
filter element and is segmented in the length direction of the
filter element (213), the segments being electrically combined in
tow subgroups (123, 124) in order to apply an electric potential to
the wall segments of the filter element to achieve a step-wise
filling of the inner volume (31) by the fluid (32). The measuring
means (50, 60) are provided to detect the length of the fluid
column (32) within the inner volume (31) of the filter element
(213).
Inventors: |
Herbert, Brian Kenneth;
(East Grinstead, GB) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8241789 |
Appl. No.: |
09/732590 |
Filed: |
December 8, 2000 |
Current U.S.
Class: |
378/158 ;
378/157; 378/159 |
Current CPC
Class: |
G21K 1/10 20130101; G02B
26/005 20130101 |
Class at
Publication: |
378/158 ;
378/157; 378/159 |
International
Class: |
G21K 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 1999 |
EP |
99309883.9 |
Claims
1. An X-ray apparatus with an X-ray source (1) for producing a beam
of X-ray radiation (2), an X-ray detector (4) to detect the X-ray
radiation and a filter (12) comprising filter elements (13)
containing an X-ray absorbing medium (32) and positioned between
the X-ray source and the X-ray detector to absorb the X-ray
radiation, in which the X-ray absorption properties of filter
elements (13) are individually controllable by changing a length of
a column of the X-ray absorbing medium (32) in a propagation
direction of the X-ray beam (2), electrical means being provided,
comprising a voltage supply means (14) for the filter elements and
an electrical measuring means (70) to measure a quantity which is
related to the X-ray absorption properties of individual filter
elements characterized in that the filter elements (13) are
tube-like structures with an inner volume (31) for containing the
X-ray absorbing medium (32) being comprised in a fluid filling
having electrically conducting and X-ray absorbing properties (132,
respectively 134), the X-ray absorption of each filter element (13)
depending on the length of the column of the X-ray absorbing fluid
in the inner volume (31), each filter element comprising a first
electrode (33) for supplying a first electric potential to its wall
(38) and a second electrode (39) for supplying a second electric
potential to the electrically conducting fluid (132), the first
electrode being separated from the electrically conducting fluid by
means of an isolator layer (44) so that an electric capacitance is
defined between the first electrode and the electrically conducting
fluid, the electrical measuring means being further embodied (70)
to measure the electric capacitance in individual filter
elements.
2. An X-ray filter (12) to be used in the X-ray apparatus of claim
1, wherein the electric measuring means (70) comprise an electric
circuit with an AC source (50) for providing an AC current to the
first and second electrodes of individual filter elements and also
comprising an AC detector (60) to measure an AC signal induced by
said current within individual filter elements.
3. An X-ray filter as claimed in claim 2, wherein the electric
measuring means (70) comprise a further circuit element connected
in series with the respective electric capacitance to form a
voltage divider.
4. An X-ray filter as claimed in claim 3, wherein the further
circuit element is an additional electric capacitance.
5. An X-ray filter as claimed in claim 2, wherein the voltage
supply means (136) comprise a high impedance for supplying the
first electric potential to individual filter elements.
6. An X-ray filter as claimed in claim 2, wherein the first
electrodes of individual filter elements comprise a stack of
segments in the X-ray propagation direction, wherein the
neighbouring segments are mutually electrically isolated (123,124),
the first electric potential being switchable between the segments
by the voltage supply means (136) to alter the length of the column
of the X-ray absorbing fluid in the X-ray propagation direction,
the amplitude of the AC signal being a function of time (90,92),
control means (101) being provided for controlling the switching of
the first electric potential between the segments (123,124) in
dependence of the behavior of the AC signal with time.
7. An X-ray filter as claimed in claim 6, wherein the control means
(101) are conceived to switch the first electric potential between
the segments triggered by the AC signal reaching a saturation
region (92).
Description
[0001] The invention relates to the field of X-ray radiography, in
particular to an X-ray apparatus with an X-ray source for producing
a beam of X-ray radiation, an X-ray detector to detect the X-ray
radiation and a filter comprising filter elements containing an
X-ray absorbing medium and positioned between the X-ray source and
the X-ray detector to absorb the X-ray radiation, in which the
X-ray absorption properties of filter elements are individually
controllable by changing a length of a column of the X-ray
absorbing medium in a propagation direction of the X-ray beam,
electrical means being provided, comprising a voltage supply means
for the filter elements and an electrical measuring means to
measure a quantity which is related to the X-ray absorption
properties of individual filter elements.
[0002] An X-ray apparatus of this type provided with an X-ray
equalization filter to be positioned between the X-ray source and
the X-ray detector is known from American patent U.S. Pat. No.
5,778,046. The known X-ray equalization filter comprises an X-ray
absorbing mask housed within a shapeable membrane which is
positioned in the primary X-ray beam, wherein the local thickness
of the mask material can be varied. The known X-ray apparatus can
be used in the field of radiodiagnostics, in particular to obtain
X-ray transmission images of human body. It is well known that some
regions of human body, in particular thorax, have structures of
quite different X-ray absorption, which result in pixels on the
X-ray detector of different blackening in the X-ray transmission
image. The range of the blackening in one X-ray transmission image
is defined as dynamic range. The function of the X-ray equalization
filter is to compensate the patient regions of low electron
density, matching the dynamic range of an X-ray imager in order to
obtain a better quality of the X-ray transmission image.
[0003] In the known X-ray apparatus a matrix of mechanical pistons
is used which can locally modify the length of the column of the
X-ray absorbing mask and by doing this, perform a local alteration
of the X-ray fluence. The pistons are driven by nickel-titanium
wires, wherein the individual length of each nickel-titanium wire
is controlled by an electric current applied to the wire. When the
electric current is applied to the wire, the wire temperature
increases and the wire length decreases. Therefore, there is a
certain time necessary for a nickel-titanium wire to shrink or to
resume an original length. Further, according to the logistics of
the known X-ray apparatus, first a low exposure X-ray image is
obtained to calculate the necessary mask profile, then the shaped
mask is positioned in the primary X-ray beam. Therefore, there is a
considerable time lost for the whole procedure. In the known X-ray
apparatus the desired two-dimensional equalization profile of the
X-ray absorbing mask is obtained within five seconds. In the field
of radiodiagnostics the timing properties of the image acquisition
play an important role, especially due to the image artifacts
resulting from the movements of the thorax.
[0004] Further, the known X-ray equalization filter comprises a
limited number of pistons, which is in the order of 8x8 or 16x16.
However, for a high quality X-ray diagnostic images this degree of
spatial resolution may not be sufficient. Next to this, for the
quality assurance purposes and operation control of the X-ray
equalization filter it is desirable to have means of a local length
detection of the column of the X-ray absorption medium within the
filter. In the known X-ray apparatus the position of each piston is
sensed using potentiometers which are specifically constructed for
that purpose. Due to the movement of the piston with respect to the
potentiometer the local resistance in the corresponding electric
circuit is varied, whereby it is assumed that the degree of the
resistance variation has a direct correlation with the length of
the column of the X-ray absorption mask.
[0005] It is an object of the present invention to provide an X-ray
apparatus with a dynamic beam attenuation filter with a high degree
of spatial resolution and a high operation speed, where the length
of column the X-ray absorption medium is directly sensed using a
simple electrical circuit. This is achieved in the X-ray apparatus
according to the invention, which is characterized in that the
filter elements are tube-like structures with an inner volume for
containing the X-ray absorbing medium being comprised in a fluid
filling having electrically conducting and X-ray absorbing
properties, the X-ray absorption of each filter element depending
on the length of the column of the X-ray absorbing fluid in the
inner volume, each filter element comprising a first electrode for
supplying a first electric potential to its wall and a second
electrode for supplying a second electric potential to the
electrically conducting fluid, the first electrode being separated
from the electrically conducting fluid by means of an isolator
layer so that an electric capacitance is defined between the first
electrode and the electrically conducting fluid, the electrical
measuring means being further embodied to measure the electric
capacitance in individual filter elements.
[0006] In the X-ray apparatus according to the invention the high
degree of spatial resolution is achieved by constructing a filter
comprising a plurality of tube-like filter elements assembled
together, each individual filter element containing an X-ray
absorption fluid with an individually controllable level of the
X-ray absorption fluid within each filter element, the actual level
of the X-ray absorption fluid within the filter element being
sensed using simple electrical means. The operation of each filter
element is based on the known physical effect of
electrowetting.
[0007] To explain the operation of the filter element different
compositions of the X-ray absorption fluid are to be considered.
Firstly, the X-ray absorption fluid with electric conduction and
X-ray absorption properties can be obtained by dissolving a salt
into a liquid (for example, water). The resulting solution is a
single component fluid with both electric conducting and X-ray
absorbing properties. Apart from this, it is possible to produce a
two-component fluid solution, the first fluid component having
exclusively electric conducting properties and a negligible degree
of X-ray absorption, the second fluid component having exclusively
X-ray absorption properties and being an insulator. The resulting
solution is a fluid with both electric conducting and X-ray
absorbing properties. It is also possible to produce a
two-component fluid from two mutually not mixable fluids, the first
fluid having mainly electric conducting properties and a negligible
degree of X-ray absorption, and the second fluid having mainly
X-ray absorption properties and being an insulator. The resulting
fluid will contain two fluid columns on top of each other with the
corresponding physical properties attributed to a corresponding
fluid component.
[0008] The essence of the phenomenon of electrowetting is the
variation of the contact angle between the electrically conducting
fluid on top of the metal and the metal and thus, the variation in
the hydrophobicity of the contact surface. For that purpose each
filter element comprises a first electrode located in the wall of
the filter element in order to bring an electric potential to the
wall. A second electrode is located in the inner volume of each
filter element in order to bring a second electric potential to the
electrically conducting fluid. The first electrode in separated
from the inner wall by means of an isolator layer. For a given
composition of the filter element there is a set of operation
voltages, namely a `fill` voltage to fill the filter element with
the electrically conducting fluid, a `hold` voltage to hold the
meniscus of the electrically conducting fluid on the desired level
and an `empty` voltage to initiate the emptying process of the
filter element. The operation of the filter element of this type is
known and is described in more detail in American patent U.S. Pat.
No. 5,625,665 (PHN 15044). In case the X-ray absorption medium is a
single component fluid or a solution of the electrically conducting
fluid component and the X-ray absorbing fluid component, there is a
single liquid column within the filter element, the level of which
defines the degree of X-ray absorption. In the case the X-ray
absorbing fluid has more than one, for example two not mutually
mixing components, there are two separate fluid columns in the
inner volume of the filter element in the X-ray propagation
direction, having a mutual separation plane perpendicular to the
X-ray propagation direction, whereas the length of the column of
the X-ray absorbing fluid component within the filter element and,
thus, the degree of the X-ray absorption is passively defined by
the length of the column of the electrically conducting fluid
component.
[0009] Due to the fact that the first electrode is separated from
the electrically conducting fluid component by the isolator layer,
an electric capacitance is created. The value of the electric
capacitance is a direct measure for the degree of filling by the
electrically conducting fluid component. An embodiment of an X-ray
filter to be used in an X-ray apparatus of the invention is
characterized in that the electric measuring means comprise an
electric circuit with an AC source for providing an AC current to
the first and second electrodes of individual filter elements and
also comprising an AC detector to measure an AC signal induced by
said current within individual filter elements. In this way in
order to sense the level of the electrically conducting fluid
component the electric measuring means comprising a simple
electrical circuit are proposed to measure the electric
capacitance.
[0010] In order to prevent the AC current from interfering with
voltage supply means of the first electric potential a further
embodiment of the filter according to the invention is
characterized in that the electric measuring means comprise a
further circuit element connected in series with the respective
electric capacitance to form a voltage divider. A preferred
embodiment for the circuit element, but not limited to it, is an
additional electric capacitance.
[0011] Also, in order to prevent an electric interference between
the first electric potential and an AC path, a fourth embodiment of
the filter according to the invention is characterized in that the
voltage supply means comprise a high impedance for supplying the
first electric potential to individual filter elements.
[0012] In many radiodiagnostic applications it may be desirable to
have a possibility of grey values per filter element, instead of
working with filter elements which are either empty or full. One of
the possible solutions for that problem is to divide the first
electrode into a stack of electrode segments in the X-ray
propagation direction. The control of the filter element filling in
this case is achieved by the proper switching of the `fill` voltage
between the segments. A fifth embodiment of the filter according to
the invention is characterized in that the first electrodes of
individual filter elements comprise a stack of segments in the
X-ray propagation direction, wherein the neighbouring segments are
mutually electrically isolated, the first electric potential being
switchable between the segments by the voltage supply means to
alter the length of the column of the X-ray absorbing fluid in the
X-ray propagation direction, the amplitude of the AC signal being a
function of time, control means being provided for controlling the
switching of the first electric potential between the segments in
dependence of the behavior of the AC signal with time.
[0013] This embodiment is based on the insight that during the
filling of a segment of the filter element by the electrically
conducting fluid component, the electric capacitance per filter
element is an increasing function with time. When a corresponding
segment is filled and no `fill` voltage is applied to the following
segment, the electric capacitance reaches the saturation region.
This region can be used as a trigger to set the `fill` voltage to
the next segment, if the fluid propagation within the inner volume
of the filter element is desired. For that purpose, a sixth
embodiment of the filter according to the invention is
characterized in that the control means are conceived to switch the
first electric potential between the segments triggered by the AC
signal reaching a saturation region. The switching of the `fill`
voltage between the consequent segments is a time critical
procedure, as the meniscus of the electrically conducting liquid
must be situated in the vicinity of the following segment in order
to feel the pulling force of the electrowetting power of the next
segment. The saturation region of the electric capacitance curve
indicates that the meniscus has arrived at the vicinity of the next
segment and, therefore, the switching can be enabled.
[0014] These and other aspects of the invention will be explained
with reference to the drawings, in which the same reference signs
indicate corresponding elements, wherein
[0015] FIG. 1 presents a schematic view of an X-ray apparatus
comprising a filter in accordance with the invention;
[0016] FIG. 2 is a diagrammatic sectional view of a filter element
of the filter of FIG. 1;
[0017] FIG. 3 gives a schematic functional representation of a
filter element of FIG. 2, where the first electrode is segmented
and forms two electric subgroups;
[0018] FIG. 4 presents an equivalent electric circuit of a filter
element of FIG. 3.
[0019] FIG. 1 presents a schematic view of an X-ray apparatus
comprising a filter in accordance with the invention. The X-ray
source 1 emits an X-ray beam 2 whereto an object 3, for example a
patient to be examined, is exposed. As a result of local
differences in the absorption of X-rays in the object 3 an X-ray
image is formed on the X-ray detector 4 which is in this case an
image intensifier pick-up chain. The X-ray image is formed on the
entrance screen 5 of the X-ray intensifier 6 and is converted into
a light image on the exit window 7, which light image is imaged on
a video camera 9 by means of a lens system 8. The video camera 9
forms an electronic image signal from the light image. The
electronic image signal is applied, for example for further
processing, to an image processing unit 10 or to a monitor 11 on
which the image information in the X-ray image is displayed.
[0020] Between the X-ray source 1 and the object 3 there is
arranged a filter 12 for local attenuation of the X-ray beam 2 by
means of various filter elements 13 in the form of tube-like
structures in line with the X-ray beam, whose X-ray absorptivity
can be adjusted by application of electric voltages to an electrode
located in a wall of each filter element. The electric voltages are
adjusted by voltage supply means 14 on the basis of, for example
brightness values of the X-ray image and/or on the basis of the
setting of the X-ray source; to this end, the adjusting circuit is
coupled to the power supply 15 of the X-ray source and to the
output terminal 16 of the video camera 9.
[0021] Part of the light of the exit window is guided, by way of a
splitting prism 19, to an exposure control system 20 which derives
a control signal from the light image in order to control the high
voltage supply on the basis of image information of the image on
the exit window. In order to receive image information of the image
on the exit window 7, the voltage supply means 14 of the filter 12
are coupled to the exposure control system 20, so that the filter
12 can be adjusted on the basis of the image on the exit window
7.
[0022] The filter 12 is constructed and oriented in such a way that
the filter elements 13 extend approximately in the propagation
direction of the X-ray beam 2; a uniform spatial resolution of the
spatial X-ray absorption pattern is thus achieved across the
cross-section of the X-ray beam. Alternatively, the filter 12 can
also be constructed in such a manner that the filter elements 13
extend approximately parallel to one another; when the X-ray beam 2
diverges, it is thus achieved that substantially all X-rays pass at
least partly through a filter element, so that the X-ray cannot
pass between two filter elements without being attenuated. The
voltage supply means apply electric voltages to the electrodes
located in the walls of the filter elements 13 so as to influence
the adhesion of the X-ray absorption fluid to the inner surface of
filter elements. In order to adjust a filter element to a high
X-ray absorptivity, an electric voltage of the first value is
applied to the wall of the relevant filter element by the voltage
supply means 14, the relevant filter element then being filled with
the X-ray absorbing fluid from the reservoir 17 via supply channels
18 by strong adhesion of the X-ray absorbing fluid to the inner
side. In order to adjust a filter element to a low X-ray
absorptivity, the voltage supply means 14 apply an electric voltage
of the second value, for example equal to the potential of a
reference electrode in the inner volume of the filter element, to
the wall of the filter element, the X-ray absorbing fluid the
exhibiting poor adhesion to the inner surface of the filter
element, so that this filter element is not filled with the X-ray
absorbing fluid from the reservoir 17. The general construction of
the filter 12 and the composition of the X-ray absorbing fluid is
described in more detail in American patent U.S. Pat. No. 5,625,665
(PHN 15.044). The construction of a filter element is described in
more detail with reference to FIG. 2.
[0023] FIG. 2a presents a schematic sectional view of a filter
element 13 of the filter 12 of FIG. 1. The filter element 13 is
filled by the X-ray absorption fluid 32 via a supply channel 30. In
this case the X-ray absorption fluid 32 forms one homogeneous
liquid which combines electrically conducting properties and X-ray
absorbing properties. For each filter element in the filter 12 one
defines a length direction z and an inner volume 31, which volume
is limited by the walls 38 of the corresponding filter element.
Each filter element comprises a first electrode 33 located in the
wall 38 in the form of an electrically conducting layer being
electrically isolated form the inner volume 31 by means of en
isolator layer 44, an inert coating layer 35 being located on the
inner surface of the walls 38 and a second electrode 39 to supply
an electrical potential to the X-ray absorption fluid 32. The
electrically conducting layer 33 of the filter element 13 is
coupled to a switch, which is this example is a drain contact 40 of
a field effect transistor 45, which source contact 41 is connected
to voltage supply means 36. The field effect transistor 45 is put
in a conduction mode, in other words the switching element is
closed by a control voltage from a control voltage supply line 37
supplied to a gate contact 34 of the field effect transistor 45. By
doing this, one applies an electric voltage to the first electrode
33 from the voltage supply means 36. When the voltage supply means
36 supply a voltage of the `fill`-voltage value, the contact angle
.theta. formed by the meniscus of the X-ray absorption fluid 32
with the inert coating layer 35 decreases and the filter element 13
is filled with the X-ray absorption fluid.
[0024] FIG. 2b shows a schematic section of a filter element 113 of
the filter 12 of FIG. 1 in case the X-ray absorption fluid
comprises two mutually not mixable first and second fluid
components, wherein the first fluid component 132 is a liquid with
electrically conducting properties and neglectable X-ray absorption
properties and the second fluid component 134 is an electrically
isolating liquid with high X-ray absorption properties, each fluid
component being supplied to the filter element 113 from its own
supply channel 120 and 121, respectively. The other functional
parts of the filter element 113 are similar to those of the filter
element of the FIG. 2a, resulting in a similar control of the level
of the first component in the inner volume 31 of the filter element
113. The level of the second fluid component in the inner volume of
the filter element is, thus, passively determined by the level of
the first fluid component therein, due to the fact that these two
fluids have a mutual separation plane 130. It is also possible to
design a filter element, where the first and second fluid
components do not have a mutual plane, but are separated by a layer
of a gaseous phase, which is further not shown in the FIG. 2b. The
degree of the X-ray absorption in these cases is determined by the
length of the column of the second component in the inner volume of
the filter element 113.
[0025] FIG. 3 gives a schematic functional representation of a
filter element of FIG. 2, where the first electrode is segmented in
the length direction z of the filter element 213 and forms two
electric subgroups 123 and 124, respectively. The function of the
filter element will be described for the case that the X-ray
absorption fluid 32 is a liquid solution between an electrically
conducting fluid component and an X-ray absorbing fluid component
and is supplied into the filter element 213 from the supply channel
30. FIG. 3a presents a schematic view on the subgroups 123 and 124
of the filter element that are controlled by the voltage supply
means 136. An electrical switch 138 alters the voltage supply from
one subgroup of electrode segments to another. Also, FIG. 3a gives
a schematic representation of the fluid level detection circuit,
comprising an AC source 50 in the form of a sinus wave generator
and an AC detector 60. A switching control unit 70 performs a
timing control of the voltage switching between subgroups 123 and
124 by the switch 138. FIG. 3b gives a temporal representation of
the voltage pulses as supplied by the voltage supply means 136 to
the corresponding subgroups of the first electrode. In order to
transport the X-ray absorption fluid 32 from the supply channel 30
into the inner volume 31 of the filter element 213 and further on
within the inner volume, the electrical switching between the
subgroups of the first electrode has to be performed in a
controlled way by the switching control unit 70, based on the
results of the fluid level detection within each segment. FIG. 3c
presents the corresponding course of the signal s from the AC
detector 60 with time. From the FIG. 3c it follows that each rising
part 90 of the signal s of the AC detector 60 corresponds to the
fluid rising within a corresponding segment and each plateau 92
corresponds to the moment in time when the fluid has filled the
corresponding segment. The insight of this interpretation is based
on the equivalent electric circuit of the filter element of this
type, which is given by FIG. 4. FIG. 4 presents an equivalent
electrical circuit for a detection of the fluid level within a
filter element, wherein the first electrode in segmented and two
electrical subgroups based on resulting segments are formed. From
FIG. 4a it follows that each subgroup of segments can be described
by a variable electric capacitance 140, which is formed between
each subgroup of segments and the fluid, the value of the
capacitance being a function of the degree of filling of that
subgroup by the fluid. The voltage supply means 136, initiating the
filling of a subgroup of the filter elements, supply the voltage to
these subgroups via switch 138. The measuring means comprise an AC
source 50 and the electrical circuit of the detectors 100 and 102.
In order to prevent the AC signal from being shorted by the voltage
supply means 136, resistors 137 and 139 in the M.OMEGA. range are
provided between the AC source 50 and the voltage supply means 136.
For measuring the electric capacitance of a filter element 140, a
capacitive voltage divider is provided, comprising the equivalent
capacitance from each subgroup 140 and an additional electric
capacitancesl33 and 135, which are in this example in the order of
20 pF. FIG. 4b presents the resulting signals Vl and V2 as measured
by the measuring means 100 and 102, respectively. In this example
the detected signal is the voltage across the segment. As the fluid
rises within the segment, the corresponding electric impedance of
the segment decreases, leading to the decrease of the voltage
across the segment. When the fluid has risen to the maximum level,
the measured voltage reaches a constant value. FIG. 4c gives a
further improvement of the detection method of the fluid level
within the filter element. According to the FIG. 4a, the measuring
means 100 and 102 can supply the signals from the detector to the
difference amplifier 101, schematically sketched in the FIG. 4c,
with the resulting signal 103 at the output of the difference
amplifier as given in the graph of FIG. 4c. By detecting the
plateau of this signal the switching between the corresponding
subgroups can be enabled.
* * * * *