Title of the invention
A multifunction image acquisition device
Background of the invention
The present invention relates to the general field
of image acquisition sensors and controlling such
sensors.
More precisely, the invention relates to an image
acquisition device enabling a dental radiological image
to be obtained, the device comprising a matrix sensor and
a control module for the sensor. The invention relates to
devices in which the matrix sensor comprises a plurality
of image acquisition photodiodes that are sensitive to
radiation, together with at least one detection
photodiode that is likewise sensitive to radiation.
Such sensors exist, in particular made using
complementary metal oxide-on-silicon (CMOS) technology,
that make it easy to integrate photodiodes having
different geometrical characteristics on a common
substrate. It is useful for the detection photodiode(s)
to present, for example, a size that is different from
the size of the acquisition photodiodes so as to obtain
higher sensitivity, enabling radiation to be detected
more quickly.
The device of the invention further comprises a
control module for controlling the matrix sensor, and
arranged to read the detection photodiode periodically
and to cause the sensor to change over between at least
two modes: a standby mode in which the acquisition
photodiodes are inhibited; and an acquisition mode in
which the energy received by the acquisition photodiodes
is used for acquiring an image.
The term "inhibit" is used to mean that any photons
received are not loaded, either by periodically purging
the acquisition photodiodes, or by blocking photon
reception by the acquisition photodiodes.

In known devices, the changeover is triggered as
soon as the detection photodiode detects irradiation by a
generator. Generally, receiving a predetermined quantity
of light means that irradiation has been detected.
Existing devices thus enable image acquisition to be
triggered as soon as the photodiode has received a given
quantity of energy in the period between two reads of the
detection photodiode.
With known devices, image acquisition is generally
performed throughout the duration of irradiation or for a
predetermined duration independently of the quantity of
energy that is actually sent towards the sensor.
Object and summary of the invention
A main object of the present invention is thus to
add to the functions of image acquisition devices for
obtaining a dental radiological image as specified in the
introduction by proposing that such a device should be
such that the detection photodiode is arranged to deliver
a periodic output signal to the control module, including
during irradiation and image acquisition by the
acquisition photodiodes, which periodic output signal has
a value that is representative of the instantaneous
received energy, and the control module uses this
periodic output signal to analyze the energy received
during acquisition.
Such an integrated matrix sensor enables the energy
received by the sensor to be tracked during image
acquisition since the diode is suitable for acting,
including during acquisition, to provide a signal that is
representative of the received energy and that is thus
quantitative. This signal is referred to below by terms
"quantitative signal".
With such an image acquisition device, the control
module is aware of the quantity of energy received by the
sensor, including during the period of irradiation. The
characteristic whereby the detection photodiode is

suitable for delivering a periodic signal, where the
period of the periodic signal is defined by the period
with which the detection photodiode is read, makes it
possible to implement all sorts of irradiation control
that were not previously possible using known devices.
In particular, the invention makes it possible to
perform quantitative analysis of the instantaneous
received energy on a permanent basis. By tracking this
instantaneous received energy, it is possible to detect
malfunctions of the generator. The invention thus makes
it possible to know the quality of the generator without
having recourse to dedicated appliances suitable for
measuring the quantity of energy that is actually
emitted.
Thus, according to an advantageous characteristic,
the control module is suitable for inserting a curve
tracking the quantity of received energy in a dedicated
zone of the acquired image.
On analyzing any image acquired with a sensor of the
invention while being irradiated by a particular
generator, this characteristic makes it possible to
extract from said image the curve tracking the received
energy. This makes it possible to evaluate the quality of
the emission by the generator in question since its
emission curve is made available.
The dedicated zone where the oscillogram curve of
the received energy is inserted is preferably a masked
zone in the image. It is either taken on the image
itself, e.g. constituting the first or the last line of
the image, or else it constitutes an additional "zero"
line added to the image.
With this advantageous characteristic, the invention
blocks off a very small portion of the image so as to
insert therein and store data relating to the
characteristics of the generator, since it relates to the
energy as received, and thus as emitted. Thus, when an
image of poor quality is obtained, it is always possible

with the invention to determine whether the poor quality
is due to poor emission by the generator or whether some
other reason needs to be found, for example the sensor
moved during acquisition.
It is recalled that the standby mode in which the
acquisition photodiodes are inhibited means that the
photons are not loaded by periodically purging them or by
blocking photon reception. Thus, in standby mode, purging
or blocking continues until the detection photodiode
detects radiation. This makes it possible to obtain a
good signal-to-noise ratio in the final image. Otherwise,
parasitic light received during the standby period
generates a background noise phenomenon on the image,
thereby degrading its quality.
It is in any event necessary and known that the
detection photodiode needs to be larger than the
acquisition photodiodes in order for it to be
sufficiently sensitive to detect radiation very quickly
while in standby mode. Under such circumstances, it is
very easily saturated. However, according to the
functional characteristic of the invention, the
photodiode is required to continue delivering a signal
that is quantitative, including during acquisition.
Thus, advantageously, the control module is arranged
to modify the resolution of the detection photodiode as a
function of the output signal from the detection
photodiode so as to ensure that the detection photodiode
does not saturate during irradiation.
Such a modification to its resolution is useful when
the sensitivity required for detecting radiation does not
make it possible to ensure there will be no saturation
during irradiation.
This characteristic enables the detection diode to
present a size that is large enough to be sufficiently
sensitive during standby mode and for it nevertheless to
be capable of delivering a quantitative signal
representative of the quantity of energy received

throughout irradiation, since this is the original and
novel function of the invention.
According to the invention, this characteristic can
be implemented in two particular manners by acting on two
distinct saturation phenomena.
The first phenomenon is the saturation phenomenon
whereby the photodiode itself saturates physically
between two reads on receiving a quantity of energy
greater than its "saturation" quantity of energy. If the
quantity of energy received between two reads is greater
than the saturation quantity of energy, then the signal
read from the photodiode can no longer be quantitative.
Typically, the photodiode read signal is
subsequently amplified by an electronic processor stage
prior generally to being sampled to produce the
photodiode output signal.
The term "photodiode read signal" is used herein to
mean the signal as read from the photodiode, while the
term "photodiode output signal" is used to mean the
signal as obtained after amplification.
The second phenomenon is the saturation phenomenon
that results from amplification of the detection diode
read signal. Amplification cannot produce a photodiode
output signal greater than its power supply voltage. If
amplifying a non-saturated photodiode read signal, i.e. a
signal that is quantitative, leads to an output signal
that is higher than the power supply voltage, then it is
the output signal that cannot be quantitative.
According to a particular characteristic of the
invention, in order to modify resolution, the control
module is arranged to increase the frequency at which the
detection photodiode is read after radiation has been
detected.
Under such circumstances, the capacity for
processing the energy received from the photodiode is
increased. By increasing the read frequency, the
detection photodiode can absorb more energy in a given

lapse of time and it may be observed that there is no
saturation phenomenon.
In the known prior art, the detection element does
not quantify the energy flux it receives, so it does not
matter that the detection element saturates during
irradiation. Indeed that is what is observed in practice
in the prior art. However that is contrary to the subject
matter of the invention, which enables the quantity of
energy that is received to be known by the control module
on a permanent basis and in quantitative manner.
Increasing the frequency thus amounts to reducing
the resolution of the photodiode, since for given
received power, the detection photodiode will be read for
smaller amounts of charge on the detection photodiode.
Nevertheless, this does not harm the accuracy of reading
during irradiation since large quantities of energy are
then received and by increasing the frequency it becomes
possible specifically for the quantities that are read to
be representative of the quantities of energy that are
actually received.
Increasing the read frequency may correspond to
multiplying it by ten, for example. Such an increase in
the read frequency makes it possible to ensure that the
photodiode saturates only when the energy received in a
given time lapse is ten times greater than when using the
initial frequency.
According to another particular characteristic of
the invention, each signal read from the detection
photodiode is amplified within a processor unit by an
electronic gain to form the output signal from the
sensor, and the control module is suitable for modifying
the electronic gain.
This characteristic makes it possible to ensure that
the output signal remains quantitative, providing the
photodiode is not itself saturated.
Typically, the gain used during standby mode is very
high in order to be able to detect radiation as quickly

as possible. If the gain is maintained at this value
during irradiation, then the output signal from the
photodiode, i.e. the amplified read signal, will very-
likely exceed the power supply voltage of the amplifier
stage and thus cease to be quantitative, even in the
presence of an increase in the frequency with which the
detection photodiode is read.
This characteristic makes it possible to resolve
conflicts between fineness of detection during standby
mode and the need to remain quantitative during
acquisition mode.
Advantageously, gain modification takes place as
soon as radiation is detected. When provision is made for
the modification in read frequency to be independent of
the level of energy received at the beginning of
irradiation, it is advantageous for gain modification to
occur before modification of the read frequency. Gain
modification is thus advantageously used as well as and
in combination with modification of the frequency at
which the photodiode is read.
The use of a detection photodiode integrated on the
same physical structure as the image acquisition
photodiodes, makes it easier to control the read
frequency or to modify the gain.
Advantageously, four levels of electronic gain are
provided in the invention. This characteristic offers
four levels of resolution for the quantity of energy read
by the photodiode and makes it possible to obtain a
quantitative output signal over a very wide range of
energy levels that are read. The extreme gain levels may
be dedicated at the highest end to resolving read
quantities of energy lying in the range 0 to
10 millivolts (mV), and at the other extreme, at the
small end, to resolving read quantities of energy lying
in the range 0 to 10 00 mV.

According to another characteristic, the detection
photodiode output signal is quantified in continuous
manner between two analog values.
This characteristic corresponds to sampling the
output signal so that it is known in the form of a
digital value enabling the received energy to be known
with fine resolution. Such sampling is advantageously-
implemented on 8 bits.
In an advantageous embodiment, the detection
photodiode is integrated at the periphery of the matrix
sensor.
This characteristic makes it possible to integrate a
rectangular photodiode of large size around the periphery
of the acquisition photodiodes that are themselves
integrated in the form of a matrix. CMOS technology makes
such integration possible.
In a particular application, the control module is
suitable for stopping acquisition mode as soon as a drop
is observed in the detection photodiode output signal.
This characteristic makes it possible to control
image acquisition as a function of the received energy.
This makes it possible to obtain images of good quality,
by ensuring that sufficient and optimum energy is
received while ensuring there is no saturation effect
that is penalizing for the acquisition photodiodes. When
using an alternating current (AC) generator, the term
"drop" in the output signal is used to mean that there is
no output signal for a duration that is longer than one
period of the generator. In particular, according to an
advantageous characteristic, analyzing the quantity of
energy received makes it possible, during acquisition, to
calculate the quantity of energy received by the sensor
so as to compare it with an optimum quantity of energy to
be received by the sensor.
This characteristic makes it possible to know when
the energy received by the sensor corresponds to the
optimum amount of energy for obtaining an image of good

quality. This can make it possible to stop acquisition
mode once said optimum amount of energy has been reached
and/or a command may be sent to the generator to cause it
to stop.
Thus, according to an advantageous characteristic of
the invention, the control module is arranged to send a
command to an irradiation generator to cause it to stop
irradiating as soon as the analysis of the received
energy shows that the optimum quantity of energy has been
received.
This advantageous characteristic makes it possible
to optimize the quantity of radiation received by the
patient since the generator itself is stopped as soon as
the quantity of energy received by the sensor is
appropriate for obtaining an image of quality.
Also advantageously, the control module is arranged
to stop acquisition mode as soon as the analysis of the
received energy shows that the optimum quantity of energy
has been received.
The invention also provides a method of controlling
an image acquisition device of the invention, the method
comprising periodic steps of sending commands for reading
the detection photodiode before and during irradiation
and image acquisition by the acquisition photodiodes and
providing a periodic output signal of value that is
representative of the instantaneous received energy, a
step of receiving said output signal, a step of
commanding the sensor to change over between standby mode
and acquisition mode, which step is triggered when the
detection photodiode detects radiation from a generator,
and an analysis step of analyzing the energy received
during acquisition by using the periodic output signal.
This method serves to track the energy received by
the matrix sensor before irradiation and throughout
irradiation.

In a preferred implementation, the various steps of
the method are determined by computer program
instructions.
Consequently, the invention also provides a computer
program on a data medium, the program being suitable for
being implemented in a control module and including
instructions adapted to implementing the steps of the
method of the invention. The program may use any
programming language and may be in the form of source
code, object code, or code intermediate between source
code and object code, such as in a form that is partially
complied, or in any other desirable form.
The invention also provides a data medium readable
by a control module and including instructions of a
computer program as mentioned above. The data medium may
be any entity or device capable of storing the program.
The medium may be a hardware element or a transmissible
medium, and in particular it may be downloaded from a
network of the Internet type. Alternatively, the data
medium may be an integrated circuit having the program
incorporated therein.
Brief description of the drawings
Other characteristics and advantages of the present
invention appear from the following description made with
reference to the accompanying figures that show an
embodiment having no limiting character, in which
figures:
• Figure 1 is a diagram of a sensor as used in an
image acquisition device of the invention;
• Figure 2 is a diagram of the relationship between
a control module as implemented in a device of the
invention, and the image acquisition sensor;
• Figure 3 is a flow chart of a method of the
invention;
• Figures 4A to 4F are timing diagrams of the
simultaneous behaviors respectively: of an alternating

current radiation generator; of a frequency with which
the detection photodiode is read in a device of the
invention suitable for modifying the resolution of the
detection photodiode by increasing the frequency with
which the detection photodiode is read and for detecting
when the irradiation stops; of the gain of the
photodiode; of the electronic gain used in a control
module of the device; of the output signal from the
detection photodiode; and of an accumulated reading of
the output signal;
• Figures 5A to 5F are timing diagrams of the
simultaneous behaviors respectively: of an alternating
current radiation generator; of a frequency with which
the detection photodiode is read in a device of the
invention suitable for modifying the resolution of the
detection photodiode by increasing the frequency at which
the detection photodiode is read, for detecting that a
predetermined received energy threshold has been reached,
and for inhibiting image acquisition on the matrix
sensor; of the gain of the photodiode; of the electronic
gain used in a control module of the device; of the
output signal from the detection photodiode; and of an
accumulated reading of the output signal;
• Figures 6A to 6F are timing diagrams of the
simultaneous behaviors respectively: of an alternating
current radiation generator; of a frequency with which
the detection photodiode is read in a device of the
invention suitable for modifying the resolution of the
detection photodiode by modifying an electronic gain for
processing the output signal from the detection
photodiode, for detecting that a predetermined received
energy threshold has been reached, and suitable for
inhibiting image acquisition on the matrix sensor; of the
gain of the photodiode; of the electronic gain used in a
control module of the device; of the output signal from
the detection photodiode; and an accumulated reading of
the output signal;

• Figures 7A to 7F are timing diagrams of the
simultaneous behaviors respectively: of a direct current
radiation generator; of a frequency with which the
detection photodiode is read in a device of the invention
suitable for modifying the resolution of the detection
photodiode by modifying an electronic gain for processing
the output signal from the detection photodiode, for
detecting that a predetermined received energy threshold
has been reached, and suitable for inhibiting image
acquisition on the matrix sensor; of the gain of the
photodiode; of the electronic gain used in a control
module of the device; of the output signal from the
detection photodiode; and an accumulated reading of the
output signal; and
• Figures 8A to 8F are timing diagrams of the
simultaneous behaviors respectively: of an alternating
current radiation generator; of a frequency with which
the detection photodiode is read in a device of the
invention suitable for modifying the resolution of the
detection photodiode by increasing the frequency at which
the detection photodiode is read at any moment during the
irradiation, for detecting that a predetermined received
energy threshold has been reached, and suitable for
inhibiting image acquisition on the matrix sensor; of the
gain of the photodiode; of the electronic gain used in a
control module of the device; of the output signal from
the detection photodiode; and an accumulated reading of
the output signal.
Detailed description of an embodiment
Figure 1 shows a sensor C of the invention in
diagrammatic manner. This matrix sensor C is in the form
of a central rectangular matrix having so-called
"acquisition" photodiodes DA integrated therein.
At the periphery of the acquisition photodiodes DA,
there is preferably integrated a single detection
photodiode referenced DD.

In another embodiment that is less favorable, it is
possible to envisage integrating a plurality of detection
photodiodes implemented in such a manner as to be read
periodically in the manner of the invention.
Nevertheless, it is desirable for the size of the
detection photodiode DD to be much greater than the size
of the acquisition photodiodes DA constituting the center
of the matrix sensor. This ensures that the detection
photodiode saturates more quickly and that it therefore
has sensitivity that is appropriate for detecting the
radiation. For given working area, it is therefore
preferable to integrate a single detection diode.
Advantageously, such a single detection photodiode DD is
integrated at the periphery of the acquisition
photodiodes DA.
Naturally, in a variant, the detection and/or
acquisition photodiodes may be replaced by any type of
photosensitive element, such as phototransistors, for
example.
The matrix sensor C is thus integrated in such a
manner as to be capable of including both types of diode,
e.g. using CMOS technology. It is sensitive to
radiological irradiation through a scintillator that
transforms the quantity of energy received in the form of
X-rays into a quantity of light.
The energy received on the detection photodiode DD
is then read periodically at the read frequency. The
analog data read from the photodiode constitutes a
photodiode read signal SL. Periodic read signals SL are
thus obtained during successive readings of the detection
photodiode. They are representative of the received
energy.
As shown diagrammatically in Figure 1, the sensor C
is associated with an electronic processor unit AD
suitable for transferring the analog data SL read from
the sensor into digital data constituting, at the output
from the processor unit AD, an output signal from the

detection photodiode, referenced NDD. This output signal
NDD is likewise periodic.
The analog-to-digital processor unit AD applies
electronic gain, written GAD, while transferring the
analog data as read from the detection photodiode DD into
a digital quantity. The unit AD thus amplifies the read
signal using the gain GAD and then samples the resulting
amplified analog value.
Advantageously, the sampling is such as to obtain a
value for the output signal NDD that is practically
analog between two extreme values, said output signal
being representative of the energy received on the
sensor.
The unit AD is advantageously an integrated portion
of the matrix sensor C, as shown diagrammatically in
Figure 1. It could also be separate therefrom on a
controller component advantageously then also including
the control module M. Furthermore, it should also be
observed that the control module M may also be integrated
on the same integrated circuit as the sensor C or it may
be integrated on a separate element, e.g. a controller
component of the sensor C, as stipulated above.
As shown in Figure 2, the sensor C of Figure 1
including the unit AD is implemented together with a
control module M, the two of them together forming an
image acquisition device of the invention. In the context
of acquisition device operation, the control module M and
the sensor C exchange signals with each other. The nature
of these signals is explained below with reference to
Figure 3.
Figure 3 is a flow chart of the method of the
invention. This method, as implemented in the control
module M of the image acquisition device of the
invention, comprises periodic steps of sending a read
command to the detection photodiode DD. The read commands
for the detection photodiode DD are thus sent regularly
and on a permanent basis.

To make Figure 3 easier to read, it is subdivided
into three portions, containing steps relating
respectively to the operation of the detection photodiode
DD, to the operation of the control module M, and to the
operation of the acquisition diodes DA. In point of fact,
all of the steps are controlled by the control module M,
but they are performed either by the detection diode DD,
or by the module M, or by the acquisition diodes DA, so
it appears more convenient to separate these steps
visually.
Thus, the periodic reading of the detection
photodiode DD under the control of the module M is
represented by a step E1, in which an output signal NDD
is obtained at an instant Ti. The periodicity of this
reading is represented diagrammatically in Figure 3 by an
incrementation step E'1 for incrementing the instant Ti
to Ti+1.
When the matrix sensor C is in standby mode, the
signal NDD is sent to the control module M for use in a
step EO having the purpose of detecting when irradiation
occurs.
When no radiation is detected (case N: no saturation
of the diode DD or no crossing of a detection threshold
or no observation of a received energy rise rate or rise
dynamics), the acquisition diodes DA are subjected to an
inhibit command written IDA in Figure 3. The acquisition
diodes DA are then either periodically purged, or
transfer of the received energy is inhibited, with
photons not being transmitted.
When radiation is detected in step EO (case Y: diode
DD saturated or detection threshold exceeded or
observation of a received energy rise rate or rise
dynamics), a changeover step E2 is triggered. This step
E2 has the effect of sending a changeover command SBA to
the acquisition diodes DA to cause them to change over
from standby mode to acquisition mode ACQ.

This changeover step E2 may also generate a command
for the detection diode DD for the purpose of modifying
its resolution. In particular, a command FDD for
modifying the read frequency of the detection photodiode
DD may then be sent. Advantageously, and even before the
read frequency modification command FDD, a command is
also generated at that moment for modifying the gain GAD
with which the read signal SL from the detection
photodiode DD is processed electronically.
Also, in order to determine whether a command for
modifying the resolution of the detection photodiode DD
is pertinent, it is useful for the value of the output
signal NDD also to be sent on a permanent basis to an
analysis unit ANA of the control module M, in which the
quantity of energy received and the rate (or dynamics) of
energy reception are analyzed within the control module
M.
The unit ANA thus advantageously operates on a
permanent basis. Nevertheless, it may also be activated
during step E2 only. Depending on the rate (or dynamics)
and the quantity of received energy, this unit ANA is
suitable for deciding, and possibly for calculating, a
modification in the frequency FDD with which the
detection diode DD is read and/or a modification in the
electronic processing gain GAD. This analysis unit ANA is
also suitable for determining whether an optimum quantity
of energy has been received or indeed, optionally, for
determining an optimum duration for image acquisition as
a function of the received energy and of the rate (or
dynamics) at which said energy is received.
In the implementation shown in Figure 3, the control
module M is suitable, in a step E3, for sending an
inhibit signal IDA to the acquisition diodes DA so as to
stop acquisition by said diodes. This signal may be sent
at the end of an optimum duration calculated by the unit
ANA, at the end of a predetermined fixed duration, or
indeed once an optimum of energy has been received by the

sensor. The value read on the acquisition diodes, written
VDA, is then sent to a memory MEM, as shown in Figure 3.
According to an advantageous characteristic, the
analysis step ANA may also lead to a step E3' that is
performed simultaneously with the step E3 and that is
drawn using dashed lines, this step causing the
generator, here referenced GEN, that irradiates the
sensor C to stop as soon as an optimum quantity of energy
has been received. This step E3' causes a stop command
STG to be sent to the generator GEN.
Figures 4 to 8 are timing diagrams of various
pertinent magnitudes showing how several embodiments of a
device of the invention operate.
Figure 4 applies to operation of the device of the
invention when an AC radiation generator GEN is used. The
energy emitted by the AC generator (referenced GEN in
Figure 4) is plotted as a function of time in Figure 4A.
By way of example, such an AC type generator emits X-rays
once every 20 milliseconds (ms), i.e. at a frequency of
50 hertz (Hz). The width of the pulses is generally about
10 ms. Figure 4 shows the operation of a device that is
suitable for detecting the end of irradiation before a
predetermined received energy threshold SPD is reached.
This predetermined threshold SPD is a function of the
size of the sensor and corresponds to an optimum quantity
of energy received by the sensor for obtaining an image
of good quality.
According to the invention, the detection photodiode
DD is read periodically at a frequency that is much
higher than the frequency of the pulses of radiation,
with the frequency FDD in this example being
100 kilohertz (kHz), as shown in Figure 4B. It can be
seen that the clock frequency of 100 kHz corresponds to
the detection photodiode being sampled at a sampling rate
that produces a measurement once every 10 microseconds
(µs) . In the embodiment described and with constant
exposure to X-rays, that corresponds to the detection

photodiode GDD having a gain that is four times greater
than at 400 kHz, as shown in Figure 4C. The output signal
NDD from the detection photodiode DD is shown in
Figure 4E. It can be seen that the value NDD is constant
and not zero so long as the AC generator is not emitting.
Step E0 is then looped back to itself, as shown in
Figure 3.
When the AC generator begins to emit, the output
signal from the detection photodiode NDD increases
strongly and quickly, as represented by a broad vertical
line in Figure 4E. Insofar as the X-ray emission lasts
for about 10 ms, the frequency at which the detection
photodiode is read enables 80 measurement samples to be
obtained during a single emission period of the generator
GEN. Such sampling enables an acceptable measurement to
be obtained in application of the Nyquist-Shannon
theorem.
The beginning of irradiation is thus detected on a
small number of read samples of the detection photodiode
DD and therefore cannot be shown in the timing diagrams
of Figure 4E other than diagrammatically by means of a
broad line. It can be seen that X-ray emission is thus
detected almost instantaneously in comparison with the
operating rate (or dynamics) of the generator and the
rate with which measurements are taken.
There are various ways of detecting the occurrence
of radiation. It is possible to assume that radiation is
detected from the moment when the output signal NDD for
at least one measurement sample exceeds a threshold value
for received energy intensity. Since the purpose is to
trigger as quickly as possible, it is useful for the gain
to be as large as possible and for the sampling frequency
to be as low as possible while complying with sampling
theorems. It should be observed here that the sampling
frequency, even at its lowest value, always remains much
greater than the radiation pulse frequency, and thus in
any event enables radiation to be detected very quickly

compared with the rate (or dynamics) at which the
radiation is generated.
It is also possible to detect the occurrence of
radiation only after tracking a small number of
measurement samples of the signal NDD, where the rate (or
dynamics) of the rise of the received energy was
analyzed. The radiation is then detected by tracking the
rate (or dynamics) at which energy is received.
This makes it possible to use the radiation rise
signature of the generator to trigger changeover to
acquisition mode. This avoids triggering acquisition mode
when the sensor is irradiated with parasitic energy other
than that coming from the scintillator and corresponding
to the X-rays emitted by the generator.
As soon as X-ray emission by the generator is
detected, as shown in Figure 3, the step E2 generates a
command signal SBA for the acquisition diodes, enabling
the beginning of acquisition ACQ to be triggered.
Simultaneously, when the analysis unit ANA that has also
received the values of the output signal NDD from the
detection photodiode becomes aware that the intensity of
the received energy exceeds or is going to exceed the
saturation threshold of the detection photodiode DD, e.g.
from the generator rise signature, the unit ANA is
arranged to send a command signal to the sensor C so as
to increase the frequency FDD with which the detection
diode DD is read, as can be seen in Figure 4B where the
frequency rises from 100 kHz to 400 kHz. The gain GDD of
the photodiode DD is then divided by four, thus ensuring
that no saturation of the detection photodiode DD is
observed throughout the acquisition step. By way of
example, the analysis unit ANA considers that the
received energy is going to exceed the saturation
threshold when it has received at least 70% of the energy
that corresponds to the saturation threshold VSAT of the
photodiode. This ensures that a quantitative value is
obtained.

If the detection photodiode DD were to saturate
during image acquisition, that would prevent the
measurement of the received energy being quantitative and
would thus make it impossible to determine the limit of
exposure accurately.
This exposure limit is advantageously determined by
a signal representing the sum of the instantaneous
received energies S. This signal is shown in Figure 4F
and is incremented on each pulse sent by the generator
GEN.
In Figure 4, image acquisition is not stopped under
the control of the signal S. The received signal is thus
tracked but it is not used for optimizing the exposure of
the acquired image. In this embodiment, it is detecting
the end of emission by the generator GEN that triggers
the end of image acquisition.
The detection photodiode DD is advantageously used
for detecting the end of emission by the generator. When
the signal NDD drops below a given value for a duration
that is longer than the emission half-period of the
generator, the analysis unit ANA is advantageously
arranged to generate a command to stop acquisition by the
acquisition diodes DA.
It should be observed that the electronic
amplification gain GAD is not modified by the control
module M. This means that the amplified read signals from
the detection photodiode do not exceed 70% of the power
supply voltage VAL of the amplifier unit AD.
Advantageously, after the radiation stops, the
analysis unit ANA is arranged so that if there is a
voltage that is less than 30% of the photodiode
saturation voltage VSAT, then the read frequency FDD is
reduced and the gain GAD is increased. This
characteristic makes it possible to return to conditions
that are favorable for the detection photodiode DD
detecting new radiation.

Figure 5 comprises timing diagrams similar to those
of Figure 4, for use of the same generator, with its
emission of radiation GEN being once more plotted in
Figure 5A. Figure 5 shows the operation of a device in
which the resolution of the detection photodiode DD is
likewise modified by increasing the frequency with which
the detection photodiode DD is read on detecting
radiation, as can be seen in Figures 5B and 5C.
The difference compared with Figure 4 consists in
image acquisition being inhibited as soon as the sum of
the received energies S reaches an optimum threshold SPD
of received energy that is predetermined to obtain an
appropriate image that is optimum from the exposure point
of view. After step E3 has been triggered, as shown in
Figure 3, the sensor C then receives a command signal IDA
that inhibits the acquisition photodiodes DA. Image
transfer then occupies about one second. It should be
observed that the detection photodiode DD may also be
inhibited. In Figure 5A, it can be seen from the curve
GEN that the generator then continues to emit two more
pulses in spite of image acquisition having come to an
end.
In an advantageous embodiment of the invention (not
shown), the control module M controlling the sensor C is
suitable for sending a command to the generator so as to
cause it to stop emitting as soon as the predetermined
optimum threshold SPD has been reached and image
acquisition has been stopped.
Figure 6 shows timing diagrams obtained using the
same generator as in Figures 4 and 5, but in this
embodiment there is a modification to the electronic gain
GAD so as to ensure that there is no saturation in the
amplification of the read signal by virtue of it
exceeding the power supply voltage of the unit AD.
In this embodiment, in standby mode, the gain GAD is
multiplied by four. This is useful for increasing
detection sensitivity. This X4 gain GAD also applies on

the saturation voltage VSAT of the detection photodiode
which therefore appears greater in the output signal NDD.
It can thus be observed that 70% VSAT is not shown at the
beginning and at the end of Figure 6D since it lies off
the scale shown. In contrast, the saturation level of the
output signal relative to the power supply voltage VAL is
not modified by applying the gain GAD. It should be
observed that here it is the output signal NDD exceeding
the value 70% VAL that is used for triggering a
modification to the resolution of the photodiode or of
the amplification, instead of exceeding the value 70%
VSAT as shown in the preceding figures.
In Figure 6D, on radiation being detected, the gain
GAD is divided by four. This modification to the gain GAD
enables energy levels as read on the detection photodiode
DD to be sampled over a different interval. This
modification makes it possible to retain the quantitative
nature of the output signal NDD for the energies read on
the photodiode that are greater than when using a gain of
four for which a small quantity of energy read on the
photodiode DD can be identified very quickly, as is
useful in standby mode.
In the absence of a modification to the gain GAD,
the amplified voltage NDD would exceed the power supply
voltage VAL which would lead to the output signal NDD
losing its quantitative nature.
In order for it to be possible to make use solely of
the modification to the electronic gain GAD, it is
necessary that the photodiode does not saturate at the
frequency used. That is why, in this figure, the
frequency used is directly 400 kHz, since this frequency
gives the smallest resolution to the diode, including
during standby mode, and also greatest capacity for
receiving energy without saturating.
If the frequency of the detection diode DD were
100 kHz, then the intensity received on the detection
photodiode DD would cause it to saturate. The pulses

observed in Figure 6 would then be seen to be peak-
limited regardless of the gain GAD used in the processing
of the signal from the detection photodiode DD.
In practice, modifications to the amplification gain
and to the read frequency are used in combination. The
gain is advantageously reduced as soon as radiation is
detected, and frequency is increased subsequently or
simultaneously. When the control module M is arranged so
that the read frequency is modified on each occasion as a
function of the received energy, it is very useful for
the gain to be diminished immediately by a very large
amount, e.g. by going from 1000 to 1, so that saturation
of the electronic amplification does not mask the
quantitative signal as read on the photodiode.
It can happen that a large gain leads to a
quantitative signal as read from the non-saturated
photodiode ceasing to be quantitative after it has been
amplified since it has reached the power supply voltage
VAL. This is harmful specifically when the output signal
NDD, i.e. the amplified read signal, is used to define
the frequency FDD with which the detection photodiode DD
is read. It would then be necessary to reduce the
frequency FDD to a much greater extent in order to obtain
a quantitative signal than would be necessary if the
amplification gain GAD were automatically reduced from
the beginning of irradiation. This is shown below in
Figure 8.
Figure 7 shows timing diagrams of the simultaneous
behaviors of pertinent magnitudes that are observed when
a direct current (DC) radiation generator is used. The
emission profile GEN from the DC generator is shown in
Figure 7A. Once more, it can be seen that the frequency
FDD at which the photodiode is read is set at 400 kHz,
and that in the embodiment that corresponds to Figure 7,
it is the electronic gain GAD that is modified to modify
the resolution of the detection photodiode DD.

In Figure 7, as soon as emission has been detected
from the generator, the gain GAD changes over so as to
reduce the resolution of the detection photodiode DD and
so as to ensure that the output signal NDD is
quantitative, as shown in Figure 7E.
In this embodiment the sum signal S is linear, being
of constant slope as shown in Figure 7F. In Figure 7, as
in Figure 4, stopping the generator causes acquisition
ACQ by the acquisition diodes DA to be stopped. Stopping
of the generator is detected by analyzing the signal NDD,
as represented by Figure 7E. Stopping of the DC generator
is detected when the received energy drops below a
predetermined floor.
Finally, it should be observed that the resolution
of the detection photodiode DD may be modified not only
at the time of changing the acquisition diodes DA over to
acquisition mode, but also during acquisition ACQ by the
acquisition diodes DA. This is useful when the intensity
from the generator rises more than expected.
This is shown in Figure 8. In Figure 8, it can be
seen that the gain GAD is equal to two with a frequency
of 100 kHz. It can then be seen that the value 30% VSAT
is doubled compared with using unit gain GAD. Thus, for
the emission GEN from the generator, as used in Figure 4
for example, sensitivity for detecting radiation is
correspondingly increased.
Once radiation has been detected, the frequency FDD
is increased in accordance with the invention, in this
embodiment being multiplied by four. Nevertheless, it
should be observed that this no longer suffices starting
from the fourth pulse from the generator since the signal
NDD has reached a voltage corresponding to 70% of the
power supply voltage VAL. Nevertheless, it can be
observed that the diode DD does not saturate physically,
so the read signal SL continues to be a representation of
the received energy that is quantitative because of the
increase in the read frequency.

The device is then suitable, as a result of the
analysis unit ANA analyzing the instantaneous received
energy, of further modifying the resolution of the
detection photodiode DD by further increasing the
frequency with which the detection photodiode DD is read,
in this example by multiplying the frequency by 1.5 as
soon as the signal NDD reaches 70% of VAL.
With this new increase in the read frequency FDD,
the signal NDD does indeed remain below 70% of the power
supply voltage VAL. The output signal NDD then remains in
the range of energies read from the photodiode that can
be amplified by the gain GAD without reaching the power
supply voltage VAL. This makes it possible to ensure that
the received energy continues to be quantitative, thereby
making it possible to determine the instant at which the
received energy corresponds to obtaining an image of
appropriate quality.
Finally, it should be observed that various
implementations can be achieved on the principles of the
invention.

CLAIMS
1. An image acquisition device (C) enabling a dental
radiological image to be obtained, the device comprising
a matrix sensor (C) integrating a plurality of image
acquisition photodiodes (DA) that are sensitive to
radiation, and at least one detection photodiode (DD)
that is likewise sensitive to radiation, the device
further comprises a control module (M) for controlling
the sensor (C) and adapted to read the detection
photodiode (DD) periodically and to cause the sensor (C)
to change over between at least two modes: a standby mode
in which the acquisition photodiodes are inhibited; and
an acquisition mode (ACQ) in which the energy received by
the acquisition photodiodes (DA) is used for acquiring an
image; with changeover being triggered (SBA) as soon as
the detection photodiode (DD) detects radiation from a
generator, the device being characterized in that the
detection photodiode (DD) is suitable for supplying a
periodic output signal (NDD) to the control module (M) ,
including during irradiation and image acquisition by the
acquisition photodiodes (DA), which periodic output
signal (NDD) has a value that is representative of the
instantaneous received energy, and in that the control
module (M) uses this periodic output signal (NDD) to
analyze (ANA) the energy received during acquisition
(ACQ), the control module (M) also being arranged to
modify the resolution of the detection photodiode (DD) as
a function of the output signal (NDD) from the detection
photodiode (DD) in order to ensure that the detection
photodiode (DD) does not saturate during irradiation.
2. A device according to claim 1, characterized in that
the control module (M) is suitable for inserting a curve
tracking the quantity(ies) of received energy in a
dedicated zone of the acquired image.

3. A device according to claim 1 or claim 2,
characterized in that, in order to modify resolution, the
control module (M) is suitable for increasing the
frequency at which the detection photodiode (DD) is read
(NDD) after radiation has been detected.
4. A device according to any preceding claim,
characterized in that each signal read from the detection
photodiode (DD) is amplified within a processor unit (AD)
by an electronic gain (GAD) to form the output signal
(NDD) from the sensor (C), and the control module (M) is
suitable for modifying the electronic gain (GAD).
5. A device according to any preceding claim,
characterized in that the detection photodiode (DD) is
integrated at the periphery of the matrix sensor (C).
6. A device according to any preceding claim,
characterized in that the control module (M) is suitable
for stopping acquisition mode (ACQ) as soon as a drop is
observed in the output signal (NDD) from the detection
photodiode (DD).
7. A device according to any preceding claim,
characterized in that the analysis (ANA) of the received
energy makes it possible, during acquisition (ACQ), to
calculate the quantity (S) of energy received by the
sensor (C) so as to compare it with an optimum quantity
(SPD) of energy to be received by the sensor (C).
8. A device according to claim 7, characterized in that
the control module (M) is suitable for sending a command
(STG) to an irradiation generator (GEN) to cause it to
stop irradiating as soon as the analysis (ANA) of the
received energy (S) shows that the optimum quantity (SPD)
of energy has been received.

9. A device according to claim 7 or claim 8,
characterized in that the control module (M) is suitable
for stopping acquisition mode (ACQ) as soon as the
analysis (ANA) of the received energy shows that the
optimum quantity (SPD) of energy has been received.
10. A method of controlling an image acquisition device
according to any preceding claim, the method comprising
periodic steps (E1) of sending commands for reading the
detection photodiode (DD) before and during irradiation
and image acquisition (ACQ) by the acquisition
photodiodes (DA) and providing a periodic output signal
(NDD) of value that is representative of the
instantaneous received energy, a step of receiving said
output signal (NDD), a step (E2) of commanding the sensor
(C) to change over (SBA) between standby mode and
acquisition mode (ACQ), which step is triggered when the
detection photodiode detects (E0) radiation from a
generator (GEN), an analysis step (ANA) of analyzing the
energy received during acquisition by using the periodic
output signal (NDD), at least one step of the control
module (M) modifying the resolution of the detection
photodiode (DD) as a function of the output signal (NDD)
from the detection photodiode (DD) so as to ensure there
is no saturation of the detection photodiode (DD) during
irradiation.
11. A computer program including instructions for
executing steps of the control method of claim 10 when
said program is executed by a control module (M) as
implemented in an image acquisition device according to
any one of claims 1 to 9.
12. A computer-readable recording medium having recorded
thereon a computer program including instructions for
executing steps of the control method according to claim
10.

The invention relates to an image acquisition device
enabling a dental radiological image to be obtained, the
device comprising a matrix sensor (C) having integrated
therein a plurality of image acquisition photodiodes (DA)
sensitive to irradiation and at least one detection
photodiode (DD) also sensitive to irradiation, the device
also comprising a control module (M) for controlling the
sensor (C) and suitable for periodically reading the
detection photodiode (DD) and for causing the sensor (C)
to change over (SBA) between at least two modes: a
standby mode and an acquisition mode (ACQ). According to
the invention, the detection photodiode (DD) is suitable
for delivering a periodic output signal (NDD) to the
control module (M), including during irradiation and
image acquisition (ACQ) by the acquisition photodiodes
(DA), which periodic output signal (NDD) has a value that
is representative of the instantaneous received energy,
and the control module (M) makes use of this periodic
output signal (NDD) to analyze the energy received during
acquisition (ACQ).