U.S. patent application number 09/826256 was filed with the patent office on 2001-08-09 for x-ray examination apparatus with x-ray image sensor matrix and correction unit.
This patent application is currently assigned to U.S. PHILIPS CORPORATION. Invention is credited to Conrads, Norbert, Schiebel, Ulrich, Weibrecht, Martin, Wieczorek, Herfried K..
Application Number | 20010012331 09/826256 |
Document ID | / |
Family ID | 8224756 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012331 |
Kind Code |
A1 |
Conrads, Norbert ; et
al. |
August 9, 2001 |
X-ray examination apparatus with x-ray image sensor matrix and
correction unit
Abstract
An x-ray examination apparatus comprises an x-ray image sensor
matrix (1) for deriving an initial image signal from the x-ray
image. The sensor elements of the x-ray sensor matrix convert
incident x-rays into electric charges. These electric charges are
read-out and converted into the initial image signal. Further a
correction unit (2) is provided for correcting the initial image
signal, notably for disturbances due to delayed transferred
charges, that have been retained in the sensor elements for some
time. The correction unit (2) is provided with a memory which
stores correction values. Further the correction provided with a
selection unit (5) for selecting appropriate correction values from
the memory (3).
Inventors: |
Conrads, Norbert; (Raeren,
BE) ; Weibrecht, Martin; (Aachen, DE) ;
Schiebel, Ulrich; (Aachen, DE) ; Wieczorek, Herfried
K.; (Aachen, DE) |
Correspondence
Address: |
Corporate Patent Counsel
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Assignee: |
U.S. PHILIPS CORPORATION
|
Family ID: |
8224756 |
Appl. No.: |
09/826256 |
Filed: |
April 4, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09826256 |
Apr 4, 2001 |
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09125482 |
Aug 19, 1998 |
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6246746 |
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09125482 |
Aug 19, 1998 |
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PCT/IB97/01595 |
Dec 23, 1997 |
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Current U.S.
Class: |
378/98.7 ;
348/E5.08; 348/E5.088; 378/207; 378/98.2 |
Current CPC
Class: |
H04N 5/325 20130101;
H04N 5/3597 20130101 |
Class at
Publication: |
378/98.7 ;
378/98.2; 378/207 |
International
Class: |
H05G 001/64 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 1996 |
EP |
96203692.7 |
Claims
1. An x-ray examination apparatus comprising an x-ray image sensor
matrix (1) for deriving an initial image signal from an x-ray
image, a correction unit (2) for deriving a corrected image signal
from the initial image signal characterised in that the correction
unit (2) includes a memory (3) for storing correction values and an
arithmetic unit (4) for computing signal levels of the corrected
image signal from signal levels of the initial image signal and at
least some of said correction values.
2. An x-ray examination apparatus as claimed in claim 1,
characterised in that the correction unit (2) includes a selection
unit (5) for selecting correction values from the memory (3) on the
basis of exposure parameters.
3. An x-ray examination apparatus as claimed in claim 2,
characterised in that the correction unit (2) is arranged to
generate a reference image signal from the x-ray sensor matrix (1),
the selection unit (5) is arranged to select the correction values
on the basis of the reference image signal.
4. An x-ray examination apparatus as claimed in any one of the
preceding claims, characterised in that the arithmetic unit (4) is
arranged to compute correction values from stored correction
values.
5. An x-ray examination apparatus as claimed in claim 4,
characterised in that the arithmetic unit (4) is arranged to
interpolate said computed correction values between stored
correction values.
Description
[0001] The invention relates to an x-ray examination apparatus
comprising an x-ray image sensor matrix for deriving an initial
image signal from an x-ray image, a correction unit for deriving a
corrected image signal from the initial image signal.
[0002] Such an x-ray examination apparatus is known from the
European patent application EP 0 642 264.
[0003] The correction unit of the known x-ray examination apparatus
counteracts disturbances in the initial image signal which are
caused by delayed emission of charges from the x-ray image sensor
matrix. Incident x-rays release electric charge carriers, i.e.
photocharges, notably photoelectrons, in the x-ray image sensor
matrix and these electric charges are detected. The signal levels
of the initial image signal represent the detected charges. A part
of the charge carriers can be trapped in a trap-state, be retained
in such a trap-state, and can escape from the trap-state at a later
stage and are detected as electric charges with a delay. If the
initial image signal with the disturbances were applied to a
monitor for displaying the image information, not only the image
information of the instantaneous image would be reproduced, but at
the same time also image information of a previously picked-up
image. As a result after-images would be displayed together with
the instantaneous image.
[0004] The correction unit of the known x-ray examination apparatus
utilises an intricate mathematical model based on physical
considerations for the trapping and subsequent release of charge
carriers so as to correct disturbances due to charge carriers
emitted in a delayed fashion. A drawback of the correction unit of
the known x-ray examination apparatus is that the computations to
obtain the corrected image signal are rather complicated so that a
powerful arithmetic unit is required which nevertheless requires a
rather long computation time. Hence, the time between forming of
the x-ray image and displaying the image information is relatively
long so that the known x-ray examination apparatus is not well
suitable for imaging rapid dynamic processes in which variations
occur within a period of time comparable to the required
computation time. Moreover, the required programming of the
arithmetic unit is complicated, so that well educated staff is
needed to set up the correction unit.
[0005] An object of the invention is to provide an x-ray
examination apparatus which requires, in comparison with the known
x-ray examination apparatus, a shorter period of time to derive the
corrected image signal from the initial image signal.
[0006] This object is achieved by means of an x-ray examination
apparatus according to the invention which is characterised in that
the correction unit includes a memory for storing correction values
and an arithmetic unit for computing signal levels of the corrected
image signal from signal levels of the initial image signal and at
least some of said correction values.
[0007] The correction values are obtained from a separate
calibration of the x-ray examination apparatus. This calibration
involves irradiating the x-ray image sensor matrix with a
predetermined calibration x-ray exposure. In particular, this
predetermined calibration x-ray exposure is carried out by way of
applying a pre-selected number of x-ray pulses with a pre-selected
pulse-length, pulse-rate and x-ray dose per pulse. Subsequent to
the calibration x-ray exposure the x-ray image sensor is read-out
so as to obtain a calibration image signal. That is, the charge
carriers in respective sensor elements of the x-ray sensor matrix
after the predetermined calibration x-ray exposure are detected.
The period of time lapsed between the calibration x-ray exposure
and the generation of the calibration image signal is recorded or
controlled. The calibration image signal represents the electric
charges emitted during read-out after said period of time since the
predetermined calibration x-ray exposure.
[0008] The physical process of trapping and releasing of electric
charges in the x-ray image sensor matrix is represented by a
mathematical model containing a small number of modelparameters.
This mathematical model has been disclosed in more detail in the
article >Measurements and simulation of the dynamic performance
of an --Si:H image sensors=in the Journal of Non-crystalline solids
Vol.164-166(1993)781-784. Values for the modelparameters are
derived from the calibration image signal, in particular by
performing a best fit to the mathematical model. On the basis of
the values of the modelparameters there are computed correction
values which represent delayed electric charges for separate x-ray
exposure circumstances and for arbitrary periods of time lapsed
since the latest x-ray exposure. This computation of the correction
values can be performed separately from the x-ray examination
apparatus, but the arithmetic unit of the x-ray examination
apparatus itself can also be used. Sets of correction values are
stored which relate to various x-ray exposure circumstances. In
particular, sets of correction values are stored for separate
values of the number of preceding x-ray pulses, the x-ray pulse
rate, respective intensities of the preceding x-ray pulses. Each
set includes correction values for several values of the time
lapsed since the latest x-ray pulse.
[0009] Instead of calculating correction values for various periods
of time lapsed since the latest x-ray exposure by way of the
mathematical model, such correction values can be derived from a
calibration signal sequence. The calibration signal sequence is
formed by reading out the x-ray image sensor matrix at several
moments after the latest x-ray exposure. The signal levels of the
calibration signal sequence represent the decay of trapped charges
as time proceeds. Hence, the correction values for separate periods
of time lapsed since the latest x-ray exposure can be derived from
the calibration signal sequence without calculations based on the
mathematical model.
[0010] The arithmetic unit computes the signal levels of the
corrected image signal from the signal levels of the initial image
signal and the correction values. Only relatively simple
computations are required such as subtracting the correction values
from the signal levels of the initial image signal. Because the
computations involving the mathematical model of the trapping and
release of charges need only to be carried-out once for the
calibration image signal, for correcting the initial image signal
only simple calculations are required. The simplicity of the
required calculations is achieved at the cost of storing a number
of correction values which is substantially larger than the small
number of modelparameters.
[0011] Since retrieving correction values from the memory can be
done very rapidly and because only simple calculations are
required, it takes only a short time to derive the corrected image
signal after the initial image signal has been read out. Moreover,
electronic memories having a large storage capacity are
commercially available and are relatively inexpensive. The x-ray
examination apparatus according to the invention is suitable to
handle x-ray images at a rate of 25-30 or even 60 images per
second. Therefore, the x-ray examination apparatus according to the
invention is especially suitable for real time x-ray imaging for
instance in interventional procedures. In particular after images
are suppressed when a fluoroscopy is performed by continuously
irradiating at low x-ray dose after one or a few brief x-ray
exposures at high-dose have been carried-out.
[0012] A preferred embodiment of x-ray examination apparatus
according to the invention is characterised in that the correction
unit includes a selection unit for selecting correction values from
the memory on the basis of exposure parameters.
[0013] The selection unit selects correction values which pertain
to an x-ray exposure which has preceded the formation of the x-ray
image. The preceding x-ray exposure has caused trapped charges
which give rise to disturbances in the initial image signal. To
select appropriate correction values from the memory, the selection
unit receives information on the preceding x-ray exposure from the
x-ray examination apparatus. Such information is readily available
from for example a central control unit of the x-ray examination
apparatus.
[0014] A preferred embodiment of x-ray examination apparatus
according to the invention is characterised in that the correction
unit is arranged to generate a reference image signal from the
x-ray sensor matrix, the selection unit is arranged to select the
correction values on the basis of the reference image signal.
[0015] The reference image signal is generated when no x-rays are
incident on the x-ray sensor matrix. Amounts of charge having
remained in respective sensor elements since a previous x-ray
exposure are detected and the signal levels of the reference image
signal represent these charges. In fact, the reference image signal
represents an after image that is still present in the x-ray sensor
matrix at the moment that the reference image signal is generated.
The reference image signal represents that amounts of electric
charge trapped in respective sensor elements can be different. Such
differences are in particular due to the fact the respective sensor
elements can receive different x-ray doses due to the previous
x-ray exposure. From the reference image signal and a few exposure
parameters, such as the x-ray pulse rate and the number of x-ray
pulses, relating to the previous x-ray exposure accurate correction
values are selected. These selected correction values correspond to
the charges that remain trapped in the sensor elements at the
moment of the generation of the initial image signal. The period of
time between the previous x-ray exposure and the generation of the
initial image signal is either recorded or controlled so as to
select correction values which take into account the decay of
trapped charges in that period of time. That decay with time is
accurately included in the mathematical model or in the calibration
signal sequence, so that the calibration provides correction values
which accurately takes into account that trapped charges decay with
time.
[0016] A preferred embodiment of x-ray examination apparatus
according to the invention is characterised in that the arithmetic
unit is arranged to compute correction values from stored
correction values.
[0017] Sets of correction values for only a limited number of
values for the exposure parameters need to be stored in the memory.
Separate sets contain correction values for respective amounts of
the exposure parameters and for several values of the time lapsed
since the previous x-ray exposure. It appears advantageous to store
separate sets of correction values relating to less than ten
different x-ray pulse rates, less than ten different values for the
number of x-ray pulses and about a few dozen different values for
the x-ray dose per pulse. This amounts to several hundreds of sets
of sets of correction values that are stored in the memory
[0018] A preferred embodiment of x-ray examination apparatus
according to the invention is characterised in that the arithmetic
unit is arranged to interpolate said computed correction values
between stored correction values.
[0019] Interpolation is a simple, rapid and accurate method to
derive a correction value from stored correction values. Thus, only
correction values for a relatively small number of values for the
modelparameters need to be stored. Correction values relating to
values of modelparameters for which no correction values are stored
are interpolated from correction values relating to values of
modelparameters which are close to the values at issue. Further,
correction values relating to an arbitrary time lapsed since the
latest x-ray exposure can be calculated from stored correction
values for particular values for said lapsed time. Preferably a
bisection method is employed for rapidly finding correction values
which are employed for the interpolation.
[0020] These and other aspects of the invention will be elucidated
with reference to the embodiments described hereinafter and with
reference to the accompanying drawing wherein
[0021] The FIGURE schematically shows an x-ray examination
apparatus according to the invention.
[0022] The FIGURE schematically shows an x-ray examination
apparatus according to the invention. An x-ray source 10 is
arranged to irradiate an object 11, in particular a patient who is
to be radiologically examined, with an x-ray beam 12. Owing to
locally varying x-ray absorption in the patient, the x-ray image is
formed on the x-ray image sensor matrix 1. The x-ray image sensor
matrix comprises a large number, e.g. 512H512, 1 k.sup.2, 2 k.sup.2
or even 3 k.sup.2 sensor elements which convert x-radiation into
electrical charges. These electrical charges are detected by way of
a multiplex circuit and from these read-out charges the initial
image signal is derived. Methods for the detection of electric
charges in an image sensor matrix so as to read out the image
information picked up are known as such, for example from the
European patent applications EP 0 440 282 and EP 0 444 720. In
particular, the initial image signal is an electronic video signal
which is supplied to a bus 13 which couples the x-ray image sensor
matrix to the correction unit 2. Owing to trapped charges, the
initial image signal is disturbed in that the initial image signal
also represents after images. The correction unit 2 removes such
disturbances from the initial image signal and supplies the
corrected image signal to a monitor 14 and/or to a buffer 15. On
the monitor 14, the image information in the x-ray image is
displayed without after images. Thus, the displayed image has a
high diagnostic quality in that small details with little contrast
are rendered well visible. The image stored in the buffer 15 may be
further processed and/or supplied to a hardcopy unit which forms a
hard-copy of the image, e.g. the hard-copy unit prints the image on
a transparent sheet.
[0023] In order to correct the initial image signal, an appropriate
correction value (CV) is selected from the memory 3 by way of the
selection unit 5. The selected correction value (CV) is supplied to
the arithmetic unit 4 together with the initial image signal (IS).
The arithmetic unit computes the signal level of the corrected
image signal (CS) from the signal level of the initial image signal
(IS) and the correction value (CV). In the event that the memory 3
does not contain correction values for the values at issue of the
exposure parameters, then an accurate correction value is obtained
by interpolation between stored correction values. The selection
unit 5 supplies a selection signal (SS) to the memory 3 so as to
indicate the appropriate address in the memory 3 which contains the
correction value at issue. The selection signal represents exposure
parameters (EP) of preceding x-ray exposures that have caused
trapped charges in the sensor elements. In particular, the control
unit 6 supplies exposure parameters such as the peak radiation
intensity of preceding x-ray pulses, the duration of the preceding
pulses, the number of preceding pulses, and/or the pulse rate and
the time lapsed since the latest x-ray pulse. The control unit 6 is
coupled with the high-voltage generator 16 of the x-ray source 10
so as to receive the relevant values of the exposure
parameters.
[0024] The control unit further arranges to form the reference
image (RI) when no x-rays are incident on the x-ray sensor matrix.
To form the reference image the control unit supplies a control
signal (CtS) to detect remaining charges in the x-ray image sensor
matrix 1. Preferably, the delay between the generation of the
initial image signal and the preceding irradiation and the delay
between the generation of the calibration image signal and the
calibration x-ray exposure are arranged to be equal. Then the
signal levels of the reference image correspond to signal levels of
the calibration image signal at the x-ray dose received at
respective sensor elements Thus, for respective sensor elements,
the relevant set of correction values is easily selected according
to the signal level of the reference image. The selected set of
correction values represents the decay with time of the trapped
charges in the sensor element at issue.
[0025] Further, the control unit 6 is arranged to switch the x-ray
image sensor matrix 1 with the correction unit between an imaging
mode and a calibration mode. In the calibration mode a
predetermined series of calibration x-ray exposures is carried-out
from which the relevant values of the modelparameters are derived
by the arithmetic unit 4. The arithmetic unit 4 subsequently
derives the correction values based on the mathematical model.
[0026] Advantageously, the arithmetic unit 4 comprises a fast
processor which is particularly suitable for real-time correction
of the initial image signal by way of the correction values. In
addition the arithmetic unit 4 comprises a separate suitably
programmed general purpose processing unit for controlling the
recording of the calibration signal and computing the
modelparameters. The general purpose processing unit is coupled to
the control unit 6 so as to receive the exposure parameters. The
general purpose processing unit is also coupled to the bus 13 so as
to receive the calibration signal. The general purpose processing
unit supplies the correction values to the memory 3. In particular,
the general purpose processing unit comprises programmable hardware
suitable to perform the calculations according to the mathematical
model.
* * * * *