IMAGE PICK-UP APPARATUS INCLUDING A CORRECTION UNIT

Abstract: The image pick-up apparatus (1) is provided with an image sensor (2) which includes a plurality of photoelectric elements which convert radiation into electrical charges. A control system (4) is arranged to read out separately electrical charges representing brightness values of the image and electrical charges representing smear. The read-out unit is arranged to derive a correction signal (SC) from the electrical charges representing smear and to derive a primary image signal (PS) from the electrical charges representing brightness values of the image. The correction unit derives a corrected image signal (VS) from the primary image signal and the correction signal. The control system (4) is especially arranged to derive the correction signal from the largest electrical charges from among the electrical charges representing smear, so that only a comparatively small number of electrical charges need be read out so as to obtain the correction signal. Preferably, the electrical charges are read out while radiation is incident on the image sensor so that only a very short period of time is required to read out the image sensor and to derive the corrected image signal.

Description: Image pick-up apparatus including a correction unit. The invention relates to an image pick-up apparatus which includes an image sensor with a plurality of photoelectric elements for converting radiation into electrical charges, a control system for reading out electrical charges representing smear from the photoelectric elements and for reading out electrical charges representing brightness values of the image from the photoelectric elements, a read-out unit for deriving a correction signal from the electrical charges representing smear and for deriving a primary image signal from electrical charges representing brightness values of the image, and a correction unit for deriving a corrected image signal from the primary image signal and the correction signal. The invention also relates to an X-ray examination apparatus provided with an X-ray detector for deriving an image signal from an X-ray image. An image pick-up apparatus of the kind set forth is known from the book "Solid-state imaging with charged-coupled devices" by Albert J.P. Theuwissen (Kluwer Academic Publishers, Dordrecht, 1995). The image pick-up apparatus picks up an image by converting radiation, such as visible light or ultraviolet or infrared radiation, whereby the image is formed, into electrical charges. The magnitude of the electrical charges corresponds to brightness values of the image. An image sensor, notably a charge-coupled image sensor, is read out by shifting electrical charges in the photoelectric elements through the image sensor to a read-out register and by deriving the primary image signal from the electrical charges in the read-out register. When radiation is incident on the photoelectric elements, notably during the shifting of the electrical charges through the image sensor, an additional electrical charge is added to the electrical charges being shifted through the image sensor. Such additional electrical charges cause a disturbance in the primary image signal. Because of this disturbance, the image represented by the primary image signal exhibits smearing of notably bright details. The additional electrical charges added during the shifting of electrical charges through the image sensor represent mainly smear. When, prior to the picking up of the image, the image sensor is read out already while radiation is already incident on the image sensor, electrical charges which represent the smear, like the additional electrical charges, are generated in the photoelectric elements. In the known image sensor a correction signal is derived from the electrical charges which represent mainly smear. A corrected image signal is derived from the correction signal and the primary image signal. Such a corrected image signal represents an image which is substantially free from smear. It is a drawback of the known image sensor that a comparatively long period of time is required so as to derive the correction signal. Consequently, the known image sensor is not suitable for deriving image signals without serious disturbances from a rapid succession of images. This makes the known image sensor notably unsuitable for use in an exposure control system of an X-ray examination apparatus. It is an object of the invention to provide an image pick-up apparatus which, in comparison with the known image pick-up apparatus, requires only a short period of time for forming an image signal which is substantially free from smear. This object is achieved by means of an image pick-up apparatus according to the invention which is characterized in that the control system is arranged to derive the correction signal from mainly the largest electrical charges representing smear. In order to derive the correction signal, the image pick-up apparatus according to the invention utilizes essentially only the largest electrical charges, representing smear, from a comparatively small number of photoelectric elements. The signal levels of the correction signal, for example, can be estimated or calculated on the basis of these largest electrical charges. Consequently, only few photoelectric elements need be read out so as to derive the correction signal. The period of time required to read out such a small number of photoelectric elements is much shorter than the period of time that would be required to read out the entire image sensor. It has been found, for example that it suffices to read out only one or a few rows of photoelectric elements so as to derive the correction signal; this means that, for example only 1/256, 1/512 or even 1/1024 portion of the image sensor is read out so as to form the correction signal. Preferably, the electrical charges representing smear are read out prior to the picking up of an image and the electrical charges representing brightness values of the image are read out after the picking up of the image. These and other aspects of the invention will be described in detail hereinafter on the basis of the embodiments as defined in the dependent Claims. After the reading out of electrical charges representing smear, electrical charges representing smear are still present in the photoelectric elements. This is because radiation is incident on the image sensor still during reading out. During the picking up of the image, subsequently electrical charges representing brightness values of the image are added to said residual electrical charges. During the reading out of the photoelectric elements after the picking up of the image, additional electrical charges representing smear are added to the electrical charges then read out. Consequently, an amount of electrical charge which is substantially equal for all photoelectric elements and will be referred to as "smear offset" hereinafter is added to the electrical charges read out which represent brightness values of the image. Such smear offset is substantially equal to the largest electrical charges representing smear. The correction signal can be simply derived from the largest electrical charges representing smear. Only few electrical charges need be read out from the image sensor for this purpose, so that little time is wasted on deriving the correction signal. The correction signal has a substantially constant signal level which corresponds to the smear offset so that the corrected image signal can be simply derived from the primary image signal by means of the correction signal. The row of photoelectric elements situated nearest to a read-out register of the image pick-up apparatus receives the largest amount of electrical charge due to the incidence of radiation on the image sensor during the shifting of electrical charges through the image sensor. This is because the photoelectric elements situated nearest to the read-out register collect electrical charges which have been generated in substantially the entire image sensor during the shifting of charge. The electrical charges in the photoelectric elements near the read-out register are substantially equal to the smear offset. The primary image signal has signal levels representing brightness values of the image as well as smear. The correction signal represents smear. The difference between the signal levels for corresponding positions in the image of the primary image signal and the correction signal accurately represents brightness values of the image. Notably when the coπection signal has a constant signal level which corresponds to the smear offset, it is particularly easy to derive the corrected image signal by subtracting the signal level of the correction signal from the signal levels of the primary image signal. It is a further object of the invention to provide an X-ray examination apparatus which enables image signals without serious disturbances to be derived from a rapid succession of X-ray images. To this end, an X-ray examination apparatus according to the invention is provided with an image pick-up apparatus as defined in one of the Claims 1, 2, 3 or 4. The X-ray source irradiates an object, for example a patient to be radiologically examined, by means of an X-ray beam. An X-ray image is formed on the X-ray detector due to local differences in the X-ray absorption within the object. The X-ray detector includes a conversion unit for converting an X-ray image into an optical image. The image pick-up apparatus derives the image signal from the optical image. The image pick-up apparatus counteracts disturbances in the image signal as caused by smearing, correction of said disturbances requiring a small amount of time only. Only a brief period of time is required between successive images so as to perform the correction. Consequently, the X-ray examination apparatus is capable of generating image signals which are hardly disturbed by smearing from a rapid succession of X-ray images. The exposure control system includes an image pick-up appartus with an image sensor for deriving a control signal for adjusting the X-ray examination apparatus from the optical image. The optical image is derived from the X-ray image by means of a conversion unit. The image signal is derived from the optical image by means of a camera which includes a variable amplifier. Preferably, the same image sensor is used to derive the image signal and the control signal from the optical image, but it is alternatively possible to use separate image sensors for deriving the control signal and the image signal. The control signal is, for example an X-ray control signal for adjusting the X-ray source to a suitable energy and/or intensity of the X-rays emitted by the X-ray source. The control signal may also be a camera control signal for adjusting the gain factor of a variable amplifier. Because the exposure control system includes an image pick-up as defined in one of the Claims 1 to 4, the control signal can be corrected for smear. The control signal is corrected for smear in the same way as the extraction of the corrected image signal from the primary image signal and the correction signal. Moreover, only a small amount of time is wasted on the correction of the control signal. Because the control signal is corrected for smear without substantial loss of time, the X-ray examination apparatus can be accurately adjusted while the X-ray source irradiates the object to be examined by means of X-rays. Because only a short period of time is required for accurate adjustment of the X-ray examination apparatus during iπadiation, the patient is exposed to only a very small X-ray dose during adjustment of the X-ray examination apparatus. Furthermore, image signals can be derived from a rapid succession of X-ray images while the X-ray examination apparatus is accurately adjusted anew for each X-ray image in the series. It is notably possible to derive image signals without an appreciable disturbance by smear because the X-ray detector is provided with an image pick-up apparatus as defined in one of the Claims 1 to 4. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described by way of example hereinafter and the accompanying drawing; therein: Fig. 1 is a diagrammatic representation of an image pick-up apparatus in which the invention is used, Fig. 2 is a graphic representation of the magnitude of the electrical charges generated in the image sensor, Figs. 2a, 2b and 2c show diagrammatically the formation of electrical charges in the image sensor, and Fig. 3 is a diagrammatic representation of an X-ray examination apparatus in which the invention is used. Fig. 1 is a diagrammatic representation of an image pick-up apparatus, for example a video camera, in which the invention is used. The image pick-up apparatus 1 includes an objective lens 10 whereby incident electromagnetic radiation, such as visible light 11 or infrared or ultraviolet radiation, is focused onto the image sensor 2. The image sensor includes a large number of, for example 1024 x 1024 or 1024 x 512 photoelectric semiconductor elements which convert the incident radiation into the electrical charges. The image sensor in the present embodiment is a charged-coupled (CCD) image sensor. The photoelectric elements are MOS diodes provided on a semiconductor substrate. The photoelectric elements are made of, for example crystalline silicon. The MOS diodes include gate contacts whereby the electrical potential distribution across the MOS diode can be influenced on the basis of an electric voltage applied to the gate contact. In order to read out the electrical charges, the electrical charges are repeatedly shifted through the image sensor to neighboring photoelectric elements until the electrical charges are ultimately transported to the read-out unit. The electrical charges are shifted by successively applying electrical potentials to gate contacts of the photoelectric elements. Under the influence of the electrical potentials, charge in one photoelectric element is shifted to an adjoining photoelectric element. The electrical charges in the photoelectric elements are read out to a read-out unit 5. The read-out unit is, for example a read-out register 5. Such a read-out register includes, for example a number of semiconductor elements with gate contacts. Electrical charges are stored in the semiconductor elements. The electrical charges can be transported through the read-out register so as to reach an output under the influence of electrical potentials on the gate contacts. For example, a read-out amplifier is connected to the output in order to derive an image signal in the form of an electrical voltage from the charges read out. The image sensor and the read-out unit are controlled by the control system so as to read out the electrical charges in the photoelectric elements in order to derive the primary image signal PS and the correction signal CS therefrom. The control system is arranged to deliver electrical gate signals to gate contacts of the photoelectric elements of the image sensor. Prior to the picking up of the image, the control system 4 ensures that there are no residual electrical charges present in the image sensor; to this end, electrical charges unintentionally present in photoelectric elements still are drained to the substrate of the image sensor. Subsequently, while radiation is incident on the image sensor, the electrical charges representing smear are shifted to the read-out register through the image sensor. The control system 4 ensures that the correction signal CS is derived from the largest electrical charges, representing smear, by means of the read-out register. For example, the correction signal is derived from the electrical charges which represent smear and originate from the row of photoelectric elements situated nearest to the read-out register. While the correction signal is being read out, electrical charges are generated again in the photoelectric element because radiation is incident on the image sensor. These electrical charges remain behind in the photoelectric elements. Subsequently, in dependence on the brightness of the image, electrical charges are integrated in the photoelectric elements during a period of time. The period of time during which electrical charges are integrated is also referred to as integration time. The duration of the integration time is dependent on a variety of circumstances. When the image signal is derived from the X- ray image, for example an integration time in the range of from 10 to 200 ms is used, depending on the brightness of the X-ray image. When the image signal is derived from the test image used for adjustment of the X-ray examination apparatus, a much shorter integration time of preferably 1 ms or less is used. The integrated electrical charges correspond to brightness values in the image picked up. The integrated electrical charges are added to the residual electrical charges. The integrated electrical charges are transferred to the read-out register, together with the residual charges. During the reading out of the integrated electrical charges, together with the residual electrical charges, the electrical charges are repeatedly shifted from one photoelectric element to neighboring photoelectric elements by the image sensor. Because radiation is still incident on the image sensor during the shifting of the electrical charges, additional charges are added to the residual and the integrated electrical charges. The read-out register then contains the integrated electrical charges, representing the brightness values in the image, and also additional electrical charges which are substantially equal for practically all photoelectric elements in the image memory. The additional electrical charges comprise the residual electrical charges and the additional electrical charges which are shifted, together with the integrated electrical charges, to the read-out register. The additional electrical charges do not represent image information but correspond to the value of the smear offset. In order to ensure that the additional charges accurately correspond to the smear offset, the integrated electrical charges are read out at the same rate as the electrical charges which represent smear in order to derive the correction signal from the largest electrical charges thereof. The image sensor can be provided with an image pick-up section 60 and an image storage section 61. Such an image sensor is also called a "storage-type" CCD sensor. The image pick-up section 60 as well as the image storage section 61 includes photoelectric elements, for example MOS diodes, whose electrical potential can be influenced by applying electrical voltages to the gate contacts of the photoelectric elements. The photoelectric elements of the image storage section 61 are shielded from the incident radiation; the image storage section 61 is provided, for example with a layer of aluminium which is disposed over the photoelectric elements. The layer of aluminium ensures that incident radiation, such as visible light, cannot reach the photoelectric elements of the image storage section 61. When radiation is incident on the image pick-up section, electrical charges are generated therein. The electrical charges in the image pick-up section 60 are shifted to the image storage section 61 wherefrom they are transferred to the read-out register. When such a storage-type CCD sensor is used, the electrical charges representing smear are shifted to the image storage section 61 and the largest one thereof is transferred to the read-out register so as to derive the coπection signal therefrom. Subsequently, electrical charges which represent brightness values of the image are integrated in the image pick-up section 60. The integrated electrical charges are added to the electrical charges left behind after the reading out of the correction signal. Subsequently, the integrated electrical charges are transferred, together with the residual electrical charges, to the image storage section 61. During this transfer additional electrical charges are formed in the image pick-up section 60 because radiation is still incident on the image sensor. The integrated electrical charges arrive in the image pick-up section 60 together with the additional electrical charges; these additional electrical charges are approximately the same for practically all photoelectric elements in the image storage section 61 and comprise the residual and the additional electrical charges. The primary image signal is derived from the integrated electrical charges with the additional electrical charges by reading out these charges from the read-out register. The signal levels of the primary image signal represent brightness values of the image picked up by means of the image pick-up apparatus, but also disturbances because additional electrical charges are generated in the photoelectric elements during the reading out of the electrical charges. This correction signal has an essentially constant signal level, being the smear offset. The read-out register 5 is connected to the coπection unit 6. The correction signal CS and the primary image signal PS are applied to the correction unit 6 and the correction unit derives the coπected image signal IS from the coπection signal and the primary image signal. More specifically, the correction unit 6 subtracts the signal level of the coπection signal CS from the signal level of the primary image signal. Figs. 2a, 2b and 2c show graphically the formation of electrical charges in the image sensor. Fig. 2a shows graphically the residual charges R left behind in the image pickup section 60 after the reading out of the electrical charges representing smear. Fig. 2a also shows the residual charge O which has been transferred to the image pick-up section 61 from the row in the image pick-up section 60 which adjoins the image storage section. The coπection signal is derived from the residual charge O. As the photoelectric elements are situated nearer to the read-out register, a larger amount of electrical charge has been left behind because the electrical charges have been shifted further through the image sensor while radiation was incident on the image sensor. Fig. 2b shows that the integrated electrical charges I have been added to the residual electrical charges R. Fig. 2c shows that the additional electrical charges E have been added to the integrated electrical charges with the residual charges. The additional electrical charges E are smaller as the relevant photoelectric elements are situated nearer to the read-out register because the electrical charges in these photoelectric elements have been displaced through the image sensor over a shorter distance. The sum of the residual electrical charges R and the additional electrical charges E represents the smear offset. Fig. 3 shows diagrammatically an X-ray examination apparatus in which the invention is used. The X-ray examination apparatus includes an X-ray source 20 for iπadiating an object to be radiologically examined, for example a patient 22, by means of an X-ray beam 21. Due to local differences in the X-ray absorption within the patient 22, an X-ray image is formed on an X-ray detector 23. The X-ray detector 23 of the embodiment shown consists of an X-ray image intensifier. The X-ray image intensifier 23 includes an entrance screen 24 which is provided with a scintillation layer 25 which converts X-rays into blue or ultraviolet light. The scintillation layer 24 is, for example a sodium-doped cesium iodide (Cs Na) layer. The entrance screen 24 also includes a photocathode 26 which is sensitive to the light emitted by the scintillation layer. Because light from the scintillation layer 25 is incident on the photocathode 26, the photocathode emits an electron beam. An electron optical system guides the electron beam to an exit window 27. A phosphor layer 28 is provided on the exit window. Because electrons from the photocathode 26 generate light in the phosphor layer 28, an optical image is formed on the exit window. This optical image corresponds to the X-ray image. The optical image on the exit window 27 is picked up by means of an image pick-up apparatus. To this end, the exit window is coupled to the image pick-up apparatus via an optical coupling 30. The image pick-up apparatus derives an image signal from the optical image, for example an electronic video signal (VS). The electronic video signal is applied to a monitor 31 for display of the optical image. The electronic video signal may also be applied to a buffer unit 32 so as to be stored in the buffer unit while awaiting further processing. In addition to the video signal, the image pick-up apparatus also supplies an image signal BS. The image signal BS is picked up at the start of iπadiation by means of X- rays, preferably during a short period of time, for example a period of time shorter than 1 ms or a few milliseconds. The image signal BS actually represents a test image, on the basis of which the X-ray examination apparatus is accurately adjusted in order to ensure that an X-ray image of high diagnostic quality is formed and that the image information in the X-ray image is reproduced with a high quality. The extraction of the image signal BS utilizes a very short integration time of, for example 1 ms or less so that a comparatively large amount of smear occurs when the electrical charges are read out so as to form the image signal. In order to ensure that the image signal BS is hardly affected by smear, first the correction signal is derived and subsequently the primary image signal. Subtracting the correction signal from the primary image signal by means of the correction unit ensures that the image signal is not disturbed by smear. The image signal IS is applied to a control unit 33. The control unit derives the X-ray control signal XCS and the camera control signal CCS from the image signal IS, said control signals being used to control the high-voltage power supply 34 of the X-ray source and the variable amplifier 35 of the image pick-up apparatus, respectively. As soon as coπect adjustment of the X-ray examination apparatus has been achieved, the primary image signal is derived again while using a longer integration time of, for example from 10 to 200 ms, depending on the brightness of the optical image.

Claims: CLAIMS: 1. An image pick-up apparatus (1) which includes - an image sensor (2) with a plurality of photoelectric elements for converting radiation into electrical charges, - a control system (4) for - reading out electrical charges representing smear from the photoelectric elements, and - reading out electrical charges representing brightness values of the image from the photoelectric elements, - a read-out unit (5) for - deriving a correction signal from the electrical charges representing smear, and - deriving a primary image signal from electrical charges representing brightness values of the image, and - a correction unit (6) for - deriving a coπected image signal from the primary image signal and the correction signal, characterized in that - the control system (4) is arranged to derive the coπection signal from mainly the largest electrical charges representing smear. 2. An image pick-up apparatus as claimed in Claim 1, characterized in that - the control system is arranged to read out, while radiation is incident on the image sensor: - the electrical charges representing smear from the photoelectric elements, and - the electrical charges representing brightness values of the image from the photoelectric elements. 3. An image pick-up apparatus as claimed in Claim 1 or 2, in which the photoelectric elements are arranged in a matrix, characterized in that - the control system is arranged to derive the coπection signal from electrical charges in a single row of photoelectric elements. 4. An image pick-up apparatus as claimed in one of the preceding Claims, characterized in that the correction unit is aπanged to - derive the coπected image signal as the difference between the primary image signal and the correction signal. 5. An X-ray examination apparatus, including - an X-ray detector for deriving an image signal from an X-ray image, characterized in that - the X-ray detector includes an image pick-up apparatus as claimed in one of the Claims 1, 2, 3 or 4. 6. An X-ray examination apparatus, notably as claimed in Claim 5, provided with - an X-ray detector for deriving an image signal from an X-ray image, and - an exposure control for adjusting the X-ray examination apparatus on the basis of brightness values in the X-ray image, characterized in that - the exposure control includes an image pick-up apparatus as claimed in one of the Claims 1, 2, 3 or 4.

Inventors: NEDERPELT CHRISTIANUS G L M, NEDERPELT, CHRISTIANUS, G., L., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H04N5
IPC codes: G01N23, H04N5, H05G1