Abstract: A system and method of medical imaging is designed to reduce a patient's X-ray exposure during scanning. The system automatically generates a default noise index from a received set of scan parameter values that specifies a desired tube current of an X-ray source to use during the scanning of a patient. The default noise index can then be overridden to select a preferred noise index based upon a desired diagnostic quality for a particular volume of interest (VOI) and a diagnostic objective.

Abstract: An apparatus for recording a 2D image of an object comprises a plurality of 1D detector units, each exposed to ionizing radiation, as transmitted through or scattered off the object, and being arranged for 1D imaging of the radiation, to which it is exposed. The detector units are distributed in an array such that the 1D images of the radiation from the detector units are distributed over a substantial portion of the 2D image. The apparatus includes a device for moving the detector units relative the object while the detector units repeatedly detect to create the 2D image of the object; and a control device for controlling the detector units to detect ionizing radiation during a short period of time before or during an initial part of the movement; calculating an optimum exposure time for the repeated detection based on the short period of time detection; and controlling the repeated detection to automatically obtain the optimum exposure time.

Abstract: An X-ray apparatus has a radiation source, an interchangeable radiation filter and an area dose measuring device having a measurement chamber with an allocated evaluation device for determining the area dose product on the basis of a measured signal provided by the measurement chamber. The measurement chamber is arranged preceding the filter in the beam path with reference to the radiation propagation direction. A detector recognizes the nature and/or the type of radiation filter that is currently inserted in the beam path. The evaluation device corrects the area dose product that is calculated from the measured signal from the measurement chamber on the basis of at least one filter-specific correction value that is selected as a result of the detection of the filter nature and/or type.

Abstract: A method and system for reducing radiation exposure from an imaging system including determining an entry location, operating the imaging system so as to cause the imaging system to emit radiation having a radiation intensity, controlling the radiation intensity in a manner responsive to the entry location so as to create image data and processing the image data so as to create processed image data. In an alternative embodiment, a medium encoded with a machine-readable computer program code for reducing radiation exposure from an imaging system, the medium including instructions for causing a controller to implement the aforementioned method.

Abstract: In a method for measuring the dose distribution in a computed tomography apparatus, wherein the dose distribution of an X-ray beam emitted by an X-ray tube with a thickness prescribed by a radiation diaphragm is acquired in the thickness direction for at least one setting of the radiation diaphragm, the dose distribution is acquired in the thickness direction with detector elements of a detector line of the computed tomography system in front of which a slit diaphragm is attached, while adjusting the radiation diaphragm so that each possible setting is set. The dose distribution can be acquired in automated fashion and without manual evaluation that is susceptible to error.

Abstract: When a phototimer unit is used for exposure control, a deterioration in a S/N ratio occurs, and optimal exposure cannot be performed due to a deviation from a proper image sensing position and the like. Therefore, there is provided a radiation image sensing apparatus comprising an X-ray image sensing panel which is capable of non-destructive reading and adapted to sense an object image by allowing radiation from an X-ray source to pass through an object, and a control circuit adapted to perform control to stop emission of radiation from the X-ray source on the basis of a signal obtained from the X-ray image sensing panel by non-destructive reading in the image sensing operation.

Abstract: An x-ray system is controlled to obtain a view of an area of a patient that is larger than a field of view of an x-ray detector. Individual images are obtained of portions of the area of the subject that, when combined, can be used to get a view of the area of the subject. Positions of individual images are determined. The positions are preferably calculated to avoid placing structures that tend to move or that are dose sensitive in an area of overlap of the individual images. Also, the positions are preferably calculated to reduce overall exposure to a subject, especially by reducing unnecessary double exposure.

Abstract: An x-ray emitting system and method for administering a predetermined x-ray dose rate are provided. The system includes an x-ray emitter, a controller operably connected to the x-ray emitter, a current sensor operably connected to the controller, and a voltage sensor operably connected to the controller. The controller determines an actual dose rate based on a received current sensor signal and a received voltage sensor signal and adjusts a supplied voltage to allow the actual dose rate to match a predetermined dose rate. The method of operating a device for emitting x-rays includes: applying a voltage from a voltage source to the device, measuring current and voltage within the device, determining an actual dose rate based on the measured current and voltage, comparing a desired dose rate to the actual dose rate, adjusting the applied voltage, and matching the actual dose rate to the desired dose rate.

Abstract: A method, system and medium for modulating the x-ray power of an imaging system so as to maintain a desired image noise in the imaging system is disclosed. In an exemplary embodiment, the method includes obtaining projection data, correcting the projection data responsive to beam hardening errors so as to create corrected projection data, processing the corrected projection data so as to create a plurality of emitter current values responsive to an imaging method and applying the emitter current values to the imaging system responsive to an object to be imaged. In another aspect, a method for determining an optimum emitter tube voltage for an imaging system includes characterizing the imaging system so as to determine a system water-equivalent path length responsive to a relative noise increase.

Abstract: The invention relates to a method for exposure control which is intended notably for dynamic X-ray examinations of an object involving a varying X-ray absorption as well as to an X-ray generator which includes an automatic exposure control unit for carrying out such a method. The method and the device are characterized notably in that an exposure kV start voltage and a maximum exposure time Tmax (for example, 100 ms) can be defined; in order to avoid motional unsharpness in the image, these variables may not be exceeded.

Abstract: For the purpose of enabling reduction of exposure dose, in an X-ray controlling method for an X-ray imaging apparatus for producing an image based on detected X-ray signals, an upper limit of an X-ray exposure dose to a subject to be imaged is set (603), and the tube current of an X-ray tube is modulated so that the exposure dose does not exceed the upper limit (606-610). The modulation of the tube current is achieved by finding an exposure dose predicted value based on an imaging protocol, and modifying a tube current set value in the imaging protocol when the predicted value exceeds the upper limit.

Abstract: A method, system and medium for modulating the x-ray power of an imaging system so as to maintain a desired image noise in the imaging system is disclosed. In an exemplary embodiment, the method includes obtaining projection data, correcting the projection data responsive to beam hardening errors so as to create corrected projection data, processing the corrected projection data so as to create a plurality of emitter current values responsive to an imaging method and applying the emitter current values to the imaging system responsive to an object to be imaged. In another aspect, a method for determining an optimum emitter tube voltage for an imaging system includes characterizing the imaging system so as to determine a system water-equivalent path length responsive to a relative noise increase.

Abstract: In a method for measuring the dose distribution in a computed tomography apparatus, wherein the dose distribution of an X-ray beam emitted by an X-ray tube with a thickness prescribed by a radiation diaphragm is acquired in the thickness direction for at least one setting of the radiation diaphragm, the dose distribution is acquired in the thickness direction with detector elements of a detector line of the computed tomography system in front of which a slit diaphragm is attached, while adjusting the radiation diaphragm so that each possible setting is set. The dose distribution can be acquired in automated fashion and without manual evaluation that is susceptible to error.

Abstract: The invention relates to a method and a device for adapting a radiation dose of an X-ray source (1). The X-ray source (1) irradiates an object to be examined, for example, a patient (4), so as to form an X-ray image (7) on an X-ray detector (5). The X-ray image (7) is subdivided into image regions (A-I) and each time the brightest image region is successively separated from the remaining image regions in an iterative method if its mean grey value forms an indication of the presence of direct radiation (2b) in the relevant image region. The image regions still remaining at the end of the iteration operation correspond to an image region of interest which can be taken into account by a control unit (6) so as to calculate the optimum radiation dose.

Abstract: The invention relates to a method and a device for the processing of X-ray images which can be used notably in medical fluoroscopy procedures since they keep the overall radiation load for a patient low. Processing of and detail enhancement in the X-ray images is preferably performed by means of a pattern matching algorithm which necessitates prior knowledge of a pattern of the detail of interest. In order to extract this pattern, at least one single image (HD) is formed (2) with a higher dose rate, said single image having an image quality which suffices for the automatic (4) or non-automatic (3) recognition of the pattern. The information extracted from this single image serves as a basis for the evaluation of X-ray images (LD2) formed with a lower dose rate.

Abstract: A radiation therapy device (2) includes a programmable power source (300) and controller (302) that monitor an injected current level and controls heater voltage in response thereto. The heater voltage is reduced in predetermined increments without affecting the beam profile.

Abstract: An object of the present invention is to acquire transmitted X-ray data by irradiating X-rays of appropriate doses determined for the portions of a section containing the major axis and the portions thereof containing the minor axis respectively. An X-ray irradiating/detecting device consists mainly of an X-ray irradiator that includes an X-ray tube and irradiates a fan-shaped X-ray beam, and an X-ray detector that has a plurality of X-ray detecting elements arrayed in a direction in which the fan-shaped X-ray beam spreads and that is opposed to the X-ray irradiator with an object of imaging between them. The X-ray irradiating/detecting device is rotated about the object in order to acquire transmission X-ray data stemming from a plurality of views. At this time, the dose of the X-ray beam is differentiated between predetermined angular ranges of a section of the object shaped like an oval, which extend with the minor axis of the oval section as a centerline, and the other angular ranges.

Abstract: A method and system for of defining, or identifying, regions of interest for exposure management in a digital x-ray imaging system, and especially in the case of multiple consecutive image acquisitions. According to the most basic embodiment of the present invention, simple geometric shapes arranged in a matrix configuration are used to aid an operator in identifying a region of interest for a diagnostic x-ray image. Each region of interest is selectable from a low-dose preshot image and may be corrected, or processed, in order to enhance the results of a subsequent diagnostic image. The processing of the preshot image allows the system to automatically make predictions for the diagnostic image exposure requirements, thereby avoiding unnecessary multiple images.

Abstract: The invention relates to a method and a device for limiting radiation applied to an object (input dose rate) during irradiation of the object by means of a radiation source (1), notably during X-ray exposures. Pairs of values of a control curve which give rise to an input dose rate on the object to be examined which exceeds a predetermined maximum input dose rate are corrected by means of a calculated factor. This results in a corrected control curve whose entire length can be used for adjusting the desired tube parameters (tube voltage, tube current or load) without giving rise to the risk of exceeding the maximum input dose rate.

Abstract: The invention relates to an X-ray device in which a set of exposure parameters is fetched from among a number of such sets stored in a first storage arrangement for APR X-ray exposures. The exposure parameters which result therefrom during the subsequent X-ray exposure and have possibly been modified by the user are stored in a second storage arrangement. The X-ray device includes means for evaluating the second sets of exposure parameters which are associated with the same first set of exposure parameters and for deriving therefrom a new set of exposure parameters which is stored in the first storage arrangement instead of the first set of exposure parameters and forms the basis for APR X-ray exposures from then on.

Abstract: Method and system aspects for achieving more accurate radiation delivery during radiation treatment by a radiation-emitting system are described. In a method aspect, and system for achieving same, the method includes providing a table of dose rate values for accumulated dosages in a treatment unit of the radiation emitting system, and controlling a dose rate of radiation emitted from the radiation emitting system through utilization of the table of dose rates. Controlling further includes determining an accumulated dosage at a sampling point, and comparing the accumulated dosage to a total desired dosage. The dose rate is adjusted based on the table of dose rate values and the determined accumulated dosage, with the dose rate ramped down as the accumulated dosage nears the total desired dosage. Accumulated dosages include fractional numbers of monitor units.

Abstract: The method consists of subjecting the organ to pre-exposure with a low dose of rays, creating in the field of the pre-exposure image a multi-cellular image in which each cell represents the mean signal level of a predetermined number of pixels of the detector, selecting from the columns the cell of minimum signal level most distant from a reference point, establishing about this cell a band parallel to the reference point and selecting from the band the cell having the absolute minimum signal level.

Abstract: An apparatus and method used in dental panoramic and cephalographic x-ray equipment provides automatic adjustment of the pre-programmed exposure factors in order to obtain optimisation of the grey scale of the latent image on various kinds of x-ray detectors, such as the radiographic film or x-ray image detector plates. A sensor, such as photo diodes, detects the intensity of the radiation passed through the patient and incident on the x-ray detector and feeds it to a control unit. The control unit provides a computed simulation of the latent image grey level and adjusts the programmed exposure factors in order to establish the grey level of the latent image at the required reference value.

Abstract: A system, method, apparatus, and means for tuning a radiation therapy device using a treatment processing unit coupled to said radiation therapy device includes selecting an operational parameter of the radiation therapy device. A signal converter is configured to receive an output signal from the radiation therapy device, the output signal depending on the operational parameter. The operational parameter is then varied while displaying the output signal. In some embodiments, the operational parameter is continually varied until the output signal reaches a desired value.

Abstract: In computed tomography a method and apparatus for registering measured data of a small region of interest, an X-ray source is rotated around an examination axis while emitting an X-ray beam that widens in a slice plane perpendicular to the examination axis. The intensity distribution of the X-rays within the X-ray beam during the measured data registration is varied and controlled dependent on the region of interest such that regions of the slice plane outside the region of interest are charged with a lower X-ray dose than the region of interest. The dose stress on the patient is substantially reduced when only small regions of the available measurement field are utilized for diagnosis. The dose reduction is independent of whether the region of interest lie in or outside the rotational center of the computed tomography apparatus.

Abstract: A process for improving a radiological examination and a device for implementing such a process.

Abstract: The invention relates to a method and a device for adapting a radiation dose of an X-ray source (1). The X-ray source (1) irradiates an object to be examined, for example, a patient (4), so as to form an X-ray image (7) on an X-ray detector (5). The X-ray image (7) is subdivided into image regions (A-I) and each time the brightest image region is successively separated from the remaining image regions in an iterative method if its mean grey value forms an indication of the presence of direct radiation (2b) in the relevant image region. The image regions still remaining at the end of the iteration operation correspond to an image region of interest which can be taken into account by a control unit (6) so as to calculate the optimum radiation dose.

Abstract: In a method for modulating a radiation dose from an x-ray tube, and in an x-ray tube-containing apparatus, the x-ray tube has at least one variable operating parameter which, when varied, modulates the radiation dose, with a modulation speed, for x-rays produced by the x-ray tube, and the x-ray tube is operated while varying this parameter through a parameter range to generate modulation speed data, representing modulation speeds of the x-ray tube respectively for different values of the operating parameter. When an examination subject is to be irradiated with an x-ray dose from the x-ray tube, an exposure effect associated with the exposure of the subject to the radiation dose is identified, this exposure effect being dependent on modulation of the radiation dose.

Abstract: A scan type digital X-ray imaging apparatus comprising an X-ray generator for emitting X-ray slit beams, an X-ray detector for detecting X-ray transmitted through an object to be examined, and an object fixing means disposed between the X-ray generator and the X-ray detector, for holding the object in an appropriate position for X-ray photography, in which X-ray beam scanning is performed in a manner that the X-ray slit beam is emitted from the X-ray generator for the object while the X-ray generator and the X-ray detector are synchronously moved in the same direction with the object interposed therebetween, thereby producing an X-ray image of the object. In the X-ray imaging apparatus, the scanning speed of the X-ray slit beam is controlled depending on the X-ray transmitted amount through the object to be examined detected by the X-ray detector during the X-ray beam scanning is performed, whereby soft and hard tissues appearing in the X-ray image are amended in each contrast and/or in each density.

Abstract: The present invention provides a method for producing a broad range of therapeutic radiation energy levels with a source of radiation that is operated to produce photons or particles at a first energy that are directed at a treatment site for a predetermined absorbed dose. The linear accelerator is then operated to produce photons or particles at a second energy directed at the treatment site for another predetermined absorbed dose. The appropriate selection of the first and second absorbed doses yields an absorbed dose at the treatment site equivalent to the dose produced by photons having an energy that is intermediate of the first and second energies. Thus, a dual energy linear accelerator may be operated according to the method to yield a continuous range of energies.

Abstract: A structure and method for performing a capacitance extraction on an integrated circuit, includes determining a parallel-plate capacitance between devices on different levels within the integrated circuit, adding extension shapes around each of the devices, reducing an area of overlapping extension shapes, multiplying a remaining area of the extension shapes by a constant to produce a fringe capacitance; and summing the parallel-plate capacitance and the fringe capacitance.

Abstract: An automatic exposure control for an x-ray system using a large area solid state x-ray detector (26) includes an exposure control (36, 34) arranged to generate data of interest within the data generated by the detector and to adjust the dosage of x-rays to a predetermined level in response to data of interest so that an x-ray image of a patient is generated using the predetermined level.

Abstract: The invention relates to an X-ray system which includes an X-ray source and an X-ray detector for deriving an image signal. The X-ray detector includes an X-ray matrix sensor, such as an FDXD, which includes a matrix of sensor elements. Preferably, the sensor elements are light-sensitive &agr;-Si sensor elements (photodiodes) and the FDXD is provided with a scintillator layer which converts X-rays into light. The sensor elements generate electric charges that are read out in the form of an image signal that represents the X-ray image. The X-ray matrix sensor is provided with an additional array of detection elements which are arranged between the sensor elements and the X-ray source. More specifically, the detection elements are also &agr;-Si light-sensitive photodiodes. The scintillator layer is arranged between the X-ray matrix sensor and the additional array of detector elements. A partly transparent coating is disposed between the additional array of detector elements and the scintillator layer.

Abstract: A dual energy scanning densitometry system for imaging and measuring bone density maintains acceptable flux by adjusting the x-ray current or scan speed according to preceding scan line data. The amount of acceptable flux is maintained within limits modified by information about the region of the body being scanned so that different precisions may be maintained for different body regions. Scan speed and current may be controlled so that scan speed is maximized within the current limits. Essentially continuous flux control can be achieved.

Abstract: An apparatus and method used in dental panoramic and cephalographic x-ray equipment provides automatic adjustment of the pre-programmed exposure factors in order to obtain optimisation of the grey scale of the latent image on various kinds of x-ray detectors, such as the radiographic film or x-ray image detector plates. A sensor, such as photo diodes, detects the intensity of the radiation passed through the patient and incident on the x-ray detector and feeds it to a control unit. The control unit provides a computed simulation of the latent image grey level and adjusts the programmed exposure factors in order to establish the grey level of the latent image at the required reference value.

Abstract: An automatic exposure control for an x-ray system using a large area solid state x-ray detector (26) includes an exposure control (36, 34) arranged to generate data of interest within the data generated by the detector and to adjust the dosage of x-rays to a predetermined level in response to data of interest so that an x-ray image of a patient is generated using the predetermined level.

Abstract: A photosensitive device comprises: photosensitive cells located at the intersection of at least one addressing conductor and one read conductor, a cell comprising a photosensitive diode connected to a read conductor and to at least one diode having a switching function connected to an addressing conductor, these diodes having a common point; addressing means connected to at least one addressing conductor; charge-reading means connected to at least one read conductor.

Abstract: A method and system for an automatic exposure control (AEC) arrangement for a matrix-addressed imaging panel having an array of sensors including use of localized regions of the imaging panel exhibiting capacitive coupling. In one embodiment, the matrix-addressed imaging panel includes one or more AEC electrode receptive field regions that provide a signal representative of exposure specific, respective AEC electrode receptive field regions. Additionally, in another embodiment, the imaging array includes data line signal monitoring regions in which capacitive coupling between electrodes in radiation sensors adjacent to the data line are read and processed to provide and AEC signal. In another embodiment, the imaging array includes both AEC electrode filed receptive regions and data line signal monitoring regions that are coupled to an AEC controller for control of the radiation source for the imaging array.

Abstract: A method for selecting an orientation of a treatment beam for intensity modulated radiation therapy based on analysis performed on a model of a patient geometry prior to the actual treatment. The model of patient geometry includes a planning target volume (PTV) containing the object to be treated by radiation, e.g., a tumor, and the structure or structures at risk, such as internal organs, bone and/or tissue. The method calls for assigning a tolerance parameter to the structure or structures at risk, subdividing the treatment beam into beamlets, selecting a treatment parameter and selecting a set of orientations for the treatment beam and deriving a score for each of the orientations in the set by weighting each of the beamlets so as to maximize the treatment parameter in the PTV while not exceeding the tolerance parameter in the structure or structures at risk at each orientation.

Abstract: The invention relates to a method for operating a radiation examination apparatus, especially an X-ray apparatus, that includes a radiation source and a detector device. The invention proposes the use of a control signal for “during pulse” radiation control, being a combination of a dose or a dose rate signal, measured by a dose rate measuring device, and an adaptive control value that is obtained, using an adaptive control algorithm, from the mean image working points within a selected region of interest of every individual preceding image within an image sequence. A detector device and a radiation examination apparatus are also claimed.

Abstract: When a phototimer unit is used for exposure control, a deterioration in a S/N ratio occurs, and optimal exposure cannot be performed due to a deviation from a proper image sensing position and the like. Therefore, there is provided a radiation image sensing apparatus comprising an X-ray image sensing panel which is capable of non-destructive reading and adapted to sense an object image by allowing radiation from an X-ray source to pass through an object, and a control circuit adapted to perform control to stop emission of radiation from the X-ray source on the basis of a signal obtained from the X-ray image sensing panel by non-destructive reading in the image sensing operation.

Abstract: A cassette assembly comprising an X-ray cassette, with a tube side and a cover, and an intensifying screen wherein the cassette assembly further comprises a compensation element adapted to the intensifying screen so as to have a cassette assembly with predetermined X-ray absorption.

Abstract: In a method and apparatus for measuring the imaging dose in the framework of the registration of a radiation image of a subject, particularly of an x-ray image, a radiation source is employed for generating radiation, a solid-state radiation detector is employed for image pickup and a sensor is employed for calculating the imaging dose, and a radiation detector on a semiconductor base that lies behind the solid-state radiation detector with reference to the direction of the radiation incident onto the solid-state radiation detector is employed, for measuring radiation penetrating through the solid-state radiation detector. The radiation sensor emits an output signal that represents the measured radiation, and the imaging dose at a location in front of the solid-state radiation detector is determined based on the output signal.

Abstract: A method of and a device for forming an image of an irradiated object in which images are formed with different exposure levels and the differently exposed images are combined so as to form a single image. The device includes a source and an imaging device which is connected to the source in order to form the image, the imaging device being arranged to form images with different exposure levels and being provided with an image processor in which the differently exposed images are combined so as to form the single image. The device produces images of a quality which is comparable to that of images acquired by means of a device having a much larger dynamic range.

Abstract: A system an method for determining the Dose Area Product (DAP) in an X-ray imaging system is provided. The present system constructs a pre-determined parameter space describing the DAP contour over ranges of typical imaging parameters. Later, when the employed in clinical imaging, a DAP processor on board the imaging system received a set of the imaging parameters being employed. The DAP applies the set of parameters to the parameter space to interpolate the DAP being delivered by the clinical imaging system. The present system may be individually calibrated to a specific X-ray imaging system to provide more optimal DAP values. Additionally, in imaging system employing an asymmetric shutter, such as a one-leaf shutter, the present system may determine a rotational scale factor for the DAP based on the rotation of the shutter. The rotational scale factor may also be calibrated to an individual X-ray imaging system.

Abstract: A method for optimizing the radiogram image of a subject is provided. The method provides for showing the location of automatic exposure control sensors on the radiograms, along with alignment and targeting projections to optimize the radiation exposure of the subject. By displaying the location of the automatic exposure control sensors on the radiograms along with the image of the subject, the observers can determine whether the radiogram image was taken with appropriate subject alignment. This method also serves as an educational aid in training radiography technicians.

Abstract: The present invention provides a system for imaging an object by irradiating it with low doses of radiation, such as x-ray, from a plurality of positions angularly distributed about the object, and analyzing the intensity of the radiation transmitted through the object. A system according to the invention can include a radiation source, a low noise detector, and an image processor. The radiation source emits radiation toward a target scene, containing an object to be imaged, from a plurality of angular positions. In one embodiment, the plurality of angular positions defines an arc about the target scene. In another embodiment, the radiation source moves in a series of steps of varying angular spacing along the arc to generate the multiple images of the scene. The detector is positioned to detect radiation transmitted through the scene and produces radiation transmission data representing the intensity of the radiation transmitted through the scene.

Abstract: Aspects for virtual wedge treatment with wedge angle optimization are described. In a method aspect, the method includes adjusting a preselected calibration factor based on chosen treatment conditions. The method further includes achieving an optimal wedge intensity distribution with the adjusted calibration factor for the chosen treatment conditions in a virtual wedge radiation treatment session. Additionally, adjusting includes determining an appropriate calibration factor for a given wedge angle and depth.

Abstract: One embodiment of the present invention is a method for measuring a radiation dose of a patient applied by a computerized tomography (CT) imaging system having a radiation source emitting a radiation beam and a radiation detector. The method includes steps of: scanning a patient with the radiation beam; imaging the patient utilizing radiation detected by the radiation detector; and estimating a patient radiation dose from said imaging utilizing a measurement of radiation delivered to only a portion of the detector during said imaging. The present invention permits the advantages of a post-patient collimated detector array to be realized while providing an accurate estimate of patient radiation dosage.

Abstract: An x-ray examination apparatus comprises an x-ray source for generating an x-ray image and an image analysis system derives brightness variations from the x-ray image and derives a dose control signal dependent on said brightness variations in order to control the x-ray source. The image analysis system derives a distribution of said brightness variations and derives the dose control signal from the distribution of brightness variations. Preferably, the image analysis system is arranged to derive the brightness variations from the processed image and a histogram analysis is employed to derive the dose control signal.