Abstract: In an X-ray system in which a source directs X-radiation through a subject to a receptor which produces a secondary planar light image, the light image is projected on the planar input face of a light intensifier after passing through the scanning aperture of a moving mask substantially at the plane of the projected light image. Preferably the system includes a moving X-ray mask with a scanning aperture between the source and receptor and means driving the X-ray and light masks in synchronism with their apertures optically superimposed.

Abstract: The invention relates to the elimination of streak-like artefacts in slit radiography which are caused by an axial excursion of the rotary anode X-ray tube used for making the radiographs. These streaks can be eliminated by adapting the speed of displacement of the X-ray beam, the number of revolutions per unit time of the anode and the intensity profile of the radiation beam to one another so that for each point the intensity modulation in the half-shade area, caused by the periodic displacement of the focal point, is compensated for.

Abstract: A dynamic scanning X-ray machine has a detector with an array of crystals which respond to X-rays and a circuit made of semiconductor material, both of which deteriorate when exposed to X-rays. A pair of blinder plates which are opaque to X-rays normally cover the detector and semiconductor material to prevent their deterioration by the X-rays. The blinder plates are separated from each other by an adjustable distance to form a gap which enables the X-rays to reach the detector. An object under study and the blinders move in synchronism through the X-ray field so that a part of the X-ray field forming the shadow of the object passes through the gap formed by the separation between the blinder plates and falls upon the detector, while the detector crystals and semiconductor material are shielded from the remainder of the X-ray field. The shielding of the detector crystals and semiconductor material does not have any effect upon the image of the object under study.

Abstract: A piezo-electric attenuation tongue system for slit radiography equipment comprises a number of tongues placed next to each other. The tongue system is manufactured by slitting a plate of piezoelectric material from one of the edges but leaving unslit a strip of the plate along the opposite edge.

Abstract: In a shadowgraphic x-ray machine the beam scans the image plane in raster fashion. A fore collimator system varies the beam cross-section during the scan to ensure that the beam footprint does not vary with position in the image plane.

Abstract: In a radiation detecting apparatus having its radiation receiving section composed of two-dimensional or one-dimentional array of radiation sensors, the whole electronic circuit is classified into a temperature-sensitive circuit portion and a temperature-insensitive circuit portion for keeping at least the temperature-sensitive circuit portion at a substantially constant temperature also during the period in which the apparatus is kept standing for detecting a radiation image. For the purpose, at least the above temperature-sensitive circuit portion is kept energized even when a radiation image is not detected; kept warmed by a heating element energized only when a radiation image is not detected, made to get in touch with a heat sink intermittently; or blown at least intermittently by a blower or blowers.

Abstract: Stimulable phosphor sheets are circulated through an image recording section for recording radiation images on the stimulable phosphor sheets, an image read-out section for scanning the stimulable phosphor sheets by stimulating rays and detecting light emitted thereby upon stimulation, and a erasing section for releasing radiation energy remaining on the stimulable phosphor sheets after read-out. The recording section is provided with at least two slits positioned such that at least one slit is located on either side of the object and drive means for synchronously driving the slits, whereby it is possible to obtain recorded radiation images free from the effect of scattered radiation.

Abstract: An apparatus and method for slit radiography, in which a body to be irradiated is scanned at least twice by means of a planar, fan X-ray beam emitted from an X-ray source (1) via a slit diaphragm (2), the hardness of which beam increases with each next scanning motion, in which an X-ray detector (6) mounted behind the body collects the radiation passed, sections of detection means (10), which sections are juxtaposed in the longitudinal direction of the X-ray detector, produce electrical signals depending upon the radiation collected by the respective X-ray detector sections, which signals are sampled during each scanning motion and are stored in a memory, and the diaphragm (2) includes controllable elements juxtaposed in the longitudinal direction of the slit in a plurality corresponding to that of the sections of the detection means, which controllable elements are each operative to locally block the slit diaphragm during a next scanning motion if a stored signal produced by the corresponding section of the

Abstract: A radiation imaging system is disclosed. A radiation source directs a thin spread beam of radiation toward a detector assembly which is aligned with the radiation beam. The detector assembly includes a printed circuit board having a bowed configuration and bearing board circuitry. An array of photodiode detector elements are attached to the board. The detector includes a front and a rear array of elements arranged one behind the other. The circuit board defines a slot between the front and rear arrays in which a radiation filter can be removably and slidably interposed. The detector elements are buried or recessed in outer surfaces of the circuit board to a depth sufficient to position contact fingers of the elements substantially flush with the unrecessed surface of the board. Radiation sensitive phosphor material is removably positioned over each of the respective front and rear detector element arrays.

Abstract: A method and system for reconstructing image data obtained from plural sequential exposures of an object by a radiation source through a multiple slit beam scanning assembly, wherein frames of original image data in the form of plural pixels are obtained for each discrete scanning step/exposure and a minimum pixel value of at least one frame is determined. Based on this determined minimum value, a cutoff value is determined. The pixel values of each frame are compared with the cutoff value, and the image is reconstructed by summation of all the pixel image data above the cutoff value for each respective pixel. In order to improve spatial resolution, the slit width of the scanning assembly is selected be the narrower than the pixel size. Pixel image data produced by exposure from radiation passing through the same slit of the scanning assembly in consecutive scanning steps are then assigned to adjacent pixels in a direction perpendicular to the scanning beam of an enlarged pixel matrix.

Abstract: Information is derived from a selected fluorescent radiation line produced when an object is illuminated by a flying spot scanner. The illuminating radiation has an energy level sufficient to produce the fluorescent line when targeted components of the object are present and illuminated. A detector senses a fluorescent radiation line emitted from the targeted components to generate electrical fluorence based signals.

Abstract: The level of relative risk of osteoporosis in human subjects can be quickly determined by: measuring the bone mineral content of the os calcis (heel)--which is highly trabecular--and; assessing relative non-violent fracture risk at substantially all skeletal sites (both spinal and non-spinal) due to osteoporosis of the subject from the os calcis mineral content determination. The bone mineral content of a cortical bone site, such as the proximal radius (forearm), can also be determined. The densitometer apparatus includes a heel holding structure, or a forearm holding structure, which may be placed between a low energy source of gamma and/or x-rays, and a detector. The source and detector are mounted on a C-shaped yoke, which yoke can be moved horizontally and vertically during a rectilinear scan, and which can be rotated about a vertical axis when computed tomography (CT) capabilities are necessary.

Abstract: A system comprising an X-ray source (1), an elongate detector tube (3) including at least one cathode (5) extending in the longitudinal direction of the tube and at least one anode (6) located opposite to the cathode, a slit diaphragm (2), and a filter (4) mounted in the path between the X-ray source (1) and the detector tube (3). The filter (4) blocks relatively low energy X-radiation in a portion of the beam emitted by the source (1). The cathode (5) is provided with an X-ray detection layer consisting of two strips (8',8") extending in the longitudinal direction of the tube (3). One strip (8") receives the radiation passed by the filter (4) and the other strip (8') receives the unfiltered radiation. The one strip (8") is of considerably greater thickness than the other strip (8').

Abstract: A digital radiographic system and method is disclosed having improved detector resolution. The system includes an X-ray source and a spaced detector assembly having several staggered columns of uniform square detector elements. The detector elements are thus arranged in interspersed rows with the center-to-center spacing between successive members of each row being about twice the lateral dimension of each square element. In response to incident X-radiation, each detector element produces an electrical charge signal indicative of the radiation. Power means actuates the source to propagate X-ray energy in a beam along a path toward the detector assembly, in which path a subject may be interposed for examination. Mechanism is provided for scanning the detector assembly, synchronously and in alignment with the X-ray beam, relative to a subject. The relative scanning motion is perpendicular to the columns of detector elements.

Abstract: In an x-ray bone densitometer, special calibration techniques are employed to accommodate variations. In one aspect, a bone-like calibration material is interposed and the system determines the calibration data from rays passing only through flesh. In another aspect, a rotating device carries the calibration material through the beam. The specific densitometer shown uses an x-ray tube operated at two different voltages to generate a pencil beam, the energy levels of the x-ray photons being a function of the voltage applied. An integrating detector is timed to integrate the detected signal of the patient-attenuated beam over each pulse, the signals are converted to digital values and a digital computer converts the set of values produced by the raster scan into a representation of the bone density of the patient. Multiple reference detectors with differing absorbers are used by the system to continuously correct for variation in voltage and current of the x-ray tube.

Abstract: Method and apparatus for the production of tomographic images includes a flying spot scanner to form, from a beam of penetrating radiation, a pencil beam sweeping over a line in space to define a sweep plane. An object to be examined is supported so that the pencil beam intersects the object along a path passing through the object and a selected slice of the object. A line collimator is provided for filtering radiation scattered by the object, the line collimator has a field of view which intersects the sweep plane in a bounded line so that the line collimator passes only that radiation scattered by elementary volumes of the object lying along the bounded line. A radiation detector responds to that portion of the scattered radiation which is passed by the line collimator.

Abstract: A radiological scanning apparatus for forming an image of an object on a film by scanning said object with a flat fan shaped beam. The film is conformed so as to form a surface of revolution. The surface of revolution is, on the one hand, moved in the same movement as the scanning movement of said beam, and on the other hand it is rotated about its axis of revolution so as to renew a surface exposed to said beam in order to reduce the dimension of said film considered in the direction of said scanning movement.

Abstract: There is disclosed a method for forming a radiogram of an object comprising the steps of energizing an X-ray source at a low level of radiation intensity wherein the X-ray source includes a slit diaphragm, pivoting the X-ray source during a first scanning of the object, moving an elongated X-ray detector in respect to the pivoting of the X-ray source to generate an output signal of intensity of the object during the first scanning thereof, measuring the output signal during at least a portion of said first scanning, energizing the X-ray source to a higher level of radiation intensity as determined by the output signal from the X-ray detector during the first scanning, pivoting the X-ray source during a second scanning of the object, moving the X-ray detector during the second scanning to generate a second output of radiation intensity and forming a radiogram from the second output signals.

Abstract: An X-ray lithography system using synchrotron radiation including X-rays is disclosed. A mirror is provided in a beam line structure between the X-ray source and the X-ray mask in the X-ray lithography apparatus. The mirror is reciprocatingly moved in the same direction as that of the X-rays from the source, or in the normal direction to that of the X-rays.

Abstract: A contour collimator has diaphragm plates arranged in displaceable fashion relative to one another. Each diaphragm plate is individually engageable with an adjustment element shared by all of the diaphragm plates for displacing one diaphragm plate relative to the remaining diaphragm plates. An interlock mechanism in engagement with the remaining diaphragm plates retains those plates in their respectively existing positions during displacement of the one plate. The adjustment element is displaceable from the one diaphragm plate to another diaphragm plate which can then be displaced by the adjustment element while the other plates are retained in position.

Abstract: A radioscopic image former comprises a slit between an X-ray source and a subject to produce an X-ray fan beam to scan the subject. The transmission of the slit is adjusted locally during the scanning depending on local absorptions in the subject. The X-ray fan beam is cyclically modulated. The local modulation frequency depends on the coordinate in the longitudinal direction of the slit. The slit comprises a number of tongues each tongue being excited at or near its resonant frequency. Each tongue has its own frequency or phase and acts as a frequency-selective element.

Abstract: The quality of x-ray shadowgraphic images is improved by beam equalization controlled to ensure that just the right amount of radiation is used to accomplish the desired result, be it a desired image density and contrast or a desired signal-to-noise ratio or some other desired characteristic. The new techniques include: (1) maintaining a substantially constant signal-to-noise ratio throughout the image by measuring both scattered and primary radiation and using the results in a feedback loop to control the x-ray tube output, (2) raster scanning an x-ray beam along straight lines at constant velocity within a line by using a curved slit rotating aperture, (3) simultaneously controlling each of the intensity and hardness of a scanned x-ray beam as determined by post-patient x-ray measurements, and (4) using a segmented fan to scan the patient in a direction transverse to the plane of the fan and individually modulating each beam segment to achieve desired image characteristics.

Abstract: A CT scanner comprises a rotatable gantry 1 carrying an X-ray source 3 which provides a fan-shaped beam of X-rays and an X-ray detector 10 which scans the X-ray intensity along a line in the plain of the beam. The gantry is rotatable about an axis A relative to a couch 2 which supports the patient under investigation. The X-ray detector 10 comprises two concentric X-ray masks exposed around a scintillator tube; the two masks having helical X-ray transmissive apertures, the helices being of opposed orientation so that rotation of the masks in opposite directions causes the exposed area of the scintillator tube to move along its length.

Abstract: Method and apparatus for imaging using penetrating radiant energy provides a resulting image with elements of intensity related to atomic number. A penetrating radiant energy source is used for generating a flying spot. A first detector is located to be responsive to transmitted energy, e.g. the flying spot traverses the first detector. A second detector is located substantially coplanar with the first detector to be responsive to scattered energy, as the flying spot scans a target. The signals produced by the first and second detectors are combined to produce an image array having elements of intensity related to atomic number. A method and apparatus for non-invasively measuring density using the apparatus already recited, is also disclosed.

Abstract: A scanning system for use in radiology incorporates two disks bearing identically patterned, radial arrays of apertures and corresponding X-ray detection materials, respectively. The disks are axially aligned and synchronously rotated on opposite sides of the radiological target. X-rays emitted adjacent to the first disk are strobed by the apertures and detected by photodiode arrays on the second disk after passing through said target. The detection array is scanned to produce the X-ray information for digital processing and imaging. The foregoing apparatus may be portable and mounted for retrofitting relative to existing X-ray installations.

Abstract: An X-ray image detecting apparatus devised so as to project an X-ray image of an object on a image receiving plane made of two-dimensionally arrayed picture elements with the object irradiated by many thin scanning X-ray beams. The beams, which penetrate the object, scan coincidentally a limited number of the picture elements of which they take a charge respectively, causing the time of scanning the whole area of the image receiving plane to be short. The image signals successively outputted from the picture elements are stored in a memory with only their respective maximum values selected. Thus, the image signals are made free from being adversely affected by the stray X-rays scattered from the inside of the object, and provide a high-contrast X-ray image.

Abstract: To determine the thickness profile of rolled metal sheets by measuring the absorption of X-rays a slit diaphragm is used which is rotatable about the imaginary center axis of the X-ray tube, the rotation consisting of a reciprocating movement lying in a range of less than 36.degree.. Between the reversal points of the rotation the rotational movement takes place in steps with rest intervals lying therebetween.

Abstract: An x-ray scanner generates a fan-shaped x-ray beam through which an object to be inspected is moved and a detector line which generates signals corresponding to the radiation attenuated by the object. The processing electronics includes a comparator for recognizing faulty detector signals by means of which the roll-in of data ito the image memory is controllable such that, given a faulty detector signal, the transfer of the information into a memory line allocated to the detector is inhibited. The x-ray scanner also includes an allocator unit which omits predetermined detector channels for the purpose of geometric balancing. Given outage of a detector, its detector channel is omitted by the allocator unit, based on a signal from the comparator, instead of a prescribed detector channel.

Abstract: An X-ray lithography system, in which an X-ray beam is separated from synchrotron radiation beams and reflected by a scanning mirror which vertically scans the reflected X-ray beam. The X-ray is irradiated into an exposure chamber via a beryllium window, which is vertically oscillated in such a manner that the beryllium window is shifted up and down in synchronization with the scanning operation of the X-ray beam.

Abstract: A flying spot generator is provided for changing a beam of x-rays into a flying spot moving in a linear direction over an object to be detected. The flying spot generator includes a rotating cylinder having even numbers of helical slots. A single long detector is provided behind the object for measuring the radiation characteristics of the object.

Abstract: An apparatus for slit radiography which comprises an X-ray source and a slit diaphragm through which a body to be examined can be scanned by a planar X-ray fan beam. The slit diaphragm is operatively associated with a plurality of juxtaposed tongue-shaped attenuation elements, which elements are arranged for having their free ends swung to a greater or lesser extent into the X-ray beam so as to effect a local attenuation of the beam. The tongue-shaped attenuation elements form at least one flared configuration as seen from the X-ray source.

Abstract: An X-ray scanner for examining objects moved through an X-ray beam on a conveyor has a detector array disposed at the opposite side of the conveyor from the X-ray source and a pulse generator connected to the motor which drives the conveyor for generating a pulse trail dependent on the speed of the motor, and hence on the speed of the conveyor. The pulse generator is connected to a scanner for the detector array and controls the scan rate dependent upon the conveyor speed. The conveyor speed is controlled such that the quotient of the scan rate and the conveying speed is maintained constant in order to hold the imaging scale constant.

Abstract: The present invention concerns an X-ray examination apparatus allowing to produce images of an object through the scanning of the object by at least two fan-shaped beams having different energy spectra, and obtained from a single X-ray source, thereby allowing to obtain simultaneously images of the object adapted to contain different informations.

Abstract: A radiotherapy machine includes a microwave powered accelerator to produce an energetic beam of charged particles, bending and focussing magnets to parallel scan the beam in a plane and collimators to make the resulting parallel scanned beam into paraxial rays of charged particles or X-rays. The beam is intensity modulated as it is scanned to control dosage spatial distribution. The subject is moved perpendicular to the scanning plane in order to treat a 3-dimensional shape.

Abstract: An X-ray imaging apparatus comprising an X-ray generator having a cylindrical rotating target, a slit plate having a slit extending in a direction perpendicular to the direction in which an X-ray focal point moves on the target, the slit plate and rotating target being moved relatively in a direction perpendicular to the direction in which the slit extends, an X-ray detector arranged opposite to the slit plate with an object interposed between them and serving to convert X-ray beams passing through the slit and the object to electrical signals, a signal processor for picking up only those signals which relate to the object from these electrical signals obtained through the X-ray detector, and a display device for displaying an X-ray image which corresponds to the signals picked up by the signal processor.

Abstract: An apparatus for slit radiography comprising an X-ray source, an X-ray detector positioned behind a body being irradiated, said detector collecting the radiation passed through the body, and a slit diaphragm placed between the X-ray source and the body being irradiated, through which diaphragm there is formed a planar X-ray fan beam, while for obtaining a complete X-ray shadow image of the desired part of the body at least the fan beam performs a scanning movement. There are provided detection means coacting with the X-ray detector and comprising a plurality of sections juxtaposed in the longitudinal direction of the strip-like portion of the X-ray detector irradiated through the slit diaphragm, each section being adapted to produce an electric signal depending upon the radiation impinging instantaneously on the associated section of the X-ray detector.

Abstract: A digital radiography system is disclosed having a multi-element curved detector assembly. The system includes an X-ray tube having a focal spot from which its X-ray energy primarily emanates, and which is spaced from the detector assembly to define a subject examination space. The detector assembly includes a number of individual detector elements each having a radiation sensitive face, the element faces collectively defining a concave curved surface oriented toward the focal spot. The assembly of elements can embody forms including a single curved line of elements, multiple similarly curved lines defining collectively a portion of a cylinder, or a three-dimensional curved array defining a portion of a sphere. Multiple layer stacked arrays can also be used. Each element responds to incident X-rays to produce an electrical charge signal indicative of the radiation.

Abstract: Apparatus are disclosed to scan an elongate portion of the patient. The apparatus have a low power x-ray source and imaging assembly, each having a collimating slit through which an x-ray beam is transmitted. Means are provided to move the patient relative to the x-ray source. A film pack is located in the imaging assembly and is mounted on a motor carrier to move the film pack progressively past the slit axis so as to record the image obtained. The low power x-ray beam is offset by a high speed film and an image intensifier in the imaging assembly, thereby reducing the amount of radiation subjected on the patient.

Abstract: Radiography apparatus includes radiation sources for substantially simultaneously generating first and second radiation beams. Collimators are interposed between the radiation source and an object to be exposed to radiation in order to convert the first and second radiation beams into parallel fan-shaped beams. Radiation detectors convert the radiation passing through the object being examined into an electrical signal containing image information. The radiation detector may have at least one scintillator to convert radiation which is passed through an object into light and at least one self-scanning array of photodiodes to convert the light into an electrical signal containing image information. Fiber optic coupling may be provided between the scintillator and the self-scanning array of photodiodes.

Abstract: An x-ray beam is scanned through a sequence of subsections of a volume. At each position the x-ray transmission is measured at two energies. The resultant transmission signals are processed to produce a signal representing the projected amount of a specific material in the volume. The two energies are chosen so that the mass attenuation coefficients of bone and soft tissue are comparable to each other.

Abstract: A collimator blade assembly for a computed tomographic X-ray scanner comprising a blade of X-ray beam stopping material having an aperture for passing X-rays and an X-ray penetrable member closing at least a portion of that aperture to reinforce the material and thereby minimize collimator blade/detector clearances while preventing undesirable collimator blade vibration.

Abstract: There is disclosed an improved control circuit for a slit radiographic apparatus wherein the slit radiographic apparatus is comprised of an X-ray source adapted to scan a body to be radiographed through a slit diaphragm with a planar fan beam wherein the slit diaphragm is divided into a plurality of attenuating sections each associated with a controllable X-ray attenuating member and wherein there is provided a detection assembly for detecting the intensity of the radiation having passed through the body and divided into sections corresponding to attenuating sections of the slit diaphragm wherein each X-ray attenuating member is controlled by the control circuit including an energizing circuit and a comparison circuit for each attenuating member of the slit diaphragm whereby the comparison circuit compares an output signal of an associated section of the detection means to a reference signal to form an output coupled to the energizing circuit to control the attenuating member of a respective attenuating secti

Abstract: An apparatus for slit radiography comprising an X-ray source by means of which a body being irradiated can be scanned through a slit diaphragm by a substantially planar, fan-shaped X-ray beam, the scanning X-ray beam passed through the body impinging upon an X-ray detector. The apparatus comprises controllable attenuation elements coacting with the slit diaphragm, each element being adapted to influence a sector of the scanning X-ray beam under the control of signals generated by a radiation detector device. The radiation detector device is disposed between the body being irradiated and the X-ray detector. The radiation detector device comprises at least one radiation detector extending at any moment into the scanning X-ray beam passed through the body and moving in synchronism with the scanning movement of the X-ray beam.

Abstract: Apparatus for the generation of an image of a blood vessel comprises an x-ray source to irradiate the vessel. X-ray image collection means receive the x-ray image of the vessel and conversion means convert the x-ray image into a visible image of the vessel. A collimator selects an elongate field of illimunation representative of a section of the blood vessel. A light detecting array receives the field of illumination and has a plurality of discreet detectors spaced along the axis of the elongated field. Each detector has a length at least equal to the width f the elongate field produces a signal upon impingement by the elongate field. The array is scanned sequentially to output the signals in series. The signals are then displayed as one line of a video display.

Abstract: A large area, digital radiography apparatus in which a prescatter and a postscatter collimator are moved simultaneously with an x-ray image intensifier tube whose output display is scanned by a stationary scanning camera to produce a digitized x-ray image over a large cross-sectional area of the patient.

Abstract: In an X-ray system with a rotating disk having sector slits transmitting fan shaped X-ray beams from a source through a subject to the image area of an X-ray receptor an improved ratio S/P of X-rays (S) scattered from the subject to primary rays (P) forming a true image of the subject at the image area is realized if the dimensions of the system fall within the following ranges of dimensionless ratios:(1) DW/RL is less than 0.5;(2) D/L is greater than 0.15;(3) LS*N/WD is greater than 0.15 and less than 0.6 whereinD is the distance of the disk from the X-ray source,W is the width of the X-ray image area spanned by the narrow beams,R is the radius of the disk,L is the distance between the X-ray source and the receptor.S* is the radially outermost slit width in the X-ray beam, andN is the number of slits whose beams simultaneously intersect the outer edge of the image area.

Abstract: The present invention provides an improvement to a digital system for reading out image information of items passing on a conveyor belt through an X-ray fan-beam. The improvement involves an oscillating screen or mask moving between the items and detector elements with the mask having openings in registration with at most one half of each of several detector elements. The mask oscillates over the detector elements to expose alternate portions of the radiation detecting surface of each of the detector elements to the incident X-rays.

Abstract: A digital X-ray scanner device for instantaneously producing an electronic image on a monitor of a object scanned by X-rays. A solid straight electronic linear array is used to detect X-ray shadows and produce an electronic signal which is processed for display. Image enhancement techniques can be utilized including displaying particular windows of the scan data and elimination of background noise and the use of color enhancement. The device can be adapted for retrofitting panoramic dental X-ray scanning machines and other uses such as veterinary scanning and scanning other portions of the human body as well as inanimate objects. Archival storage of the information allows ready access for future use.

Abstract: Scatter is reduced in an X-ray system by a relatively large light- or X-radiation-opaque mask rotating on a first axis and carrying smaller, radiation-opaque masks rotating off the first axis. The smaller masks have elongate radiation windows which are held parallel to each other and to a fixed plane through the first axis by translative drive and driven wheels on the larger and smaller masks coupled by a flexible linkage belt.

Abstract: In apparatus for measuring a void ratio by using radiation of the type comprising a hollow measuring unit through which fluid containing voids flows, a source of radiation and a radiation detector which are disposed on the opposite sides of the measuring unit, there is provided a rotary disc disposed between the measuring unit and the radiation detector and provided with a plurality of measuring openings which are equally spaced around a periphery of the rotary disc for transmitting the radiation.