referring to the drawings , fig1 depicts partially in block diagram form a typical scanning radiographic imaging system in accordance with the present invention . such apparatus 10 employs a conventional rotatable x - ray tube 12 to provide a horizontal x - ray beam 14 of suitable size corresponding in area to the overall object being scanned for exposure of the selected region , such as for a medical patient 24 . the emerging x - ray fan beam is further modulated with a collimator 16 to provide a moving x - ray fan beam 20 being employed to irradiate the medical patient . such moving x - ray fan beam results from motion of the rotating x - ray tube source and a slot opening or slit 21 provided in the collimator 16 relative to the stationary patient 24 and a detector unit . the thin fan beam 20 instantaneously cuts a slice through patient 24 . the direction within the slice which is perpendicular to the direction of the propagation of x - rays is referred to herein as the slice direction . in a pencil beam system , each raster line of a scan corresponds to a separate slice , while in a fan beam system , the slice advances continuously . a plurality of movable mechanical attenuating elements 22 modulate the slot opening with each of the attenuating elements having its movement controlled during scanning in a manner so as to substantially equalize the intensity of x - ray radiation being applied with respect to different body portions of the medical patient . while such moving controlled x - ray fan beam is depicted in the present drawings as proceeding in a generally vertical direction , it will be evident to one skilled in the art that an opposite or different travel direction can be utilized with suitable repositioning of the presently described cooperating structural components . the x - ray radiation in the moving fan beam passes through the medical patient 24 for subsequent processing in a stationary detection unit 26 which is constructed as a flat planar housing member . detection unit 26 is shown to comprise , for example , a bucky grid 28 , an equalization detector 30 , a phosphor screen 32 , and image recording means 34 which typically comprises photographic film or a film holder or cassette . an electrical power supply 36 is connected to equalization detector 30 . power supply 36 receives information concerning the position of x - ray tube 12 and / or collimator 16 , as shown by a dashed line , so that power supply 36 can selectively energize equalization detector 30 in accordance with the current position of x - ray fan beam 14 , as will be described later in more detail . electrical output signals from equalization detector 30 are provided to a control circuit 40 which generates control signals applied to variable attenuating elements 22 in a manner to equalize the signals received from equalization detector 30 . control circuit 40 is comprised of a feedback control circuit of a type having a construction which is known in the art . bucky grid 28 , scintillating phosphor screen 32 , and x - ray film or film cassette 34 are well known in the art and further description thereof will be omitted . equalization detector 30 preferably comprises an ion detection chamber having a pair of juxtaposed chamber walls and a gas contained therebetween . electrodes on each chamber wall contact the gas and are energized to provide an electrical potential across the gas resulting in an ionization region . x - rays passing through the chamber ionize the gas in proportion to the intensity of the x - rays . the electrodes collect the ions resulting in an electrode current proportional to the x - ray intensity . such an ion chamber can be placed between patient 24 and x - ray film holder 34 because x - ray beam 20 is substantially unattenuated by the ion chamber . the ability to place equalization detector 30 in front of the film holder makes the invention easily adaptable to retrofitting existing non - equalizing , film - based radiographic units for scanning equalization . fig2 is an elevation view of one preferred embodiment of an equalization detector including a first ion chamber wall 50 and a second chamber wall 52 . a gas at 51 is any gas capable of being ionized by x - rays such as air . an electrode grid network on wall 50 is comprised of a plurality of conductive runs 54 disposed on wall 50 in contact with gas 51 and extending in a direction parallel to the direction of travel of the scanning of the x - ray fan beam . a sheet electrode 56 is disposed on wall 52 in contact with gas 51 and is of substantially equal extent as the grid network of conductive runs 54 . sheet electrode 56 is coupled to a source of high voltage + hv . conductive runs 54 are grouped into a plurality of active zones 57 , 58 , and 59 by directly interconnecting groups of conductive runs corresponding to the equalization channels of the attenuating elements . three active zones are shown to simplify the drawings , however , there generally will be many more than three zones . the conductive runs in each active zone are coupled to ground or virtual ground , such as would be provided at the input of a signal amplifier ( not shown ). thus , a potential difference across the ionization region of each active zone is provided such that the conductive runs collect ions in proportion to the x - rays passing through each zone . the total current in all of the conductive runs of the respective active zone provides an output signal for input to the control circuit of the attenuating elements . the size of each active zone can be fixed , as illustrated in fig2 or , as in the embodiment of fig3 can be variable , in which case the ionization current signal from each individual conductive run is collected separately . the signals are then selectively added together in a manner allowing the detector to change the effective size and position of the active zones . such a feature would enable the invention to be used on imaging systems with a variable source - to - image distance ( sid ). as shown in fig3 the individual output signals of electrode runs 54 are each fed to a virtual - ground input of a respective amplifier 70 which provides the amplified signal for each collecting electrode to one of a plurality of electronic summing elements 72 as determined by electronically activated switches 74 . from the summing elements , the summed signals are processed by control circuit 40 as described above . the settings of switches 74 determine the grouping of conductive runs into active groups , each of which measures the exposure modulated by a single attenuating element 22 . each conductive run is connected by switches 74 to a single summing element 72 which is determined by the particular source - to - image distance being employed . electrical power is preferably supplied to amplifiers 70 , summing elements 72 , and switches 74 by power supply 36 ( fig1 ). another embodiment of equalization detector 30 is shown in fig4 wherein first chamber wall 50 and the interconnection of runs 54 may be as shown in either fig2 or fig3 . however , in this embodiment , second chamber wall 52 has an electrode grid network of conductive runs 62 disposed thereon in contact with gas 51 and extending in a direction perpendicular to the direction of travel of the fan beam . each conductive run 62 is coupled to a multiplexer 64 through a respective high voltage driver 65 . the purpose of this configuration is to provide electronic scanning of the equalization detector in order to reduce susceptibility to scattered radiation . in operation , only a portion of conductive runs 62 are energized at a time during scanning so as to limit the area of the detector which is sensitive to ionization by x - rays passing therethrough . multiplexer 64 is electronically controlled by apparatus ( not shown ) which also controls the position of the scanning fan beam in such a manner that only the conductive runs 62 which are currently directly in line with the fan beam are energized . in fig5 there is depicted a side view of an illustrative stationary detection unit employing a retrofit ion chamber detection means . such an add - on unit is easily adaptable to conventional manual and automatic film changers . for example , the add - on unit could be constructed to fit as a replacement for the automatic exposure control ionization chamber present in many existing manual and automatic film changers in non - scanning - equalization systems . the depicted stationary unit 90 employs a housing member 92 containing an equalization detector 94 according to this invention together with bucky grid means 96 . as such , the stationary detection unit is suitable for modification of existing scanning radiographic apparatus in order to provide equalization for recorded film images . the depicted housing member can thereby be joined to or physically supported by a conventional photographic film cassette 98 generally employed in such existing radiographic equipment of this type without causing substantial attenuation of the moving x - ray beam 100 before recording the radiographic image . the conventional film cassette herein illustrated employs a film member 102 operatively associated with a conventional phosphor screen member 104 . contrast equalization of the recorded film image again proceeds with the x radiation emerging from the radiographed object being simultaneously detected by the particular ion chamber detection means employed . it will be apparent from the foregoing description that broadly useful means have been provided to improve equalization detection in scanning radiography . it will be apparent from said foregoing description , however , that various modifications in the specific embodiments above described can be made without departing from the spirit and scope of the present invention . for example , it is contemplated that a moving x - ray beam having a pencil configuration can be employed . moreover , still other physical configurations of the presently improved scanning radiographic system than above specifically disclosed are possible so long as the essential dynamic relationships above disclosed are preserved between cooperating x - ray scanning means and the stationary detection unit . while preferred embodiments of the invention have been shown and described herein , it will be understood that such embodiments are provided by way of example only . numerous variations , changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention . accordingly , it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention .
a scanning type radiographic imaging system is disclosed which employs novel means to produce comparable radiation intensity in the object regions being radiographed . such result is achieved with variable radiation attenuating elements having their attenuation controlled during this scanning process with novel feedback control means . the feedback control is produced with a stationary detection unit having circuit means which generates the electrical control signals .