US2985761A - Method and apparatus for regulating X-ray exposures - Google Patents

Description

May 23, 1961 P. E. OHMART 2,985,761

METHOD AND APPARATUS FOR REGULATING X-RAY EXPOSURES Filed April 9, 1954 3 Sheets-Sheet 1 &

IN VEN TOR.

A TTORNE Y5.

May 23, 1961 P. E. OHMART 2,985,761

METHOD AND APPARATUS FOR REGULATING X-RAY EXPOSURES Filed April 9, 1954 3 Sheets-Sheet 2 INVENTOR.

May 23, 1961 METHOD AND APPARATUS FOR REGULATING X- Filed April 9, 1954 P. E. OHMART 2,985,761

RAY EXPOSURES 3 Sheets-Sheet 3 16 5N V E? TOR. BY 2 g ATTORNEYS.

United States Patent METHOD AND APPARATUS FOR REGULATING X-RAY EXPOSURES Philip E. Ohmart, Cincinnati, Ohio, assignor to The Ohm'art Corporation, Cincinnati, Ohio, a corporation of Ohio Filed Apr. 9, 1954, Ser. No. 422,068

11 Claims. (Cl. 250-95) This invention relates to X-ray photographic equipment and is particularly directed to a method and apparatus for automatically terminating the operation of an X-ray tube when a radiograph of optimum quality has been obtained.

The present invention will be explained and described particularly as it is .used in conjunction with medical radiographic equipment. Radiography is a specialized field within the medical profession, and the experts in this field of diagnosis are given special training in both the preparation and interpretation of X-ray pictures, or radiographs. In order that a radiograph may be of optimum value in diagnosing the condition of an individual being examined, it must present a maximum amount of diagnostic detail about the internal condition of his anatomy.

This diagnostic detail, which facilitates the visual interpretation of a radiograph, depends in general upon the sharpness, or definition, of the image formed of each component part, and upon the contrast that appears between parts of diiierent densities; Among the factors which afiect the contrast and detail obtained in a radiographic film are the milliamperage of the current flowing in the X-ray tube, the distance from the X-ray tube to the object, the kilovoltage applied to the tube electrodes, and the length of time during which the film is exposed.

One of the principal difliculties in obtaining consistently good X-ray photographs is that these factors are interrelated so that a change in one of them requires a corresponding change in another, in order to obtain optimum radiographic results. Over aperiod of time an expert becomes very adept in selecting the correct values of these various factors and in changing one of them whenever required to compensate for changes in another. However, the skill which is thus acquired is one of the factors rendering radiology a highly specialized field of medicine.

In many installations, it is not feasible to have such a highly trained radiologist supervising the operation of the X-ray equipment. For example, in medical clinics, in institutions, and even in the ofiices of many doctors, X-ray equipment is often operated by technicians, interns, or others whose skill is far below that of an expert radiologist. Generally, these technicians have not mastered the highly developed techniques for obtaining optimum radiographic results and as a result the radiographs which they obtain do not provide an ideal basis for making a medical diagnosis.

The principal object of the present invention is to provide a method and apparatus for automatically obtaining radiographs of optimum quality for diagnostic purposes. By utilizing the principles of this invention, even a relatively unskilled technician can consistently obtain X-ray pictures containing maximum diagnostic detail. More particularly, the present invention contemplates a method and apparatus for automatically terminating the operation of an X-ray tube, after an image of predetermined density has been formed on the photographic film or plate.

In order that the significance of this objective may be fully appreciated, it is considered helpful to briefly review the manner in which a radiograph is obtained. When preparing a radiograph, the patient is positioned so that the portion of his body to be examined is'disposed between an X-ray tube and a photographic medium such as a'film or plate. The tube is energized and the Roentgen rays or X-rays emitted by the tube penetrate the patients body where a portion of them are absorbed. The X-rays are not absorbed uniformly however, since different materials constituting the body are of different relative opacity to the rays. For example bones absorb more rays than flesh, while flesh absorbs more than blood or air; similarly diseased areas often absorb a different amount of X-rays than adjacent normal areas. Those X-rays which are not absorbed pass completely through the body and impinge upon a photographic medium. The medium in cludes a layer of a photosensitive material, such as a silver halide emulsion, which is chemically alfected by the X-rays forming an image of the subject through which the X-rays have previously passed.

The contrast and definition of the image on the developed radiograph depend not only upon the settings of the X-ray tube energization circuit, as explained above, but also upon certain characteristics of the photographic medium. The general response characteristics of most mediums are such that maximum contrast is obtained if the density is maintained at a predetermined level or within relatively narrow predetermined limits. Within these limits, even a small difference in absorption of X-rays in the subject causes appreciable difierences in the darkness and contrast of the developed X-ray picture.

The overall density, or blackness, of the radiographic image depends both upon the intensity of the radiation and the time during which radiation of that intensity impinges upon the film. It will readily be appreciated that the intensity of radiation impinging upon a photographic plate is not only dependent upon the initial adjustments of the X-ray machine, but also on the particular subject being X-rayed. Thus it is apparent that if two X-ray tubes emit identical beams and one beam passes through the chest of a 200 pound man, while theother beam passes through the chest of a pound woman or child; the photographic plate of the mans chest receives a substantially less intense beam of X-rays than a similar plate of the womans chest. For this reason the time required for an image of optimum density to be formed on the first plate is considerably longer than that required for one to be formed on the second plate.

The method and apparatus of the present invention, by effecting the termination of the X-ray tubes operation as soon as the image density on the photographic film is optimurn, automatically compensates for variations in radiographic subjects as well as for changes in the settings of the X-ray equipment. Specifically, the present invention is predicated upon the concept of generating an electric potential, which is correlated with the exposure of a photographic medium andemploying this potential to operate suitable control apparatus for automatically terminating the operation of the X-ray tube whenever the film has been subjected to the selected exposure which is considered optimum. Thus the need of a highly trained operator to judge the length of an exposure is eliminated. Rather the operator merely makes any desired adjustments in kilovoltage, milliamperage, or tube-subject distance andpresses a button to start the X-ray tube in operation. The control apparatus then takes over and automatically deenergizes the X-ray tube, when a photographic film of optimum quality has been obtained.

Empirically, the present invention is based upon the discovery and determination that if a radiant energy electric generator, or Ohmart cell is placed in alignment with a photographic medium so that X-rays passing through; the medium impinge upon the cell, a potential generated by the cell varies in exactly the same manner as the chemical changes effected in the photosensitive emulsion of the film. The potential produced by the cell thus furnishes an accurate index of the exposure of the film and the density which the film will have when developed.

Many features of the construction and operation of an Ohmart cell are described in my copending applications on Radio Electric Generator, Serial No. 233,718, filed June 27, 1951, now Patent No. 2,696,564, and Method of Converting Ionic Energy Into Electrical Energy, Serial No. 266,883, filed January 17, 1952, now abandoned. Portions of those earlier disclosures, pertinent to the present application of Ohmart cells, have been incorporated into this specification. Basically, a radiant energy electric generator, or Ohmart cell comprises two chemically dissimilar electrodes in contact with an ionizable medium. For example, the electrodes can respectively be formed from two metals such as magnesium and gold, or from a metal and metallic oxide such as zinc and lead dioxide. Among the various possible ionizable mediums, a gas such as hydrogen, argon or helium is preferred, although certain liquids such as a silicon oil or a semi-conducting solid such as germanium or a silicon semi-conductor could also be used.

It has been experimentally observed that when an ionizable medium in contact with two dissimilar electrodes is ionized by the impingement of radiant energy or by secondary radiation, in turn caused by the primary radiant energy. there is a discriminatory migration of the ions toward the electrodes. The positive ions move toward the more noble electrode and the negatively charged electrons move toward the more active electrode. I have postulated that this discriminatory migration is due to the presence of an asymmetric field established by the chemical dissimilarity of the electrode surfaces. At any rate, when the ionizable medium is converted to an ion plasma by subjecting the medium to radiations such as those emitted by an X-ray tube, a discriminatory migration of ions does occur, and this discriminatory ion migration causes a potential difference to be established between the two electrodes and also causes a current to flow through a conductor connecting the two electrodes.

It has further been determined that for every combination of compositions of electrode surfaces, a different maximum electrode potential is developed when the electrodes are in the presence of an ionic plasma. Furthermore. while this maximum difference of potential is not a function of the intensity of impinging radiation, or of the configuration of the cell, or the characteristics of the filling gas, but is exclusively a function of the chemical dissimilarity of the electrode surfaces; nevertheless, the rate at which the maximum potential between the electrodes is developed varies with the intensity of the ionizing energy. This rate of potential development also varies directly with the rate at which the silver halide of the film emulsion is chemically affected.

In an X-ray machine adapted to utilize this discovery, an Ohmart cell is disposed in alignment with an X-ray tube and photographic medium so that a portion of the radiation from the tube, passes through the photographic medium and impinges upon the Ohmart cell. The electrodes of the Ohmart cell are connected across a high impedance load resistor and are also placed in circuit connection with the control grid of a voltage amplifier. The amplifier output controls an exposure terminating relay or other electrically responsive switching device having contacts in the energization circuit of the X-ray tube.

The energization circuit of the X'ray tube also includes main contactors operable in response to the manual actuation of a push button or similar device by the operator. The exact circuit arrangement for initiating the exposure period constitutes no part of the present invention, several suitable arrangements being described in greater detail below. The contacts of the exposure terminating relay are normally closed, so that the X-ray tube is energized and the exposure period commences upon the closing of the main contacts. At the start of the X-ray exposure there is no potential difference between the electrodes of the Ohmart cell. However, as soon as the X-ray tube is energized, X-rays emitted by the tube pass through the subject and photographic film and impinge upon the Ohmart cell, ionizing a portion of the filling gas or other ionizable medium. The positive ions and electrons, which are thus formed, respectively migrate toward the positive and negative electrodes of the cell so that a difference of potential is built up between the two electrodes.

The amplifier and exposure terminating relay are adjusted so that the relay is opened when the potential difference between the two electrodes of the Ohmart cell reaches a predetermined value, less than the open circuit potential of the cell. It has been experimentally determined that no matter what the exposure conditions, the cell will always reach this potential in substantially the same time required by the film to reach a predetermined image density. Thus the control apparatus described is effective to automatically deenergize the X-ray tube as soon as the picture of the desired density has been obtained.

It will be understood that for each film density, there is a corresponding discrete potential which will be developed between the electrodes of the Ohmart cell in the same period required for the exposure of the film. That is, no matter what adjustments are made in the milliamperage, kilovoltage and subject-tube distance, and no matter what the relative opaqueness of the subject, the Ohmart cell will always arrive at a unique potential, corresponding to the selected film density, in exactly the length of time required by the film to reach that density. Thus a film of any desired density can be produced by adjusting the amplifier so that it actuates the terminating relay when the Ohmart cell develops a potential corresponding to that density.

A second unique operating characteristic of an Ohmart cell is observed when an external leakage impedance is connected across the electrodes of the cell. It has been determined that the current generated by the cell and passing through a leakage impedance varies directly with the intensity of the impinging radiation. This characteristic of an Ohmart cell is utilized in a modified type of control apparatus for automatically terminating opera tion of an X-ray tube. In the modified embodiment, an Ohmart cell is placed in alignment with the photographic film and X-ray tube as explained above. However, in this embodiment the electrodes of the cell are connected to an external circuit including a leakage resistance and a trigger capacitor. The trigger capacitor is in turn connected to an amplifier which controls the operation of an exposure terminating relay or other device in circuit connection with the energizing means of the X-ray tube. The exposure terminating relay and amplifier are arranged so that the relay is actuated when the trigger capacitor reaches a predetermined potential.

Again, it has been experimentally observed that the current generated by an Ohmart cell is effective to raise the trigger capacitor to a predetermined potential in exactly the same time required by the film to reach an .image of a selected density. When an operator closes a switch to start the X-ray exposure, the X-radiations passing through the photographic plate ionize the gas or other medium in the Ohmart cell so that a discriminatory migration of ions is established between the cell electrodes. The electrons collected at the negative electrode flow through the external circuit to the positive electrode where they neutralize the positive ions to form gas molecules. For each electron that is picked up by a positive ion, an additional electron flows through the external circuit from the negative to the positive electrode.

The amount of current flow depends upon both the intensity of the impinging radiation and the impedance of the external circuit. Since the external impedance. remains constant during the exposure period, the currentgenerated by the cell accurately reflects the intensity of the X-rays impinging upon the cell. When the current generated by the cell and flowing through the capacitor in the external circuit causes a predetermined potential to be developed across the capacitor, the amplifier actuates the exposure terminating relay to deenergize the X-ray tube. The X-ray tube is thus automatically deenergized as soon as the image on the photographic film has reached the desired density.

From a method point of view, no matter what specific form of apparatus is employed, this invention contemplates the generation of an electric potential through the use of X-rays, which have passed through a photowill be of the same density, no matter what settings are made on the X-ray tube controls, or what type of subject is being examined. To vary the density of the radiographs obtained, it is only necessary to adjust the control apparatus so that the X-ray tube is deenergized when a different potential has been developed.

This method is completely antithetical to that involved in the operation of any of the exposure control devices heretofore proposed. Each of the previously proposed devices depends for its operation upon the utilization of X-rays to vary the impedance of a circuit element, which is connected to a carefully regulated voltage supply. The current flowing through this variable impedance is then used to actuate a switching mechanism.

One of the principal advantages of the present method and apparatus is that it provides an extremely simple and reliable means for producing radiographs of optimum quality. Uniformly excellent radiograph results are obtained even when the X-ray equipment is operated by personnel relatively unskilled in radiographic techniques. By means of the present invention, the function of the operator is limited to making initial adjustments and operating a switch to initiate the exposure period. The exposure is automatically terminated, without the intervention of the operator, When a film of optimum density has been obtained.

Another advantage of the present invention is that it provides a means for obtaining uniform radiographic results throughout extremely wide ranges of operating conditions. It has been empirically observed that apparatus' operated in accordance with the present method is effective to terminate the operation of an X-ray tube when a film of the desired density has been obtained, even though the subject is relatively transparent and a high beam intensity is employed, so that the total exposure period is as short as of a second. Accurate control of film density is also obtained for low beam intensities and relatively opaque subjects requiring exposures of several seconds. Also, it has been found that the present X-ray apparatus is not adversely affected by substantial changes in kilovoltage settings, so that the need for a spectral compensator required by other types of automatic exposure devices is normally eliminated. If extreme kilovoltage variations are expected however, such a compensator can readily be incorporated into the apparatus as explained in detail below.

A still further advantage of the present invention, is that a minimum number of electrical components is required. The Ohmart cell generates its own potential so that no carefully regulated external voltage supply is needed. Moreover, since an Ohmart cell is directly 6 responsive to X-ray radiation, the need for an intermediate. energy converting medium, such as. the fluorescent screens. employed with photoelectric devices, is eliminated.

An additional advantage of the apparatus of this in vention, is that the Ohmart cell operates at a relatively low potential, generally of the order of one volt. Consequently, the cell does not present any appriciable current leakage problems, and may readily be constructed of a large cross-sectional area, to accurately measure the radiation impinging upon a large area of film. Furthermore, since the cell functions as a low voltage generator, any unavoidable leakage resistance in the'cable etc. appears in parallel with the load resistance and not in series with it as in a device of the ionization chamber type, so that variations in the leakage resistance due to temperature fluctuation, aging, and the like, do not result in any appreciable error in the operation of the device.

These and other advantages of the present invention will be more apparent from a further consideration of the following detailed description of the drawings illustrating a preferred embodiment of my invention.

In the drawings:

Figure 1 is a diagrammatic view showing the general arrangement of X-ray apparatus and one form of control circuit constructed in accordance with the present inven-v tion.

Figure 2 is a diagrammatic view similar to Figure 1 showing a modified control circuit arrangement.

Figure 3 is a top view of a preferred form of Ohmart cell for use in connection with X-ray apparatus, portions of the cell being broken away to show details of its. construction.

Figure 4 is an enlarged cross sectional view of the cell taken along line 4--4 of Figure 3.

Figure 5 is a cross sectional view of the connecting block taken along line 55 of Figure 3.

Figure 6 is a cross sectional view of the connectingblock taken along line 66 of Figure 4. V

Figure 7 is a cross sectional view of the connecting block taken along line 7-7 of Figure 6.

As shown in Figure 1, an X-ray system embodying a timer constructed in accordance with the present invention comprises an X-ray tube 10, a cassette 11 sup porting a sensitized film or photographic plate and a radiant energy electric generator, or Ohmart cell 12. Tube 10 emits a beam of X-rays which pass. through a subject 13 such as the chest of an individual being examined. A portion of the X-rays emitted by the tube It] are absorbed or scattered by the subject, while the remainder pass through the subject and impinge upon photographic film or plate 11. Some of the X-rays impinging upon the photographic film are absorbed and form a latent image on the film, while the rest of the X-rays pass through the plate and strike Ohmart cell 12.

In addition to these elements, the X-ray apparatus can be provided with an aperture plate (not shown) for regulating the dimensions of the X-ray beam. This plate, formed of lead or other opaque material, is disposed intermediate tube 10 and subject 13 and includes one or more openings through which the X-ray beam passes. In addition, it is conventional to provide a Bucky diaphragm between the subject and photographic film for screening out secondary radiation before it contacts the photographic medium so that well defined photographic images are obtained. These particular elements constitute no part of the present invention and are omitted from the drawings for the sake of clarity.

The details of construction of a preferred form of Ohmart cell 12 for use in conjunction with X-ray equipment will be explained in greater detail below. For the present purposes however, it can be stated that cell 12 comprises a housing 14, a positive electrode 15, a negative electrode 16, and an ionizable medium. The positive and negative electrodes are formed from chemically dissimilar materials such as magnesium and lead dioxide,-

the electrodes are electrically insulated from one another and are in contact with the ionizable medium such as an ionizable gas. As explained in detail below, Ohmart cell 12 generates an electric current for controlling the operation of timer circuit 17, which in turn governs the periods of energization of X-ray tube '10 in accordance with the density of the X-rays impinging upon the photographic plate and Ohmart cell.

X-ray tube includes an anode 18 and a cathode 19, which are connected across secondary Winding 20 of transformer 21. The cathode heater circuit is omitted for the sake of clarity. Alternating current is supplied to primary winding 22 of transformer 21 through power lines 23 and 24. It will be understood that the power lines include a main exposure switch (not shown) for completing the circuit to primary winding 22 and initiating the exposure period. The main switch may be in the form of a mechanical or magnetic contactor closed in response to the manual operation of an initiating switch or it may be in the form of an electronic contactor of the type shown in Fischer Patent No. 2,584,007. Alternatively, the main switch may be actuated indirectly by the closure of an initiating switch; for example, through the operation of a switch mechanically controlled by the movement of a Bucky diaphragm as shown in Gieringer Patent No. 2,591,536. It will be understood that the exact manner in which lines 23 and 24 are connected to a power source constitutes no part of the present invention.

Line 24 also contains contacts 25 and 26 of exposure terminating relay 27; these terminating contacts are in series connection with the main exposure contacts provided for initially closing the power circuit. Contacts 25 and 26 of the exposure terminating relay are normally closed so that the circuit to primary winding 22 is conditioned for completion by the main exposure switch. However, these contacts are opened to deenergize the transformer and X-ray tube, by current flow through relay coil 28 in response to the triggering of control circuit 17.

As shown, circuit 17 is controlled by the signal generated by Ohmart cell 12. Positive electrode 15 of the cell is grounded as at 30, while negative electrode 16 is connected through a shielded cable 31 to a high impedance load resistor 32. The opposite end of load resistor 32 is grounded as at 34. Cable 31, and resistor 32 are also connected through lead 35 to control grid 36 of vacuum tube 37, which constitutes the first tube of an amplifier 38. The plate-cathode circuit of tube 37 is connected across positive and negative lines 40 and 41 of an external voltage supply (not shown) through resistances 42, 43 and 44. Resistance 44 is a variable resistance including a grounded tap 45.

Plate 46 of tube 37 is connected through lead 47 to one grid 48 of tube 50; the other grid 51 of this tube is tied to the positive line 40 through resistance 52 and lead 53. Resistance 54 interconnects cathode 55 of tube 37 and lead 53. Plates 56 and 57 of tube 50 are respectively connected to positive line 40 through lead 58 and resistance 60. Cathodes 61 and 62 of the tube are tied together and are returned to negative line 41 through resistance 63 and conductor 64. Tube 50 also includes a screen 65 which is joined to line 66 and a suppressor 67 which is tied to cathode 62. Line 66 is connected in parallel to the anode-cathode circuit of tube 50, the line being connected to positive line 40 through resistance 68 and to negative line 41 through resistance 70.

A double triode 71 has one control grid 72 connected to plate 57 through lead 73 and resistance 74. The other grid 75 of tube 71 is joined to cathode 76 of the first half of the triode through connector 77. Plate 78 associated with cathode 76 and grid 72 is joined to positive line 40 through conductor 80, while plate 81 is associated with grid 75 and cathode 82 is joined to coil 28 of exposure terminating 27, the other terminal of this coil being joined to positive line 40. Cathode 76 is joined to line 83 and is returned to the negative line 41 through resistance 84. A resistance 35 is also placed between negative line 41 and control grid 72. A compensating potentiometer 86 is connected between positive line 40 and cathode 82, cathode 82 also being connected to line 41 through resistance 87. In this embodiment potentiometer 86 provides means for density adjustment as is explained in detail below in connection with a description of operation of the apparatus. A switch 39 shunts the cell electrodes and resistance 32. This switch functions to remove any residual charge from the cell electrodes and is opened prior to the exposure period.

A preferred form of radiant energy electric generator, or Ohmart cell 12 for use in connection with X-ray equipment is shown in Figures 37. As there shown, the cell 12 comprises a housing 14 enclosing a positive electrode structure 15, negative electrode structure 16 and an ionizable medium, for example an ionizable gas such as air. More specifically, housing 14 is of generally rectangular cross-section and is formed of sheets of a radiation permeable material such as Lucite. The housing includes front wall 90, near wall 91 and side walls 92. 93, 94 and 95. These walls are joined together in airtight relationship in any suitable manner, as by applying a plastic solvent to their adjoining faces and pressing the faces together.

The negative electrode assembly 16 includes a plurality of electrode members 96; each electrode member 96 is in the form of a generally rectangular sheet of magnesium foil. One end of the foil sheet is configurated to form a substantially semi-circular opening 97, while the opposite end of the foil is slit as at 93 to form two tabs 100 and 101. Each of these tabs is bent at an angle to the surface of the electrode, the tabs being disposed on opposite sides of the sheet as shown particularly in Figure 4.

The positive electrode structure preferably comprises a plurality ofv generally rectangular electrodes 102 formed of brass shim stock, both sides of which are coated with lead dioxide. These electrodes are shaped identically with the negative electrodes; that is, one end of each positive electrode member is configurated to form a substantially semi-circular opening 103, while the opposite end is slit as at 104 to form tabs 105 and 106. The tabs are bent outwardly from opposite sides of the electrode and their ends are overturned as shown particularly in Figure 4. In addition to electrode members 102, the positive electrode structure includes a coating 107 of aquadag or colloidal graphite coated on the inner surface of front wall 90. The positive electrode structure also includes a generally rectangular member 108 formed of brass stock coated with lead dioxide. Two opposite ends of this member 110 are slightly curved so that the member functions as a press plate for urging the electrode assembly together, as well as an electrode, as shown in Figure 4.

The various electrode members are held in spaced relationship with one another within the casing by means of peripheral strips 112 formed of an insulating material such as polyethylene. In the preferred form, each of these strips is of generally rectangular configuration and is provided with a longitudinal slot 113. The strips cxtend completely around the periphery of each of the electrodes in engagement with the inner walls of the housing. The endwise portions of negative electrode members 96 reside within longitudinal slots 113, while the marginal portions of positive electrode members 102 are disposed between the surfaces of adjacent blocks 112. As shown, the ends 110 of presser electrode 108 are in engagement with blocks 112, while the center portion of the electrode is in contact with rear wall 91. The arcuate configuration of this electrode causes the ends 110 to exert a spring force on blocks 112 urging them toward bottom wall 99 to hold the blocks, positive and negative electrodes, firmly in place. If desired, strips of an insulating material (not shown), such as Lucite or Teflon, having a thickness equal to the desired electrode spacing can be disposed between adjacent electrodes in the central portion of the cell.

The negative electrode members 96 are disposed within the casing so that the openings 97 formed in each of the electrodes are in alignment at one end of the casing; while the tabs res and 101 are also in alignment at the opposite end of the casing. Similarly, positive electrode members 192 are disposed within the casing so that the openings 193 formed in the electrodes are in alignment with one another as are tabs 105 and 106. However, the positions of the positive and negative electrode members within the casing are reversed so that the tabs of the positive electrodes and the openings of the negative electrodes are disposed in registry at one end of the casing, while the openings in the positive electrodes and tabs of the negative electrodes are disposed in registry at the other end of the casing.

Adjacent negative electrodes are interconnected by bending one tab of each electrode toward the opposite electrode. The two bent tabs are held in abutment and in electrical connection with one another by the resilience inherent in the electrode material. Adjacent positive electrodes are joined in the same manner; that is, by bending one tab of each of the electrodes toward the interposed negative electrode so that the two tabs are spring urged into contact with one another within the semi-circular opening 97 of the negative electrode. Electrical connection is made to the top positive electrode 108 by extending a tab 105 of the adjacent positive electrode completely through the opening 97 in the negative electrode disposed between the two and bringing the tab. into contact with an arcuate endwise portion of electrode 193. In the same manner, contact is made with the colloidal graphite coating 107 formed on wall 90 by bending tab 106 of the adjacent positive electrode 102 through slot 97 in negative electrode 102 disposed between the two.

Ohmart cell 12 actually comprises a plurality of subcells 115, 116, 117, 118, 119 and 120 connected in parallel electrical relationship. Cell 115 is formed by the colloidal graphite coating 107 on wall 90, the apposite surface of adjacent negative electrode 96, and the ionizable medium in contact with the two. Sub-cell 116 is formed by the apposed surfaces of the lowermost negative electrode 96 and the adjacent positive electrode member 102, and the ionizable medium in contact with the two. In a similar manner each of the sub-cells 117, 118 and 119 comprises the apposed surfaces of two dissimilar electrodes and the ionizable medium in contact with each.

Electrical connection is made to the positive and negative electrodes through a connecting block 122, which is mounted on wall 95 of the casing in any suitable manner such as by means of bolts 123 and 124 and nuts 125 and 126. The nuts reside in bores 127 and 128 formed in wall 95, being drilled completely through wall 95 as shown in Figures and 6. However, the bores do not interfere with the gastight construction of the cell since the inner edge 130 of wall 95 is sealed to Walls 90 and 91 by the application of a plastic solvent or in some other suitable manner. Connecting block 122 supports the end 131 of shielded cable 31. This cable is held in place by means of a set screw 132 threadably engaging a bore 133 in the connecting block.

The negative electrodes of cells 115-124} are connected to wire 135 of the insulated cable by means of.

a thin sheet of copper foil 136. Wire 135 passes through a horizontal opening 137 formed in wall 95 and is soldered to foil 136, which passes downwardly within wall 95 through a vertical opening 138. The foil passes between the innermost edge 130 of wall 95 and wall 90. The foil is then bent upwardly into engagement with 10 lowermost negative electrode member 96, to which the foil may be riveted if desired. It will be understood. that a small area of the inner surface of wall is left freeof aquadag coating so that foil 136 is insulated from the positive electrodes.

The positive electrodes of cells '120 are electrically connected to block 122 through bolt 124 and a foil strip 140. The foil strip is soldered as at 141 to either bolt 124 or nut 126 and passes downwardly through opening 128, beneath the inner edge of wall 95, and into contact with the aquadag coating 107' on wall 90.

As indicated above, when a cell 12 is installed in an X-ray machine, wall 90 is disposed parallel to the photographic plate on the side of the plate remote from the subject X-ray tube. With the cell housing positioned in this manner, the cell presents to the X-rays a plurality of progressively more absorbent layers of material. That is, an X-ray entering the cell first passes through a thin layer of aquadag 107, the ionizable medium of sub-cell 115, a thin sheet of magnesium 97, the ionizable medium of cell 116, and then the heavier, substantially more absorbent lead dioxide plate 102. Thus even a soft (low energy) X-ray energizes the mediums of sub-cells 115 and 116 before it is stopped by lead dioxide plate 102. On the other hand, a hard (high energy) X-ray, if it manages to pass through first plate 102, and the next magnesium plate 96 encounters a second dioxide plate of substantial stopping power. It has been experimentally determined that when using cells of this type, presenting successively more absorbent elements to the path of X- rays, especially those embodying heavy metals such as lead, the need for an electric compensator to adjust the exposure time for various spectral shifts caused by kilovoltage changes is eliminated.

Housing 14 can be constructed of large cross-sectional area, so that the Ohmart cell is substantially coextensive with the photographic medium. This construction is particularly desirable where it is customary to make relatively small radiographs or where, as in some clinics, it is customary to X-ray only the same part; the chest for example, of each patient. On the other hand, if the X- ray equipment is'to be used for obtaining radiographs of a wide variety of different parts such as chests, wrists, fingers and the like, it has been found desirable to utilize a plurality of smaller Ohmart cells connected in series relationship. Each cell may be constructed in a manner similar to that shown in Figure 5, or in some other suitable manner. The plurality of cells are mounted adjacent to one another on the side of the photographic medium remote to the subject and X-ray tube and are preferably of such a size and, in such number, they are substantially coextensive with the film.

The advantage of using a plurality of series of connected cells is that they automatically control the termination of the X-ray exposure in accordance with the amount of radiation passing through the densest part of the subject. This assures adequate exposure of all parts. of the film, even thoughonly a small part of the film is covered with an image of the subject, as would be the case of a radiograph of a wrist or finger.

More specifically, when using a plurality of series connected cells, the cells are connected across a load resistance and capacitor, which are in turn joined to a current amplifier of the type shown in Figure 2. The trigger capacitor is then adjusted so that the amplifier output opens the exposure terminating relay 28 when a predetermined charge corresponding to the desired film exposure has been built up on the capacitor. The circuit arrangement is described in greater detail in conjunction with a description of Figure 2.

It has been experimentally determined that the current generated by a plurality of series connected Ohmart cellsv is limited by the current generated in the least produc-. tive cell. Thus, if the radiograph of a wrist is being taken, the current effective to charge the trigger capacitor 1s limited by the current generated in the cell aligned with the densest portion of the wrist, and the X-ray tube will remain energized until the image formed on the photograph of this particular portion of the wrist is of the desired density. The substantially larger quantity of X- radiation flowing through the uncovered portion of the photographic plate in no way affects the lengths of the exposure period, since the cells upon which this radiation impinges are ineffective to cause a larger current flow through the trigger capacitor.

When operating an X-ray machine equipped with the automatic exposure control of the type shown in Figure l, the operator makes whatever adjustments are initially required in the X-ray energization circuit such as changing the milliamperage by means of a suitable control in the cathode heater circuit (not shown) by making changes in the kilovoltage setting by adjusting knob 160. This knob is shown as being mechanically connected to a tap in primary winding 22 of transformer 21; however it will readily be appreciated that the kilovoltage can be adjusted by means of an auto transformer tap, or any other suitable manner. Potentiometer 86 is then set for the desired film density. Generally, once this potentiometer has been adjusted to provide optimum film density, its setting will rarely, if ever, be changed.

The operator then places the subject 13 in position between tube and photographic medium 11. Next the operator closes an initiating switch, which either directly completes the power circuit to lines 23 and 24, or causes the power circuit to be completed through an intermediate switch. Relay contacts 25 and 26 of the exposure terminating relay are normally closed, so that when the power circuit is completed to lines 23 and 24 the X-ray tube is energized to initiate the exposure period.

Prior to the time the tube is energized, no potential difference exists between electrodes and 16 of Ohmart cell 11. However. as soon as tube 10 emits X-rays. these rays pass through the subject and photographic medium 11 and impinge upon the Ohmart cell where they ionize the gas or other medium in contact with the electrodes. The ion plasma which is thus formed consists of positive ions and negative electrons, which respectively migrate toward electrodes 15 and 16. This discriminatory migration causes a potential difference to be built up between the electrodes of the Ohmart cell, and since positive electrode 15 is grounded, causes the potential of electrode 16 to be lowered.

The potential of electrode 16 is applied to grid 36 of the first tube of amplifier 38. It can be seen that a negative potential applied to this grid causes the potential of plate 46 and grid 48 of tube 50 to rise, while the potential of cathode 55 and grid 51 of tube 50 is lowered. Thus a larger current flows between cathode 61 and plate 56 and smaller current fiows between cathode 62 and plate 57.

The potential of plate 57 is thus raised, raising the potential applied to grid 72 of tube 71. Tube 71 is a cathode follower and a rise in the potential of grid 72 causes an increase in the current flow between cathode 76 and anode 78. This results in a rise in the potential of cathode 76 and grid 75 associated with anode 81 and cathode 82. When the potential on grid 75 reaches a critical level determined by the setting of potentiometer 86, a sufliciently high current flows between anode 81 and cathode 82 and through coil 28, to pull the relay armature in and open contacts 25 and 26 of the exposure terminating relay.

The time requir d for the Ohmart cell 12 to develop a potential sufficiently large so that when amplified it will cause the exposure terminating relay to open, is exactly the same length of time required by the photosensitive emulsion on film 11 to develop a latent image of the desired density. Thus, the exposure period is automatically terminated by the opening of relay 27 when a film of optimum density has been obtained, no matter what initial 1a settings are made on the X-ray machine, or how opaque subject 13 happens to be.

Figure 2 shows a modified form of apparatus for controlling X-ray exposures in which the potential applied to the amplifier is developed by current generated in the Ohmart cell and flowing through a capacitor in the external circuit of the cell. As shown, the general arrangement of the X-ray apparatus is the same as that in the previous embodiment. X-ray tube 10, including anode 18 and cathode 19, is connected across secondary winding 20 of transformer 21, primary winding 22 of that transformer is energized through power lines 23 and 24. The power lines include an initiating switch, not shown, and contacts 24 and 26 of exposure terminating relay 27. The contacts of this relay are normally closed as in the previously described embodiment and are opened in response to a flow of current through coil 28.

As in the first embodiment, the beam of X-rays emitted by tube 10 passes through a subject 13 and a photographic plate or film 11, before impinging upon Ohmart cell 12. The Ohmart cell includes a positive electrode 15 grounded as at 30 and a negative electrode connected through shielded cable 31 to one end of load resistance 32 and a variable capacitance 149. The other end of the capacitance and resistance are connected through lead 150, and lead 83, to cathode 76 of tube 71. Thus in this embodiment, load resistance 32 and capacitance 149 form circuit elements of a feed-back loop of amplifier 151. Amplifier 151 is identical with amplifier 38 except for the provision feed-back line 15!). Consequently, it is not considered necessary to describe in detail the amplifier circuit; but for convenience, the circuit elements have been given the same numbers as the corresponding elements 38.

A further difference between the modified embodiment and the first described embodiment, is the provision of a spectral compensator. As explained previously, it has been empirically determined that with the present apparatus no compensator is required over wide ranges of kilovoltage settings. However, if it is determined that such a compensator is desirable in a specific embodiment, it can readily be incorporated as shown in Figure 2. As there shown, changes in kilovoltage settings are made by adjusting knob 160, which is mechanically connected to a tap on transformer winding 22. The knob is mechanically interconnected with potentiometer 86 in such a manner that when the kilovoltage is increased the potentiometer resistance is also increased.

When potentiometer 86 is connected to the kilovoltage setting knob in this manner, it electrically compensates for the fact that softer, less penetrating X-rays are absorbed in the back of the cassette and Ohmart cell housing. Thus, a smaller portion of the X-rays striking the film penetrate the Ohmart cell, and its potential does not quite reflect the chemical changes effected on the film. To eliminate this difiiculty, resistance 86 is adjusted so that the amplifier opens the relay at a lower potential on the capacitor. In this embodiment, the density obtained on the film is controlled by adjusting variable capacitance 149, so that a given output of the cell will produce a greater or lesser potential across the capacitor.

The operation of the device is substantially the same as that of the first embodiment. That is, the operator presses a button or otherwise closes an initiating switch to complete the power circuit to lines 23 and 24 and energize the X-ray tube. The X-rays emitted by the tube pass through the subject and strike the photographic film on which they form a latent image. A portion of the X-rays pass through the film and impinge upon the Ohmart cell forming an ionic plasma surrounding the cell electrodes. A discriminatory migration is set up within the ionic plasma, and a current flows through capacitor 149, causing a potential to be developed across that capacitor.

This potential is applied to grid 46 of tube 37; is amplified and ultimately appears on grid 75 of tube 71.

When the magnitude of the potential developed across capacitor 149 exceeds a predetermined value, the current flowing between anode 81 and cathode 82, associated with grid 75, increases to a point where the relay coil 28 is effective to attract its associated armature, opening contacts 25 and 26, deenergizing the X-ray tube. It will be noted that in the embodiment shown in Figure 2, a switch 161 in series with a resistance 162 substantially smaller than resistance 32 is connected to lead 35 and is grounded as at 163. This switch is efiective to remove any accumulated charges from capacitor 149 and is opened immediately prior to the initiation of the exposure period.

As explained previously, this circuit arrangement is of particular utility, when employed with a plurality of series connected cells, since the current supplied to capacitor 149 is controlled by the least productive cell and, therefore, the exposure period is automatically controlled to provide a predetermined minimum exposure on every portion of the X-ray film.

Having described my invention, I claim:

1. A method of regulating exposures to X-rays, said method comprising the steps of generating an electric potential by disposing two electrochemically dissimilar electrodes and an ionizable medium in contact with said electrodes in the path of said X-rays, utilizing said X- rays to form an ionic plasma in said medium, whereby a discriminatory migration of ions occurs and a potential is generated between said electrodes, and applying the potential generated by said electrodes to a control device, said control device being effective to deenergize the X-ray tube when said potential reaches a predetermined value.

2. A method of regulating the density of radiographic films exposed to X-rays, said method comprising the steps of generating an electric potential by disposing two electrochemically dissimilar electrodes and an ionizable medium in contact with said electrodes in the path of said X-rays passing through said film, utilizing said X- rays to form an ionic plasma in said medium, whereby a discriminatory migration of ions occurs and a potential is generated, and applying the potential to a control device, said control device being elfective to deenergize the X-ray tube when said potential reaches a predetermined value.

3. A method of regulating exposure to X-rays, said method comprising the steps of generating an electric potential by disposing two electrochemically dissimilar electrodes and an ionizable medium in contact with said electrodes in the path of said X-rays, utilizing said X- rays to form an ionic plasma in said medium, whereby a discriminatory migration of ions occurs between said electrodes, and connecting a capacitor to said electrodes, so that the current flowing between the electrodes builds up a potential on the capacitor, and applying said potential to a control device, said control device being eifective to deenergize the X-ray tube when said potential reaches a predetermined value.

4. Exposure controlling apparatus for use in combination with a machine including an X-ray tube and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator comprising a housing, said housing including a wall facing said X-ray tube, a coating on the interior of said Wall, a plurality of spaced electrode plates disposed within said housing parallel to said wall, adjacent electrode plates being formed of electrochemically dissimilar materials, means electrically connecting alternate plates, an ionizable medium in contact with said plates, said wall, coating, the electrode plate adjacent to said coating and the electrode plate next adjacent to said coating being formed of progressively more radiation absorbent materials, normally closed switch means in the energization circuit of said X-ray tube, an amplifier in circuit connection with two electrodes of said radiant energy electric generator and with said switch means, said amplifier being effective to open said switch means when the potential generated by the radiant energy electric generator exceeds a predetermined amount.

5. Exposure controlling apparatus for use in combination with a machine including an X-ray tube and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator comprising a housing including a wall facing said X-ray tube, a plurality of electrode plates disposed within said housing, means for supporting said electrode plates in spaced relationship parallel with said wall, adjacent plates being formed of electrochemically dissimilar materials, each of said plates being configurated to form an opening at one end and tabs at the opposite end, adjacent plates being reversed whereby the tabs of one plate are disposed in registry with the openings of adjacent plates, the tabs of alternate electrodes being bent into contact with one another, an ionizable medium in contact with said electrodes, and electrical contact means in contact with each set of alternate electrodes, normally closed switch means in the energization circuit of said X-ray tube, an amplifier in circuit connection with the contact means of said radiant energy electric generator and with said switch means, said amplifier being effective to open said switch means when the potential generated by the radiant energy electric generator exceeds a predetermined amount.

6. Exposure controlling apparatus for use in combination with a machine including an X-ray tube and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator including two electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, normally closed switch means in the energization circuit of said X-ray tube, an amplifier in circuit connection with said radiant energy electric generator and with said switch means, said amplifier being effective to open said switch means when the potential generated by the radiant energy electric generator exceeds a predetermined amount.

7. Exposure controlling apparatus for use in combination with a machine including an X-ray tube and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator including two electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, normally closed switch means in the energization circuit of said X-ray tube, an amplifier in circuit connection with said radiant energy electric generator and with said switch means, said amplifier being elfective to open said switch means when the potential generated by the radiant energy electric generator exceeds a predetermined amount, and means for varying the exposure received by said photographic medium, said means including a selectively variable impedance in circuit connection with said amplifier, said impedance being effective to change the potential generated by said radiant energy electric generator at which said amplifier opens said switch means.

8. Exposure controlling apparatus for use in combination with a machine including an X-ray tube, and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator comprising two electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, normally closed switch means in the energization circuit of said X-ray tube, a capacitor in series connection with said radiant energy electric generator, whereby a portion of the current generated by the generator passes through the capacitor, an amplifier in circuit connection with said capacitor and with said switch means, said amplifier being effective to open said switch means when the potential developed across said capacitor exceeds a predetermined amount.

9. Exposure controlling apparatus for use in combination with a machine including an X-ray tube, and a photographic medium, said apparatus comprising a radiant energy electric generator disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, said radiant energy electric generator comprising two electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, normally closed switch means in the energization circuit of said X-ray tube, a capacitor in series connection with said radiant energy electric generator, whereby a portion of the current generated by the generator passes through the capacitor, an amplifier in circuit connection with said capacitor and with said switch means, said amplifier being effective to open said switch means when the potential developed across said capacitor exceeds a predetermined amount, and means for adjusting the impedance of said capacitor for varying the exposure received by said photographic medium before said switch means are opened.

10. Exposure controlling apparatus for use in combination with a machine including an X-ray tube, and a photographic medium, said apparatus comprising a plurality of series connected radiant energy electric generators disposed in the path of X-rays emitted by said tube, each of said radiant energy electric generators comprising a cell of electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, normally closed switch means in the energization circuit of said X-ray tube, each of said generators being in alignment with a different portion of said photographic medium, a capacitor in series connection with said radiant energy electric generators, whereby a portion of the current generated by the generators passes through the capacitor, an amplifier in circuit connection with said capacitor and with said switch means, said amplifier being effective to open said switch means when the potential developed across said capacitor exceeds a predetermined amount.

11. Exposure controlling apparatus for use in combination with a machine including an X-ray tube, and a photographic medium, said apparatus comprising a, plurality of series connected radiant energy electric generators disposed in alignment with said photographic medium and said tube on the side of said medium remote from said tube, each of said radiant energy electric generators comprising a cell of electrochemically dissimilar electrodes and an ionizable gas in contact with said electrodes, each of said generators being in alignment with a different portion of said photographic medium, normally closed switch means in the energization circuit of said X-ray tube, a capacitor in series connection with said radiant energy electric generators, whereby a portion of the current generated by the generators passes through the capacitor, an amplifier in circuit connection with said capacitor and with said switch means, said amplifier being effective to open said switch means when the potential developed across said capacitor exceeds a predetermined amount, and means for adjusting the impedance of said capacitor for varying the exposure received by said photographic medium before said switch means are opened.

References Cited in the file of this patent UNITED STATES PATENTS 2,441,324 Morgan et a1. May 11, 1948 2,679,011 Fromm May 18, 1954 2,696,564 Ohmart Dec. 7, 1954 2,747,104 Jacobs May 22, 1956

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