Description:
BACKGROUND OF THE INVENTION
Medical and industrial x-ray inspection systems often employ an image converter for fluoroscopy to enable direct visualization of an examination object or of the anatomy of a subject intervening between an x-ray source and the converter. Such systems are usually equipped with means for making radiographs shortly after an image is visualized on a display screen such as a television monitor. In many systems, a film cassette or the like having light emitting radiographic intensifying screens on each side of the film is projected into the x-ray beam path when a radiograph is to be made. The screens are removed from the beam path during fluoroscopy with the converter to minimize absorption and thereby provide maximum x-ray intensity for activating the converter.
In some apparatus such as the high speed film changer described in U.S. Pat. No. 3,569,700, owned by the assignee of this application, the radiographic intensifying screens remain in the beam path at all times. The screens are separated momentarily to permit advancement of film between them after which the screens are closed to compress the film when a radiograph is to be taken. In the device shown in the cited patent, roll film is used so a segment or frame thereof will be in the beam path when the x-ray source is energized. During radiography high absorption in the film is desirable to produce the most contrasting image. On the other hand, during fluoroscopy the irremovable continuous film normally merely serves as an unwanted absorber of radiation which could desirably be activating the image converter. Heretofore, undesirable absorption by the film and image intensifying screens during fluoroscopy was compensated by increasing the x-ray tube current and thereby increasing the intensity of x-ray photons emanating from the tube. This resulted in the patient or other examination object receiving undesirably high x-ray dosage during fluoroscopy. Because of the high film cycling rate in high speed film changers and because of the mass of the parts involved, it has not been practical to insert the intensifying screens and film only during exposure and to remove them between exposure intervals during which time fluoroscopy may be conducted.
SUMMARY OF THE INVENTION
An important object of the present invention is to reduce x-ray dosage to the examination subject or object during fluoroscopy in an x-ray system adapted for conducting fluoroscopy and radiography sequentially.
A further object is to provide a method and means for conducting fluoroscopy with a fluoroscopic screen or x-ray image converter although the radiographic film intensifying screens and even the film are not removed from between the x-ray source and the fluoroscopic device.
Still another object is to minimize absorption during fluoroscopy, in other than the examination subject, by absorbers such as film and intensifying screens while at the same time maximizing responsiveness of the fluoroscopic screen or the input phosphor or the photoconductor of the image converter.
Briefly stated, the invention involves determining the principal absorbing constituents of the absorbers intervening between the examination of subject and the image converter. A filter is selected for interposition between the x-ray source and the examination object. The composition of the filter is such that it is highly transmissive or poorly absorptive of an x-radiation wavelength band which is also poorly absorbed by the interfering absorber but effects a high response by the image converter.
How the foregoing objects and other more specific objects of the invention are achieved will appear in the course of a more detailed description of a preferred embodiment of the invention in reference to the drawing.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic view of a diagnostic x-ray system in which the new fluoroscopic image enhancement features are employed;
FIG. 2 is a plot of x-ray photon energy versus absorption for various materials used in one embodiment of the new system;
FIG. 3 is a plot of x-ray photon intensity versus photon energy for a typical x-ray tube;
FIG. 4 is a schematic view of an x-ray inspection system in which the principal interfering absorber is a radiographic film and the x-ray image converter is a vidicon tube; and
FIG. 5 is a plot of x-ray photon energy versus absorption for various materials used in the system shown in the preceding figure.
Referring now to FIG. 1, a diagnostic radiography and fluoroscopy subject 10 is supported on an x-ray transmissive table top 11. A schematically represented x-ray tube 12 is supported over subject 10. The x-ray tube has the usual envelope 13, a hot cathode emitter 14 and an anode target 15. The electron beam emitted from cathode 14 is focused on target 15 at a point from which x-radiation is emitted. The boundaries of the x-ray beam are defined by a symbolized collimator 16, for example. The most common target material 15 used in x-ray tubes for general diagnostic purposes is tungsten but the principles of the present invention are applicable to systems wherein the target is made of various materials such as molybdenum, rhenium, tantalum and others.
Situated below the subject supporting table top 11 in FIG. 1 is an x-ray converter tube 17 of a well known type. Tubes of this kind usually comprise an envelope 18, an input face 19 which is transmissive to x-rays, a photocathode assembly 20 and an output phosphor 21, together with focusing means, not shown. The input side of photocathode assembly 20 usually consists of a thin layer of x-ray fluoroscent material which fluoresces differentially in accordance with the intensity of the x-ray image impinging thereon. The fluorescent material produces light when impinged by x-rays and the light falls on an adjacent layer of photoemissive material which is a photocathode from which electrons are emitted to form an image corresponding in intensity with the light image. The photoelectrons are accelerated and focused on the output phosphor 21 on which a reduced but intensely bright optical image is formed.
The visible light image on output phosphor 21 is directed by means of a collimating lens 22 to a mirror 23, usually, from whence it is redirected toward a television or video camera 24. In a well known fashion, the optical image received in the lens assembly 25 of the video camera is transformed to electric signals which are used to drive a television monitor 26 on the screen 27 of which a picture of that part of the subjects anatomy or other examination object which is being viewed can be visualized. It will be understood that the x-ray image converter 17 and its associated optical system will ordinarily be enclosed in an x-ray impervious housing, not shown, to protect the subject and attendants from scattered radiation.
Interposed between the input end face 19 of image converter 17 and the table top 11 is a pair of image intensifying screens 28 and 29 which are shown separated from each other to define a small gap 30 for receiving the radiographic film 31 between the screens. The means are not shown for cyclically separating screens 28 and 29 or for enabling advancing a radiographic film between them nor are the means shown for compressing the radiographic film between the screens during radiography. Those skilled in the art will appreciate that such cyclically operable means may take a variety of forms. A screen and film compression and decompression device is shown in detail in the above cited U.S. Pat. No. 3,569,770. In the cited patent a continuous roll film is used so there is usually a segment or frame of film between the intensifying screens 28 and 29 during radiography or film recording of the x-ray image and during fluoroscopy with the image converter 17. The film is between screens 28 and 29 and is designated by reference 31.
For reasons stated earlier, screens 28 and 29 are assumed to remain in the x-ray beam and to attenuate radiation impinging on photocathode assembly 20 in image converter 17 during radiography and fluoroscopy. As stated, when roll film is used there will also usually be a frame of film between screens during both of these procedures. In some systems, such as when individual sheets of film are fed between the intensifying screens, the film may be easily omitted during fluroroscopy to avoid exposure thereof. In some systems such as will be described later, no intensifying screens are used and only film constitutes the interfering absorber during fluoroscopy.
For present purposes it is sufficient to appreciate that in the normal operating mode of a rapid film changer, such as is schematically represented in FIG. 1, the radiographic film intensifying screens 28 and 29 remain in the x-ray beam while a rapid sequence of radiographs is taken following fluoroscopic examination. Film exposures are initiated when the screens 28 and 29 are compressed and the film is between them and, in a matter of milliseconds, the exposed film is ejected and another unexposed film frame is advanced. The exposure rate for films about nine inches square may be as high as twelve frames per second.
In the FIG. 1 embodiment, intensifying screens 28 and 29 and any plastic or aluminum sheets, not shown, that may be used to support them remain in the x-ray beam path as does the film during fluoroscopy. The film and screen are absorbers which attenuate the x-ray image impinging on the image converter 17 significantly. The invention is directed to reducing the effect of this attenuation by the use of an x-ray filter 32 as can be seen in FIG. 1. By use of a mechanism, not shown, filter 32 is operated synchronously to be in the x-ray beam during fluoroscopy and to be out of the beam when a radiograph is being made. The filter 32 automatic reciprocating means are not shown because they can be devised by any one skilled in the mechanical and electrical arts. The choice of filter 32 for minimizing absorption by interfering absorbers such as the x-ray intensifying screens 28 and 29 and the film 31 during fluoroscopy while at the same time obtaining high response from the photocathode assembly 20 of image converter 17 will now be discussed.
Generally, the invention involves determining an x-ray photon energy or wavelength at which the principal absorbing constituent of the interfering component undergoes a large jump. Of course, in a photon energy band immediately below the energy at which the jump occurs, absorption by the interfering absorber is relatively low. Next the wavelength or photon energy is determined at which an absorption jump occurs in the absorber, such as the photocathode, in which maximum absorption is desired. The absorption jump photon energies for the interfering absorber and the desired absorber define a photon energy band or window wherein, as stated, absorption by the desired absorber is high and absorption by the interfering absorber is low. In most cases there will be one or more windows evident in the graph of absorption vs. x-ray photon energy due to the L or K edges of the chemical element or elements constituting the absorbers. If any of these windows fall within the range of x-ray energies acceptable to the application, the principles of the invention apply for reducing patient dose during fluoroscopy. Thus, if the radiation emanating from the x-ray source includes a band which falls within the window energy range, this band will penetrate the interfering absorber with little attenuation and will be absorbed heavily in the desired absorber. In the present case, for example, radiographic film and x-ray intensifying screens are the undesired absorbers and the photocathode assembly or other sensitive element in the image converter are the desired absorbers in the x-ray fluoroscopy and radiographic system. Where one has control over the desired absorber such as the x-ray sensitive material in the converter, one would choose a material whose x-ray spectral response best falls within the window or windows.
The next stop is to select a filter for location in the x-ray beam near its origin or adjacent the source. In accordance with the invention, the filter is one that is highly transmissive of an x-ray photon energy band to which the interfering absorber is also highly transmissive in the window or in a photon energy band immediately preceding its absorption jump and this band should be one that is highly absorbed by the photocathode of the converter. The element from which the filter is made will usually be the same as the element which constitutes the principal absorber in the undesired absorber. Finally, in the illustrative x-ray systems, the filter should be mounted for easy removal from the x-ray beam at such time as the operator may wish to conduct fluoroscopy.
The foregoing general principles will now be related to a practical case in reference to FIGS. 1 and 2. In FIG. 1, assume that x-ray tube target 15 is tungsten, intensifying screens 28 and 29 are calcium tungstate in which the principal absorbing element is tungsten, and the fluorescent or light emitting layer in photocathode assembly 20 of converter 17 is gadoliniun oxysulfide. In FIG. 2, the several absorption curves for the materials involved are plotted in terms of x-ray photon energy impinging on them vs. percent absorption. Curve 45, constituting a dash-dash line represents the absorption characteristics of calcium tungstate of the intensifying screens 28 and 29. Of course, tungsten with an atomic number of 74 is the principal absorbing element in calcium tungstate and the curve for the latter is similar in shape, therefore, to the curve of the former. One may see that for x-ray photon energies up to 69.525 kiloelectron volts (kev) the absorption of calcium tungstate declines continuously as photon energy increases. At about 69 kev there is a sharp absorption jump for calcium tungstate which defines the upper energy limit for the window in this case.
The absorption curve for gadolinium oxysulfide in terms of photon energy vs. percent absorption is shown by a solid line marked 50. The atomic number of gadolinium is 64. The laws of x-ray physics dictate that generally absorption decreases with decreasing atomic number and that the K absorption jumps for elements will occur at impinging photon energy levels that are lower for the lower atomic number element than for the higher atomic number element. Gadolinium is the principal absorbing element in the fluorescent layer of photocathode assembly 20 in this example.
The gadolinium oxysulfide absorption curve in FIG. 2 exhibits decreasing absorption over a range from minimum photon energy to about 50.239 kev where there is a sharp absorption jump. This jump defines the lower limits of the window. One may see that the solid line curve 50 for gadolinium exhibits high absorption from about 50 kev where the absorption jump occurs to the upper window limit at about 69 kev. On the other hand, calcium tungstate curve 45 shows decreasing absorption over the window photon energy band between about 50 and 69 kev. Thus, x-radiation lying in a band between about 50 kev and 70 kev in this example will be highly transmitted by the interfering calcium tungstate screen and will be absorbed effectively by the gadolinium oxysulfide layer in the image converter.
In accordance with the invention, if during fluoroscopy the primary x-ray beam emanating from source 12 is filtered by a filter 32 so that substantially only radiation within the spectral band or window impinges on the patient or the examination subject 10, then a minimum of the radiation energy traversing the patient is wasted in the film changer intensifying screens 28 and 29 and the ratio of information output to patient dose is optimized.
The material for filter 32 is chosen on the basis of it having high transmissibility of a spectral or x-ray photon energy band coinciding with the window spectral band that is highly transmitted by the interfering absorber and highly absorbed by the desired absorber such as the photocathode of the image converter. The radiation spectrum emanating from x-ray source 12 when filter 32 is in place, in accordance with the invention is illustrated by dash-dot line 40 which is superimposed on the absorption plots in FIG. 2. The matter of choosing the characteristics of filter 32 will be discussed after a brief discussion of the x-ray physics involved in reference to FIG. 3. This figure shows the generalized continuous radiation spectrum from a tungsten target x-ray tube when, for example, an electron accelerating voltage of 100 peak kilovolts (kvp) is applied thereto. The curve is plotted in terms of photon energy vs. relative intensity of the various energies photon emanating from the x-ray tube. The shortest wavelength radiation or the highest energy x-ray photon that can be generated with a particular applied voltage is designated by λ in FIG. 3. It is well known that as applied voltage to the x-ray tube is increased, the wavelength at λ will become shorter and will shift to the left in FIG. 3 and the corresponding x-ray photons will be more energetic. The curve in FIG. 3 shows that there are varying photon intensities at the various photon energy levels. The peak intensity is at a photon energy about 3/2 times the minimum wavelength or maximum photon energy. The intensity gradually diminishes as photon energy decreases. If the x-ray tube applied voltage is above a critical level, characteristic radiation lines such as the Kα and Kβ lines will also be excited. The Kα line for this tungsten target x-ray tube occurs at about 59 kev and the Kβ line at about 69 kev which, as will be evident from FIG. 2, fall within the window. The intensities of the characteristic radiation lines is very high and this is desirable since it will result in maximum response by the photocathode of the image converter and yet will be transmitted with little attenuation to the undesired absorber. Since an element is highly transmissive of its own characteristic radiation, a filter 32 of tungsten is used and it will transmit the characteristic radiation with little attenuation. As indicated, the same is true of tungsten occurring in the calcium tungstate of the intensifying screens 28 and 29. In the absence of filter 32, the whole spectral band illustrated in FIG. 3 would be highly absorbed in the intensifying screens and little radiation would remain for exciting the gadolinium oxysulfide fluorescent input layer in the image tube. This could be compensated by increasing the x-ray tube electron current but this would also means increasing radiation dosage to the subject which is undesirable. The thickness of filter 32 must always be great enough to absorb or attenuate the lowest energy spectral band emanating from the x-ray tube since the low energy radiation is merely absorbed in the surface of the patient or examination object and increases dosage but does not contribute to exciting the x-ray film emulsion nor the image converter or other means by which the image is visualized. On the other hand, filter 32 cannot be of such composition or thickness as to attenuate the desired higher energy radiation too much or there will be inadequate intensity of radiation penetrating the examination object to excite the film or image converter. Of course, in this case, it is excitation of the image converter during fluoroscopy which is of concern since the filter 32 is removed from the beam automatically during radiography. Moreover, attempting to compensate for an excessively thick or absorptive filter 32 by increasing the output of the x-ray tube through increasing its current may result in overloading and thermal damage to the x-ray tube target.
In accordance with the invention, it has been found that when a tungsten target x-ray tube is used and when fluoroscopy is carried out in the range of about 80 kvp to 100 kvp and the tube current is about 2 milliamperes, a preferred thickness for tungsten filter 32 is about 0.040 inch thick. To balance minimizing extraneous dose to the patient against undue attenuation it appears that the filter thickness should not ordinarily be below 0.030 inch nor above 0.050 inch for the x-ray tube parameters that are generally used during fluoroscopy. If sufficient tube heat capacity is available, then the range 0.03 inch to 0.05 inch is a good one to choose. However, as often happens in angiography, tube heat capacity will very likely be strained to the limit, and the radiologist may be grateful for the smallest of performance improvements. Therefore, in practice one might expect to see machines running with filters as thin as 0.005 inch even though such a practice deviates widely from the dictates of engineering efficiency. By using a tungsten target x-ray tube and a tungsten filter 32, a radiation output curve from the filter having substantially the shape of dash-dot curve 40 in FIG. 2 will result and it will be seen that substantially all of this radiation spectrum will fall within the spectral band of the window between approximately 50 kev and 70 kev. Since the tungsten filter 32 is highly transmissive of its own tungsten characteristic radiation the intense Kα and Kβ characteristic peaks 41 and 42, respectively, in FIG. 2 also fall within the spectral band of the window to thus contribute to the maximum excitation of the gadolinium oxysulfide fluorescent layer in the photocathode of the image converter while at the same time being attenuated very little by the calcium tungstate intensifying screens whose absorption is represented by dashed line 45 in FIG. 2. The practical result is that a satisfactory image appears on the television monitor screen 27 for visualization by the radiologist even though the intensifying screens 28 and 29 are in the beam during fluoroscopy.
When the radiologist desires to make a permanent recording or a series of high speed recordings on film after fluoroscopy, he commands removal of filter 32 from the x-ray beam and effectuates whatever preset x-ray tube current and voltage and exposure time he desires for getting the best image on film 31 which is then clamped between intensifying screens 28 and 29. When roll film is used, such as in the above cited U.S. Pat. No. 3,569,700, it is, of course, not practical to remove film 31 from the beam path during fluoroscopy and the film will be partially exposed or fogged at this time. Hence, before a radiographic series is initiated, the radiologist will normally command the film to advance one frame so that unexposed film will be made available for permanent recording.
In accordance with the invention, x-ray filter 32 will be comprised of an element out of which the x-ray intensifying screens or other interfering absorber is made to assure high transmission by the interfering absorber such as the intensifying screens 28 and 29. In all cases it is desirable to use a filter 32 which passes a radiation band that is poorly attenuated or absorbed by the principal absorbing element in the material of which the intensifying screens are made. One would ideally try to arrange for the interfering absorber to have the same or slightly higher atomic number than does the anode of the x-ray tube because the higher the atomic number of the absorber the higher will be the photon energy at which the K absorption jump occurs which means that the characteristic radiation from the x-ray tube will not be absorbed or attenuated significantly in the spectral band below the photon energy at which the absorption jump of the undesired absorber occurs.
More unusual x-ray tube targets such as of lanthanum (atomic No. 57), gadolinium (atomic No 64) and terbium (atomic No. 65) would also desirably cooperate with interfering absorbers 28 and 29 of a corresponding material having the same or nearest next highest atomic number so that the K radiation of the particular target will not be attenuated significantly. Of course, in any arrangement, the input fluorescent layer of the image converter or other x-ray sensing means must be chosen to have optimum response to the spectral band that is poorly attenuated by the interfering absorber. Important input phosphors or fluorescent material are zinc cadmium sulfide, cesium iodide and zinc sulfide. An additional non-exclusive list of prospective phosphors are those based on terbium, dysprosium, holmium, thulium, ytterbium, lutetium, hafnium, tantalum, erbium, zinc, bromine, cadmium, iodine, cesium, lanthanum, cerium, europium, gadolinium and tungsten. These phosphors have atomic numbers under tungsten which is 74 and would therefore respond to tungsten radiation with reasonably high absorption in an image converter or other x-ray detector. Occasion may arise to use them in x-ray image detectors wherein the interfering intensifying screens employ calcium tungstate in which case a tungsten target x-ray tube producing the desired characteristics radiation might preferably be used. In any case, the interfering absorber should have an atomic number equal to or slightly greater than the atomic number of the filter 32 material and the x-ray detecting material should preferably be comprised of a principal absorbing element having an atomic number lower than either the filter or the absorbing element to enhance absorption in the detector and thereby minimize radiation that has to be applied to the subject in order to produce an x-ray image with reasonably good contrast.
To exemplify use of one of these elements for the input fluorescent screen or desirable absorber of an image converter, a terbium compound may be considered. Terbium has an absorption jump at about 52 kev and an atomic number of 65. The undesired absorber may again be calcium tungstate intensifying screens or other screen material in which the atomic number of the principal absorbing element is slightly in excess of the atomic number of the fluorescent screen. The absorption jump for terbium is just a little greater than that for gadolinium and the absorption curves which are shown in FIG. 2 are substantially the same for these two elements. Hence, a tungsten filter 32 may again be used if the intensifying screens were calcium tungstate but in any event it is desirable for the atomic number of the filter 32 material to be equal to or slightly greater than the atomic number of whatever x-ray tube target is used so that the characteristic radiation from the target would not be attenuated significantly by the filter.
Another embodiment of the invention will now be described in connection with FIGS. 4 and 5. FIG. 4 is a schematic view of an x-ray inspection system employing the principles of the invention to achieve minimum absorption in photographic film which is interposed in the x-ray beam during fluoroscopy. In this case the image converter or x-ray detecting device is an x-ray sensitive vidicon camera tube 60 which is shown fragmentarily and schematically since television camera tubes of this type are known. The tube comprises an envelope 61 on the input end of which there is a x-ray sensitive target 62 of the photoconductive type which may be selenium. An x-ray image falling on photoconductive target 62 varies its photoconductivity and thereby produces a differential charge pattern. By means of an electron readout beam, not shown, which sweeps over the target 62, potential variations corresponding with video signals are produced and these are processed in suitable electronic circuitry 64 and finally displayed on a television monitor 65. The vidicon tube 60 is not novel by itself but is used in a novel manner in the system depicted in FIG. 4.
Vidicon tube 60 may be used for making a fluoroscopic examination of an object 66 which is in or on x-ray transmissive supporting structure 67. In this case a fluoroscopic presentation is required for positioning the object, for determining its presence in a concealed container or for other reasons. Removal of the radiographic film 68 during fluoroscopy is not convenient. The x-ray source 69 in this case is a thin window molybdenum target tube having high intensity x-ray emission at low photon energies. The principal constituent of the interfering absorber is the thick silver bromide emulsion of a direct exposure, non-screen type, x-ray film 68. The movable x-ray filter 70 comprises silver bromide. Silver bromide at 20 milligrams per square centimeter of filter area may be used. The object of this arrangement is to get maximum response from the x-ray sensitive vidicon tube 60 during fluoroscopy while the x-ray sensitive film 68 remains in the beam. This suggests that the system may be used for luggage inspection without danger of exposing film which may be concealed in the luggage. Of course, selenium target x-ray sensitive vidicon tubes are available only in small sizes at the present time so that a scanning system is desirable for inspection of large objects.
The principal absorption spectrum of vidicon tube 60 in FIG. 4 is determined by the characteristics of its selenium photoconductive target 62. The absorption characteristics of selenium are indicated by the dash-dash line 80 in FIG. 5. One may see that there is a K absorption edge or jump at 12.653 kev for selenium after which absorption gradually declines as x-ray photon energy increases. The x-ray window is created by the absorption curve for silver bromide which is depicted as a solid line 81 and exhibits a K absorption edge or jump at 13.475 kev for bromine and another jump at 25.517 kev for silver. Thus, there are two windows created by the shape of the silver bromide solid line absorption curve 81.
Again it is necessary to match the characteristics of filter 70 to the absorption characteristics of the undesirable absorber which is the silver bromide emulsion on film 68 in this case. Radiation emanating from the molybdenum target x-ray tube 69 which is attenuated little by the silver bromide filter 70 will also be attenuated little by the silver bromide in film 68 during fluoroscopy when the filter is in the beam. The heavily prefiltered molybdenum radiation is depicted by the dash-dot curve 82 in FIG. 5 and it will be seen that the output from the molybdenum target tube including its Kα and Kβ peaks 83 and 84 will also fall within the window defined by the bromine and silver absorption edges. This radiation spectral band from the tube is more highly absorbed by the selenium target 62 as indicated by the dashed line curve 80 for selenium in FIG. 5 while the same spectral band is more poorly absorbed by silver bromide as manifested by the fact that the downwardly sloping curve 81 between the absorption edges is below the absorption curve 80 for selenium. Because low energy radiation from the molybdenum target source is used and because the objects which are being looked for usually have high density and high radiation absorption, they will be revealed in high contrast on the television display 65 while at the same time the energy will be low enough and of appropriate spectral band to avoid exposure of the film during fluoroscopy. In any event, even if the film 68 and its silver bromide emulsion is left in the beam during fluoroscopy, it will not attenuate the x-radiation falling on the vidicon tube 60 significantly. If a radiographic exposure is desired, silver bromide filter 70 is removed from the beam and the film may be properly exposed with the x-ray tube current, exposure time and voltage being set at normal values for radiography. When a radiographic film is made, of course, it will generally be desirable to increase the x-ray tube current as is customary during radiography for well known reasons. However, the system permits fluoroscopic inspection with low x-ray tube current and, hence, low photon density output which can be advantageous in a system where the object being searched for is extremely sensitive to radiation.
Besides selenium, other materials used in vidicon targets include antimony trisulfide, lead oxide, silicon, cadmium selenide and cadmium sulfide. If the target is comprised of such materials or a combination of them, the principles of the invention still apply wherein the filter 70 is comprised of an element having an atomic number smaller than or equal to the principal absorbing element or elements in the interfering absorber and larger than the principal absorbing element in the vidicon or other converter target.
Although various embodiments and materials have been suggested for use in a system to make fluoroscopic examinations with minimum dosage to a patient or other examination object, such description is intended to be illustrative rather than limiting for the principles of the invention may be variously embodied and the invention is to be limited only by interpretation of the claims which follow.