US3515874A - Method and apparatus employing an electron target and x-ray filter of the same chemical element for generating x-rays of prescribed energy - Google Patents

Description

June 2, 1970 J. R. BENS ETA!- 3, METHOD AND APPARATUS EMPLOYING AN ELECTRON TARGET AND 1 X-RAY FILTER OF'THE SAME CHEMICAL ELEMENT FOR GENERATING xRAYs OF PRESGRIBED ENERGY Filed June 13, 1967 2 Sheets-Sheet 1 JEAN 2. BEN? a. 754w -62 Al/AE PH AP 5,

June 2, 1970 R, B s ETAL 3,515,874

METHODAND APPARATUS EMPLOYING AN ELECTRONTARGET AND X-RAY FILTERKOF THE SAME CHEMICAL ELEMENT FOR GENERATING X-RAYS 0F PRESCRIBED ENERGY Filed June 13, 1967 2 Sheets-Sheet 2 United States Patent Im. Cl. H01} 5/18, /08

US. Cl. 250-86 13 Claims ABSTRACT OF THE DISCLOSURE In producing a frequency-selective X-ray beam, there is used a target made of an element having a characteristic radiation line series within the selected frequency range, the X-ray tube is energized with a voltage somewhat higher than the critical voltage required to excite said characteristic radiation, and the resulting X-ray beam is then passed through a filter made of the same element as the target in order to filter off unwanted frequencies. The common element from which the target and filter are made may be selected to have an atomic number exceeding by a very few units that of a chemical element of which a more heavily-absorbing part of the object to be X-rayed consists. In a preferred application, an X-ray diagnostic tube for mammography, the common element in the target and filter is molybdenum, and the energizing voltage is about 30* kv.

This invention deals with the selective control of the frequency spectrum of X-rays with the object of obtaining optimal contrast in X-ray images and radiographs.

In order that different constituent parts of an object may be made visible by means of X-rays, it is necessary that such parts present different degrees of absorption to the irradiating X-ray beam. Otherwise the resulting image or picture will be a uniform, meaningless blur. The degree to which a part of an object absorbs X-rays depends on three main factors: first, its chemical composition the heavier the chemical element (in terms of atomic number) the more absorbent it is; second, its density thus compact organic tissue for instance absorbs more heavily than does light tissue of similar composition; and third, thickness.

An important problem arising principally in medical (diagnostic) X-ray work but also present in the fields of laboratory and industrial X-rays is the maximization of contrast. That is, it would be of great interest in many practical applications to obtain radiographs in which parts having degrees of X-ray absorption differing by extremely small amounts could be meaningfully distinguished from each other. One such application, with which this invention is especially concerned, is the radiography of the mammary glands (which is sometimes termed mammography) for the early diagnosis of breast cancer. In the early stages of this disease an incipient tumor causes a slight hardening and opacification of the normally soft and X-ray transparent mammary tissue. It would be of great therapeutic importance to .be able to detect this condition at a very early stage when this difference in opacity (and absorption) is still extremely slight.

The penetrating ability (or hardness) of X-rays increases rapidly with frequency (approximately as the third power of frequency). It follows, as a general proposition, that in order to achieve maximum contrast in an X-ray picture it would be necessary to use an X-ray beam containing a majority of radiations at the minimum fre 3,515,874 Patented June 2, 1970 "ice quency (maximum Wavelength) required to traverse the more absorbent portions of the object, since the difference in absorption between the various portions of the object will then be greatet. The problem of maximizing contrast is thus seen to involve, basically the problem of selective control of the frequency spectrum of the X-ray beam used.

The over-all frequency spectrum of an X-ray beam as generated by any X-ray tube includes a so-called continuous, or background, spectrum which has a cutoff wavelength, and a maximum-intensity wavelength, both uniquely determined by the exciting voltage applied to the X-ray tube anode. The said over-all spectrum further includes a so-called characteristic-radiation spectrum, which consists of a number of groups of very sharp maxima, or series of spectral lines, which do not appear unless the anode-energizing voltage has exceeded a critical value. The wavelengths of these characteristic radiation lines depend only on the chemical element of which the target in the X-ray tube is made.

The usual method of controlling the frequency spectrum of an X-ray beam for purposes of maximizing contrast, has been to adjust the tube anode voltage to a value such that the maximum-intensity wavelength of the continuous spectrum is approximately situated within the region affording good contrast, as indicated above. Since almost all of the X-ray beam energy is spread over the continuous spectrum, no attempt was usually made to utilize the characteristic radiation lines, except in connection with specific problems in X-ray spectrography. In fact, excitation of the characteristic radiation lines was in general avoided, or if this was not feasible then the characteristic radiations were filtered out. As a consequence of the fact that the characteristic spectrum has not been used, the chemical nature of the target has been of minor importance and in recent years it has become customary in most or all X-ray sources for medical diagnostic purposes to construct the target uniformly out of tungsten, for reasons unrelated to the spectral characteristics of the generated X-ray beam.

The procedure for X-ray spectrum control just described, while universally used to the best of applicants knowledge in present day X-ray sources for medical purposes and most or all other applications, has serious shortcomings. Because the continuous or background spectrum is utilized, the X-ray beam contains a large amount of radiations at wavelengths outside the useful or optimal range. As chief consequences, maximum contrast is not achieved; power efiiciency is poor; the large amount of hard radiations present in the beam needlessly and dangerously increases the dose of irradiation to which the X-rayed subjects (and operators) are exposed; and, especially in radiographing soft body tissues-as in the mammography application referred to above-these hard radiations produce large amounts of diffused X-rays which considerably detract from the clearness and legibility of the radiograph.

As a further indication concerning the state of the background art, it is noted that as far as the applicants are aware the problem of contrast-maximization in the radiography of soft body tissue had not been subjected to systematic investigation prior to the recent work undertaken in this connection by the applicants at the request of and in collaboration with the medical profession, on which work the present invention is in part based.

Objects of the invention include the provision of improved methods and means for maximizing contrast in X-ray images and radiographs; at the same time maintaining higher power efficiency, i.e. lower energy consumption, than was heretofore possible with comparable results; improved method and means of X-raying soft body tissue, especially mammary tissue, whereby incipient breast tumors can be diagnosed much earlier than was heretofore thought possible; the provision of improved methods and X-ray generating assemblies for producing X-rays of controlled spectral characteristics both for medical (diagnostic and therapeutic) purposes and for all-round use, including laboratory and industrial applications.

The invention, in an important aspect, makes use of certain relationships extant between X-ray emission and absorption spectra which, though known for some years to physicists have not previously received practical application in the medical and other fields, for improving control over the spectral characteristics of X-ray beams and for improving the contrast and quality of the resulting images and radiographs.

According to the invention there is produced an X-ray beam having improved spectral characteristics, by the use of an X-ray tube in which the target is made of an element having a characteristic radiation line series within a selected frequency range, and energizing the tube anode under a voltage somewhat higher than the critical voltage required to excite said characteristic radiation. The X-ray beam thus generated is then passed through a filter made of the same element as the target in order to filter off higher (and also lower) frequencies.

In many applications, the common element from which the target and filter are made is, according to a feature of the invention, selected to have an atomic number exceeding by a very few units the atomic number of a chemical element of which a more heavily absorbing portion of the object to be X-rayed consists.

In a preferred application, the invention is embodied in an X-ray diagnostic tube for mammography; the common element in the target and filter is in such case molybdenum, and the tube anode is then excited with a voltage somewhat higher than the critical voltage (V required to excite the K line series of the characteristic spectrum of molybdenum, which is about kilovolts, a suitable anode voltage range being about from to 40 kilovolts, with kilovolts being a typically preferred value.

FIG. 1 is a graph showing a typical X-ray emission spectrum when the beam is excited under an anode voltage sufiicient to excite the K and L characteristic radiation line series for the particular chemical element from which the tube target is made;

FIG. 2 is a graph showing a typical X-ray absorption spectrum in absorbing material comprising the same chemical element as the material from which the target considered in FIG. 1 is made; in FIGS. 1 and 2 the wavelength scales of the abscissa axes correspond;

FIG. 3 is a composite graph in which the emission and absorption spectral curves of FIGS. 1 and 2 are superposed, and shows the resulting spectrum of an output beam issuing from an X-ray source assembly according to the invention; and

FIG. 4 is a general view, with a portion cut away and parts shown in section, of an X-ray diagnostic tube of generally conventional type in which the invention is shown embodied.

For an understanding of the invention it is necessary first to describe in greater detail certain known facts relating to the emission and absorption of X-rays, as at present understood.

FIG. 1 illustrates a typical emission spectrum for an X-ray beam as generated by a conventional tube. Wavelengths are plotted as abscissae and radiation intensities as ordinates. A consideration of the chart immediately shows that the spectrum can be considered as composed of a continuous spectral curve, plus a discontinuous spectral curve consisting of a number of sharp peaks or spectral lines disposed in a number of sepaarte series (two series, K and L, being shown).

First considering the continuous, or background, spectrum, this is seen to commence at a minimum or cutoff wavelength value t below which the radiation intensity is zero, from which it rises to a maximum of intensity for a wavelength A and then decreases smoothly and asymptotically to zero as the wavelength increases. This continuous spectrum is generally believed to be attributable to the bremsstrahlung radiated as the electrons of the cathode beam of the X-ray tube lose kinetic energy to the strong electric fields surrounding the atomic nuclei of the target.

The discontinuous spectrum superposed over this continuous background spectrum consists of series of very sharp peaks in the nature of the spectral lines in an optical spectrum, which are due to the radiations emitted on transition of atomic electrons in the target from higher energy states to the K, L, etc. energy states, and the wavelengths at which they occur are characteristic of the element of which the target is made. For this reason the discontinuous spectrum is also known as the characteristic spectrum for a given target element and the lines are termed characteristic radiations. The chart shows two such lines in the K series and three in the L series. For example, the K, line is emitted when a target atomic electron drops from the L orbital level to the K level, and the K line is emitted when a target atomic electron drops from the M level to the K level; analogous considerations apply to the L-series lines.

The general shape of the continuous spectrum is the same for all target elements, and the wavelength values A and A defining this continuous spectrum curve are independent of target material and depend only on the energy of the cathode rays, i.e. the source exciting voltage V, the precise dependency being given by the Duane-Hunt law indicated later herein. On the other hand, and this is of particular significance to the present invention, the characteristic wavelengths of the discontinuous spectral lines depend only on the atomic number of the targetcomposing element. The characteristic wavelength of a given line (such as the line K,) is approximately an inverse parabolic function of the atomic number of the target element, as stated by Moseleys law, indicated later herein.

Another important remark is that the characteristic radiation lines do not appear in the emission spectrum of an X-ray source unless a certain energy threshold, i.e. a definite source-energizing voltage, has been exceeded. Below this critical voltage only the continuous background emission spectrum is present in the output beam. As the energizing voltage is increased from zero the various series of characteristic lines appear in the inverse order M, L, K. The critical voltage for a given line series is, approximately, inversely proportional to the average wavelength of the series and, hence, approximately a parabolic function of the atomic number of the target element.

Since the K, L, M lines depend for their occurrence on the presence of orbital electrons at the K, L, M levels respectively of the target atoms, it will be understood that only targets composed of the heavier elements will emit the complete sequence of line series, whereas lighter elements will only display, say, K and L lines and still lighter elements only K lines.

Having thus very briefly reviewed the known physical laws governing X-ray emission, a similar brief review of X-ray absorption phenomena will be given.

When an X-ray beam traverses a sample of matter and the energy loss or absorption is plotted as a function of incident radiation wavelength, the resulting graph is termed the X-ray absorption spectrum for the sample of matter. A typical X-ray absorption spectrum is shown in FIG. 2. Consideration of the graph shows that it, rather like the emission spectrum of FIG. 1, can be regarded as resulting from the superposition of a continuous spectrum and a discontinuous one. That is, the overall absorption curve has a continuously increasing trend as the third power of wavelength (Bragg-Pierce law), this manifesting the well-known fact that high-frequency or hard X-rays are much more penetrating than are lower-frequency, soft rays. Superimposed over this third-power rising curve there are present sharp discontinuities in the form of absorption jumps or dips (such as (1 1 from a higher to a lower value.

One fact that is of prime significance to the present invention is that the wavelengths, indicated as M A in FIG. 2, at which the absorption jumps occur for a given sample material, are uniquely related to the wavelengths, indicated as x A in BIG. 1, of the characteristic radiations in the emission spectrum from a target made of that material. That is, the absorption dip wavelengths are related to the atomic number of the absorbing material by an inverse parabolic function just as the characteristic radiation wavelengths are related to the atomic number of the emissive target element (Moseleys law). Thus there is an absorption jump or dip corresponding to each of the characteristic radiation line series K, L, etc. present in the emission spectrum of the material considered. Further the absorption dip wavelengths are consistently lower, by

a small but definite amount, than the corresponding characteristic radiation wavelengths, as this is approximately indicated by a comparison of the wavelength scales of FIGS. 1 and 2. This peculiarity is explainable in quantum theory from the fact that the energy required to knock out an electron from a given orbital level in an atom of the absorbent material and then drive the knocked-out electron completely out of the atompast all of the outer levels, is obviously somewhat higher than the energy released in a similar atom, when an electron from any one of the outer levels drops to the said given energy level.

A further important property of the general X-ray absorption spectrum as experssed by the Bragg-Pierce law is that the absorption is approximately proportional to the third power of the atomic number of the absorbing material. In other words, the curve of absorption versus atomic number for a given wavelength would be approx imately the same as the curve, shown in FIG. 2, of absorption versus wavelength for a given atomic number.

A last property of X-ray absorption which will be here mentioned as being of interest to the invention, and a property shared in common with all radiation absorption effects, is that the degree of absorption is an exponential function of the thickness (and density) of the material traversed by the beam.

It is to be understood that the above theoretical description of X-ray emission and absorption phenomena has been restricted to the statement of properties having a direct bearing on the understanding of the present invention, and has accordingly been simplified. A complete disclosure of current knowledge on the subject can be obtained from any good physics textbook or theoretical X-ray manual.

In conventional X-ray tubes for medical and laboratory uses, it has been common practice to use only or predominantly the continuous (or background) spectrum of the X-ray tube output. According to the Duane-Hunt law, both the cut-off wavelength t and the maximum-intensity wavelength A of the continuous or background X-ray spectrum are inversely proportional to the tube energizing voltage V. Specifically,

where the wavelengths are expressed in angstro-rn units and the voltage in volts. Therefore by suitably selecting the voltage V, it is possible to situate the highest-intensity region of the continuous spectrum in a preselected wavelength range. This wavelength range Was selected so that the corresponding X-rays should be capable of penetrating the parts of the investigating object having greatest absorption. The total energy output of the tube is proportional to ZV where Z is the atomic number of the target metal. Tungsten (atomic number Z=74) has been by far the most widely used metal for the targets of X-ray tubes. Since almost the whole of the X-ray beam energy is distributed over the continuous spectrum it has generally been considered undesirable to use the characteristic radiation lines of an X-ray spectrum except for specialized purposes, as in X-ray spectrography.

It has been known to pass the X-ray beam from the source tube through a suitable filter in order to absorb wavelengths of the continuous X-ray spectrum below a prescribed value somewhat higher than the cutoff wavelength, in cases where the inclusion of shorter wavelengths Was objectionable, as in limited radiotherapy.

According to an aspect of the present invention, there is provided in an X-ray beam producing assembly the combination comprising an X-ray tube including a target made of a selected element, and a filter interposed in the path of the X-ray beam emitted by the target which filter comprises the same element as that from which the target is made and further the anode energizing voltage is selected at a value somewhat exceeding the critical voltage required to excite a selected series of characteristic radiation lines from said element.

The result achieved by this combination will be understood from a consideration of FIG. 3. In this graph there will be recognized at E a portion of the emission spectrum of FIG. 1 and at A a portion of the absorption spectrum of FIG. 2, both spectral curves relating to a common chemical element. As indicated above, the absorption jump wavelength )t associated with the K level, is situated just below the (average) characteristic wavelength M associated with the K line series of the emission curve. Hence, in the combination of the invention, the filter, because it is made of the same material as that of which the target is made, can be considered as providing a spectral window through which X-rays of substantially only the characteristic radiation spectrum lines of the selected material (common to both the target and the filter) will be passed.

It will be understood that in addition to the selective absorbing action due to the absorption jump, the filter will simultaneously exert a general attenuating action at all wavelengths, so that the resulting output spectrum, as indicated at R, is of somewhat lower intensity as compared to the emission spectrum E. Moreover, by suitably selecting the thickness of the filter, soft radiations of wavelengths longer than the desired characteristic K spectrum can be greatly diminished or eliminated, as indicated by the fact that the resulting output curve R drops rapidly to zero as shown.

It will thus be seen that the combination in an X-ray source assembly of a target and output filter made from a common, selected, element makes it possible to produce an output X-ray beam having a relatively narrow, selective, range of wavelengths and yet having satisfactorily high strength since the beam spectrum includes the strong characteristic spectral lines. Selection of the Wavelength range obtained is effected by a suitable choice of the common material from which the tube target and output filter are made coupled with a choice of the anode energizing voltage, as will be described in greater detail further on.

According to a further aspect of the invention, the improved selecti-ve-wavelength X-ray beams obtained as indicated above are used in order to achieve greatly improved contrast in X-ray work, in many applications. As earlier indicated, the absorption jump for a given spectral line series, say the K series, occurs at a wavelength which is an inverse parabolic function of the atomic number of the absorbing material (Moseleys law). More precisely, the K absorption jump wavelength M is related to the atomic number Z by the equation 914 Mx=m (angstrom units) It follows that when an X-ray beam produced by the method described above, using a matched target and filter assembly, is directed at an object including an element of atomic number slightly lower than the atomic number of the common element comprised in said matched target and filter, said object will present an absorption jump or dip situated within the X-ray beam spectrum, so that it will be observable with maximum contrast if surrounded by a matrix having a lower absorption characteristic.

In other Words, an X-ray source assembly according to the invention using a matched target and output filter comprising a common element of atomic number Z, will produce an image of very sharp contrast if directed at an object comprising an element of atomic number somewhat less than Z, say (Zl), (Z2), (Z3) or (Z4) placed in a less-absorbing environment.

This theoretical statement has been fully borne out by experiment, as indicated in the following non-restrictive examples.

EXAMPLE 1 A similar tube equipped with a cobalt target and cobalt output filter (atomic number 27) 6 microns thick was similarly operated under 15 kv. to observe inclusions of iron, with excellent contrast. The same equipment also gave very good results when applied to chromium and manganese inclusions as in the preceding example.

EXAMPLE 3 A similar tube provided with a copper target (atomic number 29) and copper output filter 9 microns thick was operated under 15 kv. to observe inclusions of nickel (Z=28), cobalt (Z=27) and iron (2:26) with very good contrast.

EXAMPLE 4 A similar tube fitted with molybdenum target and molybdenum output filter 30 microns thick operated under 30 kilovolts gave a very highly contrasted image of zirconium elements in a low-asborption. matrix. The atomic number of M is 42, that of Zr is 40.

EXAMPLE A similar tube fitted with a chromium target and chromium target and chromium output filter 7.5 microns thick was operated under kv. for the investigation of calcium-containing inclusions (bony tissue cuts). The resulting images were considerably more contrasty than obtainable with the conventional tungsten-target tube even though there is a difference of four units between the atomic numbers of chromium (24) and that of calcium EXAMPLE 6 The tube was fitted with a target and matched output filter made of silver (atomic number 47), the filter was 45 microns thick and the energizing voltages was kv. This source assembly enabled excellent viewing of objects containing palladium inclusions (atomic number 46).

In each of the above examples, it will be noticed that the nature of the common target-and-filter material required for optimal contrast in the X-ray image could be determined in advance from a study of the periodical table of the elements, this being possible in view of Moseleys law as earlier explained. This is because in each of the examples there was some specific element present as an inclusion to be viewed in contrast against a surrounding matrix of low absorption, and in such cases it is simply necessary, in order to preselect an element suitable for the purposes of the invention to look up, in Mendeleevs periodic table, an element appropriate for use as a target in an X-ray tube (suitably a highmelting metal), having an atomic number a few units higher than the atomic number of the inclusion element to be observed. The invention, however, is not limited to cases Where the chemical nature of the common element entering in the target and matched output filter of the improved X-ray generating assembly can be as easily predetermined from a glance at the periodic table of the elements. In other cases, preliminary experimentation is necessary in order to ascertain the particular wavelength rangeand hence the particular elementwhich Will provide optimal contrast.

A method by which this determination can be made in the general case will be described in a specific application which is of considerable importance in medicine, the radiography of the mammary glands (mammography). Because of the high incidence of breast cancer it would be of great importance to diagnose the disease in its very earliest stages when it is still susceptible to simple and sure treatment. At this stage the incipient tumor is manifested as a very slight local hardening or induration of the soft mammary tissue, which at first increases the X-ray absorption coefiicient of this tissue by only a minute amount over its normal value. It is apparent therefore that the greater the contrasts attainable in the X-ray images of mammary tissue, the earlier will it be possible to diagnose the presence of a beginning tumor, and the better will be the chances of cure. The problem is complicated by the fact that the soft and hence low-absorbing tissue tends to produce considerable amounts of diffused X-rays which further cloud the picture.

The applicants were thus confronted with the problem of determining a range of wavelengths, if any exists, within which an X-ray beam directed at soft organic tissue would afford maximum contrast. For this determination the applicants used a dummy in the form of a block of paraffin, since this substance was found to have substantially the same X-ray absorption characteristics as the tissue to be investigated. The block had two portions respectively 5 and 4.5 cm. thick in order to provide two regions of dilferential absorption the contrast between which could be measured. The dummy was irradiated with pencils of monochromatic X-rays which were obtained by reflecting a white (continuous-spectrum) X-ray beam generated by a conventional X-ray tube, at a series of different angles from a rotatable crystal and collecting the reflected rays at corresponding angles to provide individual monochromatic X-ray pencils whose wavelengths Were known by Braggs formula. The monochromatic pencils after traversing both unequal sections of the paraflin ghost were received on a film, and the contrast in brightness of the parts of film exposed through the two parafiin sections of different thickness was measured. This contrast was then plotted against the wavelength of the irradiating pencil, and it was found that the resulting curve showed a conspicuous maximum for the range of wavelengths from 0.6 A. to 0.9 A. In the composite emission-absorption graph of FIG. 3 relating to molybdenum as the common element comprising the target and the output filter, the wavelengths indicated M R and R have the values 0.61 A., 0.63 A. and 0.71 A. respectively. It is then apparent that the spectral window provided for the X-ray beam when emitted from a source constructed according to the invention using a molybdenum target and a molybdenum output filter, coincides satisfactorily with the band of wavelengths found to achieve optimal contrast in soft organic tissue, as determined by the experiments just described.

Clinical tests have borne out the applicants expectations and shown that the improved X-ray source comprising matched molybdenum target and output filter (0.03 mm. thick) provided an improvement in contrast of about six times as compared to a conventional tungsten-target tube, when applied to the radiography of mammary tissue, permitting a much earlier diagnosis of breast cancer than was heretofore possible.

It will be understood that a similar X-ray source is useful in X-raying other parts of the human body where there is a predominance of soft tissue.

FIG. 4 illustrates by way of example an X-ray diagnostic tube of generally conventional type in which the invention is embodied. The tube comprises an outer metal casing 2 provided with tubular connectors 4 and 6 for the positive and negative connector cables. The sealed evacuated glass envelope 8 of the X-ray tube is supported within the casing and spaced from its walls, and a lead shielding tube 10 is interposed between the envelope and easing. A body of insulating oil 12 is enclosed in the space surrounding the glass envelope 8. Mounted in an opening formed in the casing 2 and shield to at about mid-length thereof is a funnel-shaped annular support 14 across the smaller-radius inner end of which is mounted a window transparent to X-rays of the wavelength range used; fitted in an outer section of the window mount 14 is a filter member 18 comprising a disc-shaped strip of the selected filtering material, molybdenum in the embodiment being disclosed, mounted in the window support 14 by means of a ring mount. The molybdenum filter 18 may have a thickness of from 20 to 4071.. Within the tube 8 is a conventional cathode structure including an insulating cup support 20 in which is supported a tungsten filament 22 connected to a heating source by way of leads extending through the rigid connector posts 24, and the casing outlet 6. The anode structure includes a copper rod 26 having a bevelled end surface facing the cathode, in which is fitted the target strip 28 made of the selected target material, in this instance molybdenum. The anode 26 is externally connected to the positive high voltage source terminal by way of casing outlet 4. Sometimes, to prevent X-rays absorption by the glass bulb, a portion of thelatter in front the output window must be changed to a material more transparent to X-rays, such as beryllium.

It will be understood that the X-ray tube assembly just described is purely exemplary and that the invention may be embodied in various other types of X-ray tube.

In an X-ray source constructed and operated according to the invention, the anode energizing voltage should in each case be selected with due regard to the nature of the common element used in the target and output filter, the selected voltage value being such that it will excite the desired characteristic radiation lines of the selected target material, but yet will not excite an excessive number of higher-energy photons which would produce undesirable short wavelength X-rays in the output spectrum. That is, referring to FIG. 3, it will be understood that if there is used an exciting voltage considerably higher than the critical voltage required to excite the characteristic radiations in the K series, then the emitted spectrum will assume the form indicated in broken lines, with a greatly heightened hump of the continuous spectrum curve in a shorter wavelength region. Since in this region the filter absorption curve A is low, this would result in the presence of unwanted short-wave X-rays in the output beam. To avoid this, the exciting voltage should be held at a value not greatly exceeding the critical value for the characteristic radiation corresponding to the selected target material, such as a range of about from 1.1 to 2.5, preferably 1.2 to 1.7, times the critical voltage. Thus, in the case of the molybdenum target and filter described, the energizing voltage may be selected in the range of about from 25 to 40 kv, a preferred range being from 28 to 36 kv.

Thus, in order to determine in advance the anode energizing voltage range to be used for the purposes of the invention, it is necessary first to ascertain the critical voltage for exciting the characteristic radiation selected. This can be obtained from data available in the literature or by experiment. For convenience, two formulas from which the critical voltagefor the K-series characteristic radiations of various elements can be derived will here be indicated. A first formula giving critical voltage V in terms of characteristic wavelength Mr; is

(V in kilovolts, )\,9K in Angstrom units). A second formula gives critical voltage in terms of atomic number Z and aderived from the foregoing one by application of Moseleys law, viz.:

V =0'.014 (Z3.5) in kilovolts From either above formula, the critical voltage for the K radiation of any selected target/ filter element, and hence a suitable voltage value for use in an X-ray source assembly according to the invention, can be determined in advance.

While in the embodiments of the invention at present preferred, the K-series characteristic radiations are utilized and this series has accordingly been emphasized in the disclosure, it will be apparent that the invention may well be practiced with the use of other characteristic radiation series in case of the. heavier elements. For this purpose it is simply necessary to select the anode energizing voltage within a range high enough to excite the radiations of the desired series (such as L) but not high enough to excite the radiations of the unwanted series (say, K). Also, suitable additional precautions may have to be taken to prevent the desired long-wavelength rays from being intercepted at the output from the tube, such as suitably selecting the window material, as will be readily understood by those familiar with the art. As to the elimination of the unwanted characteristic radiation lines of longer wavelength, such as the series L, M in the disclosed embodiments of the invention where only the K series is utilized, it will be understood that this will usually not require any particular precautionary measures since such longer wavelengths are very easily arrested by making the output filter used in the invention thick enough.

As to the thickness of the output filter, as earlier indicated this should be great enough to suppress substantially all, say of the radiations at wavelengths below the desired range (or spectral window), as Well as greatly attenuating wavelengths above the desired range. However, since the filter necessarily causes a general attenuation of the output beam as indicated in FIG. 3 by the lower ordinates of curve R as compared to curve B, the filter thickness should not too greatly exceed such minimum value in the interests of efiiciency and power economy. Suitable values for the thickness of the output filters have been indicated in the examples given above for various elements, and while the indicated values have given good results, it will be evident that they can be departed from, either increased or reduced, depending on specific requirements.

What We claim is:

1. An X-ray source for generating a beam of X-rays which lie within a limited prescribed energy range said source including a sealed envelope having an X-ray transparent window in one wall thereof, an electron emissive cathode electrode and an electron target electrode mounted within the envelope such that X-rays produced within the target pass through the window, X-ray filter means positioned so as to be traversed by said X-rays, and means for applying an electron accelerating voltage between the cathode and target electrodes,

wherein the improvement comprises the features that the accelerating voltage is such that the most energetic characteristic X-ray emitted by the target forms the lower limit of said preselected energy range and that said target and said filter means consist substantially of the same chemical element chosen to have a characteristic series of X-ray emissions within said preselected range the upper limit of which occurs at an X-ray absorption jump of the element.

2. The source defined in claim :1 wherein said chemical element is molybdenum.

3. The source defined in claim 2 wherein said filter has a thickness of from 20 to 40' microns.

4. A method of generating X-rays within a limited prescribed energy spectral range including the step of directing an electron beam at a target consisting predominantly of a chemical element selected from within the group consisting of elements having a characteristic X-ray emission spectrum comprising said prescribed spectral range and having an X-ray absorption jump at the lower energy limit thereof,

accelerating said electron beam to an energy greater than the critical energy which is equal to the energy of the most energetic characteristic X-ray contained within said range,

passing the X-ray beam through a filter consisting substantially of the same chemical element as the target, whereby a filtered X-ray beam will be formed within the prescribed energy range limited at one end by the absorption jump energy of the filter and at the other end by the critical energy of the electron beam.

5. The method of claim 4 wherein the electron beam energy lies within the interval of from 1.1 to 2.0 times the critical energy.

6. The method of claim 4 wherein the electron beam energy lies Within the interval of from 1.2 to 1.7 times the critical energy.

7. The method of claim 4 wherein said characteristic X-ray emission spectrum is the K series, and the electron beam energy lies within the interval of from 1.1 to 2.0 times the critical energy.

8. In the method of analyzing an object with X-rays which lie within a limited prescribed energy range wherein the object includes a portion consisting predominantly of a chemical element of a determinable atomic number and a portion consisting predominantly of chemical elements of lower atomic number, the improvement c0mprising the steps of,

directing an electron beam at a target consisting substantially of a chemical element selected from within the group consisting of elements having a characteristic X-ray emission spectrum comprising said spectral range and having an X-ray absorption jump at the lower energy limit thereof,

accelerating said electron beam to an energy greater than the critical energy which is equal to the energy of the most energetic characteristic X-ray contained within said range, and

passing the X-ray beam through a filter consisting substantially of the same chemical element as the target, whereby a filtered X-ray beam will formed within the prescribed energy range limited at one end by the absorption jump energy of the filter and at the other end by the critical energy of the electron beam.

9. The method as defined in claim 8 wherein the target consists substantially of a chemical element selected from the group consisting of elements having an atomic number exceeding said determinable atomic number by at least one unit. p

10. The method as defined in claim 8 wherein the target consists substantially of a chemical element selected from the group consisting of elements having an atomic number exceeding said determinable atomic number by not more than two units.

11. The method as defined in claim 9 wherein the electons are accelerated to an energy within the range of from- 1.1 to 2.0 times said critical energy.

12. The method as defined in claim 8 wherein the method is mammography of the human breast, said target and filter consist substantially of molybdenum, and the electron beam is accelerated to an energy range of from 25 kev. to 40 kev. whereby the X-ray beam contains the K series radiation of molybdenum.

References Cited UNITED STATES PATENTS 2,327,568 8/ 1943 Atlee 250-86 3,114,832 12/1963 Alvarey 250-86 3,270,200 8/1966 'Rhodes 250--86 OTHER REFERENCES Roentegenography of the Breast, Victor Kremens, American Journal of Roentgenology and Nuclear Medicine, vol. 80, No. 6 December 1958 pp. 1005 to 1013.

50 RALPH G. NILSON, Primary Examiner c. E. CHURCH, Assistant Examiner

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