Rotating-anode X-ray Tube With Multiple Focal Areas

Abstract

A rotating-anode X-ray tube wherein the anode target has a first focal plane made of tungsten and a second focal plane made of molybdenum, rhodium, silver or palladium, there are disposed opposite to said first and second focal planes first and second cathodes in such a manner that each cathode impinges electrons on the corresponding focal plane, the voltage to be impressed on these cathodes can be varied by an external means, and there can be obtained, as required, a hard X-ray from the first focal plane or a soft X-ray from the second focal plane.

Description

The present invention relates to a rotating-anode X-ray tube.

Heretofore, the anode target of a rotating-anode X-ray tube chiefly used in an X-ray apparatus for medical examination has consisted of a circular disk made of tungsten or an alloy mainly composed thereof.

As is well known, the X-rays emitted from an X-ray tube provided with a tungsten target display a specific wave length distribution defined by a combination of the continuous X-ray whose wave length-intensity characteristic depends on the voltage impressed across the cathode and target of an X-ray tube and X-ray such as WL.alpha., WL.beta., WK.alpha., WK.beta., characteristic of the tungsten. The intensity of the X-rays is substantially proportionate to the X-ray tube current.

Where ordinary X-ray photographs are taken for medical examination, there are practiced various methods, for example, of selecting an X-ray tube voltage adapted for the object intended so as to obtain X--rays of the desired intensity or rendering the quality of X-rays hard using a filter made of aluminum or the like, with the view of generating X-rays having such wave length distribution as will display the best effect depending on the parts of a foreground subject or thicknesses thereof to be exposed to X-rays or the objects of taking such photographs.

With respect to the various parts of the human body such as the circulatory system including the limbs, head, dental section, chest, heart, etc. and the digestive system which have heretofore constituted the general subjects of the X-ray photography, the object of such photography has been fully attained by means of an X-ray tube having the aforesaid tungsten target and by selecting proper photographing conditions.

However, the photography or what is called mammography of the soft tissues of the breast intended for medical examination of the diseases associated with the mammary glands which have recently been taken up in radiology requires even the slightest differences in the density of the soft tissues of the foreground subject to be distinctly indicated on an X-ray photograph. Since it is necessary to this end to provide a source of soft X-rays of great intensity, there have heretofore been used the X-rays generated by impressing a voltage having a peak value of 20 to 50 kvp. on an X-ray tube provided with a target made of tungsten or alloys thereof. However, the quality of X-rays generated by the conventional X-ray tube at a peak voltage of 20 to 50 kvp. was not always suitable to photograph the aforementioned soft tissues. The reason was that the characteristic X-rays obtained at said peak voltage only consisted of very long waves exceeding 1.4 Angstrom units mainly including WL.alpha. and WL.beta.. The X-rays of such wave lengths were useless for the desired X--ray photography. On the other hand, the continuous X-ray had a certain smooth wave length distribution, and naturally restricted the parts of a foreground subject which would be effectively photographed thereby, so that this type of X-ray was, of course, unsuitable for the desired photography.

Exactly to record minute differences in the X-ray absorption by the foreground subject on an X-ray photographic film, there have heretofore been made various attempts, for example, of using an X-ray tube provided with a tungsten target at a voltage selected from the peak range of from 20 to 50 kvp. or constructing the X-ray irradiating window so as to obtain X-rays having a wave length distribution containing a great deal of a useful component for the desired photography. However, all these attempts have not achieved fully satisfactory results.

The present invention provides a rotating-anode X-ray tube which comprises an anode target whose first focal plane consists of tungsten or an alloy mainly consisting thereof and emits hard X-rays and whose second focal plane consists of molybdenum, rhodium, silver, palladium or an alloy mainly consisting of any one thereof and, if required, emits soft X-rays of great intensity, and at least one cathode so positioned as to face either of these focal planes, and wherein the first and second focal planes are differentiated in use as occasion demands, making it possible not only to emit optimum X-rays for general purpose radiography but also to take distinct mammographic pictures.

The present invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawing, in which:

FIG. 1 represents a rotating anode X-ray tube according to an embodiment of the present invention with a part indicated in section;

FIG. 2 is a diagram of an external circuit for supplying the X-ray tube of FIG. 1 with the required current and voltage; and

FIGS. 3 to 7 show the sectional views of modified targets of the present rotating-anode X-ray tube.

We have studied various materials for use as a target in the emission of soft X-rays in search for a suitable one and found that metals such as molybdenum, rhodium, silver, palladium and alloys mainly consisting of any one thereof are well adapted for use as a target in generating soft X-rays.

Comparison of the radiation Dose and qualitites of X-rays emitted from the molybdenum and tungsten targets is given in Tables 1 and 2 below.

Table 1 Radiation Dose (R/min) Tube voltage (kvp) 20 30 50 80 Target material A Tungsten 0.28 1.2 6.0 19.5 __________________________________________________________________________ B Molybdenum 0.37 2.32 12.6 32.5 B/A 1.32 1.93 2.10 1.67 __________________________________________________________________________ Table 2 Half Value Layer (HVL) of aluminum (mm.) Tube voltage (kvp.) Target Material 20 30 50 80 __________________________________________________________________________ Tungsten 0.16 0.28 0.45 0.73 __________________________________________________________________________ Molybdenum 0.20 0.35 0.55 0.65 __________________________________________________________________________

As shown in Table 1, the molybdenum target emits a larger absolute radiation quantity of X-rays than the tungsten target and the ratios of emissions by these targets progressively increase until the tube voltage rises to 50 kvp.

With respect to the quality of X-rays irradiated, the molybdenum target displays a larger half value layer of aluminum than the tungsten target within the peak voltage of from 20 to 50 kvp. as shown in Table 2, and under the same X-ray tube load, is more prominently abundant in useful energy range of X-rays Accordingly, the molybdenum target generates X-rays of great intensity in the wavelength zone around the energy of 20 kv. which is most suitable for the soft X-ray radiography, and only slightly suffers the attenuation of X-ray intensity caused by means for correcting the wave length distribution such as filtration. Also a target made of rhodium, silver, palladium, their alloys with molybdenum or alloys mainly consisting of at least one of these single metals produces useful X-rays having an enough intensity in the wave length zone around the energy of 20 to 26 kv. which is available for the desired soft X-ray radiography at a tube voltage of 20 to 50 kvp.

As previously described, however, a target made of the above-listed metals or alloys thereof is unsuitable for use at a tube voltage exceeding 80 kvp., so that it is only available for the soft X-ray radiography and barred from application in general purpose X-ray radiography.

The X-ray tube of the present invention comprises a plurality of independently operable cathodes or at least one cathode capable of emitting electrons at varying positions and a target disposed opposite to said cathode or cathodes whose focal plane corresponding to at least one of these cathodes or at least one of the varying X-ray emitting positions is made of tungsten or an alloy mainly consisting thereof and whose focal plane corresponding to at least one of the remaining cathodes or at least one of the other X-ray emitting positions consists of molybdenum, rhodium, silver, palladium or an alloy mainly consisting of any one thereof. Accordingly, the X-ray tube of the present invention is freely applicable either in the soft X-ray or general purpose radiography by selectively exciting one of the plurality of cathodes or shifting the position of at least one cathode.

There will now be described an embodiment of the present invention by reference to the appended drawing. In FIG. 1, there is rotatably supported a rotating anode section 3 by means of bearing on a stationary section 2 sealed to one end of an evacuated envelope 1, said anode section 3 being rotated by a coil 4 disposed on the outside. To the furthest end of the terminal axle section 3a of the rotating anode section 3 is fitted a circular umbrella-shaped target plate 5 whose upper surface excluding the central part gently inclines downward.

To the other end of the evacuated envelope 1 is fixed a cathode means 8 provided with two filaments 6 and 7 which are spatially arranged opposite to the target plate 5 and are each independently operable by means of an external circuit as shown in FIG. 2. The target plate 5 and filaments 6 and 7 are disposed in such relationship that the electron beams 9 and 10 emitted from said filaments 6 and 7 are concentrated on two focal points 11 and 12 spatially located in the radial direction on the inclined upper surface of the target plate 5. Where the target plate 5 rotates, the loci of the two focal points 11 and 12 define two concentric circles of different radii on the surface of the target plate 5.

According to this embodiment, the focal plane near the axial center including the inner focal point 12 on the surface of the target plate 5 consists of a circular disk 13 of tungsten and the outer focal plane including the outer focal point 11 is composed of a plate 14 of molybdenum which constitutes the base body of the target plate 5.

FIG. 2 represents an external circuit for supplying the X-ray tube with the prescribed tube current and voltage. This circuit supplies a high voltage by a high tension transformer 20 across a plate 3 and the respective ends of the filaments 6 and 7. There is introduced an electric current by a filament transformer 21 positioned between the terminals of the filaments 6 and 7. There may be contemplated various means for supplying an electric current to either of the filaments 6 and 7 as occasion demands. However, the illustrated circuit uses a change switch 22. It will be apparent that other types of external circuits generally used in the X-ray tube may also be available if only said filaments are made interchangeably usable by various known means.

There will now be described various modifications of the target plate 5 used in the aforesaid embodiment. The target of FIG. 3 has a base body composed of molybdenum and constituting the inner focal plane. The outer plane consists of an annular plate of tungsten embedded in said base body. The target of FIG. 4 has a base body made of molybdenum and constituting the outer focal plane, and the inner focal plane made of tungsten is embedded in said base body. It is also possible to fabricate the targets of FIGS. 3 and 4 by preparing a base body constituting one focal plane from tungsten and the other focal plane from molybdenum, rhodium, silver, palladium or an alloy mainly consisting of any one thereof. The relationship between the tungsten and other metals used in the target in terms of their respective requirements and positions is determined by the object of using an X-ray tube, namely, according to whether the tube is demanded to emit large amounts of soft X-rays or hard X-rays.

In the targets of FIGS. 5 and 6 the first and second focal planes have different angles of inclination so as to cause X-rays to be emitted into different fields from these focal planes. In the target of FIG. 5 the focal plane forming the base body has a smaller inclination than the other focal plane, and FIG. 6 represents a reverse arrangement. In both cases, the base body may consist of either tungsten or molybdenum.

The target of FIG. 7 as a whole has a dish shape with a cavity formed in the upper part. As illustrated, this target is fitted to an X-ray tube in inclined relationship thereby to reduce the space between the target and foreground subject and minimize loss of X-rays therebetween.

As mentioned above, the X-ray tube of the present invention causes the filaments of a cathode to be independently operated, namely, excites one of the filaments to emit X-rays from the focal point of one focal plane consisting of molybdenum for use in mammography and actuates the other filaments to release X-rays from the focal point of the other focal plane made of tungsten for use in the general photography of parts of the human body. Namely, the present invention makes it possible selectively to carry out either the soft X-ray or general purpose radiography as occasion demands, using a single X-ray tube. Moreover, the present X-ray tube has a soft X-ray focal plane made of molybdenum, rhodium, silver, palladium or an alloy mainly consisting of any one thereof, so that there are always obtained suitable soft X-rays for mammography.

The foregoing embodiment relates to the case where there were used two cathodes having the same focal lengths. However, it is also possible to cause these cathodes to have different focal lengths, or to provide three or more cathodes.

If the X-ray irradiating window of the evacuated envelope is made of beryllium it will be advantageous for the soft X-ray radiography. Or if the envelope consists of glass, the fabrication of said window with thinner walls than the other parts of the envelope will offer convenience.

Claims

What is claimed is:

1. A rotating anode X-ray tube with multiple focal areas for emitting optimum X-rays for general purpose radiography and soft X-rays for mammography, comprising:

a. an evacuated envelope, X-rays

b. a rotatable anode means housed in said envelope and provided with a target,

c. first and second concentric annular focal areas provided on said target, said first focal area made of a metal selected from the group consisting of tungsten and an alloy consisting mainly of tungsten and said second focal area made of a metal selected from the group consisting of molybdenum, rhodium, silver, palladium and alloys consisting mainly of any one thereof, and

d. a cathode means spaced from said target, and having two filaments each of which is arranged opposite to each of said focal areas and is independently operable by means of an external circuit.

2. An X-ray tube according to claim 1 wherein the upper peripheral portion of the target gently inclines outward and the focal planes are disposed on said peripheral portion.

3. An X-ray tube according to claim 2 wherein the upper peripheral portion of the target inclines in two steps and the inner inclination is sharper than the outer one.

4. An X-ray tube according to claim 2 wherein the upper peripheral portion of the target inclines in two steps and the inner inclination is more gentle than the outer one.

5. An X-ray tube according to claim 2, 3 or 4 wherein the inner focal plane consists of tungsten and the outer focal plane of molybdenum.

6. An X-ray tube according to claim 2, 3 or 4 wherein the inner focal plane consists of molybdenum and the outer focal plane of tungsten.

7. An X-ray tube according to claim 2 wherein the upper peripheral portion of the target inclines upward toward the periphery and the focal planes are disposed on said peripheral portion.

8. An X-ray tube according to claim 2 wherein the cathode member comprises two filaments provided at different positions, and as occasion demands, only either one thereof is operated by an external circuit.

9. A rotating-anode X-ray tube, comprising in combination:

a. a rotating anode section (3) sealed towards one end of an evacuated envelope (1) mounted on a stationary section (2) with electromagnet means (4) outside said envelope to rotate said anode section;

b. cathode means (8) at the other end of said evacuated envelope with spatially arranged inner and outer filaments (6 and 7) operable by means of an external circuit; and,

c. a circular target plate (5) with an inclined surface mounted on said anode opposite said cathode means (8) so that beams from said filaments (6 and 7) are concentrated on two focal points spatially separated in the radial direction, this rotation of said target plate (5) defining two concentric circles of different radii by said two focal points, forming an inner focal plane near the axial center including the inner focal point on the target plates consisting of a disc (13) made of a first metal and an outer focal plane including the outer focal point consisting of a plate (14), made of a second metal, one of said metals being mainly tungsten or an alloy mainly of tungsten while the other of said metal is a metal selected from the group consisting of molybdenum, rhodium, silver, palladium or an alloy mainly of any one thereof.