GB2042790A - Conical scanning X-ray tubes - Google Patents

Abstract

In conical (e.g. part conical) scanning X-ray tubes, space charge dispersion over the long throw (about 1.5 m) disturbing focussing is countered by magnetic fields from conical coils (11, 12) around and inside the tube cone 14. To reduce the current required in such coils it is proposed to use a beam 5 which travels at a relatively low velocity for at least three quarters of the distance from the electron gun 4 to the X-ray target and to accelerate it to a higher velocity before impact on the target. The extra acceleration potential is divided across metal rings 13 set in ceramic or glass inserts 14. Beam cross-section may be circular or ribbon shaped, the latter being maintained radial to the target by rotation of the gun or the beam shaping part thereof, or by a rotating field system. The transverse component of the beam due to thermal emission velocity, space charge repulsion and field aberrations is discussed. Computerised tomographic applications are exemplified. <IMAGE>

Description

SPECIFICATION X-ray tubes The present invention relates to X-ray tubes in which there is a relatively long distance between an electron gun, producing an electron beam, and a target on which the beam is incident to produce X-rays. The invention is particularly, although not exclusively, applicable to X-ray tubes of this kind for use in radiographic apparatus as computerised tomographic (CT) apparatus.

In British Patent Specification No. 1283915 there is described such apparatus. To achieve an examination, one or more beams of radiation are directed into the body or a patient and the intensity of the radiation is measured by suitable detectors. The proportion absorbed by each beam is thus determined and can be processed to provide a distribution of absorption coefficients for an examined slice of the body. The processing may be as described in the said Patent or improved forms such as that described in British Patent Specification No. 1471 531. To allow suitably accurate processing, beams of radiation must be directed through the body from a large number of directions and it has been the practice to orbit a radiation source around the body.

It has also been proposed to achieve at least part of that effect by scanning an electron beam over a fixed X-ray target so that the origin of the X-rays is moved in relation to the target and therefore in relation to the body being examined.

By this means the entire examination may be completed in a much reduced time, This technique may be extended to achieve a complete -.

examination by extending the X-ray target on an arc extending for at least 1 800 around the patient.

Thus a fan of X-rays, originating at the target, may be directed through the patient's body from positions disposed over at least 1 800 and the transmitted intensity measured by suitable detectors. X-ray tubes including such a target may be conical with the electron gun disposed on the cone axis and suitable deflection used to direct the electron beam to the target and to scan the beam along the target. Apparatus using such systems include that disclosed by T. A. Linuma et. ai in a paper in the Journal of Computer Assisted Tomography (Computer Tomography) 1, 4, 1977, pp 494-499.

As mentioned before, the X-ray target need only extend over 1800, although 3600 is usual, therefore 'conical' is used herein to include generally conical shapes such as part cones and cones bisected along diametrical planes.

The technique using a conical X-ray tube is straightforward in theory but more difficult in practice and problems include that in examining a patient, who must be introduced into the apparatus so that any region of interest can be irradiated, the electron beam will extend, from gun to target, over a distance of at least 1.5 metres.

Furthermore it must be brought to a suitable 'focus' (i.e. small incident spot or line) at the target and must not diverge sufficiently to strike the walls of a resonably dimensioned tube envelope.

These requirements are difficult to satisfy and it is an object of this invention to provide X-ray tubes giving satisfactory 'focussing' of such long electron beams.

According to the invention there is provided an X-ray tube including a conical vacuum chamber, and electron gun at the apex of the conical chamber to project an electron beam into the chamber at a first velocity, an annular or part annular X-ray target at the opposite end of the conical chamber, on which target the beam is incident to produce X-rays, means for setting a megnetic field in the conical chamber to reduce dispersion of the electron beam and accelerating means disposed after at least three quarters of the distance between the gun and the target to accelerate the electron beam to a second velocity before impact on said target.

In order that the invention may be clearly understood and readily carried into effect an example thereof will now be described with reference to the single figure of the accompanying drawing which shows a conical X-ray tube.

Vacuum electron guns used in conventional X-ray tubes provide electron beams having high current densities. At these current densities space charge dispersion of the beam is significant but in general the distance to the target is only of the order of a few centimetres and the dispersion is not a problem. However for gun-to-target distances of the order of 1.5 metres or more, as required for the scanning tubes discussed hereinbefore, dispersion can be a severe problem making focussing difficult. Furthermore it is not desirable to reduce current densities.

One means of reducing spreading of the electron beam is to provide along the path a magnetic field in the beam direction. This can beneficially be formed by currents flowing in opposite directions in coils wrapped around inside and outside the tube envelope. Such an arrangement is the invention of B. J. Mayo and is described in co-pending British Patent Application No. 36774/78.

In an embodiment of that arrangement for high velocity electron beams, such as 1 50 keV which is typical for such X-ray tubes, the energy needed to provide the magnetic field is very large. This invention reduces the energy requirement by reducing the velocity of the electron beam, say to about quarter of its final velocity (about 7 KeV) along most of the gun to target path and to accelerate to the final velocity immediately before the anode, after at least three quarters of the gun to anode distance.

The essential parts of a. conical X-ray tube are shown in the Figure. A conical vacuum chamber 1 comprises a mainly metal envelope, made of for example non-magnetic stainless steel. An annular anode 2 has inset thereon an annular target surface 3 to generate X-rays in response to incident electrons. The envelope extends typically for 1.5 metres from the cone apeyto the target 3.

An electron gun 4, of known type, generates a beam 5 of electrons which is appropriately focussed and deflected by scanning coils 6 to be incident on target 3. The scanning coils are arranged to scan the beam 5 to follow a circular path to traverse the point of incidence around the annular target 3. The coils take a form well known in the cathode ray tube art. The electron gun may provide a circular cross section beam being incident on the target as a point. However it is preferred to generate a ribbon shaped beam incident on a target as a line. In that case it is necessary to maintain the line of incidence radial to the axis 7 of the conical tube. That may be achieved by rotating the electron gun or the beam shaping part thereof. It is, however, preferred to achieve the effect by a suitably rotating field system.A suitable arrangement is described in our British Patent Application No. 8484/78.

The tube includes, on the inside surface of the cone, an annular X-ray window 8. X-rays generated by the incident electrons are then directed along paths, such as 9, substantially perpendicular to the axis 7. Preferably the paths are defined by collimators 10. The radiation will normally be required in the form of a fan in a plane perpendicular to the drawing, the arrangement being substantially as described in the aforesaid paper by Linuma et al. Further details, including X-ray detectors are not shown in the Figure, since they do not form part of this invention, but as the point of impact of the electrons, on the target, orbits about axis 7 the X-ray fan correspondingly orbits to properly examine a patient disposed substantially axially of the cone.

The electrons and their point of origin may be orbited continuously around the cone. However the scan is usually stepped to provide in effect as many as 2000 individual X-ray sources on the target annulus in succession in a scan time of as short as 0.01 seconds.

In order to reduce dispersion of the electron beam 5 the tube is wound with two coils, shown schematically in the figure but essentially as described in application No. 36774/78. Coil 11 is wound around the ouside of the cone and coil 12 is wound (on a former not shown) to conform to the inside surface of the cone. It will be understood that in practice the coils are more tightly wound but have been shown widely spaced for reasons of clarity. They should also extend as closely as practicable to the tube anode consistent with withdrawal of X-rays. The coils are energised with current in opposite directions as shown.

To reduce the current in the coils, needed to sufficiently limit the dispersion, the beam 5 is only acceierated through a low potential in the first part of the cone (say 7 Kv although other values may be found suitable for a particular embodiment).

However, close to the target, there are provided a series of potential setting metal rings 1 3 which divide between them the extra potential required to accelerate the electrons of beam 5 to their desired impact velocity. To allow the potential differences the rings 1 3 are set in insulating inserts 14, of ceramic or glass, in the tube envelope.

A limit to the reduction of velocity of the electron beam 5 prior to rings 1 3 is set by the transverse component of velocity which such a beam always has as a result of thermal emission velocity, space charge repulsion, field aberrations etc.

Such a component of velocity causes the beam to rotate in circles as it moves forward and the diameter of the rotation increases at lower magnetic fields so that the beam may nevertheless strike the tube walls. In practice the velocity should be as high as a convenient value of magnetic field to be generated by coils 11, 12 can retain and the additional fields set by rings 1 3 used to bring the velocity up to the finally required level for impact on the target.

Claims (7)

1. . An X-ray tube including a conical vacuum chamber, an electron gun at the apex of the conical chamber to project an electron beam into the chamber at a first velocity, an annular or part annular X-ray target at the opposite end of the conical chamber, on which target the beam is incident to produce X-rays, means for setting a magnetic field in the conical chamber to reduce dispersion of the electron beam and accelerating means disposed after at least three quarters of the distance between the gun and the target to accelerate the electron beam to a second velocity before impact on said target.

2. An X-ray tube according to Claim 1 in which the first velocity is set substantially at the highest level at which the said magnetic field can retain the said beam to not strike the tube walls.

3. An X-ray tube according to either of the preceding claims in which the first velocity is at least a quarter of a desired value of the second velocity.

4. An X-ray tube according to any of the preceding Claims in which the first velocity is provided by acceleration of the electrons through a potential of substantially 7Kv.

5. An X-ray tube according to any of the preceding claims in which the accelerating means comprise a series of rings whose axes are aligned with said tube axis.

6. An X-ray tube according to Claim 6 in which the tube is formed of non-magnetic.metal and the said rings are set in insulating inserts in said tube.

7. An X-ray tube substantially as herein described with reference to the accompanying drawings.