GB2026812A - Power supply arrangement for computerised tomographic apparatus - Google Patents

Abstract

In a rotating tomographic scanner it is possible to transfer the X- ray tube supply via slip rings. It is difficult to transfer high voltage for the tube by slip rings, yet if the mains supply is transferred the transformers required to give 70KV are excessively heavy for the gantry. It is proposed to transfer the power at 300V, high frequency and then to convert to 70KV on the gantry. The equipment then mounted on the gantry is evenly distributed there-around so that no further counterweight is required. A closed circuit oil cooling system is also provided on the gantry. <IMAGE>

Description

SPECIFICATION Improvements in or relating to radiography The present invention relates to computerised tomography, and it is particularly related to computerised tomographic (CT) apparatus in which an x-ray source is subject to many revolutions about the body of a patient to be examined.

Computerised tomography was invented by G.

N. Hounsfield. The principles of the techniques, together with several arrangements for putting them into practice, are described in British Patent No. 1-,283,915. The procedures required, that is scanning the patient to collect data representing attenuation of the radiation along many paths through a region of interest and processing the data to provide a representation of that region, are now well understood. Further developments of the basic technique have been proposed and British Patent No. 1,430,089 discloses an apparatus in which a source of radiation projects a fan-shaped radiation beam to irradiate the region of interest and be received by detectors on the other side of the body. The radiation is projected along a sufficient number of paths through the body by orbiting at least the source around the body.

This apparatus and other similar systems are capable of relatively rapid examination of the body. Such examination may require the source, and perhaps the detector, to make several orbits around the body. This can result in complex cable handling systems to provide power supplies and cooling fluid to the x-ray tube. One convenient arrangement for supplying tube power is by the use of slip-rings. These can provide problems, however, and it is an object of this invention to provide improvements to the scanning arrangement which can facilitate the rotational movement in CT apparatus of different types.

According to the invention there is provided a CT apparatus including a rotating member mounted for rotation about a patient position, an x-ray tube, mounted on the rotating member to orbit around the patient position and direct radiation therethrough, a source of electrical power at intermediate voltage and high frequency, a slip ring and brush assembly for transferring, to the rotating member, said power at said intermediate voltage and a converter assembly, mounted on the rotating member, to convert an intermediate voltage to high voltage for supplying the x-ray tube.

The x-ray tube, conversion assembly and other equipment mounted on the rotating member, such as x-ray tube cooling equipment, should be mounted on the rotating member in a distribution so as to form a substantially balanced assembly without the use of an additional countermass.

In order that the invention may be clearly understood and readily carried into effect it will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 shows in schematic form a CT apparatus with which the invention is to be used, Figure 2 shows a practical arrangement a rotating member for the apparatus of Figure 1, including the assemblies in accordance with this invention, and Figure 3 shows a rotating member as shown in Figure 2 integrated with a CT apparatus.

Referring to the drawings there is shown in Figure 1, in simplified form and in front elevation, the scanning unit of a computerised axial tomographic apparatus with which the present invention is to be used. An x-ray tube 1 is collimated, as shown at 2, to provide a substantially planar, fan-shaped spread 3 of x radiation. The collimator 2 may, if desired, include partitions to divide the spread 3 into individual beams distributed across the fan angle. The tube 1 is mounted on a ring-shaped gantry member 4 which is formed with an aperture 5 to accommodate the body of the patient. The patient is supported on couches (not shown) to lie generally horizontally with the desired part of his body in the path of the radiation.

The gantry member 4 is supported by a frame member 6, which is static, and rotates relative to the member 6 in a large annular bearing 7. The rotational movement of the member 4 is effected by means of an electric motor 8, supported by the frame member 6, which drives a heavy duty toothed belt 9, the belt 9 passing over a toothed lip 10 which extends rearwardly from the gantry member 4.

It is important for the movement of the gantry to be accurately monitored, and to that end it is provided with any convenient form of monitoring device. In this example, the gantry carries an engraved graticule 11 which cooperates with a photocell/light source combination 12, 13 to produce electrical impulses which indicate the progress of the angular movement of the gantry member.

Means are required for detecting the radiation emergent from the body along substantially linear beam paths and, in this example, those means comprise an array 14 of scintillator crystals, for example caesium iodide or sodium iodide, each coupled to a respective photo-electric converter device, such as a photodiode or photomultiplier.

The detectors 14 are collimated as indicated at 15 so that they tend to respond preferentially to radiation transmitted to them along respective straight lines from the source, rather than to scattered radiation. In this example, the detectors 14 and collimators 15 are mounted on the gantry member 4 and extend only to the extremities of the spread 3 of radiation. This need not be the case, however, and it may be preferable in some circumstances for a larger array of detectors, say extending over 1800 or 3600, to be supported by the static frame member 6 as describes in British Patent No. 1 ,540,583. In that case the source of radiation rotates relative to the detector array.

Rotation of the source tube 1 is effected about an axis 16 which passes through the aperture 5 and output data are provided by the detectors in the course of that rotation. The data are allocated to particular beam paths through the body of the patient, under the influence of the impulses from the photocell/light-source combination and are processed to yield a representation of the region of interest, perhaps as displosed in British Patent No.1,471,531.

It has been mentioned that the data may be gathered over a rotation of 3600. However the total motion of the source of radiation may be much greater than this for at least the reason that the working revolution should be relatively fast and extra rotation for acceleration and deceleration is needed.

For that reason it is inconvenient to supply power to the source 1 via cables which require complex handling arrangements. It is proposed to supply the power via slip rings, indicated schematically at 17, cooperating with brushes 18.

In the system illustrated the slip-ring arrangement also carries data from the detectors to processing circuits. However, if the detectors are fixed to the static frame that is not necessary.

It is conventional to generate the high voltage supply for an x-ray tube using mains frequency, oil-insulated transformers converting the mains voltage to that, typically 70KV, required for the tube. This voltage can be transferred via slip rings to the x-ray tube with low current (1 00mA) but the construction of slip rings suitable for 70KV presents many problems.

One alternative is to use slip rings to transfer alternating current at mains voltage and transform to high-voitage on the rotating part. This is convenient for slip ring design but problems result from the need to rotate massive mains frequency transformers and insulated rectifiers and smoothing assemblies with the X-ray tube.

To reduce the mass of the rotating parts it is here proposed to transfer high frequency alternating voltages through the slip rings. The intermediate high frequency voltage is then transformed and multiplied using a standard Cockcroft Walton multiplier circuit mounted in the gantry. The high frequency primary supplies to the transformers are in this example 300V in amplitude and the transmission of power via slip rings at this voltage level provides no significant problems.

The practical arrangement is illustrated in perspective in Figure 2 which shows the gantry member 4 with mounted thereon the x-ray tube 1.

The 300V input is fed through the slip rings to step up transformers 19 in each channel. The outputs of these are fed to the input of a positive multiplier 20 and a negative multiplier 21. A high voltage stress ring 22 is affixed to the extreme voltage end of each multiplier and then fed viad surge resistors 23 to output sockets 24. The output sockets are of the Federal 72 type and are, in this example, modified to operate in the air.

These equal and opposite outputs are fed to the x ray tube via connectors 25 (negative) and 26 (positive). An encapsulated high frequency filament isolation transformer can be seen at 27.

The output of this is added to the negative output and connected to the x-ray tube 1 via connector 25. A variable "DC" voltage (less than 50V and 1 OOVA) is also fed via the slip rings and this is chopped with an inverter mounted on 4 and fed to the primary of the filament transformer 27. The amplitudes of the filament transformer 27 input and the step up transformer 19 input are controlled by separate control systems, mounted off the gantry member. To achieve this small signals (less than 1 OV) are fed back via other slip rings in relation to X-ray voltage and beam current.

It will be realised that an x-ray tube requires cooling which typically is by a flow of cooling oil.

In this example the cooling oil circuit is a closed circuit mounted on the member 4 so that operation is possible while the assembly is rotating. The arrangement, which is also shown in Figure 2, comprises two oil pumps 28 and filter assembly 29 with an accumulator 30. The accumulator includes a rubber membrane, pressurised with nitrogen, which allows the assembly to be sealed. Cooling of the coil is provided by an annular curved radiator 31. When the system is rotating this radiator is cooled by air moving therearound.

X-ray tubes such as are used in apparatus of this kind are relatively massive and it has been usual to provide a countermass to prevent out of balance forces during rotation. However the power supply equipment mounted on the rotating part in this example is also relatively massive. This invention distributes the power supply components, oil cooling assembly and x-ray tube around part 4, as shown in Figure 2, so that they provide a substantially balanced assembly without the use of a countermass.

Other variations of the system, such as the use of a multiplier enclosed in an insulating gas like SF6, may be devised by those skilled in the art.

Figure 3 shows the manner in which a rotating gantry as shown in Figure 2 is incorporated in a rotating scanning unit of the type shown in Figure 1. In Figure 3, in which elements previously described in Figures 1 and 2 are identified by the same reference numerals, on!y the power supply components 1 8-26 have been shown on the gantry. The slip rings 17 are, in accordance with the invention, energised by a 300V high frequency supply. This supply is obtained from 50 or 60 cycle mains supply by a power supply unit 32, mounted on frame number 6 of in another convenient position.

Claims (7)

1. A CT apparatus including a rotating member mounted for rotation about a patient position, an x-ray tube, mounted on the rotating member to orbit around the patient position and direct radiation therethrough, a source of electrical power at intermediate voltage and high frequency, a slip ring and brush assembly for transferring, to the rotating member, said power at said intermediate voltage and a converter assembly, mounted on the rotating member, to convert an intermediate voltage to high voltage for supplying the x-ray tube.

2. An apparatus according to Claim 1 in which the source of electrical power is a high frequency power supply powered by an electrical supply at mains distribution voltage and frequency.

3. An apparatus according to either of the preceding claims in which the intermediate voltage is substantially 300 Volts and the high voltage is substantially 70 k volts.

4. An apparatus according to any preceding claim in which the converter assembly includes Cockroft Walton multiplier circuits.

5. An apparatus according to any preceding claim in which there is mounted on the rotating member a closed circuit oil cooling system, for cooling the x-ray tube, including a cooling radiator cooled by airflow during rotation of the said rotating member.

6. An apparatus according to any preceding claim in which power supply and other equipment mounted on the rotating member in a distribution suitable to provide an assembly substantially balanced for rotation without the use of a countermass.

7. A CT apparatus substantially as herein described with reference to the accompanying drawings.