GB2061028A - Computed tomography apparatus - Google Patents

Abstract

A computed tomography (CT) scanner apparatus includes an improved arrangement for transferring high voltage electrical energy between a stationary gantry (40) and a rotating assembly (42) which carries the X-ray source (44) and has an opening defining an aperture to receive a part of a patient. A first axis (36), about which the assembly rotates, passes through the aperture. The apparatus includes a number of slip rings (56, 58) which are used for effecting the transfer of the electrical energy. Respective portions of the gantry and rotating assembly define a cavity which contains an insulating di-electric fluid in which the slip rings are immersed. The apparatus is of compact design and, further, the rotating assembly and the slip rings can be tilted about a second axis which intersects the first axis. <IMAGE>

Description

SPECIFICATION Computed tomography apparatus This invention is directed to an improved apparatus for supplying electrical power to a rotating element such as an X-ray source, of a computed tomography (CT) scanning apparatus.

The art of computed tomography scanning has greatly improved a physician's ability accurately to diagnose the internal structure of a patient. The process of CT scanning involves transmitting X radiation through a patient from a variety of different locations, and determining the intensity of the radiation emerging from the patient with one or more X-ray detectors. Intensity data is sent to imaging electronics for image reconstruction. In many studies, the image viewed by the doctor presents greater detail than conventional X-ray techniques and therefore can be better used to diagnose the patient's condition.

In a typical computed tomography scanning environment, an X-ray source is rotated (orbited) about a patient while the patient is irradiated. The detector or detectors either orbit with the source or form a non-orbiting array of detecting units. In either configuration a substantial amount of X-ray emitting and shaping apparatus rotates with the X-ray source.

This apparatus, as well as the X-ray source, must be powered by electrical energy supplied from a source exterior to the rotating CT apparatus.

One particular computed tomography scanning unit includes an orbiting source of X radiation which emits an X-ray beam in a spread configuration. All detector units, except for a calibrating detector, are stationary relative to the X-ray source and form an annular ring of detectors about the patient. The X radiation source is positioned to transmit X-rays through the patient along a series of beam paths as it is orbited. Since a variety of beam generating, shaping and transmitting functions must be performed on the orbiting apparatus as orbiting occurs, a number of electrical signals must be supplied to this apparatus.

To create X radiation, a large potential difference on the order of 150 kilovolts must be provided. This voltage is used to accelerate electrons from an X-ray tube cathode to an anode for X radiation generation.

Typically a filament voltage and an X-ray focusing cup control are required so that a number of high voltage inputs are necessary. Providing these high voltage potential differences to the orbiting X radiation source creates design problems which have in the past required sophisticated coupling techniques.

A A number of low voltage control and energization signals must also be transmitted to the orbiting apparatus. For example, power (at low voltage relative to the voltage supplied to the X-ray tube) is supplied to a motor which rotates the X-ray tube anode.

Present state of the art CT scanning units employ slip rings to couple powerfrom an external source to the rotating X-ray source and to couple and signalling apparatus and additional power sources to other components which rotate with the X-ray source. One such arrangement is disclosed in United States patent No. 4,093,589 entitled "Axial Tomographic Apparatus." This patent shows a stationary member coupled to an assembly which bears an X-ray source, and which rotates about a single axis to a stationary member on two sets of bearings spaced apart from each other along the length of that axis.

The reference also shows the slip rings disposed at a point along the axis which is remote from the plane of the path followed by the X-ray source.

If, in the interest of reducing arcing at the slip rings, the slip rings are to be immersed in an insulating fluid, the structure shown in patent No.

4,093,589 is not adaptable to compact and convenient arrangement to confine the fluid to the housing, which now must surround the slip rings. In overcoming this and other disadvantages of such prior art CT apparatus slip ring arrangements, the present invention provides a unified, compact, slip ring arrangement for transmitting electrical energy to orbiting components in a CT scanning unit, where the slip ring arrangement is adapted for immersion in an insulating fluid.

According to the invention, respective parts of the gantry and rotatable assembly define a cavity which contains the slip rings, which cavity also maintains an insulating fluid about the slip rings.

In a preferred form of the invention, means are provided for tilting the path followed by the X-ray source and the cavity with it, with respect to the gantry, to allow a choice of scanning cross sections through the patient.

The features and advantages of the invention are brought out in the following detailed description, when considered in conjunction with the accompanying drawings, in which: Figure 1 shows schematically the elements comprising a CT scanning arrangement; Figure 2 schematically shows a source of X-rays positioned to irradiate a patient cross section; Figure 3A shows a front elevational view of elements of a CT scanner; Figure 3B shows a partially sectioned view of the CT scanner shown in Figure 3A; Figure 4 is an enlarged cross sectional view of a portion of the scanner shown in Figure 3B; Figure 4A shows a further enlarged part of the cross sectional portion shown in Figure 4; and Figures 5A-5C show wiring schematics for high voltage energization of a CT X-ray tube.

Figure 1 shows a computed tomography system 10 designed for examining the internal structure of a patient. The system comprises a scanning unit 12, a couch 16, a signal processor 20, and imager 22. The scanning unit includes a housing 13 which covers the X-ray apparatus and provides an attractive appearance to the unit. Before a CT scan the couch 16 and a patient lying on the couch are moved into an aperture 14 in the housing 13. An X-ray tube within the unit is energized and transmits X radiation, thereby irradiating the patient.

The scanning unit 12 can be tilted about an axis 24 parallel to the floor. This movement provides a flexibility in scanning without repositioning of the patient. Two support columns 23 mount the unit 12 for rotation about the axis 24. Rotational motion is applied by an AC motor 25, a right angle drive 26, and a pivot arm 27.

A series of X-ray detectors detect the intensity of the X-rays after they pass through the patient and produce electrical signals in response thereto. These signals representing patient densities are sent from the scanning unit to the signal processor 20 by an electrical connection 15. The signal processor re ceivesthese signals and utilizes known CT processing techniques to produce signals representing the variations in patient density across a patient cross section. The signal processor then sends signals to an imager 22 which provides an image of the patient.

Figure 2 schematically illustrates a CT X-ray source 30 and array of detectors 32 positioned about the patient aperture 14. The source 30 emits a spread of X radiation which passes through a collimator 34 which shapes the X radiation into a number of individual beams. One X-ray beam 33 is shown as it passes through the patient aperture and impinges upon a detector in the circular array 32 of X radiation detectors. The illustrated detectors are shown positioned on the side of the patient aperture opposed from the source 30 and therefore certain of them detect radiation intensity after that radiation has passed through the patient.

Although the Figure 2 illustration shows a finite number of detectors, so-called "stationary detector" CT designs provide an array of detectors which completely surround the patient aperture. Thus, it is possible for the X radiation detectors to remain stationary while the X radiation source 30 orbits about the patient aperture irradiating the patient from a number of different positions. The detectors are of a known design and convert X radiation into electrical signals which can be sent to the signal processor 20 for CT image formation.

All CT reconstruction algorithms require that the X radiation impinge upon the patient cross section from a number of different positions so that intensity data from radiation originating from various positions is obtained. By obtaining this multi-position intensity data it is possible to reconstruct a mapping or image of the density variations within the patient cross section. To achieve this multi-position irradiation the present invention includes a rotating assembly which supports the X-ray source 30 and is movable relative to the X-ray detector array 32.

Movement of the X-ray source in a circular path causes electrical energization problems which are compounded by the high voltage potential differences coupled to the X-ray tube.

Figures 3A and 3B illustrate a new and improved CT apparatus which facilitates the sending of potential differences to the X-ray source for X-ray generation. The CT apparatus shown includes a stationary gantry arrangement 40, a rotating assembly 42 and an X-ray tube housing 44. During operation a belt drive 48 causes the rotating assembly 42 within stationary gantry 40 to rotate about axis 36, thereby irradiating from a number of positions a cross section of interest of a part of the patient within the opening of 42 as X-ray source 44 moves through a circular path about axis 36 and circumscribing the aperture of 42. The rotating assembly comprises a frame 50 attached to an annular portion of the assembly by eight connectors 43 (see Figure 3A) spaced evenly about the patient aperture 14. The frame 50 carries the X-ray housing 44 for orbital rotation about the patient aperture 14.

One aspect of the invention is the provision of high potential differences between the cathode and anode of the X-ray tube. In one embodiment of the invention this potential difference is on the order of 150,000 volts. This potential difference is provided by positive and negative inputs, each on the order of 75,000 volts removed from ground. To transmit electrical energy to the X-ray tube the stationary gantry 40 includes both a positive 46 and negative 47 high voltage electrical receptacle or connector. The positive voltage receptacle 46 shown in Figure 3B receives a voltage input of plus 75,000 volts from an external voltage source. The negative high voltage receptacle (not shown in Figure 3B) receives an input voltage of approximately 75,000 negative volts.

The assembly further comprises positive 56 and negative 58 slip ring portions which receive these high voltage inputs and transmit them from the stationary gantry portion 40 to the rotating assembly 42 for transmittal to the X-ray tube. Each slip ring includes an annular conductor, which is disposed concentrically about axis 36, and a brush, which is biased into electrical contact with its annular conductor. In the embodiment of the invention illustrated in the drawings, each concentric annular conductor is borne by rotatable assembly 42, while each brush is borne by structure secured to gantry 40. It is to be realized that each annular conductor could be borne by gantry 40 and that each brush could be borne by rotatable assembly 42. The annular conductors of these slip ring portions are concentric with axis 36 and revolve with assembly 42.The first positive portion 56 includes only one slip ring which is coupled to the positive receptacle 46. The second negative portion 58 includes four slip rings and is designed to receive more than one negative high voltage input. The purpose of this multiplicity in high voltage slip ring configuration is to allow control of the X radiation generation by utilization of either multiple focus or grid potential voltage inputs.

The high voltage transmitted to the rotating assembly 42 is further transmitted along cabling (shown in Figure 4 at 120) to two high voltage receptacles 60, 62 mounted to the frame 50. The first receptacle 60 receives the positive voltage and the second receptacle 62 receives the negative voltage.

From these receptacles the high voltages are trans mitted to the anode (positive) and cathode (negative) of the rotating X-ray tube.

Mechanical coupling between gantry 40 and rotating assembly 42 includes a support 52 radially removed from the patient aperture by means of a suitable connector 54 such as a nut and bolt arrangement. This mounting serves to maintain the stationary gantry 40 in position relative to the patient, and (as explained below) allows the support 52 and rotating assembly 42 to be tilted about an axis 24 perpendicular to the axis 36 of CT scanning. If the support 52 is tilted while the patient is maintained in a horizontal position, the X radiation will traverse the patient aperture in a non-vertical direction and thereby provide flexibility in CT scanning. If, for example, the support 52 is tilted 200 about an axis perpendicular to the scan axis 36 the cross section of patient irradiation will also be titled 200 to the vertical.

The geometrical configuration of the frame 50 and X-ray housing 44 is such that the rotating assembly 42 is well balanced about the scan axis 36. The frame 50 is much wider at a side 51 opposed from the X-ray housing so that it counterbalances the weight of the X-ray housing and enclosed tube and provides a symmetrical mass distribution about the axis 36.

The stationary gantry 40 supports the rotating assembly 42 along an annular bearing connection 64, Figure 4, which allows free orbital rotation of the assembly 42 about the axis 36. The compact design of the apparatus including the disposition of bearing 64 and the annular slip ring portion 58 close to each other along the length axis of rotation 36 allows one bearing to provide sufficient support to the rotating assembly. The use of one bearing and the compact (with respect to axis 36) design of assembly 42, in turn, allows assembly 42 and the entire body of the single bearing to be tilted through a relatively large angle about axis 24 without interference between couch 16 or the patient.

An encoder 66 in the form of an annular ring is attached to the rotating assembly 42. This encoder 66 includes a number of marks equally spaced about the ring which indicate the angular orientation of the encoder. As the annular ring moves about the center axis, an optical encoder 67 determines the position of the ring relative to the stationary gantry 40. In this way the precise position of the X-ray source can be determined at all times during irradiation of the patient. This position data is correlated with intensity readings from the X-ray detector array and utilized in reconstruction algorithms known within the art.

Figures 5A-C illustrate three different X-ray tube input configurations for energization of an X-ray tube. Each configuration shows an anode 70 and cathode 72 coupled to high voltage energization inputs. A number of such inputs are illustrated.

In a single focus X-ray tube (see Figure 5A) three high voltage inputs are needed. A first input 76 is the positive input to the X-ray anode and in the preferred embodiment of the invention is applied through a first positive portion 56 of the slip ring arrangement.

Two negative inputs 78,80 are used to energize the cathode 72 and in the preferred embodiment are transmitted via the second portion 58 of the slip ring arrangement. A transformer 82 supplies a filament current which causes electrons 84 to be emitted thermionically from the cathode for acceleration towards the high potential anode 70.

Figure 5B illustrates a double focus X-ray tube.

Three negative high voltage inputs 86-88 are transmitted to the X-ray tube cathode 72. Through control of the voltages appearing on a primary of the transformer 82, it is possible to control the high voltage inputs 86-88 and provide a measure of X-radiation control unavailable on the single focus tube.

An X-ray tube which includes a grid control is illustrated in Figure 5C. This tube includes a high voltage positive input 76 and three high voltage negative inputs 90-92. Two inputs 90, 91 transmit a voltage appearing across the secondary of the transformer 82. A third input 92 serves to maintain a control voltage on a grid 94 within the X-ray tube.

Through adjustment of grid tube potential a means of control over electron transmittal to the anode unavailable in the single focus tube is provided.

From the illustrations in Figures 5A-5C it is apparent that a plurality of high voltage negative inputs must be available if single focus, double focus and grid X-ray tube control is to be achieved. The second negative portion 58 of the slip ring arrangement (see Figure 3B) includes a plurality of slip rings for this purpose. In the preferred embodiment four slip rings are included to provide flexibility in CT scanner design. The voltages applied to these slip rings are at many thousands of volts below ground but they differ in value, by relatively small amounts. In a single focus X-ray tube configuration, for example, the voltage separation between the two inputs 78, 80 need only be large enough to cause a filament current to flow in the X-ray tube cathode.

Figure 4 is a more detailed cross section view of the slip ring arrangement shown in Figure 3B. The latter figure illustrates the X-ray tube housing 44, and the rotating assembly 42 mounted by the bearing 64 inside the stationary gantry 40. The cross section of Figure 4 shows additional details.

Referring to Figure 4 the first slip ring arrangement 56 carries a positive high voltage to the X-ray tube and the second slip ring arrangement 58 includes 4 individual slip rings for carrying the negative high voltages to X-ray tube as discussed above. Both the first 56 and second 58 slip ring arrangement are immersed in an oil bath in a cavity 112. This oil prevents arcing between high voltage portions of the slip ring arrangement and other portions of the CT apparatus which arcing could damage both the control circuitry and the X-ray tube.

The portion of the stationary gantry 40 bordering on this cavity 112 is preferably of aluminum construction and the portion of the rotating assembly which borders the cavity is of a plastic construction.

These lightweight materials allow the apparatus to be readily tilted and the plastic rotating portions allow the system to be more easily rotated by the belt drive.

Four elastomeric seals 114-117 prevent the dielectric fluid, such as oil, from leaking from the cavity.

These seals are mounted to the nonrotating gantry 40 and are biased against the rotating assembly by means of spring biasing members 118. As the assembly rotates with respect to the stationary gantry, these springs maintain the seals in fluid tight contact with the rotating portions.

The transmission path of the positive high voltage signal is clearly illustrated in Figure 4. The high volage (typically 75,000 volts) supplied to the high voltage receptacle 46 is transmitted to the first slip ring portion 56 then through high voltage cabling to a second high voltage receptacle 60 attached to the rotating assembly for transmittal to the anode portion of the X-ray tube. The high voltage passes through a brush which is biased towards the slip ring by a spring to insure contact between the brush and rotating slip ring. One type of cabling used to transmit the high voltages between the slip ring and the X-ray tube is Federal cabling which is known in the art of X-ray CT scanning. The cabling 120 passes through a bore machined into the plastic portion of the rotating assembly.

The second portion 58 of the slip ring assembly includes four rotatable slip rings 122-125 which transmit four negative high voltages to the X-ray tube. A more detailed showing of this second portion 58 of the slip ring arrangement appears in Figure 4A. As seen in that figure, each rotatable slip ring is contacted by a biased brush 127 which in turn is connected to an electrical contact in a housing 126.

In the embodiment illustrated, a single focus tube has been utilized and therefore only two negative high potential inputs 128,129 are required with two spares available should other tubes be used.

At a location removed from the high voltage slip ring arrangement are a number of low voltage slip rings 130-135 for transmitting low voltage electrical signals from the stationary gantry to the rotating assembly. Since these slip rings transmit low voltages, they need not be immersed in an oil bath to insure electrical isolation. Although for purpose of illustration only six slip rings are shown, in a preferred embodiment of one commercial CT unit sixteen are utilized. Three of the low voltage inputs are utilized to provide power to a motor located in the rotating assembly 42 which cools the anode of the X-ray tube by rotating it. Three more of the low voltage slip rings are used as common or ground potential. Four other low voltage slip rings are utilized as general alternating current power inputs.

These inputs are used to operate a number of solenoids mounted to the assembly 42 which must be powered by AC signals.

Five remaining slip rings are used to monitor and control the condition of three switches mounted to the rotating assembly which operate a shutter, a filter, and the collimator. The functioning of these three components must be coordinated with X-ray generation in the CT imaging process. One of these five remaining inputs transmits a frequency proportional to a reference intensity from the rotating assembly 42 to the stationary imaging electronics 20. Two of the remaining four inputs are used to provide synchronization and clock signals. The remaining two slip rings operate to send and receive digital data from a multiplexing board which both controls and monitors the condition of the three switches.

Since there are 16 rotating slip rings and the function of only 15 inputs have been described, one ofthese 16 slip rings has no function in the present design but is available for future design modification.

While the embodiment described above has been characterized with some particularity, it should be appreciated to those skilled in the art that certain modifications and changes could be incorporated without departing from the spirit or scope of the invention are detailed in the appended claims.

Claims (9)

1. CT scanner apparatus for transferring electric- al energy between a gantry and a rotatable assembly mechanically coupled thereto, said assembly having an opening defining an aperture to receive a part of a patient; said assembly being rotatable with respect to said gantry about a first axis through said aperture and including an X-ray source; slip rings, each having an annular conductor concentric with said axis and a brush; one of said annular conductor and said brush of each of said slip rings being borne by said rotatable assembly and electrically connected to said X-ray source, and the other of said annular conductor and said brush of each of said slip rings being borne by said gantry; said X-ray source being disposed on said assembly to follow a path circumscribing said aperture when said assembly is rotated; input means mounted on the gantry for applying to the ones of said annular conductors and said brushes of said slip rings borne by said gantry a high voltage electrical input, so that high voltage electrical input is applied through said slip rings to said X-ray source; and means for rotating said assembly; wherein: respective portions of said gantry and of said rotatable assembly define a cavity surrounding said aperture and containing said slip rings and wherein there is an insulating fluid in said cavity in which said slip rings are fully immersed during rotation of said assembly.

2. CT scanner apparatus as in claim 1 wherein the coupling between said gantry and said assembly comprises an annular bearing disposed adjacent said path.

3. CT scanner apparatus as in claim 1 or 2 wherein the assembly is tiltable about a second axis intersecting to said first axis; and wherein there also is provided means for tilting said assembly and said bearing body about said second axis in order to allow tilting of said path about said second axis and consequent scanning through multiple patient cross-sections without changing the position of said patient relative to the direction of said first axis.

4. CT scanner apparatus as in claim 3, wherein said second axis is perpendicular to said first axis.

5. CT scanner apparatus as in any preceding claim wherein: said cavity is of annular configuration concentric with said first axis; and there are provided seals borne by said gantry biased to contact a surface of said rotatable assembly for maintaining said cavity fluid tight.

6. CT scanner apparatus as in any of preceding claim wherein: said brush of each of said slip rings is spring biased into contact with said annular conductor of its respective one of said slip rings.

7. CT scanner apparatus as in any preceding claim wherein: the electrical connection between said one of said annular conductor and said brush of each of said slip rings connected to said X-ray source comprises cabling; said cabling being of limited length because of the disposal of said X-ray source on the rotatable assembly.

8. CT scanner apparatus as in any of claims 1-6 wherein the insulating fluid in said cavity comprises oil.

9. CT scanner apparatus or method substantially as hereinbefore described with reference to Figures 3A, 3B, 4 and 4A of the accompanying drawings.