WO1999060928A1

WO1999060928A1 - X-ray imaging apparatus

Info

Publication number: WO1999060928A1
Application number: PCT/IB1999/000930
Authority: WO
Inventors: Johannes Hermanus Potgieter; Andre Booysen; Paul Van Looy; Gerhardus De Jager; Stephen James Beningfield; Peter Curt Bautz; Gillan Bowie
Original assignee: Debex (Proprietary) Limited
Priority date: 1998-05-22
Filing date: 1999-05-21
Publication date: 1999-12-02
Also published as: EP1079734A1; AU3724999A; JP2002516136A

Abstract

X-ray imaging apparatus is provided which includes a moveable C-shaped arm (16) with an X-ray source (12) mounted at one end and a linear camera array (14) at the other end. The arm is driven relative to a patient and scans the patient's body with a narrow, fanned beam. A position encoder (150) generates timing signals which are used to synchronise the motion of the arm with imaging circuits, to generate image signals of a relatively large object.

Description

X-RAY IMAGING APPARATUS

BACKGROUND OF THE INVENTION

THIS invention relates to imaging apparatus which can be used, for example, in radiological applications.

Conventional X-ray imaging apparatus is of limited versatility and is generally unsuitable for use in continuous whole-body imaging of patients at a resolution sufficient for diagnostic purposes. South African patent no. 93/8427 describes a system which is designed to facilitate whole-body imaging of a subject in order to detect smuggled articles such as diamonds concealed on the person of the subject, while at the same time minimizing the radiation dose received by the subject.

It is an object of the invention to provide an alternative apparatus which can produce images of diagnostic quality in most conventional procedures at relatively low radiation doses with a range of image sites from small to full- body. SUMMARY OF THE INVENTION

According to the invention there is provided imaging apparatus comprising:

a radiation source for generating an imaging beam;

imaging means for generating signals representing an image of a subject on which the imaging beam is incident;

a support member for supporting the radiation source and the imaging means in a predetermined spatial relationship;

drive means for moving the support member relative to the subject in a first direction; and

motion sensor means for generating a signal related to the speed of movement of the support member for use by the imaging means in generating said signals representing an image.

The motion sensor means may comprise a position encoder arranged to sense the movement of the support member and to generate a clock signal for the imaging means having a frequency related to the speed of movement of the support member.

The position encoder may be a linear encoder arranged to sense relative linear movement between the support means and a frame. The imaging apparatus may include clock signal conditioning means arranged to process the clock signal generated by the position encoder and a circuit responsive to the processed clock signal to generate digital timing signals for synchronising the operation of the imaging means with movement of the support member.

The imaging means may comprise a plurality of cameras arranged end to end and having respective outputs, a front end processor for combining the camera outputs and generating a combined output signal, and a signal processor for generating image data from the combined output signal.

The imaging apparatus may include mounting means for mounting the support member to the drive means so that the support member is rotatable about an axis coinciding with the first direction.

Preferably the radiation source is an X-ray source adapted to generate a beam which is relatively narrow in a first direction of movement of the support member, but relatively broad in a second direction transverse to the first direction of movement, so that the beam scans the full width of the subject in a single pass thereof.

The radiation source may include a light source arranged to generate a visible light beam coincident with the imaging beam, to facilitate alignment of the imaging beam with the portion of the subject to be imaged.

An X-ray translucent mirror may be arranged in the path of the imaging beam, the mirror being oriented to direct light from the light source along a path substantially coincident with X-rays passing through the mirror. A lens may be arranged adjacent the light source to spread light in a fanned beam corresponding in shape to the imaging beam.

The radiation source may include an adjustable shutter comprising at least one shutter member of X-ray opaque material movable in the second direction of movement to adjust the width of the beam.

The shutter preferably comprises a pair of shutter members movable longitudinally in a track.

Preferably, the shutter members are movable independently.

The imaging apparatus may include subject support means comprising a frame for supporting the subject, and locating means for maintaining the frame in a fixed position relative to the support means during operation of the apparatus.

Preferably, the frame defines a trolley and the locating means comprises an electromagnetic clamp means arranged to be activated during operation of the apparatus.

The electromagnetic clamp means may comprise a pair of arms each supporting an electromagnet arranged to hold complemental portions of the trolley.

Preferably, the clamp means includes a proximity switch associated with each electromagnet and arranged to activate the electromagnet only when the trolley approaches the clamping means closely.

The clamp means preferably includes sensors for detecting the position of a support surface of the trolley in use and for generating a signal to which the drive means is responsive to disable the drive means if the support surface is incorrectly located in use.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a pictorial view of imaging apparatus according to the invention;

Figure 2 is an end elevation of the apparatus of Figure 1;

Figure 3 is a similar view to that of Figure 2, showing an alternative application of the apparatus;

Figure 4 is a pictorial view of a radiological installation incorporating the apparatus of the invention;

Figure 5 is a schematic block diagram showing major mechanical components of the apparatus;

Figure 6 is a pictorial view of a trolley of a patient support and location system of the apparatus;

Figure 7 is a partial pictorial view of location and sensing components of the patient support and location apparatus;

Figure 8 is a schematic block diagram of the mechanical arrangement of the main support arm of the apparatus;

Figure 9 is a schematic sectional view of a portion of a roller mechanism of the support arm;

Figure 10 is a schematic sectional view of a portion of a drive mechanism of the support arm;

Figure 11 is a schematic sectional view of a clamping arrangement for the support arm;

Figure 12 is a schematic illustration of a beam width adjuster for the X-ray source;

Figures 13 to 15 are pictorial, end and side views, respectively, of the beam width adjuster mechanism;

Figure 16 is a schematic illustration of the arrangement of the X-ray source; Figure 17 is a schematic diagram of a light source assembly associated with the X-ray source;

Figure 18 is a schematic block diagram of the X-ray detector of the apparatus;

Figures 19a & 19b are schematic illustrations of an individual X-ray camera of the detector of Figure 18; and

Figure 20 is a schematic block diagram of electromc control circuitry of the apparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figures 1 to 3 show three different views of prototype X-ray imaging or scanning apparatus of the invention. The apparatus comprises a head 10 containing an X-ray source 12 which emits a narrow, fanned beam of X- rays towards a detector arrangement 14. The X-ray source 12 and the detector 14 are supported at opposite ends of a curved arm 16 which is generally semi-circular or C-shaped.

A frame 18 mounted on a wall 8 or another fixed structure defines a pair of rails 20 with which a motorised drive mechanism 22 engages to drive the arm linearly back and forth in a first, axial direction of movement. In addition, the drive mechanism comprises a housing 24 in which the arm 16 is movable by the drive mechanism in order to cause the X-ray source and the detector to rotate about an axis parallel with the first direction of movement of the mechanism.

A typical application of the imaging apparatus of the invention is in a radiological installation, such as that illustrated in Figure 4. The imaging apparatus is shown located in a corner of a room which may be a resuscitation area or trauma room of a hospital, for example. Alternatively, the apparatus may be located in a radiological department of a hospital or elsewhere.

Located adjacent to the imaging apparatus is a local positioning console 26, by means of which an operator can set up the required viewing parameters (for example, the angle of the arm 16, start and stop positions, and the width of the area to be X-rayed). A main operator console 28 is provided behind a screen 30 which is used by the operator to set up the required radiographic procedure. The imaging apparatus is operated to perform a scan of a subject 32 supported on a specialized trolley 36 (see below) and a confirmatory image of the radiograph is displayed on a screen at the console 28, in order to allow the operator to judge whether a successful image has been acquired.

A high resolution monitor 34 is provided for diagnostic viewing and is located so that attending clinical staff can study the radiographs being acquired. In addition, a console 38 is provided which forms part of a standard Radiological Information System which permits image viewing and archiving.

The arrangement of Figure 4 is designed for use in the resuscitation room of a trauma unit, in order to provide fast, large area X-ray images of injured patients. Once a patient has been stabilized, he or she can conveniently be placed in position, scanned, and wheeled out for further treatment, with the resulting radiograph appearing on the diagnostic screen virtually instantaneously. Due to the low X-ray dose administered by the apparatus, the risk of radiation exposure to staff and patients is reduced.

The various functional aspects of the apparatus will now be described in greater detail.

In order to exploit the potential of the invention for rapid X-ray imaging, a special trolley 36 with a low X-ray attenuation table top is provided which is height adjustable and which is provided with an electromagnetic locating arrangement to position it relative to the arm 16 of the scanning apparatus during operation.

Figures 6 and 7 illustrate the trolley and the electromagnetic locating arrangement, respectively. The trolley has a C-shaped lower frame with a main frame member 42 and end members 44 and 46. Mounted centrally on the members 44 and 46 are respective upright telescopic pillars 48 and 50 which support the table top 52. A raising/lowering mechanism is provided to allow the table top 52 to be tilted or raised and then clamped in position, and castors 54 allow easy movement of the trolley.

As shown in Figure 7, the locating arrangement comprises a pair of arms 56 and 58 which are mounted to the underside of the main frame 18 of the apparatus. The arms 56 and 58 are mounted so as to be rotatable about a vertical axis X-X and detents are provided to hold the arms in the position illustrated in Figure 7, ie. perpendicular to the direction of scanning. The detent mechanism locates the arms firmly in position, but only a moderate force is required to swivel the arms out of their extended position so that an impact with the trolley or an obstruction between the moving C-arm and the trolley during the scanning operation will cause the arms to swivel, obviating a potential safety hazard.

At the end of each of the arms 56 and 58 is an electromagnetic clamping and locating mechanism, which in each case comprises an electromagnet 60 with a pole piece or contact surface arranged to contact a complemental ferromagnetic plate 122 mounted at the end of each of the trolley frame members 44 and 46.

The locating mechanism on the arm 58 includes horizontal locating means in the form of a pair of angled surfaces 124 and 126 on either side of the electromagnet 60, the end of the trolley frame member 46 being shaped complementally so that when the plate 122 engages the electromagnet 60, the trolley is located axially (ie. in relation to the direction of scan) relative to the frame of the apparatus, while also being held the correct distance away from the apparatus. The locating mechanism at the end of the arm 56 has only a flat locating surface 128 surrounding the electromagnet 60, and serves to locate the other end of the trolley at the correct distance from the scanning apparatus.

Associated with each of the clamping/locating mechanisms is a proximity switch 130 which is operated when the trolley is engaged with the location mechanism. Correct location of the trolley results in a "trolley in position" signal from the proximity switches, so that the control system of the apparatus can automatically activate the electromagnets to hold the trolley in position.

The force exerted by the electromagnets is sufficient to hold the trolley in position in normal use, but is weak enough that a light to moderate force exerted on the trolley or the mechamcal arms 56 and 58 in the direction of scan will allow the magnetic clamps to release. In addition, the electromagnets can be deactivated via the control system.

Located at the end of the arms 56 and 58 are upwardly directed ultrasonic distance sensors 132 which are arranged to sense the position of the table top 52 of the trolley relative to the arms 56 and 58. The output signals of the sensors 132 are utilised to confirm that the table top is in its highest position, which is necessary to ensure safe rotation of the C-arm.

In the prototype system of the invention, vertical adjustment of the trolley table top relative to the scanning apparatus was required, but it will be appreciated that positive vertical adjustment of the scanner frame could be provided instead, or as an optional extra feature.

The C-shaped arm 16 is supported and controlled by drive and support mechanisms illustrated in Figures 10 and 11, and is clamped in position in use by an electromagnetic clamping assembly illustrated in Figure 12. The inter-relationship of these mechanisms is illustrated schematically in the block diagram of Figure 8. As shown in Figure 9, the C-arm 16 is supported slidably within the housing 24 on two sets of rollers 62 and 64. The rollers 62 are mounted non-adjustably on a base plate 66, and run on the inner surfaces of respective flanges 68 formed on the inner edge of the C-arm. The rollers 64 are adjustable to ensure correct tracking of the C- arm within the housing 24 and to adjust the back lash of movement of the arm.

Figure 10 shows the drive mechanism for the C-arm. The drive comprises a motor 70 which drives a pinion gear 72. A segment gear 74 is fixed to the inner edge of the C-arm 16, between the respective flanges 68, so that rotation of the pinion gear 72 moves the C-arm relative to the housing. Instead, a chain drive arrangement could be used.

Figure 11 shows an electromagnetic clamp 76 which is located in a housing 78 bolted to the housing 24, and which is arranged so that it effectively clamps a side face of the C-arm 16 when energized, to hold the C-arm in a desired position.

Once the trolley 36 has been located in position, anterior-posterior and posterior-anterior images can be taken. Should any radial images, including lateral images, be required, the table top 52 is raised to its maximum height. Once the table top is in its upper most position, the C-arm 16 can be rotated to any angle between the anterior-posterior position and the lateral position. Once in position, it is clamped electromagnetically for a scan. The shape of the arm defines a cavity which is sufficiently large to surround the body of a patient supported on the trolley 36.

With the arm 16 clamped or held in position relative to the housing 24 of the drive mechanism and the trolley 36 also located firmly in position, the linear drive mechanism is operated so that the apparatus performs a horizontal linear scan. A narrow fanned beam of X-rays irradiates a thin strip across the width of the trolley table top or patient bed as the X-ray source and the detector move from the starting point to the end point of the scan.

The normal position of operation of the apparatus is with the X-ray source 12 uppermost in order to permit the taking of A-P (anterior to posterior) full body images. The arm 16 can be rotated for different views, up to a lateral view. With the arm rotated through 90°, erect chest views are also possible, as indicated in Figure 3.

In the prototype apparatus, the thickness of the X-ray beam (in the direction of scan) on the detector is determined by a thin slit collimator which is factory preset to less than 10mm. Mounted in front of this slit is a beam width controller that determines the width of the rectangular strip to be exposed to X-rays. This width is adjustable from a minimum of 100mm to the full 680mm field of exposure, in addition to a certain amount of offset from the bed center line.

The X-ray beam width controller 80 shown in Figures 12 to 15 controls the width of the X-ray image as well as its offset from the center through adjustment by the operator. This reduces unnecessary exposure to the patient. It is also useful to limit the beam width to the X-ray camera to avoid saturation. The beam width controller consists of two elongate shutter elements 82 and 84 manufactured from an X-ray opaque material.

The shutter elements 82 and 84 have a T-section and slide in a channel 86 controlled by respective rack and pinion mechanisms. Each rack and pinion mechanism comprises a control wheel 88 connected to a shaft 90 at the end of which is a pinion gear 92. The gear 92 engages a toothed rack at the top edge of the shutter element so that rotation of the handle moves the shutter element linearly from side to side. The shutters can be moved independently, so that both the beam width and its offset relative to the center line of the trolley table top can be controlled.

Referring to Figure 16, the X-ray source consists of an X-ray tube, X-ray shutter, X-ray filter, co-incident light source, collimator and X-ray beam width controller. The X-ray tube 100 generates X-rays and is powered by a high voltage power supply. The tube uses a rotating anode resulting in a higher power rating due to more effective dissipation of heat from the anode. The X-ray shutter 102 prevents exposure of the patient to X-rays in cases when the tube is provided with power but all the operational conditions are not met, for example during power-up or the ramping up of the scanner arm speed. It also acts as a safety interlock to the control system.

The X-ray filter 104 ensures that the spectrum is filtered to the correct "hardness" by removing "soft" or low energy X-rays which would be absorbed by the patient's body and not contribute to the quality of the image. A light source 106 and an X-ray translucent mirror 108 provide a light beam which is the shape of, and coincident with, the X-ray beam. This indicates the X-ray beam's size and position to the operator. An X-ray collimator 110 blocks off the unwanted divergent X-rays which are detrimental to image quality, promote scatter and unnecessarily increase the amount of dose to the patient.

The light source/mirror arrangement of Figure 16 is shown in greater detail in Figure 17. The light source typically comprises a laser diode which produces a single beam of coherent light. Associated with the laser diode is a lens arrangement which spreads the light in a fanned beam through an arc in excess of 34°. The resulting fan of visible light is directed to the mirror 108, which is a fixed X-ray translucent mirror positioned in the X-ray path of the X-ray source 12, so that the visible light beam coincides with the X- ray beam. The distance A from the light source 106 to the mirror 108 is made equal to the distance B from the X-ray source 12 to the mirror 108. By ensuring that the focal spot sizes of the X-ray source and the light source are equal, that the sources are at right angles to one another and that the angle of the mirror 108 is at 45° to the directions of both sources, the visible light fan is made coincident with the X-ray beam. This allows visual confirmation of the position of the X-ray beam in use. The mirror 108 can be made from an appropriate material, such as aluminium, to replace the X- ray filter 104.

The X-ray detector 14 is illustrated schematically in Figure 18 and comprises a plurality of X-ray cameras 114, each with an associated CCD headboard 116, butted together end-to-end to form an elongate linear camera. The outputs of the respective headboards 116 are fed to a front end processor 118 which has a fibre optic data link output 120.

The multiple X-ray cameras are butted together in order to obtain a composite detector which is approximately 700mm wide. The gaps between the cameras are minimized to approximately 50μm in order to reduce the amount of lost information at the joints. Any remaining artifacts in the image are removed during image correction performed in an Image Post Processor. Each X-ray camera 114 detects X-rays which are transmitted through the patient and converts them to an analog electronic output signal. This output signal is amplified by the respective CCD headboard 116 which is mounted directly on the X-ray camera, and the amplified output is routed via a coaxial cable to the front end processor 118, which combines the separate outputs into a single combined output signal for further signal processing.

The arrangement of a single camera is illustrated schematically in Figures 19a and 19b. The camera comprises a scintillator 122, a fibre optic taper bundle 124 and the associated CCD headboard 116.

Charge Coupled Devices (CCD's) are effective at detecting radiation in the visible wavelength range and converting it to an analogue electronic output signal. They are less effective at detecting X-rays and are damaged by X- ray radiation. Scintillators are therefore employed to convert the X-rays to visible light. The scintillators cover the whole area over which the X-rays strike the camera. CCD's are limited in size and are extremely expensive. The image area is therefore covered by multiple CCD's, the total number being determined by a cost vs. resolution trade-off. Fibre optic taper bundles are employed in order to reduce the image size and project the image from the scintillator onto the discrete CCD's.

The CCD's are operated in a Time Delay and Integration mode in order to obtain maximum benefit from the drift scanning operation. The CCD pixels are clocked at a rate which corresponds to, and is synchronised with, the scanning speed of the C-arm 16 (see below). The CCD consists of multiple rows and columns of pixels. Pixel rows are orientated perpendicular to the scanning direction. Electrons generated by light photons are integrated in the pixel well during the time delay between the pixels' phase clocks, and are moved to the next row by the phase clocks. In each subsequent row additional electrons are generated by X-rays and the image exposure period is thereby lengthened. Image smear is prevented by clocking the pixels in accordance with the mechamcal movement, as mentioned above. The signal is therefore integrated over the whole thickness of the X-ray fan beam and substantially no X-ray signal is wasted.

Figure 20 shows the electronic circuitry of the detector 14.

The Front End Processor (FEP) 118 is mounted in the C-arm 16 in close proximity to the CCD headboards. The cables from the CCD headboards are terminated on the FEP into another amplifying/buffering stage. After this stage, a standard Correlated Double Sampling (CDS) procedure is performed on the signal. The CDS is performed making use of digital timing signals that are generated locally on the FEP in a Digital Programmable Block 134 which consists of one or more Field Programmable Gate Arrays (FPGA's) and the required memory. These timing signals are synchronized to the rest of the system by using the C-arm encoder clock 136. The physical movement of the C-arm generates this clock by means of a position encoder 150. The encoder used in the prototype system was a sealed linear encoder of the kind comprising a light source, lens and scanning reticle arranged adjacent a finely graduated grating for relative movement therebetween. In the prototype, encoders manufactured by Heidenhain were used.

A pair of linear gratings, each having a resolution of 20 microns and being shifted relative to one another by 90°, are scanned simultaneously by the LED light source, providing an effective 5 micron resolution. The output of the encoder is the encoder clock signal 136, which is utilised to synchronise the rest of the system.

The C-arm encoder clock 136 needs to be conditioned depending on what format the output from the C-arm movement encoder is. The clock needs to be TTL level compatible. The Condition Clock circuit 138 ensures this.

The gain through the analog path is variable to make provision for the different intensities due to the different dose levels required for different procedures. This gain is controlled by the control signals from the Controller.

After the CDS procedure, the analogue signal is converted to a 14-bit digital bus (containing the image data) through an analogue-to-digital converter. This CDS and A/D converter circuit is repeated for the number of CCD's that are going to be used.

The 14-bit busses are then fed to the Programmable Digital Block. In this block different digital signal processing functions are performed on the image data. These functions include binning of the pixels, etc. The programmable nature of this block makes it possible to add DSP functions and change parameters at a later stage. The image data will also be multiplexed to one 14-bit bus before it is sent to the Fibre Optic Link Interface.

The Programmable Digital Block also generates the CCD drive signals. These signals are level shifted to the appropriate levels to drive the CCD. The CCD operating voltages are generated in an external power supply. The CCD drive signals and operating voltages are then fed to the CCD headboard through a suitable cable.

On the PCB there is also a connector 144 that interfaces to the power supply unit. This provides for all the power needs of the circuit.

A controller 142 controls the different modes of operation of the FEP and performs built in test purposes. It will monitor voltage levels and the basic functions on the PCB. The status thereof is reported back to the user interface through a RS232 cable 146. It also controls the different configurations of the Programmable Digital Block 134. This is done by configuring the FPGA's differently each time a different mode of operation is required. This is necessary because of the different scanning speeds and to perform alignment correction on the data. A feedback signal 140 from the X-ray beam width control and the shutter is also fed to the controller 142. The controller 142 is implemented using a microprocessor.

The X-ray intensity feedback is converted to a digital signal. This signal is fed to a digital signal processing (DSP) block, where it is processed. The processed data can be used in the Programmable Digital Block 134 and the Image Pre-Processor to correct for the variation in the X-ray intensity.

An oscillator 148 provides an oscillator frequency which is a clock frequency which is very stable over a time period of one scan. The clock signal 136 from the C-arm encoder is then compared to this frequency. The result of the comparison indicates the variation in speed from the C-arm. This result can then be used to correct for the uneven illumination caused by the variation in speed. The oscillator frequency is also used to clock the electronic circuits when the C-arm is not moving.

Claims

CLAIMS:

1. Imaging apparatus comprising:

a radiation source for generating an imaging beam;

2. Imaging apparatus according to claim 1 wherein the motion sensor means comprises a position encoder arranged to sense the movement of the support member and to generate a clock signal for the imaging means having a frequency related to the speed of movement of the support member.

3. Imaging apparatus according to claim 2 wherein the position encoder is a linear encoder arranged to sense relative linear movement between the support means and a frame.

4. Imaging apparatus according to claim 2 or claim 3 including clock signal conditioning means arranged to process the clock signal generated by the position encoder and a circuit responsive to the processed clock signal to generate digital timing signals for synchronising the operation of the imaging means with movement of the support member.

5. Imaging apparatus according to any one of claims 1 to 4 wherein the imaging means comprises a plurality of cameras arranged end to end and having respective outputs, a front end processor for combining the camera outputs and generating a combined output signal, and a signal processor for generating image data from the combined output signal.

6. Imaging apparatus according to any one of claims 1 to 5 including mounting means for mounting the support member to the drive means so that the support member is rotatable about an axis coinciding with the first direction.

7. Imaging apparatus according to any one of claims 1 to 6 wherein the radiation source is an X-ray source adapted to generate a beam which is relatively narrow in a first direction of movement of the support member, but relatively broad in a second direction transverse to the first direction of movement, so that the beam scans the full width of the subject in a single pass thereof.

8. Imaging apparatus according to claim 7 wherein the radiation source includes a light source arranged to generate a visible light beam coincident with the imaging beam, to facilitate alignment of the imaging beam with the portion of the subject to be imaged.

9. Imaging apparatus according to claim 8 wherein an X-ray translucent mirror is arranged in the path of the imaging beam, the mirror being oriented to direct light from the light source along a path substantially coincident with X-rays passing through the mirror.

10. Imaging apparatus according to claim 9 wherein a lens is arranged adjacent the light source to spread light in a fanned beam corresponding in shape to the imaging beam.

11. Imaging apparatus according to any one of claims 7 to 10 wherein the radiation source includes an adjustable shutter comprising at least one shutter member of X-ray opaque material movable in the second direction of movement to adjust the width of the beam.

12. Imaging apparatus according to claim 11 wherein the shutter comprises a pair of elongate shutter members movable longitudinally in a track.

13. Imaging apparatus according to claim 12 wherein the shutter members are movable independently.

14. Imaging apparatus according to any one of claims 1 to 13 including subject support means comprising a frame for supporting the subject, and locating means for mamtaining the frame in a fixed position relative to the support means during operation of the apparatus.

15. Imaging apparatus according to claim 14 wherein the frame defines a trolley and the locating means comprises an electromagnetic clamp means arranged to be activated during operation of the apparatus.

16. Imaging apparatus according to claim 15 wherein the electromagnetic clamp means comprises a pair of arms each supporting an electromagnet arranged to hold complemental portions of the trolley.

17. Imaging apparatus according to claim 16 wherein the clamp means includes a proximity switch associated with each electromagnet and arranged to activate the electromagnet only when the trolley approaches the clamping means closely.

18. Imaging apparatus according to claim 16 or claim 17 wherein the clamp means includes sensors for detecting the position of a support surface of the trolley in use and for generating a signal to which the drive means is responsive to disable the drive means if the support surface is incorrectly located in use.