WO2008059263A2

WO2008059263A2 - Mri compatible manipulators

Info

Publication number: WO2008059263A2
Application number: PCT/GB2007/004376
Authority: WO
Inventors: Michael Ulrich Lamperth; Ian Robert Young; Haytham Elhawary; Tsz Ho Tse
Original assignee: Imperial Innovations Limited
Priority date: 2006-11-15
Filing date: 2007-11-15
Publication date: 2008-05-22
Also published as: EP2101671A2; WO2008059263A3; GB0622724D0

Abstract

A manipulator for moving a device (64) comprises a series of stages (10), (10a, 10c) including a first stage (10) and a final stage (10c). Each stage comprises a slider unit (12, 12a, 12c) and a motor unit (14, 14a, 14c) including a motor arranged to move the motor unit relative to the slider unit. The manipulator further comprises mounting means (61) arranged to mount one unit of the first stage (10) on a support, and attachment (means 65) arranged to attach the device (64) to one unit of the final stage (10c), wherein each stage is connected to the next in the series by connection of one unit of one of them to one unit of the other.

Description

MRI COMPATIBLE MANIPULATORS

The present invention relates to manipulators and in particular to manipulators arranged to move surgical or other instruments inside a magnetic resonance imaging (MRI) scanner.

The good soft tissue contrast of MRI makes it a very sensitive and powerful diagnostic technique. The extensive advantages and capabilities υffered by this imaging modalily are well known as are the restrictions imposed on the MRI environment by the magnetic fields of various frequencies and magnitudes which are required to obtain an image. Due to these interactions, MR safety and compatibility must be verified before any device can be introduced into the MR scanner room.

Closed cylindrical MRI scanners are often used for obtaining quality images of the patient anatomy, as they have high field strengths (typically 1-3T) . The diameter of the bore is usually around 600mm in diameter, which must contain both the patient and the scanner table. In addition, the length of most scanner bores is over 150cm, meaning that access to the part of the patient anatomy that is being scanned (which is generally located around the magnet isocentre) on behalf of the practitioner is especially difficult. These spatial constraints make free hand image guided interventions in a closed bore scanner much more challenging. A number of MRI compatible manipulators have been developed with the objective of overcoming these difficulties. Most of these mechatronic devices are designed to perform a specific procedure on a particular part of the patient, and cannot be used to perform other interventions.

Considerable progress is being made in the development of MR compatible manipulators designed to perform image guided interventions.

Various systems for prostate interventions have been reported as well as an in-bore breast biopsy system, [ Larson, B. T., Erdman, A. G. , Tsekos, N. V. , Yacoub, E. , Tsekos, P. V. , and Koutlas, I. G. Design of an MRI- compatible robotic stereotactic device for minimally invasive interventions in the breast. Trans. ASME, J. Biomech. Engng, 2004, 126(4) , 458- 465.] a manipulator for MR-guided neurosurgical interventions [Masamune, K. , Kobayashi, E. , Masutani, Y. , Suzuki, M. , Dohi, T. , Iseki, H. , et al. Development of an MRI-compatible needle insertion manipulator for stereotactic neurosurgery. J. Image Guided Surg. , 1995, 1(4) , 242-248.] and systems for liver surgery [Hata N, Ohara F, Hashimoto R, Hashizume M, Dohi T. Needle Guiding Robot with Five-Bar Linkage for MR-Guided Thermotherapy of Liver Tumor. In: Lecture Notes in Computer Science; 2004 2004/01//. pp. 161-168] . The majority of the reported devices use piezoceramic motors as an alternative to electromagnetic (EM) actuators. Although the piezoceramic elements of the motors are not affected by the magnetic field of the scanner, they are often embedded inside conductive non-magnetic materials which can result in image artifacts. The electric circuits of the motor drivers can introduce noise in the image, severely degrading the Signal to Noise Ratio (SNR) while the motors are in motion.

The piezoceramic motor most used for actuation is the USR-30 model from the Shinsei Corporation Inc. , Japan. The manipulators that incorporate this technology have generally followed one of two strategies. (i) The first consists in placing the motors at a distance from the imaging -volume in the scanner, and transferring the motion to the actuated elements via a transmission mechanism, using cables, rods or rigid links. This separation between field of view and motors is in order to avoid an excessive degradation of the SNR of the MR images. A separation of approximately Im usually proves enough distance to prevent the motors having any impact on the images, allowing for actuation while imaging is taking place. (U) The second strategy consists of introducing the motors inside the scanner bore, in direct contact with the actuated elements. In this case the motor has to be separated from the imaged region of interest by at least the size of the artifact it produces. In addition, the noise level of the scans is extremely high requiring the motors to be turned off while scanning and data acquisition is taking place. This hinders real-time image guidance as imaging and motion cannot be simultaneous.

A recent trend for the development of MRI compatible manipulators that can be used for a variety of surgical interventions can be observed. Chinzei et al ("Surgical Assist Robot for the Active Navigation in the Intraoperative MRI: Hardware Design Issues, " proc IEEE/RSJ IROS 2000, Oct 30-Nov 3, Takamatsu, Japan, pp. 727-32, 2000.) developed an MRI compatible "surgical assist" robot, capable of positioning an axisymmetric tool or pointer in an open MRI scanner. Tsekos et al ( "A Prototype Manipulator for Magnetic Resonance-Guided Interventions Inside Standard Cylindrical Magnetic Resonance Imaging Scanners" in J. Biomech Eng. , Volumel27, Issue 6, pp. 972-980, 2005. ) developed a seven degrees of freedom (DOF) manipulator dedicated to positioning an interventional tool inside a closed bore scanner. It consists of a 3 DOF Cartesian base located outside of the scanner bore, which houses all the mechatronic assemblies including the motors, and a 4 DOF arm that is introduced into the gantry on top of the patient. Hempel et al ("An MRI- compatible surgical robot for precise radiological interventions, " in Computer Aided Surgery, pp. 180-191 , Apr. 2003.) also present another mechatronic device for numerous radiological interventions, such as periradicular therapy. It also consists of an arm structure located on top of the patient inside the bore of the scanner. Piezoceramic motors are combined with pneumatic cylinders to position a needle inside the required anatomy. These three systems are capable of applying the same manipulator to a number of different procedures, making them more versatile.

The present invention provides a manipulator for moving a device. The manipulator may comprise a series of stages including a first stage and a final stage. Each stage may comprise a slider unit and a motor unit including a motor arranged to move the motor unit relative to the slider unit. The manipulator may further comprise mounting means arranged to mount one unit of the first stage on a support, and attachment means arranged to attach the device to one unit of the final stage. Each stage may be connected to the next in the series by connection of one unit of one of them to one unit of the other.

The present invention further provides a manipulator system comprising a manipulator according to the invention arranged to operate as a slave. The system may further comprise an input member arranged to be operated by a user to control the slave. The system may comprise a master motor arranged to provide haptic feedback to the input member. The system may further comprise control means arranged to control movement of the manipulator and operation of the master motor.

The present invention further provides a linear actuator comprising a slider unit and a motor unit including a motor arranged to move the motor unit relative to the slider unit, wherein the slider unit has a plurality of bearing surfaces thereon arranged to locate the motor unit relative to the slider unit, and the motor comprises a plurality of piezoelectric drive members and control means arranged to control the electrical potentials applied to the drive members to cause them to move the motor unit relative to the slider unit. The drive members may comprise drive legs. The present invention further provides an actuation system comprising a piezoelectric motor, a motor driver arranged to apply electric drive voltage to the motor, sensing means for sensing movement of the motor, and control means which includes a model of the motor and which is arranged to monitor the drive voltage and movement of the motor and to determine therefrom the load on the motor using the model.

The present invention still further provides an actuation system comprising a piezoelectric motor, a motor driver arranged to apply electric driving frequency current to the motor, sensing means for sensing movement of the motor, and control means which includes a model of the motor and which is arranged to monitor the desired velocity, and one or more parameters, for example a load condition on the motor or parameters of the drive current or voltage applied to the motor, and to determine therefrom at least one of the driving current, voltage, and frequency to be applied to the motor.

The present invention further provides a manipulator system comprising a manipulator according to the invention, an input member arranged to be operated by a user to control the manipulator, a master motor arranged to provide haptic feedback to the input member, and control means arranged to control movement of the manipulator and operation of the master motor. The manipulator may have one, two, or three or more degrees of freedom, and may be, for example, a needle firing mechanism or a robot for locating a device or system.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a partly cut away perspective view of a manipulator stage according to an embodiment of the invention;

Figure 2 is an enlargement of part of Figure 1;

Figure 3 is a diagram of the operating principle of a piezoceramic motor forming part of the manipulator stage of Figure 1;

Figure 4 is a perspective view of a manipulator including the stage of Figure 1 ;

Figure 5 is a plan view of a manipulator stage according to a further embodiment of the invention;

Figure 6 is a plan view of a manipulator including the stage of Figure 5;

Figure 7 is a perspective view of a manipulator for positioning of an endorectal probe according to a further embodiment of the invention;

Figure 8 is a schematic diagram of a scanner system including the manipulator of Figure 7;

Figure 9 is a block diagram of a control system forming part of the system of Figure 8 ;

Figure 10 is a block diagram of a speed controller forming part of the control system of Figure 9; and

Figue 11 is a diagram of a neural network forming part of the control system of Figure 8. Referring to Figures 1 and 2, a manipulator stage 10, arranged to form part of a manipulator, comprises a slider unit 12 and a motor unit 14. The slider unit 12 comprises an elongate main body 16 of constant cross section along its length with a mounting plate 18 at one end and an end stop 20 at the other. The main body 16, mounting plate 18 and end stop 20 are all made of acrylonitrile butadiene styrene (ABS) . The main body 16 is substantially rectangular in cross section having four sides 22a, 22b, 22c, 22d. One pair of opposite sides 22a, 22b is wider than the other pair 22c, 22d. Each side has a pair of flat smooth bearing surfaces 24a, 24b extending along it, each close to a respective one of the side's edges 26a, 26b where it meets one of the other sides, with a central channel 28 defined between the bearing surfaces 24a, 24b. The bearing surfaces 24a, 24b on the two wider sides 22a, 22b face away from each other in opposite directions, and the bearing surfaces 24a, 24b on the two narrower sides 22c, 22d face away from each other in opposite directions. In this embodiment the length of the slider unit is 130mm, the width 25mm and the height 20mm.

A ceramic rod 30 of rectangular cross section extends along the centre of the central channel 28 of each of the wider sides 22a, 22b. The outer surface 32 of each of these rods is flat, and the two outer surfaces 32 are parallel with each other facing in opposite directions, and aligned with each other and with the central longitudinal axis of the slider unit. A position encoding strip 34 extends along the bottom of the central channel 28 of one of the narrower sides 22c, and has a series of optically reflective markings spaced along it at predetermined intervals .

The motor unit 14 is formed from four separate blocks comprising two parallel end plates 40, 42, and two parallel side plates 44, 46, all formed of (ABS) the side plates being secured between the ends of the end plates by means of securing bolts (not shown) so as to define a generally rectangular cross section aperture arranged to receive, and fit around, the slider unit 12. Each of the end plates 40, 42 and side plates 44, 46 has a pair of parallel, inwardly facing bearing surfaces 48 thereon, which are arranged to locate against the bearing surfaces 24a, 24b on the slider unit so as to locate the motor unit 14 in all directions perpendicular to the slider unit axis, and allow the slider motor unit 14 to slide along the slider unit 12. Although not included in this embodiment, rollers or bearings could be provided to reduce the friction between the motor unit 14 and the slider unit 12.

A piezo-ceramic motor 50 is mounted in each of the end plates 40, 42 of the motor unit 14. Referring to Figure 3 each motor 50 comprises a set of four parallel legs 52 each comprising two blocks 54 of piezoelectric ceramic material. Each block 54 of each leg is connected to an electrical supply arranged to apply a variable drive voltage to it, and each block 54 can be extended or retracted depending on the electrical potential applied to, it. This allows each leg to be extended, shortened, or bent in either direction by applying the same or different potentials to the two blocks of which it is comprised. The motors 50 are arranged such that, when the legs 52 are extended they contact the top drive surface 32 of the ceramic rod 30 over which they are located, and when retracted they are lifted clear of that surface. The drive signals to the blocks 54 can be coordinated so that the legs 52 bend in one direction while extended and in contact with the drive surface 32 and bend back in the opposite direction when retracted. This causes the legs 52 to 'walk' along the surface 32 thereby moving the motor unit 14 along the slider unit 12. The speed of the motor is dependent on the phase difference, voltage amplitude and frequency of the drive signals to the motor, as well as the load applied to the motor. Typically the phase and voltage amplitude are kept constant and the frequency controlled to control the speed. Because the two motors 50 are arranged opposite each other, the forces between the drive legs 52 of the two motors 50 and their respective ceramic rods are nominally equal and opposite and substantially balance each other out. An example of a suitable motor is a Piezo LEGS (TM) motor which can produce a force of around 9N, a top speed of around 12.5mm/s and a spatial resolution of 2nm.

Referring back to Figure 2 an optical encoder 60 is mounted in one of the side plates 44 in a position so that it is located over the encoding strip 34. The encoder is arranged to detect the variations in reflectivity of the encoding strip 34 as the motor unit 14 moves along the slider unit 12 and produce an output signal that varies with the position of the motor unit 14 relative to the slider unit 12. An example of a suitable encoder is a small surface mount incremental optical encoder, such as the Agilent Technologies AEDR-8000 series. These have a size of 6.2 x 4.4 x 3.2 mm and are sufficiently small so that the magnetic material they contain only produces a small artifact. Such encoders can be used with an encoder strip of up to 180 lines per inch in quadrature giving a maximum resolution of 36 microns. End detection switches 62 are also optionally provided on both ends of the side plate 44 and are arranged to close when they contact the mounting plate 18 or the end stop 20 respectively when the motor unit 14 reaches the respective ends of its travel along the slider unit 12. These end detection switches therefore enable detection of when the motor unit 14 is in either of two reference positions, at either end of its travel, relative to the slider unit 12.

Referring to Figure 4, a number of the manipulator stages 10, which together form a modular system, can be assembled together to produce a complete manipulator with several degrees of freedom. The mounting plate 18 at one end of the slider unit 12 is arranged to be the same size and shape as one of the side plates 44 of the motor unit, and to include threaded bores in the same position as those in the side plate 44 to allow it to be bolted to the end plates 40, 42 in place of one of the side plates 44. Therefore, in order to assemble two of the stages together, one of the side plates 44 of the motor unit 14 is removed and the mounting plate 18a of the slider unit 12a of a second stage 10a is secured to the end plates 40, 42 in its place, so that the slider unit 12a of the second stage 10a is attached to the motor unit 14 of the first stage 10. This means that, if the slider unit 12 of the first stage 10 is used as a reference, the motor unit 14a of the second stage 10a can be moved in a first direction by moving itself along the slider unit 12a of the second stage, and can be moved in a second, perpendicular direction, by moving the motor unit 14 of the first stage along the slider unit 12 of the first stage. This therefore produces a manipulator with two degrees of freedom. It will be appreciated that further stages can be added to increase the number of degrees of freedom.

Referring to Figure 5, a further embodiment of a manipulator stage 10b is identical to the first embodiment 10, except that the slider unit 12b is curved with a degree of curvature which is constant along its length, and the internal features of the motor unit 14b are also curved with the same degree of curvature so that it fits onto the slider unit 12b and can slide along it in a curved path. Clearly the bearing surfaces and the ceramic rod of the slider unit will be curved, and the motor unit will have to include bearing surfaces which are adapted in shape to fit the curved slider unit 12b. Such a curved manipulator stage can be combined with others of the same or different degrees of curvature or with straight stages to provide manipulators with a very wide variety of movement. For example, referring to Figure 6, in order to achieve this the mounting plate 18b, and the side plates 44b, 46b of the motor unit 14b are designed to fit in place of those of the first embodiment, so that the curved manipulator stage 10b can be mounted on the motor unit of the first straight manipulator unit 10. In a similar manner, further manipulator stages of different lengths and different degrees of curvature can be provided, which can be assembled together in • a modular manner to provide manipulators for a wide variety of applications.

Referring to Figure 7, in order to construct a practical manipulator two end stops 61 with integral mounting legs 61a are attached to the ends of tho slider unit 12 in place of the mounting plate 18 and end stop 20. This enables the slider unit 12 to be securely mounted to a support structure, in this case an MRI scanner bed. The second stage 10a is attached to the first stage 10 as shown in Figure 4. A third stage 10c is then attached to the motor unit 14a of the second stage in the same manner. This provides a Cartesian robot 63 with three degrees of freedom in which the motor unit 14c of the third stage can be moved in three dimensions. A passive gimbal mechanism 65 is mounted on the motor unit 14c of the third stage and is arranged to support a surgical instrument or probe, for example an endorectal probe and biopsy needle 64. The gimbal mechanism 65 comprises a base 66 which is arranged to replace one of the side plates of the motor unit 14c, a first yoke 68 rotatably mounted on the base, and a second yoke 70 pivotably mounted on the first yoke. The probe 64 is supported in the second yoke 70. The probe 64 can be inserted into the rectum of a patient and the pitch and yaw angles of the probe 64 controlled by moving the three stages of the robot to pivot the probe about the anus. Translation of the probe 64 in and out of the rectum can also be controlled by the robot 63. For biopsy procedures, a needle firing mechanism (not shown) would be mounted onto the probe 64, which contains the needle, to control needle firing depth and firing.

The needle firing mechanism can include a force sensor arranged to measure the force with which the needle is being inserted into the target tissue. This can be used to provide a haptic control system in which the force experienced by the needle is used to control a force applied by an actuator to the input member that is controlled by the surgeon.

In an alternative embodiment the needle firing mechanism may be a haptic system, and may include a master with a user input member arranged to allow user input, a piezoceramic actuator arranged to drive the needle in a firing direction, and a control system arranged to control the needle firing mechanism. This system can be the same as that in the three-stage manipulator as described above with reference to Figures 9 and 10.

Referring to Figure 8, the robot 63 is arranged for use in an MRI scanner 80 which is located in a scanner room 82. The scanner room is formed from walls 83 with RF shielding to shield the scanner from stray RF fields. The scanner 80 comprises a number of magnetic coils which are located around a scanner bore 88 which is arranged to receive a patient 89 and in which they can vary the magnetic field during MRI scanning. The robot 63, including its piezoceramic motors, is located within the scanner bore 88 where it will be subject to the magnetic fields generated by the scanner and also be within the volume imaged by the scanner.

A Faraday cage in the form of an aluminium shielding enclosure 92 is located in the scanner room 82, and a controller 94, motor driver 96 and batteries 98 located within the shielding enclosure 92. The controller 94 is arranged to control the motor driver 96 which generates drive signals for the motors 90 of the robot 63. Low pass filters 100 are provided to filter the drive signals from the motor driver. Drive cables 102 are arranged to carry drive signals from the filters 100 to the motors 90 and extend from the shielding enclosure 92 and into the scanner bore 88 where they are connected to the motors. The cables 102 are shielded twisted pair cables and are less than 3m in length. The manipulator 63 is controlled remotely from a control room 110 which is outside the scanner room 82. A control input, or master 112, which is arranged be moved by the surgeon to control the manipulator, is connected to a PC 114, which is also in the control room. The PC 114 provides control signals which are converted to optical signals by an optical converter 116 connected to the PC 114. The optical converter 116 is connected via an optical cable 118 to a further optical converter 120 in the scanner room 82 which converts the signals back to electrical signals for input to the controller 94.

Referring to Figure 9, the control system for the manipulator will now be described in more detail. For the sake of simplicity only the control of one of the motors 50 will be described, and all forces and movement are considered to be in one direction, the x direction. It will be appreciated that all three of the motors would in fact be controlled in corresponding manner.

For non-backdrivable actuators such as the piezoceramic motors 50, admittance control is applied in a master slave system as shown in Figure

9. This provides force control in the master using a motor to provide the required force feedback to the user, and position control in the slave using a motor to drive the manipulator to the required position. An input member 200 is connected to a force sensor 202 which measures the force F_H applied to the input member 200 by the user. A further force sensor

204 is arranged to measure the force F_τ applied to the tissue 206 by the probe. The difference between these forces F_H and F_τ is input to a PID controller 208 which determines from them a desired velocity x_u. This desired velocity x_u is input to a speed controller 210 which computes the frequency f_M of the driving current needed to produce the desired speed in the master motor 212 under load F_H. The speed controller consists of an artificial neural network (ANN) model 220 of the speed-frequency relationship of the piezoceramic motor under varying load conditions F as shown in Figure 10. The driving frequency to the motor in the master is computed by the speed controller for the desired velocity under the load it is experiencing. One way of achieving this is that the force F in the actuator is measured by the monitoring the drive current to the actuator and monitoring the speed of movement of the actuator using the optical encoder 60. The force in actuator is estimated from these parameters. This avoids the need for a separate sensor to measure the force within the actuator. Another way is for a model of the motor to define the drive frequency that is required by the motor to achieve a desired velocity given one or more measured parameters, such as the motor speed and the drive voltage or current amplitude and frequency. This bypasses the need for a measurement of the load, but allows accurate control which takes into account variations in load that can occur and that can affect operation of the motor.

The piezoceramic motor is deemed as a black box whilst its performance can be effectively predicted provided that sufficient amount of data sets are taken for neuron training and suitable number of layers and neurons are adapted. As with nervous systems in human body, neurons are the basic element in ANN and work in parallel in a layer. The neurons are linked to pervious layer with weighted connections W. As the velocity of the motor is directly related to the phase difference, voltage amplitude and frequency of the input signal and also the load applied to the motor, the mathematical relation can be presented in equation (1) .

x = G (φ Vf F) (1)

It is desirable to know the relationship of the frequency of input signal required to go to the piezoceramic motor for a desire velocity of x Hence frequency / is set to be the subject of the equation. For simplicity of the ANN, constant voltage amplitude and phase are considered and we have

/ = G₂(x , F) (2)

The neural network is shown in Figure 11 and has corresponding weights W and transfer function F. A backpropagation algorithm is used to train the neurons to achieve each value of weight W in the network. The convergence rate and performance index of the backpropagation algorithm can be optimised by choosing a suitable number of neurons and layers in the network. Considering the nonlinearites of the motor, 200 neurons and 2 layers are adapted with two transfer functions 'logsigmoid' and 'purelin' and 1000 epochs has been computed in the backpropagation algorithm.

For position control in the slave, the displacement of the master is used to generate a setpoint for the slave system and the difference between the master displacement x_M and the slave displacement x_s is input to the slave speed controller 214 which calculates a suitable drive frequency f_s for the slave motor 216 to achieve the desired position.

Because the robot 63 is made up of stages built almost entirely of nonconducting material, including the piezo-ceramic motors, it has very little effect on the magnetic fields in the scanner bore. Filtering of noise from the electronics means that it does not significantly affect the signal to noise ratio of the scanner image even while the motors are running. The system can therefore be used for real time scanning while the robot 63 is moving the probe 64.

An alternative arrangement to that of Figure 8 is to locate the controller and motor driver and batteries outside the scanner room, but this requires significantly longer cables which have a detrimental effect on the SNR of the images produced.

The described embodiments of the present invention can provide generic MRI compatible hardware. The hardware consists of one DOF stages, each including actuation and position sensing technology for implementation of closed loop position control. They can be designed to be located very close to the scanner field of view. The stages act as building blocks of a modular system which can be inter-connected to produce multi-DOF structures and their dimensions can be varied to tailor for the specific intervention they will be used for. This speeds up development time, especially when used in conjunction with rapid prototyping (RP) manufacturing techniques .

Claims

1. A manipulator for moving a device, the manipulator comprising a series of stages including a first stage and a final stage, each stage comprising a slider unit and a motor unit including a motor arranged to move the motor unit relative to the slider unit, the manipulator further comprising mounting means arranged to mount one unit of the first stage on a support, and attachment means arranged to attach the device to one unit of the final stage, wherein each stage is connected to the next in the series by connection of one unit of one of them to one unit of the other.

2. A manipulator according to claim 1 wherein each of the motors is a piezoelectric motor.

3. A manipulator according to claim 2 wherein each of the motors comprises a plurality of piezoelectric drive legs and control means arranged to control the electrical potentials applied to the drive legs to cause them to move the motor unit relative to the slider unit.

4. A manipulator according to claim 3 wherein each of the slider units includes a drive surface extending along it, which the drive legs are arranged to contact to move the motor unit relative to the slider unit.

5. A manipulator according to claim 4 wherein each of the slider units has two bearing surfaces extending along it on opposite sides of the drive surface and arranged to locate the motor unit relative to the slider unit.

6. A manipulator according to any of claims 3 to 5 wherein the motor unit of one of the stages comprises a further motor so that it comprises two motors, the two motors being located on opposite sides of the slider unit of said one of the stages.

7. A manipulator according to any foregoing claim further comprising position determining means arranged to determine the position of each motor unit relative to its respective slider unit thereby to determine the position of the device.

8. A manipulator according to claim 7 wherein the position determining means comprises position encoding markings spaced along the slider unit and sensing means mounted on the motor unit and arranged to sense the position encoding markings thereby to determine the position of the base unit.

9. A manipulator according to claim 7 or claim 8 wherein the position determining means comprises reference position detection means arranged to detect when the motor unit is in a reference position relative to the slider unit.

10. A manipulator according to any foregoing claim wherein one of the motor units comprises a plurality of sections and fastening means arranged to secure the sections together so that the motor unit can be assembled to define an aperture therethrough, into which the slider unit is arranged to fit.

11. A manipulator according to claim 10 wherein the sections of the motor units are formed of electrically non-conductive material.

12. A manipulator according to any foregoing claim wherein the slider unit comprises a main slider body formed of electrically non-conductive material.

13. A manipulator according to any foregoing claim wherein the series of stages comprises three stages so that the device can be moved in three dimensions.

14. A manipulator system comprising a manipulator according to any foregoing claim, an input member arranged to be operated by a user to control the manipulator, a master motor arranged to provide haptic feedback to the input member, and control means arranged to control movement of the manipulator and operation of the master motor.

15. A manipulator system according to claim 14 wherein the control means is arranged to control one of the motors by: monitoring the electrical drive current to the motor and the speed of the motor, and estimating therefrom the load on the motor, thereby to determine a parameter of the drive current to be applied to the motor.

16. A manipulator system according to claim 15 wherein the control means includes a model of the motor from which the load on the motor is estimated.

17. A linear actuator comprising a slider unit and a motor unit including a motor arranged to move the motor unit relative to the slider unit, wherein the slider unit has a plurality of bearing surfaces thereon arranged to locate the motor unit relative to the slider unit, and the motor comprises a plurality of piezoelectric drive members and control means arranged to control the electrical potentials applied to the drive members to cause them to move the motor unit relative to the slider unit.

18. An MRI scanning system comprising: an MRI scanner defining a scanner volume arranged to receive a subject and including a plurality of magnets arranged to generate a variable magnetic field and at least one detector arranged to detect radiation emitted from the subject; a manipulator according to any of claims 1 to 13 located within the scanner volume and arranged to manipulate a device within the scanner volume.

19. A system according to claim 18 further comprising a motor driver arranged to generate drive signals to drive the motors, wherein the motor driver is located outside the scanner volume and the motor driver is connected to the motors by drive cabling.

20. A system according to claim 19 further comprising shielding means defining a scanner room, wherein the scanner is located within the scanner room, and the motor driver is enclosed within a Faraday cage which is also located within the scanner room.

21. A system according to claim 20 further comprising a low pass filter arranged to filter the signals on the drive cabling.

22. A system according to any of claims 18 to 21 further comprising an input member arranged to be operated by a user to control the manipulator, a master motor arranged to provide haptic feedback to the input member, and control means arranged to control movement of the manipulator and operation of the master motor, wherein the input member and master motor are located outside the scanner volume.

23. A system according to claim 22 when dependent on claim 20 wherein the input member and master motor are located outside the scanner room.

24. An actuation system comprising a piezoelectric motor, a motor driver arranged to apply electric drive current to the motor, sensing means for sensing movement of the motor, and control means which includes a model of the motor and which is arranged to monitor the drive current and movement of the motor and to determine therefrom the load on the motor using the model.

25. A manipulator stage comprising a slider unit and a motor unit including a motor and arranged to slide relative to the slider unit wherein the slider unit includes mounting means on one end and the motor unit includes attachment means, wherein the mounting means and the attachment means are arranged to be mutually compatible so that the manipulator stage can be connected to a further manipulator stage having corresponding mounting means or attachment means to form a manipulator having multiple degrees of freedom.

26. A set of manipulator stages each comprising a slider unit and a motor unit including a motor and arranged to slide relative to the slider unit wherein the slider unit includes mounting means and the motor unit includes attachment means, wherein the mounting means and the attachment means are arranged to be mutually compatible so that the manipulator stages can be connected together to form a manipulator having multiple degrees of freedom.

27. A set of manipulator stages according to claim 26 w herein the slider units of at least some of the stages have different degrees of curvature.

28. A manipulator substantially as hereinbefore described with reference to any one or more of the accompanying drawings.

29. A linear actuator substantially as hereinbefore described with reference to any one or more of the accompanying drawings.

30. An MRI scanning system substantially as hereinbefore described with reference to any one or more of the accompanying drawings.

31. A manipulator stage according to any one or more of the accompanying drawings.