TORQUE REACTION IN ROTATING MEDICAL APPARATUS

Abstract

A system and method for compensating for torque reaction forces in a medical, apparatus is disclosed. The astern may include a rotary element which rotates about an axis. A contra-rotating flywheel may be driven to rotate about the axis relative to the rotary element, wherein the flywheel is free to rotate and is accelerated and decelerated by a driver which is fixed to the rotary element. Contra-rotation of the flywheel may compensate for torque reaction forces when the rotary element is accelerated or decelerated.

Claims

1. A system for use with a medical apparatus having a rotary element which, rotates about an axis, the system comprising: a contra-rotating flywheel which is driven to rotate about the axis relative to the rotary element, wherein the flywheel is free to rotate and is accelerated and decelerated by a driver which is fixed to the rotary element.

2. The system according to claim 1, wherein the flywheel is located within an axial length of the rotary element.

3. The system according to claim 1, wherein the rotary element is cylindrical.

4. The system according to claim 3, wherein the flywheel has an outer circumference larger in diameter than an internal diameter of the cylindrical rotary element.

5. The system according to claim 4, wherein the outer circumference of the flywheel is smaller than an external diameter of the cylindrical rotary element.

6. The system according to claim 1, wherein the rotary element is a C-arm apparatus.

7. The system according to claim 1, further comprising one or more motors for driving the flywheel relative to the rotary element.

8. The system according to claim 1, wherein the flywheel comprises a rotor of an electric motor and the rotary element comprises a stator of the electric motor.

9. The system according to claim 1, further comprising one or more brakes configured to selectively decelerate the rotation of the flywheel relative to the rotary element.

10. The system according to claim 1, wherein the flywheel has a mass a moment of inertia less than that of the rotary element.

11. The system according to claim 1, further comprising a processor for controlling the rotation of the rotary element and the contra-rotation of the flywheel.

12. (canceled)

13. A method of compensating for torque reaction forces in a medical apparatus comprising a rotary element which rotates around an axis, the method comprising: driving a contra-rotating flywheel relative to the rotary element, wherein the flywheel rotates around the axis in the opposite direction of the rotary element, and wherein the flywheel is free to rotate and is accelerated and decelerated by a driver which is fixed to the rotary element.

14. The method according to claim 13, wherein the flywheel is driven to rotate at a different angular velocity than the rotary element.

15. The method according to claim 14, wherein the flywheel is driven to rotate at a higher angular velocity than the rotary element.

16. (canceled)

17. (canceled)

18. The system according to claim 1, wherein the medical apparatus is one of a medical treatment apparatus, an imaging apparatus, or a dental apparatus.

19. The system according to claim 1, wherein the rotary element is a hollow drum configured for receiving a patient therein.

20. The system according to claim 19, wherein the flywheel is annular.

21. The system according to claim 1, wherein the rotary element is a closed drum configured for rotating a gantry arm about a patient.

22. The system according to claim 21, wherein the gantry arm has a treatment or imaging device mounted thereon.

23. The system according to claim 1, wherein a treatment or imaging device is mounted upon an internal circumference of the rotary element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will now be described by way of example and with reference to the accompanying figures, in which;

[0014] FIG. 1 is a schematic diagram of a typical oncology system which is configured to rotate around a patient, and

[0015] FIG. 2 is a schematic diagram, similar to that of FIG. 1, but illustrating in principle how the present invention might be implemented,

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] FIG. 1 shows in schematic form a radiotherapy or radiation imaging apparatus comprising a generally cylindrical drum 2 which is arranged to rotate about an axis A parallel to the Z axis, along which a patient (not shown) may be positioned for treatment/imaging, and along which the isocentre lies. In many apparatuses which would be suitable for the system of this invention, the patient is supported on a support which is movable with multiple degrees of freedom (at least along the three orthogonal X, Y and Z axes, and often also around one or more of these axes); the patient is introduced inside the cylindrical drum generally along the Z axis and his/her orientation within the drum is adjusted so as to ensure the region of interest (such as a tumour) is located accurately relative to the isocentre by movements along and/or around the three axes The drum 2 is supported on wheels 4, 6 disposed to left and right of the drum along the X axis as shown; one or more of these wheels is driven so as to cause the drum 2 to rotate (a motor 16 for this purpose is shown in FIG. 2). The wheels 4, 6 are part of a base support 8 which is secured to the floor 10. Mounted to the drum 2 towards its outer circumference (sometimes on a gantry arm extending in the Z direction, and other times mounted to the axial edge or internal circumference of the cylindrical drum) is a treatment or imaging device D, which is configured to emit therapeutic or imaging radiation or energy towards the axis A (and a region of interest of the patient located (but not shown) there); there may be more than one such device D located at different angular positions around the circumference of the drum 2, and there may be imaging devices (not shown) arranged on the circumference of the drum opposite the or each device.

[0017] The drum 2 and device(s) D have a substantial mass M—in the case of a typical radiotherapy apparatus comprising a linear accelerator, this is in the order of 4 to 7 tonnes—and, in use rotates in the direction shown by the large curved arrow with an angular velocity ω. It is well known that:

E.sub.2=½I.sub.2ω.sup.2   (1)

where E.sub.2 is the kinetic energy stored in the rotating drum and gantry and I.sub.2 is the moment of inertia of the drum and gantry around the axis of rotation A.

[0018] It is also known that:

T.sub.BD=E.sub.2/Ø.sub.B   (2)

where T.sub.BD is the braking torque required to halt the rotation of the drum and gantry within an angle Ø.sub.B,

F.sub.T=T.sub.BD/R   (3)

where F.sub.BD is the tangential force required at radius R from the axis A to stop the rotation of the drum within an angle Ø.sub.B, and

F.sub.X=F.sub.T cos α  (4)

where F.sub.X is the force exerted on the floor by the base support during braking of the drum's rotation when this is carried out by the wheel 6. It will be appreciated that F.sub.T, the tangential braking force is limited by the coefficient of friction between the drum 2 and the wheel 6. In a conventional apparatus the drum typically rotates at around 1 rpm: this means that the tangential braking force F.sub.T is in the order of 8 kN and the force exerted on the floor F.sub.X is in the order of 7 kN. It can be appreciated that forces of these magnitudes are sufficient to risk undesirable slippage of the drum 2 on the wheels 4, 6, and also damage to whatever means is used to retain the base support 8 in position on the floor 10, and/or movement of the base support 8 across the floor. To increasing the rotational speed of the drum would significantly increase these risks.

EXAMPLE

[0019] A typical radiotherapy apparatus comprising a linear accelerator (˜6 tons) rotating at 1 RPM has a kinetic energy of ˜33 J. At 3 RPM the stored energy increases to ˜300 J as energy is proportional to the square of speed. The stopping distance is dependent on the reaction time of the system and the braking torque that can be applied. In the type of apparatus where (as shown in the figures) the drum is mounted on the wheels by gravity alone the braking torque is effected through wheel/rim contact. Assuming a stopping distance of 3 degrees, a rotational speed of 3 RPM and 60 ms reaction time, the drum will rotate 1 degree before actively braking over the next 2 degrees.

[0020] To oppose the rotation (brake) at 1 m distance from the centre (drum/wheel interface) a force must be applied as follows:

E=F.sub.B*(R*θ)=300 J

F.sub.B=350J/(1 m*(2n/360°)=8.6 kN

[0021] The typical force between the drum and the support wheels (such as in the case of the linear accelerator sold by Elekta AB (publ) under its Versa HD trade mark) is 17.5 kN which means that, assuming dry friction between the steel drum and the steel roller wheels gives a maximum braking force of

F.sub.T=8600*0.2=3.5 kN per wheel

the stopping torque must be transmitted via at least 3 wheels to avoid slippage between the wheels 4, 6 and the drum.

[0022] The tangential force between the wheel(s) and the drum 2 will also lead to a sideways force on the base structure 8 and thence to the floor 10 as follows:

F.sub.X=F.sub.B cos (α)=8,600*cos 31=73 kN

[0023] FIG. 2 demonstrates the principle of the present invention. The drum 2 of mass M.sub.1 rotates at angular velocity ω.sub.1; it is driven by motor 16, which acts through drive element 18 to turn wheel 6 on which drum 2 rests. A flywheel 12 of mass M.sub.2 rotates at angular velocity ω.sub.2; about the same axis A as the drum 2, but in the opposite direction as shown by the smaller curved arrow. Flywheel 12 is driven to rotate by motor 14, which is fixedly mounted to the drum 2, and which can also be used for braking the rotation of the flywheel. Because the flywheel contra-rotates relative to the drum, on starting the drum 2 and flywheel 12 rotating, the acceleration of the flywheel 12 imparts a torque on the drum in the opposite direction to the rotation of the flywheel and causes the drum to rotate in the opposite direction. Assuming for the moment that there are no losses due to friction, then the kinetic energy of the rotating flywheel E.sub.2 will equal the kinetic energy of the rotating drum E.sub.1 at any time, and the relative absolute speeds will be related to the respective moment of inertia:

ω.sub.1.sup.2/ω.sub.2.sup.2I.sub.2/I.sub.1.   (5)

[0024] Because E.sub.1=E.sub.2 at any time, when braking, or decelerating the flywheel 12, the resulting torque between the flywheel 12 and the drum 2 will result in both flywheel and drum stopping within the same stopping angle.

[0025] In practice there are frictional losses, hence embodiments of the invention require something to accelerate and decelerate rotation of the flywheel 12 relative to the drum 2, such as motor 14, and also something to drive drum 2 to compensate for frictional losses, such as motor 16, which drives drum 2 (indirectly as shown, or directly) relative to the floor 10.

[0026] The use of a contra-rotating flywheel 12 which is driven relative to the drum 2 in the way described significantly reduces the external reaction forces arising from when the drum 2 is accelerated and, more significantly, when it has to be decelerated quickly and/or within a small angle of rotation of the drum, because a large proportion of what the total external forces would be (i.e. absent the flywheel) are absorbed in the acceleration/deceleration of the rotation of the flywheel. Because it is very important to be able to stop the drum 2 rotating quickly and to cope with the torque reactions arising from this, additional means may be provided to brake the flywheel 12, such as conventional brake pads fixed relative to the drum and acting on the flywheel (or fixed relative to the flywheel and acting on the drum); the braking system may be of a conventional drum and/or disc type, with the flywheel or drum being provided or configured with a suitable braking surface. In this case, the motors would be augmented by the brakes, meaning that the motors are subject to less braking wear. We envisage that the contra-rotating flywheel 12 would be of lower mass than the drum 2, but rotates at a higher speed than the drum (as an example, a flywheel comprising a 50 kg disc with a 1 m diameter rotating at 66 RPM will contain the same energy as a 6 tonne drum rotating at 3 RPM). On braking, the energy will be lost as heat in the brakes and/or in the drive motors (to the extent these are used). Due to friction in the system, there will need to be an external drive as well as an internal drive. Furthermore, as two separate bodies are accelerated in absolute terms, twice the energy is required to accelerate a system in accordance with the invention than in a conventional apparatus.

[0027] The apparatus is controlled by a processor 20 (in the case of a radiotherapy apparatus this is usually known as a Treatment Planning Computer, or “TPC”); in FIG. 2, the processor 20 is shown operatively connected to the two drive motors 14, 16, but for clarity the other connections, such as to the device D shown in FIG. 1, are not shown in FIG. 2.

[0028] It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, although described as a flywheel, provided that the contra-rotating element has an appropriate mass and moment of inertia it could be of any shape and configuration provided it had rotational symmetry: a contra-rotating annulus might be more suitable in applications where the drum is hollow for receiving a patient, for example, whereas a more conventional flywheel might be appropriate where the drum is dosed and the gantry alone rotates around the patient. The contra-rotating dement or flywheel might be confined within the axial length of the drum, or it might be outside the axial length of the drum. It may be advantageous in some applications to have two, three or more smaller contra-rotating coaxial flywheels rather than a single larger flywheel. The drive elements 18 may be driven rollers which bear on the wheels or the flywheel, gears, or any other suitable driving connection. The drive motors 14, 16 can be electric motors as described, or hydraulic motors. The rotary element may be a generally cylindrical element, as shown, or it may be any other type of rotary element, such as a C-arm. Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.

[0029] As explained above, the present invention is applicable to any form of oncology apparatus which has a significant element of substantial size and/or mass which in use is required to rotate around a patient; thus, the invention may be implemented on a radiotherapeutic apparatus having a radiation source such as a linear accelerator (such as is described in our EP2399647), a magnetic resonance imaging linear accelerator (“MLR”—(such as is described in our EP2865419) or an isotopic source, or it may be implemented on any form of tomographic scanning or rotary imaging apparatus (whether this is used for oncological or other medical purposes) such as CT (Computed Tomography), PET (Positron Emission Tomography), SPEC (Single-Photon Emission Computed Tomography), EPID or ultrasound scanners.