ABSORBING DEVICE FOR RADIOTHERAPY

Abstract

An absorbing device (24, 24, 24, 41) for radiotherapy treatment comprising at least one layer of electromagnetic absorbing material wherein the absorbing material is for preventing interaction between a target localisation system (1) having a targeted frequency range of between about 300 kHz and 500 kHz and a treatment table (4).

Claims

1. A mattress for radiotherapy treatment comprising an absorbing device for radiotherapy treatment comprising at least one layer of electromagnetic absorbing material, wherein the absorbing material is for preventing interaction between a target localisation system having a targeted frequency range of between about 300 kHz and 500 kHz and a treatment table, wherein the absorbing device comprises a ferrite layer and at least one spacer layer positioned above the ferrite layer in a patient-facing direction away from the couch top surface on which the patient lies; wherein the ferrite layer comprises a plurality of sheets of a sintered ferrite material.

2. The mattress according to claim 1, further comprising a comfort mattress layer positioned above the ferrite layer in a patient-facing direction away from the couch top surface on which the patient lies.

3. The mattress according to claim 1 wherein the absorbing device comprises an absorber layer comprising a material having high magnetic permeability, wherein the real part of relative permeability () is greater than 200 at 300 kHz and the imaginary part of relative permeability () is less than 50 at 300 kHz.

4. The mattress according to claim 1 wherein the absorbing device comprises an absorber layer having surface resistivity greater than 1.00 E+0.9 Ohms.

5. The mattress according to claim 3 wherein the absorber layer comprises a plurality of stacked sintered ferrite sheets.

6. The mattress according to claim 1 wherein the sintered ferrite material comprises iron oxide (Fe.sub.2O.sub.3); preferably, wherein at least 50% of the total mass of the sintered ferrite material comprises iron oxide (Fe.sub.2O.sub.3); and/or wherein the ferrite material further comprises at least one of Nickel Oxide (NiO); and/or Zinc Oxide (ZnO); and/or Copper Oxide (CuO).

7. The mattress according to claim 1 wherein each sheet of sintered ferrite material has a thickness of between 0.1 mm and 1 mm.

8. The mattress according to claim 1 having a plurality of partially overlapping sheets.

9. The mattress according to claim 1 wherein each sheet is attached to at least one layer of adhesive; preferably, to at least one layer of an adhesive tape.

10. The mattress according to claim 1 wherein the spacer layer includes a reinforcing material such as epoxy, and/or polyester, and/or polymer, polymethacrylimide, and/or vinyl ester resin, and/or wood, and/or ceramic, and/or aramid, and/or glass fibre, and/or ultrahigh molecular weight polyethylene.

11. An absorbing device for radiotherapy treatment, comprising at least one layer of electromagnetic absorbing material, wherein the absorbing material is for preventing interaction between a target localisation system having a targeted frequency range of between about 300 kHz and 500 kHz and a treatment table, wherein the absorbing device comprises a ferrite layer, wherein the ferrite layer comprises a plurality of sheets of a sintered ferrite material, and wherein the absorbing device is for positioning around a patient.

12. The absorbing device according to claim 11 wherein the absorbing device comprises at least two upstanding walls and at least one transverse member therebetween for positioning over and around a patient.

13. A couch top for patient support for radiotherapy treatment comprising a mattress according to claim 1.

14. A couch top for patient support for radiotherapy treatment comprising a mattress according to claim 11.

Description

DETAILED DESCRIPTION

[0071] Example embodiments of the present invention will now be described with reference to the accompanying figures, in which:

[0072] FIG. 1 shows a side view of an example treatment room set up for a localisation system, with which the device of the present invention is configured to be used;

[0073] FIG. 2a is a schematic of the response signals from a wireless beacon used in a localisation system without the device of the present invention; and FIG. 2b is a schematic of the response signals from a wireless beacon used in a localisation system with the device of the present invention;

[0074] FIG. 3 is a graph showing the localisation error (y-axis) in the z-direction (cm) plotted against the vertical distance between the couch top and the beacon wireless beacon (x-axis) for various absorber materials;

[0075] FIG. 4 shows a further schematic cross-sectional view of the device in position between a target and the couch top, showing the additional layers forming the device of a preferred embodiment of the present invention;

[0076] FIG. 5 shows a schematic view of the multiple layers forming the absorbing device of the present invention;

[0077] FIG. 6 shows a cross-sectional view of a positioning device comprising the absorbing device of the present invention, positioned between a patient and a treatment couch;

[0078] FIG. 7 shows a cross-sectional view of an alternative embodiment of the absorbing device of FIG. 6 positioned between a patient and a treatment couch;

[0079] FIG. 8a, FIG. 8b, and FIG. 8c show further alternative embodiments of the absorbing device of the present invention.

[0080] The present invention is intended to support the use of an electromagnetic tumour tracking system in combination with a carbon-fibre couch top in radiation imaging and therapy.

[0081] Referring to FIG. 1, a target localisation system 1 for use during radiation therapy is shown. A plurality of wireless beacons 2 are implanted into the patient 3; for example, the wireless beacons 2 (also known as markers or transponders) are implanted at or near the treatment target. Alternatively, or in addition, a plurality of wireless beacons 2 are placed on the surface of the patient's skin, with the patient 3 lying on the treatment table/couch top 4. The wireless beacons 2 send signals of target motion in real time during treatment. An array of transceivers 5, often in the form of a plate, which are used to transmit, detect and measure the beacon transponder signals to enable the calculation of an objective measurement of the location of the treatment target in three dimensions. As shown in FIG. 1, in use the transceiver antenna 5 is above the patient 3 and the couch top 4 is below the patient, with the wireless beacons 2 placed between the transceiver 5 and the couch top 4.

[0082] Accurate localisation of the target means that the radiation therapist can align the patient's treatment target to the machine isocentre before radiation treatment commences. The use of the wireless beacons 2 and the transceiver 5 also allows for real time monitoring of the patient/target movement during radiotherapy treatment delivery. It is to be understood that the example shown in FIG. 1 comprises an array 5 of electromagnetic transceivers; however, in alternative embodiments of the present invention, the target localisation system comprises separate components to transmit and receive localisation signals.

[0083] In a preferred embodiment of the present invention, the or each radiofrequency beacon 2 is wireless and does not require a battery. The beacon transponders 2 are self-powered by the magnetic field of the transceiver antenna.

[0084] Referring to FIG. 1, a typical target localisation system 1 further comprises a console 7 to which the electromagnetic transceiver 5 is attached; an optical system 9, for example comprising a plurality of infrared cameras; and a tracking station 11, from where a radiation therapist or similar health professional will monitor set-up and treatment of the patient 3. In use, a radiation treatment device includes a linear accelerator (LINAC) and a movable gantry, which is powered by a motor (not shown). The treatment delivered to the patient is adjusted in real time according to the detected 3D position of the patient 3, which is given by the target localisation system 1.

[0085] Referring to FIG. 2a, the transceiver 5 of the localisation system generates a very high electromagnetic field and is placed on top of the patient's body with the patient (not shown) lying on top of the treatment table 4. The wireless beacon 2 is implanted into the patient's body or is placed on the patient's skin. The wireless beacon 2 is self-powered by the electromagnetic field generated from the transceiver 5 and does not have a battery or cable connection. The transceiver 5 and the wireless beacon 2 operate in a frequency range of between about 300 kHz and 500 kHz. FIG. 2a illustrates how the signal 13 from the wireless beacon 2 can be affected by surrounding electromagnetic noise that is generated; for example, by the motor 10 inside the treatment table, or from the LINAC gantry (not shown). This results in localisation errors. Furthermore, as shown in FIG. 2a, the electromagnetic field from the transceiver 5 travels towards the conductive couch top 4 and the LINAC gantry (not shown), such that part of the alternating electromagnetic field is transferred to a circular current at the surface of the conductive materials, which is then eventually dissipated as heat. Thus, the wireless beacon 40 receives less electromagnetic energy and may be unable to generate sufficient feedback signals, which results in localisation errors because of an incorrect calculation of the objective measurement of the location of the treatment target in three dimensions.

[0086] Unless explicitly specified to the contrary, is to be understood that the term permeability refers to the real part of relative permeability , in the context of the present invention.

[0087] It is to be understood that, in the context of the present invention, high permeability refers to the real part of relative permeability () being greater than 200 at 300 kHz, with a preferred range between 200 and 1000 at 300 kHz. It is to be understood that, in the context of this invention, low loss refers to the imaginary part of relative permeability () being less than 50 at 300 kHz, with a preferred range between 1 and 50 at 300 kHz.

[0088] The material selected for the absorbing device 24 of the present invention was selected to achieve optimum localisation accuracy. It was found that a material having high relative magnetic permeability was well-suited to the present invention. Relative permeability is a complex number and contains both a real part () and an imaginary part () as expressed in Equation 1:

=j[Equation 1]

[0089] The real part refers to the storage capability of the electromagnetic energy and the imaginary part refers to the loss behaviour of the electromagnetic energy. Unless explicitly specified to the contrary, the term permeability refers to the real part of relative permeability , in the context of the present invention. The ideal material for use in the absorbing device 24 comprises a high storage capability () and preserves electromagnetic energy with a low loss component ().

[0090] As shown in FIG. 2b, in a preferred embodiment of the present invention, the absorbing device 24 is placed on top of the couch top 4, on which the patient lies, with the transceiver 5 above the patient. Thus, the magnetic flux 14 of the electromagnetic field 14 generated from the transceiver 5 will be absorbed inside the absorber layer within the absorbing device 24, rather than being dissipated at the surface of the conductive couch top 4, as shown in FIG. 2a. With the improvement of the present invention, the wireless beacon 2 receives sufficient electromagnetic energy from the transceiver to send feedback (localisation) signals with sufficient strength for accurate localisation. Electromagnetic noise generated from the motor 10 will be absorbed locally and will be highly attenuated before reaching the wireless beacon 2. Thus, the wireless beacon 2 is significantly less affected by the surrounding conductive materials and electromagnetic noise, which significantly improves target localisation accuracy.

[0091] Various materials were investigated for the absorbing device of the present invention and the localisation errors were quantified for each possible material. Referring to FIG. 3b, the error (shown on the y-axis) was mainly found to be a localisation error in the z-direction, i.e. in the detected distance between the couch top 4 surface and the wireless beacons 2. With reference to FIG. 1, the wireless beacons 2 were mounted at a position with constant distance to the transceiver 5 and the couch top 4 was moved vertically.

[0092] The quantitative analysis to arrive at the results shown in FIGS. 3 used a dedicated measurement test set-up to evaluate the localisation error for different absorber/layer configurations. The distance between the transceiver and the wireless beacon of the localisation system is fixed and the distance between the moveable couch top and the absorber layer was varied for a range of vertical positions. The localisation error with respect to the known position of the wireless beacon was recorded each time. Measurements were taken without the use of an absorber layer for comparison purposes.

[0093] The preferred material for use in the present invention has a high storage capacity (e.g. is high) and a low loss behaviour (e.g. is low) because this minimises the electromagnetic localisation error. It has also been found that the surface resistance of the absorber layer also affects the electromagnetic field and so the localisation accuracy.

[0094] It is to be understood that in the context of the present invention, the term permeability refers to the real part of relative permeability () and the term permeable materials are the materials with a high real part of relative permeability () value. Both and are strongly dependent on the frequency of the electromagnetic field. FIG. 3 relates to testing of the absorber layer of the present invention in respect of a variety of absorber layers and measurements were also taken without the use of an absorber layer for comparison purposes. The materials and couch top combinations that are shown on FIG. 3 are: [0095] Case 1: non-conducting couch top; [0096] Case 2: conductive couch top without using absorber layer; [0097] Case 3: absorber layer with magnetically conducting nickel-iron alloy, also known as a p-metal, having a thickness of 0.1 mm, >800 at 300 kHz; [0098] Case 4: absorber layer with magnetic powder and rubber, having a thickness of 2.5 mm, =25 at 300 kHz and <3 at 300 kHz; [0099] Case 5: absorber layer with sintered ferrite panel, having a thickness of 0.2 mm, =600 at 300 kHz and =220 at 300 kHz; [0100] Case 6: absorber layer with sintered ferrite panel, having a thickness of 0.2 mm, =450 at 300 kHz and <30 at 300 kHz; [0101] Case 7: absorber layer with two sintered ferrite panels, each panel having a thickness of 0.2 mm, resulting in a total thickness of 0.4 mm, =450 at 300 kHz and <30 at 300 kHz.

[0102] The results set out in FIG. 3 show that the localization error caused by a conductive couch top appears at a couch top to wireless beacon distance of about 20 cm. When the conductive couch top is closer to the beacon and the antenna of transceiver, it causes more loss of electromagnetic field such that the localization error increases to 1.3 cm when the couch top is 3 cm closer to the beacon and transceiver antenna. FIG. 3 also shows that a material with a high and a low is suitable for minimizing localization error. In practice, the surface resistance of the absorber layer also affects the electromagnetic field. Although the p-metal of Case 3 has a very high it is conductive due to a low surface resistance and is shown to yield a large localization error. Thus, the sintered ferrite material of Cases 5, 6, and 7 is preferred in the present invention. FIG. 3 also shows, with reference to Case 7, that using a multi-layer absorber can further increase the storage capability with respect to the electromagnetic field of the ferrite panel, such that the localization error can be further reduced. The overall results shown in FIG. 3 demonstrate that the absorber layer with high , high surface resistance, low and multi-layer ferrite panels are advantageous for the absorbing device of the present invention.

[0103] Further testing of the absorbing device of the present invention was also carried out to consider potential side effects introduced by ferrite materials. For example, the induced radioactivity effect, dose dosimetry change and the potential for X-ray artefacts were considered. The ferrite material used for the absorbing layer of the present invention comprises a set of ceramic compounds composed of iron oxide (Fe.sub.2O.sub.3) combined chemically with one or more additional metallic elements, including but not limited to copper oxide (CuO), zinc oxide (ZnO) and nickel oxide (NiO).

[0104] Table 1 shows the induced radioactivity (in Sv/hour) with different ferrite material compositions. The tests carried out used an 18 MV beam generator to deliver a photon energy of 96 Gy to several ferrite samples. The induced radioactivity was then measured at the surface of the ferrite material samples. It was found that different ferrite material compositions introduce different radioactivity behaviour and that the induced radioactivity decreases to the previous background level after two hours. Further measurement also shows that both and do not change after radiation. The impact of induced radioactivity can be reduced by placing the ferrite material of the absorbing layer away from the patient's body. In the present invention, a comfort mattress layer and a spacer layer are introduced above the ferrite material layer/s to minimize such induced radioactivity. It is understood that above is in a patient-facing direction away from the couch top surface on which the patient lies.

TABLE-US-00001 TABLE 1 Time after irradiation Sample- Sample- Sample- (minutes) 1 2 3 0 1.8 1.6 0.8 10 1 1 0.5 20 0.6 0.6 0.5 120 0.2 0.2 0.2

[0105] Referring to FIG. 4, a simplified schematic of the preferred arrangement of the absorbing device of the present invention is shown. A wireless beacon 20 is shown implanted into the patient 22. During positioning and treatment, the patient 22 lies on the absorbing device 24 of the present invention, which comprises a multi-layer structure. A transceiver and corresponding antenna 5 of the electromagnetic localisation system is placed above or on top of the patient 22. The absorbing device 24 comprises a wrapper 26, which wraps around a comfort mattress layer 28; a spacer layer 30; and an absorber layer 32. The absorbing device 24 of the present invention rests, in use, oni.e. above the couch top 34. The wrapper 26 may comprise an anti-slip base layer on its couch-facing surface to minimise any movement of the absorbing device 24 when it is placed between the couch top 34 and the patient 22. Alternatively, the wrapper 26 may comprise a means for attaching the absorbing device 24 to the couch top 34 so that it is fixed in place during use. The comfort mattress layer 28 is a padded material, such as foam, or other cushioning material to improve patient comfort. The spacer layer 30 has the advantage of increasing the distance between the patient and the couch top 34, which has been found to further minimise any potential localisation error. The spacer layer 30 may also comprise a reinforcing material to prevent damage to the absorber layer 32, which may be a brittle material.

[0106] As previously described, the absorber layer 32 comprises a material having high magnetic permeability to avoid the electromagnetic energy dissipation from the transceiver or wireless beacon that may otherwise introduce localisation errors. The absorber layer 32 also comprises a material having a high surface resistivity to provide a significant resistance to current running along the surface of the couch top 34 and the absorbing device 24. In a preferred embodiment, the absorber layer 32 comprises at least one layer of a sintered ferrite material with high magnetic permeability (), high surface resistivity and low loss () to absorb magnetic flux generated from the transceiver 5. The absorber layer 32 has the following characteristics: [0107] >200 at 300 kHz; [0108] <50 at 300 kHz; [0109] surface resistance>1.00E+0.9 Ohms; [0110] ferrite material composition, comprising at least 50% of iron oxide (Fe.sub.2O.sub.3) and, more preferably, also contains Copper oxide (CuO) and/or Zinc oxide (ZnO) and/or Nickel oxide (NiO).

[0111] Referring to FIG. 5, the sintered ferrite material used for the absorber layer 32 of the present invention is produced with a thickness of about 0.1 mm. It is envisaged that, in alternative embodiments, the absorber layer has a thickness of about 0.2 mm, about 0.3 mm, or about 0.4 mm. The sintered ferrite material is attached to a sheet of adhesive material having a surface area of 120 mm120 mm to form a ferrite sheet. This 120 mm120 mm ferrite sheet is attached to a carrier panel having a greater surface area to form a ferrite panel having the required surface area of about 48 cm108 cm, which is configured to cover a typical patient's body size. In alternative embodiments, the ferrite sheets are stacked and glued on top of each other to increase the thickness of the ferrite layer.

[0112] A multi-layer ferrite panel avoids potential magnetic field saturation due to a large amount of electromagnetic energy generated from the transceiver. As shown in FIG. 5, the absorber layer 32 is separated from the patient, in use, by a mattress layer 28 and a spacer layer 30. It has been found that potential introduction of radioactivity/X-ray artefacts at the surface are avoided by careful arrangement of the mattress and spacer layers 28, 30. The mattress layer 28 and the spacer layer 30 are both non-conductive; have a surface resistance greater than 1.00E+0.9 Ohms; and have very low attenuation of X-rays. The mattress and spacer layers 28, 30 effectively form a buffer to minimise or block the induced radioactivity effects and the potential X-ray artefacts generated by the absorber layer 32. The wireless beacons implanted in the patient are further separated from the conductive couch top by the use of the mattress and spacer layers 28, 30 to further improve localisation accuracy.

[0113] In alternative embodiments of the present invention, it is envisaged that the absorbing device 24 comprises an absorber layer 32 having a high magnetic permeability but a low surface resistivity, wherein the absorber layer is modified to compensate for the low surface resistivity; for example, by being provided in small pieces surrounded by an insulating medium. It is envisaged that in this configuration the high magnetic permeability/low surface resistivity material would be usable because the material is modified to reduce the electrical conductivity. In one embodiment of the present invention, the absorber layer of the absorbing device comprises plates/tiles of a ferrite material.

[0114] Referring to FIG. 6 and FIG. 7, for reproducible and stable positioning of a patient, a vacuum cushion 40 is used in combination with the absorbing device 41 of the present invention. The vacuum cushion 40 is moulded to the patient's anatomy and allows the patient to be positioned in exactly the same position for imaging and treatment and for each subsequent treatment fraction. Examples are the BIueBAG and BodyFIX provided by Elekta AB (Publ), which comprises a mouldable vacuum cushion comprising a flexible bag of gas-impermeable material; a flowable filling material such as polystyrene grains or balls; and a valve to allow the bag to be connected to an external vacuum source or a vacuum pump.

[0115] The vacuum cushion 40 may also comprise at least one bolster (not shown). For example, the bolster can be shaped to locate a shoulder region of the patient in a comfortable, reproducible position for each treatment fraction; or support the knees of the patient in an elevated position; or be a substantially cylindrical neck roll to support underneath a neck of the patient. The use of additional bolsters or cushions reduces patient movement; improves patient comfort and provides better beam access for treatment, if appropriate. Bolsters or cushions can also be used to allow a patient to hold a panic button without introducing unwanted patient movement or increasing patient discomfort.

[0116] With reference to FIG. 6, prior to imaging or treatment, a positioning device 44 is vacuum-moulded to a couch-facing part of a patient 42, or to the entire body of the patient 42. The positioning device 44 is placed on a couch 45 and may be fixed thereto; for example at successive indexed fixing points along the couch top. The couch 45 and positioning device 44 are releasably fixed together; for example using a releasable, elongated bar with two pins projecting to be received in corresponding holes in the positioning device 44; or the positioning device 44 is releasably fixed to the couch 45 using a hook and loop fastener, such as Velcro, or similar. It is envisaged that the positioning device 44 covers substantially all and/or overlaps the couch top, such that the width of the positioning device 44 is equal to or greater than the width of the couch 45. In alternative embodiments, the positioning device 44 is moulded only to a required body region of the patient 42 and so only shields the couch 45 in that region.

[0117] Referring to FIG. 6, a patient 42 is placed on the positioning device 44, which is placed on the treatment couch 45, with the positioning device 44 in an inflated, soft and mouldable state.

[0118] With the patient 42 resting on the positioning device 44 and lying in the desired position, the vacuum cushion 40 of the positioning device 44 is connected to a vacuum source or a vacuum pump (not shown) and the gas contained therein is evacuated to compress the filling material and prevent any further movement of the filling or the positioning device 44. Thus, the patient 42 is held by the moulded vacuum cushion 40 of the positioning device 44 in a reproducible position. The vacuum cushion 40 of the patient positioning device 44 is held in a substantially rigid state until the vacuum is broken and the positioning device 44 is returned to a soft and mouldable state to allow the patient to dismount the couch 45.

[0119] As previously described, to allow for patient localisation one or more wireless beacons 46 are positioned in, on or adjacent to the patient 42. The absorbing device 41 of the present invention is incorporated within the positioning device 44 and prevents the introduction of localisation errors in the calculation of the location of the treatment target caused by the surrounding conductive materials and electromagnetic noise. An electromagnetic transceiver 47 is positioned for communication with the wireless markers 46 to allow identification and/or accurate, undistorted positional information to be detected. The transceiver 47 is held in a fixed position or is moveable using a robotic arm (not shown) to move the transceiver 47 into a desired position with respect to the wireless beacons 46 that are to be detected.

[0120] Referring to FIG. 7, in an alternative embodiment of the present invention, the patient positioning device 44 further comprises a plastic cover sheet 48. A patient 42 is placed on the positioning device 44, as previously described, and the patient 42 is covered by the transparent plastic cover sheet 48, which is adhesively secured to the positioning device 44 around its perimeter; or at least along two opposing edges of the positioning device 44. The transparent plastic cover sheet 48 allows a patient 42 to be seen through the sheet and is also transparent to wireless signals.

[0121] As previously described, with the patient 42 resting on the positioning device 44 and lying in the desired position, the vacuum cushion 40 of the positioning device 44 is connected to a vacuum source or a vacuum pump (not shown) and the gas contained therein is evacuated to compress the filling material and prevent any further movement of the filling or the positioning device 44. In this alternative embodiment, the patient 42 is held by the moulded vacuum cushion positioning device 44 and by the transparent plastic cover sheet 48 in a reproducible position. Both the vacuum cushion 40 and the cover sheet 48 are held in a substantially rigid state until the vacuum is broken. The positioning device 44 is returned to a soft and mouldable state to allow the patient 42 to dismount the couch 45.

[0122] The positioning device of the present invention can be provided to mould to and position the whole of the patient's body; or be provided for a specific body part or region, such as the hip or thorax. It is envisaged that after treatment the plastic sheet 48 can be removed and replaced and the positioning device 44 can be re-inflated and used for positioning of another patient 42.

[0123] The present invention further optimises the mass density and thickness of the absorber layer that is used. A higher mass density and a thicker ferrite absorber layer have been found to result in a greater impact on the dose dosimetry profile. These factors are carefully controlled during manufacture of the absorbing device of the present invention. The present invention also takes into account that different ferrite material compositions result in different attenuation of X-ray imaging. The risk of introducing X-ray artefacts is carefully considered and the nature of the material and the number of ferrite layers used is determined accordingly. For example, an image contrast change at 10.1 mm above the surface of the couch top was introduced when using a single-layer of ferrite material having a thickness of 0.3 mm; whereas, the image contrast change is introduced at 50.0 mm above the surface of the couch top when a laminar absorbing device having five, stacked ferrite panels is used, each having a thickness of 0.3 mm and so a total thickness of 1.5 mm.

[0124] In a further embodiment of the present invention, shown in FIG. 8a, further electromagnetic shielding is also provided around the patient body to allow for any further sources of electromagnetic noise; for example, from surrounding conductive materials. The transceiver antenna 5, which in use is above the patient body (not shown), is shielded by an absorbing device 24 above the transceiver antenna 5. As before, above is understood to be the opposing direction to the couch top 4, on which the patient lies. The absorbing device 24 comprises both lateral and transverse panels, which in use form a frame around the patient (not shown). The absorbing device 24 is an open cuboidal shape allowing the patient's body to pass into and/or through the absorbing device, when supported by the couch top 4 as shown in FIG. 8b. Alternatively, as shown in FIG. 8c, the absorbing device 24 can comprise a single panel above the transceiver antenna 5.

[0125] In a further aspect of the present invention, as an alternative or in addition to the pad or mattress accessory solution previously described, the absorbing device of the present invention can be integrated into a couch top or permanently fixed thereto. Thus, the present invention provides a combined solution whereby a carbon couch top is combined with the absorbing device, to offer a solution that is transparent to X-ray radiation; i.e. with very low X-ray attenuation, and maintains the electromagnetic field strength of a localisation system without disconnecting magnetic flux.

[0126] As previously described, the integrated couch top of the present invention comprises an absorber layer combined with an insulation layer. In use, with a patient lying on the couch top, the patient is not directly contacting the absorber layer but is separated therefrom by a non-conductive insulation layer, having a surface resistance greater than 1.00 E-0.9 Ohms. This minimises the induced radioactivity effects and blocks any potential X-ray imaging artefacts generated by the absorber layer. As described with respect to other aspects of the present invention, the insulation layer comprises any one or more of the following materials: epoxy, polyester, polymer, polymethacrylimide, vinyl ester resin, wood, ceramic, aramid, glass fibre or ultrahigh molecular weight polyethylene. The insulation layer provides additional reinforcement to the carbon fibre couch top, increasing the rigidity and loading capacity of the couch top. Furthermore, the insulation layer may comprise a non-conductive foam to act as cushioning and improve patient comfort.

[0127] The integrated couch top comprises an absorber layer having a least one ferrite panel with high magnetic permeability, wherein the real part of relative permeability () is greater than 200 at 300 kHz and the imaginary part of relative permeability () is less than 50 at 300 kHz. It is to understood that the magnetic permeability and the material selected can be further optimised according to the applied magnetic field. The ferrite material used for the absorbing layer of the present invention comprises a set of ceramic compounds composed of iron oxide (Fe.sub.2; O.sub.3); preferably with at least 50% of total ferrite material mass. In further preferred embodiments, the material of the absorber layer is also combined chemically with one or more additional metallic elements, including but not limited to copper oxide (CuO), zinc oxide (ZnO) and nickel oxide (NiO).

[0128] The integrated couch top comprises an absorber layer having a plurality of ferrite sheets; wherein each ferrite sheet has a relatively small size of about 60 cm60 cm and thickness of between about 0.1 mm and 1 mm. Prior to integrating the absorber layer into the couch top, the ferrite sheet is pre-assembled on an adhesive layer to form a larger panel. Preferably, the larger ferrite sheet is supported by a reinforcing layer. For example, the reinforcing carrier layer is rigid and comprises a conductive material, such as carbon fibre, which has very low X-ray attenuation. In alternative embodiments of the integrated couch top, the thickness of the absorber layer is adjusted according to requirements, with stacking of ferrite panels and/or layers to form a thicker layer if required.

[0129] It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.

Appendix

[0130] In addition to FIGS. 1 to 8; further evidence to demonstrate the novel approach of using a mattress and a spacer layer as a buffer to remove X-ray imaging artefacts is demonstrated in comparison with existing prior-art devices.

[0131] FIG. 9 illustrates the introduced X-ray artefacts on a head phantom with different ferrite samples. The X-ray imaging is acquired after passing through different ferrite samples. It shows that artefacts are dependent on material composition. The final ferrite material composition is the optimized embodiment described with respect to the present invention. The proposed solution using ferrite material has the potential to introduce X-ray artefacts and seriously limit the clinical usage with LINAC applications. FIG. 9ano ferrite sample; FIG. 9bferrite sample 1; FIG. 9cferrite sample 2; FIG. 9dferrite sample 3.

[0132] FIG. 10. is an illustration of the X-ray artefacts with multi-layer ferrite panels: (a) single layer ferrite panel; (b) five-layer ferrite panels

[0133] FIG. 10a illustrates that the image contrast change at 10.1 mm above the couch top is introduced by a single-layer ferrite panel. Having a thickness of 0.3 mm. FIG. 10b illustrates that the image contrast change increases to 50.0 mm above the couch top with stacking five layer ferrite panels, having a thickness of 50.3 mm, with the panels stacked on top of each other. The applicant has shown that the ferrite layer introduces X-ray artefacts and the artefacts become worse with more layers. To solve this, the present applicant introduced the use of a mattress layer and a spacer layer, which act as a buffer to effectively push; i.e. to effectively move the patient's body further away from the ferrite layer, such that there are no X-ray artefacts presented at the targeted body part.