GB2439109A

GB2439109A - Electromagnetic radiation screen

Info

Publication number: GB2439109A
Application number: GB0611617A
Authority: GB
Inventors: Peter Hobden
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2006-06-13
Filing date: 2006-06-13
Publication date: 2007-12-19
Also published as: GB2452641A; WO2007144578A2; GB0820020D0; GB0611617D0; WO2007144578A3

Abstract

A protective screen 2 that reduces the amount of radio frequency (RF) and electromagnetic field (EMF) radiation transmitted through it has a laminar structure comprising two layers 4, 6 of different materials. Layer 4 may be copper and attenuates RF radiation and layer 6 is chosen such that a magnetic or electric field induced in it by a source, such as a time varying electromagnetic field (TV-EMF) source destructively interferes with the inducing EMF so reducing the amplitude of EMF that it transmits. Layer 6 may be a diamagnetic or a paramagnetic material or alloy and may be non-magnetic stainless steel. Layers 4,6 may be a conductive glass, impregnated with suitable substances, to impart the required RF attenuation characteristic, and the ability to have electric and/or magnetic fields induced therein. The screen 2 may be used as a shield for personnel operating radiating apparatus such as NMR and MRI scanners.

Description

1 2439109 Protective Screen This invention relates to a protective

screen, and more specifically to a protective screen for individuals commonly, regularly or occupationally exposed to varying forms of radiation, in particular time-varying magnetic/electromagnetic (EM) and radio-frequency (RF) radiations, such as are often released by modern examining and scanning equipment used in the medical and other industries.

Although the following description relates primarily to the provision of a screen for the protection of individuals who may become exposed to potentially harmful radiation, it is to be mentioned that the screen may be used in other applications not specifically directed to the protection of human life, but in cases where it is desired to prevent or limit the amount of radiation penetrating through the screen into a particular space.

Additionally, while the following description is almost exclusively concerned with occupational exposure of humans to radiation emitted from apparatus commonly used in medical applications, e.g. Magnetic Resonance Imaging (MRI) scanners and the like, the invention should not be considered as limited by the type of apparatus emitting such radiation, nor by the application that such apparatus might have, or the function it might perform. lihe invention could also be used for protection of non-human components, such as electronics, where conventional methods of ferromagnetic shielding may not be possible or viable.

BACKGROUND

Nuclear magnetic resonance was first observed by Felix Bloch and Edward Mills Purcell at Berkeley in 1946. Nuclear magnetic resonance (NMR) is a physical phenomenon based upon the magnetic property of an atom's nucleus. All nuclei that contain odd numbers of nucleons and some that contain even numbers of nucleons have an intrinsic magnetic moment. The most often-used nuclei are hydrogen-I and carbon-13, although certain isotopes of many other elements' nuclei can also be observed. NMR studies a magnetic nucleus, like that of a hydrogen atom (protium being the most receptive isotope at natural abundance), by aligning it with a very powerful external magnetic field and perturbing this alignment using an electromagnetic field. The response to the field by perturbing is what is exploited in nuclear magnetic resonance spectroscopy and magnetic resonance imaging.

Since the discovery of NMR, equipment and techniques have been developed that use the phenomenon primarily but not exclusively, to image human body tissues for medical investigation. MRI is now a commonly used clinical and research tool in medicine.

An alternative use for NMR is NMR spectroscOpy, which is one of the principal techniques used to obtain physical, chemical, electronic and structural information about a molecule. It is the only technique that can provide detailed information on the exact three-dimensional structure of biological molecules in solution. More recently, NMR is one of the techniques that has been used to build elementary quantum computers.

Medical MRI scanners use static and time-varying magnetic/electromagnetic fields (TVEMF), as well as radio-frequency waves (RF), to non-invasively image soft tissues of the human body. While the intensities of such radiations are not typically large and potentially very harmful, they do nevertheless constitute a perceived threat to humans who are continually, regularly, and/or occupationally exposed thereto.

The Physical Agents Directive (PAD) 2004140/EC, in particular the branch of this directive relative to Electro-magfletiC fields (EMF) was adopted by EU in April 2004, and requires adoption of limits on exposure of persons to EMF and RF by all the various member states of Europe before April 2008.

Additionally, the International Commission on Non-lonising Radiation Protection (ICNIRP), which is part of the World Health Organisation (WHO), has prepared guidelines known as ICNIRP 1998 which define -limitations on Occupational and General Public Exposure, -restrictions based on current density within the body and Specific Absorption Rate (SAR) of energy within tissue -reference levels of Electric Field Strength, Magnetic Field Strength and Power Flux Density.

In the UK, human (patient and occupational) exposure limits to RF and TVEMFs during MRI examination are currently regulated by the Department of Health, The Health Protection Agency, and The Medicines and Healthcare Products Regulatory Agency.

From 2008 the European Parliament will enforce new levels in the European Union and UK law relating to occupationally-exposed workers' (OEWs) as published in: Directive 20041401EC Physical Agents Directive 29 April 2004 Directive 89/391/EEC.

It is an object of the present invention to provide a screen which will ensure that the abovementioned potentially harmful radiations are screened to such an extent that habitually or occupationally exposed persons receive certainly no more than the amounts of radiation defined in the abovementioned directives, and ideally much less.

It is a further objection of the present invention to provide apparatus which is capable of reducing the amplitude of TVEMFs and RE radiations to render them less harmful.

It is a possible further object of the present invention to assist in the reduction of RE and TVEMF exposure to patients undergoing MRI examinations. Although this might seem incongruous with the fundamental requirement of MRI than molecules be exposed to radiation to excite them, it has been proposed by certain regulatory bodies that new limits of exposure might be enforced to prevent patients from being over-exposed. Eor instance, the invention might be used in cases where individuals may have particular sensitivities to radiation, e.g. if pregnant.

It is a possible further object of the invention to provide a radiation screening device for providing human protection in other industries which involve the exposure of individuals to RE and TVEMEs, such as the power generation and electrical manufacturing industries.

Of course, it has long been known to provide screening for different types radiations. A most simple example is the use of sheet lead metal to absorb a certain proportion of X-rays or other more energetic radiations.

The most simple RF screen is commonly known as a Earaday cage, and in the case of MRI, such a screen is used in the Inner Controlled Area in which a patient is commonly disposed during the scanning procedure. The screen is basically a closed conductive structure, sometimes in the form of a fine conductive mesh of wire, or more commonly in the form of a continuous conductive layer such as copper sheet. This has the effect of preventing spurious signals both from foreign transmitters and from the driving electronics of the MRI system itself from introducing errors into the MRI scan readings. It is worth mentioning that the signals returned from the patient during the scan are very small, and that MRI signals can often be about 1000 times weaker than the spurious signals mentioned above.

Additionally, it is also known to provide screens for EMF and TV-EMF. In modern life, almost everyone is continuously exposed to low frequency EMF radiation, the most common manifestation of such being flickering computer screens, and of course mobile telephones. In one embodiment, screening of EMF radiations involves placing a physical barrier of shielding material between the source of interference and the affected area.

The most commonly used materials are steel and high permeability Mu-metal. EMF shielding works by diverting and deflecting the magnetic field away from the affected area. The design of a shield must take into account that a magnetic field will flow' around a partial shield, but as these fields decay rapidly with distance the extent of the shield needs to be large enough to take this into consideration. The most critical criteria when designing a shield is the level of attenuation required.

BRIEF SUMMARY OF THE DISCLOSURE

According to the present invention, there is provided a protective screen adapted for restricting and/or reducing the amount of radiation passing therethrough, said radiation including both RF and EMF components, characterised in that the screen is of laminar construction and comprising at least two Iaminae of differing materials, a first of said materials being chosen so as to attenuate RF radiation passing therethrough, and the second material being capable of having an electric and/or a magnetic field induced therein as a result of the existence of an EMF source, said induced electric and/or magnetic fields giving rise to destructive interference with the original inducing EMF thus reducing the resulting amplitude of EMF on the side of the screen being more remote from the EMF source.

It is worth mentioning that the frequency of the EMF is unaffected by the second material, and only the amplitude, and thus the power contained within the radiation (indicative of its potential to harm) is reduced.

Preferably, the first material is of copper or other well known, similar RF attenuator, and the second material is a strong dia-or paramagnetic material or alloy thereof.

Alternatively, the second material may be a non-ferromagnetic tainIess steel.

It should be mentioned that the use of a ferromagnetic material is precluded, not least because the existence of significant EMF can give rise to such large attractive forces between the EMF source and ferromagnetic objects in the vicinity that the latter can become dangerous projectiles.

In the instance where some visibility through the screen is required, it is preferable to use a conductive glass, impregnated with suitable substances which on the one hand are capable of RF attenuation, and on the other hand are capable of having electric

and/or magnetic fields induced therein.

In a more preferred embodiment, the screen shape is complementary to that of an MRI scanner around which it is to be disposed.

The screen may also be floor or ceiling mounted.

Most preferably, the screen passively interacts with the EMF (which for the purposes of this application is always considered to be time vary-ing EMF).

BRIEF DESCRIPTION OF THE DRAWINGS

A specific embodiment of the invention will now be described with reference to the accompanying drawing wherein Fig. I shows schematically a laminar structure for a screen.

DETAILED DESCRIPTION

Referring to Figure 1, there is shown a laminar structure 2 comprising two laminae 4, 6 of different materials which are ideally used in the manufacture of a screen, or of material from which screens may be manufactured, according to the present invention.

The material of the first lamina 4 comprises a material of copper or other well known, similar RF attenuator. The material of the second lamina 6 is a strong dia-or paramagnetic material or alloy thereof. Alternatively, the material of the second lamina may be a non-ferromagnetic stainless steel.

In the instance where some visibility through the screen is required, it is preferable to use a conductive or transmissive glass, impregnated with suitable substances which on the one hand are capable of RF attenuation, and on the other hand are capable of having electric and/or magnetic fields induced therein.

In a preferred arrangement which allowed light transmission, the material of the first lamina would be a conductive/transmissive glass having been impregnated with a first substance capable of attenuating RF signals, and the material of the second lamina would be a conductive/transmissive glass having been impregnated with a second substance capable of having electric and/or magnetic fields induced therein which destructively interfere with the EMF source that induced them.

The reader will be immediately aware of a number of possible applications, but the following will exemplify the invention and its uses: -Over the chest of a clinician attending to, and standing next to a patient during MRI scanning; -to protect an escort attending a nervous child or a patient suffering from claustrophobia during MRI scanning; -during intra-operative MRI techniques where surgeons and others are being exposed to EMF and RF radiations during surgery; -as a body apron or skirt for clinicians/technicians and expectant mothers; in this lattermost application, the screen is flexible, provided with fastening means along one edge adapted to be wrapped around the stomach/waist of a wearer and fastened thereat, having a roughly conical shape so as to appear as a skirt in use; -be floor mounted and transparent to allow clinicians/technicians to stand behind it and look through said screen.

Claims

CLAIMS

1. A protective screen adapted for restricting and/or reducing the amount of radiation passing therethrough, said radiation including both RF and EMF components, characterised in that the screen is of laminar construction and comprising at least two laminae of differing materials, a first of said materials being chosen so as to attenuate RE radiation passing therethrough, and the second material being capable of having an electric and/or a magnetic field induced therein as a result of the existence of an EMF source, said induced electric and/or magnetic fields giving rise to destructive interference with the original inducing EMF thus reducing the resulting amplitude of EMF on the side of the screen being more remote from the EMF source.

2. A screen according to claim 1 wherein the first material has a RF attenuating property and the second material is susceptible to electromagnetic induction such that the induced electric/magnetic field destructively interferes with the inducing source EMF radiation.

3. A screen according to any preceding claim wherein the second material is a strong dia-or paramagnetic material or alloy thereof.

4. A screen according to any of claims I -3 wherein the first material is copper.

5. A screen according to claim 1-4 wherein the second material is a non-ferromagnetic stainless steel.

6. A screen according to any of claims 1-3 wherein the first and/or second materials are of a conductive or transmissive glass, impregnated with suitable substances which on the one hand are capable of RE attenuation, and on the other hand are capable of having electric and/or magnetic fields induced therein.