WO2007144578A2

WO2007144578A2 - Protective screen

Info

Publication number: WO2007144578A2
Application number: PCT/GB2007/002120
Authority: WO
Inventors: Peter Hobden
Original assignee: Peter Hobden
Priority date: 2006-06-13
Filing date: 2007-06-08
Publication date: 2007-12-21
Also published as: WO2007144578A3; GB2439109A; GB0820020D0; GB0611617D0; GB2452641A

Abstract

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 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. Ideally the first material is Copper and the second material is a strongly dia - or paramagnetic compound or element.

Description

Protective Screen

This invention relates to a protective screen, and more specifically to a protective screen for individuals who may be exposed to varying forms of radiation, in particular time- varying electromagnetic (TV EMF) and radiofrequency (RF) electromagnetic fields - frequencies encountered in magnetic resonance imaging (MRI).

Although the following description relates primarily to the provision of a screen for the protection of individuals who may be exposed to potentially harmful radiations, it is to be mentioned that the screen technology could 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 EMF exposure of electrical or other non-biological components in certain environments where ferrous shielding may not be possible or practicable.

Additionally, while the following description is almost exclusively concerned with occupational exposure of humans to radiations from apparatus commonly used in medical apparatus, e.g. MRI scanner 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. The invention could also be used in protection of non-human components, such as electronics, where conventional methods of passive ferromagnetic shielding may not be possible or practicable.

BACKGROUND

Nuclear magnetic resonance (NMR) was first observed by Felix Bloch and Edward Mills Purcell at Berkeley in 1946. NMR is a physical phenomenon based upon the magnetic properties of certain atomic nuclei. By aligning H1 hydrogen protons in a powerful magnetic field nuclear resonance can be achieved with the use of specific electromagnetic fields. The response to the field is exploited in MRI.

Since the discovery of NMR, equipment and techniques have developed primarily, but not exclusively, in medical imaging. MRI is now a commonly used clinical and research tool in medicine. (NMR is now more commonly referred to as MRI in medicine). Medical MRI scanners use static (BO) and time-varying electromagnetic fields (TV EMFs), as well as RF, to non-invasively image soft tissues of the body. Some of these radiations constitute a potential real threat to human health e.g. the static field magnetic field poses a potential ferromagnetic missiles threat. The RF fields cause some tissue heating and conductive effects within the body. The TVEMFs (produced by the magnetic gradient coils) result in current induction in body tissue and are considered by some experts to cause other potential perceived threats to human health which are the subject of ongoing research and debate.

The Physical Agents Directive (PAD) 2004/40/EC 29 April 2004 Directive 89/391 /EEC, in particular the branch of this directive relating EMFs, was adopted by the EU in April 2004, and requires adoption of limits on exposure of occupationally-exposed persons to EMF (and RF) by all the various member states of Europe before April 2008.

Additionally, the International Commission on Non-Ionising Radiation Protection (ICNIRP), which is part of the World Heaith Organisation WHO), has prepared guidelines known as ICNIRP 1998 which define:

- limitations on occupational and public exposure - restrictions based on current density within the body and specific absorption rate (SAR) of energy within tissues

- 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 EMF during MRI examinations are regulated and overseen by the Department of Health, the Medical Devices Agency, the Medicines and Healthcare Products Regulatory Agency and the Health and Safety Executive. From April 2008 the EU are expected to enforce the new levels of exposure of Directive 2004/40/EC relating to occupationally-exposed workers.

It is an object of the present invention to provide a screen which will ensure that the abovementioned TVEMFs and RF radiations (if present) are attenuated to such a level at which occupationally-exposed workers (OEW's) can, when required, work in closer proximity to diagnostic and interventional MRI scanners by using the MRI screen system than they would otherwise be able to under the new directive without the screen. It is a further object of the present invention to provide apparatus which is capable of reducing TVEMF, and RF radiations (if present at these distances at all), to lower levels at a given distance form the source of EMF/RF.

It is a possible further object of the present invention to assist in the reduction of RF and TVEMF exposure to patients undergoing MRI examinations although currently there is no existing legislation for patient exposure it might be deemed to be prudent to protect patients where reasonably practicable. Although this might seem incongruous with the fundamental requirement of MRI that protons be exposed to radiation to resonate them, it has been proposed by certain regulatory bodies that new low limits of exposure might be enforced as a prudent measure to protect human health. For instance, the invention might be used in cases where individuals may have particular sensitivity to certain radiations and EMFs, e.g. a pregnant patient.

It is a possible further object of the invention to provide an EMF/RF screen device for the protection of individuals to RF and EMF exposure in other industries such as telecommunications.

Of course, it has long been known to provide screening for different types of radiations. A most simple example is the use of sheet lead metal to attenuate a certain proportion of X-ray energy. Additionally, a Faraday cage, being a structure commonly used in MRI equipment and consisting essentially of a conductive structure of a fine wire mesh or continuous copper sheet, will function as a simple RF screen.

It is also known to provide screens for EMF and TVEMF by ferromagnetic shielding. In modern life, almost everyone is continuously exposed to low and high frequency EMF radiation, an example being mobile phone telecommunications. In one embodiment, screening of static and low frequency EMF radiations involves placing a physical barrier of shielding material between the source and the protected area. The most commonly used materials are high μ (magnetic permeability) metals or "Mu-metals". EMF shielding works by interacting and diverting/deflecting the EMF away from the protected area. A crucial criterion when designing a shield is the level of attenuation required at specific frequencies and powers of EMF.

BRIEF SUMMARY OF 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 laminae 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, most preferably solid high-purity copper because of its inherent high conductivity and low resistivity. Alternatively, aluminium or similar metal of low conductivity and resistance may be used for the first material. The second material is preferably a strong dia- or paramagnetic material or alloy thereof. Alternatively, the second material may be a non- ferromagnetic stainless 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 large attractive/repellent forces between the EMF source and ferromagnetic objects in the vicinity.

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. A further alternative is a copper mesh material

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 the time-varying EMF produced by the magnetic gradient-generating coils, and RF if present).

BRIEF DESCRIPTION OF THE DRAWINGS

A specific embodiment of the invention will now be described with reference to the accompanying drawing wherein

Fig. 1 shows schematically a laminar structure for a screen, and

Fig. 2 shows schematically the structure of the screen at the level of a window.

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, such as Aluminium. 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 provide a window area 8, constituted by a conductive or transmissive glass, impregnated with suitable substances which on the one hand are capable of RF attenuation, and/or on the other hand are capable of having electric and/or magnetic fields induced therein, as shown in Figure 2. In a particularly preferred embodiment, the window is made of a fine copper mesh.

In a preferred arrangement which allows for 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, or alternatively a fine copper mesh, 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 present invention, at least in terms of attenuating TVEMF signals, relies on the physical principle of skin depth, a measure of the distance over which the current in a conductor falls to 1/e of its original value from the surface of said conductor. The equation for calculating the skin depth (EMF attenuating thickness) of a conductor is well known and depends on the screen conductor's permeability, resistivity and conductivity values. The skin depth will vary with EMF frequency.

In MRI, time varying magnetic gradient EMF frequencies exist] up to the KHz level (TVEMF from the magnetic gradient coils) and up to MHz levels in respect of RF radiation from the transmitting RF coil. Appropriate thicknesses of copper screen have been calculated to attenuate this range (the lower frequency EMFs resulting in thicker skin depth values than the higher frequencies). It is suggested a value of between 2 and 6mm of effective copper thickness would be appropriate for most MRI applications. Thicker screens could be used for lower frequency TV EMF protection.

The reader will be immediately aware of a number of possible applications, but the following will exemplify the invention and its uses:

- Between an MRI scanner and a clinician during MRI examinations

- Between an MRI scanner and a child's mother during an escorted MRI - During intraoperative MRI technologies where surgeons and others may be require standing close to the magnet in their work and as a result may be exposed to EMFs produced by the scanner.

- be floor or ceiling mounted so as hospital employees may look through the transparent part of the screen. - As a body apron or skirt for clinicians and other hospital personnel

- As a body apron of a pregnant worker, or for safety prudency, even a pregnant patient.

With regard to body wear the screen would have to be flexible with fastenings around the waist.

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 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.

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 1 - 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 preceding claim further including a window integrated into the screen allowing light transmission and consisting essentially of one or more of the following: - conductive glass - copper mesh.

7. 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 RF attenuation, and on the other hand are capable of having electric and/or magnetic fields induced therein.

8. A screen according to any of claims 1-3 which is substantially entirely light transmissive and is comprised of two different light transmissive layers, a first being attenuating to RF radiation, and the second being attenuating to TVEMF radiation.