GB2519976A

GB2519976A - Improvements in and relating to noise cancelling devices

Info

Publication number: GB2519976A
Application number: GB1319392.5A
Authority: GB
Inventors: Carine Henriette Wood
Original assignee: Individual
Current assignee: Individual
Priority date: 2013-11-03
Filing date: 2013-11-03
Publication date: 2015-05-13
Also published as: GB201319392D0

Abstract

An active noise cancelling (ANC) circuit 28 comprises a device (30) such as a microphone without any metallic parts, for capturing a noise input in a hostile environment such as a magnetic resonance imaging (MRI) scanner 10. The device derives a noise intensity signal; a processing unit (34) receives the noise intensity signal and produces a corresponding noise cancelling signal. Non-metallic means feeds the noise cancelling signal to the harsh environment to act as a signal to a pair of non-metallic headphones 26 or with one or more non-metallic loudspeakers. Dome shaped vibrator pads with a soft impermeable membrane may also be included, for connection to a users 12 skull via a strap or cap. A breathing mask arranged to receive a noise cancelling signal may be placed over the user, for example a patients, mouth. Additional signals may be received from a control unit that drives the source of the noise and the processing unit may be programmed to compensate for the delay both in receiving the noise input signal and delivering the inverted noise cancelling signal. Signals may be transmitted pneumatically.

Description

IMPROVEMENTS IN AND RELATING TO

NOISE CANCELLING DEVICES

This invention relates to noise cancelling devices, and particularly to those for use in an environment where no metallic objects are permitted and particularly for use by patients or animals in magnetic resonance imaging (MRI) machines.

Noise cancelling has been used in vehicles to attenuate vehicle noise, and headphones for travellers. But all these devices require a microphone or an accelerometer in situ to capture the sound waves and to convert them to an electrical signal that can be processed to produce a noise cancelling signal. Such devices are not suitable for MRI machines because of the metallic, or semiconductor content which would risk over-heating or worse as a result of the fluctuating high magnetic

fields encountered in such machines.

However, MRI machines are very noisy for the patient. Children commonly become distressed by the noise, and often have to be sedated or anaesthetised. Increasingly, animals are subjected to MRI scans. They either have to be trained for many months -at considerable cost -or heavily sedated. Sedation is not suitable where interactive brain function is to be studied.

In accordance with the invention, a noise cancelling circuit comprises a device for capturing a noise input in a high-noise environment without the use of any metallic parts and deriving a noise intensity signal, a processing unit for receiving the noise intensity signal and producing a corresponding noise cancelling signal, and non-metallic means for feeding the noise cancelling signal to the harsh environment to act as a signal to a pair of non-metallic headphones or with one or more non-metallic loudspeakers.

Such a circuit will offer a significant advantage in relation to young children who are often distressed by undergoing an MRI scan. Seriously ill children may have to undergo many such scans and can become traumatised by the repeated experience.

A truly effective noise cancelling pair of headphones will if not perhaps totally remove the distress, but it should at least attenuate it significantly, and make it a less traumatic experience. It is also expected to reduce the number of children that need to be anaesthetised.

For adults, it is not uncommon for those who have a fear of claustrophobia for this fear to be exacerbated by the noise level in an MRI scanner. It is believed if the noise intensity that they experience is reduced they will more readily undergo a scan or require reduced sedation.

It should also reduce the amount of training required for animals that are to be subjected to brain scans with little sedation.

However, small children and animals are unlikely to be able to make the adjustments to the level of the noise cancelling signal to the headphone, so in accordance with an additional feature of the invention, a feed-back loop is established by which the processing unit is programmed to adjust the noise cancelling signal by comparing the detected noise intensity signal with the noise cancelling signal received at the headphones and using the difference to correct the noise cancelling signal sent to the headphones in real time.

Whilst absolute noise cancelling by means of headphones will not totally relieve the discomfort caused by the high intensity noise produced by an MRI scanner and experienced by a patient, even where the intensity of the noise cancelling through the headphones exceeds the noise level produced by the scanner. This is because a part of the noise and vibration passes through the skull. In order to attenuate such noise and vibration, in accordance with an additional feature of the invention, additional vibrator pads are attached to the skull of the patient. These vibrator pads receive a similar noise cancelling signal to that received by the adjacent earphone.

Two or more pads may be placed over the scalp and held in place by a strap or cap.

Ideally they comprise a shallow rigid dome whose open face is covered by a soft impermeable membrane. A noise cancelling signal is fed pneumatically to the vibrator pads. Its intensity will be adjusted to produce the optimum effect, but will significantly improve the noise sensation experienced by the patient.

In severe noise environments a proportion of the noise experienced by a person travels via the mouth; in a scanner it has been observed that often a patient keeps their mouth open, thus aggravating the noise that they experience. According to an additional feature of the invention it is proposed to provide a breathing mask, preferably with an external supply of air (or other breathing gas mixture), that also receives a noise cancelling signal to cancel any residual noise effect that could travel through the patient's mouth.

It is anticipated that the processing unit may be programmed to remember' the operating conditions in an MRI scanner (after all the underlying scanning frequencies and intensities have similarities) and to adjust the noise cancelling signal accordingly.

In addition, the processing unit may advantageously be connected to or arranged to receive additional signals from the MRI control unit in order to adapt the noise cancelling signal more rapidly or to enable it to produce a more accurate signal in dynamic or unstable conditions.

The noise capturing device may conveniently comprise a pneumatic diaphragm sensor arranged to communicate the noise input signal pneumatically to the processing unit. However, pneumatic diaphragms are hardly ideal for accurately capturing a multi-frequency noise input that may vary significantly in intensity. It is contemplated that the diaphragm sensor and the communicating tube between the diaphragm and the noise detection unit in the processing unit will be calibrated externally for losses, distortions, harmonics, timing delays, etc. under simulated conditions.

In most cases, and particularly where a pneumatic signal is used for the noise input, there is a significant time delay between the detection of the signal and its processing. Equally, the noise cancelling signal may be sent pneumatically to the headphones, compounding the delay. The processing unit thus needs to be programmed to compensate for the delay both in receiving the noise input signal, and for delivering the inverted noise cancelling signal to the headphones or loudspeaker(s). The sound transmitting characteristics of headphones and the tubes connecting them with the noise generating unit will thus advantageously be calibrated externally prior to their use in the circuit.

Additional signal processing may need to be carried out in order to compensate for losses, harmonics, and/or absorption by the noise detection device, the pneumatic 3.

tubing, processing unit, or the headphones/loudspeakers(s) in order to obtain the highest quality noise reduction.

As the noise profile may differ from one side of the patient to the other, a discrete noise detection signal may need to be provided for either side of a pair of headphones in order to achieve the desired level of noise cancellation. Thus, it may be desirable to provide two separate noise cancellation circuits, one for the left side, and another for the right side, for example.

In other versions it is contemplated that the noise input signal could be derived from, or additional input, provided by the MRI control unit itself. In older units it may be convenient to derive the noise input signal directly from one or more accelerometers attached to a part of the machine that received the vibrations without being subjected to the high magnetic fields that dictate against the presence of metallic objects. It may equally be possible to use a piezo quartz crystal suitable sited to receive the vibrations.

Whilst the invention has been described primarily with reference to the use of pneumatically powered headphones, as these are widely available at relatively modest cost, the use of a plurality of small non-metallic loudspeakers would provide an advantageous solution as there would be no need to attach any apparatus to the patient (or animal).

The advantages of providing a high level of noise cancellation in an MRI scanner, for children and for animals will be readily recognised. With an intelligent system programmed for incremental improvement it is anticipated that a very high level of noise reduction will be achievable, and much distress avoided.

The invention will ne be described by way of example with reference to the accompanying drawings in which: Figure 1 shows diagrammatically a noise reduction circuit in accordance with the invention attached to a patient in readiness to undergo an MRI scan, and Figure 2 illustrates schematically the noise reduction circuit in accordance with the invention in greater detail. 4.

Figure 1 shows an MRI scanner 10 with a patient 12 lying on the moveable table 14 of the scanner, ready to progress into the core of the scanner for an MRI scan. The scanner has a magnetic field power source 16 connected to the magnetic coils 18 and a sequencer (not shown) controlled by a central processing unit 20. The MRI image is recorded below the patient 12 by a circuit 22 connected to a receiving unit 24. The central processing unit 20 has various operator controls, imaging outputs and data storage means that are not shown. The patient is provided with a pair of headphones 26 connected to a noise reduction processing circuit 28.

As shown in more detail in Figure 2. the noise reduction circuit 28 comprises a noise detection transducer 32, a processing unit 34 and a noise generator 36. In close proximity, possibly connected to or forming a part of each headphone 26 is a pneumatic noise detection sensor 30 which is connected by means of a semi-rigid tube 31 to the input signal detection transducer 32. The output from the input signal detection transducer 32 is fed to the processing unit 34 that is programmed to reverse the phase of the input signal, compensate for time delay in the signal, losses and distortion and produce a signal to a noise generator 36 that feeds the resultant noise cancelling signal to the headphones 26. The processing unit 34 also needs to take account of delays, losses and distortions both in the pneumatic noise detection and headphone circuits and their connecting links and adapt its noise cancelling output accordingly.

The noise detection sensor 30 comprises a rigid plastic dome 48 with a taught diaphragm 50 connected to the input signal detection transducer 32 by means of the semi-rigid plastic tube 31. The size and design of the noise detection sensor 30 and the shape of the dome 48 will be determined by reference to the frequency range and amplitude of the noise in the scanner and the characteristics of the input signal transducer and the tube 31.

The headphones 26 are similarly connected with the noise generator 36 by means of the semi-rigid plastic tube 37. Ideally both plastic tubes are of similar length, though they may be of different diameters. They both need to be as rigid as possible to keep losses to a minimum. 5.

An important aspect of the invention is its ability to control the amplitude of the noise cancelling effect at the headphones automatically in view of the primary application which is for very young children or for animals. For this a feed-back loop 38, from the noise detection sensor 30, and 40 from the headphone 26 is provided whereby the noise level at the noise detection sensor 30 is constantly compared at a feedback processor 42 with the amplitude of the actual cancelling noise at the headphones.

The frequency shift may also be compared and adjusted by the processing unit 34 in order to produce an optimum result without any action by the patient. A separate feed-back loop will be required for each headphone of a pair where separate noise detection and noise generation circuits are provided for each headphone.

In order to enhance the performance of the processing unit, it may be provided with a storage device 44 so that incremental improvements can be recorded as a learning' process. In addition a link 46 may be provided with the MRI central processing unit 20 whereby pre-information can be transmifted to the processing unit to improve the noise cancelling performance, and prepare for frequency or amplitude changes in the scanning.

Patients often like to listen to music or speech during a scan that will last 20 minutes, or longer. In the case of the present invention, and particularly in view of the feed-back loop, if such entertainment is required, it may be necessary to provide an additional feed to the headphones, or additional processing at the processing unit to prevent the music or speech from being cancelled out automatically through the feed-back circuit. Equally, the MRI operator may need to give verbal instructions to the patient; it is important that these instructions should not be eliminated or distorted by the noise cancellation system. Additional programming or a subsidiary channel may be required to the headphones to ensure such communication is available.

In general metal objects must not enter an MRI scanner because of the huge magnetic fields and the danger of either ferrous metal being attracted to the coils, or conductors causing electric currents and due to the magnetic fields, Thus the parts of the noise detection device that are designed to enter an MRI scanner must not include any metal, which imposes severe limitations as to the design of the noise cancelling circuit. 6.

A noise cancelling circuit as described above for use with an MRI scanner will greatly facilitate in reducing anxiety and trauma in young children and susceptible adults. It will also permit the use of MRI scanning of animals with reduced training with the prospect of enhancing and extending the scope of veterinary research. 7.

Claims

CLAIMS1. A noise cancelling circuit comprising a device for capturing a noise input in a hostile environment without the use of any metallic parts and deriving a noise intensity signal, a processing unit for receiving the noise intensity signal and producing a corresponding noise cancelling signal, and non-metallic means for feeding the noise cancelling signal to the harsh environment to act as a signal to a pair of non-metallic headphones or with one or more non-metallic loudspeakers.
2. A noise cancelling circuit in accordance with claim 1 in which the processing unit is arranged to receive additional signals from a control unit that in use drives the source of the noise in order to adapt the noise cancelling signal more rapidly or to enable it to produce a more accurate signal in dynamic orunstable conditions.
3. A noise cancelling circuit as claimed in either claim 1 or claim 2 in which the processing unit is programmed to adjust the noise cancelling signal by comparing the noise intensity signal with the noise cancelling signal and using the difference to correct the noise cancelling signal in real time.
4. A noise cancelling circuit in accordance with claim 1 in which the noise capturing device comprises a pneumatic diaphragm.
5. A noise cancelling circuit in accordance with claim 1 in which the noise cancelling signal is transmitted to the headphones or loudspeaker(s) pneumatically.
6. A noise cancelling circuit in accordance with claim 1 in which additional vibrator pads are provided and arranged to be attached to the skull of the patient whereby the vibrator pads receive a similar noise cancelling signal to that received by an adjacent earphone.
7. A noise cancelling circuit in accordance with claim 6 in which two or more pads are provided to be placed over the scalp and held in place by a strap or cap. 8.
8. A noise cancelling circuit in accordance with claim 6 in which the vibrator pads each comprise a shallow rigid dome whose open face is covered by a soft impermeable membrane and in use a noise cancelling signal is fed pneumatically to the vibrator pads.
9. A noise cancelling circuit in accordance with claim 1 in which includes a breathing mask to be placed over a patient's mouth and in use is arranged to receive a noise cancelling signal to cancel any residual noise effect that could be experienced through the patient's mouth.
10. A noise cancelling circuit in accordance with claim 1 in which the processing unit is programmed to compensate for the delay both in receiving the noise input signal, and for delivering the inverted noise cancelling signal to the headphones or loudspeaker(s).
11. A noise cancelling circuit in accordance with claim 1 in which the processing unit incoiporates or is connected to a source of music or other entertainment that piovides the latter to the headphones or loudspeakei(s) in addition to the noise cancelling signal.
12. A noise cancelling circuit in accordance with any preceding claim and substantially as herein described with or without reference to the accompanying drawings. 9.