WO2008137003A1

WO2008137003A1 - Acoustic noise attenuation system for mri scanning

Info

Publication number: WO2008137003A1
Application number: PCT/US2008/005568
Authority: WO
Inventors: Marc J. Kaufman; Blaise B. Frederick; Eric E. Ungar; Jeffrey A. Zapfe
Original assignee: Acentech, Inc.; The Mclean Hospital Corporation
Priority date: 2007-04-30
Filing date: 2008-04-30
Publication date: 2008-11-13

Abstract

A capsule for reducing the acoustic noise that is transmitted from MRI scanners to subject animals or persons ('patients') undergoing functional and/or structural examination or testing in such a system is disclosed. The capsule of the invention encloses the entire subject so as to reduce the acoustic noise transmitted to the subject's ears and other sensory organs not only via the air but also via bone conduction and body conduction. The capsule can be constructed to be compatible with the bores of existing MRI scanners, so that its use does not require modification of the scanners. Also, the capsule of the invention can be designed to accommodate a variety of accessories that are commonly used in conjunction with scanners.

Description

ACOUSTIC NOISE ATTENUATION SYSTEM FOR MRI SCANNING

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No. 60/926,939, filed April 30, 2007, the whole of which is hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

Part of the work leading to this invention was carried out with United States Government Support under Career Development Award Nos. K25 DA014013 and K02 DA017324 from the National Institutes of Health, National Institute for Drug Abuse, and under Grant No. 1 R43 MH075466-01 from the National Institute of mental Health. Therefore, the U.S. Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

MRI technology is increasingly being used for medical diagnosis and treatment evaluation, as well as for research, e.g., in studies of brain changes associated with a number of different health disorders. MRI scanning has advantages over other imaging techniques because it is noninvasive, it does not require ionizing radiation, and it provides functional and structural images in the same session. However, patients often refuse scans because of the anxiety and stress evoked by the scanner environment (Mclsaac et al., 1998), which typically consists of a patient bed that slides into a confining bore opening. Furthermore, MRI scanners are very noisy with some scans exceeding 120 dB (McJury et al . , 2000; Ravicz et al . , 2000; Moelker et al . , 2003). The stress associated with the acoustic noise of MRI scanners can affect many domains of brain function in humans and animals and thus introduces a major confound (Tomasi et al . , 2005) . Thus, there is a large incentive, both medical and economic, to reduce scan refusals, to increase patient comfort, and to minimize sound-induced research confounds. The confinement problem has been addressed by development of "open MRI" scanners, which are less likely to induce claustrophobia. However, standard systems cannot be retrofit to create open MRI units, and this strategy requires a major hardware investment, on the order of many hundreds of thousands of dollars. Also, open MRIs are limited in the magnetic field strength they can achieve. This is significant because MRI technology is advancing in directions that require higher magnetic field strengths (Sullivan, 2000) , and there are major trade-offs between scanner openness and performance .

An alternative solution involves audiovisual stimulus devices that mask scanner noise and provide comforting visual feedback. Such devices have proven successful (Harned et al . , 2001) , but they are costly and require investments on the order of tens of thousands of dollars. Also, because sound energy can be transmitted through bone and other body parts, headphones do not block all sound, particularly in intense sound fields produced by high field MRI scanners. Thus, a more complete barrier of sound energy transmission to the subject undergoing the scanning procedure would be desirable.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to a capsule for reducing the acoustic noise that is transmitted from MRI scanners to subject animals or persons ("patients") undergoing functional and/or structural examination or testing in such a system. The capsule of the invention encloses the entire subject so as to reduce the acoustic noise transmitted to the subject's ears and other sensory organs not only via the air but also via bone conduction and body conduction (Moelker et al . , 2005) . The capsule can be constructed to be compatible with the bores of existing MRI scanners, so that its use does not require modification of the scanners. Also, the capsule of the invention can be designed to accommodate a variety of accessories that are commonly used in conjunction with scanners .

In a preferred embodiment, the capsule is made largely of a transparent and easily cleaned and sterilized material, such as Lucite, to enable at least part of the subject to be observed visually. Appropriate monitoring devices (e.g., sensors, microphones and other communication devices) can also be included. Exemplary monitoring devices include sensors internal to the subject capsule, e.g., for detecting temperature or inspired/exhaled gas levels, observation cameras and microphones or RF coils integrated into the capsule. Furthermore, the materials used are chosen such that they do not interfere with MRI scanning or produce spurious signals in such scanning (Babcock et al. , 1990) .

In a preferred configuration, the capsule is substantially in the shape of a cylindrical tube with end closures, e.g., caps or plates, which are provided with acoustically treated connector parts or acoustically sealed cable pass-throughs for the provision of electrical connection with the interior capsule, e.g., to permit data acquisition from and other interactions with subjects, and for the feeding in and removal of fluids, i.e., liquids or gases .

The capsule can be of single or, preferably, of multiple- layer construction (e.g., in the form of nesting shells), with resilient connections or supports and air spaces between the layers, and with each layer consisting of a sandwich of layers separated by air-spaces or inter-layers of materials with desired acoustical and/or structural properties. The resilient supports are configured so that contact between the layers is made at locations of minimum vibration or sound transmission.

Preferably, acoustically absorptive material is provided in the capsule's interior and in the air spaces, e.g., between the layers, to reduce noise transmission into the interior and noise build-up in the interior. The absorptive material may be disposable for the sake of cleanliness or easily cleanable and, preferably, consists of pads of fiberglass or plastic foam encapsulated in bags or coverings of thin Mylar film or the like. Acoustically absorptive and mechanically resilient material can also be provided between the exterior of the capsule and the bore of the MRI scanner in order to reduce the airborne and structure- borne sound transmission to the capsule.

A cradle or support structure for the test subject or patient, as well as supports for the cradle, can be provided to facilitate insertion and removal of the subject into the capsule. This cradle is configured and supported so that it does not defeat the capsule's desired acoustical attenuation characteristics. The cradle can be made to be easily cleanable (sterilizable) and/or disposible . The end closures on one or both ends of the capsules and their connections are configured to ^' be easily and quickly removable, in order to facilitate rapid access to and removal of the subject.

The capsule material, layering, and layer interconnections are configured to provide the most significant attenuation of noise to the subject in the frequency regions in which the MRI generates the most stressful noise, where stressfulness is judged relative to the subject's hearing. For example, attenuation is provided predominantly in the frequency ranges in which the subject's hearing is most acute or in which the noise affects a physiological or psychological function most severely.

Furthermore, noise attenuation in some frequency ranges may be provided by means of dynamic vibration absorbers that are attached to the capsule structure and tuned to suppress capsule vibrations in given frequency ranges. Such a vibration absorber may consist, for example of a block of a plastic material that is fastened to the shell structure via a layer of elastic of viscoelastic material . The frequency at which such an absorber suppresses vibration (and the acoustic noise radiated as a result of this vibration) depends on the ratio of the block's mass to the stiffness of the attachment. An array of such absorbers, each tuned to a different frequency, is particularly useful for the suppression of vibration and noise over a range of frequencies (Zapfe et al . , 1997) . Similarly, tuned sound absorbers may be placed in the capsule interior or between capsule layers, in order to suppress sound in selected frequency ranges. Sound absorbers of the Helmholtz type, for example, are well known. These consist essentially of a confined volume that communicates with the ambient air via a narrow opening; tuning of such absorbers is accomplished by selection of the volume and the dimensions of the opening .

In some embodiments, active noise control may be provided. Referring to Fig. 6, acoustic sensors (microphones 62) may be located in the capsule interior and/or in the air spaces between the capsule layers, and vibration sensors 64 may be attached to various components of the capsule structure. Also, acoustic actuators (e.g., loudspeakers 66) may be located in the capsule interior and/or in the air spaces between the capsule layers, and vibration actuators 68 may be attached to various components of the capsule structure . The signals from the various sensors may be fed to a controller 70 via analog-to-digital converters, signal conditioners, filters, etc. together with signals taken from the MRI signal generator 72. Feed-forward and feedback algorithms may be implemented in a high-speed digital controller to provide signals to the loudspeakers and actuators so as to attenuate the noise in the capsule interior. The control algorithms and mechanisms are well in hand, but care must be taken to provide sensors, loudspeakers, and actuators that are not affected by the magnetic fields of the MRI scanner. All devices are of types that are not affected by the magnetic fields that are present in MRI systems. For example, optical microphones may be used to avoid any effect of magnetic fields on diaphragms of conventional microphones . In another embodiment, scanning pulse sequences can be used that produce noise in only limited frequency bands. For example, various versions of the echo planar imaging sequences can be tuned to yield primarily a single fundamental tone and its odd harmonics. These sequences can be used in conjunction with sound and vibration attenuation means that are tuned to these limited frequency bands .

In yet another embodiment, microphone (s) or other communication devices, e.g., for two-way communication with the subject, are provided in the interior space, the inner shell and/or its closure (s) or parts of these structures. Structural parts with attached piezoelectric (or similar) elements may serve as microphones and/or loudspeakers not only for communication, but also for active control purposes. In other embodiments, MRI-compatible video screens can be provided to present visual stimuli relevant to behavioral testing or to create a virtual reality environment within the capsule, which is well-known to reduce feelings of claustrophobia (Garcia- Palacios et al . , 2007). The system size can be scaled for use with small or large animal subjects and to fit small and large MRI scanner bores. The system of the invention also can be scaled for use with human subject scans that require noise isolation including research scans involving acoustic stimuli in low noise environment and clinical scans of subjects who would otherwise reject a scan because of noise stress.

The invention offers an economical means to mitigate the effects of MRI scanner acoustic noise, including the stress noise induces in human and animal subjects and the confounds noise can induce in human and animal brain imaging research (Ungar, 2006) . The invention uses a technology that is independent of MRI scanner technology and it can be easily and economically adapted for use with a wide range of MRI scanner sizes and configurations. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims, taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective view of a preferred embodiment noise attenuation system according to the invention;

Fig. 2 is an axial section schematic through the embodiment of Fig. 1;

Fig. 3 is a perspective end view of the embodiment of Fig. 1 without an end-plate,-

Fig. 4 is the section A-A through the axial section of Fig. 2,- Fig. 5 is an axial section schematic through an end-plate of the embodiment of Fig. 1, which is equipped with connection ports and pass-throughs ;

Fig. 6 is a diagram showing a sensor/activator communication pathway via a digital controller; and Fig. 7 is a graph showing the change in sound pressure level with frequency, measured at a test subject head location.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figs. 1 and 2, in a preferred embodiment, the capsule system according to the invention 10 consists of two cylindrical capsules 12, 14, with the smaller one 12 nested inside the larger one 14. Referring also to Figs. 3 and 4, the dimensions of the tested small monkey model system were chosen so that a restraint device 16 could fit inside the interior capsule 12 and the entire two-capsule assembly could fit within an approximately 50 cm diameter space in a typical MRI scanner bore. For human patients, or research subjects, the cylindrical capsule can be expanded to the maximum size that would fit into any MRI scanner to be used for clinical or research scans, in order comfortably to accommodate adult humans. The capsule will be of sufficient structural strength so that it will not crack, break, or deflect excessively under a patient's weight. For these reasons, the shells that constitute the capsule will need to be reinforced with rings and stringers (e.g., longitudinal beams), which will be located on the shells and fastened to them via resilient layers (according to principles well-known in the art, e.g., as described in L. Cremer, M. Heckl, and E. E. Ungar, 1988) so that the reinforcements interfere minimally with the noise attenuation of the shell . The material properties and dimensions of the shell are selected from both the noise attenuation and the structural strength standpoints .

An air gap 18, partially filled with acoustically absorbent material, is maintained between the two capsules 12, 14. The inner capsule 12 is supported in the outer one 14 on a small number of resilient feet 20, configured so that the inner capsule can easily be slid into the outer one. Some of the interior space that is not occupied by the test animal (or patient) and instrumentation is filled with an acoustically absorptive foam or fibrous material and, preferably, provided with a thin skin of Mylar or the like, so that it does not absorb liquids or gases and can be cleaned easily.

Both ends of each capsule are closed by circular plates or domes 22a, 24a; 22b, 24b, each configured to provide soundproof (airtight) closure while also contributing to the rigidity of the cylindrical shell and thus to its sound isolation.

The closures on at least one end of each capsule 22a, 24a are secured to their mating flanges 26a, 28a by means of quick- release provisions 30, such as latches or Velcro^® straps, enabling rapid access to and removal of the test subject (e.g., a monkey) . These end closures, and even the capsule walls, may be fully or partially transparant to enable observation of the subject. The end closures on the other side 22b, 24b can be attached to their flanges 26b, 28b in an air-tight manner, e.g., by means of plastic bolts 32, permitting these end-plates to be removed to facilitate cleaning. Referring now to Fig. 5, at least one set of end-plates, e.g., 22b, 24b, is equipped with connection ports 34a and pass- throughs 34b for the provision of, e.g., electrical and fluid access to the test subject in the inner capsule. In one embodiment, tubing 33 is inserted directly into a pass-through 34b. In another embodiment, all tubing and cables between the subject area and the exterior are run through the same thick- walled tube or pipe 36, which penetrates the two end closures 22b, 24b opposite those used for observation 22a, 24a with no gaps, or with sealant, between the outside of the pipe and the end-plate material. In addition, or as a partial alternative, the end-plates may be provided with embedded two-sided electrical connectors 38.

The portion of the pipe 36 between the two end-plates will be flexible and, perhaps, spiral or S-shaped. The portion of pipe 36 outside of the exterior capsule may be of heavy, relatively flexible plastic, may be corrugated, and may extend perhaps 6 feet from the end of the MRI scanner. This length would position the opening of the pipe an extended distance from the scanner and, thus, in an area of comparatively little noise. This long section of pipe could be connected to the exterior end-plate by a bolted or quick-release arrangement.

The following examples are presented to illustrate the advantages of the present invention and to assist one of ordinary skill in making and using the same. These examples are not intended in any way otherwise to limit the scope of the disclosure .

Experimental Results Test Instrument

4.0 Tesla MRI scanner (Varian NMR Systems, Inc., Unity/Inova, Palo Alto, CA) , at McLean Hospital, Belmont, MA (More et al. , 2006) . Test Protocol

For determining the reduction in noise that a test subject would experience using the system of the invention, the noise measured in the bore of the MRI scanner with and without the test capsules, at the approximate location of a test subject's head, was compared. Three subjectively very noisy pulse protocols that are routinely used in various human and primate brain imaging studies were chosen: two anatomical scans (a T2-weighted multi- slice axial fast spin echo sequence, and a Tl-weighted 3D FLASH sequence with initial inversion) and a standard gradient echo EPI sequence used by McLean Hospital, Belmont, MA for functional studies .

Non-Acoustical Requirements From interviews with researchers, it was determined that a capsule needs to be cleanable (sterilizable) , to permit the entry and exit of liquids and gases via tubes, to provide electrical connections, to enable visual observation of at least the face of the test subject, and also to be configured so that the test subject can be removed from the capsule in no more than 30 seconds in case the subject shows signs of distress.

Materials

Clear cast acrylic and clear PVC were identified as suitable transparent materials that would not contribute spurious signals (Babcock et al . , 1990) and were determined from scanning tests not to interfere with the scanning process.

Capsule Properties The sound pressure levels that were present in the nested capsule system at the location of the head of a test subject were measured, with the arrangement shown in Figs. 1 and 3 in the bore of a 4.0 Tesla MRI scanner and with MRI scanning carried out using the three pulse sequences described in "Test Protocol." Figure 7 shows the average of the three sound pressure levels observed in each narrow frequency band, as well as the maximum of these three. The average and maximum values may be seen not to exceed 51 dB and 57 dB, respectively, in any frequency band.

The insertion loss of the capsule arrangement was determined by measuring the noise levels at the same microphone location with the same pulse sequences, but without the capsule, and comparing the noise levels observed with and without the capsule arrangement. The insertion loss represents the difference between the noise measured in the MRI bore without the capsule and that measured in the capsule. The measured insertion loss was typically between 40 and 50 dB at frequencies below about 2700 Hz and between 30 and 40 dB at higher frequencies.

The salient dimensions of the initial monkey capsule system, selected in part on the basis of acoustical considerations and in part on the basis of ready availability of cylindrical tube stock, are indicated in Table 1.

Table 1

The experimental capsules were subjected to broadband noise in a semi-reverberant test chamber, and the noise isolation provided by a capsule was determined by comparing the noise measured inside the capsule to the noise in the test chamber volume . Several experiments were carried out in which circumferential stiffening ribs and flexible, elastically bedded, wrappings were added to the capsule . The ribs were found not to make significant differences in the results, but the wrappings were found to be helpful .

A second series of experiments conducted in the test chamber showed that the capsule's noise isolation performance improved when thicker end-plates were used. However, measurements carried out in the MRI scanner itself showed that the noise isolation with thicker end-plates differed insignificantly from that with single end-plates, apparently because in the scanner the sound field at the end-plates is considerably more benign than that around the circumference of the double capsules.

References

Babcock, E. E., et al . , 1990. Multinuclear NMR Investigations of Probe Construction Materials at 4.7T, Magnetic Resonance in Medicine 13, 498-503.

Cremer, L., et al . , 1988. Structure-Borne Sound: Structural Vibration and Sound Radiation at Audio Frequencies . Springer Verlag: New York, 2^nd Edition.

Garcia-Palacios, A., et al . , 2007. Use of virtual reality distraction to reduce claustrophobia symptoms during a mock magnetic resonance imaging brain scan: a case report. Cyberpsychol . Behav. 10, 485-8.

Harned 2nd, R. K., et al . , 2001. MRI-compatible audio/visual system: impact on pediatric sedation. Pediatr Radiol 31, 247-50. Mclsaac, H. K., et al . , 1998. Claustrophobia and the magnetic resonance imaging procedure. J Behav Med 21, 255-68.

McJury, M., et al . , 2000. Auditory noise associated with MR procedures: a review. J Magn Reson Imaging 12, 37-45.

Moelker, A., et al . , 2003. Acoustic noise concerns in functional magnetic resonance imaging. Hum Brain Mapp 20, 123-41.

Moelker, A., et al . , 2005. Importance of Bone-Conducted Sound Transmission on Patient Hearing in the MRI Scanner. Journal of

Magnetic Resonance Imaging 22, 163-169.

More, S. R., et al., 2006. Acoustic Noise Characteristics of a 4 Tesla MRI Scanner. J Magn Resn Imaging 23(3), 388-97.

Ravicz, M. E., et al . 2000. Acoustic noise during functional magnetic resonance imaging. J Acoust Soc Am 108, 1683-96.

Sullivan, F., et al . , 2000. U.S. Magnetic Resonance Imaging (MRI) Equipment Markets. Vol. 2005.

Tomasi, D., et al., 2005. fMRI-acoustic noise alters brain activation during working memory tasks. Neuroimage 27(2), 377-86. Ungar, E. E., 2006. A note on the low-frequency noise reduction of cylindrical capsules, Journal of the Acoustical Society of America 120 (6), 3467-3470.

Zapfe, J.A., et al . , 1997. Broadband Vibration Damping Using Highly Distributed Tuned Mass Absorbers. AIAA Journal 35, 1997, 753-756. While the present invention has been described in conjunction with a preferred embodiment, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof .

Claims

CLAIMSWhat is claimed is;

1. An acoustical noise attenuation system for a medical resonance imaging (MRI) scanner, said system comprising: a capsule having two ends and configured both to enclose the entire body of a subject undergoing a scanning procedure in an MRI scanner having an internal bore and to fit within said bore of said scanner, said capsule further comprising at least one separable end closure at one of said ends, wherein at least one of said end closures is fitted with one or more acoustically treated connectors or acoustically sealed pass-throughs for monitoring or communicating with said subject.

2. The noise attenuation system of claim 1, wherein said capsule shape is cylindrical.

3. The noise attenuation system of claim 1, wherein said capsule is of multi- layer construction.

4. The noise attenuation system of claim 3, wherein said capsule is formed from nested cylindrical shells.

5. The noise attenuation system of claim 3, wherein said individual layers of said capsule are separated by air spaces and resilient connecting supports.

6. The noise attenuation system of claim 5, wherein said resilient supports are configured so that contact between said layers is made at locations of minimum vibration or sound transmission between said layers.

7. The noise attenuation system of claim 1, wherein said capsule further comprises quick release end closure attachments for rapid access to said subject.

8. The noise attenuation system of claim 1, wherein a wall or end closure of said capsule is fully or partially transparent to enable observation of said subject.

9. The noise attenuation system of claim 1, wherein said connectors or pass-throughs in said end closure permit data acquisition from and interactions with said subject and the feeding in and removal of fluids .

10. The noise attenuation system of claim 1, wherein said connectors or pass-throughs in said end closure are located and configured for minimum compromise of noise isolation.

11. The noise attenuation system of claim 1, further comprising an internal sensor for temperature or oxygen.

12. The noise attenuation system of claim 1, further comprising an observation camera.

13. The noise attenuation system of claim 1, further comprising RF coils integrated into said capsule.

14. The noise attenuation system of claim 1, further comprising active noise control.