Roozen, Geerlings, Verhaar, Vliegenthart, New developments in near-field acoustic holography 1

Please leave this heading unchanged!

New developments in near-field acoustic holography

N.B. Roozen*, A.C. Geerlings, B.T. Verhaar, T. Vliegenthart. Philips Applied Technologies, High Tech Campus 7, 5656 AE Eindhoven *Both at Philips Applied Technologies and at the Eindhoven University of Technology, Den Dolech 2, 5612 AZ Eindhoven

Abstract

In the field of noise-control engineering, information about the individual strength, and

location, of the most dominant sources is of vital importance. This information allows the

acoustic engineer to take effective measures in his effort to reduce the emitted acoustic noise

levels. Near-field acoustic holography is a measurement-based tool which visualizes the

acoustic fields radiated by the noise emitting object, giving the engineer the noise source

information required. In the paper some applications of near-field acoustic holography, as

applied at Philips, will be discussed. A cylindrical version of near-field acoustic holography

was implemented and applied to a Magnetic Resonance Imaging scanner. Using an array of

microphones, the measurement time to obtain the holographic recordings was reduced

significantly, whilst the cylindrical near-field acoustic holography implementation allowed a

near-instant post-processing of the acquired data.

1. Introduction Magnetic Resonance Imaging systems are extremely helpful within Healthcare for its ability

to image the inside of the human body, especially the soft tissue, in a non-evasive way. The

imaging principle is based on atoms emitting radio frequent signals when excited in the same

way while being placed in a strong static magnetic field. The position information originates

from a superimposed, but smaller, magnetic gradient field generated by the gradient coil tube.

However the fast switching currents of the gradient coil inside the static magnetic field leads

to strong Lorenz forces on the mechanical structure, such that noise is unavoidable.

Figure 1: Patient inserted into a MRI patient bore.

2 NAG-Journaal

Since the patient is right in the middle of the gradient coil bore he (or she), as well as

the operator and the rest of the hospital environment needs to be shielded from the noise. The

space available for shielding in the patient bore is limited. Much research is focused on

optimizing the acoustic insertion loss (IL) of the gradient coil cover system.

Analyzing new cover designs in detail require advanced measurement techniques.

With laser vibrometry only vibrations can be measured. However acoustic leaks can not be

found and the radiation efficiency of shell covers is still very much a research topic. Recently

Philips Applied Technologies adapted their acoustic imaging technique such that it can be

used inside a cylindrical MRI system that was on-field (meaning that the currents in the

superconducting main magnet are running, thus producing a strong magnetic field). Besides

plate radiation also acoustic leaks have to be imaged. The resulting images help in the further

development of silent covers and will benefit the future patients.

2. Laser Vibrometry One could apply laser vibrometry to sample the structural vibrations of the gradient coil and

the cover structures. This has been done by Philips for the gradient coil itself, and also by

others (Kessels, [1]). Operational deflection shape analysis show that the coil vibrations can

be well predicted by modeling.

Sampling in this way the cover would require much more effort due to the higher

spatial sampling required for imaging the smaller bending wavelengths in the light cover

plating. However we are not as much interested in the structural cover modes as in the

radiation modes. Additionally laser vibrometry would not be able to image acoustic leaks.

For these reasons we decided to apply acoustic imaging.

3. Acoustic Imaging Wikipedia states:

“Acoustic holography is a method used to estimate the sound field near

a source by measuring acoustic parameters away from the source via

an array of pressure and/or particle velocity transducers.”

Source: Wikipedia

Acoustic holography has been applied by many researchers to localize and identify noise

sources. We call this acoustic imaging.

Using acoustics measured in the far field of a source the maximal spatial resolution

for imaging noise sources is limited by the free-field acoustic wavelength. By taking the

exponentially decaying near-field (evanescent field) into account a higher spatial resolution is

achievable, amongst others depending on the sensor spacing and the distance between the

sensors and the source as well as some post processing issues (Maynard, [2]). For instance

when we propagate the evanescent waves back to the source plane we have to magnify its

amplitude exponentially. At high spatial wavenumbers suffering from a lower S/N ratio the

imaging will deteriorate if no proper filtering is used. The better the filtering the higher

wavenumbers we can include in the imaging and the higher the resolution may become. This

filtering technique is subject to ongoing studies (Scholte, [3]).

Within Philips Applied Technologies an acoustic imaging setup has been made which

is especially devised to localize small noise sources. The technique has proven very

successful in a number of applications within the Philips community.

Below we see the case of a small noisy capacitor on a flexible PCB. The localization

of the noisy component was quickly solved with acoustic holography.

Roozen, Geerlings, Verhaar, Vliegenthart, New developments in near-field acoustic holography 3

Figure 2 Flat display (upside down, showing the

electronics which drives the flat display).

Figure 3 Reconstructed sound intensity at 13

kHz, just above the electronics of the display.

The colors indicate the intensity level (red: high

intensity, blue: low intensity), clearly showing a

single component emitting acoustic noise.

4. Cylindrical Set-up

To apply acoustic holography inside an MRI system some issues needed to be solved. First

we did not want to high frequent RF coil of the MRI imaging system be switched on, as it

would not be necessary for the acoustics anyway. However the superconducting main magnet

and the gradient coil, being the vibrational source, would be operated normally.

Figure 4: Positioning robot just outside the magnetic influence range.

4 NAG-Journaal

Another issue is the strength of the magnetic field. The positioning robot would have

to be positioned outside the field to prevent the robot to be sucked into the system

To speed up the measurement we make use of an array of 16 condenser microphones.

These microphones have been tested for operating properly inside a magnetic field.

We need a dense spatial sampling but we do not want the microphones spaced too

closely. This is why we use an array which is stepping along the axial axis while the

microphone positions are interlaced. Only when we step in the tangential direction we rotate

the array such that the microphones are perpendicular to the scanning surface.

At the lower part of the system the patient table (not installed in the picture above but which

is acoustically and operationally relevant) limits the full circular sampling.

Figure 5 The 16 channel microphone line array inside an MRI scanner.

5. Results The whole procedure to measure one configuration involved 10000 measurement points over

a surface of about 2 m2 with a resolution of about 0.02 m. This took 4 hours. The distance to

the wall was also about 0.02 m.

From the measurements we can obtain the particle velocities at the imaging surface as

can be seen in the picture below. We clearly see the central patient cover vibrating due to the

gradient coil vibrations directly behind this cover. At the end of the duct we see that the

flared cones are vibrating less since they are well isolated from the system.

Roozen, Geerlings, Verhaar, Vliegenthart, New developments in near-field acoustic holography 5

Figure 6: Surface vibrations at 670 Hz showing a clear circumferential vibration mode.

Figure 7: Reconstructed field at 312 Hz.

The above picture shows exactly what we had aimed for. Cover insulation at low frequencies

is hard to achieve in an MRI system. With this analysis we can analyze which part of the

acoustics goes through the cover and which part leaks through openings such as patient

cooling holes.

6 NAG-Journaal

6. Conclusions

Lowering the acoustics of an MRI system would be beneficial to the patients, especially

pediatric patients as to the clinical staff. Space inside the patient bore is expensive and due to

all sorts of restrictions (i.e. magnetic field, RF signals and space limitations) acoustic

insulation of the patient bore is difficult to improve.

With the acoustic holography technique made applicable to MRI studies we have a

powerful tool to study the behavior of cylindrical MRI covers systems. Due to the array

technique a high spatial resolution can be obtained, whereas the duration of an analysis is

limited to a relatively short period of time.

Acknowledgements

Kees Ham, Geert-Jan Plattel and Patrick Limpens, all from Philips Medical Systems, are

gratefully acknowledged.

References

1. Peter H.L. Kessels, 2001, “Engineering toolbox for structural-acoustic design applied to MRIscanners”,

Doctoral thesis, Technische Universiteit Eindhoven, ISBNIISSN 90-386-2682-7.

2. E.G. Williams, J.D. Maynard, 1980, “Holographic imagin without the wavelength resolution limit”,

Physical review letters, Volume 45, Number 7, pp. 554 – 557.

3. R. Scholte, I. Lopez, N.B. Roozen, H. Nijmeijer, 2007, “Wavenumber domain regularization for near-

field acoustic holography by means of filter functions and stopping rules”, submitted to the J.Acoust.

Soc. Am.