ASA Vancouver Meeting

Structural vibration isolation Structural vibration isolation retrofit for a magneticretrofit for a magnetic

resonance imaging systemresonance imaging systemChad Himmel, PE

Associate Engineer, JEACOUSTICSAustin, Texas USA

himmel<at>JEAcoustics.com

The 149th Meeting of the Acoustical Society of America19 May, 2005 Vancouver, Canada

ASA Vancouver Meeting

OccupiedSuite

Corridor

Control RoomControl Room

WaitingWaitingEquipmentEquipment

Occupied SuiteUnoccupied Suite

Outside

MRIMRI

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Existing 1.5T MRI

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Background

• Suspended concrete floor slab• Crawl space below• Noise complaints from nearby

medical exam and office suites• Unoccupied suite above (with noise)• Neighbors threaten legal action to

shut down MRI operation ➪

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Objectives

• Identify possible airborne and structure borne noise paths – “noise from above”

• Quantify and compare noise transmitted via primary airborne and structural paths

• Develop concepts for noise or vibration control to reduce transient disturbance to comply with generic criteria

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Criteria and Parameters• Continuous background noise (baseline)

– Continuous building air conditioning and heating systems’ noise < NC-35 in adjacent spaces

• Magnet noise– Magnet produces loud transient noises– Transient noise > NC-35 in adjacent occupied

spaces should be reduced• Magnet equipment noise

– Similar to building HVAC equipment• MRI manufacturer’s allowable vibration

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Existing ConditionsStructurally isolated RF shield walls

Internal partitions not decoupled from floor slab

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Existing ConditionsDuctwork

Return

Supply

Penetration

“noise from above”

VIA DUCTWORK ?

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Existing ConditionsNon-isolated magnet feet

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Existing ConditionsAmbient and transient noise

• Sound levels with and without MRI scanning

ASA Vancouver Meeting

ASA Vancouver Meeting

Existing ConditionsAmbient and transient vibration

• Wall, floor, and ceiling vibration levels with and without MRI scanning

"NC-equivalent" curves for estimated radiated sound produced by vibration acceleration of a large surface.Noise Control for Buildings and Manufacturing PlantsCh. 3, Laymon Miller, 1981Bolt Beranek and Newman Inc.

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“noise from above”

CEILING VIBRATION

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Compared Noise & Vibration

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Compared Noise & Vibration

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Existing ConditionsStructural response measurements

• Structural response to heel drop impacts

• Apparent structural resonant frequency ≈ 19 Hz

• Possible harmonic of 9.5 Hz resonance

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Proposed Design ConceptsVibration Isolation• Detach bolts at magnet feet• Insert resilient pad between

magnet feet and floor slab• Use metal (aluminum) plates to

evenly distribute large concentrated load at feet

• “De-tune” isolation pads from existing floor resonant frequency and deflection

• Identify possible flanking paths; provide isolation

MRI weight, lbs (kg) 12000 (5455)

Weight / pad, lbs (kg) 3000 (1364)

Pad size, in2 (m2) 111.7 (0.07)

Pad load, psi (kg/m2) 26.8 (18907)

Apparent 19 Hz floor resonance;<.03” (.08 mm) deflection at MRI;∴ want δ > 3x0.03” or > 0.09”

Helium exhaust (emergency dump)

➪

➪

➪

ASA Vancouver Meeting

Vibration Control Recommendations

• fsource = 150-1000 Hz• iso% = 98-99%• fnisolator = 15-100 Hz• δisolator = 0.01”-0.05”

(0.3-1.3 mm)100

iso%111

f =(Hz) fn sourceisolator

−+

Cyril Harris, Shock & Vibration Handbook, 1988, 34-7Acoust. Meas. And Noise Control, 3rd Ed, p. 29.7

2

isolatorisolator fn

3.13 =(inches)

δ δ > 0.09” still OK.

Sylomer P-25 pad deflection is approx. 0.08" at 30 psi load.

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Noise ControlRecommendations

Possible airborne noise isolation measures• Higher mass drywall, incorporating copper shielding• Internal partitions decoupled from building structure• Resiliently suspended ceiling• Acoustic door seals (RF ‘fingers’)• Ductwork modifications• Interior surface absorptionsurface absorption in MRI room

ASA Vancouver Meeting

ASA Vancouver Meeting

Cut it off !

(or raise it a little

more off the floor)

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Implementation• Two pads and aluminum

shims – inexpensive• Dump helium and lift

MRI – expensive• ~0.11” total static

deflection – image quality ? (TBD)

• Most airborne noise control measures put on hold pending evaluation of pads. Interior acoustical pads added to MRI room – “easy add”

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Vibration Measurement Results

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Noise Measurement Results

• NC 35 in exam room; NC 45 in corridor• Continuous background HVAC noise

dominates most measured noise levels

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Post-Retrofit Evaluation

• Occupants report acceptable transient MRI sound levels

• In Control Room, magnet noise is clearly audible, but not annoying

• In adjacent spaces, magnet noise is sometimes barely audible

• MRI may become more audible in winter months when HVAC is ramped down

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Conclusions

MRI vibration control concepts are simple• Can be retrofit• Must be coordinated with MRI manufacturer

Early awareness of vibration is desired• MRI sales reps• Architects and builders

Confusion of airborne and structure borne noise transmission concepts is common