Structural vibration isolation retrofit for a magnetic resonance imaging system

Transcription

1 Structural vibration isolation retrofit for a magnetic resonance imaging system Chad Himmel, PE Associate Engineer, JEACOUSTICS Austin, Texas USA himmel<at>jeacoustics.com The 149th Meeting of the Acoustical Society of America 19 May, 2005 Vancouver, Canada

2 Outside Control Room MRI Occupied Suite Equipment Waiting Corridor Unoccupied Suite Occupied Suite

3 Existing 1.5T MRI

4 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

5 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

6 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

7 Existing Conditions Structurally isolated RF shield walls Internal partitions not decoupled from floor slab

8 Existing Conditions Ductwork Supply Return Penetration noise from above VIA DUCTWORK?

9 Existing Conditions Non-isolated magnet feet

10 Existing Conditions Ambient and transient noise Sound levels with and without MRI scanning

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12 Existing Conditions Ambient 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 Plants Ch. 3, Laymon Miller, 1981 Bolt Beranek and Newman Inc.

13 noise from above CEILING VIBRATION

14 Compared Noise & Vibration

15 Compared Noise & Vibration

16 Existing Conditions Structural response measurements Structural response to heel drop impacts Apparent structural resonant frequency 19 Hz Possible harmonic of 9.5 Hz resonance

17 Proposed Design Concepts Vibration 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) (5455) Weight / pad, lbs (kg) 3000 (1364) Pad size, in 2 (m 2 ) (0.07) Pad load, psi (kg/m 2 ) 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)

18 Vibration Control Recommendations fn isolator δ isolator (Hz) = (inches) fsource iso% fn isolator Cyril Harris, Shock & Vibration Handbook, 1988, 34-7 Acoust. Meas. And Noise Control, 3rd Ed, p = 2 f source = Hz iso% = 98-99% fn isolator = Hz δ isolator = ( mm) δ > 0.09 still OK. Sylomer P-25 pad deflection is approx. 0.08" at 30 psi load.

19 Noise Control Recommendations 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 absorption in MRI room

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21 Cut it off! (or raise it a little more off the floor)

22 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

23 Vibration Measurement Results

24 Noise Measurement Results NC 35 in exam room; NC 45 in corridor Continuous background HVAC noise dominates most measured noise levels

25 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

26 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