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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
ASA Vancouver Meeting
Existing 1.5T MRI
ASA Vancouver Meeting
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 ➪
ASA Vancouver Meeting
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
ASA Vancouver Meeting
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
ASA Vancouver Meeting
Existing ConditionsStructurally isolated RF shield walls
Internal partitions not decoupled from floor slab
ASA Vancouver Meeting
Existing ConditionsDuctwork
Return
Supply
Penetration
“noise from above”
VIA DUCTWORK ?
ASA Vancouver Meeting
Existing ConditionsNon-isolated magnet feet
ASA Vancouver Meeting
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.
ASA Vancouver Meeting
“noise from above”
CEILING VIBRATION
ASA Vancouver Meeting
Compared Noise & Vibration
ASA Vancouver Meeting
Compared Noise & Vibration
ASA Vancouver Meeting
Existing ConditionsStructural response measurements
• Structural response to heel drop impacts
• Apparent structural resonant frequency ≈ 19 Hz
• Possible harmonic of 9.5 Hz resonance
ASA Vancouver Meeting
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.
ASA Vancouver Meeting
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)
ASA Vancouver Meeting
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”
ASA Vancouver Meeting
Vibration Measurement Results
ASA Vancouver Meeting
Noise Measurement Results
• NC 35 in exam room; NC 45 in corridor• Continuous background HVAC noise
dominates most measured noise levels
ASA Vancouver Meeting
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
ASA Vancouver Meeting
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