Slab on Grade for MRI 1

[lutein]

I am designing the slab on grade for MRI, which is about 12kip in weight. To size the slab on grade, I have considered:
1. punching shear from the heavy magnets.
2. make sure the slab is thick enough for the post installed anchor embedment.
3. FRP reinforcing is provided for temp and shrinkage.

I would appreciate if you could point out what other factors have i missed?

Thanks.

 

[rowingengineer]

talking with your mentor.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[cvg]

12 kip is not a huge load, why use FRP? I would think that maintaining vertical alignment and level, avoiding any settlement and vibration would be the most important considerations to protect this multi-million dollar, precision machine. make your slab / foundation thick enough to protect the investment. Get on the phone with the manufacturer as I am sure they will void the warranty if the slab is not adequate to support the machine.

 

[lutein]

frp is required for MRI, because there should be minimal or none (5psf max) of ferrous material around 5ft radius of the magnet.

vibration isolation is a good point, and yes - isolation joint is provided along ther slab edges

 

[hokie66]

I think cvg is asking why do you need to reinforce the slab, when plain concrete should work just fine. Maybe you don't mean FRP, but rather plastic fibres. If so, I suggest you might as well save whatever money the fibres cost.

 

[cvg]

I wasn't thinking of just isolation but vibration dampening. If you have ever had an MRI, it's like being surrounded by jack hammers. Tremendous noise and vibration that needs to be absorbed in the foundation. And yes, isolation is also important. I know metal is not allowed inside the radius of the magnets, but wasn't aware it was not allowed outside. There is a lot of ferrous metals next to the MRI including the table.

 

[DaveAtkins]

For the MRI projects I have worked on, steel slab reinforcing was not allowed (it messes up the imaging).

I have always used a thick (12" or thicker) slab with no reinforcing.

DaveAtkins

 

[frv]

lutein..

MRI's are frequently placed on above ground floors. How would they do that if no steel is allowed?

#4's @ 18" O.C. ea way is less than 1 PSF.

But as others have mentioned, other considerations are more important.

 

[vandede427]

my firm did a cancer treatment addition to a hospital recently. it had three rooms (more like vaults) for three radiation machines.

all six sides of the room was at least 5 ft thick concrete. there were 4" thick plates suspended in the roof of the vault.

 

[cvg]

MRI does not use radiation, no need for 5' thick concrete radiation shielding that I am aware of. There may be some EM or RF radiation

http://en.wikipedia.org/wiki/Magnetic_resonance_imaging

maybe this is the issue:

Projectile or missile effect
As a result of the very high strength of the magnetic field needed to produce scans (frequently up to 60,000 times the Earth's own magnetic field effects), there are several incidental safety issues addressed in MRI facilities. Missile-effect accidents, where ferromagnetic objects are attracted to the center of the magnet, have resulted in injury and death.[35][36] A video simulation of a fatal projectile effect accident illustrates the extreme power that contemporary MRI equipment can exert on ferromagnetic objects.

In order to help reduce the risks of projectile accidents, ferromagnetic objects and devices are typically prohibited in proximity to the MRI scanner, with non-ferromagnetic versions of many tools and devices typically retained by the scanning facility. Patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs.

Ferromagnetic detection devices are used by some sites as a supplement conventional screening techniques, and are now recommended by the American College of Radiology's Guidance Document for Safe MR Practices: 2007 and the United States' Veterans Administration's MRI Design Guide.

The magnetic field and the associated risk of missile-effect accidents remains a permanent hazard, as superconductive MRI magnets are kept permanently energized and retain their magnetic field even in the event of a power outage.

 

[doka1]

We are about to begin work on a similar project, although not on the ground floor. Slab is to be reinforced by plastic rods (not positive exact type of plastic). We have to use zip ties to tie the reinforcing and of course plastic chairs.

Have you looked into tuf strand by euclid?

 

[lutein]

doka1,
please see below for summary of design guide from advices from my other offices which have done similar projects before, conversation with MRI manufacturer (HITACHI in my case), and study of other structural engineer's drawings:

1. MRI cutsheet will typically specify maximum weight of steel reinforcing in certain perimeter from the magnet. Because ferrous material will impact the accuracy of the process. It is recommended that all reinforcing to be provided by using FRP bars per ACI 440. Make sure you specify correct splice length, strength and proporties for FRP bars. GFRP is what typically called out.

2. The slab on grade thickness is usually governed by:
a. Embedment depth of the post installed anchors, which is usually to be provided by the manufacturer. You dont want to specify a 4"slab, but the manufacturer is putting in 4" embedment anchor for seismic anchorage. it will result in a big change.
b. If vertical vibration joint is not provided around the perimeter of the room (which is strongly recommended by manufacturer), the slab on grade need to be sized to have sufficient mass to resist vibration transmitted from other places.
c. Size for weights of MRI. I was told that most MRIs are not heavy enough to drive the SOG to more than 5" or 6" thick. Certainly, punching shear of the magnets and flexural capacity with subgrade modulus should be checked, but rarely govern in SOG as we know.

3. Make sure you recess the floor for shielding, there is usually a 1/8" to 1/4" of shielding required to be inset on SOG. So, you need to show a recess in the SOG to allow for the shielding. The shielding thickness is NOT by the manufacturer, and usually from the shielding consultant, which is a separate thread hired by the architect. I have confirmed that the steel reinforcing limitation still valid even with shielding. Shielding is for rediation isolation not magnetic isolation. At the same time, if the shielding report specifies a cieling hung shielding, make sure you design the structure above to handle the weight. shielding support framing is by typicsally by UNISTRUT spacing at 4'-0".


this is all I know, just want to share with you all what i found out. thanks.

 

[TGilk]

As has been said before, a structural engineer needs to keep a few things in mind when designing a SoG for an MRI scanner.

Metal: These MRI scanners have magnetic fields tuned like Stradivarius instruments. Magnetic metals (iron, nickel & cobalt) will attract the magnetic field of the MRI and throw-off that tune. Our multi-disciplinary approach may mean that the Architectural, Mechanical and Electrical engineers each look at the maximum allowable threshold and say, 'easy, I can stay below that.' But what matters is the cumulative effect of the the metal that they all put into the job. For that reason and in order to protect the function of this $2m piece of equipment, your best bet is to eliminate steel from your slab reinforcing. FRP, carbon fiber, even Austinitic stainless are fine. No steel. This extends below the slab, too, to beams, pipes, conduits, etc...

Vibration: These machines generate their images by triangulating-in on molecule-level signals. That's remarkably challenging by itself... now imagine trying to do this while the patient is wiggling. The two primary sources for vibration are Earth-borne and structure-borne. Earth-borne vibration is a function of what vibration sources you have (highways, rail-lines, construction sites) and your soil conditions for transmitting these vibrations. Structure-borne vibration can come from pumps, air handlers, motors, fans, even foot or rolling-cart traffic. An MRI's vibrational sensitivity has spectral 'hot spots' where the amplitude and frequencies are particularly sensitive. Vibration analysis is always recommended, but requires a fully operational building around where the MRI will be at the time of study to be accurate. SoG is typically best when you know you don't have unreasonable Earth-borne vibration, and floating the slab by separating its perimeter from the building's structure helps eliminating structure-borne vibration transmission. On elevated floors, the rigidity of the entire frame must be considered relative to vibration sources and paths to the MRI.

Shield: All MRI's require radio frequency (RF) shields in a six-sided box. The load of these is negligible, but they do nothing, zero, to contain the magnetic energy of the MRI system (which is harmless to people's bodies, but can be very disruptive to medical devices like pacemakers). Magnetic shielding may be designed for the suite, and is made up of plate steel, the loads of which are not inconsequential. Remember (above) how I said to try to eliminate all steel in the slab construction? How does this square with the idea of adding large plates of steel? When steel shielding is added, it must be designed in such a way that the attractive 'pull' that it exerts on the balanced magnetic field of the MRI is equaled out. The MRI has some internal tuning ability, to correct for this, but often if there's enough steel shield material on the floor it exceeds the scanner's 'shim threshold' and requires that compensating steel be placed in the ceiling to 'balance out' the field. Because the plenum above the ceiling is much further away from the center of the magnet than is the floor, the amount of compensating steel above my be 2 - 3 times what is in the floor. Obviously this could get structurally significant very quickly. Also, the engineering and installation costs for steel magnetic shielding are very significant. Often it is less costly to increase the size of the area around the MRI than it is to attempt to shoehorn it in and correct for conflicts with steel shielding. Lastly, depending on your shield vendor and what sort of shielding you ultimately have, the slab recess should probably be 1-1/2" to 2". The shield vendor will probably want to put on their own top-seal, then install their shield, then install a protective layer over it.

Path: From a structural standpoint, the other thing to consider is the path of travel. Unlike nearly every other piece of imaging equipment, MRI's can't be 'broken down' to anything much smaller than the final product for delivery. This means that doorways often must be torn out along the delivery path. To get the MRI magnet from point-a to point-b, imagine that you will need a path that is 8-feet tall by 8-feet wide (verify the particulars with your MRI vendor's requirements). And while 12 - 24 kips isn't an impossible load to design the slab for, remember that this load is going to have to roll across the floor. On elevated slabs, this may require temporary shoring (another reason to have these on grade).

Lastly, most professional errors and omissions coverage assumes that the value of the designer's work is the building, and most of the time that's true. For MRI suites, however, the value of the professional designer's work is also in the operation of the MRI, a piece of equipment worth 2 - 4 times the value of the suite into which it's placed.

If you have any other questions about MRI suite design (issues like equipment interference, exclusion zones, screening protocols, shield vendors, design standards), I encourage you to look up

http://www.MRI-Planning.com

 

[271828]

For that reason and in order to protect the function of this $2m piece of equipment, your best bet is to eliminate steel from your slab reinforcing. FRP, carbon fiber, even Austinitic stainless are fine. No steel. This extends below the slab, too, to beams, pipes, conduits, etc...

I don't understand this part. I'm working on a project right now that has a MRI on typical composite slab and steel framing. I've also put at least 3 other MRIs on framed floors with typical construction. The MRI vendor drawings have an amount of steel that's OK and every time we've looked at the default structure, it's been OK.

I'd appreciate some elaboration on this point.

 

[271828]

If you have any other questions about MRI suite design (issues like equipment interference, exclusion zones, screening protocols, shield vendors, design standards), I encourage you to look up www.MRI-Planning.com

I went there expecting to find technical info, but it seems to be almost entirely an advertisement for a consulting company.

 

[TGilk]

One of the challenges with designing MRI suites lies in the fact that the building will last 30 - 50 years, but the equipment has a mean usable life of 7-8 years, meaning that the MRI scanner will be replaced with new versions 3 to 7 times over the (hopeful) life of the suite. Each new generation of MRI equipment has different siting thresholds, but even if you remain below the thresholds, 'acceptable' levels of interference may still compromise certain scans.

I believe that designing to the minimum acceptable standards for today's MRI equipment dramatically increases the likelihood that the next MRI, or the one after that, will necessitate significant changes to the suite that would otherwise be unnecessary had the suite been designed in anticipation of more stringent requirements of future equipment.

As for the site I suggested, it is completely different than it was just a couple weeks ago! It is quite promotional. If you Google it, perhaps you can view a few older cached pages.

 

[271828]

Thanks. You seem to have a good handle on this subject.

Do the requirements tend to get more stringent with subsequent generations of MRI?

 

[TGilk]

The criteria change. It used to be that MRI's required rooms as big as racquetball courts, now they're about 1/4 the prior size. As the sensitivity of the scanner goes up (higher field strength), the sensitivity to environmental disturbances also go up (primarily vibration). Different scan types, too, have different susceptibilities. Spectrographic analyses (growing in popularity for oncology) are more sensitive to metal disturbances than are conventional imaging scans.

In short, there's the potential that both of the major environmental siting challenges (vibration and metal) will become more stringent over time as a result of both clinical and technical changes to the MRI systems.

 

[Bobber1]

I had an MRI for a bad back a while ago. The doc said, "There's nothing wrong with your back, your spine is as straight as a rod". I looked over his shoulder at my MRI image; sure enough, I saw a No. 6 bar. Guess that's why no rebar nearby.

You can find frp reinforcing by searching plastic lumber; they use it all the time.

 

[structuresguy]

We've done several MRI installs in the last few years, both on grade and above grade, with most being installs in existing facilities. Steel reinforcing was always allowed, within certain limits published by the MRI manufacturer. We have done what we could to isolate the slab from the remaining structure, using isolation joints for slabs on grade, but for elevated installs this was not possible. We have never done any active isolation, either for on-grade or elevated installs. To date, we have not had any complaints about any of the installs. The support of the equipment is really not that complicated, and it seems that some of you are making it more complicated than necessary.

I suggest you talk with the equipment vendor about your concerns. They will give you feedback of their many, many previous installs, and let you know if they think something might be a problem.