Re: [CAD_CAM_EDM_DRO] help putting together a complete system

info.host@... wrote:

> What did it feel like having the MRI? I remember seeing on Tommorow's World
> when they first came out one of the presenters held a piece of motor bike
> lock chain a few feet away and they turned the scanner on.

Generally, in modern MRI scanners, the magnet field is on all the time,
as they use superconducting magnets cooled to about 4 Kelvin. They only
bring the magnet current down for extensive maintenance, as the machine
needs extensive recalibration any time the magnet field is changed.

> The chain
> instantly pulled taught towards the scanner, I wonder how angry they'd be if
> she'd let go by mistake? :-)

Pretty mad. They frequently get mop buckets, toolboxes, compressed gas bottles,
even an occasional forklift truck stuck in the magnets. They have also had people
killed when they didn't see a flying toolbox coming, and got caught between it and
the magnet.

> They were saying about how they plan to do
> surgery inside the scanner so they can see inside the person as they operate
> but they'd all have to use plastic knives and things from the chippy so they
> didn't stick to the scanner.

Good quality stainless can be VERY antimagnetic, but if they wanted to image
while the scalpels, retractors, etc. were inside the body, then they'd have to be
non-conductive. There ARE ceramic scalpels.

> I can imagine it now, 'I'll take three scapels,
> five sets of hold clamps and erm... give me a few portions of chips while
> your at it please.' I wonder if there is actually a measurable current
> induced in the iron compounds in your blood as you are scanned.

No, you are in a static magnetic field. There are very strong RF pulses, plus some
small fluctuations in the magnetic field caused by the gradient coils. These small
fluctuations are how the system is made spatially sensitive.

> You are
> moving through the field and it sure is one big field.

The patient does not move during the scan. You are moved into and out of the
magnet before and after scanning, though.

> I wonder what gauss (Sp?) they are?

Most medical MRIs right now are 1.5 Tesla, which I think comes out to 15000 Gauss.

> I have also always wondered why we say a transformer is
> almost 100% efficient but 50% of each coil is facing the wrong way, i.e.
> backwards to the opposite coil.

Huh? The transformer on the pole outside my house is a 25 KVA transformer.
If it wasn't nearly 100 % efficient, it would need a cooling fan the size of an
airplane prop to keep it cool. The big substation transformers at work are
about 1.5 MVA, and the fans on it only run lazily on the hottest days.

The current has to flow the opposite way in the secondary coil to cancel out
the flux caused by the current in the primary. If it didn't, then the inductance of the
core iron would prevent all but a trickle of current in the primary, the same as a
no-load condition.

Jon