Page 114 of GN1 states that as a rule of thumb, the mesh size required for a certain interference frequency can be determined from ensuring the diagonal of the square mesh should be no more than one-tenth of the wavelength. The example given is a 2m square mesh would therefore attenuate a 10 MHz signal. Could someone explain?
when you lie in an MRI scanner then you are 'illuminated' with a very strong static magnetic field - so nearby steel objects are banned - indeed folk have been badly hurt by flying oxygen bottles and so on in the early days when folk have not been thinking - now they only allow non-magnetic bottles and things in. But once the atomic nuclii are all facing along the direction of that field, we 'twang' them with about a kW or so of RF, and listen for the various frequencles that they then spin at, with radio receiver to detect weak signals that are pretty close to the theoretical limit of what can just about be received. So the screening keeps both the RF excitation ("pump frequency") in the box, so it does not jam other electronics in the building, and at the same time also protects the detector/ receiver from what is outside, and if it was not for the big magnet steel would be OK, but it isn't making things like transformers in the power supplies very tricky.
The fine structure in the received spectrum allows the concentration of various sorts of atoms, and what they are bonded to to be deduced (as they are detuned by the magnetic fields of the near neighbour atoms.)
Having designed some electronics for just such a situation, I will say that it is very challenging, as all sorts of common parts, not just iron or ferrite cored inductors but also a lot of 'innocent' looking parts like connectors fixings, mechanical parts. do actually have to be specially made in a non-magnetic variant (no nickel, no iron, no cobalt etc).
The kW of RF is why you need to be cooled while you are in the thing as inside the magnet tube is the big antenna array that is both illuminating you and listening for the nuclear magnetic resonances from the atoms inside you - it is really clever.;-) The radio frequencies, the local magnet strength, and how the antennas are used, can be scanned, swept and configured to allow regions of interest in the body to be selected with remarkable precision.
This scanner explanation is a bit of a simplification, so apologies to the many folks whose life's works are in scanner design, but the bit about the screening, what it is for and why it is not steel, is precisely accurate.
Mike.
when you lie in an MRI scanner then you are 'illuminated' with a very strong static magnetic field - so nearby steel objects are banned - indeed folk have been badly hurt by flying oxygen bottles and so on in the early days when folk have not been thinking - now they only allow non-magnetic bottles and things in. But once the atomic nuclii are all facing along the direction of that field, we 'twang' them with about a kW or so of RF, and listen for the various frequencles that they then spin at, with radio receiver to detect weak signals that are pretty close to the theoretical limit of what can just about be received. So the screening keeps both the RF excitation ("pump frequency") in the box, so it does not jam other electronics in the building, and at the same time also protects the detector/ receiver from what is outside, and if it was not for the big magnet steel would be OK, but it isn't making things like transformers in the power supplies very tricky.
The fine structure in the received spectrum allows the concentration of various sorts of atoms, and what they are bonded to to be deduced (as they are detuned by the magnetic fields of the near neighbour atoms.)
Having designed some electronics for just such a situation, I will say that it is very challenging, as all sorts of common parts, not just iron or ferrite cored inductors but also a lot of 'innocent' looking parts like connectors fixings, mechanical parts. do actually have to be specially made in a non-magnetic variant (no nickel, no iron, no cobalt etc).
The kW of RF is why you need to be cooled while you are in the thing as inside the magnet tube is the big antenna array that is both illuminating you and listening for the nuclear magnetic resonances from the atoms inside you - it is really clever.;-) The radio frequencies, the local magnet strength, and how the antennas are used, can be scanned, swept and configured to allow regions of interest in the body to be selected with remarkable precision.
This scanner explanation is a bit of a simplification, so apologies to the many folks whose life's works are in scanner design, but the bit about the screening, what it is for and why it is not steel, is precisely accurate.
Mike.
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