Summary Flashcards
What are superconductive magnets?
o Concept- A current traveling through a loop/coil of wire creates a magnetic field along the axis of the loop in line with Faraday’s law of electromagnetic induction.
o Superconductivity is achieved by reducing the resistivity of the coil of wire-ideally there is no resistivity in the coil of wire
o This reduced resistivity is achieved by cooling the coil of wire to temperatures where the coil has lowest resistance/energy
o This slow resistivity is obtained by circulating a cryogen(liquid helium- cryogen in the case of MRI)
o Superconductive materials exhibit 0 electrical resistance when cooling below a certain critical temperature-allows them to carry large currents without power loss -crucial for generating the strong magnetic fields in MRI
o Coil material used in nearly all modern superconductive magents is Naiobium titanium- becomes superconductive below 9.4 degrees calvin
Artefacts
Low field - less likely to have flow motion and chemical shift artefacts and increase motion artefact due to longer time in the bore
High field - less likely to have motion artefacts
Ultra high field - more likely to have motion artefacts
Low field mri disadvantage
• Up to 3 tesla the application of fat suppressed imaging with chemical shift selective rf is challeneged due to the decreased chemical shift of water and fat spectra- application of fat suppression pulses more difficult
How is superconductivity achieved?
superconductive state achieved by using cryogens to cool the MRI coil ensures that the magnetic field remains stable within minimal resistivity in the coil over time without fluctuations i.e. changes in resistivity that could disrupt the uniformity of the magnetic field and degrade image quality
What is superfluidity in liquid helium?
• Liquid helium shows Superfluidity – characteristic property of a fluid with 0 viscosity (thickness) and therefore flows without losing kinetic energy
Why are cryogens used?
• Cryogens are gases that have been cooled to a liquid state at very low temperatures
• Superconductive state achieved by using cryogens to cool the MRI coil ensures that the magnetic field remains stable within minimal resistivity in the coil over time without fluctuations i.e. changes in resistivity that could disrupt the uniformity of the magnetic field and degrade image quality
Materials and their magnetic domains
• Diamagnetic- 0 magnetic moments but when subject to a magnetic field they will align but in an opposite direction to the magnetic field
• Paramagnetic – have a net magnetic moment but will be randomly aligned -when subjected to a magnetic field they will align with that field
• Ferromagnetic- net magnetic moment in a domain but still randomised. Once subjected to a magnetic field – those domains that will have moments parallel to the field will grow in size whil the others which are not parallel to the field will align and roatet to be aligned with that magnetic field – strong magnetisation in that direction
Faradays 1st law
• Whenever a conductor is placed in a changing magnetic field, an electromotive force (EMF) is induced
• If the conductor is part of a closed circuit (MRI), this EMF will cause an electric current to flow
• When EMF is induced and there is a voktage within the coil – the faster the change in the magnetic field -the greater the change in the EMF coil
Faraday’s 2nd law
• Flux - is a measure of the number of electric or magnetic field lines passing through a surface in a given amount time
• Flux linkage of thr coil – the product of the number of turns in the coil and flux associated with the coil - The magnetic flux linkage is the amount of field passing through a coil of wire. It is the flux ‘linked’ to a wire.
• The magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil
• It is possible to increase EMF induced in a coil by : increasing number of turns in the coil(increasing flux), increasing the magnetic field strength, increasing the speed of relative motion between the coil and the magnet
• The flux will impact the current or the emf induced
• If we imagine a moving magnet through a coil of wire the motion of the magnetic field across the coil will induce an emf which will cause an electric current to flow through the wire and the faster the magnetic field changes (either by moving the magnet or varying it’s magnetic strength) then the greater will be the induced EMF.
Faraday’s law applicability to MRI
• The production of electricity by a rotating magnet actually describes how an MR signal is produced
• An MR scanner is an electrical generator with a computer that interprets the induced electrical currents to produce an image of in vivo magnetism.
• The creation of an MR image requires; an in vivo magnetic source, a force (torque) -hydrogen atoms that causes the magnetic source to rotate and an appropriately placed coil of wire to receive the signal
• In MRI we have active nuclei that will have a net charge and are spinning automatically, they will align to the magnetic field and they will get that magnetic moment and will align to that external magnetic field. With rf pulses we can effect that alignment and generate the current and signal that we want.
Faradays law relating to safety
• Induced currenfs – rapidly changing magnetic fields, particularly during gradient switching, can induce electrical currents – Eddy currents in conductive materials, including the human body.
• Can lead to heating and possible injury
• Therefore this is why metallic objects are harmful as rhey act as conductors in which induced currents may cause localized heating, burns or displacement forces on ferromagnetic objects.
Rf heating
• Rf pulses can induce currents in the body, potentially causing heating and burns particularly if the patient is in contact with conductive surfaces
Translational force
• Translational force- causes an object to move or translate from one location to another
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Spatial field gradient
refers to how much the magnetic field chnages over a certain distance
Torque
How an object tries to align with magnetic flux lines, which is crucial when evaluating
the safety of implants
Lenz forces
• Objects that are not magnetic can be affected by a static magnetic field
• When a conductive, non-magnetic material moves through a magnetic field,it generates an electrical current
• This electrical current in turn created a new magnetic field at 90 degrees
• The newly created magnetic field interacts with the original static magnetic field, resulting in a repulsive force
