MRI equipment Flashcards
MRI hardware includes a:
Magnet
Radiofrequency source
Image processor and
Computer system
How does MRI work?
The magnet aligns the nuclei parallel or anti-parallel.
Magnetic homogeneity is necessary so therefore a shim system is needed.
The RF system excites the nuclei and this system requires a transmitter and receiver.
Magnetic field gradients determine the spacial locations of the RF signals. You tell the MR where point 0 is.
The MRI signal is changed from the FID into an understandable format by a series of mathematical equations known as the Fourier transform. This is in CT as well
This process is overseen by a computer that also allow for operator interface.
Magnetism is an important property of
matter
What is the magnetic behavior of an atom is dictated by?
the arrangement of the orbiting electrons.
What are the coils in the magnet used for?
For location of anatomy
The changes in the electric configuration between elements classify them into different categories:
Diamagnetism
Paramagnetism
Ferromagnetism
What are the substances in the diamagnetism?
copper, silver and sulfur.
The diamagnetic substances have no net magnetic moment without?
external magnetic field present.
Their electron currents caused by their motions add to zero.
What happens when diamagnetic substances have an external magnetic field?
When an external magnetic field is applied they show a small magnetic moment that opposes the magnetic field.
They are therefore not attracted to but slightly repelled by the magnetic field.
Paramagnetic Substances :
oxygen and gadolinium (MRI Contrast)
magnetic moment of Paramagnetic Substances
They have a small magnetic moment as a result of their unpaired electrons within the atom.
What happens Without a magnetic field for a paramagnetic substances?
Without a magnetic field the magnetic moments occur random and cancel each other out.
What happens Without an external magnetic field for a paramagnetic substances?
With an external magnetic field these substances align with the direction of the field so their magnetic moments are added together.
Effect of external magnetic field on Ferromagnetic substances
Substances such as iron show a strong attraction and alignment to an external magnetic field.
They can become dangerous projectiles in the MRI environment.
They also retain their magnetism even when removed out of the magnetic field and subsequently become permanent magnets.
What are 3 Different types of magnets?
- Permanent magnets
- Electromagnetic (solenoid and resistive) magnets
- Superconducting magnets
In MRI a number of magnetic fields are used to create an MRI image:
Bo the main magnetic field.
RF field used to excite spins.
Magnetic field strength is measured in
(G) gauss(inside scan room), (KG) kilogauss or (T) tesla (outside scan room).
Permanent Magnets are?
Substances such as Nickel, cobalt and iron are used in the production of permanent magnets since these ferromagnetic substances retain their magnetism after being exposed to a magnetic field.
Main advantage of permanent magnets are?
they require no power supply and are therefore
relatively low in operating costs.
The magnetic field lines of permanent magnets run from?
The magnetic field lines runs form south to north (bottom to top) so the magnetic field is confined within the boundaries of the system and hence the scan room.
Little safety considerations and can have an open design that is good for claustrophobic and obese patients, kinematic musculoskeletal studies as well as for interventional studies
The law of electromagnetism states that
The law of electromagnetism states that moving electrical charge induces magnetic fields around themselves.
A magnetic field will be created in electromagnets if
A magnetic field will be created if a current (or moving charge) is passed through a straight wire).
The strength of the magnetic field will be proportional to what in an electromagnet?
The strength of the magnetic field will be proportional to the amount of current moving through the wire.
Superconducting Electromagnets
Construction of Electromagnets
Many loops of wires are wrapped around to form a coil in superconducting magnets.
They are often said to be resistive magnets.
Ohms law: V=IR
V = equal to the applied voltage.
I = is the current
R = the resistance within the wire.
Resistive magnets compared to permanent magnets
Resistive magnets are lighter then permanent magnets, but the operational costs are high due to large quantities of power required to maintain the magnetic field.
The magnetic flux run head to foot and follows the right hand thumb rule.
The maximum field strength for resistive magnets?
The maximum field strength is typically less than 0.3T due to its expensive power requirements. It can be turned off with the switch of a button.
The resistive system is relatively safe, but due to the orientation for the wires and hence the magnetic flux lines, there is
a considerable stray
magnetic field.
What are gradients?
x,y and z
In Superconducting electromagnets When resistance is decreased the current dissipation also
When resistance is decreased the current dissipation also decreases and therefore the resistance is reduced and the energy required maintaining the magnetic field is decreased.
