MRI I Flashcards
What is the main composition of a MRI scanner?
Magnet
Gradient coils
Radio-frequenci coils (RF coils)
Host computer
Scanner electronics
The magnet, gradient coils, and RF coil are three concentric cilinders, where the magnet is the outer layer, and the RF coil the one inside.
What is the equation that describes the relationship between the current going though a wire and the magnetic field B produces?
B ~= IR^2/(z^2+R^2)^(3/2)
What are the main characteristics of the main magnet?
They have to be strong, enough to align the patient’s protons.
They have to be homogeneous, to avoid artifacts
They have to be stable during the time of use.
They can be made from rare earth metals, resistive, or superconductive solenoids.
What is an example of a configuration of the magnet of an MRI Scanner?
And example of the layers of a magnet configuration, from the outer to innermost layer, is:
- Vacuum
- Liquid Nitrogen
- Vacuum
- Liquid Helium
- Superconducting coil
- Liquid Helium
- Vacuum
- Liquid Nitrogen
- Vacuum
How can the magnets be classified according to the strenght of the magnetic field produced?
Low field: 0.2 - 0.5 T
Intermediate field: 0.5 - 1.5 T
High field: 1.5 - 4.0 T
Ultra high: > 4.0 T
What are the three types of magnets, and their main characteristics?
Permanent:
- Made from rare earth alloys
- Low field
- No need of cooling.
- Relatively cheap
- Very heavy
- Stability and homogeneity dependent on temperature
Resistive:
- Current through conductor
- Cheap
- Low fringe field
- Require cooling, it is a major problem
Superconductive solenoids:
- Intermediate to high field
- Good uniformity: ~1 ppm
- Require cooling, as their critical temperature less than 10K.
-Large fringe field: requires shielding
- Very expensive
What is B0 and B1?
B0 is the firstmagnetic field produce, to align the patient’s protons. B1 and beyond are the magnetic fields use for imaging.
What are dielectric materials?
Those who are poor conductors because of a lack of free electrons
Whar are dialectric losses? What is their importance in MRI?
Dialectric materials tend to absorb the energy of magnetic fields and dissipate it as heat. Thus, they are of important consideration in MRI, as fat and other tissue can act as dielectric material and in an MRI they can increase their temperature, causing discomfort.
What do the RF coils do?
-> They efficiently transmit the power of the RF into a magnetic fueld whitin the subject, minimizing dielectric losses. They are in the order of KW
-> They receive the oscilating magnetization induzed by the magnetic field, minnimizing noise reception (ideally SNR = 1)
What are RF coils?
They are resonant electrical circuits, tuned to the system’s resonance frequency:
ω=1/sqrt(LC)
What are quadrature drive coils? What are the characteristics of the RF they produce?
Consist of 4 rings organized in such a way that they produce a cicularly polarized RF field. They have better homogeneity, increased SNR by sqrt(2) due to the averaging of 2 coils, and reduced transmit power.
Whar are surface coils?
Wire loops placed directly of the zone of interest. They have optimum SNR, but poor homogeneity: used as receiver
What are birdcage coils?
Consist of two wire rings connected by an even number of legs. They have reduced homogeneity, but they have almost ideal homogeneity. They are used as transmitter or transmitter and receiver
What are gradient coils?
Those that produce linear, orthogonal fields in Gx, Gy and Gz
A maxwell pair, consisting of two wired rings, produce a linear gradient in Gz. Then, saddle coils can be used to produce a gradient in Gx and Gy. A birdcage would be the combination of all three.
