1st part chapter 9 Flashcards
Homogeneity
Uniform was of magnetic field measured in parts per million
Faradays law
When there is a current running through closed loop of wire an magnetic filed will be generated
Ferromagnetic
Strong attraction to magnetic field and can become a projectile
High and positive
Can become permanent magnetized
Magnetism and types
Ability to be affected by magnetic field Paramagnetic Diamagnetic Ferromagnetic Super paramagnetic
Paramagnetic
Affects a MF by local increase in strength
Low and positive
Diamagnetic
Affects MD by local decrease in strength
Low and negative
3 primary magnets
Permanent
Resistive
Superconductive
Advantages and disadvantages of permanent magnet
Adv- low cost, negligent able fringe field
Dis- limited field strength, fixed field strength and very heavy
Adv and disadv of resistive
Adv- low cost, easy maintence and negligible fringe field
Dis- high power consumption, water cooling necessary and potential field instability
Adv and disadv of superconductive magnet
Adv- high FS, high homogeneity and low power consumption
Dis- high cost, high cryogenic cost and intense fringe filed
RF system perform 2 primary task
Excited tissue and converts signal to image
Rf receiver and preamplifiers
Receiver amplifies signal
Preamp strengthens the entire signal
Characteristics of surface coils large
More uniform
Lower SNR
Not suitable for small FOV
Applications are torso hips and long bones
Characteristics SUrface coils small
Sensitive only near body
High SNR
Small FIV
High resolution
Local coils - 5
Linear - single loop Helmholtz- two parallel loops Flexible- 1 or more linear loops around roi Quadrature- bird cage Phased array- multiple coils
Gradient coils
Slope created by electromagnetic coils
Number of loops in coil
Current passing thru
Diameter and spacing of loops
Xyz
Steps to begin scan
Light markers all centered for homogeneity
Adjust automatic tuning least path of resistance
Frequency
Transmitter adjustment
Receiver adjustment - analog to digital
What type of magnetism is gadolinium?
Paramagnetic
Measurement of magnetism
Tesla
1T = 10000 gauss
Magnetic fields of low to ultra high Ts
Low- .2-.35
Midfield- .5-.7
High- 1.-1.5
Ultra high- 3t
What is purpose of RF coils? Body coils? Surface coil?
RF to transmit and receive
Body to transmit
Surface to receive
Permanent open system used for?
Claustrophobic
Kinematic
Interventional studies
Shin coils provide?
MF homogeneity
Magnetic susceptibility
Way material behaves in external MF
Active shielding
Use of solenoid electromagnets around outside of the main magnet at each end of the bore
Located inside cryostat and superconducting
5 G threshold few ft from isocentre
Passive shielding
Surround the magnet and lining of room with steel and copper plates.
Use for high field
