Coils Flashcards
What are the properties of the main magnet
Magnetic field strength, homogeneity, temporal stability, bore size
Lamar Equation
W = y x B0
Benefits for high field strength
Faster scan times, signal form patient increases and can probe physiologic processes
Disadvantages for high field strength
bioeffects, heating, peripheral nerve stim, cost and increased susceptibility (image artefacts)
Main magnet can be three types
permanent, resistive, and superconducting
Permanent magnet adv
no power supply, low maintence and cost, large bore (good for claustro)
disadvantages for permanent magnets
low SNR, long scan times, siting - heavy
How does a superconducting magnet work
allows electrical current to run in coils without any resistance at temperatures below 250 deg (23K)
Is superconducting ever turned off
only in emergencies
What are the components of MRI system
Main magnet B0; gradient coils; radiofrequency source and image processor reconstruction system
Advantages for resistive
good uniformity, light weight, low capital costs, can be turned off in emergency
Disadv for resistive
requires cont electric power to produce magnetic field ; cost; sig amount of heat; low inherent SNR so scan times longer; stray magnetic field
Adv of superconducting
achieve high field strengths, high field uniformity
Disadv of superconducting
high initial capital and siting costs, difficulty in turning off main magnetic field in emergency and extensive fringe fields
What is a fringe field
The stray peripheral magnetic field outside the bore of the magnet
Shim coils
a device used to adjust the homogeneity of a magnetic field
Passive shimming
when magnet is shimmed with metal
Active shimming
when shimming is performed with lops of current carrying wire
From inside to out, what are the different coils in magnet
RF coil, gradient coil, shim coil and then magnet
Gradient coils enable differences
in the field to produce Larmor frequencies at different locations in the body - spatial encoding
Selecting an appropriate gradient coil
will provide a specific slice location and orientation (axial, etc.)
Gradient must have frequencies
matched
What are three types of RF coils
Surface coil, phased array coil, volume coil or bird cage coil
Volume coils are used for
transmitting RF signals over a large area
Surface coils are used for
receiving signals from a small area with high SNR
Phased array are used to
combine multiple surface coils to increase signal and improve image quality over a larger area
Volume coils transmit
RF and receive the MR signal and encompass the entire anatomy - whole body
Surface coils are
small and easily shaped to be placed near the anatomy and we can improve SNR when imaging structures near the surface
Phased array coils have multiple coils and receivers that have
individ signals that combine to form one big signal with improved SNR and good coverage
