MRI equipment Flashcards
what are the 6 main components of an MR system/room/machine
- RF shielding
- magnet
- radio-frequency transmitter and reciever
- spacial localisation/gradient coil
- computer/electronics/power amplifier/ supplier
- patient table
What is RF shielding and what material is it usually made of
- the act of shielding a device from radio frequency interference (RFI)
- copper or aluminium
what structures in the room is RF shielded
all walls, doors and windows
what is the 5 gauss line
- defines a border to an area in which the magnetic field could affect implanted devices such as pacemakers
what is the penetration panel
- any cables going in and out of the MRI room must pass through a Penetration Panel
- the panel contains waveguides and filters required to provide communication between the inside and outside of the scanner room
- this filter allows passage of all frequencies except those in a narrow range surrounding the Larmor frequency
Why must the electrical supply in an MRI be filtered?
RF spikes on images
What is the range of weight of structural support is given to MRI machine
1-4 tons
going in order from inside out, what position do the following have in an MRI machine: gradient coil, shim coil, magnet, RF coil
innermost:
RF coil
gradient coil
shim coil
magnet
outermost
what is the array processors duty in MRI
- Optional component of the computer system used to perform Fourier transformations to accelerate the processing of the received numerical data relative to the MR imaging process, to speed them up.
- a device which is capable of performing a two-dimensional Fourier transform in fractions of a second
what 2 computer/electronics are found in MRI
- array processor
- operator console
what are 3 main magnet types in MRI
- permanent
- resistive
- superconducting
describe the direction of the main magnetic field in a permanent magnet and how frequency it stays magnetised
- B0/magnetic field runs vertically (from south to north)
- remains permanently magnetised
describe the environmental conditions required for permanent magnet to work and the strength of magnetism it provides
- room temp
- temp sensitive
- 0.03 to 0.3T
how much does a permanent magnet weigh
6-100 ton
describe the direction of the main magnetic field in a resistive magnet and how frequency it stays magnetised
- B0 runs horizontally or vertically
- can be switched off without damaging windings
describe the environmental conditions required for resistive magnet to work and the strength of magnetism it provides
- works at room temp
- strength of 0.03 to 0.4 T
What is fringe field
The stray magnetic field outside the bore of the magnet
what are 2 negatives about using resistive magnet
- significant fringe field
- high running cost (but low capital cost)
what is a hybrid magnet
- uses both superconducting and resistive magnet
describe the environmental conditions required for superconductive magnet to work and the strength of magnetism it provides
- operates near absolute 0/ -273k
- required liquid helium to achieve these temps
- 0.35 to 10+ Tesla
What direction is the magnetic field using a superconductive magnet
horizontal
define homogeneity in MRI
the uniformity of a magnetic field in the center of a scanner when no patient is present
what is shimming in MRI
the process of optimization of the magnetic field homogeneity and is a two-stage procedure
What are the 2 types of shimming in MRI
- active
- passive
what is active shimming
- currents are directed through specialized coils to further improve homogeneity.
- additional power is used
what is passive shimming
- shimming small pieces of sheet metal or ferromagnetic pellets are affixed at various locations within the scanner bore
- pieces of metal to quantifiably distort B0
what is used to undergo spacial localisation?
- gradient coils in the x, y, z axes
by gradually increasing the current in separate windings towards one end of the magnet, b0 uniformly changes.
This method makes slice selection, phase and frequency encoding possible.
what 3 areas are found in high performance gradients (coils)
- high gradient strength (Gmax)
- short rise time (tR)
- high slew rate (Gmax / tR)
Define slew rate
ratio of time taken to reach maximum gradient amplitude
rise times range= 600-180 micro secs
gradient strengths range= 10mT/m - 80mT/m
slew rates range= 15m/T/msec - 200m/T/msec
What is the purpose of RF coils
they transmit and receive radio-frequency
what law do RF coils follow
faradays law of induction
what is faradays law of induction
V is proportional to B/T
V= induced voltage
B= total change in magnetic field
T= time taken to change
what is a surface coil
- a type of receive-only radiofrequency coil used in MRI to receive the radiofrequency signal
- placed closer to anatomy and has lower homogeneity than magnetic field
- higher signal to noise ratio
- general magnetic field is horizontal whilst induced magnetic field by coil is vertical
whats a helm-holt coil
- pair of conducting circular coils with many turns, each carrying a current. The coils are separated by a distance equal to the radius of the circular loops. This design produces a very uniform field in the center,
- general magnetic field is horizontal whilst induced magnetic field by both coils is vertical (facing each other)
whats a bird cage coil
- two circular conductive loops referred to as end rings connected by an even number of conductive straight elements called rungs or legs
- used volumetric resonator creating a highly homogeneous radiofrequency magnetic field in a conductive sample.
- magnetic field runs horizontal
whats a solenoid coil
- A coil of wire wound in the form of a long cylinder. When a current is passed through the coil, the magnetic field within the coil is relatively uniform. Solenoid RF coils are commonly used when the static magnetic field is perpendicular to the long axis of the body.
- useful in imaging deep structures in anatomic regions that do not fit standard head and body coils,
- general magnetic field runs vertical whit induced magnetic field running horizontal
whats an array coil
- a receive-only radiofrequency coil system which receives the radiofrequency signal in MRI
- collections of small surface coils whose signals may be combined but generally feed into independent receiver circuitry.
- high sensitivity but limited anatomical coverage
