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RAD413 > MRI equipment > Flashcards

MRI equipment Flashcards

1
Q

what are the 6 main components of an MR system/room/machine

A
  • RF shielding
  • magnet
  • radio-frequency transmitter and reciever
  • spacial localisation/gradient coil
  • computer/electronics/power amplifier/ supplier
  • patient table
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2
Q

What is RF shielding and what material is it usually made of

A
  • the act of shielding a device from radio frequency interference (RFI)
  • copper or aluminium
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3
Q

what structures in the room is RF shielded

A

all walls, doors and windows

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4
Q

what is the 5 gauss line

A
  • defines a border to an area in which the magnetic field could affect implanted devices such as pacemakers
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5
Q

what is the penetration panel

A
  • 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
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6
Q

Why must the electrical supply in an MRI be filtered?

A

RF spikes on images

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7
Q

What is the range of weight of structural support is given to MRI machine

A

1-4 tons

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8
Q

going in order from inside out, what position do the following have in an MRI machine: gradient coil, shim coil, magnet, RF coil

A

innermost:
RF coil
gradient coil
shim coil
magnet
outermost

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9
Q

what is the array processors duty in MRI

A
  • 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
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10
Q

what 2 computer/electronics are found in MRI

A
  • array processor
  • operator console
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11
Q

what are 3 main magnet types in MRI

A
  • permanent
  • resistive
  • superconducting
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12
Q

describe the direction of the main magnetic field in a permanent magnet and how frequency it stays magnetised

A
  • B0/magnetic field runs vertically (from south to north)
  • remains permanently magnetised
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13
Q

describe the environmental conditions required for permanent magnet to work and the strength of magnetism it provides

A
  • room temp
  • temp sensitive
  • 0.03 to 0.3T
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14
Q

how much does a permanent magnet weigh

A

6-100 ton

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15
Q

describe the direction of the main magnetic field in a resistive magnet and how frequency it stays magnetised

A
  • B0 runs horizontally or vertically
  • can be switched off without damaging windings
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16
Q

describe the environmental conditions required for resistive magnet to work and the strength of magnetism it provides

A
  • works at room temp
  • strength of 0.03 to 0.4 T
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17
Q

What is fringe field

A

The stray magnetic field outside the bore of the magnet

18
Q

what are 2 negatives about using resistive magnet

A
  • significant fringe field
  • high running cost (but low capital cost)
19
Q

what is a hybrid magnet

A
  • uses both superconducting and resistive magnet
20
Q

describe the environmental conditions required for superconductive magnet to work and the strength of magnetism it provides

A
  • operates near absolute 0/ -273k
  • required liquid helium to achieve these temps
  • 0.35 to 10+ Tesla
21
Q

What direction is the magnetic field using a superconductive magnet

A

horizontal

22
Q

define homogeneity in MRI

A

the uniformity of a magnetic field in the center of a scanner when no patient is present

23
Q

what is shimming in MRI

A

the process of optimization of the magnetic field homogeneity and is a two-stage procedure

24
Q

What are the 2 types of shimming in MRI

A
  • active
  • passive
25
Q

what is active shimming

A
  • currents are directed through specialized coils to further improve homogeneity.
  • additional power is used
26
Q

what is passive shimming

A
  • 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
27
Q

what is used to undergo spacial localisation?

A
  • gradient coils in the x, y, z axes
28
Q

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.

A
29
Q

what 3 areas are found in high performance gradients (coils)

A
  • high gradient strength (Gmax)
  • short rise time (tR)
  • high slew rate (Gmax / tR)
30
Q

Define slew rate

A

ratio of time taken to reach maximum gradient amplitude

31
Q

rise times range= 600-180 micro secs
gradient strengths range= 10mT/m - 80mT/m
slew rates range= 15m/T/msec - 200m/T/msec

A
32
Q

What is the purpose of RF coils

A

they transmit and receive radio-frequency

33
Q

what law do RF coils follow

A

faradays law of induction

34
Q

what is faradays law of induction

A

V is proportional to B/T
V= induced voltage
B= total change in magnetic field
T= time taken to change

35
Q

what is a surface coil

A
  • 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
36
Q

whats a helm-holt coil

A
  • 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)
37
Q

whats a bird cage coil

A
  • 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
38
Q

whats a solenoid coil

A
  • 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
39
Q

whats an array coil

A
  • 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
40
Q
A

RAD413 (24 decks)

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