MRI equipment

Question 1

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

Answer 1

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

Question 2

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

Answer 2

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

Question 3

what structures in the room is RF shielded

Answer 3

all walls, doors and windows

Question 4

what is the 5 gauss line

Answer 4

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

Question 5

what is the penetration panel

Answer 5

• 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

Question 6

Why must the electrical supply in an MRI be filtered?

Answer 6

RF spikes on images

Question 7

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

Answer 7

1-4 tons

Question 8

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

Answer 8

innermost:
RF coil
gradient coil
shim coil
magnet
outermost

Question 9

what is the array processors duty in MRI

Answer 9

• 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

Question 10

what 2 computer/electronics are found in MRI

Answer 10

• array processor
• operator console

Question 11

what are 3 main magnet types in MRI

Answer 11

• permanent
• resistive
• superconducting

Question 12

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

Answer 12

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

Question 13

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

Answer 13

• room temp
• temp sensitive
• 0.03 to 0.3T

Question 14

how much does a permanent magnet weigh

Answer 14

6-100 ton

Question 15

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

Answer 15

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

Question 16

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

Answer 16

• works at room temp
• strength of 0.03 to 0.4 T

Question 17

What is fringe field

Answer 17

The stray magnetic field outside the bore of the magnet

Question 18

what are 2 negatives about using resistive magnet

Answer 18

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

Question 19

what is a hybrid magnet

Answer 19

• uses both superconducting and resistive magnet

Question 20

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

Answer 20

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

Question 21

What direction is the magnetic field using a superconductive magnet

Answer 21

horizontal

Question 22

define homogeneity in MRI

Answer 22

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

Question 23

what is shimming in MRI

Answer 23

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

Question 24

What are the 2 types of shimming in MRI

Answer 24

• active
• passive

Question 25

what is active shimming

Answer 25

- currents are directed through specialized coils to further improve homogeneity. - additional power is used

Question 26

what is passive shimming

Answer 26

- 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

Question 27

what is used to undergo spacial localisation?

Answer 27

- gradient coils in the x, y, z axes

Question 28

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.

Question 29

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

Answer 29

- high gradient strength (Gmax) - short rise time (tR) - high slew rate (Gmax / tR)

Question 30

Define slew rate

Answer 30

ratio of time taken to reach maximum gradient amplitude

Question 31

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

Question 32

What is the purpose of RF coils

Answer 32

they transmit and receive radio-frequency

Question 33

what law do RF coils follow

Answer 33

faradays law of induction

Question 34

what is faradays law of induction

Answer 34

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

Question 35

what is a surface coil

Answer 35

- 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

Question 36

whats a helm-holt coil

Answer 36

- 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)

Question 37

whats a bird cage coil

Answer 37

- 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

Question 38

whats a solenoid coil

Answer 38

- 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

Question 39

whats an array coil

Answer 39

- 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