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MRI Physics > Chapter 9 Instrumentation > Flashcards

Chapter 9 Instrumentation Flashcards

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1
Q

MRI Scanners must all incorporate the following.

A
  1. A powerful magnet /magnetic field of over 40-50 cm spherical volume
  2. shim system
  3. Gradient system
  4. RF transmission system
  5. RF receiver coils
  6. Computer system
  7. Computer subsystem
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2
Q

What MRI system improves homogeneity of the magnetic field?

A

Shim System

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3
Q

What system in MRI creates linear slopes in field strength in any direction?

A

Gradient System

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4
Q

What MRI system generates and transmits pulses of electromagnetic radiation?

A

RF Transmission System

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5
Q

What detects signal from the patient?

A

RF Receiver Coils

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6
Q

What system allows input of parameters and displaying of images?

A

Computer System

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7
Q

System capable of coordinating the application of RF pulses and gradients and reconstructing the acquired data into images and storing them?

A

Computer Sub-System

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8
Q

What is the magnetic field in teslas (T)?

A

Bo

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9
Q

The magnetic intensity in amperes/m

A

Ho

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10
Q 
Exhibit a weak repulsion to an external magnetic field
Slightly oppose (decease) the applied magnetic field
A

Diamagnetic Compounds

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11
Q 
Exhibit a weak attraction to an external magnetic field
Add to (increase) the applied magnetic field
A

Paramagnetic Compounds

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12
Q

Have a large positive magnetic susceptibility and are powerfully attracted to an external magnetic field?
Are permanently magnetized even when removed from the magnetic field

A

Ferromagnetic Compounds

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13
Q

4 naturally occurring elements that are Ferromagnetic at a normal temperature?

A

Iron
Nickel
Cobalt
Gadolinium

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14
Q

Moving a conductor through a magnetic field induces an electrical charge in it

A
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15
Q

Moving electrical charge in a conductor induces a magnetic field around it

A
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16
Q

Three main types of MRI scanner configurations?

A

Closed-bore System
Open Systems
Extremity Systems

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17
Q

Most popular MRI system, have a tunnel shaped magnet bore almost look like a larger CT unit. Has longitudinal table movement and can generate very high magnetic field strengths over 8 T ?

A

Closed-bore System

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18
Q

Closed-bore Systems use what field strengths for clinical use and research studies?

A

Clinical Use: 1 - 3 T

Research: 8 T and above

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19
Q

Closed-Bore MRI Sytems use what type of magnet?

A

Solenoid Electromagnets that are infersuperiorly placed

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20
Q 
This system has the following features
Wider Imaging Table
Magnetic poles above and below 
Great for large people or animals
Lateral access great for biopsies
Magnets are horizontal above and below the patient
A

Open Systems

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21
Q

What type of magnet is used in Open MRI systems?

A

Large Permanent Magnets

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22
Q

What is the field strength for Open MRI systems?

A

1.2 T

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23
Q

Designed to scan limbs a much smaller unit used permanent magnets and has a field strength restricted below 1T?

A

Extremity Systems

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24
Q

MRI field strength (flux density) must be high and between?

A

1.0 - 8.0 T

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25
Q

The fringe field having a strength of 0.5 mT (5G) or greater must not extend outside the safety of what zones?

A

Zones III and IV and should be contained within the magnet room.

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26
Q

Ferromagnetic discs are known as ___________ and are mounted on a yoke that positions them directly above and below the imaging volume

A

Pole Shoes

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27
Q

The area inside the cylinder of the cryostat is known as ________ contains the patient bore and components of the MRI system that operate at room temperature.

A

WARM BORE

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28
Q

Substances used to supercool the coils of wire in superconducting magnets.

A

CYROGENS

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29
Q

Solenoid electromagnets that use supercooled coils of a wire so that there is no inherent resistance in the system. The current flows and therefore, the magnetism is generated without a driving voltage

A

Superconducting Magnets

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30
Q

Cyrogen of choice for superconducting magnets because of its extremely low temperature

A

Liquid helium

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31
Q

Main magnetic field is induced using ________

A

Solenoid Magnet

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32
Q

The process of energizing an MRI solenoid

A

RAMPING

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33
Q

Bringing an MRI magnet up to the required field strength involves a simple parallel circuit known as

A

Persistent Switch

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34
Q

Magnetic field strength is measured in two main ways?

A

Measured in Amperes (A) per m and is given the symbol H

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35
Q

Flux Density is measured in Tesla’s and given the symbol of B

A
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36
Q

The number of flux lines passing through a given area?

A

Flux Density

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37
Q

The earth’s magnetic field is usually around 0.5 G

A
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38
Q

The field strength (flux density) of and MRI magnet varies by scanner design and typically has a range of?

A

0.15 - 8T

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39
Q

In SI units the Tesla is used to quantify flux density.
In older CGS systems the gauss is used
(Where 1 T equals 10 000 G)

A
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40
Q

Magnetic Shielding occurs in 2 ways:

A
  1. Passive Shielding, requires scanner to surrounded by large steel plates
  2. Active Shielding, uses additional solenoid magnets
41
Q

Used in active shielding to constrain 5G footprint of the fringe field to within a short distance from the scanner

A

Bucking Coils

42
Q

The resistivity of electromagnetic coils is zero because they are immersed in a cryogen that reduces their temperature to almost absolute zero

A
43
Q

Law of electromagnetic induction that states charge, movement and magnetism are inextricably linked

A

Faraday’s Law

44
Q

The force exerted on a charge particle moving with velocity through an electric magnetic field

A

Lorentz Force

45
Q

1st system to be encountered within the warm bore of the inner wall of the cryostat that operates at room temperature. It ensures homogenicity of the magnetic field within the imaging volume

A

Shim System

46
Q

Process by which the evenness of the magnetic field is optimized

A

Shimming

47
Q

Uses shims to adjust for large changes in field homogeneity

A

Passive Shimming

48
Q

Uses electromagnets, additional solenoid magnets to adjust the field homogeneity

A

Active Shimming

49
Q

Another way of shimming in which another electromagnet is designed to manipulate the magnetic field during image acquisition

A

Gradient Offset (dynamic) Shimming

50
Q

Center of the warm bore, this system must localize origin of the MRI signal ?

A

Gradient System

51
Q

Closed-bore system, the y-axis lies vertically (posteroanteriorly) & x-axis horizontally from left to right
Z axis is horizontal aling the length of the magnet

A

Open System flux lines of Bo run vertically, so y-axis is horizontal, and z-axis is vertical

52
Q

Supply power to the gradient coils

A

Gradient Amplifiers

53
Q

These amplifiers help reduce heating by applying voltage to the coil in short closely spaced discrete bursts

A

Pulse Width Modulated (PWM) Amplifiers

54
Q

The percentage of time that a gradient spends at maximum amplitude

A

Duty Cycle

55
Q

Defines the power of the gradient specifically how steep a gradient slope can be achieved when the gradient coil element is activated

A

Gradient Amplitude

56
Q

The ideal gradient should be the following:

A

Powerful
Capable of high amplitude gradient slopes
Rapid to reduce scan times
(rise time, slew rate, duty cycle)

57
Q

The unit used to measure Gradient Amplitude are:

A

mT/m or G/cm

remember the unit of gauss are 10 000 times smaller than teslas

58
Q

On modern day scanners the typical values of gradient amplitude are below __________

A

40 mT/m

59
Q

The time taken to for the magnet field gradient to reach the required amplitude.
Is measured in microseconds (us)
Values are typically below 1000 us

A

Gradient Rise Time

60
Q

How quickly the gradient magnetic field can be applied at a given amplitude and over a given distance measured in meters.
Units are T/m/s

A

Gradient Slew Rate

61
Q

Strength of gradient over distance

A

Slew Rate

62
Q

Slew rate is determined by dividing the amplitude of the gradient by the gradient rise of time. Example

Amplitude: 30 mT/m divided by Rise Time 0.3 ms
= 100 mT/m/ms or 100 T/ms

A
63
Q

The time that the gradient coil is operating at required maximum amplitude and is expressed as a percentage % of the total acquisition time____________

A

Gradient Power Duty Cycle

64
Q

Acoustic Noise has been reported of intensities of up to

A

115 dB or higher

65
Q

What ate the three (3) gradients that perform several functions during a sequence ______

A

x, y, z

66
Q

The amplitude of the gradient is determined by the amount of ________________ passing through the coil

A

Current

67
Q

The _____________ of the gradient is determined by the direction of the current flowing through the coil

A

Polarity

68
Q

The amplitude of the gradient determines the ___________

A

Spatial Resolution

69
Q

Determines how fast data can be acquired_________

A

Slew Rate

70
Q

Whay system determines the scan time, slice location, spatial encoding and (indirectly) image contrast in studies using gradient echo pulse sequences (GES), perfusion, and diffusion imaging?

A

Gradient System

71
Q

This system purpose is to transmit and receive electromagnetic radiation during image acquisition and in MRI is often referred to as B1 (secondary magnetic field)

A

RF System

72
Q

Equation that states the precessional frequency of the magnetic moment of an atomic nucleus is proportional to the external magnetic field

A

Larmor Equation

73
Q

The purpose of the transmitted RF is to target a volume (slice) of tissue within the region to be examined and increase the energy in proportion of the hydrogen nuclei within that slice

A
74
Q

Hydrogen has a geometric ratio of 42.58 MHz/T

A
75
Q

Supplies power to the RF transmitter coils_____

A

RF Amplifier

76
Q

RF Amplifier typically operates at

A

18 kW or greater in 1.5 T systems

Up to 40 kW in 3T systems

77
Q

The RF pulses are described based on their function name them__________ and are timed to occur at certain time periods

A

Excitation Pulse
Rephasing Pulse
Inversion Pulse

78
Q

Known as resonator coils, goal is to transfer energy of the hydrogen nuclei.
Is circularly polarized (quadrature design) which means the coil is connected to 2 power supplies coming from the RF amplifier

A

RF Transmit Coil

79
Q

Coils that transmit RF at the resonant frequency of hydrogen to excite the nuclei and move them into high energy state___________

A

RF Transmit Coil

80
Q

Coil that both transmits RF and receives the MR signal _____

A

Transceiver

81
Q

Symmetrical birdcage arrangement of several electrically conductive elements in a transceiver coil __________

A

Birdcage Design

82
Q

Signal quality is determined by this, and it determines how well the shape and size of the coil are matched to the anatomical region under investigation. Needed to ensure good signal quality from a coil __________

A

Filling Factor

83
Q

Receives the signal returning from the patient at time TE and can also detect a certain amount of undesirable noise

A

RF Receive System

84
Q

Is caused by thermodynamic motion of electrons both in the coil and inside the body tissues_________

A

Noise

85
Q

Coil designed to receive signal from any of the anatomical areas that may be examined using the MRI scanner

A

Receiver Coils

86
Q

Receive only coils used to image surface anatomy

A

Surface Coils

87
Q

Coil that transmits and receives signal over a large volume or tissue are of the patient _________

A

Volume Coils

88
Q

Large coils provide large coverage, but poor SNR and filling factor is low

A

Small coils provide small coverage, but good SNR due to high filling factor and aliasing is more likely

89
Q

Best type of coils___ because they combine the benefits of using small coils with those using large coils

A

Phased Array Linear & Volume type

90
Q

Permit parallel data acquisition __________

A

Phased Array Coils

91
Q

Offer homogenous RF field over a large imaging volume and can image anatomy at any location and at any depth _____

A

Volume Coil

92
Q

Disadvantages of Volume Coils _______

A

Lower Filling Factor and can reduce SNR

93
Q

The large nonferromagnetic patient couch (table) that is raised and lowered to facilitate patient access and driven horizontally into the magnet bore

A

Patient Transport System

94
Q

Evenness of the magnetic field _________

A

Magnetic Homogeneity

95
Q

Center of the bore of the magnet in all planes _______

A

Magnetic Isocenter

96
Q

Entire process of MRI acquisition is completed by ________

A

Host Computer / Computer System

97
Q

Is used to identify the patient usually via a network link to the radiology information system ________

A

Graphical User Interface (GUI)

98
Q

An independent computer that is responsible for sending instructions relating to timing, amplitude, duration gradients and shape to the transmitted RF pulses and the time duration of the sampling window __________________

A

Pulse Generator Module (PGM)

99
Q

Independent drives /Used to protect form data loss _______-

A

Redundant Array

MRI Physics (1 decks)

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