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MRI > 4: Instrumentation of MRI > Flashcards

4: Instrumentation of MRI Flashcards

1
Q

Faraday’s law of Electromagnetism

A

If you pass a current of electricity through a coiled wire, the result will be a magnetic field.

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2
Q

In Faraday’s law, the direction of the magnetic field is determined by…

A

the direction in which the electric current is flowing, north vs. south

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3
Q

Faraday’s law: Right hand rule:

A

provides a visual correlation to explain the direction of current vs the direction of the magnetic field.
Curled fingers = magnetic field
Thumb = electric current.

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4
Q

Two Main type of magnets

A

Permanent Magnets and Electromagnetics

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5
Q

Permanent Magnets:

A

Constructed with many small permanent magnets surrounded a solid iron core. Vertical fields commonly used in open, low-field magnets.

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6
Q

Permanent magnet advantages:

A
  • Less power consumption
  • provides a consistent and reliable magnetic field
  • inherently weak fringe field
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7
Q

Permanent magnet disadvantages:

A
  • Always on, can not be turned off via quench
  • Presence would call for dismantling of many pieces
  • Weak magnetic field strength ~ 0.4T
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8
Q

Electromagnets:

A

Constructed with multiple loops of copper wiring with a constant power source of electricity. The standard for imaging and incorporate a horizontal magnetic field.

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9
Q

Electromagnet advantages:

A
  • High magnetic field strength, high quality imaging
  • Ability to quench in emergency situations
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10
Q

Electromagnetic disadvantages:

A
  • High power consumption
  • Costly maintenance
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11
Q

Two types of Electromagnets:

A
  1. Resistive
  2. Superconducting
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12
Q

Resistive electromagnets:

A

Constructed using an iron core encased in copper wiring. Can be either vertical or horizontal magnetic fields. Mostly vertical.

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13
Q

Resistive electromagnet advantages:

A
  • Can be quickly turned off by power switch, doesn’t need to be quenched.
  • Weak fringe field
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14
Q

Resistive electromagnet disadvantages:

A
  • high power cost
  • strict water cooling requirements
  • weak magnetic field - 0.7 - 1.2 T
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15
Q

Superconducting electromagnets:

A

Standard for high quality. Need to be stored in a liquid helium bath with a temperature of -269 C / 4 K. Constructed with low resistance coils which allows higher amounts of current to pass resulting in the ability to reach higher gradient amplitudes. Horizontal.

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16
Q

Superconducting electromagnet advantages:

A
  • Consumes the least amount of power
  • Higher magnetic fields (1.5T +)
  • Creates a stable, consistent field homogeneity
  • Ability to turn off via quench for emergency situations
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17
Q

Superconducting electromagnet disadvantages:

A
  • Stronger fringe field
  • Requires costly routine cryogen and service maintenances
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18
Q

Fringe Fields:

A

The external magnetic attraction extending in a perimeter. Directly related to the strength of the magnet.

Higher magnet strength = Stronger fringe field
Closer to the magnet = stronger fringe field

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19
Q

Gauss:

A

The unit of measurement to calculate fringe field strength

10,000 Gauss = 1 Tesla

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20
Q
A
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21
Q

2 Types of Magnetic Shielding:

A
  1. Passive
  2. Active
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22
Q

Passive Shielding:

A

Installed during installation of MRI scanner. Achieved by encasing the magnet with an iron shell, thus absorbing majority of the fringe field

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23
Q

Active Shielding:

A

Installed during installation of MRI scanner. Corporates an additional Shielding coil that works by employing an opposite current value to reduce the range of the fringe field. The LOWEST fringe field and is used on all modern MRI units

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24
Q

Which type of shielding gives the lowest Fringe Field?

A

Active

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25
Q

Which type of shielding is the standard of most modern MRI units?

A

Active

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26
Q

Radiofrequency,
high or low energy?
ionizing or nonionizing?

A

Low energy, non-ionizing type of radiation

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27
Q

Transmit coil is located…

A

Located perpendicular to the main magnetic field B0, the transmit RF pulse is termed B1
In the relation to the x, y, z gradient coils and the physical magnet, the transmitter RF coil is located DEEPEST within the MRI structural layers, right outside where the patient is laying down. Similar location to Mylar in CT

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28
Q

B1

A

transmit RF pulse

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29
Q

Transmit coil purpose:

A

transmits radiofrequency pulses to specific area of the body at the precessional frequency of tissue

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30
Q

The main biological effect of the radiofrequency pulses of the transmit coil:

A

the chance of tissue heating. Therefore, proper padding between the patient and inner bore is highly recommended. ( Elbow padding )

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31
Q

The FDA limits allowable RF absorption (SAR) to:

A

4 W / Kg

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32
Q

The FDA limits RF absorption in regards to a core body temperature of:

A

1 degree Celsius

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33
Q

Receive Coil is located…

A

manually selected an attached to the exam table by the MRI technologist. Receive coil selection depends on the body part being examined.

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34
Q

Receive coil purpose:

A

to receive the MR signal echo being emitted from the patient due to a process called relaxation.

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35
Q
A
36
Q

Transmit-Receive (T/R) Coil purpose…

A

hold a dual purpose responsibility to:
1. transmit the RF pulse
2. receive the MR signal/echo

37
Q

By selecting a T/R coil, a smaller area of the body is excited by the RF pulse which has what effect on SAR?

A

greatly reduces SAR and B1 RMS levels.

38
Q

Transmit-receive coils allow technologists to…

A

select or join exams by using a single coil.

Example: A neurovascular (NV) coil is used to link MRA head cases with MRA carotid cases. The ability to continue without changing coils reduce turnover time and reduce movement between cases.

39
Q

Three types of dedicated receive coils:

A
  1. Phase array coils
  2. Volumetric coils
  3. Surface coils
40
Q

Phase Array coil:

A

Constructed with two separate pieces that are jointed together and placed anterior and posterior to the patient. ( Sandwich ). Can increase SNR and decrease time.

41
Q

Phase Array coil advantage:

A
  • Ability to utilize parallel imaging
  • ability to cover large FOV and reduce time
42
Q

Phase Array coil disadvantage:

A
  • Expensive
  • Minimum FOV of ~ 16
43
Q

Volumetric coils:

A

Constructed and designed to the shape of specific body parts. Ideal and provide best possible signal due to closely surrounding area of interest.

44
Q

Volumetric coil advantage:

A
  • Ability to utilize motion and metal reduction techniques
  • Ability to scan at small FOV and high resolution.
45
Q

Volumetric coil disadvantage

A
  • Limited ranges for studies for coil
  • high cost
46
Q

Example of Volumetric coils:

A

Brain coils, Knee coils, Foot/Ankle coils, Wrist coils

47
Q

Surface coils:

A

Designed as long and flat in appearance and tend to cover large linear FOV ( Spines ). Constructed of multiple elements. These numbers indicate the elements that can be turned on / off manually by the technologist

48
Q

Surface coil advantages

A
  • ability to scan multiple spine exams without having to re-landmark the patient (enter new location numbers)
  • ability to use Rectangular FOV (RFOV)
49
Q

What artifact can be caused by Surface coils?

A

Annefact: produces a glaring presence due to coils left on outside of the FOV.

50
Q

Radiofrequency tuning:

A

Similar to a radio, coils must be tuned properly to transmit and receive the most optimal signal. Before the exam, each coil will adjust bandwidth to match the size and shape of the region of the body being examined and calculate a peak signal for the specific body part. In some cases, such as patient with very low body fat percentage, tuning must be done manually due to the lack of fat in the patient.

Tip: adjusting transmit gain (TG) to a range of 140 - 180 will help find an optimal peak of bandwidth

51
Q

Gradient coils:

A

physical components consisting of copper wire with passing electric current that provides the ability to create magnetic field variations in a patient’s body. Tilts the magnetic field into a positive and negative linear variation.

52
Q

What controls spatial localization? How?

A

Gradient coils, tilting the magnetic field into a positive and negative linear variation.

53
Q

The FDA’s limitation on time-varying magnetic fields (TVVMF):

A

Considered to be once a patient experiences peripheral nerve stimulation. There is no threshold.

54
Q

Time-varying magnetic field effects can include:

A

Hearing disruption
Peripheral nerve stimulation (involuntary body motion)
A sense of stars in a patient’s eyes (seeing spots)

55
Q

Gradient coil Configuration: X

A

Sagittal view, Left to right, Lateral sides of patient

56
Q

Gradient coil Configuration: Y

A

Coronal view, Anterior to posterior

57
Q

Gradient coil Configuration: Z

A

Axial view, bands around patient like the Bore at head and feet

58
Q

Gradient Amplitude:

A

the amount of current increased to create a linear variation, thus allowing spatial localization.

59
Q

The action of a gradient amplitude increasing:

A

Ramp up

60
Q

The action of when a gradient amplitude decreases:

A

Ramp down

61
Q

The time it takes a gradient to reach full amplitude:

A

Rise time

62
Q

The time it takes the gradient amplitude to turn off:

A

Fall time

63
Q

Two major factors in determining the power of a gradient:

A
  1. Slew rate
  2. Duty cycle
64
Q

Slew rate, and how is it measured?

A

the amount of time it takes an x, y, z gradient to reach its amplitude. The combination of gradient amplitude multiplied by rise time and is measured in milliTesla per meter per second.

65
Q

Slew rate formula:

A

Slew rate mT/M/s = gradient amplitude x rise time

66
Q

Greater amplitude = (higher or lower) slew rate

A

higher

67
Q

With a greater amplitude, an MRI unit has a (greater or lower) ability to scan thinner slices and smaller FOV

A

greater

68
Q

Duty cycle, and how is it measured?

A

the amount of time a gradient remains at a certain amplitude during on TR period. measured in %

69
Q

Magnetic field homgeneity, and how is it measured?

A

describes the uniformity of the magnetic field at isocenter. Measured in parts per million.

70
Q

Shimming:

A

the process in which shim coils are activated prior to scanning to balance magnetic field homogeneity or inequalities in the magnetic field

71
Q

Shimming aids in reducing:

A

any changes of magnetic field errors caused by molecular pieces of metal in the room or near the magnet

72
Q

Two types of shimming:

A
  1. Passive shimming
  2. Active shimming
73
Q

Magnetic Quenching: what is it and what harm can it do to someone?

A

the manual or incidental rapid boil off of cryogen within the MR scanner. Poor ventilation may result cryogenic gas exposure in MR room. A quench will displace helium and nitrogen throughout the room and can result in asphyxiation due to the lower oxygen levels and ruptured tympanic membranes due to increase pressure.

74
Q

Quench protocol during an incidental quench

A
  1. remove patient from magnet
  2. exit MR room low to the ground
  3. lock MR room door
75
Q

Passive Shimming

A

Physical manipulation shimming structures after a homogeneity testing. Is installed during construction of the magnet. Measurements are performed to check for any potential field inhomogeneities. When found, large shimming trays are installed to improve field homogeneity.

76
Q

Passive shimming, when is it on?

A

Passive shimming is physically installed, so it its always present and always on

77
Q

Active shimming:

A

achieved by turning on an electromagnetic coil that shims the magnetic field. This can be done by the MRI technologist at the MR console. Before scanning a particular body part, MRI technologists place a shim over the area which provides an extra current to pass through the shim coil, thus balancing the magnetic field.

78
Q

Active shimming, when is it on?

A

Active shimming is performed manually and is only active when turned on by the MRI technologist. Good to use if grainy

79
Q

Linear variation:

A

Gradient coils are induced causing an increase in gradient amplitude which allows visualization of a specific area on a slice

80
Q

Tuning

A

the adjustment of a T/R coil to maximize signal

81
Q

Fringe field

A

the magnetic field located outside the bore of the magnet.

82
Q

Magnetic shielding

A

installed to minimize fringe field

83
Q

Where is the gradient system located?

A

In the MR unit layered behind the transmit coil

84
Q

Faraday’s law of Electromagnetism states “if a loop wire is moved through a magnetic field, _________ will be created.”

A

A current of voltage

85
Q

What liquid cryogen is to maintain the temperature on superconducting magnets?

A

Helium

86
Q

layers of magnet MR system

A
87
Q
A

MRI (8 decks)

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