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FEESICKS BUSHBIRD III > ***Chapter 12 - Magnetic Resonance Basics > Flashcards

***Chapter 12 - Magnetic Resonance Basics Flashcards

1
Q

The spectroscopic study of the magnetic properties of the nucleus of the atom

A

Nuclear magnetic resonance (NMR)

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2
Q

An energy coupling that causes the individual nuclei , when placed in a strong external magnetic field, to selectively absorb, and later release, energy unique to those nuclei and their surrounding environment

A

Resonance

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3
Q

A fundamental property of matter ; it is generated by moving charges, usually electrons

A

Magnetism

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4
Q

Smallest entities of magnetism

A

Domains

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5
Q

Number of magnetic lines of force per unit area; decreases roughly as the inverse square of the distance from the source

SI Unit: Tesla (T)
1 T = 10,000 G (gauss)

A

Magnetic field strength,B (also called the magnetic flux density)

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6
Q

Earth’s magnetic field strength in mT

A

0.05 mT

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7
Q

____ of the current in the coil determines the overall magnitude of the magnetic field strength

A

Amplitude

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8
Q

Magnetic field lines extending beyond the concentrated field

A

Fringe fields

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9
Q

Heart of the MR system

A

Magnet

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10
Q

Performance criteria for magnets(3)

A
  1. Field strength
  2. Temporal stability
  3. Field homogeneity
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11
Q

Magnets which have a HORIZONTAL main field produced in the bore of the electrical windings, with the Z axis (B0) along the bore axis

A

Air core magnets

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12
Q

Magnet with a vertical field, produced between the metal poles of a permanent or wire-wrapped electromagnet; Fringe fields are confined with this design

A

Solid core magnet

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13
Q

Interact with main magnetic field to improve homogeneity over the volume used for patient imaging

A

Shim coils

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14
Q

Exists within the main bore of the magnet to transmit energy to the patient as well as to receive returning signals

A

Radiofrequency (RF) coils

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15
Q

Contains within the main bore to produce a linear variation of magnetic field strength across the useful magnet volume

A

Gradient coils

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16
Q

Obtained by superimposing the magnetic fields of two or more coils carrying a direct current of specific amplitude and direction with a precisely defined geometry

A

Magnetic field gradient

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17
Q

Describes the extent to which a material becomes magnetized when place in a magnetic field

A

Magnetic susceptibility

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18
Q

3 categories of magnetic susceptibility

A

Diamagnetic, paramagnetic and ferromagnetic

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19
Q
  • slightly negative susceptibility

- oppose the applied magnetic field, due to PAIRED electrons in the surrounding orbital electrons

A

Diamagnetic elements

*calcium, water and most organic materials (Carbon, Hydrogen)

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20
Q
  • UNPAIRED electrons
  • slightly positive susceptibility
  • enhance the local magnetic field
  • no measurable self-magnetism
A

Paramagnetic materials

*molecular oxygen, deoxyhemoglobin, methemoglobin and gadolinium-based contrast agents

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21
Q
  • “superparamagnetic”
  • augment the external magnetic field substantially
  • exhibit “self-magnetism”
  • can significantly distort the acquired signals
A

Ferromagnetic materials

  • materials containing iron, cobalt, nickel
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22
Q

For a given nucleus , it is determined thru the pairing of the constituent protons and neutrons

A

Nuclear magnetic moment

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23
Q

The principal focus for generating MR signals

A

Nucleus of the hydrogen atom, the proton

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24
Q

Describes the dependence between the magnetic field and the angular precessional frequency

A

Larmor equation

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25
Q

A stationary reference frame from the observer’s point of view

A

Laboratory frame

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26
Q

A spinning axis system whereby the x’-y’ axes rotate at an angular frequency equal to the lateral frequency

A

Rotating frame

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27
Q

Along the z direction, is the component of the magnetic moment parallel to the applied magnetic field, B0

A

Longitudinal magnetization

*at equilibrium, the longitudinal magnetization is maximal and is denoted as M0, EQUILIBRIUM MAGNETIZATION

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28
Q

The component of the magnetic moment perpendicular to B0, Mxy, in the x-y plane

A

Transverse magnetization

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29
Q

Magnetic component of the RF excitation pulse

A

B1 field

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30
Q

Corresponds to the energy separation between the protons in the parallel and antiparallel directions

A

Resonance frequency

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31
Q

Considers the RF energy as PHOTONS (quanta) instead of waves

A

Quantum mechanics model

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32
Q

Represent the degree of Mz rotation by the B1 field as it is applied along the x’-axis (or the y’-axis) perpendicular to Mz

A

Flip angles

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33
Q

A damped sinusoidal electrical signal

A

Free induction decay (FID)

34
Q

Caused by loss of Mxy phase coherence due to intrinsic micro magnetic inhomogeneities in the sample’s structures

  • individual protons in the bulk water and hydration layer coupled to macromolecules process at incrementally different frequencies arising from the slight changes in local magnetic field strength
A

FID amplitude decay

35
Q

Elapsed time between the peak transverse signal (e.g. Directly after a 90-degree RF pulse) and 37% of the peak level (1/e)

  • decay time resulting from INTRINSIC magnetic properties of the sample
A

T2 relaxation time

36
Q

Contain mobile molecules with fast and rapid molecular motion

A

Amorphous structures (e.g. CSF , highly edematous tissues)

37
Q

Decay time resulting from both INTRINSIC and EXTRINSIC magnetic field variations

38
Q

Term describing the release of energy back to the lattice (the molecular arrangement and structure of the hydration layer), and the regrowth of Mz

A

Spin-lattice relaxation

39
Q

Time needed for the recovery of 63% of Mz after a 90-degree pulse

40
Q

FREE INDUCTION DECAY

A

T2 relaxation

41
Q

RETURN TO EQUILIBRIUM

A

T1 relaxation

42
Q

Effective in decreasing T1 relaxation time of local tissues

A

Gadolinium chelated with complex macromolecules

43
Q

The period between B1 excitation pulses

  • T2 decay and T1 recovery occur in the tissues
A

Time of repetition (TR)

44
Q

Time between the excitation pulse and the appearance of the peak amplitude of an induced echo, which is determined by applying a 180-degree RF inversion pulse or gradient polarity reversal at a time equal to TE/2

A

Time of echo (TE)

45
Q

Time between an initial inversion/excitation (180 degrees) RF pulse that produces maximum tissue saturation and a 90-degree readout pulse

A

Time of inversion (TI)

46
Q

A state of tissue magnetization from equilibrium conditions

A

Saturation

47
Q

3 major pulse sequences that perform the bulk of data acquisition (DAQ) for imaging:

A
  1. Spin echo (SE)
  2. Inversion recovery (IR)
  3. Gradient echo (GE)
48
Q

Describes the excitation of the magnetized protons in a sample with a 90-degree RF pulse and production of a FID, followed by refocusing 180-degree RF pulse to produce an echo

A

Spin echo (SE)

49
Q

Pulse which converts Mz into Mxy, and creates the largest phase coherent transverse magnetization that immediately begins to decay at a rate described by T2* relaxation

A

90-degree pulse

50
Q

Applied TE/2, inverts the spin system and induces phase coherence at TE

A

180-degree RF pulse

51
Q

___ is proportional to the difference in signal intensity between adjacent pixels in an image, corresponding to different voxels in the patient

52
Q

Designed to produce contrast chiefly based on the T1 characteristics of tissues, with de-emphasis of T2 and proton density contributions to the signal

  • short TR (to maximize the differences in longitudinal magnetization recovery during the return of equilibrium)
  • short TE (to minimize T2 decay during signal acquisition)
A

T1 weighted SE sequence

53
Q

Preserves the T1 signal differences by not allowing any significant transverse (T2) decay

54
Q

T1 signal intensity from highest to lowest…

A

Fat>White matter>Gray Matter>CSF

55
Q

Relies mainly on differences in the number of magnetized protons per unit volume of tissue

  • long TR (to reduce T1 effects)
  • short TE (to reduce T2 influence in the acquired signals)
A

Proton density contrast weighting

*(CSF>fat>gray matter>white matter)
*typical PDW (TR: between 2,000 and 4,000 ms;
TE: between 3 and 30 ms)

56
Q

Sequence which achieves the highest overall signal intensity and the largest SNR

A

Proton density SE sequence

57
Q

Generated from the second echo produced by a second 180-degree pulse of a long TR spin echo pulse sequence, where the first echo is proton density weighted, with short TE

  • long TR (reduce T1 differences in tissues)
  • long TE (emphasize T2 differences)
A

T2-weighted signal

  • CSF is bright, gray and white matter are reversed in intensity
  • typical T2-weighted sequence uses a TR of approx. 2,000 to 4,000 ms and a TE of 80 to 120
58
Q

Emphasizes T1 relaxation times of the tissues by extending the amplitude of the longitudinal recovery by a factor of two

A

Inversion recovery (IR)

59
Q

***page 427

A

Inversion recovery spin-echo(IR SE)

60
Q

The delay between the excitation pulse and conversion to transverse magnetization of the recovered longitudinal magnetization

A

Inversion time (TI)

61
Q

Produces “negative” longitudinal magnetization that results in negative (in phase) or positive (out of phase) transverse magnetization when short T1 is used

A

IR sequence

62
Q

A pulse sequence that uses a very short TI and magnitude signal processing, where Mz signal amplitude is always positive

A

SHORT TAU INVERSION RECOVERY (STIR)

  • reduces distracting fat signals and chemical shift artifacts
  • typical STIR sequence uses: TI of 140 to 180 ms; TR of approx. 2,500 ms
63
Q

Materials with short T1 have a longer signal intensity (the reverse of a standard T1-weighted image), and all tissues at some point during recovery have Mz=0

A

Bounce point or tissue null

64
Q

Reduces CSF signal and other water-bound anatomy in the MR image by using a T1 selected at or near the bounce point of CSF o permit better evaluation of the surrounding anatomy

A

Fluid Attenuated Inversion Recovery (FLAIR)

65
Q

Uses a magnetic field gradient applied in one direction and then reversed to induce the formation of an echo, instead of the 180-degree inversion pulse

  • a purposeful dephasing and rephasing of the FID
A

Gradient Echo (GE)

66
Q

Magnetic field inhomogeneities and tissue susceptibilities caused by paramagnetic or diamagnetic tissues or contrast agents are emphasized in ___ imaging .

67
Q

Major variable determining tissue contrast in GE sequences

A

Flip Angle

68
Q

GRASS

A

Gradient recalled acquisition in the steady state

69
Q

FISP

A

Fast imaging with steady-state precession

70
Q

FAST

A

Fourier Acquired Steady State

71
Q

SPGR

A

Spoiled Transverse magnetization Gradient Recalled echo (SPGR)

72
Q

2 TE values in SSFP (Steady-State Free Precession)

A
  1. Actual TE

2. Effective TE

73
Q

The time between the peak stimulated echo amplitude and the next excitation pulse

74
Q

The time form the echo and the RF pulse that created its FID

A

Effective TE

75
Q

3 major GE sequences

A
  1. Coherent GE
  2. Incoherent GE
  3. SSFP
  4. Balanced SSFP
76
Q

Uses signals food the FID and the SE to produce the image, typically with contrast dependent on T2/T1 weighting and low contrast

A

Coherent GE

77
Q

Eliminates the detection of the SE, thus, providing a means to generate T1-weighted contrast from the FID signal

A

Incoherent GE

78
Q

uses the SE signal, which provides mainly T2-weighted contrast

79
Q

Uses both the gradient and stimulated echo to produce a T2/T1 weighting with symmetrically applied gradients in three dimensions

A

Balanced SSFP

80
Q

Typical magnetic field strengths for MR systems range from _____

A

0.3 to 4.0 T

FEESICKS BUSHBIRD III (13 decks)

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