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The Essential Physics of Medical Imaging > Magnetic Resonance Basics > Flashcards

Magnetic Resonance Basics Flashcards

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1
Q

This is the spectroscopic study of the magnetic properties of the nucleus of the atom.

A

Nuclear magnetic resonance (NMR)

This is not an imaging technique but rather a method to provide spectroscopic data concerning a sample placed in a small volume, high field strength magnetic device.

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2
Q

This is 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

This is a fundamental property of matter; it is generated by moving charges, usually electrons.

A

Magnetism

Magnetic properties of materials result from the organization and motion of the electrons in either a random or a nonrandom alignment of magnetic “domains”, which are the smallest entities of magnetism.

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4
Q

This is also called the magnetic flux density, can be conceptualized as the number of magnetic lines of force per unit area, which decreases roughly as the inverse square of the distance from a source.

A

Magnetic field strength

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5
Q

The SI unit for magnetic field strength is the called what?

A

Testla

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6
Q

This is a characteristic of certain metals (e.g., niobium-titanium alloy) that when maintained at extremely low temperature (liquid helium; less than 4K) exhibit no resistance to electric current.

A

Superconductivity

To achieve a high magnetic field strength (greater than 1 T) requires the electromagnet core wires to be superconductive.

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7
Q

Replenishment of liquid helium must occur continuously, because of the temperature rises above a critical value, the loss of superconductivity will occur and resistance heating or the wires will boil the helium. Which results to a what?

A

“Quench”

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8
Q

These coils interact with the main magnetic field to improve homogeneity (minimal variation of the magnetic flux density) over the volume used for patient imaging.

A

Shim coils

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9
Q

These coils exist 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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10
Q

These coils are contained 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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11
Q

This describes the extend to which a material becomes magnetized when placed in a magnetic field.

A

Magnetic susceptibility

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12
Q

What are the three categories of magnetic susceptibilities?

A

Diamagnetic
Paramagnetic
Ferromagnetic

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13
Q

These elements and materials have slightly negative susceptibility and oppose the applied magnetic field, because of paired electrons in the surrounding electron orbitals.

A

Diamagnetic

Examples of diamagnetic materials are calcium, water, and most organic materials (chiefly owing to the diamagnetic characteristics of carbon and nitrogen).

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14
Q

These materials, with unpaired electrons, have slightly positive susceptibility and enhance the local magnetic field, but they have no measurable self-magnetism.

A

Paramagnetic materials

Examples are molecular oxygen (O2), deoxyhemoglobin, some blood degradation products such as methemoglobin, and gadolinium-based contrast agents.

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15
Q

These materials are “superparamagnetic” -that is, they augment the external magnetic field substantially.

A

Ferromagnetic materials

These materials, containing iron, cobalt, and nickle, exhibit “sefl-magnetism” in may cases, and can significantly distort the acquired signals.

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16
Q

This comprises protons and neutron, and exhibit magnetic characteristics on a much smaller scale than for atoms/molecules and their associated electron distributions.

A

The nucleus

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17
Q

Magnetic characteristics of the nucleus are described by the what?

It is represented as a vector indicating magnitude and direction.

A

Nuclear magnetic moment

If the sum of the number of protons and number of neutrons in the nucleus is even, the nuclear magnetic moment is essentially zero.

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18
Q

This element ha the largest magnetic moment and greatest abundance, chiefly in water and fat.

It is by far the best element for general clinical utility.

A

Hydrogen

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19
Q

In addition to energy separation of the parallel and antiparallel spin states, the protons also experience a torque in a perpendicular direction from the applied magnetic field that causes what?

Much way that a spinning top wobbles due to the force of gravity.

A

Precession

This occurs at an angular frequency (number of rotations/sec about an axis rotation) that is proportional to the magnetic field.

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20
Q

This describes the dependence between the magnetic field and the angular precessional frequency.

A

Larmor equation

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21
Q

This is a stationary reference frame from the observer’s point of view.

A

Laboratory frame

The sample magnetic moment vector precesses about the z-axis in a circular geometry about the x-y plane.

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22
Q

This is a spinning axis system, whereby the x-y axes rotate at an angular frequency equal to the Larmor frequency.

A

Rotating frame

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23
Q

The magnetic interactions between precessional frequencies of the tissue magnetic moments with the externally applied RF (depicted as a rotating magnetic field) can be described more clearly using what frame of reference?

A

Rotating frame of reference

While the observed returning signal and its frequency content is explained using the laboratory (stationary) frame of reference.

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24
Q

The net magnetization vector of the sample is described by three components. What are these components?

A

Longitudinal magnetization
Equilibrium magnetization
Transverse magnetization

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25
Q

This is along the z direction.

It is the component of the magnetic moment parallel to the applied magnetic field.

A

Longitudinal magnetization

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26
Q

At equilibrium, the longitudinal magnetization is maximal and is denoted as what?.

A

Equilibrium magnetization

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27
Q

The component of the magnetic moment perpendicular to the magnetic field, is called what?.

A

Transverse magnetization

The vector component of the magnetic moment in the x-y plane.

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28
Q

These are result of the angular displacement of the longitudinal magnetization vector from the equilibrium position.

A

Flip angles

The represent the degree of longitudinal magnetization rotation by the B1 field as it applied along the x-axis (or y-axis) perpendicular to Mz.

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29
Q

This corresponds to the energy separation between the protons in parallel and antiparallel directions.

A

Resonance frequency

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30
Q

Elapsed time between the peak transverse signal (e.g., directly after a 90-degree RF pulse) and a 37% of the peak level is called what?

A

T2 relaxation time

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31
Q

Extrinsic magnetic inhomogeneities, such as the imperfect main magnetic field, or susceptibility agents in the tissues (e.g., MR contrast materials, paramagnetic or ferromagnetic objects), add to the loss of phase coherence from intrinsic inhomogeneities and further reduce the decay constant, known as what?

A

T2*

32
Q

This is the term describing the release of energy back to the lattice (molecular arrangement and structure of the hydration layer), and regrowth of the longitudinal magnetization.

A

Spin-lattice relaxation

33
Q

This is the time needed for the recovery of 63% of the longitudinal magnetization after a 90-degree pulse.

A

T1

34
Q

This depends on the rate of energy dissipation into the surrounding molecular lattice and hydration layer and varies substantially for different tissue structures and pathologies.

A

T1 relaxation time

35
Q

T1 is on order of 5 to 10 times _______ than T2.

Longer or shorter?

A

Longer

36
Q

Agents that disrupt the local magnetic field environment, such as paramagnetic blood degradation products, elements with unpaired electron spins (e.g., gadolinium), or may have ferromagnetic materials, cause a significant decrease in _______.

A

T2*

37
Q

This is the period between B1 excitation pulses.

A

Time of repetition

During the TR interval, T2 decay and T1 recovery occur in the tissues.

TR values range from extremely short (millisecond) to extremely long (10,000 ms) time periods, determined by the type of sequenced employed.

38
Q

Rotating at the Larmor frequency, the transverse magnetic field of the excited sample indices signal in the receiver antenna coil.

A dampened sinusoidal electronic signal is produced, known as what?

A

Free induction decay (FID)

The FID amplitude decay is caused by loss of transfer magnetization phase coherence due to intrinsic micromagnetic inhomogeneities in the sample structure, whereby individual protons in the bulk water and hydration layer couplers to macromolecules precess at incrementally different frequencies arising from the slight changes in local magnetic field strength.

39
Q

Displacement of the equilibrium magnetization occurs when the magnetic component of the RF excitation pulse, known as the ______ field, is precisely matched to he precessional frequency of the protons.

A

B1 field

40
Q

This is the 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

Excitation of protons with the B1 RF pulse creates the transverse magnetization FID signal.

To separate the RF energy deposition and returning signal, an “echo” is induced to appear at a later time, with the application of a 180-degree RF inversion pulse.

41
Q

This is the 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

During TI, longitudinal magnetization recovery occurs.

42
Q

This is a state of tissue magnetization from equilibrium conditions.

A

Saturation

43
Q

At equilibrium, the protons in a material are ________, with full M2 amplitude.

Saturated or unsaturated?

A

Unsaturated

44
Q

This occurs because the repetition time between excitation pulses does not allow for full return to equilibrium, and the longitudinal magnetization amplitude for the next RAF pulse is reduced.

A

Partial saturation - of tissues

This has an impact on tissue contrast, and explains certain findings such as unsaturated protons in blood outside of the volume moving into the volume and generating a bright vascular signal on entry slides into the volume.

45
Q

What are the three major pulse sequence perform the bulk of data acquisition (DAQ) for imaging?

A 
Spine echo (SE)
Inversion recovery (IR)
Gradient echo (GE)

When used in conjunction with spatial localization methods, “contrast-weighted images are obtained.

46
Q

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

A

Spin echo

47
Q

This is a SE sequence is designed to produce contrast chiefly based on the T1 characteristics of tissues, with de-emphasis of T2 and proton density contributions to the signal.

A

“T1-weighted”

This is achieved by using a relatively short TR to maximise the differences in longitudinal magnetization recovery during the return to equilibrium, and a short TE to minimize T2 decay during signal acquisition.

48
Q

This relies mainly on differences in the number of magnetized protons per unit volume of tissue.

T1 differences are reduced by selecting a long TR value to allow substantial recovery of longitudinal magnetization.

T2 differences of the tissues are reduced by selecting a short TE value.

A

Proton density contrast weighing

A typical proton density-weighted image has a TR between 2,000 and 4,000 ms and a TE between 3 to 30 ms.

49
Q

This follows directly from the proton density-weighing sequence: reduce T1 differences in tissues with a long TR, and emphasize T2 differences with a long TE.

A

T2 contrast weighing

A typical T2-weighted sequence uses a TR of approximately 2,000 to 4,000 ms and a TE of 80 to 120 ms.

50
Q

This emphasizes T1 relaxation time of the tissues by extending the amplitude or the longitudinal recovery by a factor of two.

A

Inversion recovery

51
Q

An initial 180-degree RF pulse inverts Mz to -Mz.

After a programmed delay, the time inversion -T1, a 90 degree RF (readout) pulse rotates the recovered fraction of Mz into the transverse plane to generate the FID.

A second 180-degree pulse (or gradient polarity reversal, GE) at TE/2 produces an echo at TE; in this situation, the sequence is called __________.

A

Inversion recovery spin echo (IR SE)

52
Q

This is a pulse sequence that uses very short TI and magnitude signal process, where longitudinal magnetization signal amplitude is always positive.

A

Short Tau Inversion Recovery (STIR)

A typical STIR sequence uses a TI of 140 to 180 ms and a TR of approximately 2,500 ms.

Compared with a T1-weighted examination, STIR reduces distracting fat signals and chemical shift artifacts.

53
Q

STIR

In this situation, materials with short TI have lower signal intensity (the reverse of a standard T1-weighted image), and all tissues at some point during recovery have Mz = 0.

This is known as the _______.

A

Bounce point or tissue null

54
Q

This sequence reduces CSF signal and other water-bound anatomy in the MR image by using a TI selected at or near the bounce point of CSF to permit better evaluation of the surrounding anatomy.

A

Fluid attenuated inversion recovery

The FLAIR sequence

55
Q

This technique uses a magnetic field gradient applied in one direction and then reversed to induced the formation of an echo, instead of the 180-degree inversion pulse.

A

Gradient echo technique

56
Q

A major variable determining tissue contrast in GE sequences is called what?_.

A

Flip angle

Depending on the desired image contrast, flip angles of a few degrees to more than 90 degrees are used, a majority of which are small angles much less than 60 degrees.

57
Q

For GE sequences with “long” TR (greater than 200 ms) and flip angles greater than 45 degrees, contrast behaviour is similar to that of SE sequences.

What is the major difference?

A

The major difference is image contrast that is based on T2* rather than T2, because external magnetic field inhomogeneities are not cancelled.

58
Q

GE acquisition with short TR less than 50 ms and flip angles up to 45 degrees produces how many signals?

A

Two signals

  1. The FID signal generated at the end of the current RF pulse, and once rephased, contains T2* or T1 information depending on the TE
  2. A stimulated echo generated from the previous RF pulse acting on the persistent transverse magnetization accruing from the RF pulse twice displaced.
59
Q

This sequence indicates the timing of the RF pulse with the dephasing and rephasing implemented by reversal of gradient polarity to generate an echo at a selectable time TE for the frequency encode gradient (FEG), where identification of proton position based upon frequency is performed.

A

Coherent GE sequence

Also involved in this sequence is the phase encode gradient (PEG), which is applied and incrementally changed for each TR to identify proton position in the direction perpendicular to the FEG based upon phase changes after the PEG is turned off.

60
Q

Under coherent GE

This is a sequence using using TR=35 ms, TE= 3 ms, and flip angle = 20 degrees shows unremarkable contrast, but blood flow shows up as a bright signal.

A

Gradient recalled acquisition in the steady state (GRASS)/ Fast imaging with steady-state precession (FISP)

61
Q

With very short TR steady-state acquisitions, T1 weighting cannot be achieved to any great extent, owing to either small differences in longitudinal magnetization with a small flip angles or dominance of the T2* effects for larger flip angles produced by persistent residual transverse magnetization created by stimulated echoes.

The T2* influence can be reduced by using a long TR (usually not an option), or by “spoiling” the steady-state transverse magnetization by introducing incoherent phase difference from pulse to pulse.

This is achieved by adding a phase shift to successive RF pulses during the excitation of protons.

What technique is this?

A

Incoherent, “Spoiled” Gradient Echo Technique

62
Q

The ability to achieve T1 contrast weighing is extremely useful for rapid three-dimensional volume imaging.

Bright blood and magnetic susceptibility artifacts are characteristics of this sequence.

A

Incoherent (spoiled) GE

63
Q

In GE sequences with short TR (less than 50 ms), the TE is not long enough to generate T2 contrast, and GE rephasing is inefficient and dominated by T2* effects.

This sequence emphasizes acquisition of only the stimulated echo, which arises from the previous RF pulse and appears during the next RF pulse at a time equal 2 x TR.

A

Steady-state free precession (SSFP)

64
Q

In this sequence, there are two TE values:

one is the actual TE, the time between the peak stimulated echo amplitude and the next excitation pulse, and the other effective TE, the time from the echo and the RF pulse that created its FID.

A

Steady-state free precession (SSFP) sequence

This sequence provide true T2 contrast weighting, are useful for brain and joint imaging, and can used with three-dimensional volumetric acquisition.

65
Q

_______ SSFP sequences generate accumulated gradient echo (FID) and stimulated echo signals with the use of symmetrical gradients in 3 spatial directions, which null phase shifts induced by flow.

A

“Balanced”

This provides T2/T1 contrast and high speed acquisition, particularly useful for cardiac imaging.

66
Q

This is essential for creating MR images and determining the location of discrete sample volumes for MR spectroscopy.

A

Spatial localization

67
Q

What are the two important properties of magnetic gradients?

A
  1. The gradient field strength

2. The slew rate

68
Q

This is determined by its peak amplitude and slow (change over distance), and typically ranges from 1 to 5 mt/m.

A

Gradient field strength

69
Q

This is the time to achieve the peak magnetic field amplitude.

A

Slew rate

Typical slew rates of gradient fields are from 5 to 25p mt/m/ms.

70
Q

As the gradient field is turned on, this currents are induced in nearby conductors such as adjacent RF coils and the patient, which produce magnetic fields that oppose the gradient field and limit achievable slew rate.

A

Eddy currents

Actively shielded gradient coils and compensation circuits can reduce problems cause by eddy currents.

71
Q

In a gradient magnetic field, protons maintain precessional frequencies corresponding to local magnetic field strength.

At the middle of the gradient, there is no change in the field strength or precessional frequency.

This is called what?

A

Null

With linear gradient, the magnetic field increases and decreases linearly from the null, as does the precessional frequency.

72
Q

Localization of protons in the three-dimensional volume requires the application of three distinct gradients during the pulse sequence.

Which are?

A

Slice select
Frequency encode
Phase encode

73
Q

RF transmitters cannot spatially direct the RF energy to a specific region in the body; rather the RF pulse, when turned on during the application of the what gradient?

This determines the slice location of protons in the tissues that absorb energy.

A

Slice select gradient (SSG)

SSG is applied simultaneously with the RF pulse of a known BW to create proton excitation in a single plane with a known slice thickness, and to localize signals orthogonal to the gradient.

It is the first of three gradients applied to the volume.

74
Q

The is applied in a direction perpendicular to SSG, along the “logical” x-axis, during the evolution and decay of induced echo.

This is also known as the readout gradient,

A

Freqency encode gradient

The SSG in concert with an incremental rotation of the FEG direction about the object can produce data projections through the object as a function of angle.

75
Q

Positions of the protons in the third orthogonal dimension is determined with what gradient?

This is applied after SSG but before FEG, along the third orthogonal axis.

A

Phase encode gradient

76
Q

MR data are initially stored in the ________ matrix, the “frequency domain” repository.

This describes a two-dimensional matrix of positive and negative spatial frequency values, encoded as complex numbers.

A

K-space

The Essential Physics of Medical Imaging (11 decks)

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