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001 MRI > EXAM #14 — PHYSICS UNIT 07 > Flashcards

EXAM #14 — PHYSICS UNIT 07 Flashcards

1
Q

Define: a. gradient magnetic field.

A

magnetic fields that change linearly over the magnet bore

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2
Q

Define: b. logical gradients.

A

slice select, frequency encoding, and phase encoding gradients

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3
Q

Define: c. spatial localization.

A

the process of locating each voxel of tissue in an imaging volume, and positioning it correctly in the image

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4
Q

Define: d. gradient slope.

A

the amount of change in strength of a gradient magnetic field

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5
Q

Define: e. magnet isocenter.

A

the center of the primary magnetic field in all three planes

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6
Q

Define: f. slice selection gradient.

when is it switched on?

A

the logical gradient that localizes tissue in the imaging plane, is switched on during RF transmission

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7
Q

Define: g. frequency encoding gradient.

when is it switched on?

A

logical gradient that localizes tissue perpendicular to the imaging plane- is switched on during signal reception

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8
Q

Define: h. readout gradient.

A

same as frequency encoding gradient

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9
Q

Define: 1. phase encoding gradient.

when is it switched on?

A

logical gradient that localizes tissue perpendicular to the imaging plane- is switched on before the refocusing pulse

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10
Q

Define: j. RF transmit bandwidth.

A

the range of frequencies of transmitted RF

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11
Q

Define: k. RF receive bandwidth.

A

the range of frequencies of received RF

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12
Q

Define: 1. slice thickness.

A

the amount of tissue that is imaged with each slice, affects spatial resolution

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13
Q

Define: m. sampling rate.

A

the frequency at which a waveform is sampled, or how many times per second the MR signal is sampled

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14
Q

Define: n. sampling time.

A

the amount of time during which a waveform is sampled

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15
Q

Define: o. k-space.

A

the frequency- and phase- encoded raw data map of an MR image

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16
Q

Define: p. image matrix.

A

the number of frequency and phase encoding steps used to spatially localize an image

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17
Q

Define: r. partial or fractional echo imaging.

A

a method of imaging whereby only half of k­ space in the frequency encoding axis is filled during acquisition, and the other half is calculated by the MR system

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18
Q

Define: s. partial or fractional averaging.

A

a method of imaging whereby only half of k­ space in the phase encoding axis is filled during acquisition, and the other half is calculated by the MR system

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19
Q

Define: u. 2D k-space filling.

A

a method of imaging whereby the first line of k-space for each slice is filled, then the second line for each slice is filled, and so on until all lines are filled

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20
Q

Define: v. 3D k-space filling.

A

a method of imaging whereby an entire volume of tissue is excited by the RF transmission, and slices are divided up after image acquisition

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21
Q

Define: w. centric ordered k-space filling.

A

filling the central phase encoding steps of k­ space first, then filling toward the outer edges

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22
Q

Define: x. Fourier transformation.

A

the mathematical calculation that converts waveforms into discrete values that describe signal strength at each frequency

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23
Q

Define: y. field of view (FOV).

A

the area of the volume to be imaged

24
Q

Define: z. slice dephasing.

A

the dephasing of spins within a slice that is caused by the gradient magnetic field

25
Q

Define: aa. slice profile.

A

the shape of the transmitted RF pulse

26
Q

Define: aa. slice profile.

A

the shape of the transmitted RF pulse

27
Q

explain the relationship between gradient magnetic fields and the local magnetic field experienced by tissues along the gradient.

A

where the gradient add to the primary magnetic field, the tissues experience a higher local magnetic field, where the gradient subtracts from the primary field, the tissues experience a lower local field strength

28
Q

explain the relationship between gradient magnetic fields and the precessional frequencies of spins along the gradient.

A

where the gradient add to the primary magnetic field, the tissues experience a higher frequency, where the gradient subtracts from the primary field, the tissues experience a lower frequency

29
Q

explain “steep” and “shallow” gradient slopes and how they are used to describe the change in magnetic field strength over the length of the gradients.

A

a steep gradient is one in which the magnetic field strength changes a lot over the length of the gradient, a shallow gradient is one that changes little

30
Q
  1. identify which physical gradient coil pair(s) perform(s) slice selection during axial, sagittal, coronal, and oblique imaging in a superconducting MRI system.
A

axial- slice select is the Z gradient

sagittal- slice select is the X gradient

coronal- slice select is the Y gradient

oblique- slice selection is performed by more than one physical gradient

31
Q
  1. describe how the slice select gradient slope affects slice thickness.
A

a steep slice select gradient produces thinner slices, a shallow slice gradient produces thicker slices

32
Q
  1. describe how the RF transmit bandwidth affects slice thickness.
A

a narrower (decreased) transmit bandwidth produces thinner slices, a wider (increased) bandwidth produces thicker slices

33
Q
  1. identify the logical gradient that is on during RF transmission.
A

slice select

34
Q
  1. identify the logical gradient that is on during MR signal reception.
A

frequency encoding or read-out

35
Q
  1. identify the logical gradient that is on before the refocusing pulse.
A

phase encoding

36
Q
  1. state the Nyquist theorem.
A

a waveform must be sampled at least twice per cycle to represent it accurately

37
Q
  1. explain the relationship between RF receive bandwidth and sampling rate that is necessary to satisfy the Nyquist theorem.
A

they must change proportionally- e.g. as bandwidth increases, sampling rate must increase in order to sample accurately

38
Q
  1. explain the relationship between sampling time and sampling rate that is necessary to satisfy the Nyquist theorem.
A

they are inversely proportional- e.g. as sampling time decreases, sampling rate must increase; as sampling rate decreases, sampling time must increase, etc.

39
Q
  1. explain the relationship between sampling time and RF receive bandwidth that is necessary to satisfy the Nyquist theorem.
A

they are inversely proportional- e.g. as bandwidth increases (wider), sampling time may decrease, as bandwidth decreases (narrower), sampling time must increase. etc.

40
Q
  1. describe a “fine” and “coarse” image matrix.
A

a fine matrix divides the image matrix into smaller pixels; a coarse matrix divides the image matrix into larger pixels

41
Q
  1. explain how the numbers of phase and frequency encodings affect the image matrix.
A

a fine matrix contains many frequency and/or phase encoding steps, a coarse matrix contains fewer frequency and/or phase encoding steps

42
Q
  1. identify the number of lines of k-space that are phase encoded during each TR period.
A

one

43
Q
  1. describe how a change in the number of phase encoding steps affect the imaging time of a sequence.

as the number of phase encoding steps increases, the scan time _____ proportionally

A
  1. describe how a change in the number of phase encoding steps affect the imaging time of a sequence.

as the number of phase encoding steps increases, the scan time increases proportionally

44
Q
  1. identify the area of k-space in which the highest amplitude of MR signal is placed.
A

the center of k-space, or the shallow phase encoding steps

45
Q
  1. identify the area of k-space that determines image resolution.
A

the outer edges, or the steep phase encoding steps

46
Q
  1. identify the area of k-space that determines tissue contrast.
A

the center, or the shallow phase encoding steps

47
Q
  1. explain how RF transmit bandwidth and sampling time affect slice profile.
A

the narrower (lower) the bandwidth the squarer the slice profile; the longer the sampling time the squarer the slice profile

48
Q
  1. explain how changes in RF transmit bandwidth affect the duration of the RF pulse.
A

a narrower (lower) bandwidth requires a longer RF pulse duration, a wider (higher) bandwidth allows a shorter RF pulse duration.

49
Q
  1. explain how changes in RF receive bandwidth affect the minimum TE and TR of a pulse sequence.
A

an wider (higher) receive bandwidth makes a shorter TE and TR possible, a narrower (lower) receive bandwidth makes a longer TE and TR necessary

50
Q
  1. explain how changes in RF transmit bandwidth affect the minimum TE and TR of a pulse sequence.
A

a wider (higher) transmit bandwidth makes a shorter TE and TR possible, a narrower (lower) transmit bandwidth makes a longer TE and TR necessary

51
Q
  1. explain how changes in sampling rate affect the minimum TE and TR of a pulse sequence.
A

a faster sampling rate makes a shorter TE and TR possible, a slower sampling rate makes a longer TE and TR necessary

52
Q
  1. describe the effect that the frequency encoding gradient and RF receive bandwidth have on the field of view (POV).
A

the FOV can be decreased by using a steeper frequency encoding gradient or a narrower (lower) receive bandwidth (or both); the FOV can be increased by using a shallower frequency encoding gradient or a wider (higher) receive bandwidth (or both)

53
Q
  1. identify which ai: as of k-space are filled with steep and which are filled with shallow phase encoding gradients.
A

the outer edges are filled with steeper phase encoding gradients; the center of k-space is filled with shallow phase encoding gradients

54
Q
  1. calculate pixel size of an image when given the POV, number of Frequency Encoding points, number of Phase Encoding steps, and slice thickness.
A

formula [POV (FE)/# FE points] x [POV (PE)/# PE steps] x [5mm slice thickness]

example [256mm / 256] x [256mm / 256] x [5mm] = 1mm x 1mm x 5mm

55
Q
  1. recognize the following on a pulse sequence diagram:
    a. frequency encoding gradient.
    b. rephasing lobe of the frequency encoding gradient.
    c. phase encoding gradient.
    d. slice select gradient.
    e. rephasing lobe of the slice select gradient.
A
  1. recognize the following on a pulse sequence diagram:
    a. frequency encoding gradient.
    b. rephasing lobe of the frequency encoding gradient.
    c. phase encoding gradient.
    d. slice select gradient.
    e. rephasing lobe of the slice select gradient.

001 MRI (15 decks)

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