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MRI > 6: Parameters and Options > Flashcards

6: Parameters and Options Flashcards

1
Q

Signal to Noise Ratio SNR:

A

the ratio between the peak frequency signal generated of the transmit coil to the amount of noise from the patient.

signal from coil: noise from patient

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2
Q

Signal to Noise Ratio SNR is reliant upon….

A

Coil placement in relation to the body part being examined

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3
Q

Poor coil placement will result in…..

A

an increase in noise, which is also termed “Grainy”

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4
Q

Contrast Resolution

A

the ability to differentiate between intensities of adjacent tissue on an MR image

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5
Q

How can contrast be adjusted?

A

TR and TE values

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6
Q

Contrast between tissues occur due to…..

A

the different relaxation times of each type of tissue

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7
Q

SNR and Contrast (direct or indirect) relationship

A

Direct: +SNR = +Contrast

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8
Q

Spatial Resolution

A

the degree of sharpness of an image

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9
Q

What controls Spatial Resolution

A

Size of pixels or voxels, matrix size

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10
Q

How to determine Voxel size?

A

FOV / Matrix X Slice thickness

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11
Q

Matrix and Resolution relationship (direct or indicrect)

A

Direct: +Matrix = +Resolution

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12
Q

Acquisition time:

A

the length of scan time for a particular sequence or total exam time

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13
Q

What affects scan time? (5)

A

TR
NEX/NSA (# of signal averages)
Phase encoding steps (#)
ETL
Slice thickness (in 3D imaging)

*Adjusting these values will result directly on acquisition time.

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14
Q

Formula to calculate 2D Scan time

A

TR x NEX x Phase Encodes
———————————————– / 60
ETL
* TR convert to seconds

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15
Q

Formula to calculate 3D scan time

A

TR x NEX x Phase Encodes x # of slices
———————————————– / 60
ETL

  • TR convert to seconds
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16
Q

When calculating scan time, if there is no FTL….

A

divide by 60

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17
Q

What would the scan time be in a 2D FSE sequence utilizing TR = 600 ms, an effective TE = 14 ms, 2 signal averages, and a matrix of 512 x 256?

A

TR x NEX x Phase / ETL / 60

  • Convert to Seconds ! *

.6 (s) x 2 (NEX) x 256 (Phase)

.6 x 2 x 256 = 307.2
307.2 / 60 = 5.12
5 mins
.12 x 60 = 7.2 seconds

Answer = 5 mins 7 seconds

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18
Q

What would the scan time be in a 2D FSE sequence utilizing TR = 3000 ms, 2 signal averages, 384 phase encoding lines of matrix with and echo train of 16?

A

2 mins 24 seconds

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19
Q

What would the scan time be in a Brain 3D FSE sequence utilizing TR = 3000ms, 2 signal averages, 192 phase encoding lines of matrix with an increase in echo train to 24 with 10 slices?

A

8 minutes

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20
Q

Repetition time (TR)? and how is it measured?

A

the time from the initial RF pulse to the initial RF pulse of the next pulse. milliseconds

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21
Q

TR value is (directly or indirectly) proportional to the amount of longitudinal relaxation allowed (63%)

A

directly

+TR = +long relaxation time = +T1 information = increase overall fat signal

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22
Q

Echo Time (TE)? how is it measured?

A

the time from the initial RF pulse to the peak resultant echo of the same pulse, measured in milliseconds

1/2 TE would be considered the time from the initial 90 degree pulse to refocusing 180 degree pulse

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23
Q

TE value is (directly or indirectly) proportional to the amount of transverse relaxation allowed (37%)

A

directly

+TE = -SNR = - T2 contrast = increase in chance of Susceptibility artifact

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24
Q

An increase in TE would have (decrease or increase) in SNR

A

decrease

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25
Q

TR and TE and used to control…

A

image contrast, “weighted” images

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26
Q

What is needed to control contrast in Inversion Recovery images?

A

TI (inversion time) TR (repetition time) and TE (echo time)

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27
Q

T1 Weighted images require:

TR
TE
ETL

Ideal for imaging…

A
  • Short TR and Short TE
  • TR: 400 - 700 ms
  • TE: 10 - 14 ms
  • ETL: 4-6

Ideal for Mass or Contrast imaging (pre and post)

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28
Q

What weighted image is best for Mass or Contrast imaging?

A

T1 Weighted

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29
Q

T2 Weighted images require
TR:
TE:
ETL:
Ideal for imaging…

A
  • Long TR and Long TE
  • TR: 4000 - 6800 ms
  • TE: 105 - 115 ms
  • ETL: 10 - 16

Ideal for Brain and Spine imaging

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30
Q

What weighted image is best for Brain and Spine imaging?

A

T2 Weighted

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31
Q

Proton Density Weighted images require

TR
TE
ETL

Ideal for imaging…

A
  • Long TR and Short TE
  • TR: 4000 - 6800 ms
  • TE: 10 - 14 ms
  • ETL: 10 - 16

Ideal for Cartilage imaging

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32
Q

What weighted images is best for Cartilage imaging?

A

Proton Density Weighted

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33
Q

Inversion Time (TI)

A

the time from the initial 180 degree FR pulse to the 90 degree pulse in an inversion recovery pulse sequence

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34
Q

Relation between TI, Fat Suppression, and T1 contrast:

A

+ TI = + Fat suppression = - T1 contrast

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35
Q

Field of View (FOV)? how is it measured?

A

the numerical value area of interest being scanned in the slice direction, measured in cm

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36
Q

FOV is constructed by interchangeable, adjustable columns and rows. These columns and rows together are considered the…

A

Frequency and phase directions of the imaging matrix.

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37
Q

Increasing FOV by a multiple of 2 will result in ______ value of signal to noise (SNR)

A

quadrupled

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38
Q

Relationship between FOV, SNR, and Resolution

A

+ FOV = + SNR = - Resolution

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39
Q

Slice Thickness? how is it measured?

A

the thickness of the slice in the slice direction, measured in millimeters

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40
Q

A thicker slice will contain (less or more) protons compared to a thinner slice, thus allowing a (higher or lower) signal to noise ratio (SNR)

A

more protons, higher SNR

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41
Q

Relationship between slice thickness, SNR, and spatial resolution:

A

+ Slice thickness = + SNR = - Spatial resolution

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42
Q

Gap

A

also termed “skip”, an adjustable spacing between consecutive slices in the slice direction

  • Can be manually entered for each sequence
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43
Q

Relationship between skip, coverage, SAR, risk of missing pathology

A

+ Skip = + Coverage = - SAR = + Risk of missing pathology

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44
Q

Crosstalk

A

an artifact of overlapping slice, creates void on intersecting

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45
Q

Matrix

A

an adjustable grid of columns and rows within a FOV that directly relates to image sharpness or resolution on an MRI image

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46
Q

The value of each matrix is reliant upon…

A

how many pixels/voxels are in each column and row

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47
Q

Matrix is (directly or indirectly) related to pixel size

A

directly

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48
Q

Fine (higher) matrices have (larger or smaller) pixel size

A

smaller

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49
Q

Coarse (lower) matrices have (larger or smaller) pixel size

A

larger

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50
Q

Relationship between Matrix, SNR, Pixel size, and Resolution:

A

+ Matrix (Fine) = - SNR = - Pixel size = + Resolution

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51
Q

Flip angle? how is it measured?

A

how far the net magnetization has been flipped from the longitudinal transverse plane, measured in degrees

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52
Q

Ernst Angle

A

the most optimal flip angle per TR and results in the best SNR

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53
Q

In regards to SAR, doubling your flip angle will result in an (decrease or increase) of body absorption by a factor of ____.

A

increase, 4

54
Q

Relationship between Flip angle, SNR, Contrast, and SAR

A

+ Flip angle = + SNR = - Contrast = + SAR (times two)

55
Q

Number of Excitations (NEX / NSA)

A

the number of times each like of k-space is sampled during one TR cycle

*Comparing MRI to taking a picture, NEX would be similar to take double or triple exposing an exact image

56
Q

Increasing NEX by double will (decrease or increase) SNR by the square root of ___.

A

increase, 2

57
Q

Relationship between NEX, SNR, and scan time

A

+ NEX = + SNR = + Scan time

58
Q

Echo Train Length (ETL)

A

the amount of echoes collected in one TR period

59
Q

As ETL increases, the resultant echoes become (stronger or weaker) as they become further away from the initial 90 degree pulse

A

weaker

60
Q

Relationship between ETL, SNR, and Scan Time

A

+ ETL = - SNR = - Scan Time

61
Q

Bandwidth? and how is it measured?

A

the range of high and low frequencies used to transmit and receive signal, measured in Hertz (Hz)

62
Q

Transmit bandwidth

A

the range of frequencies used during RF excitation

63
Q

Receiver bandwidth rBW

A

the range of frequencies used by the receiver to sample signal

64
Q

Phase Encoding contributes to… (3)

A

one direction of the imaging matrix which contributes directly to resolution or image sharpness, motion artifact, and scan time

64
Q

Relationship between Bandwidth, SNR, range, scan time

A

+ Bandwidth = - SNR = wide range = - Scan time

65
Q

Relationship between Phase, SNR, Slight resolution, and scan time:

A

+ Phase = - SNR = + slight resolution = + scan time

66
Q

Phase encoding is an important step in matrix filling as it directly corelates to ____

A

motion

67
Q

It is important to remember that while phase and frequency directions can be switched, ___ will always be ween in the ____ direction

A

motion, phase direction

68
Q

Frequency Encoding is opposite of… and contributes to…

A

opposite direction of phase encoding direction, ONLY contributes slight increase in spatial resolution

69
Q

Relationship between Frequency, SNR, resolution, time

A

+ Frequency = - SNR = slight increase on resolution = slight decrease in time

70
Q

+ TR

SNR, Contrast, Resolution, Scan time

A

+ TR = + SNR, + Contrast T1, + Scan Time

no affect on resolution

71
Q

+ TE

SNR, Contrast, Resolution, Scan time

A

+ TE = - SNR, - Contrast T2, + Scan Time

no affect on resolution

72
Q

+ TI

SNR, Contrast, Resolution, Scan time

A

+ TI = - Contrast T1, + Scan time

no affect on SNR or resolution

73
Q

+ FOV

SNR, Contrast, Resolution, Scan time

A

+ FOV = + SNR, - resolution

no affect on Contrast or Scan time

74
Q

+ Slice Thickness

SNR, Contrast, Resolution, Scan time

A

+ Slice thickness = + SNR, - Resolution, - Scan time (less slices)

No affect on Contrast

75
Q

+ Gap

SNR, Contrast, Resolution, Scan time

A

+ Gap = - Scan time (less slices)

no affect on SNR, Contrast, or Resolution

76
Q

+ Matrix

SNR, Contrast, Resolution, Scan time

A

+ Matrix = - SNR, + Resolution, + Scan time

no affect on Contrast

77
Q

+ Flip Angle

SNR, Contrast, Resolution, Scan time

A

+ Flip Angle = + SNR, - Contrast T1

no affect on Resolution or Scan time

78
Q

+ NEX / NSA

SNR, Contrast, Resolution, Scan time

A

+ NEX = + SNR, + Scan time

no affect on Contrast or Resolution

79
Q

+ ETL

SNR, Contrast, Resolution, Scan time

A

+ ETL = - SNR, - Scan time

no affect on Contrast or Resolution

80
Q

+ Bandwidth

SNR, Contrast, Resolution, Scan time

A

+ Bandwidth = - SNR, - Scan time

no affect on Contrast or Resolution

81
Q

+ Phase

SNR, Contrast, Resolution, Scan time

A

+ Phase = - SNR, + Resolution, + Scan time

no affect on Contrast

82
Q

+ Frequency

SNR, Contrast, Resolution, Scan time

A

+ Frequency = - SNR, + Resolution

no affect on Contrast or Scan Time

83
Q

2D Imaging

A

An acquisition method in which each slice is isolated and excited by creating a linear variation in combination with RF excitation in a progressive format

84
Q

What is the difference between 2D and 3D imaging?

A

the process in which data is collected in the slice direction

85
Q

In 2D imaging, increasing _____ will allow steeper gradient variation and thus the ability to sample thinner slices

A

Amplitude

86
Q

In 2D imaging, increasing amplitude will….

A

allow steeper gradient variation and thus the ability to sample thinner slices.

87
Q

3D imaging:

A

an acquisition method in which an entire region of interest is excited during each data acquisition step

88
Q

For 3D scanning, you need to excite…

A

an entire region of interest, the addition of an extra phase encoding step allows depth excitation, relaxation, and data acquisition

89
Q

What is a major benefit of 3D imaging compared to 2D?

A

the result of overall higher SNR due to a repetitive, volumetric excitation and acquisition method

90
Q

Pixel size? How is it measured?

A

the size (width and height) of one single cube within a 2-dimensional matrix, measured in millimeters

91
Q

Adjusting pixel size will directly effect…

A

spatial resolution

92
Q

Increasing pixel size will result in …

A

a decreased matrix value thus decreasing imaging sharpness and scan time

93
Q

How do you calculate pixel size?

A

Pixel size = FOV / Matrix

94
Q

Relationship between Pixel Size, Spatial Resolution, SNR, and Time:

A

+ Pixel size = - Spatial Resolution = + SNR = - Time (with FOV remaining constant)

95
Q

What is the pixel size in an MRI brain image with a FOV of 24 cm and a matrix of 256x 192?

A

240 mm / 192 mm = 1.25
240 mm / 256 mm = 0.94
1.25 x 0.94 = 1.2

1.2 mm

96
Q

Voxel size? How is it measured?

A

the size (width, height, and depth) of one single cube in a 3-dimensional matrix, measured in millimeters

97
Q

Voxel depth is adjusted through…

A

slice thickness

98
Q

How do you calculate Voxel Size?

A

Voxel size = FOV / Matrix * Slice Thickness

99
Q

What is the Voxel size in a 3d MRI Brain image with a FOV of 24 cm, a matrix of 256 x 192, and a slice thickness of 2 millimeters?

A

240 mm / 192 mm x 2 mm = 2.5
240 mm / 256 mm x 2 mm = 1.88
2.5 x 1.8 = 4.7 mm

4.7 mm

100
Q

Slice Order:

A

the order in which slices are selected and sampled during image acquisition

101
Q

Sequential slice order:

A

Within the image matrix, each pixel is acquired in the traditional 1, 2, 3, 4, 5… fashion

SLICE 1
SLICE 2
SLICE 3
SLICE 4

102
Q

Interleaved slice order:

A

within the image matrix, each pixel is acquired in a 1, 3, 5, 7, 9… followed by a 2, 4, 6, 8, 10…

*When scanning under “interleaved method”, 2 acquisitions must be used allowing 1 acquisition for odd and another acquisition for even slices

SLICE 3
———–
SLICE 5
———–
SLICE 7

GO BACK

SLICE 4
———–
SLICE 6
————
SLICE 8
————

103
Q

Interleaved slices helps reduce…

A

crosstalk

also to go back and help when patient is moving

104
Q

Saturation Pulse (Saturation Band) (Sat-band)

A

an RF pulse applied in a specific location before initial 90 RF pulse to dephase spins and minimize their signal

105
Q

Saturation bands are utilized for two reasons:

A
  1. reduction of breathing motion (l spine), perpendicular to the phase direction of the matrix
  2. reduce vascular flow artifact: place outside the FOV
106
Q

Flow compensation or Gradient Moment Nulling:

A

a flow motion reducing technique that utilizes extra adjustments to correct for flow related dephasing, adds adjustments during signal readout to limit dephasing.

107
Q

How to use Flow Compensation or Gradient Moment Nulling?
Scanning Z axis:
Scanning X or Y axis:

A
  1. Turn on Flow Compensation option
  2. If scanning in a Z-direction, choose “Slice” as flow compensation direction
  3. If scanning in an X or Y direction, choose “Frequency” as Flow Compensation direction.
108
Q

STIR: Short Tau Inversion Recovery:

A

a fat suppression technique that utilizes a short inversion time (TI) that essentially nulls signal from fat, thus creating significant contrast between fat and water.

109
Q

T2 Fat Saturation “Fat Sat” CHESS

A

creates a similar result to STIR, but demonstrates fat suppression by tuning an RF pulse and spoiler at the same frequency as fat with resultant nulling of fat

110
Q

Dixon Technique

A

newest fat suppression; collects 2-3 separate echoes at different echo times in a pulse sequence, this results in the ability to select “fat only” and “water only” or both in one sequence.

111
Q

Gaiting and Triggering:

A

the ability to reduce motion artifact caused by breathing, cardiac movement, or blood flow by synchronizing data acquisitions with cardiac or respiratory cycles with the use of respiratory belts, pulse oximeters, or biomarkers.

112
Q

Gating:

A

a PROSPECTIVE way of acquiring image data between cycles

113
Q

Triggering:

A

RETROSPECTIVE way of acquiring data consistently through the cycle and subtracting areas of increase activity

114
Q

In Phase/Out of Phase

A

normally used in abdominal imaging, is typically a gradient echo with the same TR values but different TE values. Similar to pre and post contrast imaging, it takes the same image after intervention, fat vs. water content in suspected pathological tissue.

115
Q

Aliasing; Phase Wrap:

A

an effect that causes data acquired outside of a field of view in the phase direction to become unsorted and mismapped onto the opposing side of the FOV in the phase direction

116
Q

Anti-Aliasing; Oversampling:

A

increase in data acquisition when RFOV is on to prevent phase wrap

117
Q

No Phase Wrap:

A

imaging option which compensates for any tissue from being sampled outside of the FOV in the phase direction from being in the acquired

118
Q

Parallel imaging: ARC: Grappa:

A

imaging method used to accelerate data acquisition by sampling less k-space data with the help of more receiver coils

Receiver coils placed parallel with another will sample half the area in close proximity to complete an image within a reduced time

119
Q

Parallel imaging is optimal for:

A

shortening breath holding sequences resulting in less motion artifact

120
Q

Relationship between Parallel imaging, Time, SNR, and Resolution

A

Parallel imaging on = - Time = - SNR = no effect on resolution.

121
Q

Motion is always found along… why?

A

the phase direction because phase encoding takes approximately 3x longer than frequency encoding dedicating more time to overall k-space filling in the phase direction. The chances of motion in the phase direction is higher

Phase encoding time: seconds to minutes
Frequency encoding time: milliseconds

122
Q

Propeller or Blade:

A

motion reduction techniques that utilizes radial k-space filling with an additional post processing algorithms that reduce involuntary motion during acquisition

123
Q

Filtering

A

the process of adding value to data before or after acquisition, does not inherently improve image quality, just adds value to its appearance by masking any presence of noise or contrast drop off

124
Q

Filtering: Pure

A

used to reduce variable coil intensities through the use of a calibration scan

125
Q

Filtering: SCIC

A

a filter that automatically corrects for low intensities, reduces noise and improves contrast.

126
Q

FDA limit for static magnetic field for clinical MR examination:

A

8.0 Tesla

127
Q

TI relaxation is defined once:

A

63% of the longitudinal magnetization has RECOVERED

128
Q

Transverse direction can be defined as:

A
  1. Net magnetization vector protons perpendicular to B0 or b(null)
  2. Perpendicular direction to the longitudinal direction
129
Q
A

MRI (13 decks)

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