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03 - Encoding and image formation
As previously described, for resonance to occur RF must be applied at 90 degrees to B0 at the precessional frequency of hydrogen.
70
Medical
Professional
04/24/2015

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Cards

Term

Once a slice is selected, the signal is _________ or _________ along both axes of the image. This task is performed by _________.

 

 

59

Definition
located, encoded, gradients
Term

_____________ is when the magnitude of B0 is altered in a linear fashion by the gradient coils, so that the magnetic field strength and therefore the precessional frequency experienced by the nuclei situated along the axis of the gradient can be predicted.

 

 

60

Definition
Spatial encoding
Term

The position of a ________ along a gradient can be identified according to its precessional frequency.

 

 

60

Definition
nucleus
Term

The _______ of the resulting magnetic field is the amplitude of the magnetic field gradient and it determines the rate of change of the magnetic field strength along the gradient axis.

 

 

62

Definition
slope
Term

Spatially locating (encoding) signal along the long axis of the anatomy is called ____________.

 

 

62

Definition
frequency encoding
Term

Spatially locating (encoding) signal along the short axis of the anatomy is called ____________.

 

 

62

Definition
phase encoding
Term

To achieve thin/thick slices, a steep slice select slope and/or narrow transmit bandwidth is applied.

 

 

65

Definition
thin
Term

To achieve thin/thick slices, a shallow slice select slope and/or broad transmit bandwidth is applied.

 

 

65

Definition
thick
Term

The _________ is the steepness of the slope of the frequency encoding gradient that determines the size of the anatomy covered along the frequency encoding axis during the scan.

 

 

69

Definition
frequency field of view (FOV)
Term

In ________ images the short axis of the anatomy usually lies along the horizontal axis of the magnet, therefore the X gradient performs the phase encoding.

 

 

70

Definition
coronal
Term

In _________ images the short axis of the anatomy usually lies along the vertical axis of the magnet, therefore the Y gradient performs phase encoding.

 

 

71

Definition
sagittal
Term

In _________ images the short axis of the anatomy usually lies along the vertical axis of the magnet, therefore the Y gradient performs phase encoding.

 

 

71

Definition
axial
Term

The slice select gradient is switched on during the 90º and 180º pulses in _________ pulse sequences, and during the excitation pulse only in __________ pulse sequences.

 

 

71

Definition
spin echo, gradient echo
Term

The _________ encoding gradient is switched on after the excitation pulse.

 

 

72

Definition
phase
Term

The ___________ encoding gradient is switched on during the collection of the signal (the echo).

 

 

72

Definition
frequency
Term

The duration of the readout gradient is called the __________ or _________.

 

 

75

Definition
sampling rate, acquisition window
Term

The _________ or _________ is the rate at which frequencies are sampled or digitized during the acquisition window per second, i.e. the number of samples or data points acquired per second.

 

75

Definition
sampling rate, sampling frequency
Term

__________ = 1/sampling frequency

 

 

76

Definition
sampling interval
Term

In MRI the sampling frequency is determined by the _____________.

 

 

76

Definition
Nyquist theorem
Term

The Nyquist theorem states that the sampling frequency must be at least ______ the highest frequency in the signal.

 

 

76

Definition
twice
Term

Sampling at less than once per cycle represents a completely incorrect frequency that leads to an artifact called ________.

 

 

76

Definition
aliasing
Term

The sampling frequency determines the maximum frequency that can be sampled, which is called ___________.

 

 

77

Definition
Nyquist frequency
Term

___________ = 2 x Nyquist frequency

 

 

77

Definition
sampling frequency
Term

The ___________ is the range of frequencies we wish to sample or digitize during readout.

 

 

77

Definition
recieve bandwidth
Term

When data of each signal position are collected, the information is stored in _________ as data points in the array processor of the system computer.

 

 

80

Definition
K space
Term

The _________ axis of K space is horizontal and is centered in the middle of several horizontal lines.

 

 

80

Definition
frequency
Term

The _________ axis of K space is vertical and is centered in the middle of K space perpendicular to the frequency axis.

 

 

80 - 81

Definition
phase
Term

The unit of K space is _________.

 

 

81

Definition
radians per cm
Term

The line to be filled with data in a given TR is determined by the _________ and _________ of the gradient.

 

 

81

Definition
polarity, slope
Term

The ________ gradient is usually altered every TR.

 

 

82

Definition
phase
Term

We need to alter both the _______ and the ______ of the phase gradient every TR to give the image resolution in the phase direction.

 

 

82

Definition
polarity, slope
Term

Steep gradients, both positive and negative, select the most ________ lines, while shallow gradients select the ________ lines.

 

 

82

Definition
outer, central
Term

The _______ of the slice select gradient determines which slice is excited.

 

 

82

Definition
slope
Term

The ________ encoding gradient determines which line to fill with data.

 

 

83

Definition
phase
Term

The number of data points collected determines the __________ matrix of the image.

 

 

84

Definition
frequency
Term

Longer ______ result in more time to individually excite, rephase, phase and frequency encode slices.

 

 

84

Definition
TRs
Term

As the pulse sequence continues, every TR the phase encoding amplitude is decreased/increased to step down through the lines of K space.

 

 

85

Definition
decreased
Term

To fill the bottom lines the phase gradient is switched negatively and gradually decreased/increased every TR to progressively fill the outer lines.

 

 

85

Definition
increased
Term

The number of data points horizontally/vertically in each line equals the frequency matrix.

 

 

85

Definition
horizontally
Term

The number of data points horizontally/vertically corresponds to the phase matrix selected.

 

 

85

Definition
vertically
Term

To produce an image from the acquired data points we need to complete a mathematical process called the _____________.

 

 

86

Definition
Fast Fourier transform or FFT
Term

Gradients spatially locate signal according to their _________, not their time.

 

 

86

Definition
frequency
Term

By applying the phase encoding gradient across the bore of the magnet, a change of phase over ____________ is produced.

 

 

87

Definition
distance
Term

Steep phase encoding gradients produce small/large phase shifts across a given distance in the patient and result in low/high pseudo-frequencies.

 

 

87

Definition
large, high
Term

Low amplitude phase gradients produce small/large phase shifts across the same distance and result in low/high pseudo-frequencies.

 

 

87

Definition
small, low
Term

The only gradient slope we can change is the _________ encoding gradient and by doing so we alter the       "     "       information in a line of data points.

 

 

88

Definition
phase
Term

Before FFT, each data point contains frequency information from frequency encoding and pseudo-frequency information from _________ encoding.

 

 

89

Definition
phase
Term

In each line of K space the pseudo-frequency data in each data point are unchanged/different; however, the frequency data is unchanged/different.

 

 

89

Definition
unchanged, different
Term

In each column of K space the pseudo-frequency data in each data point are unchanged/different; however, the frequency data is unchanged/different.

 

 

89

Definition
different, unchanged
Term

The _______ converts the data into a signal amplitude vs its frequency an is therefore able to calculate the grayscale associated with every pixel in the two-dimensional matrix of the image.

 

 

89

Definition
FTT
Term

Every data point contains information from the ________ slice.

 

 

90

Definition
whole
Term

Data are ____________ in K space.

 

 

90

Definition
symmetrical
Term

Frequency data digitized from the echo are the same on one side as they are on the other. The resultant symmetry is called ____________ symmetry.

 

 

90

Definition
conjugate
Term

Data acquired in the central lines contribute _________ and ________, while data acquired in the outer lines contribute _____________.

 

 

91

Definition
signal, contrast, resolution
Term

The central portion of K space contains data that have low/high signal amplitude and low/high resolution.

 

 

92

Definition
high, low
Term

The outer portion of K space contains data that have low/high signal amplitude and low/high resolution.

 

 

92

Definition
low, high
Term

The scan time is the time to fill _________.

 

 

94

Definition
K space
Term

The parameters that affect scan time in a typical acquisition are:

 

 

94

Definition
  • repetition time (TR)
  • phase matrix
  • number of excitations (NEX)
Term

The ____________ determines the number of lines that must be filled to complete the scan.

 

 

95

Definition
phase matrix
Term

The ___________ is the number of times each line is filled with data.

 

 

95

Definition
number of excitations (NEX)
Term

The amplitude of the _________ encoding gradient determines how far to the left and right K space is traversed and this in turn determines the size of the FOV in the        "    "      direction of the image.

 

 

96

Definition
frequency
Term

The amplitude of the _________ encoding gradient determines how far up and down a line of K space is filled. The steepest        " "         gradient slope in the acquisition determines the        "  "        matrix of the image.

 

 

96

Definition
phase
Term

In a _________ echo sequence the frequency encoding gradient switches negatively to forcibly dephase the FID and then positively to rephase and produce a gradient echo.

 

 

96

Definition
gradient
Term

The combination of the _________ gradient and the negative lobe of the ____________ gradient determine at what point in K space storage begins.

 

 

97

Definition
phase, frequency
Term

_________ echo imaging is performed when only part of the signal or echo is read during application of the frequency encoding gradient.

 

 

99

Definition
Partial
Term

Partial echo imaging is routinely used when very short ________ are selected, which allows for maximun T1 and proton density weighting and slice number for a given TR.

 

 

99

Definition
TEs
Term

_____________ can be used where a reduction in scan time is necessary, and where the resultant signal loss is not of paramount importance.

 

 

101

Definition
Partial averaging
Term

___________ acquisitions acquire all the data from slice 1 and then go on to acquire all the data from slice 2.

 

 

101

Definition
Sequential
Term

_____________ acquisitions fill one line of K space for slice 1, and then go on to fill the same line of K space for slice 2.

 

 

101

Definition
Two-dimensional volumetric
Term

_______________ acquisition acquires data from an entire volume of tissue, rather than in separate slices.

 

 

101

Definition

Three-dimensional (3D) volumetric

(volume imaging)

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