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

59

_____________ 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

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

60

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

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

62

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

62

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

65

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

65

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

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

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

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

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

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

72

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

72

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

75

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

__________ = 1/sampling frequency

76

In MRI the sampling frequency is determined by the _____________.

76

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

76

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

76

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

77

___________ = 2 x Nyquist frequency

77

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

77

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

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

80

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

80 - 81

The unit of K space is _________.

81

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

81

The ________ gradient is usually altered every TR.

82

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

82

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

82

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

82

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

83

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

84

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

84

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

85

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

85

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

85

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

85

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

86

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

86

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

87

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

87

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

87

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

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

89

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

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

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

Every data point contains information from the ________ slice.

90

Data are ____________ in K space.

90

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

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

91

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

92

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

92

The scan time is the time to fill _________.

94

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

94

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

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

95

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

95

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

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

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

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

97

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

99

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

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

101

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

101

_____________ 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

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

101

Three-dimensional (3D) volumetric

(volume imaging)