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Chapter 3
Encoding & Image Formation
96
Health Care
Professional
02/27/2015
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Term
For ___ to occur, RF must be applied at 90° to B0 at the precessional frequency of Hydrogen.
 Definition
Resonance
Term
The RF pulse gives hydrogen nuclei energy so that ____magnetization is created, it also puts the individual magnetic moments of hydrogen ____
 Definition
Transverse ; in phase
Term
When coherent transverse magnetization precesses at the Larmor frequency of hydrogen in the transverse plane, a ___ is induced in a receiver coil that is positioned in the transverse plane. The signal has a frequency equal to _____
 Definition
Signal ; Larmor frequency o f hydrogen
Term
The system needs to be able to locate the signal spatially in three dimensions so that is can be placed at the correct point on the image. First is locates a slice. Once a slice is selected, the signal is located or ____ along both axes of the image. These tasks are performed by ___
 Definition
Encoded ; Gradients
Term
Gradients alter the magnetic field in a linear fashion so the magnetic field strength and as a byproduct the precessional frequency can be predicted. This is called ____
 Definition
spatial encoding
Term
Nuclei that experience a ___ in magnetic field strength speed up, and those that experience a ____ in magnetic field strength slow down
 Definition
Increase ; Decrease
Term
There are 3 gradient coils situated within the bore of the magnet, and these are named according to the axis aong which they act when they are switched on, Name them
 Definition
X-axis (horizontal)
Y-axis (vertical)
Z-axis (long)
Term
The magnetic field strength at isocentre is always the same as __. Even when the gradients are switched on.
 Definition
B0
Term
FYI:
When a gradient coil is switched on, the Slope of the resulting magnetic field is the amplitude of the magnetic field gradient and it determins the rate of change of the magnetic field strength along the gradient axis.
 Definition
Term
Steep gradient slopes alter the magnetic field strength between two points __ than shallow gradient slopes
 Definition
More
Term
Gradients can be used to dephase or rephase the magnetic moment of nuclei. They also perform what three tasks in encoding? List and describe.
 Definition
1. Slice selection - locating a slice within the scan plane selected
2. Frequency encoding - Spatially locating (encoding) signal along the long axis of the anatomy
3. Phase Encoding - Spatially locating (encoding)signal along the short axis of the anatomy
Term
FYI Slice selection:
When a gradient is switched on, the magnetic field (& precessional frequency of nuclei) is altered in a linear fashion and so a specific point along the axis of the gradient has a specific precessional frequency. A slice can therefore be selectively excited by transmitting a RF with a band of frequencies coinciding with the Larmor frequencies of spins in that particular "slice" Only the nuclei in that slice resonate because their precessional frequency is different due to the applied gradient.
 Definition
Term
____gradient slopes result in large difference in precessional frequency between two points, and a ____gradient slopes result in small difference in precessional frequency between two points
 Definition
Steep ; Shallow
Term
Once a certain gradient slope is applied, the RF pulse transmitted to excite the slice must contain a range of frequencies to match the difference in precessional frequencies between two points. This Frequency range is called the ___, and as the RF is being transmitted at this point it is specifically called the ____
 Definition
Bandwidth ; Transmit bandwidth
Term
The achieve Thin slices, a ___ slice select slope and/or Narrow transmit bandwidth is applied
 Definition
Steep
Term
The achieve ____ slices, a Shallow slice select slope and/or Broad transmit bandwidth is applied
 Definition
Thick
Term
The slice is excited by transmitting RF at the center frequency corresponding to the _____ of nuclei in the middle of the slice, and the Bandwidth and gradient slope determines the range of nuclei that resonate on either side of the center
 Definition
Precessional frequency
Term
In Spine echo pulse sequence, the slice select gradient is switched on??
In gradient echo pulse sequence??
 Definition
SE - during the application of the 90° excitation pulse and the 180° rephasing pulse (to excite and rephase each slice selectively)

GE - during the excitation pulse ONLY
Term
The signal along the long axis of the anatomy is located by a process known as ___
 Definition
Frequency Encoding
Term
The frequency encoding gradient is switched on when the signal is received and is often called the____
 Definition
readout gradient
Term
FYI:
The echo is centered in the middle of the frequency encoding gradient (when the nuclei are coherent) so it is on during the rephasing and dephasing. (Example: if the frequency encoding gradient is on for 8ms, it will be on for 4ms during dephaseing and 4ms during rephasing)
 Definition
Term
The steepness of the slope of the frequency encoding gradient determines the size of the anatomy covered along the frequency encoding axis during the scan. This is called the frequency ____
 Definition
FOV (Field of View)
Term
Once the frequency encoding signal is located, the ___ encoding signal must be found along the short axis
 Definition
Phase
Term
Once the frequency encoding signal is located, the ___ encoding signal must be found along the short axis
 Definition
Phase
Term
The phase encoding gradient is usually switched ___(before/after) the application of the excitation pulse
 Definition
After
Term
FYI:
When the phase encoding gradient is switched on, the magnetic field strength (and precessional frequency of nuclei) along the axis of the gradient is altered. Nuclei that are sped up move further into their precessional path and nuclei that have slowed down move further back in their precessional path.
 Definition
Term
The steepness of the slope of the phase encoding gradient determines the degree of phase shift between two points along the gradient (Ex: is the phase is at 12 o'clock a ___ phase shift would move them to 4 & 8 o'clock, while a ___ phase shift would move them to 10 & 2 o'clock)
 Definition
Steep ; Shallow
Term
Which gradient performs the Phase encoding in the following planes
Coronal -
Sagittal -
Axial (body) -
Axial (head) -
 Definition
Coronal - X
Sagittal - Y
Axial (body) - Y
Axial (head) - X
Term
Which gradient performs the Frequency encoding in the following planes
Coronal -
Sagittal -
Axial (body) -
Axial (head) -
 Definition
Coronal - Z
Sagittal - Z
Axial (body) - X
Axial (head) - Y
Term
Which gradient performs the slice select encoding in the following planes
Coronal -
Sagittal -
Axial (body) -
Axial (head) -
 Definition
Coronal - Y
Sagittal - X
Axial (body) - Z
Axial (head) - Z
Term
The slope of the slice select gradient determines what?
 Definition
Slice thickness & slice gap (along with the transmit bandwidth)
Term
The slope of the phase encoding gradient determines what
 Definition
the degree of phase shift along the phase encoding axis
Term
The slice select gradient is switched on when?
The phase gradient?
The frequency gradient?
 Definition
Slice select - In spin echo during the 90° & 180° pulses , in gradient echo during the 90° excitation pulse ONLY

Phase - AFTER the excitation pulse

Frequency - DURING the collection of the signal (the echo)
Term
The ___ of the frequency encoding gradient determines the frequency FOV dimension
 Definition
Amplitude
Term
The frequency encoding gradient , which is ON during the time when the signal is digitized, so it is also known as the readout gradient. The duration of the readout gradient is called ____ or ____
 Definition
Sampling time or Acquisition Window
Term
Every time a sample is taken it is stored as a ___. During the acquisition window the system can sample frequencies up to 2048 times and therefore acquire ____
 Definition
data point ; 2048 data points
Term
The rate at which frequencies are sampled during the acquisition window PER second is called ____ or ___
 Definition
Sampling rate or Sampling Frequency
Term
The # of data points acquired during the acquisition window is determined by _____
 Definition
frequency matrix (so if the frequency matrix is 256, then 256 data points must be aquired during the acquisition window)
Term
Sampling frequency (rate at which freq. are sampled per second)
Frequency Matrix (# of data points collected)
Acquisition window(duration of the readout gradient)
Are all related to one another
 Definition
Example: Sprinter analogy Page 75
Term
The Sampling frequency determines how many data points can be acquired during the acquisition window (i.e. the ____ that can be achieved)
 Definition
frequency matrix
Term
The time interval between each sample is called ____
 Definition
sampling interval
Term
The time interval between each sample is called ____
 Definition
sampling interval
Term
As the sampling frequency increases, the sampling interval____
 Definition
decreases
Term
in MRI, the sampling frequency by determined by the _____
 Definition
Nyquist Theorem
Term
The nyquist theorem states:
 Definition
a frequency must be sampled at least twice in order to reproduce it reliably
Term
Sampling at less than once per cycle leads to what artifact
 Definition
aliasing
Term
FYI:
Sampling frequency = 2 X Nyquist frequency

Nyquist frequency (maximum frequency)

Receive bandwidth = 2 X _______
 Definition
Nyquist frequency (the highest frequency)
Term
The sampling frequency and the receive bandwidth are DIFFERENT but they are both equal to ____
 Definition
2x the Nyquist frequency
Term
if you increase the Bandwidth x2, you can half the ____, and if you half the Bandwidth you double the ___
 Definition
Acquisition window
Term
The acquisition window is NOT selected by the user, BUT the echo is centered in the middle of this window so the ___ affects the acquisition window indirectly
 Definition
TE
Term
The application of the 3 gradients
1. selects an individual slice
2. produces a frequency shift along one axis
3. produces a phase shift along the other axis
The system can now locate an individual signal within an image by measuring the number of times the magnetic moments cross the receiver coil(___) and their position around their precessional path (___).
 Definition
Frequency ; Phase
Term
When data of each signal position are collected, they are stored as data points in the array processor of the system computer. The data points are stored in ___
 Definition
K space
Term
K space has 2 axes perpendicular to one another. The Frequency axis is ___ and the phase axis is ____. The axes are centered in k space, much like the X,Y axis of a graft.
 Definition
Horizontal ; Vertical
Term
What is the unit of k space
 Definition
Radians per cm
Term
K space is like a chest of drawers, The # of drawers to be filled represents ___ while the # of socks (data points) used to fill the draw represents ___
 Definition
Phase ; Frequency
Term
K spaces are numbered with the highest # toward the outer edge and the lowest # near the center. Lines in the top half of k space are called ____lines, those in the bottom half ___lines
 Definition
Positive ; Negative
Term
____polarity phase encoding slopes are associated with lines in the top half, ____polarity phase encoding slopes are associated with lines in the bottom half
 Definition
Positive ; Negative
Term
The ___ encoding gradient (Polarity & Slope) is usually altered every TTR, this is necessary to fill different lines of K space with data
 Definition
Phase
Term
Phase gradient:
Positive polarity select lines in the ___ half
Negative polarity select lines in the ___half
Steep gradients select the most ___ lines
Shallow gradients Select the most __ lines
 Definition
Phase gradient:
Positive polarity select lines in the TOP half
Negative polarity select lines in the BOTTOM half
Steep gradients select the most OUTER lines
Shallow gradients Select the most CENTRAL lines
Term
Usually k space is filled in a ___ fashion
 Definition
Linear
Term
FYI: Each slice has its own Kspace (or chest of drawer)
 Definition
Term
The slope of the _____ gradient determines which slice is excited, or which chest of drawers is to be selected
 Definition
slice select gradient
Term
The ____ gradient determines which line or drawer to fill with data
 Definition
Phase encoding gradient
Term
During application of the ____gradient, frequencies in the echo are digitized to acquire data points which fill the line of k space that was selected by the phase encoding gradient
 Definition
Frequency encoding gradient
Term
Data points are laid out in a line of k space during the acquisition time, usually from left to right. The # of data ponts collected determines the ____of the image. When the sampling is complete, the frequency encoding gradient turns off and the slice select gradient is applied again to a different applitude than before
 Definition
frequency matrix
Term
FYI: K space is NOT the image!!!
(data stored in the top line does not represent the top of the image) Each data point contains information from the WHOLE slice
 Definition
Term
To produce an image from the acquired data points we need to complete a mathematical process called ___
 Definition
Fast Fourier transform (FFT)
Term
by applying the phase encoding gradient over a distance across the bore or the magnet, a change of phase over _____ is produced. This is extrapolated as a frequency by creating a sine wave formed from connecting all the phase values associated with a certain phase shift (Fig 3.23) This sine wave has a ____
 Definition
Distance ; pseudo frequency
Term
The pseudo frequency (produced by the phase encoding gradient) depends on the degree of phase shift produced by the gradient. Steep phase encoding gradients produce ____ phase shifts over a distance and results in a high pseudofrequency, while low amplitude (shallow) phase gradients produce small phase shifts and result in ___pseudofrequences.
 Definition
Large; Low
Term
in each ___ of K space the pseudofrequency data in each data point are unchanged (SAME) because they result from a particular slope of phase encoding gradient.
 Definition
line
Term
in each ___ of K space the frequency data are DIFFERENT in each data point as each data point was acquired at a different time during readout when the frequency encoding gradient was on.
 Definition
Line
Term
In each ____ of K space the frequency data are unchanged (SAME) because each data point in the ___ was acquired at the same time during readout.
 Definition
Column
Term
in each ____ of K space the pseudofrequency data are DIFFERENT because each data point was acquired with a different slope of phase encoding gradient.
 Definition
Column
Term
Summary:
Line: Same Pseudofrequency, Different Frequency
Column: Different Pseudofrequency, Same Frequency
 Definition
Term
4 Important facts about K space
 Definition
1. K space is NOT the image
2. Data are symmetrical in K space
3. Data acquired in the central lines contribute signal and contrast, while data acquired in the outer lines contribute resolution
4. The scan time is the time to fill k space
Term
Data in the top half of K space is ____ to data in the bottom half.
Data on the left side of K space are ___ to data on the right.
 Definition
Identical ; Identical
Term
Echos are symmetrical features, frequency data digitized from the echo are the same on one side as they are on the other. The resultant symmetry is called ___ and is used to reduce scan times in many image options
 Definition
Conjugate Symmetry
Term
Central lines of K space are filled using ___ phase encoding slopes and the Outer lines are filled using ___ phase encoding slopes.
 Definition
Shallow ; Steep
Term
____ slopes result in low pseudofrequencies (Because of small phase shifts) & ____ slopes result in high signal amplitudes (because of large phase shifts)
 Definition
Shallow ; Steep
Term
Outer lines of K space contribute to ___
Central lines of K space contribute to ___
 Definition
Resolution (low signal)
Signal (low resolution)
Term
List 3 parameters that affect scan time in a typical acquisitioon
 Definition
TR, Phase Matrix, NEX
Term
The maximum number of slices available depends on the __
 Definition
TR
Term
The ____ determines the number of lines that must be filled to complete the scan, One line is filled per TR.
 Definition
Phase Matrix
(Ex: phase matrix of 128 - 128 lines are filled and 128 TRs must be completed)
Term
___is the number of times each line is filled with data.
 Definition
NEX (number of excitations)
Term
If you increase the NEX the same line of kspace is filled several times, the more data in each line, the resultant image has a higher ____. The scan time is proportionally longer.
Example:
1NEX = scan time of 1 min
3Nex = scan time of 3 min
 Definition
signal to noise ratio
Term
the way kspace is transversed and filled depends on a combination of __&__of both frequency and phase encoding gradients
 Definition
Amplitude & Polarity
Term
The Amplitude of the Frequency encoding gradient determines how far Left & Right k space is traversed and this in turn determines the ___
 Definition
FOV in the frequency direction
Term
The Amplitude of the Phase encoding gradient determines how far Up & Down a line of kspace is filled. The steepest phase gradient slope in the acquisition determines the ___ of the image
 Definition
Phase matrix
Term
The Polarity of each gradient defines the direction traveled through K space as follows:

Frequency encoding gradient positive = kspace traversed from __ to __

Frequency encoding gradient negative = kspace traversed from __ to __

Phase encoding gradient positive = fills __ half of k space

Phase encoding gradient negative = fills __ half of k space
 Definition
1. FEG+ = Left to right
2. FEG- = Right to Left
3. PEG+ = Top half
4. PEG- = Bottom half
Term
The way ion which kspace is filled depends on how data are acquired and can be manipulated to suit the circumstances of the scan. Here are some examples in which k space can be filled

Good Table on Pg. 98
 Definition
Rectangular FOV
Anti-aliasing
Fast spin echo sequence
keyhole imaging
respiratory compensation
parallel imaging
Single shot and echo planar imaging
Partial echo imaging
Partial or Fractional averaging OR Half Fourier
Term
Partial Echo Imaging is used when?

How does it work (basically)
 Definition
When very short TE's are selected, Very short TE's allow for maximum T1 & PD weighting

Due to the right to left symmetry of k space, the system can extrapolate the data in the right hand side of kspace and place is also in the left (only the right half of the frequency are of kspace is filled and the system mirrors the data to the left side therefore reducing scan time.
Pg. 99
Term
Partial, fractional averaging OR half Fourier has what purpose?

How does it work (basically)?
 Definition
Purpose: to reduce scan time when SNR is good

How: It takes advantage of the mirror image of the negative & positive halves of k space. so fewer data points are acquired (less time) which results in less signal.

It does NOT extrapolate the missing data (as in partial echo imaging) because the vertical axis of kspace is that in which motion artifacts are seen, it simply fills the unacquired data with 0's
Term
List the three ways to acquire data (types of acquisitions)

Which is the most common?
 Definition
1. Sequential
2. two-dimentional volumetric
3. three-dimentional volumetric

2D is the most common
Term
____ acquisition acquire all the data from slice 1 and then go on to acquire all the data from slice 2 ect.
 Definition
Sequential
Term
____ acquisition fill one line of k space for slice one, then go on to fill the same line of k space for line 2 ect. (moving from chest to chest instead of filling one chest before moving to the next)
 Definition
2D volumetric acquisition
Term
___ acquisition acquire data from an entire volume of tissue, rather than in separate slices. The excitation pulse is not slice selective, and the whole prescribed image volume is excited.
 Definition
#D volumnetric acquisition
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