Chapter 3 Flashcards
resonance occurs when __
an RF pulse is applied at the Larmor precession frequency
a resonating RF pulse gives H nuclei enough energy so that __ is created and so that nuclear moments __
transverse magnetization; precess in phase
a voltage is induced in the receiver coil of the scanner at the Larmor frequency, regardless of __
the spatial origin of the signal
to identify the spatial origin of the signal, we use
spatial encoding using gradients
first step in locating signal, then what
slice selection, then encoding along both axes of the image
gradients are __ and are generated by __
alterations to the static field; wire coils located within the magnet bore
the flow of electric current through gradient coils induces a magnetic fields which __
either increases of decreases the strength of Bo
in spatial encoding, the magnitude of Bo is __ by the gradient coils, so that __ can be predicted
altered linearly; magnetic field strength and precessional frequency
the three gradient coils
Z gradient = long, Bo
Y gradient = vertical
Z gradient = horizontal (short)
the magnetic isocenter is located __
at the center of the magnet’s bore, and the center of the gradient coordinate system
magnitude of Bo at the isocenter is __ by gradients
unaltered
the slope of the gradient (steep vs shallow) determines __
how fast the magnitude of B changes along the gradient direction
when gradient coils are switched on, every point along each axis has a specific __
precessional frequency associated with it because the nuclei at that location have a specific Larmor frequency
slices are selectively excited when __
an RF pulse is transmitted at the Larmor frequency of spins in the slice defined by the slice gradient
the scan plane determines __
which gradient performs slice selection
if the slice selection gradient is Z, then you get __ slices
axial
if the slice selection gradient is X, you get __ slices
sagittal
if the slice selection gradient is Y, you get __ slices
coronal
oblique slices can also be selected by __
activating several gradients simultaneously, each at different strengths
(at the same transmit bandwidth)
steep gradient = __ slice
shallow gradient = __ slice
thin; thick
to give each slice a thickness, __
a band of nuclei is excited by the RF pulse
the steepness of a gradient’s slope determines __
the difference in Larmor frequency across space along that gradient’s direction
once a gradient is applied, the RF pulse transmitted to excited the slice must contain a certain __
transmit bandwidth (a range of frequencies to match the precessional frequency between two points)
at same slope,
narrow bandwidth = __ slice
wide/broad bandwidth = __ slice
thin; thick
the slice gap is determined by __ (2)
gradient slope and slice thickness
slice select gradients are turned on __
during all RF pulses
once a slice has been selected, the signal must be located along both axes of the image. this is done using __
frequency and phase encoding
for coronal slices, the freq gradient is __
Z
for axial slices, the freq gradient is __
Y
for sagittal slices, the freq gradient is __
X
the frequency encoding gradient is applied/turned on when __, and thus is called the __
the signal is received; readout gradient
the readout gradient/freq gradient is applied at the __
echo time
the portion of space (size of anatomy) covered along the freq encoding axis is called
the field of view (FOV)
once slice selection and freq encoding have been performed, the image must be __, this is performed using __
localized along the remaining axis of the image; phase encoding
when the phase encoding gradient is turned on, nuclei along the gradient __
precess at different frequencies along the gradient direction because their Larmor frequencies are different
(phase encoding)
slower precessing nuclei __, faster precessing nuclei __
lose/ trail behind in the phase; gain/ move ahead in the phase
phase encoding gradient is applied __
after initial dephasing after 90 pulse, before 180 pulse
steep phase encoding gradient results in __
shallow gradient results in __
big phase shift from one end of the gradient to the other; small phase shift
sampling interval I is the __
duration between successive samples
the sampling interval is the inverse of __
the sampling frequency (I = 1/f)
as the sampling interval increases, the sampling frequency __
decreases
to sample frequencies accurately, __
the highest frequency must be sampled at least twice as fast (twice per cycle)/ you must sample at a rate that is twice as fast as the highest frequency
if sampling is performed slower than twice the highest frequency, __ results and the recorded frequency __
aliasing; is not the true frequency because sampling was implemented inappropriately
the maximum frequency which can be sampled is called __
the Nyquist frequency
sampling frequency = (equation)
2 x Nyquist frequency
on an MR scanner, do we specify the Nyquist frequency or the receiver bandwidth?
the receiver bandwidth
receiver bandwidth =
the range of frequencies we wish to sample
to specify a receive bandwidth, we select __
a center frequency
the acquisition window is __
the amount of time that the readout gradient is on
the duration of the acquisition window affects __
the Te (ex: echo must occur halfway through the freq encoding gradient/acquisition window, so if the window is increase, Te increases because the peak of the echo occurs later)
the number of times that the magnetic moments of nuclei cross the receiver coil indicates __
the frequencies encoded in the slice
the positions of the magnetic moments along their precessional paths indicates __
their phases
these data (signal frequency, phase) are stored in __, which constitutes a __
k space; spatial frequency domain
vertical axis of kspace = __
horizontal axis of kspace = __
phase; frequency
frequency encoding gradient (Gr) is turned on while the system __
records the frequencies present in the signal and digitizes them
the duration of the readout gradient Gr is called __
the sampling time or acquisition window
during the acquisition window, the scanner samples frequencies a large number of times, and the value of each frequency __
is stored as a data point
the sampling frequency is _-
the rate at which frequencies are sampled and digitized every second
the number of data points (__) is determined by the __
frequencies recorded; frequency matrix
if the freq matrix has 256 columns, __
256 frequencies are recorded during the acquisition window
the spatial interval over which the reconstructed image repeats itself is the __ (__ equation)
field of view/FOV; 1/deltak = FOV
phase gradients are turned on __ in order to fill different k space __ with data
every Tr; lines
phase gradients with positive (negative) polarity aid in recording the __
top (bottom) half of k space
steep (shallow) phase gradients select the __
outer (inner) lines of k space
usually, k space is filled __, but there are many ways to fill it
in a linear fashion
chest of drawers analogy: slice select gradient __, phase encoding gradient __, freq encoding gradient __
chooses which chest; chooses with drawer; choose which point in drawer to put sock
an MR image consists of a series of __
pixels
the number of pixels is determined by __
the size of k space
size of k space = number of lines filled (__) and number of data points in each line (__)
phase matrix; frequency matrix
FFT allows one to __
assign an image intensity corresponding to the amplitude of specific frequencies coming from the spatial location corresponding to the pixel whose intensity is being determined
FFT converts time-domain information to __
frequency-domain information
in kspace, within a row = __ pseudofreq, __ freq; along a column = opposite
same; different
in each row of kspace, the pseudofreq data in each data point are unchanged because __
they result from a particular slope of phase encoding gradient
in each row of kspace, the freq data are different because _-
each data point was acquired at a different time during readout while freq encoding gradient was on
in each column of kspace, the pseudofreq data in each data point are different because __
they were acquired with a different slope of the phase encoding gradient
in each column of kspace, the freq data in each data point are the same because __
they were acquired at the same time during readout while freq encoding gradient was on
conjugate symmetry
kspace is symmetric with respect to the horizontal and vertical center lines
data acquired in central lines contribute __, data acquired in outer lines contribute __
signal and contrast; resolution
phases of same amplitude but different polarity yield __ pseudofreq
the same
shallow gradient slopes __ phase differences, and the resulting signal has __, thereby contributing more to __
minimize; high amplitude; image intensity and contrast
steep gradient slopes __ phase differences, resulting signal has __ and thus does not contribute much to __
maximize; low amplitude; signal intensity and contrast
the presence of large phase differences indicates that two points close together in space will __. this is why __
be well-differentiated in the image; outer lines of kspace provide resolution information
TR is actually the time __
required to get a line of kspace for each slice, before going back to fill the next line of each slice in the next TR
the amplitude of the freq encoding gradient determines how __ k space is filled (i.e. __)
far to the left and right (i.e. what the FOV is in the freq direction)
the amplitude of the phase encoding gradient determines how __
far up and down k space is filled
gradient polarity determines __ k space is filled
the direction in which
positive freq gradient = __ traversal
left to right
negative freq gradient = __ traversal
right to left
positive phase gradient = filling the __
top
negative phase gradient = filling the __
bottom
partial echo allows for __
a very short TE
partial echo allows the freq gradient to be switched on at a normal time but have the peak signal occur __
earlier rather than at the middle of the acquisition window
partial echo: because k space has conjugate symmetry, __
we only need to fill half of it with data and the remainder can be extrapolated
NEX (number of excitations) indicates the __
fraction of kspace to fill
the lower the NEX, the __ the scan time
shorter
though scan time can be reduced by reducing NEX, __
fewer data are acquired and signal is weaker
(partial averaging) one cannot extrapolate data as with partial echo, because __
each k space line takes a Tr to acquire and so subject movement is very likely to have occurred between the beginning and the end of scan
in partial averaging, instead of extrapolating the data to the missing lines, __
they are filled with zeroes
3 types of acquisition
sequential, 2D volumetric, and 3D volumetric
sequential acquisition
data are acquired in order for each slice and line of k space
2D volumetric acquisitions
one line of kspace is filled for all slices first, then the procedure is repeated for subsequent lines of kspace
3D volumetric acquisitions
excitation pulse does not perform slice selection and the entire volume is excited instead. after all data have been acquired, the slice select gradient is turned on and slices are identified based on their phase. this is called slice encoding.
