chap 5 & 6 Flashcards
used when a patient can’t hold still, filled as a block & rotated about the central axis of k space, decreasing scan time, increasing the SNR & decreasing motion
propeller
sensitivity encoding, fills multiple lines of k-space per TR by assigning to coils. reduction or acceleration factor
parallel
T/F: data in K space is/are symmetrical
true
T/F: phase encoding gradient positive fills the bottom half of K space
false
scan time in 2D imaging =
Tr x M(p) x NSA
scan time in FSE =
TR x M(p) x NSA / ETL
scan time in 3D imaging =
TR x M(p) x NSA x Ns
scan time in EPI =
TR x #shots x NSA
positive frequency encoding gradient
left to right
negative frequency encoding gradient
right to left
positive phase encoding gradient
top half
negative phase encoding gradient
bottom half
how fast do we sample frequencies according to the Nyquist theorem
2x/cycle
T/F: K space is not the image
true
data acquisition; all data from 1 slice then all data from 2nd slice etc
sequential
data acquisition; 1 line of k-space for slice 1 then the same for slice 2 etc
2D
data acquisition; no slices, aquires and entire volume
3D
partial fourier + FSE, half the lines are acquired and half are transposed. reduces SAR but has SNR penalty
single shot fast spin echo
k space filling where we fill high signal amplitudes starting in the center of our K space and filling outward to the periphery
Centric filling
simplest form, k space filled in a linear manner from top to bottom or bottom to top
Cartesian
T/F: the scan time is the time to fill k space
true
encodes along the short axis of anatomy
phase
encodes along the long axis of anatomy, FOV
frequency
T/F: the top half of data in k space is identical to the bottom half
true
concerning k space, data in the central lines contribute
to signal & contrast
concerning k space, data in the outer line contribute to
resolution
locates a slice in the scan plane selected
slice select
locate a signal along the long axis of the image
frequency encoding
locate a signal along the short axis of the image
phase encoding
when is the slice select gradient switched on during a spin echo pulse sequence
during the 90 & 180 degree pulses
what does the slope of the frequency encoding gradient determine?
- the size of the anatomy covered along the frequency encoding axis
- the FOV
a shallow or weaker gradient selects a ________ bandwidth
broader
a steeper or stronger gradient will select a __________ bandwidth
narrow
which gradient performs phase encoding for coronal slices
X gradient
which gradient locates signal along the long axis of the anatomy
frequency encoding gradient
amplitude of the frequency encoding gradient
frequency FOV
number of different phase encoding steps
phase matrix
amplitude of the steepest phase encoding gradient positively & negatively
phase resolution (pixel size)
amplitude of the slice select gradient & transmit BW
slice thickness & slice gap
steep sloping TBW
thin slices
gradual sloping TBW
thick slices
when does the slice select gradient turn on
at the same time as the RF excitation pulse (and the rephasing pulse in SE)
when does the frequency encoding gradient turn on
switched on during the echo
when does the phase encoding gradient turn on
switched on any time, usually after the RF
slice selection for sagittal image
X
phase axis on sagittal image
Y
frequency axis on sagittal image
Z
which gradient is the readout gradient
frequency
the slice select for a coronal image
Y
phase encoding for coronal image
X
frequency encoding on coronal image
Z
slice select for an axial brain image
Z
phase encoding gradient on axial brain
X
frequency encoding gradient for axial brain
Y
steepness of the slope of the frequency encoding gradient determines the size of the anatomy
FOV
duration of the readout gradient (acquisition window)
sampling time
mathematical process that converts time into a frequency domain
FFT
artifact caused by sampling too infrequently (less than 2)
aliasing
the mathematical process that converts time into a frequency domain
Fast Fourier Transform
