Chapter 4 Flashcards
Which is the equation for scan time?
scan time = TR * NEX * # phase encodings * # slice encodings
Match the TR, TE and flip angle to SNR … long TR short TE
increases SNR
Match the TR, TE and flip angle to SNR … short TR, long TE, lower the flip angle
reduces SNR
TRUE or FALSE: A small coil has more SNR.
True
Match SNR, signal and noise … SNR
ratio of amplitude of signal to amplitude of noise
Match SNR, signal and noise … signal
voltage induced in the coil by the precession of the NMV in the transverse plane
Match SNR, signal and noise … noise
exist randomy
TRUE of FALSE: increasing NEX increases SNR
True
what increases spatial resolution?
- thin slices
* fine matrix
What is scan time proportional to? Choose all that apply …
- TR
- NEX
- number of phase encodings
How does reducing the slice thickness (small voxel) affect spatial resolution and SNR? Choose all that apply …
- increases resolution
* decreases SNR
Doubling the NEX ___ scan time and ___ SNR ___
doubles, increases, by the sq root of 2
what maximizes resolution?
decreasing slice thickness
TRUE or FALSE: A large coil will include more area of received signal but this is more susceptible to artifacts.
True
What is spatial resolution determined by? Choose all that apply …
- FOV
- matrix size
- slice thickness
Match the MRI tradeoffs concerning maximizing SNR … increase NEX
max’s SNR but increases scan time
Match the MRI tradeoffs concerning maximizing SNR … increase slice thickness
max’s SNR but decreases resolution
Match the MRI tradeoffs concerning maximizing SNR … decrease TE
max’s SNR but decreases T2 weighting
Match the MRI tradeoffs concerning maximizing SNR … increase TR
max’s SNR but decreases T1 weighting
How do we calculate the pixel area?
FOV/matrix
What happens when you increase the phase matrix? Choose all correct …
increase resolution but increases scan time and decreases SNR
Match the following concerning changes in TE values … increase TE
increase T2-weighting, reduce SNR
Match the following concerning changes in TE values … decrease TE
decrease T2-weighting, increases SNR
Match the following concerning volume imaging … isotropic
Symmetrical voxels. Equal resolution in every plane
Match the following concerning volume imaging … anisotropic
Voxels not symmetrical
TRUE or FALSE: I should study the Trade Off Chart located at the end of Chapter 4 …
True
Match the change to SNR to a change in NEX and RBW … half the RBW
increase SNR by 40%
Match the change to SNR to a change in NEX and RBW … doubling the NEX
increase SNR by sq root of 2
Match the MRI tradeoffs concerning minimizing scan times … decrease TR
minimize scan time but increases T1-weighting, lowers SNR and less # slices
Match the MRI tradeoffs concerning minimizing scan times … decrease phase matrix
decreases resolution but increases SNR
Which is used in short scan times?
- short TR
- coarsest matrix
- reduce NEX
What happens when you increase the NEX? Choose all that apply …
- increase SNR
* increase the time
Match the following concerning changes in TR values … increase TR
decreases T1-weighting
Match the following concerning changes in TR values … decrease TR
increases T1-weighting
A thicker slice has a better SNR than a thinner slice.
True
Choose the term that best describes “a unit volume of patient tissue” from the list below.
Voxel
Increasing the field strength increases SNR because:
The NMV increases in size because the low energy population of spins increases
(Increasing the field strength increases the energy jump between high and low energy spins. Hence fewer nuclei have enough energy to align their magnetic moments in opposition to the stronger field. The low energy population therefore increases in size relative to the high energy population so increasing the NMV.)
Do we achieve greater SNR with a long or a short TR?
long
(Long, because a long TR allows full recovery of the longitudinal magnetization so that more is available to be flipped the next repetition. Therefore, SNR is improved as TR increases.)
Which of the following factors affect voxel size (choose all that apply)?
- Slice thickness
- Number of pixels or matrix
- FOV
(See page 126 in the text for more information.)
Pixel area = ____________________ / matrix size.
FOV
Decreasing receive bandwidth by half increases the SNR by how much?
40%
Halving a square FOV reduces the SNR by:
3/4
(Halving the FOV halves its dimension along both phase and frequency encoding axes. Therefore the voxel volume is reduced <em>to</em> 1/4 or <em>by</em> 3/4.)
Doubling the number of signal averages changes the SNR by how much?
√2
(As noise is random the SNR is only increased by a square root of 2 or 1.4 (40%) when the number of signal averages is doubled.)
Of the following parameters, which would give the best spatial resolution?
256 × 256, 3 mm slice thickness, 12 cm FOV, 1 NEX
(Choosing the “even” matrix (i.e. 256X256, which would result in square voxels, which have better resolution), the thinner slice, and the smaller FOV would result in better spatial resolution.)
Which of the following does NOT occur when the receive bandwidth is reduced?
Chemical shift artifact improves
(SNR increases as reducing the receive bandwidth reduces the number of noise frequencies that are sampled. Chemical shift artifact gets worse as the receive bandwidth is reduced as fewer frequencies are mapped across each pixel. TE increases as reducing the receive bandwidth reduces the digital sampling rate and therefore the sampling time and the TE increase. Slice number available will decrease as the TE increases; fewer slices may be acquired for a given TR.)
TR controls the amount of T2 weighting.
False
TE controls the amount of T2 weighting.
Of the following parameters, which would give the highest SNR?
256 × 128, 8 mm slice thickness, 40 cm FOV, 4 NEX
For the highest SNR, choose the thickest slice, the coarsest matrix, largest FOV, and more NEX.
Decreasing the phase matrix does which of the following?
Makes the scan shorter
(If you <em>decrease</em> your phase matrix, you are “filling fewer drawers” or lines of K space. This would result in a coarser matrix. We know that a coarse matrix has larger voxels, so our spatial resolution would decrease and our SNR would increase. We have to fill fewer lines of K Space, so our scan time would decrease. See page 132 for more information.)
Square pixels always provide better spatial resolution than rectangular pixels.
True
See explanation on page 127 in the text for more information.
A long TE increases SNR and a short TE decreases SNR.
False
A long TE reduces SNR and a short TE increases SNR. See page 111 in the text.
Which of the following factors affect scan time (choose all that apply) during a sequential acquisition?
- TR
- Phase Matrix
- NEX
(See page 132 for more information regarding the three factors that affect scan time during a sequential acquisition.)
A fine matrix is one with a low number of frequency encodings and/or phase encodings, and results in a small number of pixels in the FOV.
False
(A fine matrix is one with a high number of frequency and phase encodings, and results in a large number (i.e. many) of pixels in the FOV.)
Choose three factors that affect the SNR from the list below (there are many factors).
- Magnetic field strength of the system
- Proton density of the area under examination
- Receive bandwidth
(Magnetic field strength of the system, proton density of the area under examination, voxel volume, TR, TE, flip angle, NEX, receive bandwidth, and coil type.)
Which of these is a consequence of decreasing the TR from 2500 ms to 500 ms?
Scan time decreases
(The others are all consequences of increasing the TR. TR is a scan time parameter as it dictates the time interval between filling a line of K space for a particular slice.)
If we double the NEX, what does that do to our scan time?
Doubles the scan time
See page 114 in the text for more information.
