Chapter 4 Flashcards
signal-to-noise ratio (SNR) is the ratio of __ and is dependent on __ (2)
signal amplitude to the average noise amplitude; number of acquisitions N and the noise variance
as the magnetic field strength increases, the energy difference between __
spin up and spin down states grows and spin excess increases
a spin excess increase leads to __
larger Mo (magnetization) and higher SNR at high B
low proton densities yield __
low signal amplitudes and thus low SNR
the voxel is the MRI unit of volume, defined as __
the product of pixel area and slice thickness
the pixel area is equal to __
the FOV divided by the matrix dimension
the larger the voxel, the __ the SNR because _-
higher; higher signal due to more protons/spins
greater slice thickness = better __ but poorer __ (and vice versa)
SNR; resolution
7 ways to maximize SNR and their consequences
increase NEX (but increase scan time); decrease matrix size (decrease scan time, but decrease resolution); increase slice thickness (but decrease resolution); decrease receive bandwidth (but increase TE and chemical shift); increase FOV (but decrease resolution); increase TR (but decrease T1 weighting and increase # of slices); decrease TE (but decrease T2 weighting)
3 ways to maximize resolution (assuming a square FOV) and their consequences
decrease slice thickness (but decrease SNR); increase matrix size (but decrease SNR and increase scan time); decrease FOV (but decrease SNR)
4 ways to minimize scan time (and their consequences)
decrease TR (but increase T1 weighting, decrease SNR, and decrease number of slices); decrease phase matrix (but decrease resolution and increase SNR); decrease NEX (but decrease SNR and increase movement artifacts); decrease slice number in volume imaging (but decrease SNR)
benefits of increasing TR
higher SNR, more slices
limitations of increasing TR
increases scan time, decreases T1 weighting
benefits of increasing TE
increase T2 weighting
limitations of increasing TE
decrease SNR
benefits of increasing NEX
higher SNR and higher signal averaging
limitations of increasing NEX
longer scan time
benefits of increasing slice thickness
higher SNR, higher coverage
limitations of increasing slice thickness
lower resolution and higher partial voluming
benefits of increasing FOV
higher SNR, higher coverage, decreased aliasing
limitations of increasing FOV
lower resolution
benefits of increasing phase matrix
better resolution
limitations of increasing phase matrix
longer scan time, lower SNR if pixel small
benefits of increasing receive bandwidth
lower chemical shift, lower minimum TE
limitations of increasing receive bandwidth
lower SNR
benefits of using a large coil
more area of received signal
limitations of using a large coil
lower SNR, more sensitive to artifacts, more prone to aliasing with small FOV
when the anatomy has a smaller dimension in one axis vs another, a __ may be desired. this can __
a rectangular FOV; reduce scan time a lot (imaging spinal cord)
spatial resolution is influenced by __ (3)
slice thickness, FOV, and matrix size
square pixels provide better spatial resolution because __
the image is equally resolved along both the freq and phase axes
receiver coils should be positioned to maximize the __, according to __
EMF which they can record; Faraday’s law
the contrast-to-noise ratio (CNR) can be increased by __ (3)
using T2 weighting (since tumor contrast is better), using contrast agents, using more sophisticated techniques such as magnetization transfer contrast (MTC)
narrow receive bandwidth gets better SNR because __
same amount of signal, but less noise under the curve
doubling the NEX increases the SNR only by __, so __
square root of 2; increasing the NEX is not an efficient way to increase SNR
if TE is too long, __
signal intensity is too low and therefore SNR is too low too
if TR is too short
no signal
the closer to 90 degrees the flip angle is, the __. the closer to 0/180 the flip angle is, vice versa
greater signal (because greater transverse component
