3. Basic Principle
⢠relatively intermediate intensity
structures look bright on a dark
background
âimportant to remember with fatsat
⢠relatively intermediate intensity
structures look dark on a light
background
4.
5.
6.
7.
8.
9.
10. Caveat
⢠windowing affects the relative
contrast of tissues
âintensity values of pixels are relative
to one another, unlike CT
⢠windowing can make a solid tumor
look like a âcystâ
14. Summary
⢠need visible differences in intensity
to discriminate tissues
⢠surrounding tissues can make an
intermediate signal tissue appear
dark or bright
⢠windowing affects image and tissue
contrast
15. Noise
⢠constant at a given machine setup
⢠reduces the ability to visualize low
contrast structures
⢠adds to or subtracts from the
average signal intensity of a given
pixel
16. Noise
⢠increasing the available signal will
reduce the relative effects of noise
⢠machine parameters must be
chosen to maximize signal without
significantly extending exam times
⢠S/N is a relative measure allowing
for comparison in a variety of
circumstances
33. Stronger Main Magnet
S/N effect Downside
⢠linear increase ⢠less T1 weighting
at high fields
⢠increased
chemical shift
effects in RO
direction
34. Coil Selection
S/N effect Downside
⢠increase in signal
with surface coils
⢠quadrature
provides 40%
increase S/N over
linear
⢠phased array
increased over
quadrature
⢠limited coverage
with surface coils
⢠more complex
coils are more
expensive
35. Larger Voxel Size
S/N effect Downside
⢠linear increase in
either RO or PE
direction
⢠linear increase
with increased
slice thickness
⢠decreased
resolution
36. Decreased Phase Encodings
S/N effect Downside
⢠square root
increase in signal
to noise
⢠linear decrease in
scan time
⢠decreased
resolution in PE
direction
⢠Gibbâs
phenomenon in
PE direction
37. Increased Signal Averages
S/N effect Downside
⢠square root
increase in signal
to noise
⢠linear increase in
scan time
38. Decreased Receiver BW
S/N effect Downside
⢠square root
increase in signal
to noise
⢠increase in
chemical shift
artifact in RO
direction
39. Pulse Sequence Parameters
⢠SE imaging
âincreased TR provides nonlinear
increase in SNR with linear increase
in scan time
âdecreased TE provides nonlinear
increase in SNR with no effect on
scan time and less T2 weighting
40. Pulse Sequence Parameters
⢠GE imaging
âcomplex effects
âmaximum SNR typically between 30
and 60 degrees
âlong TR sequences (2D)
⢠increase SNR with increased flip angle
âshort TR sequences (TOF & 3D)
⢠decreased SNR with increased flip angle
41. SNR Application
⢠pituitary imaging
âbaseline:
⢠16 cm FOV, 3 mm slice thickness, 192
phase encodes, 4 NEX
ânew goal:
⢠reduced scan time, same SNR
43. Fat Suppression and SNR
⢠non fat-suppressed image
âeach image pixel comprised of signal
from water and fat in the imaging
voxel
⢠fat-suppression
âreduces total signal by suppression of
fat from the voxel
âreduces SNR