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Tissue Contrast
• intrinsic factors
–relative quantity of protons
• tissue proton density
–relaxation properties of tissue...
Contrast
• the ability to discriminate different
tissues based on their relative
brightness
Basic Principle
• relatively intermediate intensity
structures look bright on a dark
background
–important to remember wit...
Caveat
• windowing affects the relative
contrast of tissues
–intensity values of pixels are relative
to one another, unlik...
T1 SET2 FSE
“CYST”“CYST”
T1 SET2 FSE
CYST?CYST?
T2 FSE T2 FSE
CYST?CYST?
Summary
• need visible differences in intensity
to discriminate tissues
• surrounding tissues can make an
intermediate sig...
Noise
• constant at a given machine setup
• reduces the ability to visualize low
contrast structures
• adds to or subtract...
Noise
• increasing the available signal will
reduce the relative effects of noise
• machine parameters must be
chosen to m...
frequency
SI
frequency
SI
Signal versus Noise
• high signal
• high SNR
• low signal
• low SNR
Noiseless Conditions
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity
High Signal/Low Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity
Low Signal/High Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity
Noiseless Conditions
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
Rela...
High Signal/Low Noise
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
Rel...
Low Signal/High Noise
0
10
20
30
40
50
60
70
80
90
100
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Relative Pixel Location
Rel...
Image Contrast
100% noise
Image Contrast
80% noise
Image Contrast
60% noise
Image Contrast
40% noise
Image Contrast
20% noise
Image Contrast
0% noise
Factors Affecting SNR
• strength of main magnet
• coil selection
• voxel size
• phase encoding
• number of averages
• rece...
SNR
SNR K voxel size
measurements
bandwidth
= • •( )
• stronger main magnet
• proper imaging coil
• larger voxel size
• decreased phase encoding
• increased number of averages...
Stronger Main Magnet
S/N effect Downside
• linear increase • less T1 weighting
at high fields
• increased
chemical shift
e...
Coil Selection
S/N effect Downside
• increase in signal
with surface coils
• quadrature
provides 40%
increase S/N over
lin...
Larger Voxel Size
S/N effect Downside
• linear increase in
either RO or PE
direction
• linear increase
with increased
slic...
Decreased Phase Encodings
S/N effect Downside
• square root
increase in signal
to noise
• linear decrease in
scan time
• d...
Increased Signal Averages
S/N effect Downside
• square root
increase in signal
to noise
• linear increase in
scan time
Decreased Receiver BW
S/N effect Downside
• square root
increase in signal
to noise
• increase in
chemical shift
artifact ...
Pulse Sequence Parameters
• SE imaging
–increased TR provides nonlinear
increase in SNR with linear increase
in scan time
...
Pulse Sequence Parameters
• GE imaging
–complex effects
–maximum SNR typically between 30
and 60 degrees
–long TR sequence...
SNR Application
• pituitary imaging
–baseline:
• 16 cm FOV, 3 mm slice thickness, 192
phase encodes, 4 NEX
–new goal:
• re...
FOV RO PE
Slice
Thickness
(mm) NEX
Imaging Time
(TR=500
msec)
Relative
SNR
160 256 192 3 4 6.40 43.30
160 256 170 4 2 2.83...
Fat Suppression and SNR
• non fat-suppressed image
–each image pixel comprised of signal
from water and fat in the imaging...
frequency
SI
frequency
SI
Fat Suppression
• without fat
suppresion
• high SNR
• with fat
suppression
• lower SNR
water
plu...
Tissue contrast
Tissue contrast
Tissue contrast
Tissue contrast
Tissue contrast
Tissue contrast
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Tissue contrast

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Tissue contrast

  1. 1. Tissue Contrast • intrinsic factors –relative quantity of protons • tissue proton density –relaxation properties of tissues • T1 & T2 relaxation • secondary factors –flow –contrast agents
  2. 2. Contrast • the ability to discriminate different tissues based on their relative brightness
  3. 3. Basic Principle • relatively intermediate intensity structures look bright on a dark background –important to remember with fatsat • relatively intermediate intensity structures look dark on a light background
  4. 4. Caveat • windowing affects the relative contrast of tissues –intensity values of pixels are relative to one another, unlike CT • windowing can make a solid tumor look like a “cyst”
  5. 5. T1 SET2 FSE “CYST”“CYST”
  6. 6. T1 SET2 FSE CYST?CYST?
  7. 7. T2 FSE T2 FSE CYST?CYST?
  8. 8. 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
  9. 9. 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
  10. 10. 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
  11. 11. frequency SI frequency SI Signal versus Noise • high signal • high SNR • low signal • low SNR
  12. 12. Noiseless Conditions 0 10 20 30 40 50 60 70 80 90 100 Tissue A Tissue B Tissue Type RelativeSignalIntensity
  13. 13. High Signal/Low Noise 0 10 20 30 40 50 60 70 80 90 100 Tissue A Tissue B Tissue Type RelativeSignalIntensity
  14. 14. Low Signal/High Noise 0 10 20 30 40 50 60 70 80 90 100 Tissue A Tissue B Tissue Type RelativeSignalIntensity
  15. 15. Noiseless Conditions 0 10 20 30 40 50 60 70 80 90 100 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Relative Pixel Location RelativeSignalIntensity
  16. 16. High Signal/Low Noise 0 10 20 30 40 50 60 70 80 90 100 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Relative Pixel Location RelativeSignalIntensity
  17. 17. Low Signal/High Noise 0 10 20 30 40 50 60 70 80 90 100 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Relative Pixel Location RelativeSignalIntensity
  18. 18. Image Contrast 100% noise
  19. 19. Image Contrast 80% noise
  20. 20. Image Contrast 60% noise
  21. 21. Image Contrast 40% noise
  22. 22. Image Contrast 20% noise
  23. 23. Image Contrast 0% noise
  24. 24. Factors Affecting SNR • strength of main magnet • coil selection • voxel size • phase encoding • number of averages • receiver bandwidth • pulse sequence parameters
  25. 25. SNR SNR K voxel size measurements bandwidth = • •( )
  26. 26. • stronger main magnet • proper imaging coil • larger voxel size • decreased phase encoding • increased number of averages • decreased receiver bandwidth • (pulse sequence parameters) Factors INCREASING SNR
  27. 27. Stronger Main Magnet S/N effect Downside • linear increase • less T1 weighting at high fields • increased chemical shift effects in RO direction
  28. 28. 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
  29. 29. Larger Voxel Size S/N effect Downside • linear increase in either RO or PE direction • linear increase with increased slice thickness • decreased resolution
  30. 30. Decreased Phase Encodings S/N effect Downside • square root increase in signal to noise • linear decrease in scan time • decreased resolution in PE direction • Gibb’s phenomenon in PE direction
  31. 31. Increased Signal Averages S/N effect Downside • square root increase in signal to noise • linear increase in scan time
  32. 32. Decreased Receiver BW S/N effect Downside • square root increase in signal to noise • increase in chemical shift artifact in RO direction
  33. 33. Pulse Sequence Parameters • SE imaging –increased TR provides nonlinear increase in SNR with linear increase in scan time –decreased TE provides nonlinear increase in SNR with no effect on scan time and less T2 weighting
  34. 34. Pulse Sequence Parameters • GE imaging –complex effects –maximum SNR typically between 30 and 60 degrees –long TR sequences (2D) • increase SNR with increased flip angle –short TR sequences (TOF & 3D) • decreased SNR with increased flip angle
  35. 35. SNR Application • pituitary imaging –baseline: • 16 cm FOV, 3 mm slice thickness, 192 phase encodes, 4 NEX –new goal: • reduced scan time, same SNR
  36. 36. FOV RO PE Slice Thickness (mm) NEX Imaging Time (TR=500 msec) Relative SNR 160 256 192 3 4 6.40 43.30 160 256 170 4 2 2.83 43.39 190 256 192 3 2 3.20 43.18 160 256 144 3 3 3.60 43.30 SNR Example
  37. 37. Fat Suppression and SNR • non fat-suppressed image –each image pixel comprised of signal from water and fat in the imaging voxel • fat-suppression –reduces total signal by suppression of fat from the voxel –reduces SNR
  38. 38. frequency SI frequency SI Fat Suppression • without fat suppresion • high SNR • with fat suppression • lower SNR water plus fat water only

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