SHAPE Society

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. Contrast
• the ability to discriminate different
tissues based on their relative
brightness

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

10. 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”

11. T1 SET2 FSE
“CYST”“CYST”

12. T1 SET2 FSE
CYST?CYST?

13. T2 FSE T2 FSE
CYST?CYST?

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

17. frequency
SI
frequency
SI
Signal versus Noise
• high signal
• high SNR
• low signal
• low SNR

18. Noiseless Conditions
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

19. High Signal/Low Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

20. Low Signal/High Noise
0
10
20
30
40
50
60
70
80
90
100
Tissue A Tissue B
Tissue Type
RelativeSignalIntensity

21. 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

22. 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

23. 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

24. Image Contrast
100% noise

25. Image Contrast
80% noise

26. Image Contrast
60% noise

27. Image Contrast
40% noise

28. Image Contrast
20% noise

29. Image Contrast
0% noise

30. Factors Affecting SNR
• strength of main magnet
• coil selection
• voxel size
• phase encoding
• number of averages
• receiver bandwidth
• pulse sequence parameters

31. SNR
SNR K voxel size
measurements
bandwidth
= • •( )

32. • 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

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
• Gibb’s
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
–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

40. 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

41. SNR Application
• pituitary imaging
–baseline:
• 16 cm FOV, 3 mm slice thickness, 192
phase encodes, 4 NEX
–new goal:
• reduced scan time, same SNR

42. 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

43. 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

44. frequency
SI
frequency
SI
Fat Suppression
• without fat
suppresion
• high SNR
• with fat
suppression
• lower SNR
water
plus
fat water only