MRI Image quality

1. MRI Image Quality
GGamalamal FFathallaathalla MM..
MMahdalyahdaly
gamal_mahdaly@hotmail.comgamal_mahdaly@hotmail.com
Experta Medica

2. Factors affecting digital image QualityFactors affecting digital image Quality
 Signal to Noise Ratio:Signal to Noise Ratio:
The amount of true data building the imageThe amount of true data building the image
relative to the amount of false datarelative to the amount of false data
interfering with the image data duringinterfering with the image data during
image reconstruction.image reconstruction.
In MRI, being unavoidable, noise should notIn MRI, being unavoidable, noise should not
exceed signal & the ratio SNR should beexceed signal & the ratio SNR should be
around 1.around 1.
In CT, the noise sources are much less, soIn CT, the noise sources are much less, so
a different noise index was proposed.a different noise index was proposed.
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3. Factors affecting Signal to NoiseFactors affecting Signal to Noise
Ratio (SNR)Ratio (SNR)
 Receive Bandwidth (RBW) describes theReceive Bandwidth (RBW) describes the
window of frequencies, the receive coilwindow of frequencies, the receive coil
tuned to receive.tuned to receive.
 The received frequencies are of bothThe received frequencies are of both
Signal & Noise.Signal & Noise.
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4. Sources of the Noise in MRISources of the Noise in MRI
• Random fluctuations in electrical current (electronic
noise): exists in all electrical conductors including the MR
coils with which we measure the signal, but it also includes the
electrically conducting tissues of the patient.
•Human tissue contains many ions such as sodium, potassium
and chloride which are electrically charged atomic particles
carrying electrical currents within the body, e.g. in nerve
conduction.
•These currents generate fluctuating magnetic fields which
induce a noise voltage in the coil.
•Pulsatile flow is also implicated in noise generation as the
pulsed spins generate random electric fluctuations as well.
• Remedy Tips: use a small or dedicated anatomy coil.
Where large fields of view are essential, phased array coils are
usually best.
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5. Receive Bandwidth (RBW)Receive Bandwidth (RBW)
 The frequency range of the receiver canThe frequency range of the receiver can
be related to the frequency used tobe related to the frequency used to
encode each pixel and thereforeencode each pixel and therefore
determine the extent of the chemical-shiftdetermine the extent of the chemical-shift
artifact.artifact.
 For example, for a 256 frequency matrixFor example, for a 256 frequency matrix
and bandwidth of 32 kHz, there are 125and bandwidth of 32 kHz, there are 125
Hz per pixel so the fat–water shift isHz per pixel so the fat–water shift is
approximately 2 pixels. Typicalapproximately 2 pixels. Typical
bandwidths vary from 6.5 kHz to 1 MHz.bandwidths vary from 6.5 kHz to 1 MHz.
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6. Bandwidth ( back to Larmor Equation)
ν = γ/2π Βο
Where ν is the larmor frequency
Gyromagnetic ratioGyromagnetic ratio γ/2π = 42.57 MHz / Tesla for proton= 42.57 MHz / Tesla for proton
RBWRBW being the whole repertoire of received frequencies, we can say:being the whole repertoire of received frequencies, we can say:
RBW = 42.57RBW = 42.57 Βο
For 1T systems, RBW should be in the range of 42.57 KHzFor 1T systems, RBW should be in the range of 42.57 KHz
For 1.5T systems, RBW is in the range of 64 KHzFor 1.5T systems, RBW is in the range of 64 KHz
For 3T systems, RBW is in the range of 128 KHzFor 3T systems, RBW is in the range of 128 KHz
The question is how can we select the suitable RBW from these ranges & is itThe question is how can we select the suitable RBW from these ranges & is it
dependable on the pulse sequence?dependable on the pulse sequence?
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7. Effect of RBW on SNR
We should consider three factors:
•The Fat/water phase shift
•The SNR
•The Spatial Resolution
Good SNR with Narrow RBW Poor SNR with Wide RBW
Noise remains always Constant & Fluctuating
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8. 88
Effect of RBW on SNR
Sampling rate controls the receive bandwidth
•To narrow the receive bandwidth the system samples
slower & consequently the read out time increases & vice
versa
•Slower sampling requires
a longer readout time
•Faster sampling allows
a shorter readout time
Select BW to balance minimum TE, minimum ES versus SNR
Consider the impact on chemical shift
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9. Factors affecting Signal to Noise Ratio
(SNR)
RBW : As the RBW widens, SNR decreases.
Number of Excitations ( NEX): As NEX increases, SNR also
increases because each excitation generates part of the
signal with constant fluctuating noise.
Field of View (FOV): As FOV increases, SNR also increases
due to increase of signal producing spins.
Slice Thickness: As it increases, SNR also increases.
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10. Spatial Resolution
It is the ability to discriminate between 2 neighboring
points in any image i.e. to see image details.
To be read on a 2D screen, the 3D voxel content is interpreted into 2D
pixel
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11. Voxel and pixelVoxel and pixel
Voxel =Voxel =
(Volume element)(Volume element)
Pixel =Pixel =
(Picture element)(Picture element)
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12. To be read on a 2D screen, the 3D voxel content is interpreted into 2D
pixel
The system averages
the intensities
In the pixel, thereby
decreasing spatial
resolution
Partial volume averaging (PVA)
Most approaches to enhance spatial resolution are in deed to overcome PVA

13. Field of view (FOV):Field of view (FOV):
The diameter of reconstructed imageThe diameter of reconstructed image
MatrixMatrix::
 Orderly array of cells fashioned in rowsOrderly array of cells fashioned in rows
and columns.and columns.
 In MRI, we have a frequency matrix &In MRI, we have a frequency matrix &
a phase matrixa phase matrix

14. Spatial Resolution
Factors affecting spatial resolution:
• RBW: as RBW widens, spatial resolution increases as the
data building the image ( signal frequencies) increases.
• Matrix (Phase & Frequency dimensions): as
matrix increases, PVA decreases increasing spatial
resolution.
• Thickness (Slice dimension): as thickness
decreases, PVA decreases increasing the spatial resolution.
• FOV: as FOV decreases, PVA decreases, increasing the
spatial resolution

15. 128 256 512

16. 2020
512
512
256
256
Matrix ZIP is a zero-fill interpolation to reconstruct images in a 512x512
or 1024x1024 matrix even though acquired in a different gradient
encoding configuration ( i.e. to spread image data on a larger matrix).
Spatial Resolution Matrix ZIP

17. 2222
Matrix ZIP increases the visibility of resolution inherent in the
Image: Note the increased sharpness of the vessel edges
256 encoded/512 recon 256 encoded/256 recon
Spatial Resolution Matrix ZIP

18. Contrast Resolution
Contrast or Grey Scale Resolution is the ability to
resolve the grey color into more color grades.
Contrast Resolution demonstrates the ability to
differentially diagnose different lesions according
to their color index on the grey scale.
In MRI, we have several contrasts in each pulse
sequence.
Each pulse sequence has its weighting (contrast –
creating) factor(s).

19. Grey Scaling
The MRI reconstruction process results in a 2D matrix of floating
point numbers in the computer which range from near 0.0 up to
value equal to 1.0
These numbers correspond to the average signal intensities of the
tissue contained in each voxel
The MR images are normalized and truncated to integer values that
encompass 4096 values, between -1000 and 3095 (typically)
Rescaling these values to definite shades of grey produces the
Grey Scale.

20. Window width / window levelWindow width / window level
The human tissue MR signal intenseties extend over a narrow range (window) of t
Window width represents the MR signal intensities of all the tissues of interest wh
Tissues with intensities outside this range are displayed as either black or white.
Window Level represents the central number of all the intensities covered by the w
Both WW & WL can be set independently & their respective settings affect the con
Unlike CT, in MRI, we have multiple contrasts in each pulse sequence with differen

21. The Concept of Contrast (or Weighting)The Concept of Contrast (or Weighting)
 ContrastContrast = difference in RF signals — emitted= difference in RF signals — emitted
by water protons — between different tissuesby water protons — between different tissues
 T1 weighted example: gray-white contrast isT1 weighted example: gray-white contrast is
possible becausepossible because T1T1 is different between theseis different between these
two types of tissuetwo types of tissue

22. T2
Decay
MR
Sign
al
T1
Recovery
MR
Sign
al
50 ms50 ms 1 s1 s
Static Contrast Imaging Methods

23. Optimal TR and TE for Proton Density
Contrast
0 10 20 30 40 50 60 70 80 90 100
0
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0
0.5
1
1.5
2
2.5
T2 Decay
MRSignal
t (ms)t (s)
MRSignal
TRTR TETE
T1
Recovery
Optimum TR corresponds to the
end of T1 recovery
Optimum TE corresponds to the
beginning of T2 decay

24. Proton Density ContrastProton Density Contrast
• Technique: use very long time between
RF shots (large TR) and very short delay
between excitation and readout window
(short TE)
• Useful for anatomical reference scans
• Several minutes to acquire 256×256×128
volume
• ~1 mm resolution

25. Proton Density Weighted ImageProton Density Weighted Image

26. Optimal TR and TE for T2* and T2 Contrast
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T2 Decay
MRSignal
MRSignal
T1
Recovery
TRTR TETE
T1 ContrastT1 Contrast T2 ContrastT2 Contrast

27. T2* and T2 ContrastT2* and T2 Contrast
• Technique: use long TR and intermediate to
long TE in SE & FSE
• Use an itermediate TE & TR for GRE with a
narrow flip angle ( 15-30)
• In F-GRE, use a very short TE & TR & very
narrow flip angle.
• Useful for functional (T2* contrast) and
anatomical (T2 contrast to enhance fluid contrast)
studies
• 1mm resolution for anatomical scans or 4 mm
resolution [better is possible with better gradient
system, and a little longer time per volume]

28. T2 WeightedT2 Weighted
ImageImage

29. Optimal TR and TE for T1 Contrast
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70 80 90 100
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
T1 contrast T2 contrast
T2 Decay
MRSignal
MRSignal
T1
Recovery
TRTR TETE

30. T1 Contrast
• Technique: use intermediate timing
between RF shots (intermediate TR) and
very short TE, also use large flip angles for
GRE & MEMP
• Useful for creating gray/white matter
contrast for anatomical reference
• ~1 mm resolution

31. T1 WeightedT1 Weighted
ImageImage

32. -S-So
SSo
S = SS = Soo * (1 – 2 e* (1 – 2 e –t/T1–t/T1
))
S = SS = Soo * (1 – 2 e* (1 – 2 e –t/T1’–t/T1’
))
Inversion Recovery for Extra T1Inversion Recovery for Extra T1
ContrastContrast

33. T2T2
Inversion RecoveryInversion Recovery
(CSF Attenuated)(CSF Attenuated)

34. In summary, TR controls T1 weighting andIn summary, TR controls T1 weighting and
TE controls T2 weighting. Short T2 tissuesTE controls T2 weighting. Short T2 tissues
are dark on T2 images, but short T1 tissuesare dark on T2 images, but short T1 tissues
are bright on T1 images.are bright on T1 images.

35. Motion Contrast Imaging MethodsMotion Contrast Imaging Methods
 Can “prepare” magnetization to makeCan “prepare” magnetization to make
readout signal sensitive to different motionreadout signal sensitive to different motion
propertiesproperties
– Flow weighting (bulk movement of blood)Flow weighting (bulk movement of blood)
– Diffusion weighting (scalar or tensor)Diffusion weighting (scalar or tensor)
– Perfusion weighting (blood flow into capillaries)Perfusion weighting (blood flow into capillaries)

36. MR AngiogramMR Angiogram
•Time-of-FlightTime-of-Flight
ContrastContrast
•Phase ContrastPhase Contrast

37. Time-of-Flight ContrastTime-of-Flight Contrast
No Flow
Medium
Flow
High
Flow
No
Signal
Mediu
m
Signal
High
Signal
Vessel
Acquisitio
n
Saturation Excitatio
n
Vessel Vessel

38. 90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
90o
Saturation
Time to allow fresh
flow enter the slice
Pulse Sequence: Time-of-Flight
Contrast

39. Phase Contrast (Velocity Encoding)
Externally Applied
Spatial Gradient G
Externally Applied
Spatial Gradient -G
Blood Flow v
2
0
2
)()(
GvT
dtvtxGdtvtxG
T T
T
γ
γγφ
=
+−+= ∫ ∫
Time
T
2T0

40. 90o
Excitation
Phase
Image
Acquisition
RF
Gx
Gy
Gz
G
-G
Pulse Sequence: Phase Contrast

41. MR AngiogramMR Angiogram

42. Diffusion Weighted ImagingDiffusion Weighted Imaging
SequencesSequences
Dtl 2=
Externally AppliedExternally Applied
Spatial GradientSpatial Gradient GG
Externally AppliedExternally Applied
Spatial Gradient -Spatial Gradient -GG
TimeTime
TT
2T2T00
322
3
2
TGD
oeSS
γ⋅−
=

43. Pulse Sequence: Gradient-Echo DiffusionPulse Sequence: Gradient-Echo Diffusion
WeightingWeighting
90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
-G

44. 90o
Excitation
Image
Acquisition
RF
Gx
Gy
Gz
G
180o
G
Pulse Sequence: Spin-Echo Diffusion
Weighting

45. Advantages of DWIAdvantages of DWI
1.1. The absolute magnitude of the diffusionThe absolute magnitude of the diffusion
coefficient can help determine proton poolscoefficient can help determine proton pools
with different mobilitywith different mobility
2.2. The diffusion direction can indicate fiber tracksThe diffusion direction can indicate fiber tracks
3.3. DWI defines the diffusion characteristics of tumorsDWI defines the diffusion characteristics of tumors

46. Diffusion Anisotropy

47. Determination of fMRI Using
the Directionality of Diffusion
Tensor

48. Display of Diffusion Tensor Using
Ellipsoids

49. Diffusion ContrastDiffusion Contrast

50. PerfusionPerfusionDiffusionDiffusion
Diffusion and Perfusion ContrastDiffusion and Perfusion Contrast

51. Part III.3Part III.3
Some fundamental acquisition methoSome fundamental acquisition metho
And their k-space viewAnd their k-space view

52. k-Space Recap
Kx =Kx = γγ/2/2π ∫π ∫00
tt
Gx(t) dtGx(t) dt
Ky =Ky = γγ/2/2π ∫π ∫00
tt
Gx(t) dtGx(t) dt
Equations that govern k-space trajectoryEquations that govern k-space trajectory
These equations mean that the k-space coordThese equations mean that the k-space coord
are determined by the area under the gradienare determined by the area under the gradien
dxdyeyxIkkS
ykxki
yx
yx )(2
),(),(
+−
∫∫=
π

53. Gradient Echo ImagingGradient Echo Imaging
 Signal is generated by magnetic fieldSignal is generated by magnetic field
refocusing mechanism only (the use ofrefocusing mechanism only (the use of
negative and positive gradient)negative and positive gradient)
 It reflects the uniformity of the magnetic fieldIt reflects the uniformity of the magnetic field
 Signal intensity is governed bySignal intensity is governed by
S = So eS = So e-TE/T2*-TE/T2*
wherewhere TETE is the echo time (time fromis the echo time (time from
excitation toexcitation to
the center of k-space)the center of k-space)
 Can be used to measure T2* value of theCan be used to measure T2* value of the
tissuetissue

54. MRI Pulse Sequence for GradientMRI Pulse Sequence for Gradient
Echo ImagingEcho Imaging
digitizer ondigitizer on
ExcitationExcitation
SliceSlice
SelectionSelection
FrequencyFrequency
EncodingEncoding
PhasePhase
EncodingEncoding
ReadoutReadout

55. K-space view of the gradient echoK-space view of the gradient echo
imagingimaging
Kx
Ky
1
2
3
.
.
.
.
.
.
.
n

56. Multi-slice acquisitionMulti-slice acquisition
Total acquisition time =Total acquisition time =
Number of views * Number of excitations * TRNumber of views * Number of excitations * TR
Is this the best we can do?Is this the best we can do?
Interleaved excitation methodInterleaved excitation method

57. readoutreadout
ExcitationExcitation
SliceSlice
SelectionSelection
FrequencyFrequency
EncodingEncoding
PhasePhase
EncodingEncoding
ReadoutReadout
readoutreadout readoutreadout
…………
…………
…………
TRTR

58. Spin Echo ImagingSpin Echo Imaging
♦Signal is generated by radiofrequency
pulse refocusing mechanism (the use of
180o
pulse )
♦It doesn’t reflect the uniformity of the
magnetic field
♦Signal intensity is governed by
S = So e-TE/T2
where TE is the echo time (time from
excitation to
the center of k-space)
♦Can be used to measure T2 value of the
tissue

59. MRI Pulse Sequence for Spin Echo
Imaging
digitizer ondigitizer on
ExcitationExcitation
SliceSlice
SelectionSelection
FrequencyFrequency
EncodingEncoding
PhasePhase
EncodingEncoding
ReadoutReadout
9090
180180

60. K-space view of the spin echo imaging
Kx
Ky
1
2
3
.
.
.
.
.
.
.
n

61. Fast ImagingFast Imaging
How fast is “fast imaging”?How fast is “fast imaging”?
In principle, any technique that can generate anIn principle, any technique that can generate an
entire imageentire image
with sub-second temporal resolution can be calledwith sub-second temporal resolution can be called
fast imaging.fast imaging.
For fMRI, we need to have temporal resolution on theFor fMRI, we need to have temporal resolution on the
order oforder of
a few tens ofa few tens of msms to be considered “fast”. Echo-planarto be considered “fast”. Echo-planar
imaging,imaging,
spiral imaging can be both achieve such speed.spiral imaging can be both achieve such speed.

62. Echo Planar Imaging (EPI)Echo Planar Imaging (EPI)
 Methods shown earlier take multiple RF shots to readoutMethods shown earlier take multiple RF shots to readout
enough data to reconstruct a single imageenough data to reconstruct a single image
– Each RF shot gets data with one value of phase encodingEach RF shot gets data with one value of phase encoding
 If gradient system (power supplies and gradient coil) areIf gradient system (power supplies and gradient coil) are
good enough, can read out all data required for one imagegood enough, can read out all data required for one image
after one RF shotafter one RF shot
– Total time signal is available is aboutTotal time signal is available is about 22⋅⋅T2*T2* [80 ms][80 ms]
 Must make gradients sweep back and forth, doing allMust make gradients sweep back and forth, doing all
frequency and phase encoding steps in quick successionfrequency and phase encoding steps in quick succession
 Can acquire 10-20 low resolution 2D images per secondCan acquire 10-20 low resolution 2D images per second

63. ......
...
Pulse Sequence K-space View

64. Why EPI?Why EPI?
 Allows highest speed for dynamic contrastAllows highest speed for dynamic contrast
 Highly sensitive to the susceptibility-Highly sensitive to the susceptibility-
induced fieldinduced field
changes --- important for fMRIchanges --- important for fMRI
 Efficient and regular k-space coverageEfficient and regular k-space coverage
and goodand good
signal-to-noise ratiosignal-to-noise ratio
 Applicable to most gradient hardwareApplicable to most gradient hardware

65. Spiral ImagingSpiral Imaging
t = TE
RFRF
GxGx
GyGy
GzGz
t = 0

66. KK-Space-Space
Representation ofRepresentation of
Spiral ImageSpiral Image
AcquisitionAcquisition

67. Why Spiral?Why Spiral?
• More efficientMore efficient kk-space trajectory to improve-space trajectory to improve
throughput.throughput.
• Better immunity to flow artifacts (no gradient atBetter immunity to flow artifacts (no gradient at
the center of k-space)the center of k-space)
• Allows more room for magnetization preparation,Allows more room for magnetization preparation,
such as diffusion weighting.such as diffusion weighting.

68. Under very homogeneous magnetic field,
images look good …

69. Gradient-Recalled EPI Images Under Homogeneous Field

70. Gradient Recalled Spiral Images Under Homogeneous Field

71. However, if we don’t have a homogeneous field …
(That is why shimming is VERY important
in fast imaging)

72. Distorted EPI Images with Imperfect x-Shim
A
B
C
D

73. Distorted Spiral Images with Imperfect x-Shim
A
B
C
D

74. Any Question???.
Again Any Question???.
Thank you!

75. Have a nice dayHave a nice day