allen w. song, phd brain imaging and analysis center duke university
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MRI: Image Formation. Allen W. Song, PhD Brain Imaging and Analysis Center Duke University. What is image formation?. To define the spatial location of the sources that contribute to the detected signal. A Simple Example. But MRI does not use projection, reflection, or refraction - PowerPoint PPT PresentationTRANSCRIPT
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Allen W. Song, PhDBrain Imaging and Analysis CenterDuke University
MRI: Image Formation
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What is image formation?
To define the spatial location of the sourcesthat contribute to the detected signal.
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But MRI does not use projection, reflection, or refractionmechanisms commonly used in optical imaging methodsto form image …
A Simple Example
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MRI Uses Frequency and Phase to Construct Image
= = tt
The spatial information of the proton pools contributing to MR signal is determined by the spatial frequency and phase of their magnetization.
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Three Gradient Coils
Gradient coils generate spatially varying magnetic field so that spins at different location precess at frequencies unique to their location, allowing us to reconstruct 2D or 3D images.
X gradient Y gradient Z gradient
x
y
z
x
z z
x
y y
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The Use of Gradient Coils for Spatial Encoding
w/o encoding w/ encoding
ConstantMagnetic Field
VaryingMagnetic Field
MR Signal
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Spatial Decoding of the MR Signal
FrequencyDecomposition
a 1-D Image !
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How Do We Make a Typical MRI Image?
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First Step in Image Formation:Slice Selection
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Slice Selection – along Slice Selection – along zz
zz
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Determining Slice Thickness
Resonance frequency range as the resultResonance frequency range as the resultof slice-selective gradient:of slice-selective gradient: f = f = HH * G * Gslsl * d * dslsl
The bandwidth of the RF excitation pulse:The bandwidth of the RF excitation pulse: /2/2
Matching the two frequency ranges, the slice Matching the two frequency ranges, the slice thickness can be derived asthickness can be derived as ddslsl = = / ( / (HH * G * Gslsl * 2 * 2))
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Changing Slice Thickness or Selecting Difference Slices
There are two ways to do this:There are two ways to do this:
(a)(a) Change the slope of the slice selection gradientChange the slope of the slice selection gradient
(b)(b) Change the bandwidth of the RF excitation pulseChange the bandwidth of the RF excitation pulse
Both are used in practice, with (a) being more popularBoth are used in practice, with (a) being more popular
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Changing Slice Thickness or Selecting Difference Slices
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Second Step in Image Formation:
Spatial encoding and resolving one dimension within a plane
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Spatial Encoding of the MRI Signal:An Example of Two Vials
w/o encoding w/ encoding
ConstantMagnetic Field
VaryingMagnetic Field
Continuous Sampling
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Spatial Decoding of the MR Signal
FrequencyDecomposition
a 1-D Image !
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It’d be inefficient to collect data points continuously over time, actually, if all weneed to resolve are just two elements in space.
There is a better way to resolve these twoelements discretely.
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t
2t
G
Element 1 Element 2
G
Element 1 Element 2
A B
A
B
time 0
S0 = A + B
Time point 1S1 = | A*exp(-i1t) + B*exp(-i2t) |
Time Point 2
lag
lead
It turns out that all we need is just two data points:
1 = G x, where x is determined by the voxel size
A B
time t
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The simplest case is to wait for time t such that A and B will point along opposite direction,
A B
A
B
such that S0 = A + B, S1 = A – B,
resulting in A = (S0 + S1)/2, and B = (S0 – S1)/2
t
t
time 0 time t
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GG
A1
time t1=0 time t256
S1 = A1 + ... + A9
Time point 1, S1
S9 = | A1*exp(-i1t9) +... + A9 *exp(-i9t9) |
Time Point 9, S9
1t2
9t2
GA1
A9
time t2
S2 = | A1*exp(-i1t2) +... + A9 *exp(-i9t2) |
Time Point 2, S2
A9... ...
1t9A1
A9
...9t9
. . .
Now, let’s extrapolate to resolve 9 elements along a dimension …Now, let’s extrapolate to resolve 9 elements along a dimension …
A1 A2 A3 A4 A5 A6 A7 A8 A9
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A typical diagram for MRI frequency encoding:Gradient-echo imaging
readoutreadout
ExcitationExcitation
SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding
ReadoutReadout
TETE
Data points collected during thisData points collected during thisperiod corrspond to one-line in k-spaceperiod corrspond to one-line in k-space
………………Time point #1Time point #1 Time point #9Time point #9
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Phase Evolution of MR DataPhase Evolution of MR Data
digitizer ondigitizer on
Phases of spinsPhases of spins
GradientGradient
TETE
………………Time point #1Time point #1 Time point #9Time point #9
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Image Resolution (in Plane)
Spatial resolution depends on how well we can separate frequencies in the data S(t) Stronger gradients nearby positions are better separated in
frequencies resolution can be higher for fixed f Longer readout times can separate nearby frequencies better
in S(t) because phases of cos(ft) and cos([f+f]t) will be more different
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Summary: Second Step in Image FormationFrequency Encoding
After slice selection, in-plane spatial encoding begins During readout, gradient field perpendicular to slice selection
gradient is turned on Signal is sampled about once every few microseconds, digitized,
and stored in a computer• Readout window ranges from 5–100 milliseconds (can’t be longer than
about 2T2*, since signal dies away after that) Computer breaks measured signal S(t) into frequency components
S(f ) — using the Fourier transform Since frequency f varies across subject in a known way, we can
assign each component S(f ) to the place it comes from
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Third Step in Image Formation:
Resolving the second in-plane dimension
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Now let’s consider the simplest 2D image
A B
C D
A B
C DA
B
C
D
S0 = (A + C) + (B + D)
Time point 1
S1 = (A + C) - (B + D)
Time point 2
S0 S1
TimeGx
x
Gx
x
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A B
C D
G
xS0 = (A + C) + (B + D)
Time point 1
S1 = (A + C) - (B + D)
Time point 2
S0 S1
t
S2 = (A + B) + (C + D)
Time point 3
S3 = (A + B) - (C + D)
Time point 4
S2 S3
t
y
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A B
C DA
B
C
D
S0 S1
TimeGx
x
Gx
xA
B
CD
A B
C D
S2 S3
y
Gy
y
Gy
TimeGx
x
Gx
x
S0 = (A + C) + (B + D)
S1 = (A + C) - (B + D)
S2 = (A + B) - (C + D)
S3 = A – B – C + D
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A Little More Complex Spatial EncodingA Little More Complex Spatial Encoding
x gradientx gradient
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y gradienty gradient
A Little More Complex Spatial EncodingA Little More Complex Spatial Encoding
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Physical SpaceA 9A 9××9 case9 case
Before EncodingBefore Encoding After Frequency Encoding After Frequency Encoding (x gradient)(x gradient)
So each data point contains information from all the voxelsSo each data point contains information from all the voxels
MR data spaceMR data space 1 data point1 data point another data pointanother data point
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Physical SpaceA 9A 9××9 case9 case
Before EncodingBefore Encoding After Frequency EncodingAfter Frequency Encodingx gradientx gradient
After Phase EncodingAfter Phase Encodingy gradienty gradient
So each point contains information from all the voxelsSo each point contains information from all the voxels
MR data spaceMR data space 1 data point1 data point 1 more data point1 more data point another pointanother point
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A typical diagram for MRI phase encoding:Gradient-echo imaging
readoutreadout
ExcitationExcitation
SliceSliceSelectioSelectionnFrequencyFrequency EncodingEncoding
PhasePhase EncodingEncoding
ReadoutReadout………………
Thought Question: Why can’t the phase encoding gradient be
Thought Question: Why can’t the phase encoding gradient be
turned on at the same time with the frequency encoding gradient?
turned on at the same time with the frequency encoding gradient?
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Summary: Third Step in Image Formation Phase Encoding
The third dimension is provided by phase encoding: We make the phase of Mxy (its angle in the xy-plane) signal
depend on location in the third direction This is done by applying a gradient field in the third direction ( to
both slice select and frequency encode) Fourier transform measures phase of each S(f ) component of
S(t), as well as the frequency f By collecting data with many different amounts of phase encoding
strength, can break each S(f ) into phase components, and so assign them to spatial locations in 3D
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A Brief Introduction of the Final MR Data Space (k-Space)
ImageImage k-spacek-space
Motivation: direct summation is conceptually easy,but highly intensive in computation which makes it impractical for high-resolution MRI.
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PhasePhaseEncodeEncodeStep 1Step 1
PhasePhaseEncodeEncodeStep 2Step 2
PhasePhaseEncodeEncodeStep 3Step 3
Time Time point #1point #1
Time Time point #2point #2
Time Time point #3point #3
…………....
Time Time point #1point #1
Time Time point #2point #2
Time Time point #3point #3
…………....
Time Time point #1point #1
Time Time point #2point #2
Time Time point #3point #3
…………....
……
……
....
Frequency Encode
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Contributions of different image locations to the raw k-space data. Each data point in k-space (shown in yellow) consists of the summation of MR signal from all voxels in image space under corresponding gradient fields.
..
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..
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.+Gx-Gx 0
0
+Gy
-Gy .
Physical SpaceK-Space
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Acquired MR Signal
dxdyeyxIkkS ykxkiyx
yx )(2),(),(
From this equation, it can be seen that the acquired MR signal,From this equation, it can be seen that the acquired MR signal,which is also in a 2-D space (with kx, ky coordinates), is the which is also in a 2-D space (with kx, ky coordinates), is the Fourier Transform of the imaged object.Fourier Transform of the imaged object.
For a given data point in k-space, say (kx, ky), its signal S(kx, ky) is the For a given data point in k-space, say (kx, ky), its signal S(kx, ky) is the sum of all the little signal from each voxel I(x,y) in the physical space, sum of all the little signal from each voxel I(x,y) in the physical space, under the gradient field at that particular momentunder the gradient field at that particular moment
Kx = Kx = /2/200ttGx(t) dtGx(t) dt
Ky = Ky = /2/200ttGy(t) dtGy(t) dt
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Two Spaces
IFTIFT
FTFT
k-spacek-space
kkxx
kkyy
Acquired DataAcquired Data
Image spaceImage space
xx
yy
Final ImageFinal Image
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Two Spaces
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ImageImage KK
Two Spaces
HighHighSignalSignal
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Full k-spaceFull k-space Lower k-spaceLower k-space Higher k-spaceHigher k-space
Full ImageFull Image Intensity-Heavy ImageIntensity-Heavy Image Detail-Heavy ImageDetail-Heavy Image
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FOV = 1/k, x = 1/K
FOV
k
Field of View, Voxel Size – a k-Space Perspective
K
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Image Distortions: a k-Space Perspective