basic pulse sequences i · radiology pulse sequence definitions • tr - repetition time –...
TRANSCRIPT
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UCLA Radiology
Basic Pulse Sequences I Saturation & Inversion Recovery
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UCLA Radiology
Gradient HardwareY-Gradient
Transceiver
Patient
Z-Gradient
X-Gradient
http://www.magnet.fsu.edu
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UCLA Radiology
GradientsGradients are a special kind of inhomogeneous field whose z-component varies linearly along a specific direction called the gradient direction.
BG,z(x) = Gxx
x-gradient amplitude
B-field from a
gradient
Points along the z-direction
Varies with the
x-direction
x-distance from isocenter
} } } } }
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UCLA Radiology
Free Precession & Gradients~Beff =
~!rot
�+ ~Brot
=��B0k0
�+⇣B0 + ~G · ~r
⌘k0
=⇣~G · ~r
⌘k0
@Mrot
@t= ~Mrot ⇥ � ~Beff
= ~Mrot ⇥ �⇣~G · ~r
⌘k0
My0 = M0x0 sin
⇣�� ~G · ~r
⌘+M0
y0 cos⇣�� ~G · ~r
⌘Mx0 = M0
x0 cos⇣�� ~G · ~r
⌘�M0
y0 sin⇣�� ~G · ~r
⌘
Mz0 = M0z0
2
64
dMx0dt
dMy0
dtdMz0dt
3
75 =
������
i0 j0 k0
Mx0 My0 Mz0
0 0 � ~G · ~r
������
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UCLA Radiology
Gradients - Frequency & Phase
My0 = M0x0 sin
⇣�� ~G · ~r
⌘+M0
y0 cos⇣�� ~G · ~r
⌘Mx0 = M0
x0 cos⇣�� ~G · ~r
⌘�M0
y0 sin⇣�� ~G · ~r
⌘
Mz0 = M0z0
!~G (~r) = ��⇣~G · r
⌘k0
The frequency of free precession in the rotating frame is a function of space ( ) in the presence of an applied gradient ( ).~G~r
�~G =
Z tgrad
0~!~G(~r, t)dt
= �Z tgrad
0� ~G(t) · ~r(t)dt
The phase of the spin in the rotating frame is a function of position (x) and gradient duration (tgrad) in the presence of an applied gradient (Gx).
�~G(x, t) = ��Gx · x · tgrad
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UCLA Radiology
Lecture #6 Learning Objectives• Appreciate the definition of image contrast. • Explain what a T1 or T2-weighted image is. • Describe what a pulse sequence is. • Understand the saturation recovery pulse
sequence and the saturation condition. • Describe the inversion recover sequence. • Distinguish between STIR and FLAIR.
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Image Contrast
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UCLA Radiology
Why Image Contrast?Visual Area
of the Thalamus
Visual Cortex
Retina
Optic nerve
Optic tract
Optic chiasm
The human visual system is more sensitive to contrast than absolute luminance.
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UCLA Radiology
Why Image Contrast?
Which is brighter A or B?
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UCLA Radiology
CNR, Object Size, and Noise
Noise Free Noisy
Large high-contrast objects are easier to see in the presence of noise.
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UCLA Radiology
CNR, Resolution, and Noise
Small low-contrast objects are easier to see with higher resolution.
Image signal-to-noise is constant.
High Resolution Low Resolution
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UCLA Radiology
Image Contrast
CAB = f(�, T1, T2, T�2 , D, ...)
CAB =|IA � IB |
Iref
CAB � f(T1) CAB � f(T2)A central goal in MRI is to limit image contrast to a single mechanism.
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Pulse Sequences
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UCLARadiology
What is a pulse sequence?
Sheet music is a timing diagram for playing the piano.
A pulse sequence is a timing diagram for running the scanner.
TETR
RF
X
Y
Z
t
t
t
t
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UCLARadiology
Pulse Sequences
Contrast Module Imaging Module
Inversion Recovery T2-preparation Fat Saturation
Tagging
Spoiled Gradient Echo SPGR, FLASH, T1 FFE
bSSFP FIESTA, Balanced FFE, True FISP
Spin Echo Fast or Turbo Spin Echo
timeGra
dien
ts &
RF
Pul
ses
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How do we keep track of the magnetization’s history?
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UCLA Radiology
Pulse Sequence Definitions
M(n)z (0�)
M(n)z (0+)
M(n)xy (0+)
M(n)xy (0�)
Longitudinal magnetization before the nth event.
Longitudinal magnetization after the nth event.
Transverse magnetization after the nth event.
Transverse magnetization before the nth event.
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UCLA Radiology
Free? Forced? Relaxation?• We’ve considered all combinations of:
– Free or forced precession – With or without relaxation – Laboratory or rotating frames
• Which one’s concern M219 the most? – Rotating frame
• Free precession with relaxation
• Forced precession without relaxation
Mz (t) = M0z e
� tT1 +M0
⇣1� e�
tT1
⌘Mxy (t) = M0
xye�t/T2
~M (n)(0+) =
2
4c2✓ + s2✓c↵ c✓s✓ � c✓s✓c↵ �s✓s↵c✓s✓ � c✓s✓c↵ s2✓ + c2✓c↵ c✓s↵
s✓s↵ �c✓s↵ c↵
3
5 ~M (n)(0�)
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Typical Pulse Sequence…RF RFTR
ɠ ɡ
t
B1(t)
~M (1)(0�) = ~M0 =
2
400M0
3
5
~M (1)(0+) = RF↵✓
~M (1)(0�)
~M (2)(0�) = E(T1, T2, TR) ~M (1)(0+)
~M (2)(0+) = RF↵✓
~M (2)(0�)
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Saturation Recovery
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UCLA Radiology
Pulse Sequence Definitions
• TR - Repetition Time – Duration of basic pulse sequence repeating block – At least one echo acquired per TR
• TE - Echo Time – Time from excitation to the maximum of the echo – Data is recorded at time TE to form an image
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Saturation Recovery
Relax
ɠ ɡ
RF90�
0� RF↵0� RF↵
0�
Relax
To The Board...
TR TR
(90o � TR)N
ɢ
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Saturation Condition
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Saturation Condition• The saturation condition states:
M(n)z (0+) = 0, n�1
Mz is ZERO after the event (RF pulse).
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UCLA Radiology
Saturation Condition
• This is true if the Mxy is “gone” before the next 90° RF-pulse is applied:
– No Mxy to convert to Mz – How? TR>>T2
• The saturation condition states:
M(n)z (0+) = 0, n�1
Mz is ZERO after the event (RF pulse).
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UCLA Radiology
Saturation Condition
• This is true if the Mxy is “gone” before the next 90° RF-pulse is applied:
– No Mxy to convert to Mz – How? TR>>T2
• What if TR<~3T2? – Mxy can be converted back to Mz – Corrupts/complicates image contrast – Solution? Spoiler gradients to disperse Mxy
• Steady-state solution arises if the saturation conditions are met/enforced
• The saturation condition states:
M(n)z (0+) = 0, n�1
Mz is ZERO after the event (RF pulse).
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Saturation Recovery
Relax
ɠ ɡ
RF90�
0� RF↵0� RF↵
0�
Relax
To The Board...
TR TR
(90o � TR)N
ɢ
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SR Contrast
Afid / M0z
⇣1� e�TR/T1
⌘/ ⇢
⇣1� e�TR/T1
⌘
I (~r) / ⇢ (~r)⇣1� e�TR/T1(~r)
⌘
• Afid – Signal amplitude immediately after the 90°. • ρ – proton density. • If the process of imaging doesn’t perturb the magnetization:
Eqn. 7.13
Eqn. 7.14
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SR Contrast
I (~r) / ⇢ (~r)⇣1� e�TR/T1(~r)
⌘
I (~r)TR!1 / ⇢ (~r)
The final image is the product of ρ(r) and f(T1(r)).
The image pure pure ρ(r) contrast under this limit.
• Note only one parameter adjusts contrast – Longer T1s appear darker with short TRs
• Long T1 will be dark. • Short T1 will be bright.
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SR Contrast
TRopt =ln
⇣T1,A
T1,B
⌘
1T1,B
� 1T1,A
I (~r)TR!TRopt/ Maximum T1 contrast
Eqn. 7.19
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Saturation Recovery - Applications
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SASHA - Normal Subject
Chow K et al. Mag. Reson. Med., v71, i6, p2082-2095, 2014
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SASHA - Myocardial Infarct
Chow K et al. Mag. Reson. Med., v71, i6, p2082-2095, 2014
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UCLA Radiology
Chow K et al. Mag. Reson. Med., v71, i6, p2082-2095, 2014
SAturation recovery single-SHot Acquisition (SASHA)
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UCLA Radiology
SAturation recovery single-SHot Acquisition (SASHA)
Chow K et al. Mag. Reson. Med., v71, i6, p2082-2095, 2014
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UCLA Radiology
SAturation recovery single-SHot Acquisition (SASHA)
Chow K et al. Mag. Reson. Med., v71, i6, p2082-2095, 2014
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Inversion Recovery
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Spin Echo Inversion Recovery
TI=25ms
Spin Echo with TE=12ms, TR=2000ms
WM – 790ms GM – 925ms
Mz (t)
M0
�M0
TI [ms]
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UCLA Radiology
Spin Echo Inversion Recovery
TI=25ms
Spin Echo with TE=12ms, TR=2000ms
TI=200ms
WM – 790ms GM – 925ms
Mz (t)
M0
�M0 WMGM
TI [ms]
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Spin Echo Inversion Recovery
TI=25ms
Spin Echo with TE=12ms, TR=2000ms
TI=200ms TI=500ms
WM – 790ms GM – 925ms
Mz (t)
M0
�M0 WMGM
TI [ms]
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Spin Echo Inversion Recovery
TI=25ms
Spin Echo with TE=12ms, TR=2000ms
TI=200ms TI=500ms TI=1000ms
WM – 790ms GM – 925ms
Mz (t)
M0
�M0 WMGM
TI [ms]
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UCLA RadiologyTE=12ms, TR=2000ms
TI=25ms TI=200ms TI=500ms TI=1000ms
Spin Echo Inversion Recovery
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Inversion Recovery
Relax
RF180�
0� RF90�
0�
ɠ
Relax
RF180�
0�
Relax
RF90�
0�
ɡ ɢ ɣ
To The Board...
(180o � TI� 90o � TD)N
TI TI
TR
TD
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IR Contrast
Afid / ⇢⇣1� 2e�TI/T1 + e�TR/T1
⌘
I (~r) / ⇢ (~r)⇣1� 2e�TI/T1(~r) + e�TR/T1(~r)
⌘Eqn. 7.21
The final image is the product of ρ(r) and f(T1(r)).
The final image contrast is controlled by TI and TR.
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IR Signal Nulling Effect
TInull =hln 2� ln
⇣1 + exp�TR/T 0
1
⌘iT 0
1
TInull = [ln 2] T 01 , if TR �!1
I (~r) = 0, if T1 (~r) = T 01 (~r)
Target T1
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SR vs. IR
Mz (t)
M0
�M0
Saturation RecoveryInversion Recovery
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Inversion Pulse - Applications• Greater T1 contrast than SR
• T1 species nulling/attenuation – FLAIR (Fluid Attenuated Inversion Recovery)
– STIR (Short Tau Inversion Recovery)
• IR is better than SR for generating contrast when: – ρ(A)=ρ(B) and T2(A)=T2(B)
– AND
– T1(A) and T1(B) are slightly different
• Quantitative T1 mapping
I (~r) / ⇢ (~r)⇣1� 2e�TI/T1(~r) + e�TR/T1(~r)
⌘Eqn. 7.21
The final image is the product of ρ(r) and f(T1(r)).
The final image contrast is controlled by TI and TR.
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STIR Pulse Sequence180° 180°TR
ɠ ɡ
t
B1(t)
t
TI
Fat CSF
Mz(t)
Short Tau Inversion Recovery (STIR) is used to null fat.
TE
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UCLA Radiology
STIR Images
http://www.svuhradiology.ie/wp-content/uploads/2015/04/STIRmetscombo.jpg
T2-Weighted STIR
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UCLA Radiology
FLAIR Pulse Sequence
ɠ ɡ
t
B1(t)
t
TI
Fat CSF
Mz(t)
FLuid Attenuated Inversion Recovery (STIR) is used to CSF.
180° 180°TR
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UCLA Radiology
FLAIR Images
http://www.neuroradiologycases.com/2011/11/intracranial-lipoma.html
FLAIR can distinguish fat from CSF.
Images courtesy of Balaji Anvekar
CSF?
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FLAIR ImagesLong T2 is bright on T2w.
http://www.neuroradiologycases.com/2011_08_01_archive.html
T2 T1 FLAIR
Short T1 is bright on T1w. Long T1 is dark on FLAIR.
Lesion has long T2 and intermediate T1. Not fat. Not CSF. Cerebral hydatid.
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UCLA Radiology
Thanks
Daniel B. Ennis, Ph.D. [email protected] 310.206.0713 (Office) http://ennis.bol.ucla.edu
Peter V. Ueberroth Bldg. Suite 1417, Room C 10945 Le Conte Avenue