1 Seite 1 RUPRECHT-KARLS- UNIVERSITY HEIDELBERG Computer Assisted Clinical Medicine Prof. Dr. Lothar Schad 12/9/2008 | Page 1 Master‘s Program in Medical Physics Chair in Computer Assisted Clinical Medicine Faculty of Medicine Mannheim University of Heidelberg Theodor-Kutzer-Ufer 1-3 D-68167 Mannheim, Germany [email protected]www.ma.uni-heidelberg.de/inst/cbtm/ckm/ Physics of Imaging Systems Basic Principles of Magnetic Resonance Imaging V Prof. Dr. Lothar Schad RUPRECHT-KARLS- UNIVERSITY HEIDELBERG Computer Assisted Clinical Medicine Prof. Dr. Lothar Schad 12/9/2008 | Page 2 Imaging Contrast Imaging Contrast
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Seite 1
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 1Master‘s Program in Medical Physics
Chair in Computer Assisted Clinical MedicineFaculty of Medicine Mannheim University of HeidelbergTheodor-Kutzer-Ufer 1-3D-68167 Mannheim, GermanyLothar.Schad@MedMa.Uni-Heidelberg.dewww.ma.uni-heidelberg.de/inst/cbtm/ckm/
Physics of Imaging Systems
Basic Principles of Magnetic Resonance Imaging V
Prof. Dr. Lothar Schad
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 2
Imaging Contrast
Imaging Contrast
2
Seite 2
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 3
kx
k-space: after 1. TR k-space: after 2. TR k-space: after 256. TRky
Spin-Echo Sequence
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 4
PwTR = 2775 ms
TE = 17 ms
T2wTR = 2775 msTE = 102 ms
T1wTR = 575 msTE = 14 ms
Spin-Echo Images
32143421factorT2
2TE/T
factorT1
1TR/TSE ]1[S
−
−
−
− ⋅−⋅= eeρ
- SE gold standard technique for T1 and T2 morphology
3
Seite 3
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 5
TR
TE
90
-
60ms
40
-
10ms
T1-weighted
T2-weighted
proton-weighted
300 - 800 ms 1500 - 3000 ms
no contrastSNR low
Spin-Echo Contrast: TR, TE
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 6Spin-Echo: T1 Dependency
source: Reiser and Semmler. “Magnetresonanztomographie” 2002
T1-factor:[1-exp(-TR/T1)]
GM: T1 = 970 msWM: T1 = 600 ms
contrastBA
BA
SSSSC
+−
=
T1 - factor
WM
T1 - contrast
GM
4
Seite 4
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 7Spin-Echo: T2 Dependency
T2-factor:exp(-TE/T2)
GM: T2 = 110 msWM: T2 = 90 msT2 - factor
WM GM
T2 - contrast
source: Reiser and Semmler. “Magnetresonanztomographie” 2002
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 8Spin-Echo: TR, TE
source: Schlegel and Mahr. “3D Conformal Radiation Therapy: A Multimedia Introduction to Methods and Techniques" 2007
5
Seite 5
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 9Spin-Echo: TR, TE
SE contrast betweengrey and white matter
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 10Spin-Echo: Multi-Slice
measurement #1
measurement #2
- interleaved SE measurement
- multi-slice SETR = 600 ms, TE = 10 ms→ about 60 slices can be measured
simultaneously !
1. slice
2. slice
3. slice
4. slice
5. slice
6
Seite 6
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 11Inversion Recovery Sequence
Mz
|Mz|
source: Reiser and Semmler. “Magnetresonanztomographie” 2002
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 12Inversion Recovery Images
321444 3444 21factorT2
TE/T2
factorT1
TR/T1TI/T1IR ]21[S
−
−
−
−− ⋅+−⋅= eeeρ
7
Seite 7
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 13Inversion Recovery Contrast
- WM and GM contrast dependency on TI, TR and TE- IR gold standard technique for T1 contrast
source: Reiser and Semmler. “Magnetresonanztomographie” 2002
T1-contrast
T2-contrast
contrast [a.u.]
T2-contrast
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 14
α
- example: α = 20° Mz - reduction by 6%Mxy- value 34% of Mz!!
M
x´,y´
z
Mz
Mxy
Gradient-Echo Sequence
spoiler
8
Seite 8
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 15
txGtdxGtx x
t
x ⋅⋅⋅−=⋅⋅−⋅= ∫ γγφ0
ˆ),(τ≤≤ t0
)(
ˆ),(
τγτγ
γγφτ
−⋅⋅⋅+⋅⋅⋅−=
⋅⋅⋅+⋅⋅⋅−= ∫txGxG
tdxGtxGtx
xx
t
xxττ 2≤≤ t
source: Liang and Lauterbur. “Principles of Magnetic Resonance Imaging” 2000
in rot. frame:
Gradient-Echo: Phase
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 16Gradient-Echo: Steady-State
αcos1)1(MM TR/T1
TR/T10SS
z −
−
−−⋅
=e
enumber of RF
example: WMT1 ~ 600 msTR = 25 ms
*TE/T2SSzxy sinMMS −⋅=∝ eα
Mz steady-state:
Mxy steady-state:source: Reiser and Semmler. “Magnetresonanztomographie” 2002
dephasing oftransversal
magnetization
9
Seite 9
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 17Gradient-Echo: FLASH
Haase et al. JMR 1986
FLASH signal: 3214434421 actor*T2
*TE/T2
actorT1
1
1FLASH cosE1
sin)E1(Sf
f
e−
−
−
⋅−−
⋅=ααρ with E1 = exp(-TR/T1)
Ernst-angle: ( )TR/T1E arccos −= eα
FLASH: “fast low angle shot”
contrast [a.u.] contrast [a.u.]
T1-contrast
ρ-contrast
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 18
SE GE
α = 90° / 180°TE = 20 ms
TR = 600 ms
Taq: minutes
α = 25°TE = 7 ms
TR = 20 ms
Taq: seconds !!
Gradient-Echo: Measuring Time
10
Seite 10
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 19Gradient-Echo: Examples I
spin-echoTR = 680 msTE = 15 msα = 90°
T1-contrast
3D FLASHTR = 60 msTE = 40 msα = 25°
T2*-contrast
2D FLASHTR = 170 ms
TE = 5 msα = 70°
T1-contrast
- no T2 contrast with gradient-echo technique !
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 20
CT SETE = 20 ms
GETE = 20 ms
normal bone
osteoporosis
Gradient-Echo: Examples II
susceptibilityeffect
11
Seite 11
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 21Gradient-Echo: FISP
- FISP: “fast imaging with steady precession”- steady-state free precession sequence: SSFP- no spoiler, all phases are compensated- Mxy contributes to Mz→ more signal !- FISP signal for TR<<T1 and TR<<T2*:
Oppelt et al. Electromedica 1986
321444444 3444444 21 factor*T2
*TE/T2
factor*T1/T2
FISP cos)T1/T2*1(*T1/T21sinS
−
−
−
⋅⋅−++
⋅= eα
αρ
- FISP contrast: T1/T2*water: ~ 1, all other tissues: ~ 10
slice selection gradient
gradient-echo
phase encoding gradient
frequency encoding gradient
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 22Gradient-Echo: FISP Problem
concept• transverse magnetization is not spoiled
(as in FLASH)• complete refocusing of all gradients in
one TR
problem• signal equation yields T1/T2 contrast• susceptibility differences can cause
artifacts• high flip angles (SAR)
solution• strong gradients• very short TR (< 5 ms)
12
Seite 12
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 23Gradient-Echo: FISP Contrast
- FISP sequence parameters:TR/TE/α = 2.6 ms / 1.4 ms / 55°⎟⎟
⎠
⎞⎜⎜⎝
⎛+−
=1*T1/T21*T1/T2arccosoptα
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
- chemical shift between protons of water H2O and fatty tissue CH2 is about 3.5 ppm or 220 Hz at 1.5 T
- 1/220 Hz = 4.55 ms →TE = 4.55, 9.1 … ms fat / water: “in-phase”TE = 6.8, 11.4 … ms fat / water: “opposed-phase”
B0
90°
α
RF
S
fat water
W WFF
“in-phase” TE = 9.1 ms
“op.-phase” TE = 6.8 ms
13
Seite 13
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 25Spin-Echo: Fat-Suppression
- no fat-water separation possible since refocusing 180° pulse- preparation by frequency selective 90°-pulse to select only fat resonance (CHESS) and to destroy fat signal by spoiling
B0
90°180°
90°180°
RFS
fat water W
W
F
F
no fat-suppression with fat-suppression
x
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 26Spin-Echo: Contrast Agent
- intravenous injection of 0.1 mmol/kg paramagnetic contrast agent Gd-DTPA- shortens locally T1 (ca. 1000 ms → 100 ms)- signal increase at SE imaging by a factor of 2 - 4 !- detects disruption of blood brain barrier → tumor grading !- also ferromagnetic nano-particles (SPIO ~ 100 nm, USPIO < 50 nm) available → liver
+
++
+
++
++
+
e-N S
Gd-DTPA complex
Gd
14
Seite 14
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 27
(by the different changes in relaxation times after their injection)
2. negative contrast agents- short T2 relaxation time - dark on MRI (decreased signal intensity)- small aggregates often termed superparamagnetic iron oxide(SPIO) and ultrasmall superparamagnetic iron oxides (USPIO) - crystalline iron oxide core and a shell of polymer (e.g. dextrane)
1. positive contrast agents- a reduction in the T1 relaxation time (increased signal intensity)- bright on MR images- small molecular weight compounds containing gadolinium, iron ormanganese
- unpaired electrons in their outer shells
Contrast Agent: Classification
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 28
• good solubility in water • high thermodynamic and possibly kinetic stability to ensure against the in vivo release of toxic Gd3+ ions and free ligand molecules
• short T1 relaxation time
gadolinium:
• large magnetic moment • long electron spin relaxation time (~10-9 s at the magnetic field strengths of interest for MRI applications)
• optimum efficiency for nuclear spin relaxation of the interacting nuclei
Parker and Williams. J Chem Soc 1996 Aime et al. Chem Soc Rev 1998
Contrast Agent: Gd Requirements
15
Seite 15
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 29
- all considered safe enough for clinical use
- octadentate ligands: one water molecule in the inner coordination sphere - rapid exchange with the bulk solvent to affect the relaxation of all solvent protons
Paul-Roth and Raymond. Inorg Chem 1995
Contrast Agent: Gd Ligands and Complexes
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 30
Gadopentetate(Magnevist)
courtesy: Weinmann, Schering AG
Contrast Agent: Gd Spherical Models
Gadoterate(Dotarem)
Gadodiamide(Omniscan)
Gadoteridol(ProHance)
16
Seite 16
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 31
• octadentate coordination of gadolinium
• one free coordination side is occupied by a water molecule
• relationships between the chemical structure and the factors determining the ability to enhance the water protons relaxation rates
• for a positive CA → short T1 time → high longitudinal relaxation rate
Contrast Agent: Gd-DTPA Complex
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 32
dipolar interaction between the paramagnetic ion and the proton nuclei of coordinated water molecules
- GRASE: “gradient and spin-echo” technique- TGSE: “turbo-gradient-spin-echo” technique- TSE technique mixed with GE’s- k-space variability: SE’s in center of k-space
GE’s at border of k-space
- volunteer high resolution imaging:GRASE: 2030/112/90° ms
- resolution: 0.4 x 0.4 x 2 mm3 !
Oshio and Feinberg. MRM 1991
spin-echo
gradient-echoes
27
Seite 27
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 53
conventional1 echo / TR
hybridmultiple echoes / TR
single-shotall echoes / TR
SEGE / FLASH
FLAIRMP-RAGE
FISPPSIF
TSETGSE / GRASE
HASTEEPI
Summary: K-Space Scan Options
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 54Summary: K-Space / Speed
spin-echoes
gradient-echoes
spee
d
28
Seite 28
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad
12/9/2008 | Page 55Sequence Family
RUPRECHT-KARLS-UNIVERSITY HEIDELBERG
Computer Assisted Clinical MedicineProf. Dr. Lothar Schad