Bildgebende Systeme in der Medizin

Transkript

1 Hochschule Mannheim 11/24/2011 Page 1/27 Bildgebende Systeme in der Medizin Magnet Resonanz Tomographie II: Anwendungen RF Methoden und Bildgebung Lehrstuhl für Computerunterstützte Klinische Medizin Medizinische Fakultät Mannheim, Universität Heidelberg Theodor-Kutzer-Ufer 1-3 D Mannheim, Deutschland 11/24/2011 Page 2/27 Spin-Echo Sequence k-space: after 1. TR k-space: after 2. TR k-space: after 256. TR k y k x

2 11/24/2011 Page 3/27 Spin-Echo Images S TR/T1 TE/T2 = ρ [ e e123 SE ] T1 factor T2 factor Pw TR = 2775 ms TE = 17 ms T2w TR = 2775 ms TE = 102 ms T1w TR = 575 ms TE = 14 ms - SE gold standard technique for T1 and T2 morphology 11/24/2011 Page 4/27 Spin-Echo Contrast: TR, TE TE ms 40 - no contrast SNR low T1-weighted T2-weighted proton-weighted 10 ms ms ms TR

3 Spin-Echo: T1 Dependency 11/24/2011 Page 5/27 T1 - factor GM: T1 = 970 ms WM: T1 = 600 ms WM GM T1-factor: [1-exp(-TR/T1)] T1 - contrast contrast C = S S A A S + S B B source: Reiser and Semmler. Magnetresonanztomographie 2002 Spin-Echo: T2 Dependency 11/24/2011 Page 6/27 T2 - factor GM: T2 = 110 ms WM: T2 = 90 ms T2-factor: exp(-te/t2) WM GM T2 - contrast source: Reiser and Semmler. Magnetresonanztomographie 2002

4 Spin-Echo: TR, TE 11/24/2011 Page 7/27 source: Schlegel and Mahr. 3D Conformal Radiation Therapy: A Multimedia Introduction to Methods and Techniques" 2007 Spin-Echo: TR, TE 11/24/2011 Page 8/27 SE contrast between grey and white matter

5 Spin-Echo: Multi-Slice 11/24/2011 Page 9/27 - interleaved SE measurement measurement #1 1. slice measurement #2 2. slice 3. slice 4. slice 5. slice - multi-slice SE TR = 600 ms, TE = 10 ms about 60 slices can be measured simultaneously! Inversion Recovery Sequence 11/24/2011 Page 10/27 M z M z source: Reiser and Semmler. Magnetresonanztomographie 2002

6 Inversion Recovery Images 11/24/2011 Page 11/27 S TI/T1 TR/T1 TE/T2 = ρ [1 2e + e e IR ] T1 factor T2 factor Inversion Recovery Contrast 11/24/2011 Page 12/27 contrast [a.u.] T1-contrast T2-contrast T2-contrast - WM and GM contrast dependency on TI, TR and TE - IR gold standard technique for T1 contrast source: Reiser and Semmler. Magnetresonanztomographie 2002

7 11/24/2011 Page 13/27 Gradient-Echo Sequence spoiler z M z α M M xy x,y - example: α = 20 M z - reduction by 6% M xy - value 34% of M z!! 11/24/2011 Page 14/27 Gradient-Echo: Phase in rot. frame: 0 t τ τ t 2τ t φ ( x, t) = γ G x dtˆ = γ G 0 φ ( x, t) = γ G x x t + γ G = γ G x x τ x τ + γ G t x x x x t x dtˆ x ( t τ ) source: Liang and Lauterbur. Principles of Magnetic Resonance Imaging 2000

8 Gradient-Echo: Steady-State 11/24/2011 Page 15/27 example: WM T1 ~ 600 ms TR = 25 ms dephasing of transversal magnetization M z steady-state: number of RF M SS z TR/T1 M0 (1 e ) = TR/T1 1 e cosα source: Reiser and Semmler. Magnetresonanztomographie 2002 M xy steady-state: S M xy = M SS z sinα e TE/T2* Gradient-Echo: FLASH 11/24/2011 Page 16/27 contrast [a.u.] contrast [a.u.] T1-contrast ρ-contrast (1 E1)sinα TE/T2* FLASH signal: SFLASH = ρ e 1 E cosα T2* factor Haase et al. JMR 1986 T1 factor TR/T1 Ernst-angle: α = arccos ( e ) E with E 1 = exp(-tr/t1) FLASH: fast low angle shot

9 11/24/2011 Page 17/27 Gradient-Echo: Measuring Time SE GE α = 90 / 180 TE = 20 ms TR = 600 ms T aq : minutes α = 25 TE = 7 ms TR = 20 ms T aq : seconds!! Gradient-Echo: Examples I 11/24/2011 Page 18/27 spin-echo TR = 680 ms TE = 15 ms α = 90 T1-contrast 2D FLASH TR = 170 ms TE = 5 ms α = 70 T1-contrast - no T2 contrast with gradient-echo technique! 3D FLASH TR = 60 ms TE = 40 ms α = 25 T2*-contrast

10 11/24/2011 Page 19/27 Gradient-Echo: Examples II CT SE TE = 20 ms GE TE = 20 ms normal bone susceptibility effect osteoporosis Gradient-Echo: FISP 11/24/2011 Page 20/27 slice selection gradient gradient-echo - FISP: fast imaging with steady precession - steady-state free precession sequence: SSFP - no spoiler, all phases are compensated - M xy contributes to M z more signal! - FISP signal for TR<<T1 and TR<<T2*: phase encoding gradient frequency encoding gradient S FISP sin α TE/T2* = ρ e 1+ T1/T2* + (1 T1/T2*) cos α T2* factor T1/T2* factor - FISP contrast: T1/T2* water: ~ 1, all other tissues: ~ 10 Oppelt et al. Electromedica 1986

11 Gradient-Echo: FISP Problem 11/24/2011 Page 21/27 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) Gradient-Echo: FISP Contrast 11/24/2011 Page 22/27 α opt T1/T2 * 1 = arccos T1/T2 * FISP sequence parameters: TR/TE/α = 2.6 ms / 1.4 ms / 55

12 Gradient-Echo: Fat-Water Separation 11/24/2011 Page 23/27 - chemical shift between protons of water H 2 O and fatty tissue CH 2 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 in-phase TE = 9.1 ms RF S α op.-phase TE = 6.8 ms fat B 0 water 90 F W F W Spin-Echo: Fat-Suppression 11/24/2011 Page 24/27 x - 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 no fat-suppression with fat-suppression RF S B 0 fat water F W F W

13 11/24/2011 Page 25/27 Spin-Echo: Contrast Agent 11/24/2011 Page 26/ Gd N e - S + Gd-DTPA complex - intravenous injection of 0.1 mmol/kg paramagnetic contrast agent Gd-DTPA - shortens locally T1 (ca 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

14 11/24/2011 Page 27/27 Contrast Agent: Classification (by the different changes in relaxation times after their injection) 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 or manganese - unpaired electrons in their outer shells 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) 11/24/2011 Page 28/27 Contrast Agent: Gd Requirements good solubility in water high thermodynamic and possibly kinetic stability to ensure against the in vivo release of toxic Gd 3+ 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

15 11/24/2011 Page 29/27 Contrast Agent: Gd Ligands and Complexes - 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 all considered safe enough for clinical use 11/24/2011 Page 30/27 Contrast Agent: Gd Spherical Models Gadopentetate (Magnevist) Gadodiamide (Omniscan) Gadoterate (Dotarem) Gadoteridol (ProHance) courtesy: Weinmann, Schering AG

16 11/24/2011 Page 31/27 Contrast Agent: Gd-DTPA Complex 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 11/24/2011 Page 32/27 Gd-DTPA Relaxation Mechanism dipolar interaction between the paramagnetic ion and the proton nuclei of coordinated water molecules 1. the number of metal bound water molecules q 2. molecular reorientation time τ R 3. electron-spin relaxation time τ S 4. and chemical exchange time τ M described in detail by the Solomon-Bloembergen-Morgan theory Bloemenbergen and Morgan. J Chem Phys 1961 Solomon. Phys Rev 1955

17 Spin-Echo: Gd-DTPA Example 11/24/2011 Page 33/27 - typical measuring protocol in brain diagnostics SE T1 w SE T2 w SE T1 w patient: glioblastoma intravenous injection of 0.1 mmol/kg Gd-DTPA 11/24/2011 Page 34/27 MRI: Advantage - Disadvantage - advantage - disadvantage best soft tissue contrast of all imaging modalities multi-planar slice orientation no ionizing radiation high flexibility of the method due to complex signal dependency of physical parameters costs availability contraindication It s fun!!

18 RUPRECHT-KARLS 11/24/2011 Page 35/27 Summary V source: Liang and Lauterbur. Principles of Magnetic Resonance Imaging 2000 RUPRECHT-KARLS 11/24/2011 Page 36/27 Finale