Part III: Sequences and Contrast

Transcription

1 Part III: Sequences and Contrast Contents T1 and T2/T2* Relaxation Contrast of Imaging Sequences T1 weighting T2/T2* weighting Contrast Agents Saturation Inversion Recovery

2 JUST WATER? (i.e., proton density PD) Basic Sequences and Contrast

3 MRI Contrast Mechanisms Proton density (PD) Chemical Shift Imaging (water-fat) Susceptibility Imaging (SWI) Spin-Lattice Relaxation Diffusion weighted Imaging Flow / Motion Imaging Spin-Spin Relaxation Temperature Mapping Functional Imaging (BOLD) Relaxation from Macroscopic Field inhomogeities Elastography (MRE) Perfusion Imaging Contrast enhanced MRI, cell tracking, SPIOs, USPIOs, MR Angiography Spectroscopy and many more!

4 Basic Contrast Mechanism: PD, T1, T2 After excitation, the magnetization returns back to thermal equilibrium M () t M e x x,0 M () t M e y y,0 tt / tt / 1 M t M M e M z 2 tt / () ( ) 2 z,0 0 0 Equilibrium M 0 PD

5 Basic Contrast Mechanism: PD, T1, T2 PD

6 Basic Contrast Mechanism: PD, T1, T2 PD

7 Basic Contrast Mechanism: PD, T1, T2 PD

8 Basic Contrast Mechanism: PD, T1, T2 PD

9 Basic Contrast Mechanism: PD, T1, T2 PD

10 Basic Contrast Mechanism: PD, T1, T2 PD

11 Basic Contrast Mechanism: PD, T1, T2 PD / M () t M 1 e tt z 0 1 / M () t M (0) e tt xy xy 2

12 Basic Contrast Mechanism: PD, T1, T2 An MRI sequnce consists of a series of (i) excitation pulses (RF pulses), (ii) gradients and (iii) signal readout events (ADC: analog digital converter). Consider the following simple sequence scheme ( Analysis under the Assumptions: T2* = T2 ) waiting time >> T1 (full recovery) 90 pulse 90 pulse TR: Repetition time of RF pulse (time between excitations) TE: Echo-Time (time between excitation and signal acquisition)

13 Basic Contrast Mechanism: PD, T1, T2 PD S TR TE PD 1 exp( ) exp( ) T1 T2

14 Basic Contrast Mechanism: PD, T1, T2 S TR TE PD 1 exp( ) exp( ) T1 T2

15 Basic Contrast Mechanism: PD, T1, T2 S TR TE PD 1 exp( ) exp( ) T1 T2

16 Basic Contrast Mechanism: PD, T1, T2 image weighting T1 T2 PD seq. property TR TE

17 Basic Contrast Mechanism: PD, T1, T2 image weighting T1 T2 PD seq. property TR TE TR ~ T1 (~1000ms) no effect TR >> T1 (~5000ms) S TR TE PD 1 exp( ) exp( ) T1 T2

18 Basic Contrast Mechanism: PD, T1, T2 image weighting T1 T2 PD seq. property TR TE TR ~ T1 (~1000ms) no effect no effect TE ~ T2 (~70ms) TR >> T1 (~5000ms) TE << T2 (~10ms) S TR TE PD 1 exp( ) exp( ) T1 T2

19 A Gradient Echo (GRE) Sequence TE TR TR: Repetition time TE: Echo time : excitation (flip) angle

20 Signal of the Gradient Echo Sequence Case I: = 90, TR >> T1 (full recovery) M z M 0 M xy M E M xy M z * 2 0 (saturation recovery) S PD E * 2 Convention: The uppercase + ( - ) denotes the magnetization immediately after (before) the RF pulse * * E1( t) exp( t/ T1) E2( t) exp( t/ T2 ) E1: exp( TR/ T1) E : exp( TE/ T ) * * 2 2

21 Signal of the Gradient Echo Sequence Case II: = 90, TR ~ T1 (partial recovery), TR >> T2 (full decay) M z (1 E ) M 1 0 M xy xy M z M E M * 2 0 (saturation recovery) * 1 2 S PD (1 E ) E Convention: The uppercase + ( - ) denotes the magnetization immediately after (before) the RF pulse * * E1( t) exp( t/ T1) E2( t) exp( t/ T2 ) E1: exp( TR/ T1) E : exp( TE/ T ) * * 2 2

22 Signal of the Gradient Echo Sequence Case III: < 90, TR ~ T1 (partial recovery), TR >> T2 (full decay) T1 recovery T1 decay polarization M M z xy cos M sin M z z M z EM M 0(1 E1) 1 z Action of RF pulse Action of TR M z 1 E1 xy 1 E1 M 0 m 1 xy E1 cos M0 E1 M sin 1 cos Convention: The uppercase + ( - ) denotes the magnetization immediately after (before) the RF pulse * * E1( t) exp( t/ T1) E2( t) exp( t/ T2 ) E1: exp( TR/ T1) E : exp( TE/ T ) * * 2 2

23 Contrast Behaviour of the Gradient Echo Sequence: PD, T1 S 1 E1 PD sin 1 E cos 1 PDw Ernst angle (max. signal): 1 E cos E1 T1w TR=3 T 1 saturation TR=0.2 T 1

24 Contrast Behaviour of the Gradient Echo Sequence: PD, T1 TR [msec] [deg]

25 Contrast of the Gradient Echo Sequence: T2* k-space GRE reads FID: T2*-weighted T2 : dephasing from field inhomogeneities T2: loss of transverse magnetization

26 Contrast of the Gradient Echo Sequence: T2* TE G x ADC T2* weighted GE: TR=200ms, =35 TE = 10 ms TE = 20 ms TE = 40 ms TE = 60 ms

27 A Spin-Echo Sequence Parameters: Repetition time (TR) & Time to Echo (or echo time TE)

28 A Spin-Echo Sequence k-space Echo time TE: time between excitation (0) and arrival a k-space center (3)

29 A Spin-Echo Sequence

30 A Multi-Echo Spin-Echo Sequence

31 Contrast of the Spin-Echo Sequence: T2 or T2* macroscopic field inhomogeneities changes are in the range of sec and thus much longer than the typical TE of the sequence phase changes from macroscopic field inhomogeneities are thus deterministic T T T * microscopic field fluctuations Interactions have very short correlation times: c ~ [sec] phase changes from microscopic field fluctuations are thus deterministic

32 Contrast of the Spin-Echo Sequence: T2 or T2* What happens to the spin echo? Phase evolution of a single spin (magnetic moments) spin-echo Macroscopic field inhomogeneities are rephased by the 180 pulse!

33 Contrast of the Spin Echo (SE) Sequence k-space SE reads echo: T2-weighted T2 : dephasing due to field inhomogeneities (between 90 and 180 pulse) T2 : rephasing due to field inhomogeneities (between 180 and spin echo) T2: loss of transverse magnetization

34 Contrast of the Spin Echo (SE) Sequence: PD, T2 SE: TR=6000ms, =90 TE = 10 ms TE = 40 ms TE = 70 ms TE = 100 ms

35 Contrast of the Spin Echo (SE) Sequence: PD, T1, T2 TR [msec] TE [msec]

36 T2* versus T2 weighting Gradient Echo Spin Echo TR=500ms, TE=10ms Advanced Imaging: GRE sequences can be used for susceptibility imaging Metallic Implants: GRE sequences show strong artifacts (signal loss)

37 Contrast Modification

38 Contrast Modification Tissue properties (nativ): PD, T1, T2 Contrast Agents contrast-enhanced (ce): PD, T1, T2

39 Contrast Agents (CA) MR Signal Intensity: PD, T1, T2 Principle: artificial shortening of T1 and T2 with paramagnetic contrast agent Gadolinium-DTPA

40 Contrast Agents (CA) MR Signal Intensity: PD, T1, T2 CA Paramagnetic agents: 1/ T 1/ T [ CA] r 2 2, nativ 2 1/ T 1/ T [ CA] r 1 1, nativ 1 Gadolinium-DTPA r 1 ~ r 2 ~ T [CA] : 0.0M (native) T 1 = 1000 ms [CA] : 0.5 M (not diluted): [CA] : 0.05 M (10 x diluted): [CA] : M (100 x diluted): T 1 = 0.44 ms T 1 = 4.4 ms T 1 = 42 ms

41 Contrast Agents (CA) MR Signal Intensity: PD, T1, T2 CA Paramagnetic agents: 1/ T 1/ T [ CA] r 2 2, nativ 2 1/ T 1/ T [ CA] r 1 1, nativ 1 Positive CA: low concentrated Gd-chelates. Predominantly reduction in T1 (electron-proton dipolar coupling). (Positive contrast in T1w-image)

42 Contrast Agents (CA) MR Signal Intensity: PD, T1, T2 CA Paramagnetic agents: 1/ T 1/ T [ CA] r 2 2, nativ 2 1/ T 1/ T [ CA] r 1 1, nativ 1 Positive CA: low concentrated Gd-chelates. Predominantly reduction in T1 (electron-proton dipolar coupling). (Positive contrast in T1w-image) Negative CA: superparamagnetic agents (SPIO, USPIO) in small crystalline structures (iron-oxide). Predominantly reduction in T2 (increased. (Negative contrast in T2w-image)

43 Contrast Modification Tissue properties (nativ): PD, T1, T2

44 Contrast Modification Tissue properties (nativ): PD, T1, T2

45 Contrast Modification: Saturation spoiler Magn. Preparation Host Sequence: ANY Spatial Saturation, Fat Suppression, Water Suppression,

46 Contrast Modification: Inversion Recovery TI T 1 (gray, white matter) Inversion time (TI) Magn. Preparation Host Sequence: ANY

47 Contrast Modification: Inversion Recovery Inversion time (TI) Magn. Preparation Host Sequence: ANY Inversion Pulses are used to induce T1-weighting onto Host - Sequence

48 Contrast Modification: MPRAGE (magnetization prepared rapid gradient echo) TI T 1 (gray, white matter) Inversion time (TI) Magn. Preparation Host Sequence: PD weighted 3D GRE Example 1: T1-weighting on GRE

49 Contrast Modification: MPRAGE (magnetization prepared rapid gradient echo) Inversion time (TI) Magn. Preparation Host Sequence: PD weighted 3D GRE Induce T1-weighting onto 3D PD GRE images to allow for fast acquisition of high resolution whole brain T1 images PD T1

50 Contrast Modification: FLAIR (fluid attenuated inversion recovery) Inversion time (TI) TI TR : S( CSF) 1 2 exp( ) 0! T1 Magn. Preparation Host Sequence: 2D mslc T2 (T)SE Example 2: T1-weighting on (T)SE

51 Contrast Modification: FLAIR (fluid attenuated inversion recovery) Inversion time (TI) Magn. Preparation Host Sequence: 2D mslc T2 (T)SE Supress the very strong hyperintense signal from fluids in T2w images T2 FLAIR

52 Contrast Modification: Overview PD, T1, T2, T2* CA native tissue PD, T1, T2, T2* magn. prep. imaging sequences sequence PDw, T1w, T2w, T2*w

53 Summary: Part III Contrast of gradient-echo and spin-echo sequences is modified from T1 and T2 relaxation. Gradient echo sequences use gradient recalled echoes, whereas spin-echo sequences use a spin-echo for signal readout. The echo in GRE sequences is T2*-weighted, whereas the echo in spin-echo sequences is T2-weighted. As a result, spin-echo sequences are less prone to susceptibility effects as compared to gradient echo sequences Contrast in gradient echo sequences is modified by the flip angle, by the repetition time and the echo time. Contrast in spin-echo sequences depends on the repetition time and the echo time.

54 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo Should the slice-selection gradient be applied before, during or after the RF excitation pulse? Before the RF excitation. During the RF excitation. After the RF excitation.

55 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo How is the slice thickness increased? Increase the transmitted RF bandwidth or the slice selection gradient strength. Increase the transmitted RF bandwidth, or decrease the slice selection gradient strength. Decrease the transmitted RF bandwidth, or increase the slice selection gradient strength.

56 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo How do signal differences between tissue types arise from differences in T1? We wait for different amounts of signal decay to occur before taking a signal measurement. We wait for different amounts of magnetisation recovery to occur before starting the MRI signal measurement process. We change the T1 of certain tissues.

57 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo How do signal differences between tissue types arise from differences in T2? We wait for different amounts of signal decay to occur before taking a signal measurement. We wait for different amounts of magnetisation recovery to occur before starting the MRI signal measurement process. We change the T2 of tissues

58 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo What does the frequency encoding gradient do? It moves net magnetisations into the xy-plane. It reads out the MRI signal. It causes a range of Larmor frequencies to exist.

59 Exercises: Part II & Part III Topics: imaging, k-space, gradient-echo, spin-echo How is proton-density weighting achieved? Short TR, short TE. Short TR, long TE. Long TR, short TE