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M R I Pulse Sequences Jerry Allison Ph.D.. 1017 pages 1017 pages ©2004 2.

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Presentation on theme: "M R I Pulse Sequences Jerry Allison Ph.D.. 1017 pages 1017 pages ©2004 2."— Presentation transcript:

1 M R I Pulse Sequences Jerry Allison Ph.D.

2 1017 pages 1017 pages ©2004 2

3 Outline I. Spin Echo Imaging MultiplanarMultisliceOblique II. Inversion Recovery (IR) III. Gradient Recalled Echo IV. Three Dimensional (Volume) Techniques Techniques V. Fast Imaging Techniques VI. Echoplanar Imaging

4 Image Contrast Image contrast in radiography and CT is based upon a few properties of the tissues or contrast agents involved: Image contrast in radiography and CT is based upon a few properties of the tissues or contrast agents involved: - physical density (g/cc) - physical density (g/cc) - electron density (electrons/cc) - electron density (electrons/cc) - atomic number - atomic number 4

5 Image Contrast Contrast in MRI is more complex and depends on many properties/parameters, which can be classified into “intrinsic” properties and “extrinsic” parameters. Intrinsic properties relate directly to the tissue. Extrinsic parameters relate to the characteristics of the MR imager and the details of the “MRI Pulse sequence” used for imaging. Contrast in MRI is more complex and depends on many properties/parameters, which can be classified into “intrinsic” properties and “extrinsic” parameters. Intrinsic properties relate directly to the tissue. Extrinsic parameters relate to the characteristics of the MR imager and the details of the “MRI Pulse sequence” used for imaging. 5

6 Proton density Proton density T 1 relaxation T 1 relaxation T 2 relaxation T 2 relaxation T 2 * relaxation T 2 * relaxation - magnetic susceptibility - magnetic susceptibility Diffusion Diffusion Magnetization transfer Magnetization transfer -cross relaxation -cross relaxation 6 Intrinsic Properties

7 Chemical Shift Chemical Shift Temperature Temperature Perfusion Perfusion Changes in tissue composition (e.g. age) Changes in tissue composition (e.g. age) Viscosity Viscosity Physiologic motion Physiologic motion Bulk flow Bulk flow Blood Blood CSF CSF 7

8 Extrinsic Parameters Magnetic field strength Magnetic field strength -static field -static field -gradient field -gradient field Magnetic field homogeneity Magnetic field homogeneity Hardware and software parameters Hardware and software parameters -coil selection -coil selection -number of slices acquired -number of slices acquired -slice thickness and gap -slice thickness and gap 8

9 Extrinsic Parameters Hardware and software parameters Hardware and software parameters -slice location -slice location -slice orientation -slice orientation -number of averages or excitations -number of averages or excitations -RF pulse shape (#sinc lobes) -RF pulse shape (#sinc lobes) -RF transmitter bandwidth -RF transmitter bandwidth -RF receive bandwidth -RF receive bandwidth -pixel size -pixel size -matrix size -matrix size -field of view -field of view 9

10 Extrinsic Parameters -acquisition mode ( 2D / 3D ) -acquisition mode ( 2D / 3D ) -artifact suppression -artifact suppression -physiologic triggering / gating -physiologic triggering / gating -orientation of phase and frequency -orientation of phase and frequency encode gradients encode gradients 10

11 Extrinsic Parameters RF pulse sequences RF pulse sequences -inversion recovery -inversion recovery -spin echo -spin echo -gradient recalled echo -gradient recalled echo -fast scan sequences -fast scan sequences -echoplanar (single shot techniques) -echoplanar (single shot techniques) 11

12 Extrinsic Parameters Pulse sequence parameters Pulse sequence parameters -repetition time (TR) -repetition time (TR) -echo time (TE) -echo time (TE) -inversion time (TI) -inversion time (TI) -flip angle (  ) -flip angle (  ) -echo train length -echo train length Contrast enhancing agents Contrast enhancing agents 12

13 MRI Pulse Sequences An MRI pulse sequence dramatically impacts the appearance of an MRI image. An MRI pulse sequence dramatically impacts the appearance of an MRI image. 13

14 14 Spin Echo Pulse Sequences Spin Echo Pulse Sequences T2 weighted PD weighted T1 weighted TR 510 TE 14 2min 7sec for 17 slices TR 4500 TE 15eff (ETL7) 2min 39sec for 24 slices TR 4500 TE 105eff (ETL7) 2min 39sec for 24 slices

15 15 Inversion Recovery Gradient Echo Pulse Sequence TR 12.1 TE 5.4 3min 11sec for 160 slices

16 MRI Pulse Sequences More specifically, an MRI pulse sequence is a “sequence” of temporal waveforms: More specifically, an MRI pulse sequence is a “sequence” of temporal waveforms: Radiofrequency (RF) pulses Radiofrequency (RF) pulses Gradient (magnetic field) pulses Gradient (magnetic field) pulses Data acquisiton intervals Data acquisiton intervals 16

17 17 Here is a pulse- sequence diagram. This shows a timeline for: 1) RF pulses; 2) gradient amplitudes for Gx, Gy, Gz; 3) the readout (i.e., A/D), and 4) the signal of the excited nuclei.

18 Multiplanar Imaging Axial, sagittal, and coronal images can be acquired as follows: Axial, sagittal, and coronal images can be acquired as follows: 18 Notice that for each plane, the choice of axis for phase and frequency encoding can vary.

19 MRI Image Weighting Many MRI images are described as: Many MRI images are described as: Proton density weighted Proton density weighted T1 weighted T1 weighted T2 weighted T2 weighted (and T2* weighted) (and T2* weighted) 19

20 20 Spin Echo Images Spin Echo Images T2 weighted PD weighted T1 weighted TR 510 TE 14 2min 7sec for 17 slices TR 4500 TE 15eff (ETL7) 2min 39sec for 24 slices TR 4500 TE 105eff (ETL7) 2min 39sec for 24 slices

21 Proton Density Weighting Images are (largely) weighted by the mobile hydrogen content of the tissues (water and fat). Images are (largely) weighted by the mobile hydrogen content of the tissues (water and fat). PD: FAT < WM < GM < CSF PD: FAT < WM < GM < CSF PD images: CSF > GM > WM > Fat PD images: CSF > GM > WM > Fat 21

22 Proton Density Weighting Proton Density Proton Density -The nucleus of most hydrogen atoms is a single particle: the proton -The nucleus of most hydrogen atoms is a single particle: the proton -The number of “mobile” hydrogen nuclei per voxel directly affects the intensity of the voxel in an MRI image (for all image weightings). -The number of “mobile” hydrogen nuclei per voxel directly affects the intensity of the voxel in an MRI image (for all image weightings). -Proton Density Weighting emphasizes proton density (as opposed to t1, t2 or T2*) -Proton Density Weighting emphasizes proton density (as opposed to t1, t2 or T2*) -Total proton densities -Total proton densities -CSF g H/cc -CSF g H/cc -Grey Matter g H/cc -Grey Matter g H/cc -White Matter g H/cc -White Matter g H/cc -Fat 0.1 g H/cc -Fat 0.1 g H/cc - Protons in lung tissue volume ~ 0.01 g H/cc - Protons in lung tissue volume ~ 0.01 g H/cc So, one of many problems with lung imaging is the low proton density per volume, leading to very low SNR. 22

23 Proton Density Weighting -Although white matter and grey matter have very similar proton density; they are differentiated in MRI by their lipid and water content. -Although white matter and grey matter have very similar proton density; they are differentiated in MRI by their lipid and water content. Lipid (g H / cc) Water (g H / cc) Lipid (g H / cc) Water (g H / cc) Grey Matter Grey Matter White Matter White Matter

24 T1 Weighting Images demonstrate good contrast between soft tissue types (because different tissues have different “T1” values). Images demonstrate good contrast between soft tissue types (because different tissues have different “T1” values). 24

25 T2 Weighting Images demonstrate good contrast between normal tissue and pathology (because many pathologies have elevated “T2” values due to increased free water content). Images demonstrate good contrast between normal tissue and pathology (because many pathologies have elevated “T2” values due to increased free water content). 25

26

27 T1, T2 Weighting In images of the head In images of the head T1: FAT < WM < GM < CSF T1: FAT < WM < GM < CSF T1 images: FAT > WM > GM > CSF T1 images: FAT > WM > GM > CSF T2: FAT < WM < GM < CSF T2: FAT < WM < GM < CSF T2 images: CSF > GM > WM > FAT T2 images: CSF > GM > WM > FAT Careful: CSF or Fat can be suppressed Careful: CSF or Fat can be suppressed 27

28 Pulse Sequence Families Spin Echo: SE Spin Echo: SE Gradient Echo: Gradient Echo: GEGE Gradient Recalled Echo (GRE)Gradient Recalled Echo (GRE) Field Echo (FE)Field Echo (FE) Inversion Recovery: IR Inversion Recovery: IR STIR: short tau inversion recoverySTIR: short tau inversion recovery Fat suppressionFat suppression FLAIR: fluid attenuated inversion recoveryFLAIR: fluid attenuated inversion recovery Fluid (CSF) suppressionFluid (CSF) suppression 28

29 Spin Echo Imaging Easy to control image weighting with SE Easy to control image weighting with SE T1 weightedT1 weighted T2 weightedT2 weighted PD weightedPD weighted 29

30 Spin Echo Imaging The Spin Echo imaging technique has the advantage that it is not as sensitive to static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue. The Spin Echo imaging technique has the advantage that it is not as sensitive to static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue. 30

31 31 Spin Echo Imaging

32 Spin Echo Imaging Spin Echo Imaging The pulse sequence must be repeated many times to produce an MRI image. The time interval between each execution of the pulse sequence is termed the Repetition Time (TR). The pulse sequence must be repeated many times to produce an MRI image. The time interval between each execution of the pulse sequence is termed the Repetition Time (TR). 32

33 Spin Echo Imaging Spin Echo Imaging The value of the repetition time (TR) and the echo time (TE) can be varied to control contrast in spin echo imaging. For example: The value of the repetition time (TR) and the echo time (TE) can be varied to control contrast in spin echo imaging. For example: TR = 2000 msec TE = 20 msec Proton Density Weighting TR = 2000 msec TE = 20 msec Proton Density Weighting TR = 2000 msec TE = 80 msec T 2 Weighting TR = 2000 msec TE = 80 msec T 2 Weighting TR = 600 msec TE = 20 msec T 1 Weighting TR = 600 msec TE = 20 msec T 1 Weighting 33

34 34 Fast Spin Echo Pulse Sequence (FSE) Turbo Spin Echo (TSE) Careful: Fat can be excessively bright on FSE images (j-coupling)

35 Gradient Recalled Echo Gradient recalled echo techniques have great versatility. A variety of contrasts can be produced while imaging rapidly. Gradient recalled echo techniques have great versatility. A variety of contrasts can be produced while imaging rapidly. GRE techniques include: GRE techniques include: GRASS, SPGR, FLASH, FISP, PSIF and many, many others. GRASS, SPGR, FLASH, FISP, PSIF and many, many others. 35

36 36

37 37 MIP MIP (Maximum Intensity Projection) Gradient Recalled Echo Images 2D-FLASH TR 25msec TE 9msec a = 35 o 5.7sec per slice

38 38 Multi Planar GRASS mixed T1/T2 weighting TR 500msec TE 13msec 2NEX a=60 o 3min 14 sec for 15 slices Gradient Recalled Echo Image

39 Gradient Recalled Echo Exceptions are: Exceptions are: 1. The creation of the echo is accomplished solely by gradient magnetic fields (no 180 o RF pulse). 1. The creation of the echo is accomplished solely by gradient magnetic fields (no 180 o RF pulse). 2. Deposition of RF energy in the patient is lower since the 180 o RF pulses are not used (less heating of patient tissues). 2. Deposition of RF energy in the patient is lower since the 180 o RF pulses are not used (less heating of patient tissues). 3. Static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue are NOT corrected by gradient recalled echo techniques. 3. Static inhomogeneity of the magnet and inhomogeneity caused by magnetic susceptibility of patient tissue are NOT corrected by gradient recalled echo techniques. 4. T2 contrast becomes T2* contrast. 4. T2 contrast becomes T2* contrast. 39

40 Gradient Recalled Echo 4. The initial flip angle is frequently chosen to be less than 90 o. The flip angle in gradient recalled echo techniques is called . 4. The initial flip angle is frequently chosen to be less than 90 o. The flip angle in gradient recalled echo techniques is called . The optimum value of  for a particular TR and tissue having spin lattice relaxation T 1 is called the Ernst angle. The optimum value of  for a particular TR and tissue having spin lattice relaxation T 1 is called the Ernst angle. 40 e -TR T1T1T1T1 cos(  e ) =

41 Gradient Recalled Echo 5. 3D or volume imaging can be accomplished (resulting in thinner slices). 5. 3D or volume imaging can be accomplished (resulting in thinner slices). 41 Vancouver, BC courtesy of Dr. Rawson Vancouver, BC courtesy of Dr. Rawson

42 Three Dimensional Volume Techniques 3D voxels are isotropic (or nearly isotropic). The voxels are the same size in all 3 dimensions. The dimensions of a typical 3D voxel are 3D voxels are isotropic (or nearly isotropic). The voxels are the same size in all 3 dimensions. The dimensions of a typical 3D voxel are 1 mm x 1 mm x 1 mm. The acquisition of isotropic voxels enables the data set to be reformatted into any oblique plane without significant loss of resolution using Post Processing Techniques. 1 mm x 1 mm x 1 mm. The acquisition of isotropic voxels enables the data set to be reformatted into any oblique plane without significant loss of resolution using Post Processing Techniques. 42

43 43 MPRAGE: Magnetization Prepared Rapid Gradient Echo TR 11.4msec TE 4.2msec a=12 o 1.4mm 6min 55sec for 120 slices (168mm slab) Uses Inversion Recovery Three Dimensional Volume Image

44 Inversion Recovery (IR) Inversion recovery pulse sequences are useful for: Inversion recovery pulse sequences are useful for: Suppression of selected tissues (e.g. orbital fat, liver screening, fatty tumors, CSF) Suppression of selected tissues (e.g. orbital fat, liver screening, fatty tumors, CSF) Creation of heavily T1 weighted images without a dominant contribution from fat (e.g. brain, liver and musculoskeletal imaging). Creation of heavily T1 weighted images without a dominant contribution from fat (e.g. brain, liver and musculoskeletal imaging). 44

45 Inversion Recovery (IR) A basic IR spin echo pulse sequence consists of a 180 o inversion pulse, followed by an inversion time TI, then a 90 o RF pulse. A basic IR spin echo pulse sequence consists of a 180 o inversion pulse, followed by an inversion time TI, then a 90 o RF pulse. 45

46 46 Consider two voxels, one of fat and one of H 2 O This method of fat suppression is sometimes called “short TI” inversion recovery or STIR imaging.

47 Inversion Recovery (IR) In spin echo inversion recovery imaging sequences, the 90 o pulse is followed by a 180 o pulse in order to produce a spin echo at time TE following the 90 o pulse In spin echo inversion recovery imaging sequences, the 90 o pulse is followed by a 180 o pulse in order to produce a spin echo at time TE following the 90 o pulse 47

48 48

49 49 FLAIR: fluid attenuated IR (T2 weighted spin echo) Inversion time: 2.5sec (CSF is suppressed) TR 10sec TE 119msec (ETL7) 3min 49sec for 19 slices IR Image vs STD T2 weighting

50 50 GE MRI Image Annotation

51 51 GE MRI Image Annotation

52 52 GE MRI Image Annotation

53 53 GE MRI Image Annotation

54 54 GE MRI Image Annotation

55 55 GE MRI Image Annotation


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