MRI. When to use What sequences. Outline 2012/09/19. Sequence: Definition. Basic Principles: Step 2. Basic Principles: Step 1. Govind Chavhan, MD

Transcription

1 MRI When to use What sequences Govind Chavhan, MD Assistant Professor and Staff Radiologist The Hospital For Sick Children, Toronto Planning Acquisition Post processing Interpretation Patient history and clinical question Properties of the sequences to answer clinical question Outline Basic principles of MR imaging 4 groups of sequences and their subtypes Contrast, resolution and speed of acquisition offered by each sequence Comparison of sequences Why should we use a sequence in particular situation Time chart of interplay of parameters LM RF pulses Gradients for localization K-space filling Sequence: Definition Patient is placed in the magnet Longitudinal Magnetization formed Basic Principles: Step 1 RF pulse is sent Transverse Magnetization is formed Basic Principles: Step 2 Longitudinal Magnetization 1

2 Basic Principles: Steps 3 and 4 Signal from patient received by the coil and stored in k-space Fourier Transformation to form image TR TE Transverse Relaxation Basic Principles: Few other things Basic Principles: time No. of TRs No. of k-space lines 128x128 matrix = 128 k-space lines Spin-echo Gradient echo Inversion Recovery Echo Planar Sequences: Classification Spin-echo Sequences Gradient echo Spin-echo sequence: Design pulses Spin-echo sequence: Modifications Dual SE Fast (Multi) SE FRFSE Single-shot FSE ETL Turbo Factor 2

3 SNR Speed Soft Tissue 2012/09/19 TE Spin-echo sequences: Trade names Spin-echo sequence: Images SEQUENCES SIEMENS* GE * PHILIPS* T1-w SE Spin Echo sequences Conventional SE ( RF pulses) Double SE (90 0 followed by two RF pulses) Multi SE (90 0 followed by multiple RF pulses) SE PD/T2 Turbo SE SE PD/T2 Fast SE SE PD/T2 Turbo SE T1 SE Multi SE with flip-back 90 0 pulse RESTORE FRFSE DRIVE Single-shot Multi SE (Multi SE with half k-space filling) HASTE Single Shot FSE Ultrafast SE ETL <5 T2-w SE T2-w SE High TE T2 TSE TE 90 FRFSE GB TSE 3D SS FSE SS TSE slab Gradient Echo sequence: Design No pulse No 90 0 pulses T2* relaxation Gradient Echo sequence: Types Depends on what is done to residual TM after signal is received Spoiled- T1-w SS- T2-w GRE Sequences Residual TM SPOILED Residual TM REFOCUSED 3

4 Gradient Echo sequences: Trade names T1-w GRE Pre-Gd Late arterial Gradient Echo Sequences Siemens GE Philips A. Incoherent spoiled TM FLASH SPGR T1-FFE 3D versions 3D FLASH VIBE LAVA FAME THRIVE Portal venous Delayed venous B. Coherent/Rephased TM 1. Post excitation refocused (FID sampled) FISP GRASS FFE T1 FLASH 8 min 11 min 2. Pre-excitation refocused (Spin echo sampled) PSIF SSFP T2-FFE Dyn VIBE 3. Fully refocused (both FID & spin echo sampled) True FISP FIESTA Balanced FFE In-phase TE 4.6ms Out-phase TE 2.3ms T2-w GRE Inversion Recovery sequence: Design TrueFISP MPGR PSIF Inversion pulse T1 differences increased Contrast between H2O and fat increased Tissue suppression TI = Time to invert T2 FFE DESS Inversion Recovery sequence: Types IR sequences: STIR vs FLAIR Depending on TI used Short TI ( ms) e.g. STIR Medium TI ( ms) e.g. MPRAGE Long TI ( ms) e.g. FLAIR 4

5 Resolution Speed T1 contrast Speed 2012/09/19 IR sequences: STIR vs FLAIR Echo Planar : Design STIR Short TI of ms Combined T1 and T2 weighting Fat, white matter are suppressed Mainly used in body imaging Cannot be used in post contrast Vs FLAIR Long TI of ms Heavily T2 weighted images CSF, water is suppressed Used in neuroimaging Can be used in post contrast imaging. Single TR SE-EPI GRE-EPI All k-space lines filled in a single TR Limitations: Very sensitive to susceptibility artifacts, Low resolution and SNR Multishot EPI to improve SNR- a portion of k-space is filled per TR Hybrid EPI combines SE (to reduce susceptibility) and GRE (speed) Diffusion, Perfusion and fmri T1-w 3-5 Min There is no free lunch in MRI 1-3 Min T1 FLASH Sec T1 VIBE T2 TSE T2-w 3-5 Min T2*-w STIR 4-6 Min T2FFE MPGR EPI T2* TruFisp SS TSE EPI T2 b= Sec In Sec BH In Sec BH 5

6 T1 contrast Speed 2012/09/19 Post Gd T1-w 3-5 Min What sequence? T1 TFE 1-3 Min Solid organ imaging Bowel imaging Fluid imaging imaging Vessel imaging Marrow imaging Cartilage imaging Sec THRIVE Solid organ imaging: Abd Lesion detection Characterization by pre and post Gd signal intensity T1-w: TSE, In-out-phase, pre dynamic 3D GRE T2 FSE FS T2-w: T2 TSE FS, STIR, Post Gd: dyn- T13D GRE, T1TSE FS Opt: single-shot if patient uncooperative and irregular breathing, Balanced- vascular anatomy Trufi T2 SS FSE FS Solid organ imaging: Brain T1-w: TSE, 3D T1 GRE T2-w: T2 TSE, FLAIR Post Gd: T13D GRE, T1TSE FS Congenital malformation/ dysplasia- Medium TI IR Opt: single-shot if patient uncooperative Lesion detection Characterization by pre and post Gd signal intensity Moving target Bowel Fetal Moving target Heart T1-w: TFE>TSE, pre 3D T1 GRE T2-w: Single-shot, Balanced Post Gd: T13D GRE, T1TFE FS T2 FSE FS T2 SS FSE Balanced SSFP T2/T1 T2 SS FSE FS Gd+ T1 FS Gd+ Thrive FS 6

7 SNR Thin sections T2 T1 T2 T2 Soft tissue, bone & vessel anatomy Thin sections T1-w and T2-w 3 planes TSE>TFE, FSE>SS Balanced- vascular anatomy T1-w and T2-w 3 planes TSE>TFE, FSE>SS Balanced- vascular anatomy T1 TFE T2 FSE T2 FSE FS T2 FSE FS T2 FFE Marrow STIR/T2 FS T1 Vessel MRA/MRV CEMRA Balanced TFE T1 SE thin continuity Patency T2 FSE FS T1 FSE T2 FFE 2D TOF MRV 7

8 Vessel Cartilage GRE PD 3D WATS MEDIC DWI 2D TOF MRV T2 FFE PD T2 FSE FS Cartilage OCD Sequences: Summary Sequences offer different contrast, resolution and speed of acquisition They should be used to suit our needs In general, SE offer better contrast and resolution and GRE offer speed T2 FFE PD T2 FSE FS 8