MRI Sequences: What to use for what

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1 MRI Sequences: What to use for what MRI basics T 1 and T 2 relaxation Common Imaging Protocols Mechanical function (cine) Tissue characterization LGE Edema imaging (T 2 weighted) T1 Special protocols MRA / Flow Summary Outline MRI Basics Measures signal from water (protons) Body is 60% water Each proton is like a small magnet Signal (contrast) depends on: Concentration Flow Local environment Blood vs. fat 3D vdw.png 1

2 MRI Basics 3 Easy Steps 1) Prepare sample Line up H + in strong magnetic field 2) Excite sample Excite H + using radio waves Excited H + precess in unison 3) Acquire signal Radio waves emitted by excited H + Turn signal into pictures Net Magnetization T 1 Relaxation Spin lattice or longitudinal relaxation Recovery of longitudinal magnetization Typical values (1.5T): Blood: 1650 ms Myocardium: 1175 ms Fat: 250 ms T 2 (and T 2* ) Relaxation Spin spin, or transverse relaxation Decay of excited transverse magnetization Typical values (1.5T): Blood: 250 ms Myocardium: 50 ms Fat: 60 ms 2

3 Mechanical Function Injury/Viability Anatomy Blood Velocity Perfusion Cine Imaging: LV RV Function Mechanical Function Short Axis Long Axis 3

4 LV Function Protocol: (15 min) Quantitative: Volumetrics: (ml/m2) Indexed to BSA LVEDVI LVESVI RVEDVI RVESVI LV mass / wall thickness LA volume LV Function Protocol: (15 min) Qualitative Morphology wall motion abnormalities Visual assessment of Mitral and Tricuspid valve function/morphology Indications: Monitor medical treatment Assess for ICD Cardio oncology Myocardial Infarction Healthy Cine RV stack + LV RV Short axis function: ARVC protocol RV volume RV Wall motion abnormalities Marcus FI, et al. Circulation 2010; 121 4

5 Cine Imaging Let s make a movie! Image Acquisition Single shot Segmented Gated segmented Imaging Image 1 Image 2 Image 3 A portion of several images is acquired every heartbeat Each image has a different cardiac phase Requires no motion between heartbeats (good breath holding!) 5

6 LGE + LV function: Chronic Cardiomyopathy Protocol Ischemic vs non ischemic vs infiltrative cardiomyopathies LGE: Myocardial Fibrosis Contrast agent (Gd) reduces T 1 values Contrast cannot enter intact cells Greater accumulation of contrast in scarred tissue Weinsaft JW et al. Cardiol Clin 2007;25:35 56 Inversion Recovery 250 ms 425 ms 1000 ms Image contrast varies with inversion time Healthy myocardium can be made dark by choosing a specific inversion time 6

7 LGE Imaging: Identifying Etiology Ischemic vs non ischemic vs infiltrative cardiomyopathy White, JA, Patel, MR. CMR in Heart Failure and the Cardiomyopathies Cardiology Clinics, 2007 Limitations of Weighted Imaging (More) Healthy? Scar Healthy? LGE imaging identifies scar relative to healthy myocardium Anatomic Late Enhancement Global or diffuse disease may not be readily identifiable T2 + (LV function + LGE) : Acute Cardiomyopathy Protocol T2 imaging: Highlights edema like/water signal T2 STIR vs T2 Mapping vs SPAIR 7

8 T 2 Imaging T 2 imaging is sensitive to pathological tissue changes Turbo spin echo imaging has good T 2 contrast, but is sensitive to flow artifacts?? Simonetti O et al. Radiology 1996;199:49 57 Triple Inversion Recovery Global INV Slice INV Global INV IMG Additional inversion can null both blood and fat Signal to noise penalty Termed Short Tau Inversion Recovery (STIR) STIR When to add... T2 Weighted Imaging Ischemic Non Ischemic Disease Acuity? Disease reversibility? LGE LGE T2 T2 8

9 Perfusion Imaging Short Axis Special Protocols: T2* Free breathing + Tagging T1 mapping MRA Flow T2* + LV Function: Iron Overload Protocol 9

10 T2* imaging of iron overload T2* decay (calculate slope of line) T2* imaging of iron overload > 20ms is normal at 1.5T 10 20ms is possible >10ms is positive Tagging + Free breathing cine: Pericardial Disease LV Function/cine images: septal bounce Pericardial thickening (enhancement) 10

11 Tagging + Free breathing cine: Pericardial Disease Real time Cine (free breathing, with deep inspiration) Looking for D shaped septum constrictive physiology Tagging + Free breathing cine: Pericardial Disease Tagged imaging Direct visualization of pericardial adhesions T1 Mapping + LGE + LV Function: Chronic Cardiomyopathy ( esp Amyloidosis or Fabry s) LV RV function + LGE Sarcoidosis myocarditis T1 mapping: Amyloidosis Fabry s disease 11

12 T1 Mapping Pulse Iron Fat Normal Fibrous (scar) Edema (water) Bulluck H, et al Circulation Journal (2015) 3D Gadolinium enhanced MRA / MRV 3D volumetric acquisition allows multiplanar post processing Imaging of the Aorta Aortopathy/aneurysm/dissection Coarctation/Bicuspid aortic valve 3D Gadolinium enhanced MRA / MRV Imaging of the Aorta Aortopathy/aneurysm/dissection Coarctation/Bicuspid aortic valve Pulmonary vessels Pulmonary vein mapping AFib Pulmonary arteries congenital 12

13 Bright blood (cine/trufi) Dark blood double inversion pulse Null signal of blood Reduces susceptibility artefact from clips Vessel wall evaluation 13

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16 Cine Valve: (Bicuspid Aortic Valve) 16

17 Flow Assessment Flow Assessment: (Aortic regurgitation) +218 mls - 52 mls (24% regurgitant fraction) Flow Assessment: Valvular stenosis regurgitation in plane or through plane Quantify pressure gradients at areas of stenosis (modified Bernoulli Equation) Qp:Qs quantify flow through MPA vs Ascending aorta 17

18 T2/STIR Cine SUMMARY LGE Tagging T1 Map T2* Black blood Flow Perfusion MRA Summary Many different MRI sequences Ways to generate image contrast permit tissue characterization ** a unique feature of MRI! Tailor the protocol to answer the clinical question. Each sequence takes time patients can only tolerate so long in the scanner Important to use an MRI scanner to capacity. Having a preset group of protocols may benefit workflow New sequences and means to generate image contrast are still being discovered! Acknowledgements: Stephenson Cardiovascular MRI Centre, Director Dr. James White Kelvin Chow, Senior Scientist, MR R&D Collaborations, Siemens Medical Solutions 18

19 SUMMARY: Cine LV RV function LGE (+ LV RV function) Chronic CM T2 (+ LGE + LV RV function) acute CM 3D MRA + Flow Special protocols (T2*, T1, Tagging, Free breathing, perfusion). Post contrast T1 Mapping Pulse Post contrast T1 Native T1 (pre contrast) Bulluck H, et al Circulation Journal (2015) Calculating ECV Pulse Post contrast T1 ECV Native T1 (pre contrast) myocardial ECV = (1 hematocrit) (ΔR1myocardium/ΔR1blood), where R1 = 1/T1 19

20 CMR = Versatility Ischemia Inflammation Infiltration Cardiac Ischemia Masses CMR s Incremental Role LGE Ischemic Non Ischemic Disease Etiology Disease activity Disease reversibility Response to Rx Risk stratification Impact Therapy White, JA, Patel, MR. Cardiology Clinics,

21 When to add... T1 Weighted Imaging ARVC: The 3 F s Disease Etiology Task Force Disease activity Fat Disease reversibility Response to Rx Function Risk stratification Marcus FI, et al. Circulation 2010; 121 Impact Therapy Fibrosis Beyond Diagnosis LGE Does CMR Alter Therapeutic Decisions? Disease Etiology Patients yo male Presenting with SCD, normal with Resuscitated echo and normal SCD or cath Unstable VT Disease activity Disease reversibility Response to Rx Risk stratification Impact Therapy White, JA, Fine N, Warren H, et al: Circulation: Card Imaging 2012 T 2 Imaging and Bright Blood T 2 imaging is sensitive to pathological tissue changes Turbo spin echo imaging has good T 2 contrast, but is sensitive to flow artifacts?? Simonetti O et al. Radiology 1996;199:

22 Cardiac MRI Sequences: 3D Balanced steady state free precession (3D b ssfe) ECG triggered, navigator gated free breathing For assessment of morphology 3D volumetric acquisition allows multiplanar postprocessing Cardiac MRI Sequences: Phase Contrast Allows measurement of intracardiac shunts Pulmonary : Systemic blood flow ratios (Qp:Qs) Quantification of reguritant fractions Quantify pressure gradients at areas of stenosis (modified Bernoulli Equation) MRI Contrast Agents Paramagnetic materials have unpaired electrons that catalyze proton interactions Result in shorter T 1 and T 2 times Gadolinium (Gd) has 7 unpaired electrons! Contrast agents are chelated Gd so that they can be biologically safe 22

23 Myocardial Perfusion Heart requires constant blood flow to function Bolus IV contrast flows along with the blood Contrast causes increase in brightness Repeated imaging can image contrast dynamics Kellman P et al. J Magn Reson Imag 2012;36: Quantitative T 2 Imaging Used to assess severity of edema Can detect global edema Standard imaging for all studies Long axis planes HLA, VLA, LVOT, LVOT coronal, (RVOT +/ RV inflow if right sided lesions) Need two perpendicular views of the valve(s) in question LV & RV function 23

24 Don t forget the aorta in your valve assessment! Aortic disease Residual root dissection in a patient with a previous type A dissection repair (interpositional graft) Mitral regurgitation Standard methods of quantification are indirect: 1) Regurgitant flow = LVSV - Ao systolic flow (independent of other valve lesions) 2) Regurgitant flow = LVSV - RVSV Mitral stenosis Can assess mitral valve area by direct planimetry Important to ensure correct slice positioning at MV tips (as for echo) Diastolic flow (volume and velocity) is feasible though temporal resolution is lower than echo 24

25 Mitral stenosis (2) Planimetry of the MV tips: Pulmonary stenosis Horizontal RVOT planned RVOT planned from transverse slices from previous RVOT Good visualisation of pulmonary valve motion Accurate velocity assessment RVOT sizing for potential surgery / balloon valvuloplasty / percutaneous valve replacement Pulmonary regurgitation Quantification of PR Size & shape of RVOT?percutaneous stentvalve replacement Size & function of RV Forward flow: 72mls Regurgitant flow: 27mls (38% regurgitant fraction) 25

26 Pulmonary valve disease (3) CMR is also important for: determining RV mass & volumes assessing RVOT morphology Dilated RV secondary to chronic PR Supravalvular Complex stenosis with pulmonary previous surgical disease widening Now recurrent supravalvular stenosis & valvular regurgitation Dilated post stenotic pulmonary artery Tricuspid disease Severe TR (note low velocity causes minimal turbulence from dephased spins). Also has pericardial effusion Regurgitation can be quantified similarly to mitral regurgitation (RVSV pulmonary flow) Stenosis can be assessed with direct planimetry of the tips RV volumes & function for all 26

27 Multiple valve disease Detailed assessment of severity of each lesion & LV function Mixed aortic and mixed mitral valve disease Proceed sequentially through assessment of each lesion, including LV/RV funciton assessment Global INV Slice INV IMG Double Inversion Recovery Double inversion has no effect on imaging slice Inverted blood flows into slice Imaging occurs when inverted blood has no signal (due to T 1 recovery) Aortic Valve 27

28 Aortic stenosis (1) SA pilot LVOT view Coronal LVOT view Plan initial LVOT view from short axis pilot scan, with the plane through the aortic root/valve The second LVOT (coronal) plane is planned through this, aligned with the stenotic jet. There is often a central core in the jet comprising laminar flow, with turbulent flow (black/low intensity on gradient echo) surrounding this Align planes with AS jet rather than Ao root Aortic stenosis (2) Choose the best LVOT view for in plane flow assessment (the one with the best view of the core jet) Aortic stenosis (3) Measure the peak velocity, either from the in plane flow itself, or using the in plane flow to identify the point of peak velocity and acquire a through plane flow sequence at this point: Position for through plane flow acquisition 28

29 Aortic stenosis (4) Measure the valve area by direct planimetry, by acquiring a thin (5 6mm) slice through the tips of the aortic valve in systole, piloted from the 2 LVOT views. It is important to ensure you are at the tips, as you may overestimate the valve area otherwise Need still image of valve in systole here, including planimetry Valve tips in systole area = 1.0cm 2 Aortic stenosis (5) Correct alignment with AS jet Accurate trans valvular velocity (in plane / through plane) avoids underestimation with angulated roots Valve orifice area (direct planimetry) LV mass & volumes to assess impact on LV Aortic regurgitation Through plane flow measurement Allows quantification of regurgitation 29

30 Saturation recovery imaging Short T 1 tissue recover magnetization faster and are therefore brighter Longer T 1 means slower recovery and are darker Image brightness can be used to calculate T 1 and also contrast concentration Quantitative T 1 Imaging T 1 imaging is used to assess fibrosis Extracellular volume can also be calculated 30