Transcript Slide 1
Contrast Mechanisms An Introduction to MRI Physics and Analysis Michael Jay Schillaci, PhD Monday, February 18th, 2008 Contrast Mechanisms Static Contrasts Motion Contrasts Sensitive to movement of spins in space E.g., Dephasing, Diffusion, Perfusion Endogenous Contrast Sensitive to type, number and relaxation of spins E.g., T1, T2 Depends upon intrinsic properties of tissue E.g., BOLD fMRI Exogenous Contrast Uses injection of to track changes E.g., Nuclear Medicine (NMR) Static Contrast The Concept of Contrast Contrast = difference in signals emitted by water protons between different tissues For example, gray-white contrast is possible because T1 is different between these two types of tissue Static Contrast Imaging Methods MR Signal MR Signal T2 Decay transverse T1 Recovery longitudinal 50 ms time 1s time Most Common Static Contrasts 1. Weighted by the Proton Density 2. Weighted by the Transverse Relaxation Times (T2 and T2*) 3. Weighted by the Longitudinal Relaxation Time (T1) The Effect of TR and TE on Proton Density Contrast TE 2.5 2.5 2 2 MR Signal MR Signal TR 1.5 T1 Recovery 1 T2 Decay 1 0.5 0.5 0 1.5 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 t (s) 0 10 20 30 40 50 60 70 80 90 100 t (ms) Inversion Recovery to Boost T1 Contrast So S = So * (1 – 2 e –t/T1) S = So * (1 – 2 e –t/T1’) -So IR-Prepped T1 Contrast T1 and T2 Values Equilibrium magnetization Depends on field Depends on H20 content N z2 M0 B0 pH 2O B0 V k BT T1 and T2 Values for Various Tissues and Fields1 Material % H2O2,3 T1 ( ms )4 B0 = 0.5 T T2 ( ms )4 B0 = 1.5 T B0 = 0.5 T B0 = 1.5 T White matter 84.3 500 600 74 80 Grey Matter 70.6 650 900 87 100 CSF 99.0 1800 4000 600 2000 1Table Adapted from: http://members.lycos.nl/mri/Nieuw/T1eng.htm Matter: http://www.fmrib.ox.ac.uk/~stuart/lectures/lecture4/sld004.htm 3CSF Value: http://www.ivis.org/special_books/Braund/tipold/chapter_frm.asp?LA=1 4Values From: Huettel Chapter 5 and http://members.lycos.nl/mri/Nieuw/T1eng.htm 2White/Grey Image Formation - General Integrate magnetization to get MRI signal Select Z “slice” and form image of XY plane variations t S (t ) M x, y, t e XY i xGX yGY dt 0 dxdy Area Contrast comes from difference in magnetization values Measurement at different times gives different contrast MRI Picture Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm Static Contrast – T1 and T2 T1 Contrast Weighting T1 Contrast Echo at T2 min Repeat at T1 max TR TE T2 Contrast Echo at T2 max Repeat at T1 min Max T1 Contrast Min T2 Contrast T2 Contrast Weighting TR TE Magnetization is given by M XY TR TE M 0 1 e T 1 e T 2 re cov ery decay Min T1 Contrast Max T2 Contrast Static Contrast Images Examples from the Siemens 3T T1 and T2 Weighted Images T1 Weighted Image (T1WI) T2 Weighted Image (T2WI) (Gray Matter – White Matter) (Gray Matter – CSF Contrast) Flip Angle RF Pulse Determines Flip Angle Duration and magnitude are important Rotation determines amount of magnetization measured +z M B0 MZ +y BC MXY +x M Z M cos M XY M sin Adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm Field Strength Increased field strength Net magnetization in material is greater Increased contrast means signal is increased Image1 resolution is better Muscle Tissue 1MRI adapted from: http://www.mri.tju.edu/phys-web/1-T1_05_files/frame.htm Static Contrast - T2* Relaxation T2* accounts for magnetic defects and effects 1 1 1 1 T 2* T 2 T 2 M T 2 MS T2 is relaxation due to spin-spin interaction of nuclei T2M is relaxation induced by M inhomogeneities of main magnet T2MS is relaxation induced by M magnetic susceptibility of material B0 m Motion Contrast Motion Contrast - Dephasing Dephasing of H2O and Fat MRI signal is a composite of Fat and H2O signals H2O and Fat resonate at different frequencies T1F = 210 ms, T1W = 2000 ms ( b/c T1F > T1W → fat is brighter … ) Relative phase gives TE dependence MF ΦFW MW Parallel ( ΦFW = 0o ) Anti-Parallel (ΦFW = 180o ) @ TE = 13.42 ms @ TE = 15.66 ms Motion Contrast Imaging Methods Prepare magnetization to make signal sensitive to different motion properties Flow weighting (bulk movement of blood) Diffusion weighting (water - random motion) Perfusion weighting (blood flow into capillaries) Flow Weighting: MR Angiogram • Time-of-Flight Contrast • Phase Contrast Time-of-Flight Contrast Saturation Acquisition Excitation No Flow No Signal Medium Flow Mediu m Signal High Flow High Signal Vessel Vessel Vessel Pulse Sequence: Time-of-Flight Contrast 90o Time to allow fresh flow enter the slice 90o RF Excitation Gx Saturation Gy Gz Image Acquisition Phase Contrast (Velocity Encoding) T 2T 0 T G( x vt)dt G( x vt)dt GvT 2 Blood Flow v Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G T 0 2T Time Pulse Sequence: Phase Contrast 90o RF Excitation G Gx -G Phase Image Gy Acquisition Gz MR Angiogram Random Motion: Water Diffusion Diffusion Weighting l 2Dt S So e 2 D 2G 2T 3 3 Externally Applied Spatial Gradient -G Externally Applied Spatial Gradient G 0 T 2T Time Pulse Sequence: Gradient-Echo Diffusion Weighting Excitation RF 90o G Gx G - Image Gy Acquisition Gz Large Lobes Pulse Sequence: Spin-Echo Diffusion Weighting 180o 90o RF Excitation G G Gx Image Gy Gz Acquisition Diffusion Anisotropy Determination of fMRI Using the Directionality of Diffusion Tensor Advantages of DWI 1. The absolute magnitude of the diffusion coefficient (ADC) can help determine proton pools with different mobility 2. The diffusion direction can indicate fiber tracks ADC Anisotropy Fiber Tractography DTI and fMRI D A B C Perfusion The injection of fluid into a blood vessel in order to reach an organ or tissue, usually to supply nutrients and oxygen. In practice, we often mean capillary perfusion as most delivery/exchanges happen in the capillary beds. Perfusion Weighting: Arterial Spin Labeling (ASL) Imaging Plane Labeling Coil Transmission Arterial Spin Labeling Can Also Be Achieved Without Additional Coils Pulsed Labeling Imaging Plane Alternating Inversion Alternating Inversion FAIR EPISTAR Flow-sensitive Alternating IR EPI Signal Targeting with Alternating Radiofrequency Pulse Sequence: Perfusion Imaging 180o 90o 180o RF Gx Image Gy Odd Scan Alternating opposite Distal Inversion Gz Even Scan 180 90o o 180 o RF Gx Image Gy Gz Alternating Proximal Inversion Odd Scan Even Scan Advantages of ASL Perfusion Imaging 1. It is non-invasive 2. Combined with proper diffusion weighting to eliminate flow signal first, it can be used to assess capillary perfusion Perfusion Contrast Perfusion Map Diffusion Perfusion Summary of Time Characteristics Spin-Lattice Relaxation (T1) Spin-Spin Relaxation (T2) gradients increase/decrease coherence Echo Time (TE) nuclei quickly become incoherent Magnetic Effects and Defects (T2*) nuclei realign with the magnetic field when DAQ occurs Repeat Time (TR) time between RF pulses Image adapted from: http://www.med.nagasaki-u.ac.jp/radiolgy/MRI%20of%20the%20FOOT/MRI-CDNUH/Fig9.html Endogenous Contrast Hemoglobin and Magnetism The Hemoglobin (Hb) Molecule An organic molecule containing four heme groups (with iron in each) and globular protein (globin). Oxygen Characteristics Oxygen bound - oxyhemoglobin (Hb) No oxygen bound - deoxyhemoglobin (dHb) Magnetic Properties Hb is diamagnetic - no dipole dHb is paramagnetic - slight dipole Oxygen and Field Strength Apply magnetic field to brain Blood oxygen level differs dHb is paramagnetic field increased Hb diamagnetic Field decreased Endogenous Contrast - fMRI Depends on internal biological compound Blood deoxygenation affects T2 Recovery Decreasing Relaxation Time T2 T1 Increasing Blood Oxygenation Level BOLD - Endogenous Contrast Blood Oxygenation Level Dependent Contrast dHb is paramagnetic, Hb is less Susceptibility of blood increases linearly with oxygenation BOLD subject to T2* criteria Oxygen is extracted from capillaries Arteries are fully oxygenated Venous blood has increased proportion of dHb Difference between Hb and dHb states is greater for veins Therefore BOLD is result of venous blood changes BOLD - T2* Contrast MR Signa l MR Signal T2 Decay T1 Recovery 50 ms 1s