Nuclear Magnetic Resonance-Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course Signal from Flow Signal from Flow The MR signal from moving fluids (vascular and CSF)is Low signal intensities:high-velocity signal loss complicated by many factors: ation(nothing ◆Flow velocity ◆Vessel orientation .Laminar vs.turbulent flow patterns Flow turbulence:flow voids Pulse sequences Dephasing of spins in blood(confused spin alignment) Image acquistion modes Black-blood:double IR (TI=600 ms) Flow related mechanisms combine with image IR sequence prefaced with non-selective.volume 180pulse acquisition parameters to alter contrast Flow-related enhancement ."Bright-blood'to black-blood' Even-echo rephasing (prominent in slow laminar flow-veins) .Can be a source of artifacts Gradient ech Exploited to produce MR angiography images 色制 Perfusion and Diffusion Contrast Perfusion and Diffusion Contrast Perfusion of cells via capillary bed Subtract post-stimulus image from pre-stimulus image Exogenous tracer methods Color-coded overlay to a grayscale anatomic image ◆2H.3He.17oand1年experimental procedures Intravascular blood-pool agents:Gd-DTPA demonstrate activity(t)correlating with stimulus(t) Endogenous tracer methods Diffusion depends on the random motion of H2O molecules in tissues Labeling of inflowing spins (black blood"):tagging ◆Tagged spins perfuse into tissues→！MR signal intensity Interactions of the local cellular structure with the BOLD(Blood Oxygen Level-Dependent) diffusing H2O molecules produ uces anisotropic, generated by bl netabolism in brair directionally dependent diffusion Diffusion-weighted sequences use a strong gradient- Areas of metabolic activity-correlated signal (functional MR) signal differences based on mobility/directionality 9,19and26May2005 17Nuclear Magnetic Resonance – Bushberg Chapter 14 Diagnostic Radiology Imaging Physics Course 9, 19 and 26 May 2005 17 © UW and Brent K. Stewart, PhD, DABMP 65 Signal from Flow ™ The MR signal from moving fluids (vascular and CSF) is The MR signal from moving fluids (vascular and CSF) is complicated by many factors: complicated by many factors: ™ Flow velocity ™ Vessel orientation ™ Laminar vs. turbulent flow patterns ™ Pulse sequences ™ Image acquisition modes ™ Flow related mechanisms combine with image Flow related mechanisms combine with image acquisition parameters to alter contrast acquisition parameters to alter contrast ™ ‘Bright ‘Bright-blood’ to ‘black-blood’ ™ Can be a source of artifacts ™ Exploited to produce MR angiography images © UW and Brent K. Stewart, PhD, DABMP 66 Signal from Flow ™ Low signal intensities: high-velocity signal loss ™ Nuclei move out of slice during echo reformation (nothing Nuclei move out of slice during echo reformation (nothing focused in Mxy plane → no or little FID signal no or little FID signal) ™ Flow turbulence: flow voids ™ Dephasing of spins in blood (confused spin alignment) Dephasing of spins in blood (confused spin alignment) ™ Black-blood: double IR (TI blood: double IR (TI ≈ 600 ms) 600 ms) ™ IR sequence prefaced with non-selective, volume 180° pulse ™ Flow-related enhancement related enhancement ™ Even-echo rephasing (prominent in slow laminar flow (prominent in slow laminar flow – veins) ™ Gradient echo images (unsaturated blood): Gradient echo images (unsaturated blood): ∝ velocity, slice velocity, slice ‘thinness’ and TR © UW and Brent K. Stewart, PhD, DABMP 67 Perfusion and Diffusion Contrast Perfusion and Diffusion Contrast ™ Perfusion of cells via capillary bed ™ Exogenous tracer methods ™ 2H, 3He, 17O and 19F experimental procedures ™ Intravascular blood-pool agents: Gd-DTPA ™ Endogenous tracer methods ™ Labeling of inflowing spins (‘black blood’): tagging ™ Tagged spins perfuse into tissues Tagged spins perfuse into tissues → ↓ MR signal intensity ™ BOLD (Blood Oxygen Level-Dependent) ependent) ™ Differential contrast generated by blood metabolism in brain ™ Oxyhemoglobin Oxyhemoglobin → deoxyhemoglobin (paramagnetic) increases deoxyhemoglobin (paramagnetic) increases magnetic susceptibility and induced signal loss (increased T2*) magnetic susceptibility and induced signal loss (increased T2*) ™ Areas of Areas of ↑ metabolic activity metabolic activity → correlated signal (functional MR) correlated signal (functional MR) © UW and Brent K. Stewart, PhD, DABMP 68 Perfusion and Diffusion Contrast Perfusion and Diffusion Contrast ™ Subtract post Subtract post-stimulus image from pre-stimulus image ™ Color-coded overlay to a grayscale anatomic image coded overlay to a grayscale anatomic image demonstrate demonstrate activity(t activity(t) correlating with ) correlating with stimulus(t stimulus(t) ™ Diffusion depends on the random motion of H2O molecules in tissues ™ Interactions of the local cellular structure with the Interactions of the local cellular structure with the diffusing H2O molecules produces anisotropic, O molecules produces anisotropic, directionally dependent diffusion ™ Diffusion-weighted sequences use a strong gradient weighted sequences use a strong gradient → signal differences based on mobility/directionality