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Published byDerrick Wheeler Modified over 9 years ago
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MRI “Magnetic Resonance Imaging”
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Nuclei with nuclear spin: elementary magnets Magnetic moment: =magnetogyric ratio L=angular momentum
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In absence of magnetic field: Random orientation of elementary magnets In magnetic field: elementary magnetsenergy levels orientsplit B0B0 parallel antiparallel EE B0B0 E B
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Precession Precession or Larmor frequency:
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B0B0 M Low energy state parallel in case of proton High energy state antiparallel in case of proton Net magnetization (M) due to spin excess in different energy states
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Excitation using radio frequency (RF) radiation Resonance condition: Larmor frequency M Net magnetization
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Spin-lattice relaxation T1 or longitudinal relaxation t MzMz T1 relaxation time: depends on interaction between elementary magnet (proton) and its environment
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Spin-spin relaxation T2 or transverse relaxation M xy t “free induction decay” (FID) T2 relaxation time: depends on interaction between elementary magnets (protons)
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1970: detection of lengthened relaxation times in cancerous tissues 1972: theoretical development of human in vivo 3D NMR 1977: first human MRI image Inventor of MRI: Raymond V. Damadian (1936-)
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MRI: Net magnetization of the human body takes place “indomitable”
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Paul C. Lauterbur (1929-) 1971: development of spatially resolved NMR
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voxel: volume element pixel: picture element Image MRI imaging I: Spatial resolution
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Definition and addressing of elementary 3D image points (voxels): by using gradient magnetic fields MRI imaging I: Spatial resolution ByBy BxBx BzBz
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MRI imaging II: Color (grayscale) resolution (contrast) Based on relaxation times
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MRI imaging II: Color (grayscale) resolution (contrast) Based on spin density and relaxation times T1-weighing T2-weighing Proton density- weighing
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MRI technology Magnet: superconducting (liquid He) Resolution enhancement: with surface RF coils Excitation with pulse sequences 90˚ Detection and analysis: Fourier transform of temporal signal t
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MRI: Image manipulation I Reslicing in perpendicular plane
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MRI: Image manipulation II Spatial projection („volume rendering”)
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Blood flow Image slice Saturated spins Unsaturated spins MRI: Non-invasive angiography
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MRI: Non-invasive angiography arteria carotis Circulus arteriosus Willisii
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MRI movie Based on high time resolution images Opening and closing of aorta valve
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Functional MRI fMRI High time resolution image sequences recorded synchronously with physiological processes Effect of light pulses on visual cortex
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ISOTOPE-BASED DIAGNOSTIC IMAGING TECHNIQUES Gamma camera SPECT PET
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Gamma Camera Lead collimatorPMT matrixScintillation detector Kidney scintigram PROBLEM: Summation image
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SPECT: Single Photon Emission Computed Tomography gamma camera The detector circles the body in the transaxial plane
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PET: Positron emission tomography photon (0.51 MeV) electron (e - ) positron ( + ) 18 F (positron emitter) Detector NB: annihilation radiation conservation of momentum Half life of isotopes: 20-110 min. 1-2 mm
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PET imaging Coincidence
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PET: Tomographic imaging and 3D reconstruction Image slices Typical voxel (3D volume element): 8 mm x 8 mm x 14 mm
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Studying brain glucose metabolism with PET
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Superposition of different 3D image information 1. Based on common fiducial points 2. Simultaneus acquisition with two methods (PET&CT or PET&MRI) MRI PET + =
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Superposed MRI and PET image sequence PET activity: during eye movement Volume rendering
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