Published on October 22, 2008
(Some of) The Physics of Magnetic Resonance Imaging : (Some of) The Physics of Magnetic Resonance Imaging Jolinda Smith Lewis Center for Neuroimaging The Robert and Beverly Lewis Center for Neuroimaging : The Robert and Beverly Lewis Center for Neuroimaging Part of the University of Oregon Brain-Biology-Machine Initiative Construction began in July, 2001 MRI unit installed and operating in March, 2002 NMR/MRI : NMR/MRI Nuclear: any nucleus with a non-zero nuclear spin: 1H, 31P, 13C, 19F Magnetic: uses an external magnetic field to interact with nuclear magnetic moments Resonance: exploit resonance to perturb nuclear spins Imaging: vary the external field as a function of position to localize signals Siemens Allegra 3T Scanner : Siemens Allegra 3T Scanner B0 = 3 T = 30,000 gauss Earth’s magnetic field = 0.5 gauss Magnetic fields are powerful! : Magnetic fields are powerful! A nucleus in a magnetic field : A nucleus in a magnetic field A nucleus with non-zero spin has angular momentum and a magnetic moment These nuclei will tend to align with an external magnetic field For every 100,000 protons at 3T, there are 2 “extra” in the low energy state The Larmor Equation : The Larmor Equation = B0 is the Larmor frequency B0 is the strength of the external magnetic field is the gyromagnetic ratio and depends on the nucleus 1H has the highest gyromagnetic ratio of any nuclear species and is highly abundant in the human body Resonance : Resonance Net magnetization: Free induction decay : Free induction decay Details of the relaxation depend on the local environment. We can exploit these differences to emphasize different types of contrast in images. MRI Hardware : MRI Hardware RF Coil : RF Coil RF Coils are used to excite the spins and to receive the signal A typical coil is a tuned LC circuit and may be considered a near field antenna Gradient coils : Gradient coils Gradient coils produce magnetic field gradients that are used to add spatial information to the signal Selective excitation: only excite those spins within a thin “slice” of the subject Frequency encoding: make the Larmor frequency depend on position Phase encoding: make the RF signal phase depend on position Slice selection : Slice selection During excitation, a linear gradient is applied. As a result, only a “slice” of the sample is excited. Frequency encoding : Frequency encoding m(x) x B = B0 + Gxx The resonant frequency depends on location, so a Fourier transform of the signal tells us where the nuclei are in one dimension During signal reception, a gradient is applied in one direction: Phase encoding : Phase encoding Repeated pulses of a gradient applied between excitation and reception advance the phase of the spins by an amount depending on location. Pulse sequences : Pulse sequences 2D Fourier Imaging : 2D Fourier Imaging MRI of the Brain : MRI of the Brain Functional MRI : Functional MRI Functional MRI is the use of MRI equipment to detect changes in brain activity The most common fMRI technique utilizes blood oxygen level dependant (BOLD) contrast Time Series and Activation Maps : Time Series and Activation Maps Off Off Off Off On On On On Scan Number Signal Intensity
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... http://lcni.uoregon.edu/downloads/science_teachers_2005/Mri_rough_guide.ppt ... •During MRI scanning the patients were told or read jokes of ...