Lucas Parra CCNY City College of New York BME I5000 Biomedical Imaging Lecture 8 Magnetic Resonance Imaging mostly NMR Lucas C Parra parra ccny cuny edu Blackboard http cityonline ccny cuny edu 1 Lucas Parra CCNY City College of New York Schedul e 1 Introduction Spatial Resolution Intensity Resolution Noise 2 X Ray Imaging Mammography Angiography Fluoroscopy 3 Intensity manipulations Contrast Enhancement Histogram Equalisation 4 Computed Tomography 5 Image Reconstruction Radon Fourier Transform Filtered Back Projection 6 Nuclear Imaging PET and SPECT 7 Maximum Likelihood Reconstruction 8 Magnetic Resonance Imaging 9 Fourier reconstruction k space frequency and phase encoding 10 Optical imaging Fluorescence Microscopy Confocal Imaging 11 Enhancement Point Spread Function Filtering Sharpening Wiener filter 12 Segmentation Thresholding Matched filter Morphological operations 13 Pattern Recognition Feature extraction PCA Wavelets 14 Pattern Recognition Bayesian Inference Linear classification 2 Lucas Parra CCNY City College of New York Biomedical Imaging Imaging Modality Year Inventor Wavelength Physical principle Energy X Ray 1895 R ntgen Nobel 191 3 100 keV Measures variable tissue absorption of X Rays Single Photon Emission Comp Tomography SPECT 1963 Kuhl Edwards 150 keV Radioactive decay Measures variable concentration of radioactive agent Positron Emission Tomography PET 1953 Brownell Sweet 150 keV SPECT with improved SNR due to increased number of useful events Computed Axial Tomography CAT 1972 Hounsfield Cormack Nobel 1979 keV Multiple axial X Ray views to obtain 3D volume of absorption Magnetic Resonance Imaging MRI 1973 Lauterbur Mansfield Nobel 2003 GHz Space and tissue dependent resonance frequency of kern spin in variable magnetic field Ultrasound 19401955 many MHz Measures echo of sound at tissue boundaries 3 Lucas Parra CCNY City College of New York MRI History Nuclear Magnetic Resonance NMR Felix Block and Edward Purcell 1946 atomic nuclei absorb and re emit radio frequency energy in an external magnetic field 1952 Nobel prize in physics 1971 NMR Tumor detection Damadian Bloch Purcell Lauterbur Mansfield Magnetic Resonance Imaging MRI 1973 Lauterbur suggests NMR could be used to form images 1977 clinical MRI scanner patented 1977 Mansfield proposes echo planar imaging EPI to acquire images faster 2003 Nobel Price in Medicine Functional MRI fMRI 1990 Ogawa observes BOLD effect with T2 blood vessels became more visible as blood oxygen decreased 1991 Belliveau observes first functional images using a contrast agent 1992 Ogawa et al and Kwong et al publish first functional images using BOLD signal Adapted from Jody Culham http defiant ssc uwo ca Jody web fmri4dummies htm Ogawa 4 Lucas Parra CCNY City College of New York MRI Equipment 4T magnet RF Coil gradient coil inside Magnet Gradient Coil RF Coil Source Joe Gati photos Adapted from Jody Culham http defiant ssc uwo ca Jody web fmri4dummies htm 5 Lucas Parra CCNY City College of New York MRI Basic Recipe 1 Put subject in big magnetic field When protons are placed in a constant magnetic field they precess at a frequency proportional to the strength of the magnetic field at typical radio frequencies They also align somewhat to generate a bulk magnetization 2 Transmit radio waves into subject about 3 ms Exposure to radio frequency magnetic field will synchronize this precession 3 Turn off radio wave transmitter The coherent precession continues but decays slowly due to interactions with magnetic moments of surrounding atoms and molecules tissue dependent 4 Receive radio waves re transmitted by subject 10 110ms The coherent precession oscillation generates a current in an inductive coil The detected signal is called magnetic nuclear resonance 5 Store measured radio wave data vs time Now go back to 2 to get some more data with different magnetic fields and radio frequencies here lies the Art of MRI 6 Process raw data to reconstruct images Source Robert Cox s web site 6 Lucas Parra CCNY City College of New York MRI Nuclear Spin Nucleus has a quantum mechanical property called spin quantized by I I 1 2 for a proton in H2O Spin can be thought of as a spinning mass with an angular momentum J J h 2 J I I 2 Since the particle is electrically charged this spinning will generate a magnetic moment J The gyromagnetic ratio is specific to each nucleus As we will see the magnetic fields and radio frequency RF are tuned to a specific value of i e to a specific nucleus 7 Lucas Parra CCNY City College of New York MRI Nuclear Spin Properties on nuclei found at high abundance in the body Nucleus Proton 1H Phosphorus 31P Carbon 12C Oxygen 16O Sodium 23Na Atomic Number 1 15 6 8 11 Atomic Mass 1 31 12 16 23 0 0 3 2 2 MHz T MRI Signal 42 58 17 24 yes yes no no yes 11 26 MRI can be performed with odd odd atomic mass non zero spin 1 H 13C 19F 23Na 31P Most frequent medical imaging is performed with 1H proton abundant high concentration in human body high sensitivity yields large signals 1 5T magnet uses RF at 3 87 MHz for proton imaging 8 Lucas Parra CCNY City College of New York MRI Big Magnet Very strong 1 Tesla T 10 000 Gauss Earth s magnetic field 0 5 Gauss 4 Tesla 4 x 10 000 0 5 80 000X Earth s magnetic field Continuously on Main field B0 Robarts Research Institute 4T x 80 000 B0 Source www spacedaily com Adapted from Jody Culham http defiant ssc uwo ca Jody web fmri4dummies htm 9 Lucas Parra CCNY City College of New York MRI Nuclear Spin in Magnetic Field When a spin is placed in a homogeneous external magnetic field B0 it precesses at a frequency 0 The effect is analogous to a spinning mass in a gravitational field J 0 B0 gravity Quantum mechanics however dictates that the valued for the zorientation of J and can only be with m for I h z J z mI 2 graphic from http www ecf utoronto ca apsc courses bme595f notes 10 Lucas Parra CCNY City College of New York MRI Nuclear Spin in Magnetic Field Given the quantization of and z the spin can only be at angles 54 7o with external field Bo z Bo z z h 4 h 4 h 3 4 y x These are called parallel and anti parallel states They have different energy levels h B0 E z B o 4 11 Lucas Parra CCNY City College of New York MRI Bulk Magnetization In a macroscopic sample with many nuclei the number of nuclei with parallel or anti parallel spin configuration is given by the Boltzman distribution Np h B0 h B0 E exp exp 1 Na kT 2 kT 2 k T For a 1 5T field we find that in 1 million protons there are only 5 more parallel