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CUNY BME I5000 - Magnetic Resonance Imaging

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Slide 1Slide 2Slide 3History of NMRNecessary EquipmentRecipe for MRIProtonsSlide 8The Big MagnetMagnetic Resonance (MR)Slide 11Slide 12Slide 13Slide 14Slide 15Cox鈥檚 Swing AnalogySlide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 321Lucas Parra, CCNYCity College of New YorkBME I5000: Biomedical ImagingLecture 8Magnetic Resonance Imaging(mostly NMR)Lucas C. Parra, [email protected] Blackboard: Parra, CCNYCity College of New YorkSchedule1. Introduction, Spatial Resolution, Intensity Resolution, Noise2. X-Ray Imaging, Mammography, Angiography, Fluoroscopy3. Intensity manipulations: Contrast Enhancement, Histogram Equalisation4. Computed Tomography5. Image Reconstruction, Radon & Fourier Transform, Filtered Back Projection6. Nuclear Imaging, PET and SPECT7. Maximum Likelihood Reconstruction8. Magnetic Resonance Imaging9. Fourier reconstruction, k-space, frequency and phase encoding 10. Optical imaging, Fluorescence, Microscopy, Confocal Imaging11. Enhancement: Point Spread Function, Filtering, Sharpening, Wiener filter12. Segmentation: Thresholding, Matched filter, Morphological operations13. Pattern Recognition: Feature extraction, PCA, Wavelets14. Pattern Recognition: Bayesian Inference, Linear classification3Lucas Parra, CCNYCity College of New York Biomedical ImagingImaging Modality Year Inventor Wavelength EnergyPhysical principleX-Ray 1895 Röntgen (Nobel 191)3-100 keV Measures variable tissue absorption of X-RaysSingle Photon Emission Comp. Tomography (SPECT)1963 Kuhl, Edwards150 keV Radioactive decay. Measures variable concentration of radioactive agent.Positron Emission Tomography (PET)1953 Brownell, Sweet150 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 1940-1955many MHz Measures echo of sound at tissue boundaries.4Lucas Parra, CCNYCity College of New YorkNuclear Magnetic Resonance (NMR)Felix Block and Edward Purcell1946: atomic nuclei absorb and re-emit radio frequency energy in an external magnetic field1952: Nobel prize in physics1971: NMR Tumor detection (Damadian)Magnetic Resonance Imaging (MRI)1973: Lauterbur suggests NMR could be used to form images1977: clinical MRI scanner patented1977: Mansfield proposes echo-planar imaging (EPI) to acquire images faster2003: Nobel Price in MedicineFunctional MRI (fMRI)1990: Ogawa observes BOLD effect with T2*: blood vessels became more visible as blood oxygen decreased1991: Belliveau observes first functional images using a contrast agent1992: Ogawa et al. and Kwong et al. publish first functional images using BOLD signalBloch PurcellAdapted from Jody Culham, MansfieldMRI - History5Lucas Parra, CCNYCity College of New YorkMagnet Gradient Coil RF CoilSource: Joe Gati, photosRF Coil4T magnetgradient coil(inside)MRI - Equipment Adapted from Jody Culham, Parra, CCNYCity College of New York1) Put subject in big magnetic fieldWhen 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 transmitterThe 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. timeNow 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 imagesSource: Robert Cox's web siteMRI – Basic Recipe7Lucas Parra, CCNYCity College of New YorkNucleus 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. Since the particle is electrically charged this spinning will generate a magnetic moment µ:The gyromagnetic ratio γ is specific to each nucleus. As we will see the magnetic fields and radio frequency (RF) are tuned to a specific value of γ, i.e. to a specific nucleus.MRI – Nuclear Spinμ=γ J∣J∣=h2 π√I2+ I+J8Lucas Parra, CCNYCity College of New YorkProperties on nuclei found at high abundance in the body:Nucleus Atomic Number Atomic Mass γ/2π(MHz/T) MRI SignalProton, 1H 1 1 ½ 42.58 yesPhosphorus, 31P 15 31 ½ 17.24 yesCarbon, 12C 6 12 0 noOxygen, 16O 8 16 0 noSodium, 23Na 11 23 3/2 11.26 yesMRI can be performed with odd odd atomic mass (non-zero spin)1H, 13C, 19F, 23Na, 31PMost frequent medical imaging is performed with 1H (proton)abundant: high concentration in human bodyhigh sensitivity: yields large signals1.5T magnet uses RF at 3.87 MHz for proton imaging. MRI – Nuclear Spin9Lucas Parra, CCNYCity College of New Yorkx 80,000 = 4 Tesla = 4 x 10,000 ÷ 0.5 = 80,000X Earth’s magnetic fieldRobarts Research Institute 4TVery strongContinuously onSource: www.spacedaily.com1 Tesla (T) = 10,000 GaussEarth’s magnetic field = 0.5 GaussMain field = B0 B0MRI – Big Magnet Adapted from Jody Culham, Parra, CCNYCity College of New YorkMRI – Nuclear Spin in Magnetic FieldWhen a spin is placed in a homogeneous external magnetic field B0 it precesses at a frequency ω0. Quantum mechanics however dictates that the valued for the z-orientation of J (and µ) can only be:with m = ±½ for I = ½.ω0=γ B0Jgravitygraphic fromμz=γ Jz=γh2 πmIThe effect is analogous to a spinning mass in a gravitational field:11Lucas Parra, CCNYCity College of New YorkMRI – Nuclear Spin in Magnetic FieldGiven the quantization of |µ| and µz the spin can only

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