Lec1Ch1 2IntroandHardware

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Published on October 15, 2007

Author: Pumbaa

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MRI History and Hardware Basic Safety Issues Introduction to fMRI:  MRI History and Hardware Basic Safety Issues Introduction to fMRI John VanMeter, Ph.D. Center for Functional and Molecular Imaging Terms Used for MRI:  Terms Used for MRI NMR (Nuclear Magnetic Resonance) MR (Magnetic Resonance) MRI (Magnetic Resonance Imaging) Felix Bloch & Edward Purcell - 1945:  Felix Bloch & Edward Purcell - 1945 Detection of Nuclear Magnetic Resonance Signal (Voltage in a coil) in “Bulk Matter” Basis of Nuclear Magnetic Resonance Spectroscopy Raymond Damadian - 1971 :  Raymond Damadian - 1971 Discovery: Rat Tumor has a RELAXATION TIME longer than normal tissue Differences in relaxation time provides one form of tissue contrast - T1 Paul Lauterbur - 1973 :  Paul Lauterbur - 1973 Field GRADIENTS to distinguish spatially localized signals  PHASE ENCODING Zeumatography  FREQUENCY ENCODING Both techniques needed to encode spatial location of signals First MR Image - 1973:  First MR Image - 1973 Lauterbur created image by applying gradients at different angles to produce 1D projections Combining projections forms image Inefficient as time needed for each angle equivalent to a single acquisition Sir Peter Mansfield - 1974:  Sir Peter Mansfield - 1974 Selective Excitation  SLICE SELECT Identifies where in a 3D object to collect signal from Richard Ernst - 1975:  Richard Ernst - 1975 2D-FT  TWO-DIMENSIONAL FOURIER TRANSFORMATION Needed to reconstruct images, which are encoded with frequency and phase Peter Mansfield - 1976:  Peter Mansfield - 1976 Developed very efficient way to collect data using technique called echo planar imaging (EPI) Transmits 1 RF pulse per slice Rapidly switches gradients and records EPI used today in fMRI! Damadian - 1977:  Damadian - 1977 First ever MRI image of human body Created using the “Indomitable” scanner Field strength was 0.05T Homogeneous part of field very limited so patient table was moved to collect each voxel! Took 4hrs to collect single slice FDA Clears First MRI Scanner - 1985:  FDA Clears First MRI Scanner - 1985 Minicomputers such as the PDP-11 and VAX become readily available GE develops first “high-field” (1.5T) commercial MRI scanner (1982) Medicare starts paying for MRI scans (1985) VAX 11/750 (1982) 1990’s :  1990’s FUNCTIONAL IMAGING 4 Nobel Laureates for MRI:  4 Nobel Laureates for MRI Bloch, Purcell (1952) Lauterbur, Mansfield (2003) Nobel Controversy - 2003:  Nobel Controversy - 2003 Damadian took out full page ads in NY Times and Washington Post protesting award to Lauterbur and Mansfield “This Year’s Nobel Prize in Medicine. The Shameful Wrong That Must Be Righted” “The Nobel Prize Committee for Physiology or Medicine chose to award the prize, not to the medical doctor/research scientist who made the breakthough discovery on which all MRI technology is based, but to two scientists who later made technological improvements based on his discovery” "I know that had I never been born, there would be no MRI today" Basic MRI Hardware:  Basic MRI Hardware Magnet Large magnetic field that is homogeneous over a large area Aligns protons in the body Radiofrequency (RF) coils Transmit and Receive RF energy into and from the body Gradients Induce linear change in magnetic field Spatial encoding Computer System and Console Patient Handling System Types of Magnets:  Types of Magnets Permanent Iron Core Low Field “Open” Resistive Electromagnet Up to 0.2T Superconducting Magnet Cools wire coil with cryogens 0.5T to 35T Electromagnets:  Electromagnets Field proportional to number of loops relative to cross-section area of each loop Increases in current also increases field strength Field highest at center of coil Properties of Superconducting Magnets:  Properties of Superconducting Magnets Very high field strengths generated Cool magnet’s wire coil using cryogens (liquid helium and possibly nitrogen) to near absolute zero Reduces resistance to zero for certain metals Provides stable and homogeneous magnetic field over a relatively large area Once ramped up no electricity used (relatively cheap) MAGNET ALWAYS ON! New dangers specific to these types of magnets RF (Radiofrequency) Coils:  RF (Radiofrequency) Coils Used to transmit and receive RF energy Needed to create images Coil Designs:  Coil Designs Closer coil is to object being imaged the better signal Variety of coils designed for specific body parts Coil Design Affects Images:  Coil Design Affects Images Gradient Coils:  Gradient Coils Induce small linear changes in magnetic field along one or more dimensions Results in two types of spatial encoding referred to as Frequency and Phase Encoding Inside an MRI Scanner:  Inside an MRI Scanner Gradients Body Coil Passive Shims Cyrostat MRI Safety:  MRI Safety Static B0 Field Projectiles Implants/other materials in the body RF Field tissue heating Gradient fields peripheral nerve stimulation acoustic noise Forces on Ferrous Objects:  Forces on Ferrous Objects Crash cart meets a 1.5T magnet Floor Buffer:  Floor Buffer Welding tank:  Welding tank Preventing Accidents Due to Ferrous Metallic Objects:  Preventing Accidents Due to Ferrous Metallic Objects Train ALL personnel who work in the facility Perform MRI safety screening on everyone prior their entering the MRI scanner room Limit access to the scanner facility based on training and need ACR guidelines establish 4 MRI Safety Zones and limit access to each zone MRI Safety:  MRI Safety Static B0 Field projectiles RF Field tissue heating Gradient fields peripheral nerve stimulation acoustic noise RF Exposure Standards:  RF Exposure Standards The FDA limits RF exposure to less than a 1 degree C rise in core body temperature RF Exposure Standards:  RF Exposure Standards 4W/Kg whole body for 15 min 3W/Kg averaged over head for 10 min 8W/Kg in any gram of tissue in the head or torso for 15 min 12W/Kg in any gram of tissue in the extremities for 15 min MRI Safety:  MRI Safety Static B0 Field projectiles RF Field tissue heating Gradient fields peripheral nerve stimulation acoustic noise Stimulation Caused by the Switching Gradient Fields:  Stimulation Caused by the Switching Gradient Fields Nerve stimulation Acoustic trauma Burn from looped cables be careful when using anything with electrical wires or cables in the scanner Functional Brain Mapping with MRI: Requirements:  Functional Brain Mapping with MRI: Requirements fMRI ready scanner (rapid switching of gradients) Stimulus Presentation Hardware and Software Subject Response Hardware and Software Analysis Tools Visualization Tools Basic fMRI Experiment:  Fixation Thumb movement time Basic fMRI Experiment Slide42:  fMRI BOLD Contrast Mapping Analysis of Functional Data:  Analysis of Functional Data Correct for Subject Motion Spatial and Temporal Filtering Transform into a Common Reference Space/Atlas Compute Statistical Significance Fusion of Structure & Function Key Concepts:  Key Concepts Basis for the Signal Contrast Temporal Resolution Spatial Resolution Basis of MRI Signal:  Basis of MRI Signal Underlying measure of all MRI is the precession of hydrogen protons in the magnetic field Basis of fMRI Signal:  Basis of fMRI Signal Underlying measure quantified with fMRI is changes in the oxygenation of hemoglobin Changes in oxygenation are related to changes in blood flow Changes in blood flow are related to neuronal activity Contrast:  Contrast In a structural MRI contrast is a measure of how easy it is to distinguish two tissues Functional Contrast:  Functional Contrast Functional contrast provides some physiological measure related to neuronal activity fMRI contrast is derived from changes in blood oxygenation Functional contrast more important in fMRI than spatial contrast Temporal Resolution:  Temporal Resolution Rate at which data can be acquired Most whole brain fMRI studies acquire data at the rate of 2-4 seconds per volume Single slice fMRI studies are collected on the order of 10’s of milliseconds Faster is desirable Spatial Resolution:  Spatial Resolution Smallest change in physical space that can be measured Higher spatial resolution desirable fMRI Compared to Other Functinal Techniques:  fMRI Compared to Other Functinal Techniques

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