MRI History and Hardware Basic Safety Issues Introduction to fMRI

1. MRI History and HardwareBasic Safety IssuesIntroduction to fMRI John VanMeter, Ph.D. Center for Functional and Molecular Imaging

2. Outline • History of MRI • Hardware components of an MRI scanner • Basic MRI Safety • Introduction to fMRI

3. Chapter 1: History of MRI

4. Pauli, Stern and Gerlach - 1920’s • Pauli postulated that atomic nuclei (e.g. H, C, etc) have two properties: spin and magnetic moment • Further, the rate of spin occurs at a given frequency depending on the nuclei • Stern & Gerlach demonstrate this in pure gases • Shot beam of gas through a static magnetic field • Produced multiple smaller beamlets

5. Rabi - 1937 • Rabi showed that nuclei absorb energy if the frequency matched the “resonant frequency” of the nuclei • Showed resonance frequency is dependent on static magnetic field strength • Measured resonance frequency of the lithium nucleus

6. Edward Purcell - 1945 • Detected resonance frequency in bulk matter • Used current passing through paraffin wax in a strong magnetic field • Changed strength of magnetic field over time • At first did not see any change in current but hypothesized it would take some time for relaxation of the spins to occur • Repeated experiment after leaving wax in magnetic field overnight and had success • Fundamental basis of Nuclear Magnetic Resonance Spectroscopy and MRI

7. Felix Bloch - 1945 • Similar experiment to Purcell’s except using water in a brass box inside a magnetic field • Used a transmitter coil to send electromagnetic energy into the box and receiver coil to measure changes in energy absorbed by the water • Was also able to measure magnetic resonance effect • This basic setup is the basis of NMR spectrometers used in biochemistry • With some additional refinements it is also the basis modern MRI scanners

8. Raymond Damadian - 1971 Discovered tumors in a rat had relaxation time longer than normal tissue Differences in relaxation time provides one form of tissue contrast - T1

9. Paul Lauterbur - 1973 • Used GRADIENTS to distinguish spatially localized signals PHASE ENCODING • Also, used GRADIENTS to manipulate the frequency of the spins to localize signals. He referred to this as Zeumatography FREQUENCY ENCODING Both techniques needed to encode spatial location of signals

10. First MR Image - 1973 • Lauterbur created an image by applying gradients at different angles to produce 1D projections • Combining projections forms image (back-projection reconstruction technique) • Inefficient as time needed for each angle equivalent to a single acquisition

11. Sir Peter Mansfield - 1974 Devised selective excitation of a slice again using gradients  Slice Select Identifies where in a 3D object to collect signal from

12. Richard Ernst - 1975 Used 2D-FT Two-Dimensional Fourier Transformation Needed to reconstruct images, which are encoded with frequency and phase Faster alternative to back-projection technique

13. Sir 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 signal • EPI used today in fMRI!

14. Damadian - 1977 • First ever MRI image of the 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!

15. FDA Clears First MRI Scanner - 1985 • Minicomputers such as the PDP-11 and VAX become widely available • GE develops first “high-field” (1.5T) commercial MRI scanner (1982) • Medicare starts paying for MRI scans (1985) VAX 11/750 (1982)

16. 1990’s Functional imaging using MRI is first demonstrated – initially by injecting a contrast agent and later using properties of the blood itself

17. 5 Nobel Laureates for MRI Rabi (1944) Bloch, Purcell (1952) Lauterbur, Mansfield (2003)

18. 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 breakthrough 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"

19. Chapter 2: MRI Hardware

20. 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

21. 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

22. Electromagnets • Field proportional to number of loops relative to cross-section area of each loop • Increases in current also increases field strength • Field highest and most homogenous at center of coil

23. Properties of Superconducting Magnets • Very high field strengths generated • Cool magnet’s wire coil using cryogens (liquid helium and in older scanners 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

24. RF (Radiofrequency) Coils • Used to transmit and receive RF energy • Needed to create images

25. Surface Coil Volume Coil (aka Birdcage Coil) Coil Designs • Closer coil is to object being imaged the better signal • Variety of coils designed for specific body parts

26. Coil Design Affects Images

27. Gradient Coils • Induce small linear changes in magnetic field along one or more dimensions • Produces two types of spatial encoding referred to as Frequency and Phase Encoding

28. Under the Hood of an MRI Scanner Cyrostat Gradients Body RF Coil Passive Shims

29. Under the Hood of Our MRI Scanner Quench Pipe Cold Head

30. Computer System and Console • Image reconstruction and post processing is computationally intensive • Standard workstation sufficient for basic clinical MRI system • Multi-processor systems with gigabytes of memory needed for functional MRI and DTI (Diffusion Tensor Imaging) scanning • Console computer coordinates everything

31. Patient Handling System • Methods to get patient in and out of the scanner • Alignment of the body part to be scanned with isocenter of the scanner • Labeling of scans with appropriate identifiers and anatomic labels

32. MRI Safety

33. MRI Safety • Static B0 Field • Projectiles • Implants/other materials in the body • RF Field • tissue heating • Gradient fields • peripheral nerve stimulation • acoustic noise

34. Forces on Ferrous Objects Crash cart meets a 1.5T magnet

37. Welding tank

40. 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

41. MRI Safety • Static B0 Field • projectiles • RF Field • tissue heating • Gradient fields • peripheral nerve stimulation • acoustic noise

42. RF Exposure Standards • The FDA limits RF exposure to less than a 1 degree C rise in core body temperature

43. 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

44. MRI Safety • Static B0 Field • projectiles • RF Field • tissue heating • Gradient fields • peripheral nerve stimulation • acoustic noise

45. Creates voltage, current and heat Changing B field V ~ (Area) x (dB/dt) Stimulation Caused by the SwitchingGradient Fields • Nerve stimulation • Acoustic trauma • Burn from looped cables • be careful when using anything with electrical wires or cables in the scanner

46. Introduction to Functional MRI

47. Difference BetweenMRI & fMRI From: Daniel Bulte Centre for Functional MRI of the Brain University of Oxford

48. Tools Necessary for fMRI • High-field MRI (1.5T or greater) scanner • BOLD effect (fMRI signal) increases with field strength though not linearly • Fast imaging sequence • Echo Planar Imaging (EPI) • Stimulus presentation equipment • Projector to show visual stimuli • Response devices such as button box to record subject’s response • Headphones for auditory stimuli (and hearing protection)

50. Functional Brain Mapping with MRI • Basic concept - changes in neuronal activity produces a measurable change in MR signal • Collect 100-500 MRI scans continuously (1 every 2-3s; each typically cover 30-50 slices) • Experimenter induces changes in activity at known points in time by having subject perform some cognitive or motoric task • Analyses statistically tests for MR signal changes that corresponding to experimental task