1. MRI History and Hardware Basic Safety Issues Introduction to fMRI
John VanMeter, Ph.D.
Center for Functional and Molecular Imaging

2. Terms Used for MRI
NMR (Nuclear Magnetic Resonance) MR (Magnetic Resonance) MRI (Magnetic Resonance Imaging)

3. 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
Produced multiple smaller beamlets

4. Rabi
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

5. 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
Basis of Nuclear Magnetic Resonance Spectroscopy and MRI

6. Felix Bloch
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

7. Raymond Damadian
Discovery: Rat Tumor has a relaxation time longer than normal tissue
Differences in relaxation time provides one form of tissue contrast - T1

8. Paul Lauterbur
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

9. First MR Image
Lauterbur created 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
Back projection

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

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

12. Sir Peter Mansfield
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!

13. 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

14. 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)

15. 1990’s
FUNCTIONAL IMAGING

16. 5 Nobel Laureates for MRI
Rabi (1944)
Bloch, Purcell (1952)
Lauterbur,
Mansfield (2003)
Rabi, Bloch, Purcell, Lauterbur, Mansfield

17. Nobel Controversy
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"

18. MRI Hardware

19. Basic MRI Hardware Magnet Radiofrequency (RF) coils Gradients
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

20. Types of Magnets Permanent Iron Core Resistive Electromagnet
Low Field “Open”
Resistive Electromagnet
Up to 0.2T
Superconducting Magnet
Cools wire coil with cryogens
0.5T to 35T

21. 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
Fringe fields…

22. 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
What is a major expense for an MRI scanner operation?

23. RF (Radiofrequency) Coils
Used to transmit and receive RF energy
Needed to create images
Single loop LC-circuit (inductor, capacitor)

24. Coil Designs Closer coil is to object being imaged the better signal
Variety of coils designed for specific body parts
Surface Coil Volume Coil
(aka Birdcage Coil)
Why does proximity matter?

25. Coil Design Affects Images
Where surface was located to make image on right?

26. 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

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

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

29. 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

30. 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

31. MRI Safety

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

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

35. A supposedly ambulatory patient told a technologist that she was feeling rather tired and weak. She asked if she could sit down while the tech was entering data into the computer. So the tech let her sit in one of the console chairs. Since the woman was rather tired, the technologist pushed the patient into the room on the chair. Since this was an actively shielded magnet, she thought it would be OK. The patient got off the chair, sat on the table and gave the chair a little push with her feet. The chair rolled away and just kept right on rolling on a curved path to the magnet bore and flew right in. Fortunately, no one was hurt but it took almost a full day to ramp the magnet down, pull the chair out, ramp back up and check out the system.

36. Welding tank

39. 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

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

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

42. 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

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

44. 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
Changing B field
Creates voltage,
current and heat
V ~ (Area) x (dB/dt)

45. Introduction to Functional MRI

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

47. Tools Necessary for fMRI
High-field MRI (1.5T or greater) scanner
BOLD effect (fMRI signal) increases with field strength
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)

48. Here is a typical set of the stimulus presentation system – the subject is lying in the scanner and looking up at set of mirrors
that allow the subject to see a rear projection screen. In addition, the subject is also outfitted with headphones for presentation stimuli
auditorily and some noise cancelation. Lastly, the subject holds in each hand a button box. For each of tasks we want some way of measuring subject compliance
and accuracy. If we know that subject is not performing the task then the results of the analysis will not be interpretable.
NOW, I’m going to show you what is under the hood so to speak in terms of the analysis and visualization parts of the system

49. Functional Brain Mapping with MRI
Basic concept - changes in neuronal activity produces a measurable change in MR signal
Collect MRI scans continuously (1 every 2-3s each typically cover 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

50. Basic fMRI Experiment time Fixation Thumb movement
The layout of a typical fMRI experiment experiment is shown in this slide.
In the upper left corner is a diagram of what the subject sees. Initially the subject, only sees a cross on the screen and
is instructed to fixate their gaze on the cross at all times. They are also instructed to tap their thumb each time a circle
appears. The stimulus presentation system is programmed to alternate between periods of just fixation and periods of
movement. The time course of the periods is shown in the diagram at the bottom of the slide.
Through this time the scanner is continuously collecting images. In the system we developed a total of 50 whole-head
Volumes is collected in a little over 3 minutes.
This image in the upper right corner shows the results of the analysis. The voxels in color show the areas in the
Brain that show an increase in blood flow as measured with the BOLD contrast method.
time

51. Data Analysis
Identify voxels with signal changes matched to the timing of experiment
Tapping Tapping Tapping
Rest Rest Rest

52. Unimanual Thumb Flexion
Right Thumb Left Thumb
L R

53. fMRI Compared to Other Functional Techniques
How can you have high spatial and temporal resolution but poor functional resolution?

54. Examples of fMRI

55. Activity in a Vegetative State

56. Super Bowl Ads
Marco Iacoboni at UCLA used fMRI to examine the brain’s response to different super bowl ads
Ranked ads based on brain responses
Found differences in the ads that stimulated the brain most and those people reported as liking the most

57. Brain Activity During Disney Ad
Mirror Neurons

58. Brain Activity During FedEx Ad
Fear response in Amygdala during scene where the human is squashed by the dinosaur

59. Caution Needed
Interpretation of the signal changes depends on a lot of factors
Communication of results with public needs to be approached with care
McCabe & Castel (2008, Cognition) brain imaging increased perceived credibility of research compared to bar graphs