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

MRI History and HardwareBasic Safety IssuesIntroduction to fMRI

John VanMeter, Ph.D.Center for Functional and Molecular

Imaging

Terms Used for MRI

NMR (Nuclear Magnetic Resonance)

MR (Magnetic Resonance)

MRI (Magnetic Resonance Imaging)

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

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

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

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

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

• 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

First MR Image - 1973 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

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

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

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 EPI used today in fMRI!

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

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)

1990’s

FUNCTIONAL IMAGING

5 Nobel Laureates for MRI

Bloch, Purcell (1952)

Lauterbur, Mansfield (2003)

Rabi (1944)

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"

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 Permanent Iron Core

Low Field “Open” Resistive Electromagnet

Up to 0.2T Superconducting Magnet

Cools wire coil with cryogens 0.5T to 35T

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

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 Used to transmit and receive RF energy Needed to create images

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)

Coil Design Affects Images

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

Under the Hood of an MRI Scanner

Gradients

Body RF Coil

Passive Shims

Cyrostat

Under the Hood of Our MRI Scanner

Quench Pipe

Cold Head

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

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

MRI Safety

MRI Safety

Static B0 FieldProjectilesImplants/other materials in the body

RF Fieldtissue heating

Gradient fieldsperipheral nerve stimulationacoustic noise

Forces on Ferrous Objects

Crash cart meets a 1.5T magnet

Welding tank

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 Static B0 Field

projectiles RF Field

tissue heating Gradient fields

peripheral nerve stimulationacoustic noise

RF Exposure Standards

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

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 Static B0 Field

projectiles RF Field

tissue heating Gradient fields

peripheral nerve stimulationacoustic noise

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

Changing B field Creates voltage,current and heat

V ~ (Area) x (dB/dt)

Introduction to Functional MRI

Difference BetweenMRI & fMRI

From: Daniel Bulte

Centre for Functional MRI of the Brain

University of Oxford

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)

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

Fixation

time

Basic fMRI Experiment

Thumb movement

Data Analysis

430435440445450455460465470475480

0 20 40 60 80 100

Time

AU

Identify voxels with signal changes matched to the timing of experiment Tapping Tapping

Tapping

Rest Rest Rest

Unimanual Thumb Flexion

L R

Right Thumb Left Thumb

fMRI Compared to Other Functional Techniques

Examples of fMRI

Activity in a Vegetative State

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

Brain Activity During Disney Ad

Mirror Neurons

Brain Activity During FedEx Ad

Fear response in Amygdala during scene where the human is squashed by the dinosaur

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