MRI History and Hardware Basic Safety Issues Introduction 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 Back projection

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

Two-Dimensional Fourier Transformation 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 Rabi (1944) Bloch, Purcell (1952) Lauterbur, Mansfield (2003) Rabi, Bloch, Purcell, Lauterbur, Mansfield

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 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

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

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…

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?

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

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?

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

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 Cyrostat Gradients Body RF Coil Passive Shims

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 Field RF Field Gradient fields Projectiles Implants/other materials in the body RF Field tissue heating Gradient fields peripheral nerve stimulation acoustic noise

Forces on Ferrous Objects Crash cart meets a 1.5T magnet

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.

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 RF Field Gradient fields projectiles tissue heating Gradient fields peripheral nerve stimulation acoustic 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 RF Field Gradient fields projectiles tissue heating Gradient fields peripheral nerve stimulation acoustic noise

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)

Introduction to Functional MRI

Difference Between MRI & 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)

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

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

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

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

Unimanual Thumb Flexion Right Thumb Left Thumb L R

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

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