Download An Introduction to Functional MRI

An Introduction to Functional
MRI
Brain Imaging and Analysis Center
FMRI Graduate Course
Summary of the Course
• Combines lectures and laboratory sessions
– Laboratories will illustrate concepts from lectures
• Grading basis
–
–
–
–
Participation in course sessions (attendance, discussion)
Completion of laboratory exercises
One take-home test (mid-term)
Practicum research project at end of semester
• Course web page (www.biac.duke.edu/education)
– BIAC Logins
• Readings
– Buxton, Introduction to fMRI
– Original papers (generally posted on the web page)
– Full reading list to be posted over upcoming weeks
Outline for Today
•
•
•
•
•
Why use fMRI to image brain function?
Key concepts of fMRI
History of fMRI
Parts of a MR scanner
MR safety
• Laboratory: Scanner Visit (Dr. Jim Voyvodic)
– Scanner hardware
– Stimulus presentation and recording hardware
– Demonstration of real-time fMRI
What is fMRI?
• A technique for measuring changes in brain
activity over time using principles of
magnetic resonance.
• Scanning procedures and restrictions are
generally similar to clinical MR.
• Most fMRI studies use changes in BOLD
contrast, although other measures exist.
Growth of fMRI : Published Studies
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
…
2002
0
100
200
300
400
500
600
700
800
900
Why the Growth of fMRI?
Why the Growth of fMRI?
• Powerful
– Improved ability to understand cognition
– Better spatial resolution than PET
– Allows new forms of analysis
• High benefit/risk ratio
– Non-invasive (no contrast agents)
– Repeated studies (multisession, longitudinal)
• Accessible
– Uses clinically prevalent equipment
– No isotopes required
– Little special training for personnel
New Cognitive Analyses
• Sampling rate affects experimental design
– PET: >30s/data point ; fMRI: 1s/data point
– Cognitive processes being measured must change more
slowly than sampling rate
• New forms of analyses
– Event-related: sorting trials by accuracy, response time,
type of condition
– Rapid stimulus presentation
• Allows creation of process models of activity
– Difference in activation timing between regions is often
on order of 100-1000ms
Cheng, Waggoner, & Tanaka (2001) Neuron
Sakai, Rowe, & Passingham (2002) Nature Neuroscience
Image provided by Dr. James Voyvodic (Duke BIAC)
Key Concepts
•
•
•
•
Contrast
Spatial Resolution
Temporal Resolution
Functional Resolution
Anatomical Contrast
Definition: The ability to distinguish between two (or more)
different properties of tissue.
Blood Oxygenation Level Dependent
(BOLD) Contrast
From Mosley & Glover (1995)
Design Effects on Functional Contrast
Contrast should really be considered as “contrast to noise”: how effectively can
we decide whether a given brain region has property X or property Y?
Spatial Resolution: Voxels
Voxel: A small rectangular prism that is the basic sampling unit of fMRI.
Typical functional voxel: (4mm)3. Typical anatomical voxel: (1.5mm)3.
Spatial Resolution: Examples
~8mm2
~4mm2
~1.5mm2
~2mm2
~1mm2
Temporal Resolution
• Importance depends upon research question
– Type I: Detection
• Temporal resolution is only indirectly important if your study
investigates whether or not a given brain region is active.
– Type II: Estimation
• Temporal resolution is extremely important when attempting to
understand the properties of an active region.
• Determining factors
– Sampling rate, usually repetition time (TR)
– Dependent variable, usually BOLD response
• BOLD response is sluggish, taking 2-3 seconds to rise above
baseline and 4-6 seconds to peak
– Experimental design
From Jezzard et al., 2001
Functional Resolution
The ability of a measurement technique to identify
the relation between underlying neuronal activity
and a cognitive or behavioral phenomenon.
Functional resolution is limited both by the intrinsic
properties of our brain measure and by our ability
to manipulate the experimental design to allow
variation in the phenomenon of interest.
History of Magnetic
Resonance Imaging
Timeline of MR Imaging
1924 - Pauli suggests
that nuclear particles
may have angular
momentum (spin).
1972 – Damadian
patents idea for large
NMR scanner to
detect malignant
tissue.
1937 – Rabi measures
magnetic moment of
nucleus. Coins
“magnetic resonance”.
1944 – Rabi wins
Nobel prize in
Physics.
1952 – Purcell and
Bloch share Nobel
prize in Physics.
1920
1930
1940
1950
1946 – Purcell shows
that matter absorbs
energy at a resonant
frequency.
1946 – Bloch demonstrates
that nuclear precession can be
measured in detector coils.
1960
1959 – Singer
measures blood flow
using NMR (in
mice).
1985 – Insurance
reimbursements for
MRI exams begin.
1973 – Lauterbur
publishes method for
generating images
using NMR gradients.
MRI scanners
become clinically
prevalent.
NMR becomes MRI
1970
1980
1973 – Mansfield
independently
publishes gradient
approach to MR.
1975 – Ernst
develops 2D-Fourier
transform for MR.
1990
2000
1990 – Ogawa and
colleagues create
functional images
using endogenous,
blood-oxygenation
contrast.
Discovery of Nuclear Magnetic
Resonance Absorption (1946)
• Bloch and Purcell independently
discovered how to measure nuclear
moment in bulk matter (1946)
– Determined relaxation times.
• They showed that energy applied
at a resonant frequency was
absorbed by matter, and the reemission could be measured in
detector coils
Felix Bloch
• They shared the 1952 Nobel Prize
in Physics
Edward Purcell
Timeline of MR Imaging
1924 - Pauli suggests
that nuclear particles
may have angular
momentum (spin).
1972 – Damadian
patents idea for large
NMR scanner to
detect malignant
tissue.
1937 – Rabi measures
magnetic moment of
nucleus. Coins
“magnetic resonance”.
1944 – Rabi wins
Nobel prize in
Physics.
1952 – Purcell and
Bloch share Nobel
prize in Physics.
1920
1930
1940
1950
1946 – Purcell shows
that matter absorbs
energy at a resonant
frequency.
1946 – Bloch demonstrates
that nuclear precession can be
measured in detector coils.
1960
1959 – Singer
measures blood flow
using NMR (in
mice).
1985 – Insurance
reimbursements for
MRI exams begin.
1973 – Lauterbur
publishes method for
generating images
using NMR gradients.
MRI scanners
become clinically
prevalent.
NMR becomes MRI
1970
1980
1973 – Mansfield
independently
publishes gradient
approach to MR.
1975 – Ernst
develops 2D-Fourier
transform for MR.
1990
2000
1990 – Ogawa and
colleagues create
functional images
using endogenous,
blood-oxygenation
contrast.
Early Uses of NMR
• Most early NMR was used for chemical analysis
– No medical applications
• 1971 – Damadian publishes and patents idea for using
NMR to distinguish healthy and malignant tissues
– “Tumor detection by nuclear magnetic resonance”, Science
– Proposes using differences in relaxation times
– No image formation method proposed
• 1973 – Lauterbur describes projection method for creating
NMR images
– Mansfield (1973) independently describes similar approach
The First ZMR NMR Image
Lauterbur, P.C. (1973). Image formation by induced local interaction: Examples employing
nuclear magnetic resonance. Nature, 242, 190-191.
Early Human MR
Images (Damadian)
Mink5 Image – Damadian (1977)
Timeline of MR Imaging
1924 - Pauli suggests
that nuclear particles
may have angular
momentum (spin).
1972 – Damadian
patents idea for large
NMR scanner to
detect malignant
tissue.
1937 – Rabi measures
magnetic moment of
nucleus. Coins
“magnetic resonance”.
1944 – Rabi wins
Nobel prize in
Physics.
1952 – Purcell and
Bloch share Nobel
prize in Physics.
1920
1930
1940
1950
1946 – Purcell shows
that matter absorbs
energy at a resonant
frequency.
1946 – Bloch demonstrates
that nuclear precession can be
measured in detector coils.
1960
1959 – Singer
measures blood flow
using NMR (in
mice).
1985 – Insurance
reimbursements for
MRI exams begin.
1973 – Lauterbur
publishes method for
generating images
using NMR gradients.
MRI scanners
become clinically
prevalent.
NMR becomes MRI
1970
1980
1973 – Mansfield
independently
publishes gradient
approach to MR.
1975 – Ernst
develops 2D-Fourier
transform for MR.
1990
2000
1990 – Ogawa and
colleagues create
functional images
using endogenous,
blood-oxygenation
contrast.
Using MRI to Study Brain Function
Visual Cortex: Kwong, et al., 1994
Somatosensory Cortex: Hammeke, et al., 1994
Parts of a MR Scanner
BIAC 1.5T Scanner
MRI Safety
Issue: The appropriate risk level for a research participant is
much lower than for a clinical patient.
Hospital Nightmare
Boy, 6, Killed in Freak MRI Accident
July 31, 2001 — A 6-year-old boy died after
undergoing an MRI exam at a New Yorkarea hospital when the machine's powerful
magnetic field jerked a metal oxygen tank
across the room, crushing the child's
head. …
ABCNews.com
MR Incidents
• Pacemaker malfunctions leading to death
– At least 5 as of 1998 (Schenck, JMRI, 2001)
– E.g., in 2001 an elderly man died in Australia after being twice
asked if he had a pacemaker
• Blinding due to movements of metal in the eye
– At least two incidents (1985, 1990)
• Dislodgment of aneurysm clip (1992)
• Projectile injuries (most common incident type)
– Injuries (e.g., cranial fractures) from oxygen canister (1991, 2001)
– Scissors hit patient in head, causing wounds (1993)
• Gun pulled out of policeman’s hand, hitting wall and firing
– Rochester, NY (2000)
Issues in MR Safety
• Magnetic Field Effects
• Known acute risks
– Projectiles, rapid field changes, RF heating,
claustrophobia, acoustic noise, etc.
• Potential risks
– Current induction in tissue at high fields
– Changes in the developing brain
• Epidemiological studies of chronic risks
– Extended exposure to magnetic fields
• Difficulty in assessing subjective experience
– In one study, 45% of subjects exposed to a 4T scanner
reported unusual sensations (Erhard et al., 1995)
Possible Effects of Magnetic Fields
• Physiological
– Red blood cells (especially sickled) may alter shape in a
magnetic field
– Some photoreceptors may align with the field.
• Sensory (generally reported in high-field)
–
–
–
–
Nausea
Vertigo
Metallic taste
Magnetophosphenes
Risks of MRI
•
•
•
•
•
•
•
Projectile Effects: External
Projectile Effects: Internal
Radiofrequency Energy
Gradient field changes
Claustrophobia
Acoustic Noise
Quenching
Projectile Effects: External
Chaljub (2001)
Schenck (1996)
“Large ferromagnetic objects that were reported as having been
drawn into the MR equipment include a defibrillator, a wheelchair,
a respirator, ankle weights, an IV pole, a tool box, sand bags
containing metal filings, a vacuum cleaner, and mop buckets.”
-Chaljub et al., (2001) AJR
Chaljub (2001)
Radiofrequency Energy
• Tissue Heating
– Specific Absorption Rate (SAR; W/kg)
• Pulse sequences are limited to cause less than a one-degree rise in
core body temperature
• Scanners can be operated at up to 4 W/kg (with large safety margin)
for normal subjects, 1.5 W/kg for compromised patients (infants,
fetuses, cardiac)
– Weight of subject critical for SAR calculations
• Burns
– Looped wires can act as RF antennas and focus energy in a small
area
• Most common problem: ECG leads
• Necklaces, earrings, piercings, pulse oximeters, any other cabling
Projectile/Torsion Effects: Internal
• Motion of implanted medical devices
– Clips, shunts, valves, etc.
• Motion or rotation of debris, shrapnel, filings
– Primary risk: Metal fragments in eyes
• Swelling/irritation of skin due to motion of
iron oxides in tattoo and makeup pigments
Acoustic Noise
• Potential problem with all scans
– Short-term and long-term effects
• Sound level of BIAC scanners
– 1.5T: 93-98 dB (EPI)
– 4.0T: 94-98 dB (EPI)
• OSHA maximum exposure guidelines
– 2-4 hours per day at BIAC levels
• Earplugs reduce these values by 14-29 dB,
depending upon fit.
Gradient Field Changes
• Peripheral nerve stimulation
– May range from distracting to painful
– Risk greatly increased by conductive loops
• Arms clasped
• Legs crossed
• Theoretical risk of cardiac stimulation
– No evidence for effects at gradient strengths
used in MRI
Claustrophobia
• Most common subject problem
– About 10% of patients
– About 1-2% of BIAC subjects
• Ameliorated with comfort measures
–
–
–
–
Talking with subject
Air flow through scanner
Panic button
Slow entry into scanner
Quenching
• Definition: Rapid decrease in magnetic field strength due to
loss of superconductivity
– Only initiated voluntarily due to danger to participant’s life or health
• Effects
– Magnets heat up with loss of current
– Cryogenic fluids (Helium) boil off and fill the scanner room
• Displaces breathable air from room
• Cooling of room, condensation reduces visibility
– Physical damage to the scanner may occur
– Safety personnel must be cognizant of room conditions
Scanner Visit
• Anyone with implanted metal should see
me before going to the scanner
– Pacemaker, cochlear implant, shunt, clip, etc.
– Dental work and piercings are fine
• Please do not bring backpacks, books, etc.,
– You can leave them here in the room, which we
will lock
• MR center is a working patient environment