Transcript Intro slides from Ryan D`Arcy

Human Functional Brain Imaging
Dr. Ryan C.N. D’Arcy
NRC Institute for Biodiagnostics (Atlantic)
Hemodynamic measures:
Positron Emission Tomography (PET)
Magnetic Resonance Imaging (MRI)
Functional Magnetic Resonance Imaging (fMRI)
Near-infrared light (NIR)
Single Photon Emission Tomography (SPECT)
Electrophysiological measures:
Electroencephalography (EEG), Evoked Potentials (EPs), Event-related Brain Potentials (ERPs)
Magnetoencephalography (MEG), Evoked Magnetic Fields (EMFs)
Transcranial Magnetic Stimulation (TMS)
• MRI uses a combination of
very strong magnetic fields
and radio frequency (RF)
pulses of energy
• In the case of NRC’s system
the magnetic field is about
80000 times stronger than the
earth’s field
• Unlike X-Ray CT or Nuclear
Medicine it does not involve
ionizing radiation
• MRI is non-invasive, repeat
studies, multiple sources of
information
NMRI = Nuclear Magnetic Resonance Imaging
Felix Bloch and Edward Purcell
In 1946 they simultaneously performed
experiments showing that atomic nuclei
absorb and re-emit radio frequency
energy
nuclear: properties of nuclei of atoms
magnetic: magnetic field required
resonance: interaction between magnetic
field and radio frequency
imaging: gradient magnet fields for spatial
encoding (xyz)
NMR Imaging  Why is it not NMRI?
“Nuclear” had bad connotations…
especially during the cold war.
Clinicians thought it would scare patients
away.
4T magnet
RF Coil
gradient coil
(inside)
• Need a REALLY strong magnet (the stronger the magnet, the bigger the signal,
the better the image)
• Need a gradient coil to pulse another magnet field that varies linearly with
space
• Need a RF coil that will pulse radio frequency energy into the subject, and then
measure the signal
Free Induction
Decay (FID)
1/2
Fourier
Transform
Time (s)
L
Frequency (Hz)
Larmor Frequency: L = B0
Quantum mechanically NMR operates by stimulated transitions between
Zeeman energy levels
E  hB0
• T1 (Spin-Lattice Relaxation): The time required for the system to return to its
equilibrium state, by exchanging the energy with its environment (the “lattice”).
• T2 (Spin-Spin Relaxation): The time required for the system to come to an
internal equilibrium, at which point the system has lost all “phase coherence”.
This governs the rate of decay of observable magnetization.
• T2* (Effective Spin-Spin Relaxation): Takes into account additional factors
which will lead to a loss of phase coherence, such as magnetic field
distortions/inhomogeneities.
Bo
Mz
90o
RF Pulse
Bo
Mx,y
Spatial resolution
VOXEL
(Volumetric Pixel)
In-plane resolution
e.g., 192 mm / 64
= 3 mm
3 mm
6 mm
IN-PLANE SLICE
Slice Thickness
e.g., 6 mm
SAGITTAL SLICE
Matrix Size
e.g., 64 x 64
Field of View (FOV)
e.g., 19.2 cm
3 mm
Contrast agents (exogenous and endogenous)
• oxy-Hgb (four O2) is
diamagnetic  no B
effects
• deoxy-Hgb is
paramagnetic  if
[deoxy-Hgb]  
local B 
Hemoglobin (Hgb):
- four globin chains
- each globin chain contains a heme group
- at center of each heme group is an iron atom (Fe)
- each heme group can attach an oxygen atom (O2)
Blood Oxygen Level Dependent signal
Mxy
Signal
Mo
sin
T2* task
T2* control
Stask
Scontrol
S
TEoptimum
time
Source: Jorge Jovicich
Source: fMRIB Brief Introduction to fMRI
~2s
ROI Time
Course
fMRI
Signal
(% change)
Time
Condition
Statistical Map
superimposed on
anatomical MRI image
Time
Region of interest (ROI)
~ 5 min