Human Functional Brain Imaging 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 ImagingFelix 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 atomsmagnetic: magnetic field requiredresonance: interaction between magnetic field and radio frequencyimaging: 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.

RF Coil

4T magnet

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

Larmor Frequency: L = B0

FourierTransform

Time (s) Frequency (Hz)

Free Induction Decay (FID) 1/2

L

Quantum mechanically NMR operates by stimulated transitions between Zeeman energy levels

0hBE =

• 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.

Mz

Bo90o

RF Pulse Mx,y

Bo

Slice Thicknesse.g., 6 mm

SAGITTAL SLICE

IN-PLANE SLICE

Field of View (FOV)e.g., 19.2 cm

VOXEL(Volumetric Pixel)

3 mm

3 mm6 mm

Matrix Sizee.g., 64 x 64

In-plane resolutione.g., 192 mm / 64

= 3 mm

Spatial resolution

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)

• oxy-Hgb (four O2) is diamagnetic no B effects

• deoxy-Hgb is paramagnetic if [deoxy-Hgb] local B

Contrast agents (exogenous and endogenous)

Source: fMRIB Brief Introduction to fMRI

Blood Oxygen Level Dependent signal

time

Mxy

SignalMo sin

T2* taskT2* control

TEoptimum

Stask

ScontrolS

Source: Jorge Jovicich

Statistical Mapsuperimposed on

anatomical MRI image

~2s

Time

Condition 1

Condition 2 ...

~ 5 min

Time

fMRISignal

(% change)

ROI Time Course

Condition

Region of interest (ROI)