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 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.
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 = B 0 Fourier Transform Time (s) Frequency (Hz) Free Induction Decay (FID) 1/2 L L Quantum mechanically NMR operates by stimulated transitions between Zeeman energy levels
T 1 (Spin-Lattice Relaxation): The time required for the system to return to its equilibrium state, by exchanging the energy with its environment (the “lattice”). T 2 (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. T 2 * (Effective Spin-Spin Relaxation): Takes into account additional factors which will lead to a loss of phase coherence, such as magnetic field distortions/inhomogeneities. MzMz BoBo 90 o RF Pulse M x,y BoBo
Slice Thickness e.g., 6 mm SAGITTAL SLICE IN-PLANE SLICE Field of View (FOV) e.g., 19.2 cm VOXEL (Volumetric Pixel) 3 mm 6 mm Matrix Size e.g., 64 x 64 In-plane resolution e.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 (O 2 ) oxy-Hgb (four O 2 ) is diamagnetic no B effects deoxy-Hgb is paramagnetic if [deoxy-Hgb] local B Contrast agents (exogenous and endogenous)
Source: fMRIB Brief Introduction to fMRIfMRIB Brief Introduction to fMRI Blood Oxygen Level Dependent signal time M xy Signal M o sin T 2 * task T 2 * control TE optimum S task S control SS Source: Jorge Jovicich
Statistical Map superimposed on anatomical MRI image ~2s Time Condition 1 Condition 2... ~ 5 min Time fMRI Signal (% change) ROI Time Course Condition Region of interest (ROI)