Functional Neuroimaging

nuclei

anatomically discrete and identifiable clusters of neurons within the brain that typically serve a particular function

resistance vessels

arterioles that control the flow of blood through the capillary bed

spin & spin system

atomic nuclei that possess the NMR property; that is, they have both magnetic moment angular moment; spin system is a collection these these nuclei in a spatial location

pericytes

contractile cells that wrap around the endothelial cells of capillaries and that can constrict the vessel thus influencing local blood flow

MR Signal

current measured in a detector coil following excitation and reception

vascular tone

degree to which a blood vessel resists blood flow, dictated by constriction and relaxation of vascular smooth muscle (altering relative diameter)

Net Magnetization

determined by the difference between the number of spins in the parallel state and the number of spins in the antiparallel state (aka bulk magnetization); the more spins in the parallel state, the larger the M

Shimming Coils

electromagnetic coils that compensate for inhomogeneities in the static magnetic field

Gradient Coils

electromagnetic coils that create controlled spatial variation in the strength of the magnetic field; oriented along cardinal directions (represented z=parallel to main field, x & y=perpendicular to main field)

Radiofrequency Coils

electromagnetic coils used to generate and receive energy at the sample's resonant frequency, which for field strengths typical to MRI is in the radiofrequency range

phase-encoding gradient

gradient that is applied before the data acquisition period so that spins can accumulate differential phase offsets over space

frequency-encoding gradient

gradient that is applied during the data acquisition period so that the spin precession frequencies change over space

precession

gyroscopic motion of a spinning object, in which the axis of the spin itself rotates around a central axis (like a spinning top); protons precess around an axis parallel to the main magnetic field

MRI Physics Video

https://www.youtube.com/watch?v=Ok9ILIYzmaY https://www.youtube.com/watch?v=djAxjtN_7VE

acetylcholine

important neurotransmitter used throughout the central and peripheral nervous systems and at the neuromuscular junction; within the brain, ACh projections from certain cell groups in the basal forebrain may stimulate widespread changes in blood flow

magnetoencephalography (MEG)

measures changes in magnetic fields caused by neural electrical activity; measures rapid changes in magnetic flux and valuable for studying the timing of processes

electroencephalography (EEG)

measures electrical potential in the brain, usually through electrodes places on the surface of the scalp; measures rapid changes in electrical potentials & good for studying the timing of processes

contrast-to-noise ratio

megnitude of the intensity difference between different quantities divided by the variability in measurement (e.g., an airplane flying overhead obstructing the wall shadow)

single cell records

placing electrode near or within a single neuron to record change in electrical potential associated with the firing of that neuron

Reception

process of receiving electromagnetic energy emitted by a sample at its resonant frequency (also called detection). As nuclei return to a low-energy state following the cessation of the excitation pulse, they emit energy that can be measured by a receiver coil when high-energy spins fall back to the low-energy state, they emit photons whose energy is equal to the energy difference between the two states

Excitation

process of sending electromagnetic energy to a sample at its resonance frequency (also called transmission). The application of an excitation pulse to a spin system causes some of the spins to change from a low energy to a high energy state radiofrequency coils within an MRI bombard spins in the magnetic field with photons, which are actually electromagnetic waves that are adjusted so they oscillate at the resonant frequency of the nucleus of interest

aerobic glycolysis

process, consisting of glycolysis, the TCA cycle, and the electron transport chain, that breaks down glucose in the presence of oxygen, resulting in a gain of 36 ATP molecules

180 degree excitation pulse

quantity of electromagnetic energy that, when applied to a spin system during MR excitation, results in a flipping of the usual net magnetization, such that there are now more nuclei in the high energy state than in the low energy state

90 degree excitation pulse

quantity of electromagnetic energy that, when applied to a spin system during MR excitation, results in equal numbers of nuclei in the low- and high-energy states

Specific Absorption Rate

quantity that describes how much electromagnetic energy is absorbed by the body over time

vascular conducted responses

rapid spread of vasodilation or constriction along small blood vessels that is not dependent on neural activity or a passive response to changes in blood flow

nitric oxide

rapidly diffusing soluble gas that is produced in pyramidal cells, interneurons, astrocytes, and endothelial cells; possibly a molecular modulator of blood flow

functional neuroimaging

research technique that creates images of the brain's functional properties, notably different aspects of cognition and related information processing (fMRI, PET, optical imaging); has advantage of revealing short-term physiological changes associated with the active functioning of a given area

Larmor frequency

resonant frequency of a spin within a magnetic field of a given strength. It defines the frequency of electromagnetic radiation needed during excitation to make spins change to a high-energy state, as well as the frequency emitted by spins when they return to the low-energy state

TCA cycle

second step in aerobic glycolysis; it involves the oxidation of pyruvate (aka citric acid cycle/Krebs cycle)

Pulse Sequences

series of changing magnetic field gradients and electromagnetic pulses that allow the MRI scanner to create images sensitive to a particular physical property

pulse sequence

series of changing magnetic field gradients and oscillating electromagnetic (RF) pulses that allow the MRI scanner to create images sensitive to a particular physical property

HDR Initial Dip

short-term decrease in MR signal immediately following the onset of neuronal activity, before the main positive component of the hemodynamic response. The initial dip may result from initial O2 extraction before the later over-compensatory response

slice

single slab within an imaging volume (thickness of slice defined by strength of gradient and bandwidth of electromagnetic pulse used to select it)

arterioles

small arteries (largest artery - aorta ~ 1 inch - has a diameter 2500x that of smallest arteriole)

venules

small veins

capillaries

small, thin-walled blood vessels; extraction of oxygen and glucose from the blood and removal of waste carbon dioxide occurs in capillaries 60,000 miles worth of capillaries; so small that red blood vessels have to deform to pass through; pyramidal cell on average no more than 1-2 cell widths from nearest capillary, thus demonstrating their density

B0

strong static magnetic field generated by an MRI scanner

net magnetization

sum of all magnetic moments of all spins within a spin system

voxel

three-dimensional volume element; total signal recovered from a voxel is proportional to its size, and voxels that are too small may have insufficient signal to create high-quality images (1mm for structural MRI & 3mm in fMRI)

t2 (Decay)

time constant that describes the decay of the transerve component (e.g., transverse relaxation) of net magnetization due to accumulated phase differences caused by spin-spin interactions

t2* (Decay)

time constant that describes the decay of the transerve component of net magnetization due to both accumulated phase differences and local magnetic field inhomogeneities. T2* is always shorter than T2. BOLD contast fMRI relies on t2* contrast

t1 (Recovery)

time constant that describes the recovery of the longitudinal component of net magnetization over time

Magnetic Moment

torque (rotational force) exerted on a magnet, moving electrical charge, or current carrying coil when it is placed in a magnetic field

pixel

two-dimensional picture element

functional contrast

type of contrast that provides information about a physiological correlate of brain function (e.g., changes in blood oxygenation)

slice selection

combined use of spatial magnetic field gradient and radiofrequency pulse to excite spins within a slice

electron transport chain

third step in aerobic glycolysis; generated an additional 34 ATP molecules

HDR Balloon Model

A model of the interaction between changes in blood volume and changes in blood flow associated with neuronal activity.

Vascular response to neuronal activity

1. flow-controlling subtances released by neurons into the extracellular space (influenced by neurons & astrocytes) 2. Neurons may directly alter flow in arteries/arterioles/capillaries (fMRI consequences unknown) 3. Arterial supply of oxygenated hemoglobin (O2 extracted in the capillaries)

fMRI & doxygenated hemoglobin

Changes in concentrations of deoxygenated hemoglobin form the basis of fMRI. Blood flow & cerebral metabolism well-coupled at rest. Functional hyperemia during stimulation (divergence from ideal metabolic rates), which may be to meet energy demands (emphasizes delivery of O2 & glucose to active tissue through blood flow) or due to neural control (emphasizes disassociation between metabOlic need & O2/glucose delivery).

Two Criteria for Suitable Magnetic Field

Field Uniformity (Homogeneity) - if not uniform, signal from part of body could change depending on where it is located in magnetic field (Helmholtz pair & solenoid generate homogeneous fields) Field Strength - superconducting electromagnets whose wires are cooled to temperatures near absolute zero, resulting in disappearance of resistance in wires (create strong, stable, lasting electric current with no power requirements and at minimal cost)

Magnetization Vectors

Longitudinal - parallel or antiparallel to magnetic field Transverse - perpendicular to magnetic field

Three Main Components of MRI

M - Main static field (electromagnetic coils carry large currents around bore of scanner); when a human body is palced, atomic nuclei become aligned with the magnetic field R - delivery of energy at resonance frequency of targeted molecule I - Image formation, requires alteration of magnetic field strength over space by turning on and off the magnetic gradient coils

Magnetic Properties of Blood Cells

Oxygenated Hemoglobin - diamagnetic (no unpaired electrons & 0 magnetic moment) Deoxygenated Hemoglobin - paramagnetic (both unpaired electrons & significant magnetic moment) Deoxygenated blood has 20% greater magnetic susceptibility than fully oxygenated blood

Paramagnetic Substances & Protons

Paramagnetic substances (e.g., hemoglobin) distort magnetic field & impact nearby protons, resulting in more reapid decay of transverse magnetization (e.g., MR signal greater for highly oxygenated blood). However, process of displacing deoxygenated hemoglobin that had been suppressing MR signal intensity actually results in greater T2 contrast with increased neural activity. Sometimes, local deoxygenated hemoglobin accumulates without concomitant increases in blood flow, thus leading to a negative "initial dip" in BOLD time course.

Radiofrequency Coil Arrangments

Surface Coils - placed on the surface of the head (excellent sensitivity to nearby regions, poor for distant regions) Volume Coils - surrounds entire sample (similar sensitivity throughout) *Phased Array - arranging multiple surface detector coils to improve spatial coverage while maintaining high sensitivity

HDR Undershoot

The decrease in MR signal amplitude below baseline due to the combination of reduced blood flow and increased blood volume.

HDR Peak

The maximal amplitude of the hemodynamic response, typically occurring 4-6 sec following a short-duration event

spatial resolution

ability to distinguish changes in an image across different spatial locations

temporal resolution

ability to distinguish changes in signal over time; in fMRI, even if we sampled more quickly, hemodynamic changes may occur to slowly to make inferences about more rapid neuronal activity

functional resolution

ability to map measured physiological variation to underlying mental processes or behaviors

glycolysis

after glucose in trasported from capillaries to cytoplasm of neuron, glyolysis occurs where glucose is proken down into other compounds to produce ATP; glycolysis uses two ATP molecules, but produces 4

Angular Momentum

because proton has an odd-numbered atomic mass, its spin has angular momentum (multiplying mass of spinning body by its angular velocity - H=mass of 1)

veins

blood vessels that carry blood from the body to the heart; blood in the veins (except for pulmonary veins) is partially deoxygenated

anastomosis

branching and reconnection of blood vessels

Hemodynamic Response (HDR)

change in MR signal on T2 images following local neuronal activity. The hemodynamic response results from decrease in the amount of deoxygenated hemoglobin present within a voxel

Relaxation

change in net magnetization over time (transverse magnetization quickly loses coherence, and the longitudinal magnetization slowly recovers)

pulse sequence

changes is magnetic gradients and oscillating electromagnetic fields; depending on electromagnetic field frequency, energy is absorbed by atomic nuclei (e.g., most scanners are tuned to hydrogen frequency); after absorption, electromagnetic energy is released and the amount correlates to the number and types of nuclei present; alters quantum properties but has no influence on neuronal firing/blood flow

manipulation techniques

changing brain structure/function & observing effects on behavior

structural neuroimaging

class of research and clinical techniques that create images of the brain's physical structure, often to provide insight into the locations and distribution of different types of tissue; often used to relate neurological disorders to the patterns of brain injury that cause them

static magnetic field

field created by MRI scanner expressed in units of tesla (3.0 T is normal; range is 1.5-7)

sampling rate

frequency in time with which a measurement is made (e.g., fMRI one brain volume every 1-2 sec, PET changes in brain metabolism every 3-10 minutes; both are much slower than directly measuring electrical activity)

positron emission tomography

functional neuroimaging technique that creates images based on the movement of injected radioactive material; disadvantage is invasiveness of radioactive injections, expense of generating radioactive isotopes, and slow speed; still used for targeting specific chemicals or metabolites like neurotransmitters & relating concentrations to behavior/genetics/individual differences; measures how much energy is being used by a part of the brain by change sin glucose levels

circle of Willis

interconnection between the basilar artery and the carotid arteries at the base of the cranial vault

Nuclear Magnetic Resonance Property (NMR)

label for atomic nuclei that have both magnetic moment and angular momentum, which together allow them to exhibit nuclear magnetic resonance effects (nucleus with even number protons & neutrons causes magnetic moment to be cancelled & nucleus would be invisible to MRI)

arteries

large, thick-walled blood vessels that carry oxygenated blood from the heart to the rest of the body

sinuses

long venous channels formed by meningeal coverings that form the primary draining system for the brain; air-filled cavities in the skull

spatial gradient

magnetic field whose strength varies systematically over space (note: since a given spatial location only experiences one magnetic field, which represents the sum of all fields present, spatial gradients in MRI act to change the effective strength of the main magnetic field over spacE)

Fourier transform

mathematical technique for converting signal into its power spectrum

flux

measure of strength of a magnetic field over an area of space

noradrenaline (NA)

neurotransmitter used extensively in the cnetral and peripheral nervous system; within the brain, NA projections from the locus coeruleus nuclei of the brain stem plays a role in a number of psychological processes, including attention and alertness (aka norepinephrine)

functional MRI

noninvasive technique that usually measure changes in blood oxygenation levels over time (blood O2 changes rapidly following neural activity); fMRI allows localization of brain activity on a second-by-second basis & within milliseconds of its origin; measures how much energy is being used in a part of the brain by change sin oxygen

adenosine triphosphate (ATP)

nucleotide containing three phosphate groups that is the primary energy source for cells in the human body

parallel/antiparallel state

parallel has lower energy level than protons in an antiparallel state; parallel state is more stable and there will always be more protons in this state (exact proportion depends on temperature & strength of magnetic field)

Echo-Planar Imaging (EPI)

technique that allows collection of an entire two-dimensional image by changing spatial gradients rapidly following a single electromagnetic pulse from a transmitter coil

transcranial magnetic stimulation

technique to temporarily stimulate a brain region to disrupt its function; uses an electromagnetic coil placed close to the scalp, currents pass through coil and generate magnetic field in nearby brain tissue producing localized electric currents

contrast

the intensity difference between different quantities being measured by an imaging system; imaging techniques sensitive to small gradations in quantity being measured will have good contrast

measurement techniques

using equipment to measure brain fx while subjects engage in task