Chapter 12 p. 2 (fMRI) Flashcards
neurovascular coupling is the phenomenon whereby changes in __ are modulated by changes in __
neural activity; vascular parameters controlling blood delivery to the brain
in response to local neuronal activity, __ ( a type of __) mediate changes in __, leading to enlargements of __ and increased delivery of __
astrocytes; glial cells; blood vessel diameter; arterioles; glucose to neurons
__ (especially __) also have a substantial effect upon neurovascular coupling
neurotransmitters; dopamine
dopamine terminals are found next to small intracortical arterioles and capillaries in physical adjacency to __, which are __ cells able to __ capillaries and thereby to influence __
pericytes; contractile; constrict or to dilate; local flow patterns
blood contains __ (2)
oxygenated hemoglobin (Hb) and deoxygenated hemoglobin (dHb)
Hb is __ and has __
diamagnetic; no magnetic moment
dHb is __ and has __
paramagnetic; magnetic moment
concentrations of Hb and dHb affect the __ in the brain and cause changes in the __ of brain tissue
local magnetic field; local relaxation constants
as the percentage of Hb in the blood increases, the __ increases, so that the __ goes down
transverse relaxation time; transverse relaxivity (1/T2)
as paramagnetic substances __ the local magnetic field, spins precess at __ and thus lead to __ of the transverse magnetization
distort; different frequencies; more rapid decay
__ is essentially unaffected by the percentage of oxygenated blood because it is predicated on __
T1 recovery; spin-lattice interactions
following neuronal stimulation, the concentration of dHb __ at stimulus onset (__ at 2 seconds), and then __ at 6 seconds after onset, then returns to baseline at about 10 seconds
increases rapidly; peaking; declines to a minimum
Hb concentrations begin to __ shortly after stimulus and __ after 5 seconds, with a __ at 10 seconds
rise; peak; slow decline
the changes in d/Hb after stimulation is due to the fact that active neurons utilize __ to support their initial activity, so the initial metabolic response of neurons is __
whatever oxygen is already present; oxidative
neuronal activity may lead to prolonged reductions in __ compared with flow, and these changes persist even __
glucose metabolism; after activity ceases
under normal conditions, Hb is converted to dHb __
at a constant rate within the capillary bed
when neurons become active, the vascular system supplies __
more Hb than is needed by the neurons through an overcompensatory increase in blood flow
the overcompensatory increase in blood flow results in an increase in the amount of __ and a corresponding decrease in __, leading to a __ fMRI image
dHb; signal loss due to T2* effects; brighter
the BOLD signal contrast occurs not because __ but rather because __
Hb increase the MR signal; Hb displaces dHb which had been suppressing MR signal intensity prior to neuronal activation
after a single exogenously-triggered neuronal activation, the BOLD response exhibits __
a characteristic rise and fall followed by an undershoot and return to baseline
the shape of the BOLD response varies with the __ and the classical BOLD response may have a different __ depending on __
nature of the stimulus; temporal profile; activation pattern
after a block of successive event-related activations, the BOLD response changes shape by __
developing a plateau instead of a peak
the length of the BOLD plateau is proportional to the __, as well as the __
number of events which trigger the neuronal activation; frequency of these events
to appropriately model the BOLD response after a complex stimulus, __ is often required
complex mathematical modeling of the response itself
cerebral blood flow (CBF) indicates the amount of __
blood supply to the brain within a specified period of time
cerebral blood volume (CBV) indicates the amount of __
blood which is present at any given time within a specified brain tissue volume
the BOLD signal depends not only on the rate of __ but also on __ and __ levels
oxygen extraction; CBF; CBV
after stimulus offset, __ levels return quickly to baseline levels but __ levels off more slowly
CBF; CBV
elevated CBV relative to CBF causes an increase in __ in the tissue, which is probably the cause of the __ in the BOLD signal
total amount of dHb present; post-stimulus undershoot
simultaneous ECoG and fMRI recordings indicate the relationship between __ (4)
BOLD response, local field potentials (LFPs), multi-unit activity (MUA), and single-unit activity (SUA)
SUA is due to the __
firing of individual neurons close to recording electrodes
MUA is due to the __
collective firing of large PY neurons within a few hundred microns of the ECoG electrodes
LFPs are __
synchronous changes in integrative activity from cells within a few mm of the electrode
the longer the stimulus, the more sustained the __ (4) are, though the neural response always __
SUA, MUA, LFP, and BOLD responses; precedes the BOLD response, which is much slower
even in cases where cells in neural tissue are activated by a very precise and well-defined stimulus, there remains __
substantial variability in the amplitude and form of the hemodynamic response across events
these individuals events must be averaged in order to retrieve __ and to increase the __ of the experiment
the ‘classical’ shape of the BOLD response; SNR
generally, single-trial fMRI data do not provide __, so __ must be employed instead
a sufficiently accurate basis for interpretation and analysis; multi-trial paradigms
__ noise factors make the true BOLD response __
intrinsic and extrinsic; very difficult to isolate from single-trial experiments
the functional resolution of fMRI is strongly dependent upon the __
concordance of neuronal activity and vascular responses to that activity
the maps of neuronal activations generated using __ do not overlap very well with those create via __
electrophysiological methods; fMRI
the size of activation maps is larger when measured via __ than via __
fMRI; ECoG
spatial filtering effects of the vascular system cause the fMRI signal to be __
spatially blurred over a larger spatial extent than ECoG signals
because the typical fMRI voxel size is larger than ideal, __ are often encountered because decreasing voxel size results in reduced __ and increased __
partial volume effects; SNR; acquisition time
the BOLD signal is due to __, which are present in __ and __ but not in __
dHb levels; capillaries; veins; arterial blood
dHb molecules are __ and generate __ which unfortunately extend __
paramagnetic; magnetic field changes; far into surrounding tissue
the primary mechanism of the BOLD signal is the __ within __ molecules as these molecules diffuse through __
dephasing of spins; water; these gradient fields
spins located within __ give rise to the __ of the BOLD signal, while spins in __ (__) give rise to the __ of the signal
vessels; intravascular component; surrounding tissues (parenchyma); extravascular components
both intravascular and extravascular components originate in __, which are very __ and __. thus, the absolute limit for measuring hemodynamic activity is the average __ and this cannot be __
capillaries; small in diameter; close together; separation between capillaries; improved
as brain activations are elicited, __ blood enters the venous system and displaces __, thus __ BOLD signals __ from active neurons
Hb-rich; dB; increasing; downstream
some of the BOLD response can be due to __, not to neuronal activation, a phenomenon known as __
venous drainage; the large vessel effect
sequences have been designed to attenuate the large vessel effect by exploiting the fact that the magnetic field due to __ changes __ over space as it extends into __. for this reason, nearby __ spins within diffusing water experience relatively __ as they move
dHb in large vessels; slowly; surrounding tissues and fluid; extravascular; small B field changes
SE sequences can reverse the loss of phase coherence due to __ but not due to __ because the latter generate __ B field gradients and the lost phase coherence cannot be completely refocused by __
large vessels; small vessels; large; 180 degree SE rephasing pulses
the TR has a very strong effect upon the __ of fMRI signals, particularly because the __ are difficult to resolve with __ sampling, which makes modeling the fMRI signal difficult if the stimuli have a complex time profile
time resolution; initial and final undershoots; low-frequency
when more than one stimulus is presented within a short period of time, it leads to __
an overlap of BOLD signals
generally, the response due to two distinct stimuli is the __ (i.e. the BOLD response has the property of __)
sum of the individual responses due to each stimulus; linearity
if stimuli are presented very closely together, the combined response may not be __ and we may notice, in fact, a __ due to the so-called __
additive; reduction in hemodynamic amplitude; BOLD refractory effect
