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fMRI Data and Statistics > Ch8 - Signal, Noise, and Preprocessing of fMRI Data > Flashcards

Ch8 - Signal, Noise, and Preprocessing of fMRI Data Flashcards

1
Q

blood-oxygenation-level-dependent (BOLD) contrast

A

differences in signal on t2*-weighted images as a function of the amount of deoxygenated hemoglobin

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2
Q

preprocessing

A

computational procedures that are applied to fMRI data following image reconstruction but before statistical analysis
- intended to reduce variability in data not associated with experimental task
- preparation for statistical testing

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3
Q

signal

A

meaningful changes in some quantity
- fMRI: incl. changes in intensity associated with the BOLD response across a series of t2* images

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4
Q

noise

A

nonmeaningful changes in some quantity

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5
Q

Signal-to-Noise ratio (SNR)

A

relative strength of a signal compared to other sources of variability in data

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6
Q

raw SNR

A

ratio between MR signal intensity associated with the sample (brain) and thermal noise that is measured outside the sample

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7
Q

contrast-to-noise ratio (CNR)

A

magnitude of the intensity difference between different quantities divided by variability of their measurement

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8
Q

voxel-wise analysis

A

evaluation of statistical test at the level of individual voxels

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9
Q

region-of-interest (ROI) analysis

A

evaluations of hypotheses about the functional properties of brain regions
- often chosen to reflect a priori distinctions within the brain

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10
Q

functional signal-to-noise ratio (functional SNR)

A

ratio between the intensity of signal associated with changes in brain function and the variability in the data due to all sources of noise

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11
Q

functional resolution

A

ability to map measured physiological variation to underlying mental processes

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12
Q

partial volume effect

A

combination, within voxel, of signal contributions from 2+ distinct tissue types or functional regions

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13
Q

spatial extent

A

number of active voxels within a cluser of activation

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14
Q

susceptibility artifacts

A

signal losses on T2* -dependent images due to magnetic field inhomogeneities in regions with air/air adjacent tissue

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15
Q

sinuses

A
  1. air filled cavaties in the skull
  2. long venous channels that form the primary draining system for the brain
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16
Q

power spectrum

A

representation of the strenght of different frequency components within a signal
- Fourier transform converts a signal into its power spectrum

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17
Q

thermal noise

A

fluctuations in MR signal intensity over space or time that are caused by the thermal motion of electrons within the sample or scanner hardware

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18
Q

system noise

A

fluctuations in MR signal intensity over space or time that are caused by imperfect functioning of the scanner hardware

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19
Q

scanner drift

A

slow changes in voxel intensity over time
(part of system noise)

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20
Q

physiological noise

A

fluctuations in MR signal intensity over space and time due to physiological activity of the human body
- motion, respiration, cardiac activity, metabolic reactions

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21
Q

aliasing

A

the sampling of a signal at a rate insufficent to resolve the highest frequencies that are present
- energy at those frequencies become artifically expressed at lower frequencies, distorting the measured signal

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22
Q

quality assurance (QA)

A

set of procedures designed to identify problems with fMRI data so that they don’t compromise experimental analyses

23
Q

phantom

A

object used for testing MR systems
- mostly filled with liquids or gel with known properties so problems with the scanner system can be readily identified

24
Q

ascending/descending slice acquisition

A

collection of data in consecutive order such that slices are acquired sequentially from one end of the imaging volume to the other

25
Q

interleaved slice acquisition

A

collection of data in alternating order
- first acquired from the odd-numbered slices, then even numbered slices
- minimizes influence of excitation pulses on adjacent slices

26
Q

temporal interpolation

A

estimation of the value of a signal at a time point that was not originally collected
- using data from nearby time point

27
Q

coregistration

A

spatial alihnment of 2 images or image volumes

28
Q

reference volume

A

target image volume to which other image volumes are aligned

29
Q

rigid-body transformation

A

spatial transformation that doesn’t change the size or shape of object
- 3 translational parameters & 3 rotational parameters

30
Q

translation

A

movement of an object along an axis in space

31
Q

rotation

A

turning of an object around an axis

32
Q

cost function

A

quantity that determines the amount of residual error in a comparison

33
Q

mutual information

A

in context of MRI:
amount of information about 1 image that is provided by knowledge of another

34
Q

spatial interpolation

A

estimation of the intesity of an image at a spatial location that was not originally sampled, using data from nearby locations

35
Q

shimming coils

A

electromagnatic coils that compensate for inhomogeneities in the static magnetic field

36
Q

magnetic field mapping

A

the collection of explicit information about the strength of the magnetic field across space

37
Q

field map

A

an image of the intensity of the magnetic field across space

38
Q

bias field estimation

A

technique for estimating inhomogeneities in the magnetic field based on intensity variations in collected images

39
Q

segmentation

A

the process of partitioning an image into constituent parts
- typically types of tissue or topographical divisions

40
Q

normalization

A

transformation of MRI data from an individual subject to match the spatial properties of standardized image

E.g.:
averaged brain derived from a sample of many individuals

41
Q

stereotaxic space

A

precise mapping of system using 3D coordinates

42
Q

Talairach space

A

commonly used space for normalization of fMRI data
- coordinates based on measurements from a single postmortem human brain

43
Q

MNI space

A

commonly used space for normalization of fMRI data
- coordinates derived from an average of MRI structural images from more than 100 individuals

44
Q

cytoarchitecture

A

the organization of the brain based on cell structure

45
Q

filter

A

within MRI: algorithm for removing temporal or spatial frequency components of data

46
Q

Nyquist frequency

A

highest frequency that can be identified in a digitally sampled signal
-defiend as one-half of the sampling rate

47
Q

task frequency

A

rate of presentation of a periodic experimental task

48
Q

prewhitening

A

application of filters to remove autocorrelation in a time series of data
- in fMRI: applied before removing task-unrelated noise

49
Q

precoloring

A

introduction of autocorrelation to a time series of fata so that the time series will have known statistical properties
(less common)

50
Q

spatial smoothing

A

blurring og fMRI data across adjacent voxel to improve the validty of statistical testing and maximizing functional SNR
- at cost of spatial resolution

51
Q

matched filters

A

principle that a filter of the same frequency as the signal of interest provides the max. signal-to-noise ratio

52
Q

brain extraction

A

step during preprocessing that removes unwanted parts of imaging volume

E.g.:
Skull

53
Q

multiple comparison problem

A

increase in number of false-positive results with an increasing number of statistical tests
- particular consequence of voxel-wise fMRI analysis

fMRI Data and Statistics (11 decks)

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