fMRI: Biological Basis and Experiment Design Lecture 17: Baselines, SNR & CNR • Visualizing data • Trial triggered average • Baselines • SNR and CNR – Block vs. ER 1 light year = 5,913,000,000,000 miles?
FMRI: Biological Basis and Experiment Design Lecture 17: Baselines, SNR & CNR Visualizing data Trial triggered average Baselines SNR and CNR –Block vs.
Dec 19, 2015
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fMRI: Biological Basis and Experiment DesignLecture 17: Baselines, SNR & CNR
• Visualizing data• Trial triggered
average• Baselines• SNR and CNR
– Block vs. ER
1 light year = 5,913,000,000,000 miles?
Localizers scans define ROI
Sample data – selected voxels, impulse response
Subject 3
Subject 4
Subject 2
Subject 1
voxe
l
time (seconds ... scans concatenated)
Scan 1 Scan 2 Scan 3 Scan 4
Sample data – ROI average
Subject 3
Subject 4
Subject 2
Subject 1
voxe
l
time (seconds ... scans concatenated)
Scan 1 Scan 2 Scan 3 Scan 4
Sample data – response average (60 trials)
Subject 3
Subject 4
Subject 2
Subject 1
time (seconds)
Sample data – response average (30 trials)
Subject 3
Subject 4
Subject 2
Subject 1
time (seconds)
Sample data – response average (15 trials)
Subject 3
Subject 4
Subject 2
Subject 1
time (seconds)
Effect of averaging
Subject 3
Subject 4
Subject 2
Subject 1
NM
Error bars are S.E.M. (M)
Analyzing each scan separately
Subject 3
Subject 4
Subject 2
Subject 1
Baselines and contrasts
20s block cycles 10s block cycles
Trade-off between CNR and block length
20s block cycles, 40 repetitions
10s block cycles, 80 repetitions
More contrast, fewer samples
Less contrast, more samples
Signal-to-noise ratio
• Thermal noise– white– independent of voxel size– independent of tissue type– independent of voxel location– trivial with 3 x 3 x 3mm voxels– a problem with 1.5 x 1.5 x 1.5 mm voxels (1/8th volume)– for the sake of argument:
• SNRthermal,3mm = 160
• SNRthermal,1.5mm = 20
SNRthermal: block and ER design
Contrast-to-noise ratio: block and (slow) ER design
2.10083.0
01.0
3.70083.0
06.0
ER
ERER
block
blockblock
N
CCNR
N
CCNR
2.10083.0
01.0
ER
ERER N
CCNR3.7
0083.0
06.0
block
blockblock N
CCNR
Response estimate: single voxel (SNR = 120)
Sample data – block experiment
Scan 2Scan 1
time (seconds ... 2 scans concatenated)
voxe
l
ROI average predicted from HIRF
Block response estimated with different HIRFsHIRF HRF
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