BOLD-Signal Flashcards
What are T1- and T2*-weighted images used for?
- T1: structural images
- T2*: functional images
How do oxygenated and deoxygenated blood cells differ?
- oxygenated blood: diamagnetic (pushed by magnet, weaker)
- desoxygenated blood: paramagnetic (pulled by magnet, stronger)
desoxygenated blood has higher relaxation rate / lower relaxation time / faster dephasing of transverse magnetization / less MR signal (dark)
What are T1 and T2?
- T1: longitudional relaxation time (time taken for spinning protons to realign with external magnetic field)
- T2: transverse relaxation time (time taken for spinning protons to lose phase coherence among nuclei spinning perpendicular to the external magnetic field)
What are the differences between T1- and T2-weighted scans?
- T1: short TE and TR, enhances signal of fatty tissue (white matter) and suppresses signal of water/liquids
- T2: longer TE and TR, enhances signal of water/liquids (CSF) and suppresses signal of fatty tissue
CSF is dark on T1-weighted imaging and bright on T2-weighted imaging.
Which contrast do you need to use to detect the BOLD signal?
T2* because the acquired signal is different for oxygenated and deoxygenated blood. In T1, it is not due to refocusing.
What are the 3 relaxation effects regarding T1, T2 and T2*?
- T1: slow recovery of longitudinal magnetization due to spin-lattice interactions
- T2: fast dephasing of spins due to spin-spin interactions
- T2*: fast dephasing of spins due to combination of spin-spin interactions and magnetic field inhomogeneities (influence by BOLD effect)
During increased brain activity, why does the BOLD signal not change as expected? (i.e. more neural activity -> less oxygen -> hydrogen spin relaxation faster -> darker image)
- neurovascular coupling (neural activity increases cerebral blood flow) leads to displacement of deoxygenated blood / less deoxygenated blood -> hydrogen spin relaxation slower -> brighter image
- T2* does not measure oxygen consumption, but increase in oxygen
What is the haemodynamic response?
- neural activity first leads to increase in deoxy blood compared to oxy (initial dip, 0-2s)
- afterwards, blood flow increases, deoxy blood is quickly displaced and more and more oxy blood comes in (overshoot, peak ~ 4-6s)
- then, blood flow decreases again, less oxy blood comes in (decay), eventually increasing the proportion of deoxy blood (undershoot 10-20s, signal suppression after stimulation) until it goes back to baseline
1mm^3 of cortex contains …
- 10k - 100k neurons
- 100 mio. - 1 billion synapses
- 300 m of dendrites
- 4000 m of axons
- 0.4 m of capillaries
What is the partial volume effect?
The partial volume effect is the loss of contrast between two adjacent tissues in an image caused by insufficient resolution so that more than one tissue type occupies the same voxel (or pixel). That may induce a partial volume artifact, dependent on the size of the image voxel. If fat and water spins occupy the same voxel, their signals interfere destructively. A small amount of water signal may be eliminated by a larger lipid signal from the same voxel, resulting in a voxel that appears to contain only lipid. The partial volume effect is minimal with thin slice thickness and sufficiently high resolution, so that fat and water or other different structures are unlikely to occupy the same voxel.
The width of the BOLD signal correlates with
response time (estimation of relative duration of neural activity possible)
Variability of HRF
- HRF is variable between different subjects
- no correlation between peak latency and amplitude
Linearity of BOLD signal
- Nonlinearity of BOLD at <6s after stimulus onset for different stimulus durations, but after 6s, do differences between stimulus durations
- Linearity quite good approximation and very successful in application
spatial precision of BOLD signal
- submillimeter precision, but might not completely agree with electrophysiology
temporal precision of BOLD signal
- in same region and subject: relative precise onset
- different for brain regions and subject (and maybe conditions)
