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Functional Brain Signal Processing: EEG & fMRI
Lesson 11
Kaushik Majumdar
Indian Statistical Institute Bangalore Center
M.Tech. (CS), Semester III, Course B50
T1 and T2 Relaxation Time
Buxton, 2009
Clearly, T2 << T1. Let T2*
be partly due to T2 of homogeneous magnetic property of the tissue and partly due to inhomogeneous magnetic property of the same. So
*2 2 2
1 1 1
T T T
where T’2 is time delay due to pure inhomogeneity. So
T2* < T2 << T1.
Buxton, 2009, p. 148
Free Induction Decay (FID)
Buxton, 2009
About one million oscillations during the T2
*.
Repetition Time (TR)
Buxton, 2009
Spin Echo and Echo Time (TE)
Buxton, 2009
Fast fMRI
In conventional phase encoding if TR = 2 s and 256 lines are to be drawn, it will take 8 minutes 32 seconds for generating one image. If we reduce the number of lines to 64 the time required is 2 minutes 8 seconds.
Our ultimate goal is to get as much information from fMRI as possible from LFP.
One way to address the problem is to go for ‘fast’ fMRI, at least, as fast as we can. EPI is one paradigm for fast fMRI.
Fast Spin Echo Technique
Mezrich, 1995
Echo Planar Imaging (EPI)
Deichmann et al., 2010
K space in EPI
EPI (cont)
Deichmann et al., 2010
Safety Issues in EPI
Specific absorption rate (SAR). Up to 130 dB acoustic noise due to rapid switching
(about 1000 Hz) of gradient magnetic field. (A major issue in EPI.)
Nerve stimulation. Y gradient is generally not used for frequency encoding in EPI. (A major issue in EPI.)
Generation of heat in body tissues due to RF pulses is always there in MR imaging including in EPI.
Buxton, 2009, p. 249
References
R. B. Buxton, Introduction to Functional Magnetic Resonance Imaging, 2e, Cambridge University Press, Cambridge, UK, 2009.
THANK YOU
This lecture is available at http://www.isibang.ac.in/~kaushik