Transcript Lesson 11

M.Tech. (CS), Semester III, Course B50
Functional Brain Signal
Processing: EEG & fMRI
Lesson 11
Kaushik Majumdar
Indian Statistical Institute
Bangalore Center
[email protected]
Buxton, 2009
T1 and T2 Relaxation Time
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
1
1 1


*
T2 T2 T2
where T’2 is time delay due
to pure inhomogeneity. So
T2* < T2 << T1.
Buxton, 2009, p. 148
Buxton, 2009
Free Induction Decay (FID)
About one million
oscillations during the
T 2 *.
Buxton, 2009
Repetition Time (TR)
Buxton, 2009
Spin Echo and Echo Time (TE)
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.
Mezrich, 1995
Fast Spin Echo Technique
Deichmann et al., 2010
Echo Planar Imaging (EPI)
K space in EPI
Deichmann et al., 2010
EPI (cont)
Buxton, 2009, p. 249
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.
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