Functional Brain Signal Processing: EEG & fMRI

Lesson 11

Kaushik Majumdar

Indian Statistical Institute Bangalore Center

kmajumdar@isibang.ac.in

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