Towards the Direct Detection of Neuronal Activity in the Brain

D. Konn, P. Gowland, R.Bowtell.

Introduction• Present fMRI techniques rely on BOLD contrast:

– Indirect effects of neuronal activity.– Uncertainties regarding location of activity.– Haemodynamic delay - poor temporal resolution.

• Direct detection of neuronal activity:– Remove uncertainties in the location of the

activity.

– Information on dynamics of brain.

Effect of Neuronal Activity on MR Image

Local current flow

Magnetic field

Change proton precession rate and phase

2 effects on GE experiment

Net phase change Loss of signal

Postsynaptic potentials.Current dipole in a conducting volume.

Characteristics of neuronal currents

• Evoked potentials.– 10nAm. (Hamalainen)

– Activity extended over several mm3 of cortex.

– Time locked to stimulus.

• Spontaneous Alpha waves (7-13Hz).– Point dipole 100 nAm (MEG) (Salmelin and Hari).

– Relaxed, with eyes shut.

Salmelin R., Hari R., Electroencephalography and Clinical Neurophysiology, 91, 237-248, 1994.

M. Hamalainen, Rev. Mod. Phys., 65(2), 413-487, 1993.

Theory

Thank you Dezhong Yao.Yao D., IEEE Trans Biomed Eng, 47(7), 964-966, 2000.

•Effect on image from current in straight wire. (Bodurka) J. Bodurka et al. J. Magn. Reson., 137, 265 - 271, 1995.

•B(x,y,z) point current dipole in a conducting homogeneous sphere.

– Primary current- trivial.–Volume currents-

•B(x,y,z) extended current dipole.–Summation of point dipoles every 0.1 mm.–e.g. 18,000 dipoles for 3 x 3 x 2 mm3 dipole.

B0

z

y

x

•Displaced 5cm along x in an 8cm radius sphere.•10 nAm, 3 x 3 x 2mm3, along y.

Y-variation of Bz(x,y,z) at 1mm along x from centre of different primary current spatial extents, Q = 10 nAm.

•Change in phase

•Net phase change in any voxel- CNRphi

•Dephasing across voxel- CNRmag

•CNRphi >> CNRmag (CNRphi/CNRmag > 2000 Q = 10 nAm).

•Concentrate on phase images.

),,(**),,( zyxBTEzyx z

(TE=50 ms, TR=200ms, 4 minutes, 3x3x3mm3, SNR=50)

Dipole spatial extent max(Q= 10nAm) Qmin (nAm) Point dipole 0.0036 3.23 x 3 x 2 mm3 0.0019 6.26 x 6 x 2 mm3 0.0010 11.5

Experiments• 6.4 cm sphere, physiological saline, 5mM CuSO4.

• Current conducted along glass tubes.• Primary dipole 2 cm from bottom of sphere.

• DC. • TR = 1s, TE = 55 ms. 3 x 3 x 5 mm3.• Q = 15Am. 150 images.

• 10 Hz AC. • TR = 45 ms, TE = 36 ms, 4 x 4 x 9 mm3.

• Q = 80 nAm. 9600 images, 7 mins. CNRphi agrees with theory.

EEG•Sample every 4 ms.•Good temporal resolution.

EEG Results- ERP’s and Alpha Waves

•ERP’s - LED goggles. 7.8 Hz•Alpha Waves- eyes open/shut•Scanner noise

In Vivo MR Imaging

Conclusions• Calculated and verified magnetic field from extended dipole inside sphere.• Phase images more sensitive.• Can detect effect from dipole strength of same order of magnitude as we expect from

neuronal activity.• Can detect dipoles with temporal characteristics similar to those of alpha waves.• Sensitivity depends on primary current spatial extent and orientation.• Found paradigms with chance of causing contrast in MR images.• Problems to overcome.

Acknowledgements

P. Gowland

R. Bowtell

R. Swainson

G. Jackson

S. Jackson