Functional Spectroscopy of Brain Activation Following a Single light Pulse: Examinations of the Mechanism of the Fast Initial Response.

J.Henning,C.Janz,O.Speck,and T.Ernst

In other words, how does the visual cortex respond to light?

Principal GoalsWhat is proton spectroscopy?How does the visual cortex respond to visual stimulation?What is the key limitation of this paper?

Nuclear Dipole

When placed in an external magnetic field :

Finally the FID signal:

•The RF coil is switched off

•An antenna(coil) receives the electrical signals

•Magnetization vector goes back to its initial vertical state.

How to get the one dimensional spectrum? B0 B(r) =B0 + Gxx (r) = (B0 + Gxx) -

-spatial dependence of used to map proton distribution.

Functional spectroscopy:

-Time evolution of water signal from a small voxel in the activated brain is measured.

-Led to the discovery of fast early response

BOLD:

Blood oxygen level dependent contrast

PRESS sequence :

-Point RESoveled Spectroscopy used to get FID from a voxel

90-TE1-180-(TE1+TE2)-180-TE2-Acq

IN THIS PAPER:

-Used PRESS sequence to measure the FID of water signal

-FID ‘s measured as function of echo time (TE) and length of stimulus(TS).

-An initial response observed at 400ms .

-No change in T2 observed with a minor change in T2*

-Mainly change in apparent spin density and/or T1.

MATERIALS AND METHODS:

-2T whole body system with shielded gradients(10mT/m)

-FID obtained from water signal from 8cm3 voxel in the visual cortex

-TR =400ms

-Visual Stimulus-2x3 light diodes

-TS (time of stimulus)--varied from 0.5 to 1 to 2sec.

-TE (time to echo)--varied of 30,60,90msec for TS=1-sec

Continued…

-Experiment sequence (paradigm):

16 acquisitions --- steady state.

Stimulation cycle:

12 acquisitions– prior to stimulus.

26 acquisitions--onset of stimulus

-the stimulation cycle was repeated 6 times

--number of FID ‘s 16+6x38=244FID ‘s lasting 97.6 sec per experiment.

-A total of 167 experiments(1002 stimulation periods)performed on 14 volunteers in 20 expt, sessions.

RESULTS:

-Of total 167 experiments ,106 could be used for further evaluation the rest were discarded .

TE(col)TS(row)

30 ms 60ms 90ms

500ms 17

1000ms

23 16 19

2000ms

15 16

Figure shows a stacked plot of FIDs for 23experiments at TS=1sec

400ms(initial response)

4400ms(activation)

10000ms undershoot

Figure shows the results of experiment as a function of TE.

-initial response as well as the positive BOLD effect reduced with TE.

-curve A represents a mechanism characterized by pure T2 effect

-curve B represents a pure amplitude effect.

-- The observed effect was primarily an amplitude effect as represented by B.

Here the intensity of the initial response was measured as a function of TS.

--no significant change was found when going from 500 to 1000ms.

--At 2000ms the initial drop was seen to vanish.

--In contrast the positive (BOLD) response increases.

DISCUSSION:

-The fast early response follows a mechanism different from the

slower BOLD effect.

Physiologic mechanism( hypotheses ) to explain behavior.

- fast influx of Na+ during activation

- concordant change in cell size .

-The observation of no initial dip for TS=2sec

CONCLUSION:

-The initial signal drop was verified at short repetition times.

Components of the early response:

•A small BOLD effect.

•A decrease in the observed spin density

•And/or an increase in T1.

-A possible explanation -- fast influx of Na+

-More direct view of neuronal activation.

LIMITATIONS :

1) Fails to say precisely whether the effect is due to T1 alone or Proton density alone or both.

2) Fails to determine the cause of the fast response

3) Explanation for no initial dip for TS=2sec contradicts with present studies by Beau Ances and John Detre.

(Ref: Ances B.M. ,Detre J.A. ;Strategy for enhancement of the early negativity in the BOLD fMRI response using short inter-stimulus intervals, NeuroImage human Brain mapping 2000 meeting)

Principal GoalsWhat is proton spectroscopy?How does the neural cortex respond to visual stimulation?What is the key limitation of this paper?