Alternative Neuroimaging Techniques PET TMS EEG SPECT

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Alternative Neuroimaging Techniques PET TMS EEG SPECT Powered By Docstoc
					Imaging the brain before, during and after
                  TMS
                           TMS

• Thompson (1910)
placed head between
two coils and stimulated
at ~ 42 Hz
• saw flashing lights –
magnetophosphenes
• was probably
stimulating the retina
and not the visual
cortex




                                 Cowey and Walsh, 2001
                                   TMS




                   circular coil       induced current

• the induced current in the tissue is in the opposite direction to that
of the coil and degrades towards the outside and centre of the coil
(i.e., the highest focus is on the circumference of the coil itself)

                                                              Cowey and Walsh, 2001
                               TMS               little or
                                                no change

   maximum
hyperpolarization




   maximum
  depolarization




 • the flow of the current must cross the axon to cause
 stimulation or interruption of function (N3 will not be stimulated)

                                                         Cowey and Walsh, 2001
            Transcranial magnetic stimulation (TMS)
• Typically an interruption of function – creating temporary lesions in the healthy
brain (can also stimulate brain function).

• Great for pinpointing regions involved in specific components of tasks or for
mimicking neurological disorders.

• Single vs. rapid-pulse TMS – inherent dangers in rapid-pulse TMS

• Poor spatial resolution – vitamin E tablets and MRI help! (and magnetic dipole
modeling as in VEPs)
                 Imaging before TMS

• spatial extent of of induced electric field
    – drops ~ 75% within 10 mm
    – affects 600 mm2 of neural tissue

• for single pulse TMS duration of stimulation = 1 msec, but
affects motor cortex for up to 100 msec

• for rapid pulse TMS stimuli are delivered in trains with
frequencies from 1 to 25 Hz (1 – 25 times per second)

• duration for rapid pulse TMS anywhere from msec to
several seconds
                   Imaging before TMS

• fiducial frame
    – subject is scanned in MRI wearing a bite bar with MR
    contrast agents
    – same bite bar co-ordinate system used to position the coil
    outside the magnet

• frameless stereotaxy
    – anatomical landmarks (bridge of nose, tragus of the ears
    and the nasion) that can be seen both in MRI images and on
    the head are co-registered using optical imaging devices

• spatial accuracy of frameless stereotaxy is slightly worse
than the fiducial frame but allows for compensation of
subject’s head movements and movements of the coil
      Frameless stereotaxy and fMRI



• areas can be
identified functionally
and then used to
position the coil in a
TMS study using the
frameless stereotaxy
method
                Imaging during TMS

• several authors have used TMS in conjunction with PET

• main issue is the strong (up to 2 Tesla!) but brief (~ 200µs)
pulse can disrupt the PET imaging process (so fewer
collisions of positrons and electrons are recorded)

• aligning coil with main field of photon detectors can avoid
this issue but limits the number of positions for the coil

•TMS and ERP’s – saturation of amplifiers by the magnetic
pulse can be compensated for
                  Imaging after TMS

• very little is known about the long term effects of TMS

• typically only rapid pulse TMS produces any long
term changes

• some evidence that rTMS may benefit depression – a
milder form of ECT?

(watch W-Five on CTV at 9:00 Friday for a discussion of this)

				
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posted:11/24/2012
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