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

• 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)

circular coil induced current

Cowey and Walsh, 2001

TMS

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

maximum depolarization

maximum hyperpolarization

Cowey and Walsh, 2001

little or no change

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)