New sources of data for neurosurgical planning

Functionally important cerebral regions can be

mapped in many ways, such as using MRI (Mag-

netic Resonance Imaging) or PET (Positron Emission

Tomography). Speech regions can also be located by

using the Wada test, where alternate hemispheres of

the brain are anaesthetized by injecting a barbitu-

rate into one of the carotid arteries. Tests can then

determine which hemisphere is crucial for speech.

The BioMag Laboratory at Helsinki University Central

Hospital has developed a method for transcranial

magnetic stimulation (nTMS), which can be used

clinically to map functionally important area when

planning neurosurgery, particularly epilepsy patients.

The cerebral cortex is stimulated by delivering mag-

netic pulse through the cranium.

TMS has a long history, and equipment has already

been commercialised. Development work has been

continued in SalWe’s Mind and Body Programme.

Basic research began at Helsinki’s BioMag Labora-

tory in 1994 under Professor Risto Ilmoniemi, and

a spin-off company, Nexstim Oy, was established in

2000. In 2009 the equipment that it has commercial-

ised for nTMS was approved in the USA for mapping

motor cortex.

Parts of the human cerebral cortex are critical for everyday activities, such as movement, speech and understanding. When a tumour or epileptic focus is located near these regions, they need to be mapped by function before surgery.

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“Extra individual information about the precise loca-

tion of functional cerebral regions is valuable,” says

Jyrki Mäkelä, head of the BioMag Laboratory. “If the

neurosurgeon has this information before surgery,

the patient can be told about the treatment options

available. To leave part of a tumour intact is risky but

to excise the whole growth sometimes has its own

risks, for example to the patient’s ability to speak.”

BioMag development started with mapping motor

cortex, which Mäkelä says is slightly easier than find-

ing speech regions. Motor regions are mapped by

subjecting the cortex to a rapidly fluctuating mag-

netic field, targeted according to a three-dimensional

brain scan of the patient. The magnetic field induces

a weak electric current in the targeted region of the

cortex.

“The results of TMS are combined and compared with

patient data obtained by other mapping methods. The

comparison takes time and requires close cooperation

between attending physicians.”

Challenges of speech mapping“Mapping the parts of the cortex used in speech is

hard, because their anatomy is poorly understood.

There are individual variations in speech and they

aren’t arranged as systematically as they are in motor

cortex,” Mäkelä points out.

“Speech is hard to disrupt with TMS, so we have de-

veloped a system at BioMag where we deliver pulses

to a variety of cerebral regions. We video the stimu-

lation, its target in a three-dimensional map of the

brain and the patient’s performance as speech tests

are given. The results are compiled afterwards by

studying the videos.”

Strength through cooperationThe BioMag Laboratory has developed speech map-

ping together with international partners. The cor-

respondence between video nTMS and speech region

mapping performed during surgery was analysed in

collaboration with researchers from the Charité Uni-

versity Hospital of Berlin and the neurosurgical unit

of Munich Technical University.

Nexstim has been involved in the project to map

speech regions. It has commercialised a speech mod-

ule for nTMS that received the FDA approval in 2012.

The method is already in clinical use in 17 hospitals

in the USA, Germany, Italy, Sweden, Finland, among

others.

BioMag development began with finance from

Helsinki University Central Hospital but SalWe has

facilitated its continuation. SalWe has created

closer cooperation with Nexstim and also promoted

Nexstim’s commercialisation of the speech applica-

tion.

BioMag nTMS screenshots and mapping have been

integrated in Helsinki University Central Hospital’s

picture archiving and communication system PACS

since spring 2013. The results can therefore be trans-

ferred via the hospital’s imaging network to the clin-

ics that need them and to a neuronavigator that

guides the work of a neurosurgeon in the operating

room.

More information

Jyrki Mäkeläassociate professorBioMag Laboratory, HUS Medical Imagingjyrki.makela@hus.fi+358 50 427 9051

SalWe - Strategic Centre for Science, Technology and Innovation in Health and Well-being