Direct electrical stimulation is considered the gold standard for brain mapping during tumor resection near eloquent brain regions. Preoperative techniques like fMRI, DTI, EEG, MEG, and TMS provide information on functional areas of the brain. Intraoperative methods include direct electrical stimulation, electrocorticography, intraoperative MRI, and real-time neuropsychological testing. Awake craniotomy allows for more accurate brain mapping compared to asleep craniotomy, though it requires a highly trained team and can cause patient discomfort. The goal of brain tumor surgery has shifted to more extensive or total resections while preserving quality of life using advanced brain mapping techniques.

1. Brain Mapping Techniques
Presented by – Dr. Soumen Kanjilal
Moderated by – Dr. Kuntal Kanti Das

2. Introduction
• Importance of extent of resection for improved outcomes in both low- and
high-grade gliomas.
• Supra-total removal of gliomas i.e resection beyond the MRI-based tumor
border.
• To maximize the extent of resection while minimizing morbidity.
• Intraoperative mapping has become the “gold standard” for resection of
tumours in or near eloquent brain regions.
Yordanova YN, Moritz-Gasser S, Duffau H. Awake surgery for WHO Grade II gliomas within “noneloquent” areas in the
left dominant hemisphere: toward a “supratotal” resection. Clinical article. J Neurosurg. 2011;115(2):232-239.

3. Preoperative techniques
• functional Magnetic Resonance Imaging (fMRI),
• Diffusion Tensor Imaging (DTI),
• Electroencephalography (EEG),
• Magnetoencephalography (MEG),
• Transcranial Magnetic Stimulation (TMS) and
• Neuropsychological testing (NPT).

4. Intraoperative methods
• Direct Electrical Stimulation (DES),
• Electrocorticography (ECoG),
• Intraoperative Magnetic Resonance Imaging (IoMRI), and
• Real-time NPT

5. Neuropsychological Testing
• NPT refers to a detailed assessment of functional domains that can be
performed prior to and during surgical resection.
• NPT is a straightforward way to detect deficits in both basic and higher order
cognitive function in patients with brain tumors.
• Three main forms of NPT:
• subjective assessments,
• screening and
• batteries.

6. Neuropsychological Testing
• Subjective assessments are reports from the patient regarding the types of deficits and
limitations they have in their everyday life.
• Screening is the most widely used form of NPT and can be performed at bedside.
• Screening tests include the
• Mini-Mental State Examination (MMSE) and
• Montreal Cognitive Assessment (MoCA).
• Results outside of the reference range are a strong indicator that neurocognitive
impairment exists.

7. Neuropsychological Testing
• Full neurocognitive battery testing – comprehensive test administered by a trained
clinical psychologist or psychiatrist
• examines a variety of domains including
• speed and executive function,
• learning and memory,
• visuospatial functioning,
• verbal fluency,
• mood and in some cases social cognition.
• The next frontier of NPT is to translate simplified versions of the comprehensive
battery into the operating room with real-time interpretation during DES.

8. Functional Magnetic Resonance Imaging (fMRI)
• Most common preoperative technique.
• Increased neural activity leads to increased local concentrations of deoxygenated
hemoglobin.
• BOLD images are formed.
• It can be co-registered on an anatomic MRI image to create a map of where the
neural activity.

9. Functional Magnetic Resonance Imaging (fMRI)
• Advantages –
• gives insight into the functional anatomy
• relationship with the tumour with high
spatial precision.
• allows simultaneous study of all regions
of the brain

10. Diffusion tensor imaging (DTI)
• Diffusion tensor imaging (DTI) is a useful MRI-based technique,
• It takes advantage of water displacement along white matter fiber tracts,
where it was noted that diffusion is increased parallel to tracts compared to
that along a perpendicular axis – Fractional anisotrophy
• DTI helps to determine if the tumour had only displaced the white matter
tracts versus invading and disrupting the underlying tracts.

13. Diffusion tensor imaging (DTI)
• Limitations—
• DTI only displays anatomic localization and provides no information on
functional significance,
• Tumour invasion or edema can alter results and occasionally give an
incorrect picture and
• Incorporating DTI into a neuronavigation system, once craniotomy has
been performed brain shifts distort the relationships.

14. Electroencephalography (EEG)
• EEG is a tool where electrodes are attached to the scalp surface and neuronal
electrical activity measured during a given task (i.e., evoked potentials)
• it can examine the entire cortical surface simultaneously,
• the measurement is more physiologic as it directly captures electrical activity
as opposed to the indirect measurement through blood flow with fMRI.

15. Electroencephalography (EEG)
• Limitations : -
• The current from the neurons on the cortical surface must travel through several
layers before reaching the scalp surface, and these layers have their own inherent
distortions and resistances.
• There is a time lag given the multiple resistive layers, causing poor temporal
resolution and phase information.

16. Magnetoencephalography (MEG)
• MEG is similar to EEG except that the patient is placed in a
device that detects small changes in magnetic fields
produced by primary neuronal electrical activity.
• MEG has much improved spatial resolution and thus can be
quite useful in localizing cortical function.
• MEG has been shown to correlate well with DES and
navigated TMS.
• Limitations –
• high cost.

17. • TMS is a technique where electrical current is non-invasively applied to the cortical surface
• either provoke positive effects, such as hand muscle activation
• or inhibit activity, thereby creating a temporary lesioning effect, such as speech arrest with
ventral premotor cortex stimulation.
From a coil
Electric
current
Passes through skin,
skull, meninges
Magnetic
field
In the underlying
cortical surface
Induces
electric
field
Transcranial Magnetic Stimulation (TMS)

18. Transcranial Magnetic Stimulation (TMS)
• TMS and DES differ by only an average of 4–8mm for motor cortex mapping.
• Disadvantage—
• TMS can cause patient discomfort and/or reliability issues due to temporalis muscle
contraction
Krieg SM, Shiban E, Buchmann N, et al. Utility of presurgical navigated transcranial magnetic brain stimulation for the
resection of tumors in eloquent motor areas. J Neurosurg. 2012;116 (5):994–1001.

19. Direct Electrical Stimulation
• Intraoperative DES is the current “gold standard” technique
• DES is a technique that relies on the surgeon stimulating the cortical surface
directly.
• Hand-held bipolar probe is used
• Motor mapping – either awake or asleep conditions.
• Language mapping – patient is kept awake and during language tasks
stimulation is performed, thereby causing a temporary and reversible “virtual
lesion” effect.

21. Direct Electrical Stimulation
• Disadvantages :-
• requires craniotomy,
• patients must be awake during the procedure,
• DES can only give information about the region of the cortex that is exposed during
surgery and thus available for stimulation.
• Finally, depending on the technique used to determine the appropriate current
threshold, there is a potential for false positives and negatives.

22. Electrocorticography (ECoG)
• ECoG is essentially the same concept as EEG except electrodes are applied
directly onto the cortical surface following a craniotomy.
• The major drawback of ECoG is that -
• not been well studied as a mapping modality for speech/language in clinical scenarios
and generally,
• requires a significant amount of postprocessing, making real-time mapping
challenging.

24. Intraoperative MRI (IoMRI)
• MR images are obtained prior to surgery and displayed intraoperatively after registering to a
reference frame.
• During the course of the operation, brain shift can occur as a result of
• intrinsic edema,
• mass resection and
• drainage of cerebrospinal fluid.
• These shifts can lead to inaccurate localization as defined by standard neuro-navigation
systems and may lead to inaccurate resections.
• IoMRI was developed to avoid this issue by updating anatomic information during surgery.

25. Awake v/s Asleep Craniotomy
AWAKE CRANIOTOMY ASLEEP CRANIOTOMY
1. Reduced morbidity.
2. Reduction of severe postoperative neurological deficits.
3. Accurate brain mapping.
4. Better probability to increase extent of resection
5. Higher rates of gross total resection
6. Better postoperative seizure control
7. Better postoperative KPS score
8. Improved postoperative quality of life
9. Reduction in hospital LoS
10. Reduction in overall health-care costs
1. Avoid emotional distress
2. Reduce discomfort during prolonged procedures
3. Alternative for failed awake craniotomies
4. Viable option for positions with extreme head
turns (e.g., park bench or prone)
• Advantages

26. Awake v/s Asleep Craniotomy
AWAKE CRANIOTOMY ASLEEP CRANIOTOMY
1. Need of a highly trained multidisciplinary team
2. Unable to control breathe rate and CO2 resulting in brain
swelling.
3. Patient discomfort
4. Not recommended with certain surgical positions
1. Unable to perform speech mapping
2. Lower accuracy for motor mapping as only MEPs can be
performed
3. Evaluation for neurological deficits can only be performed
after the patient is completely awake
4. Lower probability to achieve a favorable extent of resection
5. Poorer postoperative neurological outcomes
6. Slower rate of recovery
7. Higher overall health-care costs
8. Longer hospital stay
9. Worse postoperative seizure control
• Disadvantages

27. Conclusion
• Advances in brain mapping have allowed an increased extent of resection
while maintaining or even improving quality of life.
• The goal of intrinsic tumor surgery has changed from conservative
lesionectomy or biopsy-based approaches to aggressive or supratotal
resections.
• Any function important to the patient can be tested intraoperatively under
awake conditions, awake mapping should become the rule rather than the
exception for surgical resection of gliomas.

28. Thank you