MAGNETIC RESONANCE IMAGING FOR INTRACRANIAL NAVIGATION
G. Rees Cosgrove, M.D.,
F.R.C.S.(C), Bradley R. Buchbinder M.D., Hong Jiang, Ph.D.
Departments of Neurosurgery and Radiology, Massachusetts General
Harvard Medical School, Boston, Massachusetts
Address correspondence to:
Address for Correspondence:
N. Eskandar, M.D.
Massachusetts General Hospital
15 Parkman St. ACC # 331
Boston, MA 02114
Patient Appointments: 617.724.6590
| Stereotactic Surgery | Parkinson's
Disease | Intractable
Epilepsy | Movement Disorder
Guestbook | Selected
Publications | Links
Historically, neurosurgeons have
mapped cortical function invasively by direct electrical stimulation
of the cortex, either intra-operatively using a hand held stimulator
or extra-operatively using chronically implanted subdural grids.
These methods have proven value but obvious technical limitations.
Classical cortical mapping requires a craniotomy under local or
light general anesthesia with its associated discomfort, risk
and expense. Only limited areas can be tested and fully 2/3 of
the grey matter or the exposed surface is found along the depths
of the sulci and is inaccessible to stimulation. Most importantly,
the information cannot be used preoperatively for risk assessment,
therapeutic decision-making and surgical planning.
Non invasive cerebral mapping techniques
have thus evolved to localize functionally important cortical
areas. They attempt to resolve the spatial and temporal distribution
of neuronal activity during behavioral and cognitive tasks by
measuring one of two physiologic responses: electromagnetic response
or hemodynamic/metabolic response. Magneto-encephalography (MEG)
or electroencephalography (EEG) measure electromagnetic response
and positron emission tomography (PET) measures the hemodynamic/metabolic
response. Both can localize certain cortical functions non-invasively
but require dedicated, expensive units that are not widely available.
To facilitate anatomic localization and clinical use, these modalities
are typically correlated with magnetic resonance imaging (MRI)
and/or computerized tomography (CT) using some type of image co-registration
and fusion algorithm.
Modern MRI scanners can provide
exquisite anatomical detail of normal cortex as well as demonstrate
pathology. Images are generally viewed in the traditional 2-D
orthogonal views ( axial, sagittal, coronal) or 3-D representations
with surface renderings. Recent advances have now made noninvasive
functional mapping feasible with MRI in the clinical setting.
7,11 Functional MRI (fMRI) can combine detailed anatomical information
with precise physiological information to create a structural
and functional model of an individual's brain. This information
can be used preoperatively for surgical planning and intra-operatively
for guiding surgical resection strategies. The combination of
anatomical and physiological information makes fMRI an ideal tool
for intracranial navigation.
Principles of fMRI
Basic neurophysiologic studies over
many years have shown that there is tight coupling between cerebral
activity, blood flow and metabolism.14 Since neuronal activity is
associated with changes in regional cerebral blood flow (rCBF),
hemodynamic measurements can provide an indirect assessment of neuronal
activity.15 This principle forms the basis of the hemodynamic/metabolic
mapping techniques that have been employed for many years in activation
PET studies with excellent results.2,4,9 PET measures cerebral blood
flow using tracers containing the positron-emitting isotope O15.
The basic technique is elegant and simple: (1) acquire CBF images
during a control task (e.g. rest) and one or more activation states
(e.g. moving the fingers of the left hand); then (2) subtract the
control CBF images from the activated CBF images to reveal regions
of increased rCBF associated with task specific neuronal activation
( i.e. activated rCBF = task rCBF - baseline rCBF).
Technological Requirements for fMRI
In 1991, Belliveau and coworkers
at the Massachusetts General Hospital first reported the sensitivity
of MRI to functional brain activity.1 Using MR images obtained
rapidly after intravenous bolus injection of a paramagnetic contrast
agent, they were able to demonstrate signal change in the human
visual cortex after activation with photic stimulation. This signal
change was presumably due to the increased concentration of contrast
agent secondary to increased rCBF and was called the "contrast
bolus tracking" technique. In 1992, Ogawa et al. described a second
fMRI technique that did not require injection of an exogenous
contrast material.13 This technique takes advantage of the inherent
paramagnetic qualities of deoxyhemoglobin for use as an endogenous
contrast agent. The underlying concept is that blood flow and
deoxyhemoglobin concentration in cerebral capillaries and veins
affects the signal intensity with magnetic resonance imaging.
These signal intensity changes can then be used to indirectly
measure changes in regional cerebral blood flows secondary to
neuronal activity. The term blood oxygenation level dependent
(BOLD) technique was coined to distinguish this from the contrast
bolus tracking technique.12 BOLD imaging is completely noninvasive
and has subsequently become the fMR imaging method of choice.
The theoretical basis for BOLD imaging
is thought to be due to net changes in deoxyhemoglobin concentration
that accompany changes in regional CBF.
In resting brain, there is a close
correlation between rCBF and regional cerebral metabolic rates
for glucose(rCMRglc) and oxygen (rCMRO2).15 With physiologic activation,
rCBF may increase by as much as 50% in response to increased neuronal
activity, far in excess of oxygen metabolic demands. The physiologic
mechanisms by which this increase occurs is not fully understood
although various mediators including neurotransmitters such as
serotonin, acetylcholine, neuroactive peptides and nitric oxide
produced by cortical neurons have all been implicated.6 During
transient neuronal activity however, it is suspected that the
brain may utilize anaerobic metabolism. The increased rCBF with
only slightly increased O2 extraction results in a overall increase
in oxyhemoglobin concentration and a relative decrease in deoxyhemoglobin
concentration in the capillary and venous beds of the activated
cortex.5 The signal changes appear to come from the microvasculature,
capillaries and venules < 10 microns at 4 T and from larger venules
and veins at 1.5 T.10 The net decrease in paramagnetic deoxyhemoglobin
concentration then causes increased signal intensity on T2*- weighted
MR images. Since these signal changes are quite small (2-6%),
successful imaging requires acquisitions with high signal to noise
ratio and subtraction techniques to detect the signal changes
between a resting state and the activated state over time. The
biological time constant of these changes is in the order of 2-3
Initial fMRI studies were carried out
on high field strength magnets (2.0 - 4.0 T). With higher field
strengths, the magnitude of the functional signal change is significantly
increased resulting in enhanced signal to noise ratio and improved
spatial resolution. With stronger shielded gradient coils, improved
electronics and better surface coils, conventional 1.5 T MR scanners
can be used to obtain fMR images 3 but optimal studies still require
high field strength magnets or special echo-planar technology. Echo-planar
technology implies "fast" imaging with image acquisition times on
the order of 70 - 100 ms.16 With appropriate relaxation times, this
means complete volumes can be imaged every 1.5 seconds. This rapid
echo-planar imaging (EPI) has several advantages including increased
temporal resolution, shorter imaging time, opportunity for more
signal averaging and increased sample volume. Depending on the acquisition
parameters, this allows for 13 - 20 slices of the brain to be imaged
simultaneously with average voxel dimensions of 3 x 3 x 7 mm3. In
all of our studies to date, we have used a GE 1.5 T Signa Scanner
modified for EPI by advanced NMR Systems, Inc.
Methodology of fMRI
Functional MRI generates large volumes
of data and therefore computer hardware requirements include adequate
memory with additional data storage capacity. Image processing,
registration and rendering requires high-end graphics workstations.
To obtain fMR images for clinical use
requires special imaging acquisition sequences followed by detailed
computer image analysis.
After careful immobilization of
the patient in the scanner, a structural MR data set is created
using two volumetric T1-weighted SPGR imaging sequences. The first
sequence ( TR 25, TE 5, flip angle 30o, FOV 22, 256 x192, 1 NEX)
takes multiple thin axial sections ( 1.2 mm x 124 slices) and
is designed to render cortical gyral anatomy and related pathology.
The second sequence ( TR 28, TE 13, flip angle 30o, FOV 22, 256
x192, flow compensated, 1 NEX) is obtained after gadolinium administration
to visualize the cortical veins. These 3-D structural image sets
serve as the template upon which the functional date is superimposed.
For functional data sets, T2*-weighted
echo-planar images using the BOLD technique are acquired in a
standard head coil using an asymmetric spin-echo pulse sequence
(TR 20, TE 70, offset -25ms, FOV 20, 64 x64). Usually 8 - 13 slices
(7mm thick, slice interval 0 - 2.5 mm) are acquired during alternating
epochs (30 s duration) of the resting and activated state for
a total duration of 2.5 - 4.0 minutes.
A variety of activation tasks can
be carried out depending on the requirements of the clinical situation
and the area of the cortex one wishes to map. Primary sensory
tasks involve presentation of cutaneous, visual or auditory stimuli
while combined sensorimotor tasks involve active repetitive movements
of the appropriate body part (tongue, hand or foot @ 1 Hz). Language
testing generally involves a verb generation task and memory testing
a visual encoding paradigm.
Computerized Image Analysis
After image acquisition is complete,
detailed image processing, co-registration and rendering is required.
First, all image volumes within and between acquisitions are corrected
for motion using an automated algorithm. Next, the 3-D volumetric
T1-weighted SPGR imaging sequences are segmented to display soft
tissue, bone and cortical surfaces. Regions of interest (ROIs)
including pathological lesions and cortical veins are also segmented.
Next, the functional imaging sequences are averaged and sites
of activation identified by statistical analysis of the signal
- time course on a voxel by voxel basis using the Kolmogorov-Smirnov
test. Changes in the signal intensity by more than 2 - 3 standard
deviations from baseline are considered significant and segmented
out as a cortical activation (ROI).
Finally, the functional MR images
are then co-registered, fused and volume rendered with the anatomic
images using both surface and volume based matching techniques
[ANALYZE, Mayo Clinic] 8. All image processing is carried out
on computer workstations (Hewlett Packard 740 Series) and presented
in a variety of formats including two dimensional orthogonal slices
along the principal axes, volume rendered image or movie clip.
Final image selections can be displayed either on the monitor
or printed out as hard copy on a color printer.
Since March 1993, we have performed
fMR imaging studies as part of the presurgical evaluation in 31
patients with lesions involving or adjacent to critical cortical
regions. There were 17 males and 14 females ranging in age from
9 - 62 years. Lesional pathology included 18 tumors, 5 AVMs, 4 cavernous
angiomas, 3 cortical atrophy and 1 cortical heterotopia. Functional
MRI was successful in defining the cortical function of interest
in nearly all patients and was confirmed at surgery with conventional
intraoperative cortical mapping (bipolar stimulation, 50 Hz, biphasic
square wave, 2 ms duration, 1-12 ma) in 26 patients. In 5 patients,
surgery was deferred because of the perceived risk of disabling
neurologic deficit that might result from surgical intervention.
In those patients who underwent surgery, there was excellent correlation
between the cortical gyral and venous anatomy observed at surgery
with the anatomy predicted by fMRI. Sensorimotor tasks activated
contralateral primary motor cortex in all patients and supplementary
motor areas in the majority. Language tasks demonstrated more variable
localization in Broca's area, Wernicke's area, premotor cortex,
temporal neocortex and cerebellum. The selection of language paradigms
was of utmost importance in determining activation areas. There
was close correlation (3-5 mm) between the functional mapping by
cortical stimulation and that predicted by fMRI. The fMRI studies
accurately predicted the location and extent of all subcortical
lesions not evident even on close visual inspection of the cortex.
No patient suffered any unexpected deficit post-operatively.
Case 1: This 15 year old right
handed boy had a 3 year history of intractable supplementary motor
seizures with speech arrest and secondary generalization. Routine
MRI demonstrated a small lesion in the left superior frontal gyrus.
Video/EEG monitoring suggested left frontal onset and surgical
resection of the lesion was planned. Preoperative fMRI demonstrated
the lesion in the supplementary motor area anterior to primary
motor cortex. (Figure 1) At surgery under light general anesthesia,
electrocorticography was initially unrevealing and the lesion
was not evident upon visual inspection or palpation of the cortex.
Recognition of the cortical topography and venous anatomy localized
the lesion and insertion of an intracerebral electrode demonstrated
nearly continuous epileptiform discharges. Motor stimulation of
the pre-Rolandic cortex caused flexion contraction of the thumb,
fingers and wrist while stimulation of the presumptive supplementary
motor area via the depth electrode caused tonic elevation of the
right arm. Lesionectomy and minimal surrounding corticectomy was
performed. Pathology demonstrated a dysembryoplastic neuroepithelial
tumor. The patient made an uncomplicated recovery from surgery
without motor deficit and has remained seizure free for one year.
In this case, the fMRI accurately
identified the primary and supplementary motor areas but more
importantly allowed for precise localization of the subcortical
lesion by correlation of the exposed cortex with that predicted
by the fMRI.
Case 2: This 39 year old
right handed woman had a 10 month history of focal sensory seizures
beginning in her left foot and then gradually ascending up her
leg to her trunk. Routine MRI revealed a non-enhancing lesion
in the right superior parietal lobule. FMRI was performed during
hand and foot activation to demonstrate the relationship of
the tumor to the primary Rolandic cortex. (Figure 2) Surgery
was performed under local anesthesia with cortical mapping which
confirmed the location of the central sulcus and the somatotopic
distribution of sensorimotor function as predicted by fMRI.
The lesion involved the post-central gyrus but complete resection
of the tumor was accomplished. Pathology revealed oligodendroglioma.
Patient made a good recovery from her surgery with the expected
cortical sensory loss in the left leg but only temporary motor
deficit in the foot.
This case demonstrates the ability
of fMRI to localize sensorimotor function even in the presence
of an infiltrating tumor and its ability to accurately define
its subcortical extent.
Case 3: This 36 year old
right handed women experienced several months of mild cognitive
and memory impairment. A large left intraventricular meningioma
was discovered on routine MRI. fMRI was carried out to localize
language cortex in order to plan the optimal approach and select
the safest area in which to make a cortical incision. (Figure
3) At surgery, a parasaggital high parietal incision was made
avoiding presumed language cortex and the tumor was completely
excised. Post-operatively, she had mild transient confusion
and a temporary visual field cut but no dysphasia.
This case demonstrates the usefulness
of fMRI to clearly define complex anatomical and functional
relationships to allow the neurosurgeon to select the optimal
approach to a difficult subcortical lesion in the dominant hemisphere.
Case 4: This 22 year old
right-handed man had a 9 month history of focal seizures characterized
by an unusual sensation in his throat followed by involuntary
reflex coughing and speech arrest. After a generalized tonic-clonic
seizure, a MRI revealed a small enhancing lesion in the left
frontal opercular region. FMRI was performed to localize the
lesion and its relationship to sensorimotor tongue and language
cortex.(Figure 4) At surgery under local anesthesia, the primary
sensorimotor tongue and anterior language cortex was confirmed
by cortical stimulation and the lesion was localized by recognizing
the unique gyral topography overlying the lesion. A small corticectomy
and removal of a gangliocytoma was accomplished without motor
or language deficits.
This case exemplifies the exquisite
combination of structure and function that can be obtained with
fMRI. It is important to note however, that only the anterior
language cortex was activated because the language specific
task was a word generation task rather than a comprehension
task. Wernicke's area, although present just posterior to the
lesion resection and confirmed by cortical mapping, showed no
activation demonstrating the importance of careful task paradigm
construction and interpretation.
As has been demonstrated, fMRI can
be used non-invasively to localize cortical function with respect
to a surgical target. In this way, a more accurate assessment of
risk relative to the planned surgical procedure can be formulated.
The 3-D display of structure and function also allows for more accurate
preoperative planning of the surgical approach and optimal exposure.
Determination of functional cortex allows for selection of the most
appropriate and safe cortical incision and trajectory. In many cases,
fMRI can lateralize and localize language function thereby determining
hemispheric dominance. If initial success with paradigms for testing
memory are validated then intra-carotid amobarbital tests may soon
Intra-operatively, regions of interest
can be superimposed on the patients soft tissue/scalp rendering
and their locations with respect to normal anatomical landmarks
(i.e. eyes, glabella, occiput, ears) can be seen, measured and
used to plan scalp incisions and bone removal. Once the dura is
opened, the fMR images display gyral anatomy and superficial veins
which can direct cortical incisions and define appropriate resection
margins. Visualization of these cortical veins is very important
and they prove to be valuable stereotactic landmarks. As the brain
shifts after withdrawal of CSF, following administration of mannitol
or during resection of tumor, the veins will shift along with
the cerebral structures and the relative anatomy of the vein to
the functional areas or lesion remains intact. fMRI provides even
more information then visual inspection of the cortex because
it can display the subcortical location of the lesion and its
extent with respect to the cortical surface.
One of the most interesting applications
that has yet to be fully investigated is the potential that fMR
might be able to predict recovery from a variety of neurological
events. These would include recovery after a stroke, head injury,
tumor surgery, or other neurosurgical interventions. The ability
to follow cortical function over time and its possible reappearance
may provide a unique opportunity to observe reparative CNS processes
A final application will be in the
area of medical education where students can employ "electronic
brain cutting" to study CNS anatomy and function. The ability
to cut sections in any direction through the volumetric data set
and to remove specific structures would be a valuable interactive
teaching aid to learning basic neuroanatomy. The neurosurgical
resident could attempt, using a variety of computer tools, to
perform an operation electronically just prior to a planned procedure.
This would prepare him/her for the actual surgery as well as explore
optimal neurosurgical approaches for a variety of pathological
Navigation is the science of determining
the exact position of an object in space and then charting a course
for guiding that object safely and expeditiously from one point
to another. The ideal intracranial navigation system must perform
in a similar fashion. It must accurately display cerebral structures
and function that can be easily integrated and utilized intra-operatively
by the surgeon. Neurosurgeons like navigators, not only must have
a thorough knowledge of the science but also considerable experience
Functional MRI is a new and powerful
neuroimaging technique that can create an anatomical and functional
model of an individual patient's brain. The concurrent 3-D rendering
of cerebral topography, cortical veins and related pathology gives
an unprecedented display of critical relational anatomy. Since
stereotaxy means simply the three dimensional arrangement of objects,
then fMRI may be the ultimate stereotactic system. It allows us
to see through the scalp and cortex into subcortical areas which
are not visually apparent. It accurately predicts cortical gyral
and venous anatomy as well as the subcortical location and extent
of lesions. But most importantly, it is capable of mapping specific
cortical functions to anatomical regions thereby combining form
This information can be used preoperatively
to assess whether the patient is a candidate for surgery or whether
some form of alternative therapy i.e. radiotherapy or radiosurgery
might be more appropriate. It is extremely useful for presurgical
planning by demonstrating lesions and eloquent cortex in relationship
to a soft tissue rendering of the patient's head. Intra-operatively,
this allows for optimal placement of the scalp and bone flaps.
After exposure of the cortex, an optimal approach to the lesion
can be selected based on the representation of the subcortical
lesion with respect to critical functional areas. fMRI demonstrates
relational anatomy and any intraoperative shift will affect all
anatomical structures in the field therefore minimizing this source
of localization error that can occur with frameless stereotactic
Although direct visual inspection
of the computer rendered image is all that is generally necessary
in making intraoperative decisions, the integration of these data
sets with a frameless stereotactic device may be useful in the
future. In most instances however, displaying exactly what the
surgeon will see at surgery obviates the need for an electronic
pointing device. When one can identify the gyrus and adjacent
vein as the exact area of cortex overlying a deeper lesion, there
is little need of a frameless stereotactic localizing system to
point at what your eyes can clearly see.
fMR has many advantages over other
non-invasive methods of mapping cortical function. In comparison
to activation PET studies, fMRI is widely available and does not
use any radioactive isotopes. PET facilities are limited and have
poor temporal resolution. In order to provide appropriate structural
and anatomical information, PET studies must be co-registered
with MRI or CT. fMRI will be widely available, is non invasive
and easily repeatable. Localization is possible even in the presence
of local pathology including arteriovenous malformations and cavernous
angiomas. FMRI has no ionizing radiation and provides both structural
and functional information with the same imaging modality.
The major limitations of fMRI is that
it is labor intensive and requires a dedicated group of physicians
and imaging specialists.. Currently, the average functional MRI
study requires 60-90 minutes for image acquisition and then 4-8
man hours for image registration, segmentation and rendering. It
is therefore not appropriate for all patients and should be restricted
to those patients with lesions involving or adjacent to eloquent
cortex. Movement artifact can be a major problem and therefore patient
cooperation and stable head fixation is imperative.
Automation of the image segmentation
and processing will undoubtedly reduce the time and expense involved
in creating these images. Methods for quantification of the signal
changes during fMRI need to be developed so that one might be able
to predict the outcome for various pathological conditions including
stroke and head injury. Paradigms must be developed to investigate
higher cognitive functions. The system should be integrated with
frameless stereotactic devices and heads up display for maximal
usage in the operating room.
Functional magnetic resonance imaging
can accurately represent cerebral topography, cortical venous structures
and underlying lesions. Functional activation appears to accurately
localize appropriate cortical areas and these studies are feasible
in the presence of local pathology. It is extremely useful in presurgical
planning as well as assessment of operability. Intra-operatively,
it shows a great promise in being able to define the exact location
and extent of lesions with respect to surrounding functional cortex.
- (1) Belliveau JW, Kennedy DN, McKinstry
RC et al: Functional mapping of the human visual cortex by magnetic
resonance imaging. Science 254:716-719,1991.
- (2) Colebatch JG, Deiber M-P, Passingham
RE et al: Regional cerebral blood flow during voluntary arm and
hand movements in human subjects. J Neurophysiol 65(6):1392- 1401,1991.
- (3) Connelly A, Jackson GD, Frackowiak
RS, et al: Functional mapping of human primary cortex with a clinical
MR imaging system. Radiology 188:125-130,1993.
- (4) Fox PT, Burton H, Raichle M:
Mapping human somatosensory cortex with positron emission tomography.
J Neurosurg 67:34-43,1987.
- (5) Fox PT, Raichle ME: Focal physiologic
uncou[KHORMC1]pling of cerebral blood flow and oxidative metabolism
during somatosensory stimulkation in human subjects. Proc Natl
Acad Sci 83:1140-1144,1986.
- (6) Iadecola C: Regulation of the
cerebral microcirculation during neural activity: Is nitric oxide
the missing link. Trends Neurosci 16:206-214,1993.
- (7) Jack CR, Thompson RM, Butts
RK eet al: Sensory motor cortex: correlation of presurgical mapping
with functional MR imaging and invasive cortical mapping. Radiology
- (8) Jiang H, Robb RA, Holton KS:
A new approach to 3-D registration of multimodality medical images
by surface matching. Visualization in Biomedical Computing SPIE
- (9) Leblanc R, Meyer E: Functional
PET scanning in the assessment of cerebral arteriovenous malformations.
J Neurosurg 73:615-619,1990.
- (10) Menon RS, Ogawa S, Tank EW,
et al: Tesla gradient recalled echo characteristics of photic
stimulation induced signal changes in the human primary visual
cortex. Magn Reson Med 12:380-386,1993.
- (11) Morris GL, Mueller WM, Yetkin
FZ, et al: Functional magnetic resonance imaging in partial epilepsy.
- (12) Ogawa S, Lee LM, Kay AR et
al: Brain magnetic resonance imaging with contrast dependent on
blood oxygenation. Proc Natl Acad Sci 87:9868-9872,1990.
- (13) Ogawa S, Tank DW, Menon R,
et al: Intrinsic signal changes accompanying sensory stimulation:
functional brain mapping with magnetic resonance imaging. Proc
Natl Acad Sci 89:5951-5955, 1992.
- (14) Raichle ME: Circulatory and
metabolic correlates of brain function in normal humans. In: Mouncastle
VB, Plum KF Geiger SR (eds): Handbook of Physiology-The Nervous
System V. Bethesda, MD: American Physioloical Society, 643-674,1988.
- (15) Sokoloff L: Relationships among
local functional activity, energy metabolism and blood flow in
the central nervous system. Fed Proc 40:2311-2316,1981.
- (16) Stehling MK, Turner R, Mansfield
P: Echo-planar imaging: magnetic resonance imaging in a fraction
of a second. Science 254:43-50,1991.
Figure 1: Vertex view illustrating
lesion (yellow) in superior frontal gyrus just anterior to the
supplementary motor areas (red). Right (green) and left (blue)
hand activation tasks define the central sulcus.
Figure 2: Vertex view of
oligodendroglioma (yellow) arising in the right superior parietal
lobule and invading the post-central gyrus. Right (red) and left
(blue) hand activation tasks define the primary sensorimotor cortex.
Note the appropriate homologous representation along the central
A) Lateral view of laarge left intaventricular
meningioma (yellow) with location of the primary sensorimotor
cortex defined by hand activation (red) and speech cortex defined
by specific language tasks (green).
B) Vertex view with superimposed
cortical veins demonstrating the area for optimal cortical incision
to approach tumor while avoiding important sensorimotor (red)
and language (green) cortex.
A) Lateral view with soft tissue
rendering to demonstrate relative location of lesion (yellow),
tongue sensorimotor cortex (red) and Broca's area (green) on the
scalp. This can be useful to plan incision and craniotomy for
B) Lateral view with soft tissues
removed to demonstrate cortical location of lesion (yellow) in
the subcentral gyrus with respect to tongue sensorimotor cortex
(red), Broca's area (green) and superficial veins.