Determination of Cognitive Hemispheric Lateralization by
“Functional” Transcranial Doppler Cross-Validated
by Functional MRI
Peter Schmidt, MD; Timo Krings, MD; Klaus Willmes, PhD; Florian Roessler;
Jürgen Reul, MD; Armin Thron, MD
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Background and Purpose—Changes of blood flow velocity in the right and left middle cerebral artery (MCA) induced by
cognitive demands are detectable by means of “functional” transcranial Doppler sonography (fTCD). Functional MRI
(fMRI) is an alternative method for mapping brain activity. The purpose of this study was to determine whether fTCD
can detect hemispheric lateralization and to cross-validate fTCD with fMRI.
Methods—Bilateral continuous MCA monitoring of 14 healthy, right-handed subjects with TCD was performed while the
subjects underwent a visuospatial task, and the hemispheric blood flow velocity shift was calculated. Identical stimulus
and response patterns were used in fMRI. Blood oxygenation level– dependent fMRI was performed with the use of a
gradient-echo echo-planar sequence on a 1.5-T scanner. Statistical maps were computed on a voxel-by-voxel basis,
hemispheric ratios for activated pixels were computed, and a group study was performed separately for the male and
female subgroups.
Results—Statistical analyses (t test) showed a significantly higher mean peak blood flow velocity increase (P,0.05) of the
right MCA (111.367.0%) compared with the left MCA (107.166.1%). fMRI demonstrated bilateral activation in the
superior parietal lobulus (Brodmann area 7) with a right/left ratio of 1.95. Concordant differences between the female
and male subgroups could be visualized with both methods.
Conclusions—Both methods succeeded in discriminating a blood flow shift to the right hemisphere induced by a complex
cognitive visuospatial task. fMRI cross-validates the findings of fTCD. Our study suggests that fTCD can investigate
the close relationship between brain activity and blood flow and lateralize higher cognitive functions reliably. (Stroke.
1999;30:939-945.)
Key Words: cerebral blood flow n magnetic resonance imaging n ultrasonography
W
hile the determination of functional human hemispheric lateralization by invasive radiological procedures such as the Wada test is already well established,1,2 the
noninvasive determination by a transcranial Doppler (TCD)
assessment is still in the process of clinical validation.3,4 The
clinical establishment of new alternative diagnostic tools that
could possibly help to avoid the highly invasive Wada test
should be encouraged for the benefit of patients. Since it is
well known that specific changes of blood flow velocity in
the right and left middle cerebral artery (MCA) induced by
cognitive demands are detectable by means of “functional”
transcranial Doppler sonography (fTCD),5–7 this method
could establish its role as a noninvasive, inexpensive, and
easy-to-use alternative to determine hemispheric lateralization. Data to validate fTCD, however, are still lacking.
Functional MRI (fMRI) is a noninvasive method for
mapping brain activity by means of hemodynamic changes
induced by cognitive tasks.8 –10 Although fMRI is a relatively
new methodology, its possible applications for mapping the
human brain have been validated by established methods.
Examples include the correlation between fMRI and positron
emission tomography studies, the Wada test, transcranial
magnetic stimulation, and direct electric cortical stimulation.11–14 These data confirmed the ability of fMRI to localize
and lateralize brain activity. Since both fMRI and TCD prove
brain activity by means of changes in cerebral hemodynamics, we decided to validate fTCD with fMRI.
In this study a visuospatial discrimination task for which
right hemispheric functional specialization is assumed15 was
used to investigate the cerebral blood flow shift lateralization
induced by brain activity with both fMRI and fTCD. The
purpose of this study was to cross-validate the presumed
ability of fTCD to detect hemispheric lateralization with the
findings of fMRI to further establish TCD in its clinical use
Received November 24, 1998; final revision received February 8, 1999; accepted February 8, 1999.
From the Department of Neuroradiology (P.S., T.K., J.R., A.T.), Interdisciplinary Center for Clinical Research—Central Nervous System (T.K., K.W.,
F.R., J.R., A.T.), and Division of Neuropsychology of the Department of Neurology (K.W.), University Hospital of the RWTH Aachen, Aachen,
Germany.
Correspondence to Dr P. Schmidt, Department of Neuroradiology, Klinikum der RWTH Aachen, Pauwelsstr 30, D-52057 Aachen, Germany. E-mail
[email protected]
© 1999 American Heart Association, Inc.
Stroke is available at http://www.strokeaha.org
939
940
TCD and fMRI for Hemispheric Lateralization
as a tool for the noninvasive investigation of lateralization of
cortical functions.
Subjects and Methods
Subjects
Fourteen healthy, right-handed subjects (6 women, 8 men) whose
ages ranged from 22 to 33 years (mean6SD, 27.063.3 years) were
included prospectively in this study after their written informed
consent was obtained. Right-handedness was confirmed with the use
of the Edinburgh Handedness Inventory Assessment.16 None had a
history of neurological or psychiatric disorders or a family history
of left-handedness.
Transcranial Doppler
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All subjects were examined with a TCD device (MultiDopX, DWL
Inc) that enabled simultaneous and continuous measurement of the
blood flow velocity of both the right MCA and the left MCA. Both
pulsed-wave 2-MHz Doppler probes were fixed with an elastic
headband over the temporal ultrasound window. The MCA was
monitored 10 mm proximal to the depth where the reverse signal of
the anterior cerebral artery was recorded. For 10 cycles of alternating
epochs of rest and activation lasting 15 seconds each, the blood flow
velocity of the middle cerebral artery (VMCA) was recorded online
and stored digitally for subsequent offline analysis.
Functional MRI
fMRI studies were performed on a 1.5-T system (Philips ACS NT
1.5 Gyroscan, Philips Inc) equipped for echo-planar imaging. The
subjects were scanned in a standard head coil while the head was
immobilized with the use of hook-and-loop fastener straps and foam
rubber pads. For anatomic reference, 10 contiguous T1-weighted
spin-echo slices were obtained (repetition time [TR], 100 ms; echo
time [TE], 14 ms; flip angle [FA], 90°; matrix, 2563256; field of
view [FOV], 2503175 mm; slice thickness, 5 mm; interslice gap,
0 mm). To detect large draining vessels, a 3-dimensional phasecontrast gradient-echo angiographic sequence was subsequently
performed (imaging parameters: TR, 20 ms; TE, 8.6 ms; FA, 15°;
matrix, 2563256; flow sensitivity, 10 mm/s). Additionally, a
3-dimensional gradient-echo T1-weighted scan that covered the
whole brain was obtained (TR, 30 ms; TE, 4.5 ms; FA, 30°; matrix,
2563256; 120 slices; 1.5-mm slice thickness). During functional
imaging, a blood oxygenation level– dependent (BOLD) contrast
multislice echo-planar gradient-echo T2*-weighted sequence was
performed covering the whole brain (TE, 35 ms; TR, 456 ms; FA,
45°; FOV, 2503175 mm; slice thickness, 5 mm; interslice gap,
0 mm). During total imaging time of 3.18 minutes for functional
images, 72 images were generated (2.7 s per volume). The functional
run was repeated twice for each subject.
Cognitive Task
The task paradigm consisted of a variation of subtest No. 1 from the
BET (Berufseignungstest) of Schmale and Schmidtke,17 which represents an adaptation of the US vocational fitness test.18 This task is
assumed as a right-hemispheric perceptual speed and visual discrimination task.5 During activation epochs, the subjects had to determine
whether identically shaped tool-like figures (hammer, pipe wrench,
nut, square nut, bolt, saw, screw, spanner, ax, drill) with arbitrarily
placed black and red areas were similar in the placement of the
colored areas (Figure 1). While 50% of the presented items were
identical, 50% varied in small details. The order of the presented
identical and nonidentical items was randomized. All subjects were
confronted with the same randomized item order.
Procedures
All subjects first performed the fMRI procedures, followed by the
Doppler procedures. Four examples were given to explain the task.
Subjects were instructed to try to solve each task correctly and as fast
as possible. Identical stimulus and response patterns were used in the
fMRI and Doppler procedures: to minimize unilateral hemispheric
Figure 1. Two examples of the task items. Top, Items differ in
placement of colored areas. Bottom, Identical items.
motor activation, subjects had to elevate both index fingers to
indicate that they made their decision. Immediately after their
response, the next item was presented.
fMRI Procedures
A mirror mounted on the head coil allowed the subjects to see the
items that were projected onto a rear projection screen. During 3
alternating cycles of rest and activation, 72 images of the whole brain
were obtained in a total scanning time of 3.18 minutes. Thus, each
cycle lasted 66 seconds, consisting of 33 seconds of activity and 33
seconds of resting. During each cycle, 24 scans were obtained.
Activation and control (rest) epochs were included in the same
functional run. During rest states, the subject looked passively at an
item with a single tool-like figure. Projection of the single tool-like
figure indicated the start and end of the activity epoch. During rest
states, the subjects were instructed to elevate both index fingers
irregularly for obtaining the same bilateral motor activation as in the
activation state.
fTCD Procedures
The subjects were placed in comfortable seating conditions in a silent
and dim room in front of a computer screen. After the left and right
2-MHz probes were fixed with an elastic headband, the MCA signal
was detected, and the continuous and simultaneous recording of the
bilateral VMCA was begun. After a habituation period of 1 minute,
10 successive cycles of rest and activation without interruption were
performed as described above. Each cycle consisted of an epoch of
15 seconds of activity and an epoch of 15 seconds of resting; thus,
each cycle lasted 30 seconds. During rest periods, the subjects were
instructed to close their eye and to relax. The examiner indicated
each activity or rest state with the sentence, “Open your eyes” or
“Close your eyes,” respectively. Activation and control epochs were
evaluated in 2 different functional runs. To estimate the effect of
opening and closing the eyes on changes of VMCA in the left and
right MCA, the same procedure was repeated without cognitive
demands (control). With their eyes open, the subjects were instructed
look passively at the monitor screen, on which a single tool was
presented.
Data Analysis
fMRI Data
An automatic realignment algorithm for each set of functional data
was to account for the effects of interimage motion-related artifacts
Schmidt et al
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in the brain.19 After motion correction, each slice was processed on
a voxel-by-voxel basis by comparing the MRI signal time course
with the time course of the task paradigm. Statistical maps were
computed with the use of both the XDS software developed by the
Massachusetts General Hospital Nuclear Magnetic Resonance Imaging Center at Harvard University using the nonparametric
Kolmogorov-Smirnov test and the statistical parametric mapping
(SPM) software developed by Friston and coworkers20 using the
parametric t test. Statistical maps were overlaid on the anatomic and
angiographic MR images series. Only regions that lay over gray
matter were further evaluated. Voxels exhibiting a 2ln P value of
.5 (corresponding to a P,0.0067) on the Kolmogorov-Smirnov test
were assigned as statistically significant and therefore “active”
voxels. The total number of active voxels, the mean, and the peak
2ln P values were computed for each hemisphere separately, and the
ratio of the number of activated voxels between the 2 hemispheres
was calculated for each subject. For SPM96 analysis, the data were
resampled into a Cartesian matrix and then processed with a
2-dimensional fast Fourier transform. Once individual images were
reconstructed, the time series from each pixel was correlated with a
reference waveform. The reference waveform was calculated by
convolving a square wave representing the time course of the
alternating task conditions with a data-derived estimate of the
hemodynamic response function.20 The resulting correlations were
transformed into a Z-score map (SPM{Z}).20 Pixels that satisfied the
criterion of Z .3.0 were selected and overlaid on the corresponding
T1-weighted structural image. For display purposes, the SPMs were
processed with a median filter with spatial width of 3 pixels to
emphasize spatially coherent patterns of activation. Stereotaxic
coordinates for clusters of activation within these averaged SPMs
were obtained with the use of the coordinate system of the Talairach
and Tournoux atlas.21 Significance was assessed with the delayed
box-car reference function of the SPM96 software.22 An ANCOVA
with 2 experimental conditions (comparison of 2 tools versus
inspection of 1 tool) and global flow per scan as covariate was
performed with the use of the SPM96 software. Pixels were
considered significant when they had a correlation of their time
series with the reference function exceeding a Z score of 3.05
(uncorrected P,0.001) and belonged to clusters with a significance
level of 0.05 (corrected). For visualization, color-coded quantitative
maps of positive contrasts were superimposed onto the corresponding T1-weighted templates. Data analysis was performed separately
for the male and the female subgroups.
fTCD Data
During the 10 alternating epochs of activation and rest, the VMCA
of both right and left MCA was recorded (Figure 2). Because of
inevitable slightly different insonation angles of the ultrasound beam
and the MCA of the left and right sides of one subject, the measured
velocities expressed in centimeters per second do not represent the
real blood flow velocities due to the cosines dependence of the
Doppler signal. Therefore, all velocity data were transformed into
changes of VMCA percentages for subsequent data evaluation. The
value of the VMCA at the end of the resting phase of each cycle was
considered 100% in each subject. Subsequently, an averaged cycle of
VMCA change during activation and rest phases was calculated for
the left and right MCA on the basis of data of the 10 recorded cycles
and expressed in percent change of baseline VMCA. A calculated
left/right ratio given automatically online by the system indicated the
relative hemispheric blood flow shift (Figure 3). After the individual
VMCA change curves were obtained, the fTCD data were pooled,
and mean VMCA increase curves for (1) the whole group, (2) the
male subgroup, and (3) the female subgroup were calculated.
Because of difficulties in determining the start and end points of the
activation plateau phase, we used the mean peak VMCA change as
the parameter for subsequent data analysis. All data were statistically
analyzed by means of Student’s t test for paired samples. A value of
P,0.05 was considered significant, and a value of P,0.01 was
considered highly significant.
May 1999
941
Figure 2. Recorded VMCA of left and right MCA in centimeters
per second of one subject during task performance (top: upper
curve indicates VMCA of left MCA; lower curve indicates VMCA
of right MCA) during 10 alternating phases of rest and activation
(bottom) of one subject. Note the different blood flow velocity
levels referring to the cosines dependence of Doppler ultrasonography; for comparability, transformation of the data in percentage of VMCA change is obligatory.
Results
fMRI Results
In all investigated subjects, the left and right superior parietal
lobe showed task-related hemodynamic changes, with the
right area exhibiting a larger region of significant blood flow
changes than the left. No activation was seen in the primary
sensorimotor region or the primary visual cortex. For all
subjects, mean percent signal change over the right parietal
area was 2.7% (SD 1.8%), and mean percent signal change
over the left parietal area was 2.2% (SD 1.7%). Mean
hemispheric ratios (right/left) of the number of activated
pixels for all subjects were 1.7460.27. In all subjects, the
right hemisphere exhibited a larger region of activation than
the left. Onset of task-related hemodynamic changes was seen
4 seconds after the first images were presented. A delayed
return to baseline was obvious in all trials. After spatial
normalization to the standard Talairach stereotaxic atlas, a
group study was performed with all subjects and with both
the male and female subgroups. The anterior cingulum and
both left and right extrastriate association cortex showed
task-related hemodynamic changes (Figure 4). Brodmann
area 7 (superior parietal lobulus) was bilaterally activated.
The right side exhibited a larger region of activated pixels.
For the whole group, the calculated right/left ratio of activated regions was 1.95. When sex-related differences were
evaluated, it was found that the male subgroup differed in 2
aspects from the female subgroup. First, the area of activation
over the right hemisphere was larger in the male subgroup
than in the female subgroup, indicating more significant
changes in blood flow between rest and activation states in
this region. Second, the right/left ratio of the activated region
was lower for the male subgroup, indicating a larger activation of the left superior parietal cortex compared with the
female subgroup (Figure 5).
fTCD Results
In all subjects, VMCA increase started after 1 to 3 seconds of
activity and peaked at 10 to 12 seconds. After 25 to 28
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TCD and fMRI for Hemispheric Lateralization
Figure 3. VMCA curves of left and right MCA of one subject after averaging and transformation of centimeters per second into percentage of blood flow velocity change.
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seconds of the cycle, VMCA was again similar to preactivity
baseline VMCA. Compared with the resting phase, the task
led to a VMCA increase of both left MCA (mean, 59.968.0
cm/s for resting condition versus 67.769.8 cm/s for activation condition) and right MCA (mean, 55.3611.5 cm/s for
resting condition versus 64.9613.2 cm/s for activation condition). After transformation in percentage of VMCA change
and determination of the mean peak VMCA increase, the left
MCA showed a VMCA mean peak increase up to
107.166.2%, with increase in values ranging from 3.6% to
19.4%. The right MCA showed a VMCA mean peak increase
up to 111.367.0%, with increase in values ranging from
8.6% to 32.7%.
Statistical analyses showed highly significant increases of
the VMCA during task performance compared with resting
condition in both left and right MCA (left MCA: P,0.005,
t53.8; right MCA: P,0.001, t57.0). Evaluation of the mean
peak VMCA showed a significant higher increase of the right
MCA compared with the left MCA (P,0.01, t522.8)
(Figure 6). The mean right/left ratio of the mean peak VMCA
was 1.1560.06, ranging from 1.36 to 1.06.
Regarding the sex of the subjects, the male subjects
showed a VMCA mean peak increase of the left MCA up to
Figure 4. Three-dimensional reconstruction of fMRI group study
(SPM96 software) after spatial normalization of the individual
anatomic variations to the Talairach atlas. Red areas indicate
the active voxels.
107.566.2%, with values ranging from 5.6% to 19.4%. The
right MCA showed a VMCA mean peak increase up to
112.867.6%, with values ranging from 8.6% to 32.7%. The
female subjects showed a VMCA mean peak increase of the
left MCA up to 106.866.5%, with values ranging from 3.6%
to 17.7%. The right MCA showed a mean peak VMCA
increase up to 110.867.5%, with values ranging from 9.0%
to 24.8%.
In both sex subgroups, statistical analyses showed significant increases of the VMCA during task performance compared with resting condition in both left and right MCA
(male: left MCA: P,0.005, t54.6; right MCA: P,0.0005,
t57.5; female: left MCA: P,0.05, t52.9; right MCA:
P50.005, t54.4). In the male subgroup, the comparison of
the mean peak VMCA of the left and right MCA showed no
significant difference, while in the female subgroup the mean
peak VMCA of the left and right MCA differed significantly
(P,0.05, t52.88) (Figure 7).
Evaluation of the VMCA during opening and closing of the
eyes without cognitive demands showed a mean peak increase of the left MCA up to 102.363.3% and of the rMCA
up to 103.866.0%. Statistical analyses revealed no significant difference between right and left MCA.
Right hemisphere dominance for the investigated task was
found in all subjects with the use of fTCD. These results are
Figure 5. fMRI group study (SPM96 software) showing sex differences of male (top) and female (bottom) subgroups: gray and
black areas indicate voxels with statistically significant taskrelated hemodynamic changes. Note that the area of activation
over the right hemisphere is larger in the male subgroup than in
the female subgroup, indicating more significant changes in
blood flow between rest and activation states in this region.
These results are in good correspondence with the fTCD results
(Figure 7).
Schmidt et al
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Figure 6. VMCA of left and right MCA during task and results of
statistical analyses averaged across all subjects. Mean maximum increase for left MCA was 105.4365.18%, and that for
right MCA was 109.5967.26%.
in close correspondence with the fMRI results, in which the
right hemisphere exhibited a larger region of activation than
the left in all investigated subjects as well. Correlation
analysis of the individual results of both fTCD (mean peak
VMCA increase) and fMRI (size of activated region) revealed that the right-left differences of TCD velocity changes
and fMRI activation sizes were correlated across subjects
(Spearman’s rank correlation rs50.54; P50.02).
Discussion
There is general consent that the left and right human
hemispheres differ in anatomy and function: verbal analytical
functions are primarily related to the left hemisphere, while
nonverbal integrative visuoperceptive and visuocognitive
functions are related to the right hemisphere.23–27 As a review
of the literature reveals, right hemispheric functional specialization tasks supply a marked and consistent increase of the
VMCA shift to the right hemisphere. Because some authors
describe a more pronounced effect of right hemispheric
lateralization tasks than left hemispheric lateralization
tasks,3,7,28,29 we decided to use this kind of cognitive task for
this study. The right hemispheric lateralization of this visuo-
May 1999
943
spatial task was validated with 2 different methods, suggesting this paradigm as a consistent and strong stimulus for
further hemispheric functional lateralization investigations.
The paradigm appears to be more subtle than other simple
language tasks and might therefore be better applicable in
cognitive lateralization studies.
However, the primary aim of this study was to confirm the
findings of fTCD with fMRI because validation of fTCD is
still necessary before further clinical establishment of this
method can be realized.
fMRI detects increased blood flow and blood volume in
those areas associated with cognitive tasks. The basic principle of fMRI with the use of the BOLD technique is based on
neuronal activation of the brain, which leads to localized
changes in the cerebral blood flow, blood volume, and
oxygenation.30,31 Since deoxygenated hemoglobin is more
paramagnetic than oxygenated hemoglobin and the surrounding brain tissue, the hemodynamic phenomena resembling
brain activation can be visualized with MR techniques that
are sensitive to changes in local field homogeneity.32 These
sequences are typically T2* weighted. A decrease in T2*
signal reflects a decrease in field homogeneity and thereby an
increase in deoxyhemoglobin concentration.33 During neuronal activation, the brain has considerable “luxus perfusion” to
those areas being activated.30,31 This extra perfusion results in
an increase of venous oxygenation and a consecutive increase
in T2* signal.8,34
In fTCD blood flow, velocity changes reflect blood flow
volume changes induced by cognitive activity, if one assumes
that the diameter of the large branches of the MCA does not
change during mental activity. Therefore, both techniques
should detect similar differences in cerebral hemodynamics
during cognitive activation. This theoretical concept was
proven in our study.
Both methods succeeded in discriminating a significant
blood flow shift to the right hemisphere induced by a
complex cognitive visuospatial task. Regarding the fTCD
results, we found a significant blood flow velocity shift to
the right hemisphere. These results are in concordance
with other different studies in which TCD was used.3–5 In
concordance with these fTCD findings, the fMRI results
reveal a significantly higher activation of the right superior
parietal cortex (Brodmann area 7) compared with the
Figure 7. VMCA changes of left and
right MCA during task performance and
results of the statistical analyses of the
male and female subgroups. Note a
larger increase in VMCA of the right
MCA in the male subgroup than in the
female subgroup and the higher statistical significance of differences between
the VMCA changes of right and left MCA
in the female subgroup. These results
are in good correspondence with the
fMRI results (Figure 5) and indicate that
activation over the right hemisphere was
larger in the male subgroup than in the
female subgroup.
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TCD and fMRI for Hemispheric Lateralization
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contralateral cortex in all subjects and in the fMRI group
study. Since the parietal lobe is supplied by the MCA, we
have shown a concordant shift of hemispheric blood flow
during this cognitive task with both functional methods.
The parietal activity presumably reflects activation of the
cortical regions involved in visuospatial discrimination
and/or perceptual speed, both being cognitive demands of
the given task. The fMRI activation found in the anterior
cingulum may be due to activation of the spatial working
memory recruited by the task. Working memory refers to
a system responsible for the maintenance of information
and involves different components, one of which is primarily responsible for maintaining visual images, the
so-called visuospatial sketchpad. In different studies in
which positron emission tomography was used, the anterior cingulate showed significant activation during typical
working memory tasks (match to sample).35–37
The comparison between fTCD and fMRI was concordant
in all investigated subjects, ie, fTCD demonstrated the same
individual hemispheric lateralization as did fMRI in all
subjects. These results demonstrate the potential of fTCD to
identify lateralized cognitive brain activation. In addition to
the concordance in lateralization of hemispheric function
demonstrated by both methods, we found an additional
similarity regarding the results of the sex subgroups. fTCD
shows a higher VMCA increase of the right MCA in the male
subgroup compared with the female subgroup, corresponding
to the larger area of activation found in the fMRI results in the
male subgroup. One might hypothesize that the difference in
laterality might be due to functional and/or anatomic differences in the male and female brain, eg, corpus callosum
morphometry.38
Regarding the signal response patterns of both fTCD and
fMRI, there is also an obviously similar time course: both
methods show an signal increase 1 to 3 seconds after task
onset, and in both methods the signal reached baseline again
after 5 to 8 seconds. This suggests that both methods are
sensitive to the same quality of task-induced signal change:
cerebral blood flow, which the used Doppler technique
already implicates because of its underlying principle of
detecting frequency shifts caused by moving ultrasound
reflectors (blood cells in the vessel lumen). This may be of
importance for further fMRI studies investigating the sensitivity of BOLD sequences in cerebral blood flow studies.
We conclude that with fTCD the close relationship between brain activity, metabolism, and blood flow can be
reliably investigated since fMRI as a hemodynamically based
method to determine cognitive task-induced changes in regional cerebral blood flow and volume has confirmed the
findings of fTCD.
The concordance of fMRI and fTCD results proves that
fTCD offers the potential not only to study laterality of higher
cortical functions but also to study patterns of cerebral
lateralization as a noninvasive, reliable, and cost-effective
method. This may be of clinical interest in the field of
preoperative diagnosis of hemispheric functional lateralization before brain surgery, where until today the invasive
Wada test has been the gold standard. fTCD studies of a
larger number of subjects are still necessary to further
confirm the potential of fTCD to determine hemispheric
lateralization reliably; in particular, cross-checks with confirmed right-hemispheric, language-dominant subjects are
desirable.
Acknowledgments
The authors would like to thank A. Klusmann, K. Specht, I.
Wurdack, and S. Erberich for technical assistance during the performance of the studies.
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Determination of Cognitive Hemispheric Lateralization by ''Functional'' Transcranial
Doppler Cross-Validated by Functional MRI
Peter Schmidt, Timo Krings, Klaus Willmes, Florian Roessler, Jürgen Reul and Armin Thron
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Stroke. 1999;30:939-945
doi: 10.1161/01.STR.30.5.939
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