(Stroke. 1999;30:939-945.)
© 1999 American Heart Association, Inc.
Original Contributions |
Determination of Cognitive Hemispheric Lateralization by "Functional" Transcranial Doppler Cross-Validated by Functional MRI
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 p-schmidt{at}altavista.net
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Abstract |
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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 (111.3±7.0%) compared with the left MCA (107.1±6.1%). 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.
Key Words: cerebral blood flow • magnetic resonance imaging • ultrasonography
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Introduction |
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While 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 6 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 9 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 12 13 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 as a tool for the noninvasive investigation of lateralization of cortical functions.
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Subjects and Methods |
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Subjects
Fourteen healthy, right-handed subjects (6 women, 8 men) whose ages ranged from 22 to 33 years (mean±SD, 27.0±3.3 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
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, 256x256; field
of view [FOV], 250x175 mm; slice thickness, 5 mm;
interslice gap, 0 mm). To detect large draining vessels, a
3-dimensional phase-contrast gradient-echo angiographic sequence was
subsequently performed (imaging parameters: TR, 20 ms; TE,
8.6 ms; FA, 15°; matrix, 256x256; 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, 256x256; 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, 250x175 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.
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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 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 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 -ln 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 -ln 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.
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Results |
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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 1.74±0.27. 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).
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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 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, 59.9±8.0 cm/s for resting condition versus 67.7±9.8 cm/s
for activation condition) and right MCA (mean, 55.3±11.5 cm/s for
resting condition versus 64.9±13.2 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 107.1±6.2%, with increase in values ranging from 3.6%
to 19.4%. The right MCA showed a VMCA mean peak increase up to
111.3±7.0%, 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,
t=3.8; right MCA: P<0.001, t=7.0).
Evaluation of the mean peak VMCA showed a significant higher increase
of the right MCA compared with the left MCA (P<0.01,
t=-2.8) (Figure 6). The mean
right/left ratio of the mean peak VMCA was 1.15±0.06, ranging from
1.36 to 1.06.
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Regarding the sex of the subjects, the male subjects showed a VMCA mean peak increase of the left MCA up to 107.5±6.2%, with values ranging from 5.6% to 19.4%. The right MCA showed a VMCA mean peak increase up to 112.8±7.6%, with values ranging from 8.6% to 32.7%. The female subjects showed a VMCA mean peak increase of the left MCA up to 106.8±6.5%, with values ranging from 3.6% to 17.7%. The right MCA showed a mean peak VMCA increase up to 110.8±7.5%, 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, t=4.6; right MCA: P<0.0005,
t=7.5; female: left MCA: P<0.05,
t=2.9; right MCA: P=0.005, t=4.4). 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, t=2.88) (Figure 7).
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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 102.3±3.3% and of the rMCA up to 103.8±6.0%. 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 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 rs=0.54; P=0.02).
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Discussion |
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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 24 25 26 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 visuospatial 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 4 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 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 36 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.
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Acknowledgments |
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The authors would like to thank A. Klusmann, K. Specht, I. Wurdack, and S. Erberich for technical assistance during the performance of the studies.
Received November 24, 1998; revision received February 8, 1999; accepted February 8, 1999.
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