Functional Magnetic Resonance Imaging of Sensorimotor Cortex at
1.5T: A Comparison of Functional Venography and BOLD-Imaging
K. T. Baudendistel, J. R. Reichenbach, R. Metzner, J. Schröder,
and L.R. Schad
The measurement of brain activation using blood oxygen level dependent
contrast (BOLD, 1) relies on the signal change arising from
activation-induced decrease in paramagnetic deoxyhemoglobin
concentration. This change is not limited to the activated region's
parenchyma, but extends towards offleading venous vessels. Imaging of
venous vessels at high spatial resolution can be achieved, using the
deoxyhemoglobin concentration as source of image contrast (2).
Venograms obtained in rest and during a finger tapping paradigm reflect
changes in vessel contrast near the regions delineated by brain
activation maps obtained with conventional BOLD-EPI-fMRI.
Mapping of venous structure was performed using a 1.5T clinical
EPI-whole body scanner (Magnetom Vision, Siemens Medical Systems,
Germany) equipped with the standard head coil. The 3D-gradient-echo
sequence described in (2) was used for MR-venography with FOV=240mm, 16
3D-partitions (effective slice thickness=2mm), NEX=2, TA=10min, and a
matrix size of 512x320. For improvement of SNR, minimum intensity
projections of three consecutive slices were calculated, resulting in
6mm thick slices. A venogram was acquired in resting state followed by
a second, obtained during performance of self-paced finger tapping of
one hand. BOLD-EPI-fMRI was performed using a FID-echo planar imaging
(EPI) sequence with same FOV, TH=3mm, MAT=128², TE=54ms. Twelve
consecutive slices were acquired every 3s. fMRI series of 60 images
were acquired in cycles of ten images in resting state followed by ten
images during task performance. The activation paradigm consisted of
the finger tapping described above. Prior to brain map calculation, a
motion correction was performed using the MCW-AFNI software package
(3). Brain activation maps were calculated using an unpaired Student's
t-test. To obtain comparable slice thickness, two consecutive t-maps
Four healthy volunteers (age 20-30ys) were investigated. All volunteers
showed a reduction in venous vessels contrast near the regions of brain
activation. Finger tapping resulted in an increased activation of the
corresponding sensorimotor cortex and the supplementary motor area (see
arrows). A Student's t brain activation map superimposed on the anatomy
is presented by Fig.1c. Only pixels exceeding Student's t of 2.0 are
shown. The venograms show a corresponding reduction in venous vessel
contrast, corresponding to a decrease in the concentration of
deoxyhemoglobin under task performance (Fig. 1b) as compared to the
resting condition (Fig. 1a).
MR venography allows direct observation of deoxygenation concentration
change under task performance with high spatial resolution. Combination
of venography and BOLD-fMRI-experiments may offer possibilities for
separation of venous vessel and brain parenchyma contribution to the
observed BOLD-signal, leading to improved assessment of brain tissue
|| Fig.1: Compared to resting state (a), high
resolution images showing venous vessels, obtained during finger
tapping (b), show a reduction in venous vessel contrast near the
regions of activation depicted by BOLD-EPI-fMRI (c) (see arrows).
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