Biomedical Engineering Reference
In-Depth Information
of operating factors producing patterns of correlation (fractal
concept)
(6)
. Since most of the biological systems rely on a con-
certed action of large number of factors (degrees of freedom), we
focus on fractal analysis of dynamic signals.
4. Basic
Requirements of
Experimental
Methods
In order to capture complex dynamics in time and in space,
the process needs to be sampled at a proper rate and resolu-
tion
(6, 14)
. Non-invasive experimental methods should be pre-
ferred in that their use does not disturb the delicate patterns
in vivo. To capture the complex patterns, these tools should
produce spatial and/or temporal data sets of high resolution.
Tools of data analysis of complexity cannot compensate for an
inadequate spatial and/or temporal sampling. In our experi-
mental work, having kept pace with advancing technologies,
optical imaging (laser Doppler flowmetry (LDF), laser speckle
imaging (LSI), near infrared spectroscopy (NIRS)) and functional
magnetic resonance imaging (fMRI) proved adequate in this
regard.
The LDF and LSI methods monitor the blood perfusion
in the brain tissue. Both methods rely on the frequency analy-
sis of the back scattered laser light: LDF monitor evaluates the
Doppler shift of the photons, whereas LSI analyzes the blur-
ring effect of the photons' interference pattern called “speckle”.
NIRS and fMRI measure the intensity changes of the photon
flux at given wavelengths, which are in the visible and radiofre-
quency ranges of the electromagnetic energy spectrum. All of
these methods yield large data sets as required by the fractal
analysis.
The detailed description of our animal and human experi-
ments are found elsewhere
(15-18)
. In brief, in the animal stud-
ies, all rats were anesthetized and artificially ventilated. In the
optical studies, the animal's head was fixed in a stereotaxic frame,
the scalp was removed and the skull was thinned to transparency.
The probe of the LDF monitor (Model MBF3D, Moor Instru-
ments, Millwey, Axminster, Devon, UK) and an custom made
LSI system
(12)
detected cerebral hemodynamics from the pari-
etal somatosensory cortex. In the fMRI experiments, the blood
oxygenation level dependent (BOLD) signal was collected on a
9.4T Bruker horizontal-bore spectrometer (Billerica, MA) with
1
H resonator surface coil (1.4 cm diameter) using a echo planar
imaging (EPI) sequence, where the repetition delay and gradi-
ent echo time were 200 and 15 ms, respectively. The coronal slice
(2 mm) slice thickness was selected at the level of the bregma. The
NIRS experiments were conducted in awake human volunteers
