the figure shows the absolute frequency signal obtained by
taking FFT along the lateral direction (B-scan direction). The
biological tissue is often optically heterogeneous in nature,
which means that the magnitude of the backscattered light
and the refractive index are functions of the time variable.
Thus, the intensity captured by the CCD camera will be
modulated by the heterogeneous properties of the sample
along each B-scan. The spatial frequency components of a
static tissue sample, which we call the heterogeneous frequencies, will exhibit as a randomly distributed function around
zero frequency with a bandwidth of BW.
On the other hand, the moving scatterers produce a
frequency shift, which is caused by the Doppler effect of the
moving particles and shifts them away from the heterogeneous
frequencies of the static scatterers. Next, the structural signal
is filtered out in the frequency domain. The cut-off frequency
depends upon the heterogeneity of the static scatterers. Then
we use the Hilbert transform to convert the flow signal to an
analytic signal, which includes both the amplitude and phase
of the flow signal.
For the flow signal, the inverse Fourier transforms have
positive and negative frequencies. After inverse transforming
from the frequency domain, we apply the conventional spectral
Fourier transform along the depth direction to retrieve the
strength of the flow signal. Based on the signal processing,
the real flow signal is transformed to an analytic signal by the
Hilbert transform, which enables the bidirectional flow configuration. In other words, the positive and negative flows can
be separated in different imaging planes.
OMAG imaging of vasculature
Imaging 3-D functional blood flow in the vessels and tissues
of small animals has helped researchers to understand the
mechanisms of human vascular diseases. Here, we show some
examples of how to use OMAG to describe transcranial blood
perfusion at capillary-level resolution without removing or
thinning the cranium. In the experiments, the OMAG system
was used to image progressive slices of a mouse brain through
an intact cranium in order to collect a raw 3-D spectral interferogram dataset that took about 25 s.
After evaluating the spectrogram data slice by slice, we
re-combined the processed slices to yield 3-D OMAG images.
The top figure on the right shows the imaging results obtained
from a cortical tissue volume of 1. 8
3 1. 8
3 2.0 mm3. These
images demonstrate the potential of OMAG to assess the morphological parameters of the blood vessels that innervate the
perfused tissue, including vessel diameters, vessel density and
the volume of blood flowing in vessels.
OMAG has also been demonstrated for imaging cerebrovascular blood perfusion in mice with the intact skin and skull in
vivo. OMAG offers researchers the opportunity to visualize the
cerebral blood perfusion through the skin and skull.
OMAG imaged a 1. 8
3 2.0 (x-y-z) mm3 volume of an adult
mouse brain through an intact cranium in vivo. (Left) A 3-D
volumetric rendering of imaged cerebral blood vasculature and
(right) a 2-D x-y projection image of blood flow in the scanned
tissue; the image shows the detailed blood vessel network,
including the capillaries over the cortex, with the skull left intact.
(a, b) Projection views of blood perfusion from within the skin
and the brain cortex, respectively. Capillary blood flow can be
seen in (b). It took 7. 5 min. to acquire the 3-D data to obtain
(a) and (b) using the current system setup. (c) Photo taken right
after the experiments; viewing the vasculatures through the
skin is impossible. (d) Blood vessels over the cortex after the
skull and skin of the same mouse were carefully removed. The
superficial major blood vessels show excellent correspondence
with those in (b). The area marked with a dashed white box
represents 4. 2 x 7. 2 mm2.