In Vivo Photoacoustic Imaging:
Brain Research Application
Vassiliy Tsytsarev,
E-mail: tsytsarev@umaryland.edu
University of Maryland School of Medicine
Most of the presented data have been obtained in:
Optical Imaging Laboratory
Department of Biomedical Engineering
Washington University, St. Louis
http://oilab.seas.wustl.edu -- 1
What is Photoacoustic?
Photoacoustic imaging - a hybrid biomedical
imaging modality, is developed based on the
photoacoustic effect
Photoacoustic effect – discovered by Alexander
Bell in 1880; he showed that thin discs emitted
sound when exposed to a beam of sunlight that
was rapidly interrupted with a rotating slotted disk.
http://oilab.seas.wustl.edu -- 2
How it works?
Laser
Ultrasound trasducer
Ultrasound
http://oilab.seas.wustl.edu -- 3
PA Imaging: three types of scanning
PACT image of the cortical
vasculature in a living adult intact rat
Dark-field AR-PAM
(Wang et al., 2003; Stein et al., 2008)
OR-PAM (Open Brain)
http://oilab.seas.wustl.edu -- 4
Scheme of optical-resolution photoacoustic
microscopy (OR-PAM)
Wavelength-tunable
laser system
Condenser lens
Pinhole
Microscope objective
Correction lens
Right-angle prism
Silicone oil layer
Ultrasonic transducer
Water tank
Acoustic lens
Polyethylene membrane
Scanner
Hu et al, 2007; (Wang et al., 2003; Stein et al., 2008)
http://oilab.seas.wustl.edu -- 5
Intact Rat PA Imaging
http://oilab.seas.wustl.edu -- 6
Open Brain PA Imaging
http://oilab.seas.wustl.edu -- 7
Functional Brain Imaging: Transcranial Application
In vivo PACT image of the cerebral vascular response to right-side whisker
stimulation (intact rat)
The hemodynamic response due to whisker stimulation is shown in blue
and red and is superimposed on the cortical vascular image shown in gray.
(Hu and Wang, 2009)
http://oilab.seas.wustl.edu -- 8
PA Microscopy: 2-Wavelengths Functional Imaging
Hu, Maslov, Tsytsarev and Wang, 2009
http://oilab.seas.wustl.edu -- 9
Photoacoustic imaging of the vascular response
Real-time monitoring of photoacoustic signals
at specific excitation wavelengths reveals
vascular dynamics, such as changes in blood
volume and oxygen saturation, in response to
electrical or physiological stimulation or
epileptic seizures
http://oilab.seas.wustl.edu -- 10
Neurovascular coupling: response to
electrical stimulation
Electrode
Astrocyte
Neuron
Glu
GABA
Glu
ATP
K+, NO
Blood vessel
Electrical stimulation may cause neurons to release
various neurotransmitters (Glutamate [Glu], GABA, ATP
and NO). These reactions drive the vessel to either
vasoconstriction or vasodilatation
http://oilab.seas.wustl.edu -- 11
Photograph of exposed mouse brain surface
with introduced microelectrode
Cranial opening
Microelectrode
5 mm
http://oilab.seas.wustl.edu -- 12
Photoacoustic imaging of the brain
microvasculature
SO2
1.0
0.6
Min
Optical absorption
Max
400 μm
The oxygen saturation
(SO2) mapping is shown
as a superposition in
color scale. A line-scan
monitoring of the
vascular response was
performed along the
dashed yellow line.
1mm
Photograph of the microelectrode
http://oilab.seas.wustl.edu -- 13
Vascular response to electrical stimulation
Axial
Transverse
Axial
Transverse
Stimulation
10 μm
http://oilab.seas.wustl.edu -- 14
Vascular response to electrical stimulation
http://oilab.seas.wustl.edu -- 15
Vasoconstriction and vasodilatation
100 µA
Electrical
stimulations
150 µA
50 s
50 s
126 s
130 s
150 s
150 s
217 s
230 s
250 s
280 s
10 s
50 µm
25 µm
Min
Max
Optical absorption
http://oilab.seas.wustl.edu -- 16
Time courses of the vessel cross-sectional
area under various stimulation intensities
Stimulation 1
Stimulation 2
Each stimulation consisted
of a train of four 0.3 ms
pulses at 300 Hz
150 μA
300 μA
350 μA
110 μA
Vasoconstriction and
vasodilatation are
observed, and the
response duration is
positively correlated
with stimulation
intensity
100 μA
http://oilab.seas.wustl.edu -- 17
Transcranial imaging of the single blood vessel
(b)
(a)
Studied
vessel
medial
400 μm
rostral
Transcranial images of
stimulation-induced
vasodilatation
1.6
1.4
(c)
1.2
1.0
a)
b)
c)
0
10
20
time (s)
Transcranial brain image
Crossectional monitoring
Time courses of the electricalstimulation-induced vessel
size
Stimulation http://oilab.seas.wustl.edu -- 18
Epileptic Seizures: Single Vessel Monitoring
http://oilab.seas.wustl.edu -- 19
Summary
l
Optical-resolution photoacoustic microscopy (ORPAM) imaged two types of vascular response to
electrical stimulation:
n Vasoconstriction
n Vasodilatation
l
OR-PAM clearly and reliably imaged the vascular
response to electrical stimulation at the capillary
level with a temporal resolution of one second.
l
OR-PAM is a promising tool for in vivo studies of
neurovascular coupling under a variety of
experimental conditions invasively as well as
transcranially
http://oilab.seas.wustl.edu -- 20
Epileptic seizures accompanied by two
vessels vasodilatation (PA Imaging)
EEG
10 s / 2 mV
Vessel 1
Vessel 2
25 μm
http://oilab.seas.wustl.edu -- 21
Conlusions and Perspecives:
- Current application: animal experiment
- Clinical application: questionable
- Perspective: biomarcer hybridozation
http://oilab.seas.wustl.edu -- 22