Chemistry Reference
In-Depth Information
OH
O
H
O
HO
N
N
N
S
OH
S
N
Coelenterate
N
Firey
HO
fIgure 11.39
Structures of two commonly used luciferins for molecular imaging from different types of bioluminescent organisms.
NH
HO
OH
+
N
N
OH
HO
NH
n
ICG
Melanin
Na
+
O
-
O
S
S
O
O
O
O
-
fIgure 11.40
The structures of two commonly used organic probes for molecular photoacoustic imaging.
11.8
photoacoustIc IMagIng
Photoacoustic imaging is a new hybrid biomedical imaging technique that has been developed based on photoacoustics,
where the absorption of electromagnetic energy transforms into acoustic waves (pressure wave or sound). rF or short laser
pulses are used for photoacoutic imaging excitation in soft tissues, usually pulsed on a nanosecond timescale, because the
waves are non-ionising in these regions, which is safer for humans and can provide high contrast and enough penetration
depths. The rF properties of biological tissues can provide the information of the physiological nature of their electrical
properties [172].
In biomedical applications, photoacoustic imaging offers a number of advantages. For example, it combines ultrasonic
resolution and high contrast from light or rF and does not rely on the photons for excitations. Also, photoacoustic imaging
can overcome the problems existing in the conventional optical such as micro-absorption spectrometry and dark-field
microscopy and ultrasound imaging. It produces images of high electromagnetic contrast at high ultrasonic resolution at
large depth. The resolution and imaging depth of photoacoustic imaging is tunable by controlling the frequency of the ultra-
sound transducer used. When a tissue is irradiated by an incident laser pulse, absorption of the laser energy will cause a rapid
temperature rise, consequently leading to transient thermoelastic expansion and the concomitant generation of broadband
ultrasound waves inside the tissue. Optical properties (e.g., the optical absorption) of the tissue that is used to characterise
biological tissues can then be estimated by the waves generated.
In general, there are two types of contrast agents for photoacoustic imaging: endogenous (e.g., haemoglobin and melanin)
and exogenous (e.g., indocyanine green [ICG], various gold nanoparticles, single-walled carbon nanotubes [SWNTs],
quantum dots [QDs], and fluorescent proteins). Applications of haemoglobin in photoacoustic imaging can greatly facilitate
brain-and-blood dynamics-related research, such as measuring microvascular blood flow, visualising brain structure and
lesions, monitoring haemodynamics, delineating tumour vasculature, and imaging small animals. recently, haemoglobin
has been found useful in monitoring burn recovery by using multi-wavelength photoacoustic measurements. Haemoglobin-
based photoacoustic imaging for tumour angiogenesis is another case in point.
The second main endogenous contrast agent is melanin. It is employed mainly for diagnosis, prognosis, and treatment
planning of melanotic melanoma (>90% of all melanomas). Furthermore, more parameters for the detection of melanomas
can now be obtained by the mixed use of haemoglobin and melanin. Noninvasively three-dimensional images of subcutaneous
melanomas and their surrounding vasculature in living nude mice can now be obtained by dual-wavelength reflection-mode
photoacoustic imaging, because there is a vast difference between the absorption coefficients of blood and melanin-pigmented
melanomas when two wavelengths at 584 nm and 764 nm are used for haemoglobin and melanin respectively (Figure 11.40) [174].
