Chemistry Reference
In-Depth Information
time (how quickly the contrast agent is tumbling in solution), and
susceptibility effect (also caller outer-sphere effect) [40].
5.1.3
Optical Imaging
Optical imaging, which is based on the detection of light photons
after their interaction with biological tissue, has been used in
biomedical diagnostics for years [1, 41]. It provides high sensitivity
for detection and is a nonionizing method. An optical imaging
instrument is relatively cheap and easy to use. Limitations of optical
imaging include low penetration depth and scattering of light in the
tissue [42]. Therefore, as the sensitivity of detection is primarily
dependent on the fluorphores, the development of novel fluorphores
with suitable functionalities and properties is paramount. Ideal
fluorphores should have good solubility in biological media, specific
association with target molecules or cells, typical absorption
wavelengths and fluorescence emissions, high fluorescence quantum
yields and lifetimes, and photochemically stable [43] .
A wide variety of fluorophores for bioimaging applications
are currently available. Traditional fluorophores include organic
dyes, fluorescent proteins, and lanthanide chelates, which are still
the most commonly used fluorescent labels. They have small size,
good water solubility, and are easy to use [44]. However, these
conventional fluorophors suffer from several limitations such as
broad absorption spectrum, high photobleaching susceptibility, and
poor photochemical stability. Novel inorganic fluorophores, on the
contrary, have the potential to overcome these problems. Examples
of inorganic fluorophores include QDs, rare earth doped inorganic
nanoparticles, and silica nanobeads [45-48].
5.1.4
X-Ray Computed Tomography
X-ray computed tomography is another clinical imaging modality. It
employs X-rays, together with computer software, to create images of
tissues. The X-ray tube rotates around the object, and the transmission
profiles are sampled from up to 360
angles in a digitized form using
a detector during imaging acquisition. Tomography was created by
computer processing. It was invented by Sir Godfrey Hounsfield,
who later received the Nobel Prize in medicine in 1979 [6]. Since
then, the modality has gained popularity in medical imaging to
supplement X-rays and medical ultrasonography. Contrast agents
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