F O C U S O N M O L E C U L A R I M A G I N G
Optical Imaging: Current Applications and Future
Optical techniques, such as bioluminescence and fluorescence,
are emerging as powerful new modalities for molecular imaging
in disease and therapy. Combining innovative molecular biology
and chemistry, researchers have developed optical methods for
imaging a variety of cellular and molecular processes in vivo, in-
cluding protein interactions, protein degradation, and protease
activity. Whereas optical imaging has been used primarily for
research in small-animal models, there are several areas in which
optical molecular imaging will translate to clinical medicine. In this
review, we summarize recent advances in optical techniques for
molecular imaging and the potential impact for clinical medicine.
Key Words: bioluminescence imaging; fluorescence imaging;
optical imaging probes
J Nucl Med 2008; 49:1–4
Recent advances in genomics and proteomics have identified
a large number of molecules and signaling pathways that
potentially promote or limit diseases such as cancer, athero-
sclerosis, and infectious disease. A key challenge for basic
scientists and clinicians is defining which pathways, among many,
control disease initiation and progression in intact animals or
patients, rather than in reductionist systems. Ongoing discoveries of
potential target molecules in disease also drive pharmaceutical and
biotechnology companies to find leading agents that selectively
regulate these molecular pathways in vitro and in vivo. Optical tech-
niques, including bioluminescence and fluorescence, are emerging
as key technologies to meet these challenges and advance molec-
ular imaging in preclinical research and patient care. This review
will discuss basic methods in optical imaging for preclinical and
clinical applications and highlight recent advances for in vivo
imaging of specific molecular processes and signaling pathways.
OPTICAL IMAGING TECHNOLOGIES FOR PRECLINICAL
AND CLINICAL APPLICATIONS
The focus on optical imaging techniques for molecular imaging
is driven in large part by the sensitivity for imaging optical
contrast agents and reporter molecules in vivo. The lower limits
of detection for optical imaging may reach picomolar or even
femtomolar concentrations of an optical reporter or contrast agent.
Combined with the minimal background of techniques such as
bioluminescence imaging and fluorescence imaging in the near-
infrared spectrum, the signal-to-background ratio for detecting
specific molecular signals equals or exceeds that which can be
achieved with other molecular imaging modalities.
A keychallenge for optical imaging probes and instrumentation,
particularly those targeted toward eventual clinical applications, is
overcoming attenuation and scattering of light by tissues. For light
in the visible spectrum, absorption by hemoglobin and other mol-
ecules may reduce optical signals by approximately 10-fold per
centimeter of tissue (1). To image fluorescence in deeper tissues,
investigators have developed strategies for imaging near-infrared
fluorescence (NIRF) with emission wavelengths between 650 and
900 nm. At these wavelengths, absorption of light by hemoglobin,
lipids, and water is lowest, and tissue autofluorescence also is
greatly reduced.Asaresult,thesensitivity forNIRFimagingagents
is greatly enhanced, potentially allowing for tomographic optical
imaging signals to be detected at depths of 7–14 cm (2).
Differential absorption of light by tissues also produces images
that are weighted toward optical reporters and probes that are
located closer to the surface of a subject. While this limitation is
being overcome with 3-dimensional imaging and analysis tech-
niques such as fluorescence molecular tomography (FMT) (3),
optical techniques typically allow relative quantification of imag-
ing signals, rather than absolute quantification possible with PET.
Despite these challenges, optical techniques have growing roles in
molecular imaging research and clinical translation.
Bioluminescence imaging is commonly used for preclinical
cellular and molecular imaging in small animals. Bioluminescence
refers to light produced by the enzymatic reaction of a luciferase
enzyme with its substrate (Fig. 1). Firefly (Photinus pyralis) lucif-
erase is the most frequently used luciferase for molecular imaging.
This enzyme oxidizes its substrate, luciferin, in a reaction that
requires oxygen and adenosine triphosphate (ATP), emitting light
with abroad emission spectrum anda peak at ?560 nm. Because of
tissue attenuation, red and far-red emissions from firefly luciferase
are detected preferentially for imaging in small animals. Luciferin
distributes throughout an animal rapidly after intraperitoneal
injection and passes across blood–tissue barriers including the
brain and placenta. Light from firefly luciferase peaks ?10–12 min
after injection of luciferin and decreases slowly over 60 min (4),
providing a broad time window for acquiring images. The com-
bination of enzymatic amplification of signals from luciferase and
the almost negligible background bioluminescence in vivo makes
bioluminescence imaging with firefly luciferase a highly sensitive
method for small-animal molecular imaging.
There are a variety of luciferase enzymes from other organisms
that possess unique spectral characteristics and substrate require-
ments. Luciferases from a click beetle, Pyrophorus plagiophthala-
mus, havebeen optimizedtoproducegreen–orange (544nm) or red
(611 nm) light after oxidizing luciferin. Despite the potential
advantages of 611-nm emission for in vivo imaging, firefly
luciferase remains the preferred enzyme for bioluminescence
Received Aug. 1, 2007; revision accepted Oct. 22, 2007.
For correspondence contact: Gary D. Luker, MD, University of Michigan,
109 Zina Pitcher Pl., A526 BSRB, Ann Arbor, MI 48109-2200.
COPYRIGHT ª 2008 by the Society of Nuclear Medicine, Inc.
OPTICAL MOLECULAR IMAGING • Luker and Luker1
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