Chemistry Reference
In-Depth Information
15
non-nanopartIcle-Based dual-ModalIty
IMagIng agents
Reinier Hernandez
Department of Medical Physics, University of Wisconsin-Madison, Madison, WI, USA
Tapas R. Nayak and Hao Hong
Department of Radiology, University of Wisconsin-Madison, Madison, WI, USA
Weibo Cai
Departments of Radiology and Medical Physics, University of Wisconsin-Madison, Madison, WI, USA
15.1
IntroductIon
In the first decade of the 21st century, molecular imaging, which embraces a variety of techniques such as positron emission
tomography (PET), single-photon emission computed tomography (SPECT), optical bioluminescence, optical fluorescence,
targeted ultrasound, magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), and many others that are
under active development, has been an extremely dynamic interdisciplinary field [1-3]. Each of these imaging modalities
has its intrinsic advantages and disadvantages in terms of spatial/temporal resolution, tissue penetration, sensitivity, cost,
dependence on infrastructure, as well as other parameters. For example, nuclear medicine techniques (PET and SPECT)
have very high sensitivity and excellent tissue penetration that can allow the acquisition of whole-body images. In addition,
accurate quantification is another hallmark of nuclear medicine imaging techniques, in particular PET. However, PET and
SPECT imaging have rather poor resolution, in the order of a few millimetres (mm), which is inadequate in many
scenarios.
On the other hand, real-time images with resolution in the sub-mm range can be acquired using optical techniques
in superficial tissue; however, the resolution and sensitivity deteriorates very rapidly with increasing depth due to
many factors such as tissue absorption, scattering, and autofluorescence [4]. The near-infrared (NIR; 700-900 nm)
region is optimal for
in vivo
optical imaging because the absorbance spectra for all biomolecules reach a minimum,
thus providing a clear optical window for small animal studies and limited clinical scenarios (e.g., breast imaging,
endoscopy, surgical guidance) [5]. In addition to better tissue penetration of light when compared to optical imaging
in the visible range, there is also significantly less background signal from tissue autofluorescence in the NIR window.
MRI can offer high resolution (sub-mm), three-dimensional (3D) imaging of anatomical structures with exquisite soft
tissue contrast. In addition, it does not involve ionising radiation. However, the sensitivity of MRI is many orders of
magnitude lower than the nuclear medicine and optical imaging techniques. Clearly, the combination of two or more
molecular imaging modalities is synergistic and can overcome many of the abovementioned limitations for a given
technique.
