Molecular imaging contrast enhancement

Fig. 14. Targeting of microparticles (e.g., bubbles and emulsion droplets) destined for molecular imaging and drug delivery Schematic simultaneous binding to a microparticle of a targeting device (antigen-specific ligands), of stealth-providing elements (e.g., PEG strands), and of drugs and markers (e.g., a Gd + chelate for MRI contrast enhancement).

The complex quantity, y6br = e (y(3)r) + i Im (x r), represents the nuclear response of the molecules. The induced polarization is resonantly enhanced when the Raman shift wp — ws matches the frequency Qr of a Raman-active molecular vibration (Fig. 6.1A). Therefore, y(3)r provides the intrinsic vibrational contrast mechanism in CRS-based microscopies. The nonresonant term y6bnr represents the electronic response of both the one-photon and the two-photon electronic transitions [30]. Typically, near-infrared laser pulses are used to prevent the effect of two-photon electronic resonances. With input laser pulse frequencies away from electronic resonances, y(3)nr is independent of frequency and is a real quantity. It is important to realize that the nonresonant contribution to the total nonlinear polarization is simply a source for an unspecific background signal, which provides no chemical contrast in some of the CRS microscopies. While CARS detection can be significantly effected by the nonresonant contribution y6bnr [30], SRS detection is inherently insensitive to it [27, 29]. As will be discussed in detail in Sects. 6.3 and 6.4, this has major consequences for the image contrast mechanism of CARS and SRS microscopy, respectively. 

Mulder WJ, Strijkers GJ, van Tilborg GA, Griffioen AW, Nico-lay K. Lipid-based nanoparticles for contrast-enhanced MRI and molecular imaging. NMR Biomed. 2006 19 142-164. 

Daldrup H, Shames DM, Wendland M et al. Correlation of dynamic contrast-enhanced MR imaging with histologic tumor grade comparison of macromolecular and small-molecular contrast media. AJR Am J Roentgenol 1998 171 941-949. 

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