In constructing their PET–MRI tracer, the investigators began with MnFe2O4 Inhibitor Library ic50 and coated the surface with cross-linked serum albumin for stabilization resulting in a 32-nm probe appropriate for lymphatic imaging. The PET radionuclide 124I can then be directly conjugated to the tyrosine residue on the serum albumin to generate a dual-modality probe. The authors present in vivo data in a rat model showing both MR and PET localization of probe within the brachial and axillary lymph nodes. An example of a cell-surface targeted
PET–MRI probe was developed and applied in vivo by Lee et al. [74]. Polyaspartic-acid-coated iron oxide nanoparticles were synthesized, and the surface amino groups were coupled to the arginine–glycine–aspartic peptide sequence for active targeting to the ανβ3 integrin. (The integrins are known to play a fundamental role in angiogenesis, and many groups have developed tracers and contrast agents to specifically image them,
particularly, ανβ3, in order to assess their expression [75].) DOTA was again used to chelate Selleck TSA HDAC 64Cu. The in vivo data showed that the investigators were able to achieve specific targeting (though some nonspecific accumulation was observed) of the receptor in mice bearing U87MG tumors. A final dual-modality example to consider is the probe developed by Frullano et al. [76]. They noted that a PET–MRI agent could potentially allow for quantification of both concentration and relaxivity which would enable a host of possible applications, including quantitative
pH imaging. In these initial studies, simultaneous PET–MRI measurements were acquired in phantoms with known pH, and the PET signal was used to determine the absolute concentration Phosphoprotein phosphatase of the tracer, which was then combined with MR relaxation measurements to determine the pH of the phantoms. The authors showed good correspondence between the pH measured by an electrode and that calculated from imaging data. The last example is particularly important because it simplifies the measurement of pH which is difficult by using just one of the modalities. Another, similar, utility for a dual PET–MRI tracer would be to remove the ambiguity inherent in pharmacokinetic modeling of contrast-enhanced MRI studies. As the contrast agent is not directly measured in an MRI experiment (its presence is merely inferred based on its effect on relaxation times), its concentration is difficult to quantify absolutely. This fact limits the ability to perform quantitative modeling in, for example, dynamic (T1-weighted) contrast-enhanced MRI studies or in dynamic (T2-weighted) susceptibility contrast MRI studies. However, the counts registered in a PET study are directly proportional to the concentration of tracer present in the voxel or ROI, so quantification of tracer concentration is straightforward in PET.