In vivo medical imaging has made great progress due to advances in the engineering of imaging devices and developments in the chemistry of imaging probes. Several modalities have been utilized for medical imaging, including X-ray radiography and computed tomography (X-ray CT), radionuclide imaging using single photons and positrons, magnetic resonance imaging (MRI), ultrasonography (US), and optical imaging. To extract more information from imaging,“contrast agents” have been employed. For example, organic iodine compounds have been used in X-ray radiography and computed tomography, superparamagnetic or paramagnetic metals have been used in MRI, and microbubbles have been