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Molecular imaging is a non-invasive technique employed to visualize, characterize, and quantify biological processes at the molecular and cellular levels within humans or other living organisms. In essence, injecting and following molecular imaging tracer, which comprises a targeting vehicle joined to a detection label, is a convenient functional molecular imaging approach. In recent years, various molecular imaging tracers have been developed for (pre)clinical research fields such as oncology, cardiology, neurology, or rheumatology.

SdAb as In Vivo Molecular Imaging Tracer
The rapidly growing field of in vivo molecular imaging has inspired the development of many imaging tracers, each with its advantages and limitations. In this area, sdAbs match the requirements of the ideal molecular imaging tracer because of their unique properties:

sdAbs bind tightly to the targets and have affinities in the low nanomolar to high picomolar range.
sdAbs are highly stable to tolerate numerous labeling strategies (e.g., rationale, near-infrared fluorophores, and fluorescent proteins) to meet the various in vivo imaging requirements modalities.
sdAbs can rapidly distribute through the bloodstream, reaching tissues homogenously.
sdAbs easily penetrate tissues, also accessing cryptic antigens (e.g., located behind the blood-brain barrier).
sdAbs from camelid share a higher degree of sequence identity with human VHs and have a low immunogenic potential.
sdAbs are cleared from blood rather quickly when unbound due to the small molecular weight, resulting in a contrast-enhanced imaging signal and reduced accumulation of labeled fragments in the liver, lowering radiation burden.
Collectively, these attributes render sdAbs highly suitable for in vivo molecular imaging probes in both preclinical and clinical settings. Nonetheless, a significant limitation of sdAb-based imaging is their high non-specific uptake in the kidneys and bladder. Potential solutions to this issue include the development of multivalent sdAbs or sdAb-Fc fusion constructs. The fusion of sdAbs with Fc fragments appears particularly promising due to this option can also extend half-life and enhance tumor penetration capabilities.

In the evolving landscape of integrated diagnosis and treatment, sdAbs are poised to play pivotal roles. They are instrumental in tumor diagnosis, assessment, and prediction prior to initiating therapeutic protocols, as well as in dynamic monitoring during treatment to detect potential disease recurrence. To date, various diagnostic imaging techniques have been developed and employed using sdAbs as in vivo tracers. These include nuclear imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) with radionuclides like 99mTc, 89Zr, or 68Ga; optical imaging with near-infrared (NIR) fluorescent dyes; ultrasound imaging using microbubbles; computed tomography (CT); and magnetic resonance imaging (MRI).