## 1 Introduction  
Molecular Imaging (MI) is a type of medical procedure that provides the visualization, characterization, and measurement of biological processes at the molecular and cellular levels in living systems [1]. Nuclear medicine is a branch of MI that, through the use of radiopharmaceuticals, allows physicians to see and to measure functional, metabolic, chemical, and biological processes within the body and/or to treat malignant tumours, cancer, and other diseases [2; 3; 4; 5]. In general, a radiopharmaceutical is a medicinal product that is, usually, administered intravenously to the patient. The radiopharmaceutical is the result of the linkage of two elements, a carrier and at least one radioactive atom that, with its nuclear properties, defines the diagnostic and/or therapeutic nature of the radioactive compound. The carrier plays an important role in the selective transport of the radionuclide to a specific biological target (Figure 1). Radioactive atoms decay through a nuclear process that leads to the production of \(\gamma\)-rays used in the diagnostic imaging field. In this case, the radioactivity coming from the radiopharmaceutical, carrying the diagnostic information outward from the body, is detected and transformed in a very precise picture of the radiopharmaceutical's distribution in the body by means of so-called \"gamma cameras\" working with sophisticated computerized algorithms. On the contrary, radionuclides decaying through the emission of massive particles, such as electrons or alphas, can be used in nuclear medicine for radionuclide therapy to treat certain types of cancer and other diseases [6; 7; 8]. Ideally, for diagnostic nuclear medicine imaging applications, each nuclear decay should yield a monochromatic gamma-ray in the energy range 100-600 keV, and the half-life of the radionuclide should be on the order of a few hours to avoid unnecessary exposure to radiation. Radiopharmaceuticals for single-photon computed tomography (SPECT) are generally composed of single small molecules (the size can range from 10-12 to 10-6 m) radiolabelled with a gamma-emitting isotope, such as indium-111, iodine-123, gallium-67, or technetium-99m. Advanced methods to specifically and selectively incorporate medical radionuclides, such as radiometals, into targeting vectors, based on an improved understanding of their coordination chemistry and the development of creative labelling strategies, are the core of modern radiopharmaceutical research [9; 10; 11]. Among the SPECT isotopes currently in use, technetium-99m has become the workhorse of diagnostic nuclear medicine and technetium-99m-based radiopharmaceuticals are still the most used radioactive chemical compounds in hospitals' clinical practice [12; 13; 14; 15]. The main reasons for \({}^{99m}\)Tc's continuing usage are its ideal nuclear properties and its convenient supply method through a commercial generator system. Techentium-99m decays through the emission of 140 keV \(\gamma\)-rays (89% abundance), which is ideal for imaging with medical gamma cameras, and can be administered to patients in low-radiation doses. Moreover, its 6-h half-life is sufficient for the preparation of \({}^{99m}\)Tc radiopharmaceuticals (in hospitals or centralized radiopharmaceuticals), their possible distribution, the performance of quality controls, administration to the patient, accumulation in the target organ, and image acquisition. Even if the use of radiopharmaceuticals labelled with positron emitters radionuclides challenged the field of SPECT tracers, over 70% of diagnostic investigations are still performed with this single isotope for imaging of bone, renal, hepatic, hepatobilary, cardiac, and oncological diseases or other pathologies.