Using fluorine in MRI has several advantages as ¹⁹F is a spin ½ nucleus, which means that it has an odd number of protons and neutrons and spherical charge distribution, and a gyromagnetic ratio close to the proton. Thus, for a given magnetic field value, the spin processes at almost the same frequency, permitting the use of a conventional proton spectrometer to detect fluorine [6]. Fluorine has 100 % natural abundance and an extremely broad chemical shift and is not naturally present in biological tissues and therefore has no biological background signal; the traces of fluorine in bone and teeth exhibit a very short spin-spin relaxation time (T₂) and are not visible to conventional MRI methods [2]. In all, fluorine MRI gives a very high contrast-to-noise ratio and specificity, in a situation where a fluorinated element is introduced as a contrast agent . Such agent requires a very high density of ¹⁹F nuclei on the molecule in addition to a high tissue concentration in order to produce an image quality such as the one of ¹H MRI. However, it is important to realize that the administered levels of ¹⁹F will still be many orders of magnitude lower than the amount of ¹H present in biological tissues, and thus, despite all its advantages in terms of imaging properties, the low concentrations of ¹⁹F frequently result in images that hover at the detection limit with low signal-to-noise ratios (SNRs). Finally, some fluorocarbons show a T₁ sensitivity to oxygen tension, which may allow its use as an in vivo sensor [2]; alternatively, this T₁ variability can be a nuisance, hindering accurate quantification.

An example of fluorinated compounds includes fluorinated peptides, polymers, and also fluorinated small molecules, such as sugars; however these result in much lower cell loading and, as a result, a lower detection sensitivity. Small fluorinated molecules have lower ¹⁹F content than larger fluorocarbons, such as perfluorocarbons (PFCs); however their advantage is that they can be used to study cell functionality [5]. Despite this benefit, the insufficient sensitivity of the small molecules makes them less attractive for in vivo use. Recent developments in small fluorinated molecules include fluorinated albumin [7] and self-assembling nanoparticles sensitive to their environment [8], although neither of these have been applied to cell tracking . Some examples are listed in Table 1.

There have been limited studies using fluorinated drugs (see Table 1). The limit here is the high reactivity and electronegativity of fluorine, which when incorporated into a drug can significantly alter drug chemistry and subsequent metabolism. Chemical shift artifacts can also be an issue. For all these reasons, such small molecules are seldom used for cell tracking.

Perfluorocarbons (PFCs) are synthetic molecules mainly composed of carbon and fluorine atoms (see Table 2 for a summary of their physical properties). Depending on their chemical structure and molecular weight, PFCs exist in a gaseous, liquid, or even solid form. The compounds are extremely stable, generally have low vapor pressures, and are inert. PFCs do not react with living tissues due to their carbon-fluorine bonds, which cannot be metabolized in vivo, and they have low intermolecular forces, and hence, the surface tension of the liquid form of PFCs is extremely low. Most PFCs have a surface tension of 14–18 dyne/cm [9]. As gases, PFCs show some toxicity, while in a liquid form they are relatively safe [10]. Clearance of PFC agents from the body generally occurs via the mononuclear phagocyte system (MPS) uptake (phagocytic cells primarily in lymph nodes, spleen, and liver) and finally lung exhalation. Some PFCs can also bind and dissolve oxygen. This led to their early use as blood substitutes, although the idea has been commercially abandoned. PFCs are still used clinically, for example for partial liquid ventilation [LiquiVent, Alliance Pharmaceutical Corporation], eye surgery [Perfluoron™, Alcon^® Inc., USA], and as ultrasound contrast agents as gas bubbles [Sonovue^®, Bracco International B.V.] (Fig. 2a).

PFCs have now found their use as imaging labels for ultrasonography, computed tomography, and ¹⁹F MRI. Their use in the latter imaging method is especially beneficial, as PFCs have a high payload of fluorine atoms, which makes ¹⁹F MRI a promising modality for in vivo quantitative imaging, including in vivo cell tracking. Here, liquid PFCs are more suitable, as they result in the most favorable imaging characteristics.