In vivo imaging and tracking of transfected liver cells is a critical part of assessing the success of liver transfection techniques. The ability to track the cells over time can provide important information about the distribution of the transfected cells, the duration of transgene expression, and the response of the host organism. Several techniques are commonly used:

1. Bioluminescence Imaging (BLI): Bioluminescence imaging involves introducing a gene that encodes a bioluminescent protein (such as luciferase) into the cells. The protein emits light when it reacts with a substrate, which can be detected and quantified using a special camera. This technique allows for non-invasive, real-time monitoring of the transfected cells in living organisms.
2. Fluorescence Imaging: Similar to bioluminescence, fluorescence imaging involves introducing a gene that encodes a fluorescent protein (such as GFP – green fluorescent protein). The protein emits light when exposed to specific wavelengths, which can be detected using fluorescence microscopy. This technique is often used in preclinical models, such as transgenic mice.
3. Magnetic Resonance Imaging (MRI): MRI can be used to track cells labeled with magnetic particles. This technique provides high-resolution, three-dimensional images and can be used in larger animals and potentially in humans. However, it requires the use of contrast agents and is more technically demanding than some other methods.
4. Positron Emission Tomography (PET) and Single-Photon Emission Computed Tomography (SPECT): These nuclear imaging techniques can be used to track radiolabeled cells or transgene products in vivo. They provide three-dimensional images and can be used in both preclinical models and humans.
5. Computed Tomography (CT): CT scans can be used in combination with contrast agents to visualize the distribution of transfected cells, although this technique is less commonly used for cell tracking compared to the others.

In vivo tracking of transfected liver cells can provide valuable insights into the behavior of these cells and the efficacy of gene therapy treatments. However, these techniques also come with challenges, such as the potential for the signal to decrease over time (due to dilution of the reporter gene during cell division, for example), potential toxicity of the reporter gene or contrast agent, and the need for specialized equipment and expertise. As of my knowledge cut-off in September 2021, research is ongoing to develop new and improved methods for in vivo cell tracking.