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Perfusion can be considered a subclass of flow-based methods. In the context of this discussion, the definition of perfusion is relaxed slightly to include the passage of a fluid through microfluidic systems for the purpose of transporting nutrients and waste. Therefore, such systems can be used to maintain biological samples under conditions conducive to normal behavior, enabling long-term measurements of the system under normal and stimulated situations. Systems may include cell populations/layers/clusters and may increase in complexity up to tissue slices, organ-on-a-chip, and small organisms. Long-term measurement of such systems while using MR-compatible technical systems enables spatially resolved, longitudinal monitoring of morphology as a function of interesting stresses.

Starting from spectroscopy, perfusion-enabled microfluidic devices for long-term monitoring of biological systems have been used to monitor metabolic flux. Kalfe et al. [49] monitored a single tumor spheroid with diameter 500 µm over 24 h to characterize the transition from oxidative to glycolytic metabolism. Given the small volumes, it is important to ensure that the metabolic activity is reflective of the actual biological system and not perturbed, for example, by a lack of oxygen in the culture medium. This issue has been directly addressed by Yilmaz and Utz [47], who used a gas permeable membrane in combination with a micro-stripline [46] to ensure adequate oxygen supply for in situ cell culture NMR spectroscopy. Using 3D printing to produce MR-compatible microfluidic systems Montinaro et al. [45], demonstrated spectroscopy of small organisms, achieving detection volumes of 100 pl and therefore substructure spectroscopy of Caenorhabditis elegans in their work.

Incubator systems have been implemented for improved tissue slice MR microscopy. To enable long-term microscopy of a biologically viable tissue, incubator systems must not only manage perfusion but also gas concentration and temperature control. Flint et al. [48]. developed such an incubator system compatible with a 600-MHz NMR spectrometer. They demonstrated diffusion-weighted imaging of rat cortical slices with 31.25 µm isotropic resolution (1.5 h measurement time) over 21 h. The challenge for soft tissue incubation in vertical bore NMR systems is preventing tissue deformation caused by gravity. This can be addressed, for example, by physically clamping the tissue taking care not to unnecessarily perturb the tissue function. This challenge can be circumvented by ensuring gravity is perpendicular to the tissue surface, easily achieved in horizontal bore systems. Kamberger et al. [35] implemented an incubator for mouse brain slice imaging under this condition, with the added feature of a LL for magnetic field-focusing and improved SNR [33]. Using a 9.4-T MRI system, T1-weighted images could be obtained in 8 min with 0.5 mm slice thickness and in-plane resolution of 0.1 × 0.1 mm, importantly, with a factor of 10 improved SNR yielded by the LL (Figure 1.7). In a clever use of capillary forces, tissue–air interfaces perpendicular to B₀ were avoided by allowing the perfusion medium to slightly overfill the tissue chamber thereby eliminating magnetic susceptibility-induced imaging artifacts.