Table 4.
Preclinical imaging modalities, their properties and application in relation to biodegradable implants.
| CT | PET | US | PAI | MRI | |
|---|---|---|---|---|---|
| Depth | - [72] | - [72] | >50 mm [74] | >50 mm [60] | - |
| Spatial resolution | 0.05 mm [75] | 1–2 mm [72] | Up to 0.15 mm at 25 MHz [76] | 1/200th of depth [60,61] | >0.1 mm [77] |
| Target | |||||
| Bone tissue | Implants and tissues of different attenuation | Molecular activity with target tracers | Surface topography | Surface topography | Unsuitable |
| Soft tissues | Molecular activity with target tracers | Structures and borders | Structures and molecular chromophores | Highly efficient | |
| Implant | Surface topography | Mostly surface topography | Unsuitable | ||
| Biomarkers | |||||
| Inflammation | Visual oedema and liquid accumulation, liquid iodine contrast [41] | e.g., 18F-FDG [78] | Visual oedema and liquid accumulation | Oxygen level dynamics [79] | Visual oedema and liquid accumulation |
| Bone healing | Visual changes | e.g., 18F-NaF [44, 72.73] | Exceptional for early ossification [57] | Chromophores; tissue morphology changes during remodelling | Contrast of low signal from bone with high signal from soft tissues [80] |
| Angiogenesis | blood vessels with iodine contrast [81] | e.g., 68Ga-NODAGA-RGD [47]; 68Ga-NOTA-RGD [47,82]; 68Ga-DOTA-E[c(RGDfK)]2 [83] | Vascular imaging using microbubbles and binding agents [84,85] | Integrin-binding chromophores; blood oxygenation | e.g., Gadolinium contrast, perfusion capacity [86] |
| Advantages | Simple, fast | Large choice of radiotracers for different targets, possible to design tracers for specific targets; metabolic data | Simple; non-invasive; no contraindications | Can image molecular activity; differentiate targets based on absorption spectra | High resolution and depth; very good soft tissue differentiation; possible to use contrast agents |
| Disadvantages | Radiation; requires high attenuation between target and surroundings; contrast agents require injections; implant artefacts [35,36,37,40,78] | Radiation; high price; complexity; injections; long duration; requires either in-house isotope production and tracer laboratory or be within transportation range | Limited depth; can’t image inside hard objects | Very limited depth; light absorption; complexity; exogenous contrast agents require injections | High price; complexity; unsuitable for ferromagnetic implants [35]; poor imaging of targets lacking H+ protons; better resolution requires longer imaging time; contrast agents are injected |