Rheumatologists in Europe and the USA increasingly rely on fluorescence optical imaging (FOI, Xiralite) for the diagnosis of inflammatory diseases. Those include rheumatoid arthritis, psoriatic arthritis, and osteoarthritis, among others. Indocyanine green (ICG)-based FOI allows visualization of impaired microcirculation caused by inflammation in both hands in one examination. Thousands of patients are now documented and most literature focuses on inflammatory arthritides, which affect synovial joints and their related structures, making it a powerful tool in the diagnostic process of early undifferentiated arthritis and rheumatoid arthritis. However, it has become gradually clear that this technique has the potential to go even further than that. FOI allows visualization of other types of tissues. This means that FOI can also support the diagnostic process of vasculopathies, myositis, collagenoses, and other connective tissue diseases. This work summarizes the most prominent imaging features found in FOI examinations of inflammatory diseases, outlines the underlying anatomical structures, and introduces a nomenclature for the features and, thus, supports the idea that this tool is a useful part of the imaging repertoire in rheumatology clinical practice, particularly where other imaging methods are not easily available.

Fluorescence imaging has a long history in diffuse optics.^{1 – 4} The main success of this technology in clinical applications is based on the broad acceptance of the contrast agent indocyanine green.^5,6 This fluorescent dye is applied as a marker for cancer tissue^{7 – 10} and for sentinel lymph node detection in fluorescence-guided surgery,^{11 – 13} and as a perfusion agent, e.g., for the detection of rheuma.^14,15 With its absorption and fluorescence band in the near-infrared (NIR) spectral region the application of indocyanine green (ICG) is not limited to superficial tissue regions. It can be also used for imaging through even centimeter thick tissue regions as successfully demonstrated by in vivo breast tumor characterization.^5,16