Peripheral arterial disease (PAD) is one of the most common diseases in erlderly people, with an overall prevalence of around 3-10% and a prevalence of 15-20% in people over 70 years. It is associated with an increased risk of cardiovascular morbidity and mortality. Current diagnostic tools include the ankle brachial index (ABI), computed tomography (CT), magnetic resonance imaging (MRI) or digital subtraction angiography (DSA), which mainly examine the macrocirculation. Despite these various diagnostic options, the diagnosis of patients with unclear leg pain when walking remains difficult. In these cases, a diagnostic tool for direct assessment of the affected organ, namely the lower leg muscles in PAD, would be helpful. Multispectral optoacoustic tomography (MSOT) promises to overcome these diagnostic challenges as it enables non-invasive, quantitative imaging of the composition and oxygenation of target tissues such as muscle. Since hemoglobin concentration and oxygenation status are markers of perfusion, MSOT-based imaging of these parameters could provide a highly sensitive and reliable method for analyzing muscle perfusion in patients with peripheral arterial disease (PAD), as initial studies by the working group have shown. 
Whether there is heterogeneity in the distribution of hemoglobin or oxygenation status in the examined muscles and whether this heterogeneity plays a role in the diagnosis of claudication remains unknown. Therefore, a new approach of three-dimensional (3D) MSOT imaging of the calf muscle will be investigated in this study.  
Since 3D MSOT imaging is limited to either specialized 3D transducers with limited field of view or costly or bulky tracking devices for 3D reconstruction, a novel optical pattern specifically designed for easy and inexpensive 3D reconstruction of conventional 2D MSOT images ("tattoo" tomography) will be used here. The pattern printed with special ink only absorbs light in a specific wavelength range and thus enables both anatomical and functional information from MSOT image sections. Using this approach, 3D longitudinal scans along the muscle can be created, providing further insight into the microperfusion, the degree of degeneration and the volume of the muscle. Furthermore, this technique is not limited to PAD patients, but can potentially be used for other clinical applications such as tumor diagnosis or assessment of intestinal perfusion.

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