## 1 Introduction  
Alzheimer's disease (AD) is the leading cause of dementia accounting for up to 70% of cases [1]. In 2015, up to 47 million people were affected worldwide [2; 3; 4]. This number is expected to double every 20 years, resulting in a dramatically increased social as well as financial burden [5]. AD is characterizedby the degradation of neurons, especially in the cerebral cortex, basal forebrain, hippocampus and amygdala, critical regions for memory and learning as well as cognitive functioning [6; 7; 8; 9; 10; 11; 12]. The disease manifests with mild cognitive deficits progressing to significant cognitive impairments over time [13]. As the disease progresses, patients become increasingly dependent on caregivers [14]. Current research focuses on developing targeted therapies for the disease and improving diagnostics of AD, for instance by investigating imaging biomarkers [15]. Key hallmarks of AD are the deposition of intracellular neurofibrillary tangles and extracellular accumulation of pathological amyloid-beta protein [16]. The gold standard for the definitive diagnosis of AD is the histological analysis of different brain regions including multiple cortical regions [17]. For this purpose, the tissue has to be fixed in formalin, dehydrated and embedded in paraffin. Using immunohistochemistry, thin sections of the tissue are then stained in order to determine the presence or absence of amyloid-beta protein in the brain [18]. Usually, a further section is stained with Congo red dye to visualize a subclass of amyloid-beta plaques, so-called neuritic plaques. Due to the parallel arrangement of the amyloid fibrils within them, neuritic plaques exhibit the physical phenomenon known as birefringence [19]. This birefringence is amplified by Congo red and can therefore be visualized with polarized light microscopy [20]. However, the histological procedure requires formalin fixation, which terminates cell metabolism, does not preserve lipid structures such as cell membranes, and shrinks the tissue [21; 22]. An alternative to conventional histology is the sectioning of fresh frozen tissue. This method does not require any of the previously mentioned tissue preparation procedures, which makes it a rapid and powerful approach for intraoperative diagnostics. However, studies have shown that the frozen-section procedure is not as accurate as conventional formalin-fixed and paraffin-embedded histological analysis of tissue and that pathologists often have to deal with artifacts due to the rapid freezing process [23; 24]. Optical coherence tomography (OCT) is an optical imaging technique, which was introduced in the early 1990s [25] and has since become a standard diagnostic tool in ophthalmology [26]. OCT has also been used in many other fields including dermatology, endoscopy and neuroimaging [27; 28; 29]. OCT data can be acquired in three-dimensional (3D), in real time, and with micrometer resolution [25]. Most importantly, tissues can be imaged non-destructively in an imaging range of a few millimeters. OCT image contrast is based on the inherent backscattering properties of the sample, so that extensive tissue preparation such as fixation is not required. A functional extension of OCT is polarization-sensitive (PS) OCT [19; 30]. PS-OCT visualizes specific tissue types based on how they alter the polarization state of the incident light beam. For example, birefringence exhibited by fibrous tissues and depolarization produced by melanin pigments are sources of PS-OCT image contrast and can be assessed by PS-OCT [25; 31]. Several recent reports have demonstrated that amyloid-beta plaques in both human and mouse brain tissue can be visualized using either conventional OCT, PS-OCT [19; 32; 33; 34; 35; 36] or other functional extensions such as spectroscopic OCT [32] or directional OCT [36]. Label-free imaging of cerebral amyloid-beta plaques has been achieved using extended-focus optical coherence microscopy (OCM) in the near infrared range at 800 nm [34].