## 1 Introduction  
Raynaud's phenomenon (RP) is a common vascular disorder consisting of recurrent, long-lasting, and episodic vasospasm of the fingers and toes often manifesting as discoloration and pain [1]. RP is typically induced by cold exposure and emotional stress [2]. It affects approximately 5-10% of the population (prevalently females) [3], with a worldwide distribution, although its prevalence is elevated in cold climates where the risk of exposure to low ambient temperatures is greater [4]. RP is classified as primary RP (PRP) if there is no known underlying illness and secondary when associated with a disorder detected upon assessment [5]. The distinction is important because prognosis, severity, and treatment can all be affected [6]. Secondary RP can be associated with many systemic rheumatic diseases. The most frequent association is with systemic sclerosis (SSc) [7]. SSc is a complex autoimmune connective tissue disease that is characterized by progressive generalized obliterative vasculopathy and widespread aberrant tissue fibrosis [8]. Although SSc is a heterogeneous disease, RP occurs in most patients, affecting \(\sim\)96% of them [9]. RP is considered the most common and one of the earliest symptoms of this disease [10]. SSc-associated RP typically has a lag period that can last several years before additional SSc organ-specific disease manifestations emerge [10, 11]. Whereas in primary or 'diopathic' RP, tissue ischemia is transient or reversible, in secondary RP persistent tissue ischemia can occur, resulting in digital ulceration and/or gangrene [12]. Frequently in the early stages of disease there is not a clear-cut difference between PRP and secondary PR. Indeed, many RP patients have no sign of systemic disease, although they do present with subtle nailfold abnormalities. Conversely, the differential diagnosis of PRP versus secondary RP is of utmost importance to allow for the earliest successful treatment of this condition and the associated underlying disease. Since RP impacts the finger thermoregulatory system, the evaluation of the finger thermoregulatory impairment is crucial to distinguish between PRP and RP secondary to SSc [13]. To this aim, finger temperature can be monitored through thermal infrared (IR) imaging technique. Thermal IR imaging is a contactless, non-invasive technique which provides a map of a body's superficial temperature by measuring its emitted infrared radiation [14; 15; 16]. It has been widely used in medicine to assess cutaneous temperature and its topographic distribution as well as to monitor the psychophysiological state of an individual [17; 18]. Considering that the skin temperature depends on local blood perfusion and thermal tissue properties, thermal IR imaging provides important indirect information concerning circulation and thermoregulatory functionality of the cutaneous tissue [14]. This technique has been employed to differentiate primary from secondary RP, often by monitoring the finger response to a controlled cold challenge [19]. Indeed, SSc, PRP, and healthy controls (HC) show different thermal recovery to the same functional stimulation (i.e., cold challenge) [13]. Therefore, the differentiation among HC, SSc, and PRP was usually performed based on statistical analysis of simple descriptors of the cutaneous temperature recovery (e.g., lag time, time to reach a given recovery threshold) [19]. However, such an analysis procedure involves identifying the regions of interest on the fingers from which the statistical descriptors of the temperature recovery are computed as well as choosing the best descriptors to use for the classification. Hence, the intervention of an expert operator is required, thus introducing operator-dependent bias on the classification outcome. Furthermore, since in PRP or SSc patients the blood vessels in the extremities are over-sensitive to changes in temperature, the administration of a cold challenge can be a very uncomfortable, and frequently painful, process.