## 1 Introduction  
Parathyroid glands are located on the posterior surface of both thyroid lobes. They are represented by two pairs of structures: upper and lower parathyroid glands with oval shape on 2D images or ellipsoid shape in 3D volume. The size of parathyroid gland varies between 2 and 8 mm in length. There are four typically located glands in the majority (80-97%) of healthy humans, but there are various exceptions in number and location [1]. In the literature cases of up to 12 glands per patient were reported [2]. The main function of parathyroid gland is the maintenance of body's calcium and phosphate levels and it is achieved by the production of parathyroid hormones (PTH) [3]. In parathyroid disorders, depending on etiology, uncontrolled growth of parathyroid gland or hyperplasia of all parathyroid glands with transition into adenomas is observed [4; 5; 6]. Surgery remains the main treatment option, and it is a relatively simple procedure if the location of parathyroid pathology is previously known. The main challenge is the preoperative localization of parathyroid pathology. Ultrasonography [7; 8], magnetic resonance imaging (MRI) [9], computed tomography (CT), and scintigraphy [10] are used as preoperative imaging techniques. Modern imaging approaches use Single-Photon Emission Computed Tomography (SPECT) or Single-Photon Emission Computed Tomography combined with Computed Tomography (SPECT/CT) [11; 12] for improved localization. Ultrasonography remains the first-line parathyroid imaging technique due to low cost, good availability, lack of radiation, quickness of the procedure, and high anatomical detail. However, the method is operator-dependent. \({}^{99m}\)Tc-MIBI Planar and SPECT/CT with low dose protocol is usually the second-line technique. The method is not widely available, more expensive, time-consuming, and involves radiation exposure. It provides data about the metabolic activity of the hyperactive parathyroid gland [12]. Combination of SPECT and low dose CT enables the anatomical localization and good surgery results. According to the literature, sensitivity of 100% was achieved in patient-based analysis. However, in the analyzed group of patients, the lesions volume was considerably large with mean 3D volume \(1.2\pm 0.1\) cm\({}^{3}\)[13]. As a next step to reduce radiation exposure for patients and staff during SPECT/CT study, there was an attempt of 80% reduction of the administered activity. In the opinion of authors of the presented paper, phantoms test should be performed to prove the applicability of the approach [14]. Traditional contrast-assisted CT is rarely used as a single modality. It is very helpful for surgeons as it provides an excellent anatomical map, yet there are disadvantages such as high radiation exposure. The data regarding sensitivity are shown in Table 1[15]. Specificity and sensitivity of the SPECT imaging of parathyroid gland depends on scintigraphy protocol [16; 17; 18]. F18-Choline PET/CT is a novel method which might be an alternative for \({}^{99m}\)Tc-MIBI in view of better spatial resolution, allowing detection of smaller lesions. To date, the method has not been widely used because of its cost [19]. SPECT allows metabolic analysis of lesions and CT helps with anatomic localization [20; 21; 22]. The tracer used for localization of hyperfunctioning parathyroid gland/glands before the first surgery is lipophilic cation complex (MIBI: methoxy-isobutyl-isonitrile) labeled with pertechnate--\({}^{99m}\)Tc (gamma emitter with a half-life of 6 h, decays by isomeric transition to technetium-99). The factor determining tumor cell uptake of \({}^{99m}\)Tc-MIBI is a strong electrostatic attraction between the (+) charged